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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Chem.</journal-id>
<journal-title>Frontiers in Chemistry</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Chem.</abbrev-journal-title>
<issn pub-type="epub">2296-2646</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">859969</article-id>
<article-id pub-id-type="doi">10.3389/fchem.2022.859969</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Chemistry</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Efficient Coagulation Removal of Fluoride Using Lanthanum Salts: Distribution and Chemical Behavior of Fluorine</article-title>
<alt-title alt-title-type="left-running-head">Zhong et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">Fluoride Removal Using Lanthanum Salts</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Zhong</surname>
<given-names>Xiaocong</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Chen</surname>
<given-names>Chen</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Yan</surname>
<given-names>Kang</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zhong</surname>
<given-names>Shuiping</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Wang</surname>
<given-names>Ruixiang</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1642785/overview"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Xu</surname>
<given-names>Zhifeng</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>Faculty of Materials Metallurgy and Chemistry</institution>, <institution>Jiangxi University of Science and Technology</institution>, <addr-line>Ganzhou</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>State Key Laboratory of Separation and Comprehensive Utilization of Rare Metals</institution>, <addr-line>Guangzhou</addr-line>, <country>China</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>Jiangxi College of Applied Technology</institution>, <addr-line>Ganzhou</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/61218/overview">Florent Allais</ext-link>, AgroParisTech Institut des Sciences et Industries du Vivant et de L&#x2019;environnement, France</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/654108/overview">Flemming Jappe Frandsen</ext-link>, Technical University of Denmark, Denmark</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1651773/overview">Hamid Rashidi Nodeh</ext-link>, Standard Research Institute (SRI),&#x20;Iran</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Ruixiang Wang, <email>jxustpaper@163.com</email>; Zhifeng Xu, <email>jxustxzf@163.com</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>04</day>
<month>03</month>
<year>2022</year>
</pub-date>
<pub-date pub-type="collection">
<year>2022</year>
</pub-date>
<volume>10</volume>
<elocation-id>859969</elocation-id>
<history>
<date date-type="received">
<day>22</day>
<month>01</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>14</day>
<month>02</month>
<year>2022</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2022 Zhong, Chen, Yan, Zhong, Wang and Xu.</copyright-statement>
<copyright-year>2022</copyright-year>
<copyright-holder>Zhong, Chen, Yan, Zhong, Wang and Xu</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<abstract>
<p>
<bold>Abstract:</bold> La-loaded absorbents have been widely reported for fluoride removal due to the strong affinity of La<sup>3&#x2b;</sup> towards fluoride ion. Herein, chemical removal of fluoride from flue gas scrubbing wastewater using lanthanum salt is investigated. The retaining free F<sup>&#x2212;</sup> concentration, phase composition and morphology of filtration residues, and the distribution of fluorine have been investigated using ion-selective electrode, analytical balance, scanning electron microscopy, and X-ray diffractor. The results show that at La/F molar ratio &#x2265;1:3.05, the majority of fluorine exists as LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> complexes, leading to the failure of fluoride removal. At 1:3.20 &#x2264; La/F molar ratio &#x2264;1:3.10, the formation of LaF<sub>3</sub> is facilitated. However, co-existing LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> tends to absorb on the surface of LaF<sub>3</sub> particles, leading to the formation of colloidal solution with large numbers of LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> suspended solids. At an optimized La/F molar ratio of 1:3.10, a fluoride removal of 97.86% is obtained with retaining fluorine concentration of 6.42&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>. Considering the existing of positively charged LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>, coagulation removal of fluoride is proposed and investigated using lanthanum salts and negatively charged SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O colloidal particles, which is <italic>in-situ</italic> provided <italic>via</italic> Na<sub>2</sub>SiO<sub>3</sub> hydrolysis at pH near 5.5. At a La/F molar ratio of 1:3.00 and Na<sub>2</sub>SiO<sub>3</sub> dose of 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, a fluoride removal of 99.25% is obtained with retaining fluorine concentration of 2.24&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>. When Na<sub>2</sub>SiO<sub>3</sub> dose increases to 1.00&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, the retaining fluorine concentration could be further reduced to 0.80&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>.</p>
</abstract>
<kwd-group>
<kwd>fluoride removal</kwd>
<kwd>LaF<sub>x</sub>
<sup>3-x</sup> complexes</kwd>
<kwd>coagulation</kwd>
<kwd>colloidal particles</kwd>
<kwd>precipitation</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>Fluorine is one of the main contaminants in ground water and industrial effluents (<xref ref-type="bibr" rid="B14">Kong et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B30">Wan et&#x20;al., 2021</xref>). Long-term intake of high-fluoride level water may lead to dental fluorosis, bone fluorosis and even neurological damage (<xref ref-type="bibr" rid="B6">D&#xed;az-Flores et&#x20;al., 2021</xref>). According to a standard issued by the World Health Organization (WHO), the fluorine content in drinking water should be less than 1.5&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B13">Kim et&#x20;al., 2020</xref>).</p>
<p>Following the large-scale exploitation of sphalerite ores in recent decades, the available sphalerite ores present a decreasing grade and contain complicated components (<xref ref-type="bibr" rid="B10">Hu et&#x20;al., 2017</xref>). Among the typical impurities in sphalerite ores, the fluoride has attracted extensive attention due to its detrimental effects on the peel-off of cathodic product (<xref ref-type="bibr" rid="B22">O&#x27;Keefe and Han, 1992</xref>) and the corrosion of lead-based anodes during the zinc electrowinning process (<xref ref-type="bibr" rid="B36">Zhong et&#x20;al., 2015</xref>; <xref ref-type="bibr" rid="B35">Zhong et&#x20;al., 2017</xref>). During the roasting of sphalerite ores, about 70% of the fluorine and chlorine in the ores enter the flue gas, and the others incorporate into the zinc calcine. In order to further reduce the content of fluorine and chlorine, the zinc calcine is usually treated in a rotary/tube furnace at elevated temperature, where fluorine and chlorine would be volatilized into flue gas in the form of HF and HCl, respectively (<xref ref-type="bibr" rid="B5">&#xc7;inar&#x15e;ahin et&#x20;al., 2000</xref>; <xref ref-type="bibr" rid="B12">Kahvecioglu et&#x20;al., 2013</xref>). Similarly, the zinc recovery processes from Waelz oxide, zinc oxide fumes, electric arc furnace dust, and other zinc-bearing dusts also discharge flue gas containing HF and HCl (<xref ref-type="bibr" rid="B32">Wei et&#x20;al., 2010</xref>; <xref ref-type="bibr" rid="B19">Martins et&#x20;al., 2021</xref>). During the scrubbing of above-mentioned flue gas, fluorine and chlorine transfer into the liquid phase, yielding a large amount of fluorine-containing acidic scrubbing wastewater. Due to the potential threat of fluorine on human&#x2019;s health, the fluoride ions must be selectively removed before being discharged. According to China&#x2019;s &#x2018;Emission Standard of Pollutants for Copper, Nickel and Cobalt Industries&#x2019; (GB25467-2010), the fluoride ion content in discharge water should be below 5.0&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B18">Liu., 2021</xref>).</p>
<p>Varied literatures have reported methods to efficiently remove fluoride from aqueous solutions, such as chemical precipitation (<xref ref-type="bibr" rid="B28">Turner et&#x20;al., 2005</xref>; <xref ref-type="bibr" rid="B4">Chang and Liu., 2007</xref>; <xref ref-type="bibr" rid="B11">Huang et&#x20;al., 2017</xref>), coagulation (<xref ref-type="bibr" rid="B27">Sandoval et&#x20;al., 2019</xref>), adsorption (<xref ref-type="bibr" rid="B24">Ravuru et&#x20;al., 2021</xref>; <xref ref-type="bibr" rid="B34">Yang et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B25">Sadhu et&#x20;al., 2021</xref>), ion exchange (<xref ref-type="bibr" rid="B26">Samadi et&#x20;al., 2014</xref>) and membrane separation (<xref ref-type="bibr" rid="B7">Feng et&#x20;al., 2008</xref>; <xref ref-type="bibr" rid="B2">Arahmana et&#x20;al., 2016</xref>). Among these methods, chemical precipitation has been widely used in metallurgical industry due to its advantages of simple operation, low cost, and applicability for high-fluorine wastewater. At present, the most widely used precipitant for fluoride removal is lime. However, due to the limitation of solubility product of CaF<sub>2</sub>, it is difficult to achieve a retaining fluoride content below 7.5&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B29">Wajima et&#x20;al., 2009</xref>) via lime precipitation, which is higher than the regulation limit of fluoride&#x20;level.</p>
<p>As a highly electropositive metal, La<sup>3&#x2b;</sup> has a strong affinity towards fluoride ion (<xref ref-type="bibr" rid="B21">Nagaraj et&#x20;al., 2017</xref>). Consequently, a large number of La-loaded absorbents have been reported for fluoride removal from ground water (G.J.&#x20;<xref ref-type="bibr" rid="B20">Millar et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B33">Yan et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B31">Wang et&#x20;al., 2018</xref>; <xref ref-type="bibr" rid="B8">He et&#x20;al., 2019</xref>). Absorption has been regarded as one of the most effective methods to treat ground water with relative low fluoride concentration (&#x3c;100&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>), which features low retaining fluoride concentration, low cost, and easy operation (<xref ref-type="bibr" rid="B37">Zhou et&#x20;al., 2018</xref>). In a typical flue gas scrubbing wastewater, the fluoride content could be higher than 300&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, La-loaded absorbents are not suitable to treat this aqueous solution because of its relatively small absorption capacity, which could lead to a large consumption of absorbents and long operation time. Hence, chemical precipitation of fluoride ion using lanthanum salt was proposed to remove fluoride in this work. In spite of extensive report on La-loaded absorbents, the distribution and chemical behavior of fluorine in the presence of La<sup>3&#x2b;</sup> remains unclear.</p>
<p>In the present work, the aqueous equilibrium diagrams of F-H<sub>2</sub>O and La-F-Cl-H<sub>2</sub>O systems were made to understand the species distribution of fluorine and lanthanum element in La-F-Cl-H<sub>2</sub>O system at varied pH values. The retaining fluorine concentration, precipitate morphology and structure, and distribution of fluorine in La-F-Cl-H<sub>2</sub>O system at different La/F molar ratios (1:3.20 &#x2264; La/F molar ratio &#x2264;1:2.40) were investigated and analyzed. Based on the experimental results mentioned above, the distribution and chemical behavior of fluorine in the presence of La<sup>3&#x2b;</sup> was discussed in detail. Furthermore, coagulation strategy was proposed to efficiently remove fluoride by adding Na<sub>2</sub>SiO<sub>3</sub> and La(NO<sub>3</sub>)<sub>3</sub>&#xb7;H<sub>2</sub>O. The retaining fluorine in the equilibrium solution can be reduced to 0.80&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>.</p>
</sec>
<sec id="s2">
<title>Experimental Section</title>
<sec id="s2-1">
<title>Reagents</title>
<p>NaF, NaCl, La(NO<sub>3</sub>)<sub>3</sub>&#xb7;6H<sub>2</sub>O, Na<sub>2</sub>SiO<sub>3</sub>, NaOH, HNO<sub>3</sub> of analytical grade were purchased (Sinopharm Group, China) and used without further purification. The fluorine-containing synthetic solutions were prepared to stimulate flue gas scrubbing wastewater with NaF, NaCl, and deionized water, and its pH was adjusted to 2.0&#x20;&#xb1; 0.1 using HNO<sub>3</sub>. The fluorine and chlorine concentrations of synthetic solutions were set as 300&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> and 1,200&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> respectively, closing to those of the industrial flue gas scrubbing wastewater.</p>
</sec>
<sec id="s2-2">
<title>Aqueous Equilibrium Diagram</title>
<p>The aqueous equilibrium diagrams of F-H<sub>2</sub>O and La-F-Cl-H<sub>2</sub>O systems were made using Hydra database and MEDUSA<sup>&#xa9;</sup> software. Specifically, the Hydra database (<xref ref-type="bibr" rid="B23">Puigdomenech, 2004</xref>) provided thermodynamic data such as composition data, possible solid phases formed, potential chemical reactions and their equilibrium constants. In the F-H<sub>2</sub>O system, the main species considered were HF, F, H<sub>2</sub>F<sub>2</sub>, HF<sub>2</sub>
<sup>&#x2212;</sup>, H<sup>&#x2b;</sup> and OH<sup>&#x2212;</sup>. In the La-F-Cl-H<sub>2</sub>O system, F-, HF, H<sub>2</sub>F<sub>2</sub>, HF<sub>2</sub>
<sup>&#x2212;</sup>, La<sup>3&#x2b;</sup>, LaF<sup>2&#x2b;</sup>, LaF<sub>2</sub>
<sup>&#x2b;</sup>, LaF<sub>3</sub>, LaF<sub>4</sub>
<sup>&#x2212;</sup>, La(OH)<sup>2&#x2b;</sup>, La(OH)<sub>2</sub>
<sup>&#x2b;</sup>, La(OH)<sub>3</sub>, La(OH)<sub>4</sub>
<sup>-</sup>, La<sub>5</sub>(OH)<sub>9</sub>
<sup>6&#x2b;</sup>, LaCl<sup>2&#x2b;</sup>, LaCl<sub>2</sub>
<sup>&#x2b;</sup>, H<sup>&#x2b;</sup> and OH<sup>&#x2212;</sup> were taken into account. The chemical reactions and corresponding equilibrium constants are listed in <xref ref-type="table" rid="T1">Table&#x20;1</xref>. These data were sent to MEDUSA<sup>&#xa9;</sup> software to make diagrams based on principles of mass conservation, simultaneous chemical equilibrium, and electronic charge neutrality. The distribution of fluorine species in the F-H<sub>2</sub>O system, and the distribution of La or F species in the La-F-Cl-H<sub>2</sub>O system were analyzed.</p>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>Chemical reactions in La-F-Cl-H<sub>2</sub>O aqueous system and their equilibrium constants.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Equation no</th>
<th align="center">Reaction</th>
<th align="center">Lg K</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">(1)</td>
<td align="center">2H<sup>&#x2b;</sup> &#x2b; 2F<sup>&#x2212;</sup> &#x3d; H<sub>2</sub>F<sub>2</sub>
</td>
<td align="char" char=".">6.77</td>
</tr>
<tr>
<td align="left">(2)</td>
<td align="center">H<sup>&#x2b;</sup> &#x2b; F<sup>&#x2212;</sup> &#x3d; HF</td>
<td align="char" char=".">3.18</td>
</tr>
<tr>
<td align="left">(3)</td>
<td align="center">H<sup>&#x2b;</sup> &#x2b; 2F<sup>&#x2212;</sup> &#x3d; HF<sub>2</sub>
<sup>&#x2212;</sup>
</td>
<td align="char" char=".">3.62</td>
</tr>
<tr>
<td align="left">(4)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x3d; H<sup>&#x2b;</sup> &#x2b; La(OH)<sup>2&#x2b;</sup>
</td>
<td align="char" char=".">&#x2212;8.66</td>
</tr>
<tr>
<td align="left">(5)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x3d; 2H<sup>&#x2b;</sup> &#x2b; La(OH)<sub>2</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">&#x2212;18.14</td>
</tr>
<tr>
<td align="left">(6)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x3d; 3H<sup>&#x2b;</sup> &#x2b; La(OH)<sub>3</sub>
</td>
<td align="char" char=".">&#x2212;27.91</td>
</tr>
<tr>
<td align="left">(7)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x3d; 4H<sup>&#x2b;</sup> &#x2b; La(OH)<sub>4</sub>
<sup>-</sup>
</td>
<td align="char" char=".">&#x2212;40.86</td>
</tr>
<tr>
<td align="left">(8)</td>
<td align="center">5La<sup>3&#x2b;</sup> &#x3d; 9H<sup>&#x2b;</sup> &#x2b; La<sub>5</sub>(OH)<sub>9</sub>
<sup>6&#x2b;</sup>
</td>
<td align="char" char=".">&#x2212;71.20</td>
</tr>
<tr>
<td align="left">(9)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; F<sup>&#x2212;</sup> &#x3d; LaF<sup>2&#x2b;</sup>
</td>
<td align="char" char=".">3.85</td>
</tr>
<tr>
<td align="left">(10)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; 2F<sup>&#x2212;</sup> &#x3d; LaF<sub>2</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">6.65</td>
</tr>
<tr>
<td align="left">(11)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; 3F<sup>&#x2212;</sup> &#x3d; LaF<sub>3</sub>
</td>
<td align="char" char=".">8.69</td>
</tr>
<tr>
<td align="left">(12)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; 4F<sup>&#x2212;</sup> &#x3d; LaF<sub>4</sub>
<sup>&#x2212;</sup>
</td>
<td align="char" char=".">10.35</td>
</tr>
<tr>
<td align="left">(13)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; Cl<sup>&#x2212;</sup> &#x3d; LaCl<sup>2&#x2b;</sup>
</td>
<td align="char" char=".">0.29</td>
</tr>
<tr>
<td align="left">(14)</td>
<td align="center">La<sup>3&#x2b;</sup> &#x2b; 2Cl<sup>-</sup> &#x3d; LaCl<sub>2</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">&#x2212;0.03</td>
</tr>
<tr>
<td align="left">(15)</td>
<td align="center">H<sub>2</sub>O &#x3d; H<sup>&#x2b;</sup> &#x2b; OH<sup>&#x2212;</sup>
</td>
<td align="char" char=".">&#x2212;14.00</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
<sec id="s2-3">
<title>Experimental Procedure and Characterization</title>
<p>The pH of the fluorine-containing synthetic solution was firstly adjusted to 5.5&#x20;&#xb1; 0.1 using NaOH. Afterwards, different amounts of La(NO<sub>3</sub>)<sub>3</sub>&#xb7;6H<sub>2</sub>O were added to 200&#xa0;ml of synthetic solution under agitation (200&#xa0;rpm) based on a specific La/F molar ratio. The fluoride removal reaction proceeded for 1&#xa0;h at 30&#x20;&#xb1; 1&#xb0;C. During the reaction, fluoride ion selective electrode and pH meter were used to monitor the variation of pF (-log(a<sub>F-</sub>), measured continuously without pH adjustment) and pH value with reaction time. At the end of reaction, the solution was vacuum filtrated using inorganic filtration membrane (pore diameter of 0.22&#xa0;&#x3bc;m, Jinteng&#xae;). The filtration time was recorded, and the filtration residue was collected. The final pF of the solution was measured using ion selective electrode after adjusting the pH of filtrate to 5.5&#x20;&#xb1; 0.1. The filtration residues collected were dried overnight in an oven at 80&#xb0;C. After this, the weights of filtration residues were obtained using weight loss method. The morphologies and phase structures of filtration residues were investigated using scanning electron microscopy (SEM, MIRA 3) and X-ray diffraction (XRD, MiniFlex 600-C). At a La/F molar ratio &#x2265;1:3.05, there was no precipitate obtained, extra NaF (0.133&#xa0;g) was added into the filtrate to produce LaF<sub>3</sub> precipitates, which were further collected, dried, weighed and analyzed as mentioned&#x20;above.</p>
<p>Removal of fluoride from synthetic solution using Na<sub>2</sub>SiO<sub>3</sub> and La<sup>3&#x2b;</sup> was also investigated. Firstly, different doses of Na<sub>2</sub>SiO<sub>3</sub> (0.25&#x2013;1.00&#xa0;g&#xa0;L<sup>&#x2212;1</sup>) were added into the synthetic solution under agitation (200&#xa0;rpm). After 30&#xa0;min of agitation, the solution pH was adjusted to 5.5&#x20;&#xb1; 0.1. Then La(NO<sub>3</sub>)<sub>3</sub>&#xb7;6H<sub>2</sub>O was added to target a La/F molar ratio of 1:3.00. After 1&#xa0;h of reaction, the solution was vacuum filtrated. The filtration residue and filtrate were analyzed as mentioned above. To confirm the presence or absence of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>, extra NaF (0.133&#xa0;g) was added to the filtrate, and whether precipitate formed was checked.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>Results and Discussion</title>
<sec id="s3-1">
<title>Aqueous Equilibrium Diagram</title>
<p>
<xref ref-type="fig" rid="F1">Figure&#x20;1A</xref> shows the distribution of fluorine species in F-H<sub>2</sub>O equilibrium system. At pH &#x3c; 4.0, fluorine element exists in the form of HF, F<sup>&#x2212;</sup> and H<sub>2</sub>F<sub>2</sub>. The proportion of H<sub>2</sub>F<sub>2</sub> decreases as pH increases from 1.0 to 4.0 and descends to zero at pH &#x3e; 4.0. As a weak acid, most of fluorine exists as HF at low pH range (<xref ref-type="bibr" rid="B15">Li et&#x20;al., 2013</xref>). Over the pH range 1.0&#x2013;5.5, the proportion of HF descends rapidly as pH increases, while the proportion of F<sup>&#x2212;</sup> exhibits a reverse variation. At pH &#x3e; 5.5, HF becomes negligible, and almost all the fluorine exists as free F<sup>&#x2212;</sup>. <xref ref-type="fig" rid="F1">Figures 1B, C</xref> show the distribution of F and La species in La-F-Cl-H<sub>2</sub>O equilibrium system. The molar ratio of La/F in the system is set as 1:3.00, and the total concentration of F and Cl is 0.0158 and 0.0338 M, respectively, same as the synthetic solution. As shown in <xref ref-type="fig" rid="F1">Figures 1B,C</xref>, at pH &#x3c; 3.0, a small part of fluorine exists as H<sub>2</sub>F<sub>2</sub>, while the majority of fluorine exists as LaF<sub>3</sub>. Over the pH range 3.0&#x2013;8.3, almost all fluorine exists in the form of LaF<sub>3</sub>. Interestingly, as pH further increases (&#x3e;8.3), La<sup>3&#x2b;</sup> tends to hydrolyze to La(OH)<sub>3</sub>, resulting in a decrease of LaF<sub>3</sub> proportion. Based on the thermodynamic diagrams of La-F-Cl-H<sub>2</sub>O equilibrium system, it can be confirmed that at acidic environment (pH &#x3c; 3.0), part of fluorine exists in the form of H<sub>2</sub>F<sub>2</sub>, which could hinder the combination of fluorine with La<sup>3&#x2b;</sup>. Nevertheless, in the pH range over 8.3, La<sup>3&#x2b;</sup> hydrolysis reaction would compete with LaF<sub>3</sub> precipitation reaction, resulting in a low fluoride removal and a high retaining fluorine concentration. Therefore, the pH range for fluoride removal using La<sup>3&#x2b;</sup> should be controlled between 5.5 and 8.3. Given the pH of the synthetic solution is about 2.0, in order to reduce the neutralization cost, the pH for fluoride removal is optimized to 5.5. Similarly, several literatures have also reported the largest adsorption capacity and highest defluorination efficiency of La-loaded adsorbents near pH of 5.5 (<xref ref-type="bibr" rid="B3">Cai et&#x20;al., 2018</xref>).</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Aqueous equilibrium diagram: <bold>(A)</bold> fraction of fluorine species in F-H2O system at 298&#xa0;K, [F]T &#x3d; 0.01 M; fraction of fluorine species <bold>(B)</bold> and lanthanum species <bold>(C)</bold> in La-F-Cl-H2O system at 298&#xa0;K, [La]T &#x3d; 0.0053 M, [F]T &#x3d; 0.0158 M, [Cl]T &#x3d; 0.0338&#xa0;M.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g001.tif"/>
</fig>
</sec>
<sec id="s3-2">
<title>Fluoride Removal at La/F Molar Ratios &#x2265;1:3.00</title>
<p>
<xref ref-type="fig" rid="F2">Figure&#x20;2</xref> shows the variation of solution pF and pH with time during the reaction at La/F molar ratios &#x2265;1:3.00. At the initial stage, pF increased rapidly, indicating a sharp decreasing of free F<sup>&#x2212;</sup> concentration (<xref ref-type="fig" rid="F2">Figure&#x20;2A</xref>). Meanwhile, the solution pH value exhibits a quick drop (<xref ref-type="fig" rid="F2">Figure&#x20;2B</xref>). The decreasing pH could be explained by the enhanced ionization of HF due to the decreasing concentration of free F<sup>&#x2212;</sup>, which results in a larger H<sup>&#x2b;</sup> activity. When the reaction time exceeds 300&#xa0;s, the solution pF and pH change slightly, suggesting an equilibrium state obtained between La<sup>3&#x2b;</sup> and fluorine species. As the La/F molar ratio increases from 1:3.00 to 1:2.85, the stable pF increases apparently, and the pH exhibits a reverse change. This result indicates that addition of excessive La<sup>3&#x2b;</sup> can promote the combination of F<sup>&#x2212;</sup> and La. However, when the La/F molar ratio further increases, the stable pF increases slightly, indicating that increasing lanthanum dose has a small effect on the stable pF at La/F molar ratio &#x2265;1:2.85.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Variations of pF <bold>(A)</bold> and pH <bold>(B)</bold> of the solution with time after adding lanthanum nitrate to the fluorine-containing synthetic solutions with different La/F molar ratios (La/F &#x2265; 1:3.00).</p>
</caption>
<graphic xlink:href="fchem-10-859969-g002.tif"/>
</fig>
<p>As mentioned above, during the fluoride removal at La/F molar ratios &#x2265;1:3.00, the stable pF is larger than 4.5, namely, the free F<sup>&#x2212;</sup> concentration could be reduced to below 0.5&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>. However, after 1&#xa0;h of reaction the solutions with La/F molar ratios &#x2265;1:3.00 remained clear and transparent (shown in <xref ref-type="sec" rid="s10">Supplementary Figure S1</xref>). During the vacuum filtration using inorganic membranes with a pore diameter of 0.22&#xa0;&#x3bc;m, the mass of filtration residue collected from five solutions with different La/F molar ratios (&#x2265;1:3.00) is neglectable.</p>
<p>Due to the low free F<sup>&#x2212;</sup> concentration and the absence of precipitate, it is reasonable to assume that most of fluorine retains in the solution as soluble species. In order to verify this assumption, extra NaF (0.133&#xa0;g) was added to each solution. The digital photos of solutions after adding extral NaF are shown in <xref ref-type="sec" rid="s10">Supplementary Figure S1</xref>. Interestingly, flocculent precipitates formed immediately upon the addition of extra NaF. The XRD pattern and SEM image of the precipitates are shown in <xref ref-type="sec" rid="s10">Supplementary Figure S2</xref>. It is apparent that characteristic peaks of LaF<sub>3</sub> are observed in the XRD pattern of the precipitates (<xref ref-type="sec" rid="s10">Supplementary Figure S2A</xref>), signifying the formation of LaF<sub>3</sub> with extra addition of NaF. As shown in <xref ref-type="sec" rid="s10">Supplementary Figure S2B</xref>, LaF<sub>3</sub> precipitates exist as irregular crystalline bulks and amorphous particles simultaneously. The crystalline bulks may be resulted from the recrystallization of amorphous floccules during the vacuum filtration and drying process (<xref ref-type="bibr" rid="B9">Hermans and Weidinger., 1946</xref>). The mass of precipitates formed by adding extra NaF in different solutions is shown in <xref ref-type="sec" rid="s10">Supplementary Figure S3</xref>. It is found that the mass of precipitates is close to the theoretical LaF<sub>3</sub> production. These results confirm that at La/F molar ratio &#x2265;1:3.00, fluorine retains in the solution as soluble species. However, reducing the La/F molar ratio by adding extra NaF could facilitate the formation of&#x20;LaF<sub>3</sub>.</p>
<p>Considering the fact that fluoride ion could complex La<sup>3&#x2b;</sup>, which results in the formation of LaF<sup>2&#x2b;</sup> and LaF<sub>2</sub>
<sup>&#x2b;</sup>, it could be inferred that in aqueous solutions with La/F molar ratios &#x2265;1:3.00, most of fluorine retains in solution as LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> complexes. Similarly, <xref ref-type="bibr" rid="B1">Antu&#xf1;ano et&#x20;al. (2016)</xref> and <xref ref-type="bibr" rid="B17">Liu et&#x20;al. (2020)</xref> reported the presence of AlF<sub>
<italic>n</italic>
</sub>
<sup>3&#x2212;<italic>n</italic>
</sup> complexes in Al-F-H<sub>2</sub>O system. <xref ref-type="table" rid="T2">Table&#x20;2</xref> shows the distribution of fluorine in the solution after 1h of reaction with La/F molar ratios &#x2265;1:3.00. It could be found that over 99% fluorine retains in solution as LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>. As the La/F molar ratio increases, the proportion of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> slightly increases. Since all fluorine retains in the solution in the form of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or free F<sup>&#x2212;</sup>, the fluoride removal efficiencies of solutions with La/F molar ratios &#x2265;1:3.00 are all&#x20;zero.</p>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>Distribution of fluoride element in aqueous solutions with La/F molar ratio &#x2265;1:3 after reaction for 1&#xa0;h.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">La/F molar ratio</th>
<th colspan="3" align="center">Free F<sup>&#x2212;</sup>
</th>
<th colspan="2" align="center">LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>
</th>
<th align="center">Retaining fluoride<xref ref-type="table-fn" rid="Tfn3">
<sup>c</sup>
</xref>
</th>
<th align="center">Removal</th>
</tr>
<tr>
<th align="center">
<italic>C</italic>
<xref ref-type="table-fn" rid="Tfn1">
<sup>a</sup>
</xref>/mg L<sup>&#x2212;1</sup>
</th>
<th align="center">Mass/mg</th>
<th align="center">Proportion/%</th>
<th align="center">Mass<xref ref-type="table-fn" rid="Tfn2">
<sup>b</sup>
</xref>/mg</th>
<th align="center">Proportion/%</th>
<th align="center">mg L<sup>&#x2212;1</sup>
</th>
<th align="center">%</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">1:3.00</td>
<td align="char" char=".">0.45</td>
<td align="char" char=".">0.090</td>
<td align="char" char=".">0.15</td>
<td align="char" char=".">59.910</td>
<td align="char" char=".">99.85</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:2.85</td>
<td align="char" char=".">0.26</td>
<td align="char" char=".">0.052</td>
<td align="char" char=".">0.09</td>
<td align="char" char=".">59.948</td>
<td align="char" char=".">99.91</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:2.70</td>
<td align="char" char=".">0.20</td>
<td align="char" char=".">0.040</td>
<td align="char" char=".">0.07</td>
<td align="char" char=".">59.960</td>
<td align="char" char=".">99.93</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:2.55</td>
<td align="char" char=".">0.18</td>
<td align="char" char=".">0.036</td>
<td align="char" char=".">0.06</td>
<td align="char" char=".">59.964</td>
<td align="char" char=".">99.94</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:2.40</td>
<td align="char" char=".">0.16</td>
<td align="char" char=".">0.032</td>
<td align="char" char=".">0.05</td>
<td align="char" char=".">59.968</td>
<td align="char" char=".">99.95</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn1">
<label>a</label>
<p>Calculated with final pF of filtrate measured at pH 5.5&#x20;&#xb1; 0.1.</p>
</fn>
<fn id="Tfn2">
<label>b</label>
<p>Calculated with the fluorine balance <inline-formula id="inf1">
<mml:math id="m1">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mn>60</mml:mn>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>m</mml:mi>
<mml:mi>g</mml:mi>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>e</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>F</mml:mi>
<mml:mo>&#x2212;</mml:mo>
</mml:mrow>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>o</mml:mi>
<mml:mi>n</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>u</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xd7;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mn>57</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mn>196</mml:mn>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula>.</p>
</fn>
<fn id="Tfn3">
<label>c</label>
<p>Including free F<sup>&#x2212;</sup>and LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>.</p>
</fn>
</table-wrap-foot>
</table-wrap>
</sec>
<sec id="s3-3">
<title>Fluoride Removal at La/F Molar Ratios &#x2264;1:3.00</title>
<p>It is confirmed that at La/F molar ratios &#x2265;1:3.00 the fluorine could be not removed in the form of LaF<sub>3</sub> precipitates. Therefore, it is necessary to investigate the chemical behavior of fluorine at La/F molar ratios &#x2264;1:3.00.</p>
<p>
<xref ref-type="fig" rid="F3">Figure&#x20;3</xref> shows the variation of pF and pH with time in solutions with La/F molar ratios &#x2264;1:3.00. Similar to the results obtained in solutions with La/F molar ratios &#x2265;1:3.00, the reaction between La<sup>3&#x2b;</sup> and fluorine get an equilibrium state in 5&#xa0;min. The stable pF decreases as the La/F molar ratio decreases, while the final pH shows the reverse change. Notably, at La/F molar ratio &#x2264;1:3.00, the La/F molar ratio has a more significant influence on the stable pH and pF of the solutions. Specifically, as La/F molar ratio decreases from 1:3.00 to 1:3.05, the stable pF decreases from 4.50 to 3.75, indicating a much higher concentration of free F<sup>&#x2212;</sup> retaining in the solution.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Variations of pF <bold>(A)</bold> and pH <bold>(B)</bold> of the solutions with time after adding lanthanum nitrate to the fluorine-containing synthetic solutions with different La/F molar ratios (La/F &#x2264; 1:3.00).</p>
</caption>
<graphic xlink:href="fchem-10-859969-g003.tif"/>
</fig>
<p>
<xref ref-type="sec" rid="s10">Supplementary Figure S4</xref> shows the photos of the solutions with different La/F molar ratios (&#x2264;1:3.00) after 1&#xa0;h of reaction. It is apparent that a large number of white floccules suspend in the solutions with La/F molar ratios &#x2264;1:3.05. As La/F molar ratio decreases, the solution becomes more and more turbid. It is worth noting that after standing for 2&#xa0;h, the suspended solids could not settle. Therefore, it can be inferred that colloidal solutions were formed at La/F molar ratio &#x2264;1:3.05. The formation of colloidal solution could be explained as follows. As mentioned above, in solutions at La/F &#x2265; 1:3.00, the insufficient fluorine results in formation of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>, such as LaF<sup>2&#x2b;</sup> and LaF<sub>2</sub>
<sup>&#x2b;</sup>. In solution of La/F &#x2264; 1:3.05, the presence of sufficient fluorine promotes the formation of LaF<sub>3</sub>. However, the coexisting LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> tends to adsorb on the surface of LaF<sub>3</sub>, resulting in negative charges on the surface of suspended solids (LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>). This would enhance the stability of suspended solids and result in the formation of a colloidal solution.</p>
<p>The suspended solids were collected by vacuum filtration using inorganic membrane. Interestingly, filtration residues could only be obtained from solutions with La/F molar ratio &#x2264;1:3.10. The XRD patterns and SEM images of the filtration residues obtained from solutions with La/F molar ratio of 1:3.10, 1:3.15 and 1:3.20 are shown in <xref ref-type="fig" rid="F4">Figures 4</xref>, <xref ref-type="fig" rid="F5">5</xref>, respectively. It can be found that the main phase of the filtration residues obtained in three solutions is LaF<sub>3</sub>. There is no other apparent characteristic peak, signifying the high purity of LaF<sub>3</sub> precipitates. As shown in <xref ref-type="fig" rid="F5">Figure&#x20;5</xref>, the LaF<sub>3</sub> residues presents two kinds of structures, part of LaF<sub>3</sub> exists as irregular bulks with high crystalline, while the others exhibit amorphous structures. As La/F molar ratio decreases, excessive fluoride accelerates the formation of LaF<sub>3</sub>, resulting in smaller particle size and a larger crystalline degree of LaF<sub>3</sub>. It is noteworthy that, at La/F molar ratio of 1:3.15 and 1:3.20, the filtrate obtained in vacuum filtration remains light pale, suggesting a small number of suspended solids remain in the solution. This could be explained with the smaller size of colloid particles formed with large fluorine excess coefficient, which could not be intercepted by the filtrate cake and membrane.</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>XRD patterns of filtration residues obtained in solutions at La/F molar ratio of 1:3.10, 1:3.15 and 1:3.20 after reaction for 1&#xa0;h.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g004.tif"/>
</fig>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>SEM images of filtration residues obtained in solutions with La/F molar ratios of 1:3.10 <bold>(A)</bold>, 1:3.15 <bold>(B)</bold> and 1:3.20 <bold>(C)</bold> after reaction for 1 h.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g005.tif"/>
</fig>
<p>
<xref ref-type="table" rid="T3">Table&#x20;3</xref>; <xref ref-type="fig" rid="F6">Figure&#x20;6</xref> show the distribution of fluorine in solutions with La/F molar ratios &#x2264;1:3.00 after 1&#xa0;h of reaction. It could be found that, as La/F molar ratio decreases (the excess coefficient of fluorine increases), the free F<sup>&#x2212;</sup> concentration of the equilibrium solution increases. At La/F molar ratio of 1:3.00 and 1:3.05, no fluorine is removed from the solution in the form of filtration residues, and over 98% fluorine retains in solution as LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>. Remarkably, at La/F molar ratio of 1:3.10, about 97.86% fluorine is removed from the aqueous system in the form of filtration residue, and the retaining fluoride concentration is about 6.42&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, slightly higher than the emission standard established in GB25467-2010. As La/F molar ratio further decreases, the proportion of free F<sup>&#x2212;</sup> increases obviously, while the proportion of filtration residues decreases, resulting in a higher retaining free F<sup>&#x2212;</sup> concentration and a decreasing fluoride removal efficiency. Specifically, when La/F molar ratio increases to 1:3.20, the retaining fluoride concentration is about 32.16&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, much higher than the emission standard. Although the presence of excess fluorine could facilitate the precipitation of LaF<sub>3</sub>, it leads to a significant increase in the retaining free F<sup>&#x2212;</sup> concentration and fine suspended colloid particles (LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>). Therefore, the La/F molar ratio is optimized as 1:3.10 for precipitating fluoride using lanthanum&#x20;salts.</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>Distribution of fluorine element in aqueous solutions with La/F molar ratios &#x2264;1:3.00 after reaction for 1&#xa0;h.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">La/F molar ratio</th>
<th colspan="3" align="center">Free F<sup>&#x2212;</sup>
</th>
<th colspan="2" align="center">LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or fine LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>
</th>
<th colspan="3" align="center">Filtration residues</th>
<th align="center">Retaining fluoride<xref ref-type="table-fn" rid="Tfn7">
<sup>d</sup>
</xref>
</th>
<th align="center">Removal</th>
</tr>
<tr>
<th align="center">
<italic>C</italic>
<xref ref-type="table-fn" rid="Tfn4">
<sup>a</sup>
</xref>(mg L<sup>&#x2212;1</sup>)</th>
<th align="center">Mass (mg)</th>
<th align="center">(%)</th>
<th align="center">Mass<xref ref-type="table-fn" rid="Tfn5">
<sup>b</sup>
</xref>(mg)</th>
<th align="center">(%)</th>
<th align="center">Mass<xref ref-type="table-fn" rid="Tfn6">
<sup>c</sup>
</xref> (mg)</th>
<th align="center">Filtration time (min)</th>
<th align="center">(%)</th>
<th align="center">mg L<sup>&#x2212;1</sup>
</th>
<th align="center">%</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">1:3.00</td>
<td align="char" char=".">0.51</td>
<td align="char" char=".">0.10</td>
<td align="char" char=".">0.17</td>
<td align="char" char=".">59.90</td>
<td align="char" char=".">99.83</td>
<td align="char" char=".">0</td>
<td align="center">-</td>
<td align="char" char=".">0</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:3.05</td>
<td align="char" char=".">1.19</td>
<td align="char" char=".">0.24</td>
<td align="char" char=".">0.40</td>
<td align="char" char=".">59.76</td>
<td align="char" char=".">98.60</td>
<td align="char" char=".">0</td>
<td align="center">-</td>
<td align="char" char=".">0</td>
<td align="char" char=".">300</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">1:3.10</td>
<td align="char" char=".">3.30</td>
<td align="char" char=".">0.66</td>
<td align="char" char=".">1.10</td>
<td align="char" char=".">0.62</td>
<td align="char" char=".">1.03</td>
<td align="char" char=".">58.72</td>
<td align="char" char=".">158</td>
<td align="char" char=".">97.86</td>
<td align="char" char=".">6.42</td>
<td align="char" char=".">97.86</td>
</tr>
<tr>
<td align="left">1:3.15</td>
<td align="char" char=".">7.14</td>
<td align="char" char=".">1.43</td>
<td align="char" char=".">2.38</td>
<td align="char" char=".">3.98</td>
<td align="char" char=".">6.63</td>
<td align="char" char=".">54.59</td>
<td align="char" char=".">95</td>
<td align="char" char=".">90.98</td>
<td align="char" char=".">27.06</td>
<td align="char" char=".">90.98</td>
</tr>
<tr>
<td align="left">1:3.20</td>
<td align="char" char=".">12.35</td>
<td align="char" char=".">2.47</td>
<td align="char" char=".">4.12</td>
<td align="char" char=".">3.96</td>
<td align="char" char=".">6.60</td>
<td align="char" char=".">53.57</td>
<td align="char" char=".">76</td>
<td align="char" char=".">89.28</td>
<td align="char" char=".">32.16</td>
<td align="char" char=".">89.28</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn4">
<label>a</label>
<p>calculated with final pF of filtrate measured at pH 5.5&#x20;&#xb1; 0.1.</p>
</fn>
<fn id="Tfn5">
<label>b</label>
<p>calculated with the fluorine balance <inline-formula id="inf2">
<mml:math id="m2">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mn>60</mml:mn>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>m</mml:mi>
<mml:mi>g</mml:mi>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>e</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>F</mml:mi>
<mml:mo>&#x2212;</mml:mo>
</mml:mrow>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>o</mml:mi>
<mml:mi>n</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>u</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xd7;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mn>57</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mn>196</mml:mn>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula>.</p>
</fn>
<fn id="Tfn6">
<label>c</label>
<p>calculated with the mass of precipitate <inline-formula id="inf3">
<mml:math id="m3">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>o</mml:mi>
<mml:mi>n</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>u</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xd7;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mn>57</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mn>196</mml:mn>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> based on the assumption that precipitate contains LaF<sub>3</sub>&#x20;only.</p>
</fn>
<fn id="Tfn7">
<label>d</label>
<p>including free F<sup>&#x2212;</sup>, LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and fine LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>Distribution of fluorine element in solutions with different La/F molar ratios (&#x2264;1:3.00).</p>
</caption>
<graphic xlink:href="fchem-10-859969-g006.tif"/>
</fig>
</sec>
<sec id="s3-4">
<title>Chemical Reactions Between F<sup>&#x2212;</sup> and La<sup>3&#x2b;</sup>
</title>
<p>Based on the above results, it can be found that the chemical reactions between F<sup>&#x2212;</sup> and La<sup>3&#x2b;</sup> are sensitive to the La/F molar ratio. As shown in <xref ref-type="fig" rid="F7">Figure&#x20;7</xref>, at La/F molar ratio &#x2265;1:3.05, the fluorine is relatively insufficient, there is not enough fluorine participate in the precipitation reaction (<xref ref-type="disp-formula" rid="e16">Eq. 16</xref>), resulting in the formation of LaF<sub>2</sub>
<sup>&#x2b;</sup> (<xref ref-type="disp-formula" rid="e17">Eq. 17</xref>), or even LaF<sup>2&#x2b;</sup> (<xref ref-type="disp-formula" rid="e18">Eq. 18</xref>). Under this condition, although the free F<sup>&#x2212;</sup> concentration in the aqueous system can be reduced to a very low level, most of fluorine retains in the solution in the form of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> complexes, resulting in the failure of fluorine removal. At 1:3.20 &#x2264; La:F &#x3c; 1:3.05, sufficient fluoride enhances the precipitation of LaF<sub>3</sub>. However, coexisting LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> in the aqueous solution could adsorb on the surface of LaF<sub>3</sub>, resulting in the formation of colloidal solution with LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> suspended solids (<xref ref-type="disp-formula" rid="e19">Eqs 19</xref>&#x2013;<xref ref-type="disp-formula" rid="e20">20</xref>).<disp-formula id="e16">
<mml:math id="m4">
<mml:mrow>
<mml:msup>
<mml:mrow>
<mml:mtext>La</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>3</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
<mml:mo>&#x2b;</mml:mo>
<mml:msup>
<mml:mrow>
<mml:mn>3</mml:mn>
<mml:mtext>F</mml:mtext>
</mml:mrow>
<mml:mo>&#x2212;</mml:mo>
</mml:msup>
<mml:mo>&#x3d;</mml:mo>
<mml:msub>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>3</mml:mn>
</mml:msub>
</mml:mrow>
</mml:math>
<label>(16)</label>
</disp-formula>
<disp-formula id="e17">
<mml:math id="m5">
<mml:mrow>
<mml:msup>
<mml:mrow>
<mml:mtext>La</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>3</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
<mml:mo>&#x2b;</mml:mo>
<mml:mn>2</mml:mn>
<mml:msup>
<mml:mtext>F</mml:mtext>
<mml:mo>&#x2212;</mml:mo>
</mml:msup>
<mml:mo>&#x3d;</mml:mo>
<mml:msubsup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:msubsup>
</mml:mrow>
</mml:math>
<label>(17)</label>
</disp-formula>
<disp-formula id="e18">
<mml:math id="m6">
<mml:mrow>
<mml:msup>
<mml:mrow>
<mml:mtext>La</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>3</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
<mml:mo>&#x2b;</mml:mo>
<mml:msup>
<mml:mtext>F</mml:mtext>
<mml:mo>&#x2212;</mml:mo>
</mml:msup>
<mml:mo>&#x3d;</mml:mo>
<mml:msup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
</mml:mrow>
</mml:math>
<label>(18)</label>
</disp-formula>
<disp-formula id="e19">
<mml:math id="m7">
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>3</mml:mn>
</mml:msub>
<mml:mo>&#x2b;</mml:mo>
<mml:msubsup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:msubsup>
<mml:mo>&#x3d;</mml:mo>
<mml:msub>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>3</mml:mn>
</mml:msub>
<mml:mo>&#x22c5;</mml:mo>
<mml:msubsup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:msubsup>
</mml:mrow>
</mml:math>
<label>(19)</label>
</disp-formula>
<disp-formula id="e20">
<mml:math id="m8">
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>3</mml:mn>
</mml:msub>
<mml:mo>&#x2b;</mml:mo>
<mml:msup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
<mml:mo>&#x3d;</mml:mo>
<mml:msub>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mn>3</mml:mn>
</mml:msub>
<mml:mo>&#x22c5;</mml:mo>
<mml:msup>
<mml:mrow>
<mml:mtext>LaF</mml:mtext>
</mml:mrow>
<mml:mrow>
<mml:mn>2</mml:mn>
<mml:mo>&#x2b;</mml:mo>
</mml:mrow>
</mml:msup>
</mml:mrow>
</mml:math>
<label>(20)</label>
</disp-formula>
</p>
<fig id="F7" position="float">
<label>FIGURE 7</label>
<caption>
<p>Schematic diagrams of species in La-F-Cl-H<sub>2</sub>O aqueous system.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g007.tif"/>
</fig>
</sec>
<sec id="s3-5">
<title>Coagulation Removal of Fluoride Using La<sup>3&#x2b;</sup> and Na<sub>2</sub>SiO<sub>3</sub>
</title>
<p>At the optimized La/F molar ratio of 1:3.10, the retaining fluorine concentration in the filtrate is 6.42&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, exceeding the emission standard level (GB25467-2010). Therefore, chemical precipitation using lanthanum salt fails to sufficiently remove fluoride from aqueous solution. Given that fluorine could exists in the form of positively charged LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> (<italic>x</italic>&#x20;&#x3d; 1 or 2), SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O, as a well-known negatively charged colloidal particle (<xref ref-type="bibr" rid="B16">Lin et&#x20;al., 2018</xref>), is introduced to facilitate the coagulation of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> based on electrostatic interaction. In practice, Na<sub>2</sub>SiO<sub>3</sub> was firstly added into the synthetic solution. After 0.5&#xa0;h of agitation, the solution pH was adjusted to 5.5&#x20;&#xb1; 0.1 to produce active colloidal particles (SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O) <italic>in situ</italic>. Afterwards, the lanthanum salt is added to target a La/F molar ratio of 1:3.00, rather than 1:3.10 in order to reducing the retaining free F<sup>&#x2212;</sup> concentration.</p>
<p>
<xref ref-type="sec" rid="s10">Supplementary Figure S5</xref> shows the digital photos of solutions with different doses of Na<sub>2</sub>SiO<sub>3</sub> after 1&#xa0;h of reaction. In the absence of Na<sub>2</sub>SiO<sub>3</sub> (<xref ref-type="sec" rid="s10">Supplementary Figure S5A</xref>), the solution remains clear and transparent. Correspondingly, during the vacuum filtration, there is no filtration residues obtained from this solution. In the presence of 0.25&#xa0;g&#xa0;L<sup>&#x2212;1</sup> Na<sub>2</sub>SiO<sub>3</sub> (<xref ref-type="sec" rid="s10">Supplementary Figure S5B</xref>), about 0.10&#xa0;g&#xa0;L<sup>&#x2212;1</sup> filtration residues were collected from the solution. As Na<sub>2</sub>SiO<sub>3</sub> dose further increases, the solutions at equilibrium state become more and more turbid, signifying the formation of suspended particles. Consequently, much more filtration residues were obtained from these solutions.</p>
<p>The XRD patterns and SEM images of the filtration residues were shown in <xref ref-type="fig" rid="F8">Figures 8</xref>, <xref ref-type="fig" rid="F9">9</xref>. In the XRD patterns of filtration residues obtained from solutions with 0.50&#x2013;1.00&#xa0;g&#xa0;L<sup>&#x2212;1</sup> Na<sub>2</sub>SiO<sub>3</sub>, only the featured peaks of LaF<sub>3</sub> appears, demonstrating the main phase of the filtration residues is LaF<sub>3</sub>. The absence of characteristic peaks of SiO<sub>2</sub> may be explained by the low content of SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O or the amorphous structure of SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O. The morphologies of filtration residues obtained with Na<sub>2</sub>SiO<sub>3</sub> addition are shown in <xref ref-type="fig" rid="F9">Figure&#x20;9</xref>. It can be found that the presence of Na<sub>2</sub>SiO<sub>3</sub> has an obvious effect on the structures of filtrate residues. A large number of fine irregular particles agglomerates, resulting in the lumps in large size (&#x2265;5&#xa0;&#x3bc;m). However, the dose of Na<sub>2</sub>SiO<sub>3</sub> does not show obvious influence on the morphologies of filtration residues.</p>
<fig id="F8" position="float">
<label>FIGURE 8</label>
<caption>
<p>XRD patterns of filtration residues obtained from solutions with La/F molar ratio of 1:3.00 after 1&#xa0;h reaction in the presence of different doses of Na<sub>2</sub>SiO<sub>3</sub>.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g008.tif"/>
</fig>
<fig id="F9" position="float">
<label>FIGURE 9</label>
<caption>
<p>SEM images of filtration residues obtained at La/F molar ratios of 1:3.00 with different doses of Na<sub>2</sub>SiO<sub>3</sub>. <bold>(A)</bold> 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, <bold>(B)</bold> 0.75&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, (<bold>C</bold>) 1.00&#xa0;g&#xa0;L<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g009.tif"/>
</fig>
<p>
<xref ref-type="fig" rid="F10">Figure&#x20;10</xref> shows the distribution of F, Si and La element on the surface of filtration residues obtained from solution with La/F molar ratio of 1:3.00 and 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup> Na<sub>2</sub>SiO<sub>3</sub>. It is observed that the F and La element is uniformly distributed on the surface of filtration residues, further demonstrating the formation of LaF<sub>3</sub> precipitates. In spite of the absence of featured peaks of SiO<sub>2</sub> in the XRD pattern, the Si element is detected on the surface of filtration residues, which testifies the participation of colloidal particles (SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O) in the coagulation of LaF<sub>3</sub>. The EDS results indicate that the contents of Si in filtration residues are 2.87&#xa0;wt%, 3.67&#xa0;wt% and 4.25&#xa0;wt%, corresponding to the addition of 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, 0.75&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, and 1.00&#xa0;g&#xa0;L<sup>&#x2212;1</sup> Na<sub>2</sub>SiO<sub>3</sub>, respectively.</p>
<fig id="F10" position="float">
<label>FIGURE 10</label>
<caption>
<p>SEM image <bold>(A)</bold> and elements mapping <bold>(B&#x2013;D)</bold> of filtration residues obtained at La/F molar ratios of 1:3.00 in the presence of 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup> Na<sub>2</sub>SiO<sub>3</sub>. <bold>(B)</bold> F, <bold>(C)</bold> Si, <bold>(D)</bold> La.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g010.tif"/>
</fig>
<p>
<xref ref-type="table" rid="T4">Table&#x20;4</xref>; <xref ref-type="fig" rid="F11">Figure&#x20;11</xref> shows the distribution of fluorine element in solutions with La/F molar ratio of 1:3.00 and different doses of Na<sub>2</sub>SiO<sub>3</sub>. In the absence of Na<sub>2</sub>SiO<sub>3</sub>, even though free F<sup>&#x2212;</sup> concentration could be reduced to 0.51&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, nearly 99.83% fluorine retains in the solution as LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>. When Na<sub>2</sub>SiO<sub>3</sub> dose reaches 0.25&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, about 20.12% fluorine is removed from the solution in the form of filtration residues, and 79.25% remains in the solution in the form of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>. As Na<sub>2</sub>SiO<sub>3</sub> dose increases to 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, a fluorine removal efficiency of 99.25% could be obtained. When adding extra NaF into the filtrate, the solution keeps clear and transparent, proving the absence of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>. The retaining fluoride concentration in equilibrium solution is 2.24&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, lower than the emission standard established in GB25467-2010. As Na<sub>2</sub>SiO<sub>3</sub> dose further increases, the fluorine removal efficiency ascends slightly, and the retaining fluoride concentration could be reduced to 0.8&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, lower than the limited fluorine level established by the WHO. However, it is noteworthy that, at a Na<sub>2</sub>SiO<sub>3</sub> dose &#x2265;0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, the filtration time is longer than 5&#xa0;h, which could be explained by the formation of a large amount of sticky SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O colloidal particles.</p>
<table-wrap id="T4" position="float">
<label>TABLE 4</label>
<caption>
<p>Distribution of fluorine element in aqueous solutions (La/F molar ratio &#x3d; 1:3.00) after 1&#xa0;h of reaction in the presence of Na<sub>2</sub>SiO<sub>3</sub>.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">Na<sub>2</sub>SiO<sub>3</sub> dose (g L<sup>&#x2212;1</sup>)</th>
<th colspan="3" align="center">Free F<sup>&#x2212;</sup>
</th>
<th colspan="2" align="center">LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or fine LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>
</th>
<th colspan="3" align="center">Filtration residues</th>
<th align="center">Retaining fluoride<xref ref-type="table-fn" rid="Tfn11">
<sup>d</sup>
</xref>
</th>
<th align="center">Removal</th>
</tr>
<tr>
<th align="center">
<italic>C</italic>
<xref ref-type="table-fn" rid="Tfn8">
<sup>a</sup>
</xref> (mg L<sup>&#x2212;1</sup>)</th>
<th align="center">Mass (mg)</th>
<th align="center">(%)</th>
<th align="center">Mass (mg)</th>
<th align="center">(%)</th>
<th align="center">Mass (mg)</th>
<th align="center">Filtration time (min)</th>
<th align="center">(%)</th>
<th align="center">(mg L<sup>&#x2212;1</sup>)</th>
<th align="center">%</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">0</td>
<td align="char" char=".">0.51</td>
<td align="char" char=".">0.10</td>
<td align="char" char=".">0.17</td>
<td align="center">59.90<xref ref-type="table-fn" rid="Tfn9">
<sup>b</sup>
</xref>
</td>
<td align="char" char=".">99.83</td>
<td align="center">0</td>
<td align="center">-</td>
<td align="char" char=".">0</td>
<td align="char" char=".">300.00</td>
<td align="char" char=".">0</td>
</tr>
<tr>
<td align="left">0.25</td>
<td align="char" char=".">1.89</td>
<td align="char" char=".">0.38</td>
<td align="char" char=".">0.63</td>
<td align="center">47.55<xref ref-type="table-fn" rid="Tfn9">
<sup>b</sup>
</xref>
</td>
<td align="char" char=".">79.25</td>
<td align="center">12.07</td>
<td align="char" char=".">2.3</td>
<td align="char" char=".">20.12</td>
<td align="char" char=".">271.15</td>
<td align="char" char=".">20.12</td>
</tr>
<tr>
<td align="left">0.50</td>
<td align="char" char=".">2.24</td>
<td align="char" char=".">0.45</td>
<td align="char" char=".">0.75</td>
<td align="center">0</td>
<td align="char" char=".">0</td>
<td align="center">59.55<xref ref-type="table-fn" rid="Tfn6">
<sup>c</sup>
</xref>
</td>
<td align="char" char=".">340</td>
<td align="char" char=".">99.25</td>
<td align="char" char=".">2.24</td>
<td align="char" char=".">99.25</td>
</tr>
<tr>
<td align="left">0.75</td>
<td align="char" char=".">2.33</td>
<td align="char" char=".">0.47</td>
<td align="char" char=".">0.78</td>
<td align="center">0</td>
<td align="char" char=".">0</td>
<td align="center">59.53<xref ref-type="table-fn" rid="Tfn6">
<sup>c</sup>
</xref>
</td>
<td align="char" char=".">362</td>
<td align="char" char=".">99.22</td>
<td align="char" char=".">2.33</td>
<td align="char" char=".">99.22</td>
</tr>
<tr>
<td align="left">1.00</td>
<td align="char" char=".">0.80</td>
<td align="char" char=".">0.16</td>
<td align="char" char=".">0.27</td>
<td align="center">0</td>
<td align="char" char=".">0</td>
<td align="center">59.84<xref ref-type="table-fn" rid="Tfn6">
<sup>c</sup>
</xref>
</td>
<td align="char" char=".">325</td>
<td align="char" char=".">99.73</td>
<td align="char" char=".">0.80</td>
<td align="char" char=".">99.73</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn8">
<label>a</label>
<p>calculated with final pF of filtrate measured at pH 5.5&#x20;&#xb1; 0.1.</p>
</fn>
<fn id="Tfn9">
<label>b</label>
<p>Calculated with the fluorine balance <inline-formula id="inf4">
<mml:math id="m9">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mn>60</mml:mn>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>m</mml:mi>
<mml:mi>g</mml:mi>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>e</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>F</mml:mi>
<mml:mo>&#x2212;</mml:mo>
</mml:mrow>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>o</mml:mi>
<mml:mi>n</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>u</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xd7;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mn>57</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mn>196</mml:mn>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> based on the assumption that precipitate contains LaF<sub>3</sub> only (Si is neglected).</p>
</fn>
<fn id="Tfn10">
<label>c</label>
<p>Calculated with the fluorine balance <inline-formula id="inf5">
<mml:math id="m10">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mn>60</mml:mn>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>m</mml:mi>
<mml:mi>g</mml:mi>
<mml:mo>&#x2212;</mml:mo>
<mml:mi>m</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>s</mml:mi>
<mml:msub>
<mml:mi>s</mml:mi>
<mml:mrow>
<mml:mi>f</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>e</mml:mi>
<mml:mo>&#xa0;</mml:mo>
<mml:mi>F</mml:mi>
<mml:mo>&#x2212;</mml:mo>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula>, the absence of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> or LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> is proved by adding extra NaF.</p>
</fn>
<fn id="Tfn11">
<label>d</label>
<p>Including free F<sup>&#x2212;</sup>,LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and fine LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3-<italic>x</italic>
</sup>.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<fig id="F11" position="float">
<label>FIGURE 11</label>
<caption>
<p>Distribution of fluorine element in aqueous solutions (La/F molar ratio &#x3d; 1:3.00) after 1&#xa0;h reaction in the presence of different doses of Na<sub>2</sub>SiO<sub>3</sub>.</p>
</caption>
<graphic xlink:href="fchem-10-859969-g011.tif"/>
</fig>
<p>Above results verify our assumption that the presence of negatively charged SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O colloidal particles could interact with positively charged LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>, promoting their aggregation and settlement. It is demonstrated that coagulation removal of fluoride could be achieved through adding Na<sub>2</sub>SiO<sub>3</sub> and lanthanum salts. However, the presence of SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O colloidal particles render the difficulty in solid/liquid separation, the operation parameters for coagulation removal of fluoride using Na<sub>2</sub>SiO<sub>3</sub> and lanthanum salts should be optimized in future&#x20;works.</p>
</sec>
</sec>
<sec sec-type="conclusion" id="s4">
<title>Conclusion</title>
<p>Pyrometallurgical treatment of low-grade sphalerite ores and zinc-bearing dusts discharges a large amount of flue gas containing HF and HCl. During the flue gas scrubbing step, fluorine and chlorine transfer into the liquid phase, yielding a large amount of fluorine-containing scrubbing wastewater. In this work, precipitation removal and coagulation removal of fluoride from flue gas scrubbing wastewater (300&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> fluorine) using lanthanum salt is investigated.</p>
<p>The chemical reaction between La<sup>3&#x2b;</sup> and F<sup>&#x2212;</sup> has been discussed based on the distribution of fluorine in solutions with different La/F molar ratios. At acidic environment (pH &#x2264; 4.0), part of fluorine exists as HF and H<sub>2</sub>F<sub>2</sub>, retarding the combination of La<sup>3&#x2b;</sup> and fluorine. Nonetheless, at pH &#x3e; 8.3, La<sup>3&#x2b;</sup> hydrolysis reaction would compete with LaF<sub>3</sub> precipitation reaction, resulting in a low fluoride removal. Consequently, the pH for fluoride removal is optimized to 5.5. At La/F molar ratio &#x2265;1:3.05, most of fluorine retains in the solution in the form of LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> complexes, resulting in the failure of fluorine removal. At 1:3.20 &#x2264; La:F ratio &#x3c;1:3.05, sufficient fluoride enhances the precipitation of LaF<sub>3</sub>. However, LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> coexisting in the aqueous solution could adsorb on the surface of LaF<sub>3</sub>, resulting in the formation of colloidal solution with large numbers of LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> suspended solids. In summary, at optimized La/F molar ratio of 1:3.10, about 97.86% fluorine is removed from the aqueous system in the form of filtration residue, and the retaining fluoride concentration is about 6.42&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, slightly higher than the emission standard established in GB25467-2010.</p>
<p>Considering the existing of positively charged LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup> and LaF<sub>3</sub>&#xb7;LaF<sub>
<italic>x</italic>
</sub>
<sup>3&#x2212;<italic>x</italic>
</sup>, coagulation removal of fluoride is proposed and investigated using lanthanum salts and negatively charged SiO<sub>2</sub>&#xb7;nH<sub>2</sub>O colloidal particles (<italic>in-situ</italic> produced <italic>via</italic> Na<sub>2</sub>SiO<sub>3</sub> hydrolysis at pH near 5.5). At a La/F molar ratio of 1:3.00 and Na<sub>2</sub>SiO<sub>3</sub> dose of 0.50&#xa0;g&#xa0;L<sup>&#x2212;1</sup>, a fluoride removal of 99.25% is obtained with retaining fluorine concentration of 2.24&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>. When Na<sub>2</sub>SiO<sub>3</sub> dose increases to 0.20&#xa0;g, the retaining fluorine concentration could be further reduced to 0.80&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, lower than the limited fluoride level established by the WHO. During the coagulation removal of fluoride using lanthanum salts and Na<sub>2</sub>SiO<sub>3</sub>, the presence of SiO<sub>2</sub>&#xb7;<italic>n</italic>H<sub>2</sub>O colloidal particles render the difficulty in solid/liquid separation, the operation parameters for coagulation removal of fluoride should be optimized in future&#x20;works.</p>
<p>Generally, coagulation removal of fluoride has a huge potential to be adopted in metallurgical industry due to high removal efficiency, low consumption of lanthanum salts, and relative low cost of lanthanum salts and Na<sub>2</sub>SiO<sub>3</sub>. However, the influence of impurities, especially anions such as SO<sub>4</sub>
<sup>2-</sup>, Cl<sup>&#x2212;</sup>, CO<sub>3</sub>
<sup>2-</sup>, and NO<sub>3</sub>
<sup>&#x2212;</sup>, on the chemical behavior and distribution of fluoride during coagulation removal of fluoride should be fully evaluated before industrialized application of this process.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/<xref ref-type="sec" rid="s10">Supplementary Material</xref>, further inquiries can be directed to the corresponding authors.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>RW: Writing review and editing, Funding acquisition. CC: Investigation, Writing original draft. XZ: Supervision, Methodology, Formal analysis, Writing review and editing. KY: Validation. SZ: Resources, Formal analysis. ZX: Supervision, Writing review and editing.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This work was financially supported by the National Key Research and Development Program of China (2019YFC1908405); Natural Science Foundation of Jiangxi Province (Youth Program; 20202BAB214015); Training plan for academic and technical leaders of major disciplines in Jiangxi Province (Youth Program; 20212BCJ23006, 20212BCJL23052); Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology (JXUSTQJYX2020015); Research Fund of State Key Laboratory of Rare Metals Separation and Comprehensive Utilization (GK-201905), China; Distinguished Professor Program of Jinggang Scholars in institutions of higher learning, Jiangxi Province.</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
<sec id="s10">
<title>Supplementary Material</title>
<p>The Supplementary Material for this article can be found online at: <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/fchem.2022.859969/full#supplementary-material">https://www.frontiersin.org/articles/10.3389/fchem.2022.859969/full&#x23;supplementary-material</ext-link>
</p>
<supplementary-material xlink:href="DataSheet1.docx" id="SM1" mimetype="application/docx" xmlns:xlink="http://www.w3.org/1999/xlink"/>
</sec>
<ref-list>
<title>References</title>
<ref id="B1">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Antu&#xf1;ano</surname>
<given-names>N.</given-names>
</name>
<name>
<surname>Cambra</surname>
<given-names>J.&#x20;F.</given-names>
</name>
<name>
<surname>Arias</surname>
<given-names>P. L.</given-names>
</name>
</person-group> (<year>2016</year>). <article-title>Fluoride Removal from Double Leached Waelz Oxide Leach Solutions as Alternative Feeds to Zinc Calcine Leaching Liquors in the Electrolytic Zinc Production Process</article-title>. <source>Hydrometallurgy</source> <volume>161</volume>, <fpage>65</fpage>&#x2013;<lpage>70</lpage>. <pub-id pub-id-type="doi">10.1016/j.hydromet.2016.01.008</pub-id> </citation>
</ref>
<ref id="B2">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Arahmana</surname>
<given-names>N.</given-names>
</name>
<name>
<surname>Mulyati</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Lubis</surname>
<given-names>M. R.</given-names>
</name>
<name>
<surname>Takagi</surname>
<given-names>R.</given-names>
</name>
<name>
<surname>Matsuyama</surname>
<given-names>H.</given-names>
</name>
</person-group> (<year>2016</year>). <article-title>The Removal of Fluoride from Water Based on Applied Current and Membrane Types in Electrodialysis</article-title>. <source>J.&#x20;Fluorine Chem.</source> <volume>19197</volume>, <fpage>102</fpage>. <pub-id pub-id-type="doi">10.1016/j.jfluchem.2016.10.002</pub-id> </citation>
</ref>
<ref id="B3">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Cai</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhao</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>Q.</given-names>
</name>
<name>
<surname>Pan</surname>
<given-names>B.</given-names>
</name>
</person-group> (<year>2018</year>). <article-title>Enhanced Fluoride Removal by La-Doped Li/Al Layered Double Hydroxides</article-title>. <source>J.&#x20;Colloid Interf. Sci.</source> <volume>509</volume>, <fpage>353</fpage>&#x2013;<lpage>359</lpage>. <pub-id pub-id-type="doi">10.1016/j.jcis.2017.09.038</pub-id> </citation>
</ref>
<ref id="B4">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Chang</surname>
<given-names>M. F.</given-names>
</name>
<name>
<surname>Liu</surname>
<given-names>J.&#x20;C.</given-names>
</name>
</person-group> (<year>2007</year>). <article-title>Precipitation Removal of Fluoride from Semiconductor Wastewater</article-title>. <source>J.&#x20;Environ. Eng.</source> <volume>133</volume> (<issue>4</issue>), <fpage>419</fpage>&#x2013;<lpage>425</lpage>. <pub-id pub-id-type="doi">10.1061/(asce)0733-9372(2007)133:4(419)</pub-id> </citation>
</ref>
<ref id="B5">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>&#xc7;inar&#x15e;ahin</surname>
<given-names>F.</given-names>
</name>
<name>
<surname>Derin</surname>
<given-names>B.</given-names>
</name>
<name>
<surname>Y&#xfc;cel</surname>
<given-names>O.</given-names>
</name>
</person-group> (<year>2000</year>). <article-title>Chloride Removal from Zinc Ash</article-title>. <source>Scand. J.&#x20;Metall.</source> <volume>29</volume> (<issue>5</issue>), <fpage>224</fpage>. <pub-id pub-id-type="doi">10.1034/j.1600-0692.2000.d01-26.x</pub-id> </citation>
</ref>
<ref id="B6">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>D&#xed;az-Flores</surname>
<given-names>P.</given-names>
</name>
<name>
<surname>Arcibar-Orozco</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Flores-Rojas</surname>
<given-names>A.</given-names>
</name>
<name>
<surname>Rangel-M&#xe9;ndez</surname>
<given-names>J.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Synthesis of a Chitosan-Zeolite Composite Modified with La(III): Characterization and its Application in the Removal of Fluoride from Aqueous Systems</article-title>. <source>Water Air Soil Poll</source> <volume>232</volume>, <fpage>235</fpage>. <pub-id pub-id-type="doi">10.1007/s11270-021-05185-1</pub-id> </citation>
</ref>
<ref id="B7">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Feng</surname>
<given-names>C.</given-names>
</name>
<name>
<surname>Khulbe</surname>
<given-names>K. C.</given-names>
</name>
<name>
<surname>Matsuura</surname>
<given-names>T.</given-names>
</name>
<name>
<surname>Gopal</surname>
<given-names>R.</given-names>
</name>
<name>
<surname>Kaur</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Ramakrishna</surname>
<given-names>S.</given-names>
</name>
<etal/>
</person-group> (<year>2008</year>). <article-title>Production of Drinking Water from saline Water by Air-gap Membrane Distillation Using Polyvinylidene Fluoride Nanofiber Membrane</article-title>. <source>J.&#x20;Membr. Sci.</source> <volume>311</volume> (<issue>1-2</issue>), <fpage>1</fpage>. <pub-id pub-id-type="doi">10.1016/j.memsci.2007.12.026</pub-id> </citation>
</ref>
<ref id="B8">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>He</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>An</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Wan</surname>
<given-names>G.</given-names>
</name>
<name>
<surname>Zhu</surname>
<given-names>W.</given-names>
</name>
<name>
<surname>Luo</surname>
<given-names>Y.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Enhanced Fluoride Removal from Water by Rare Earth (La and Ce) Modified Alumina: Adsorption Isotherms, Kinetics, Thermodynamics and Mechanism</article-title>. <source>Sci. Total Environ.</source> <volume>688</volume>, <fpage>184</fpage>&#x2013;<lpage>198</lpage>. <pub-id pub-id-type="doi">10.1016/j.scitotenv.2019.06.175</pub-id> </citation>
</ref>
<ref id="B9">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hermans</surname>
<given-names>P. H.</given-names>
</name>
<name>
<surname>Weidinger</surname>
<given-names>A.</given-names>
</name>
</person-group> (<year>1946</year>). <article-title>On the Recrystallization of Amorphous Cellulose</article-title>. <source>J.&#x20;Am. Chem. Soc.</source> <volume>68</volume> (<issue>12</issue>), <fpage>2547</fpage>&#x2013;<lpage>2552</lpage>. <pub-id pub-id-type="doi">10.1021/ja01216a037</pub-id> </citation>
</ref>
<ref id="B10">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hu</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Peng</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Kong</surname>
<given-names>L.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Removal of Fluoride from Zinc Sulfate Solution by <italic>In Situ</italic> Fe(III) in a Cleaner Desulfuration Process</article-title>. <source>J.&#x20;Clean. Prod.</source> <volume>164</volume>, <fpage>163</fpage>&#x2013;<lpage>170</lpage>. <pub-id pub-id-type="doi">10.1016/j.jclepro.2017.06.213</pub-id> </citation>
</ref>
<ref id="B11">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Huang</surname>
<given-names>H.</given-names>
</name>
<name>
<surname>Liu</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>P.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>D.</given-names>
</name>
<name>
<surname>Gao</surname>
<given-names>F.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Investigation on the Simultaneous Removal of Fluoride, Ammonia Nitrogen and Phosphate from Semiconductor Wastewater Using Chemical Precipitation</article-title>. <source>Chem. Eng. J.</source> <volume>307</volume>, <fpage>696</fpage>&#x2013;<lpage>706</lpage>. <pub-id pub-id-type="doi">10.1016/j.cej.2016.08.134</pub-id> </citation>
</ref>
<ref id="B12">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Kahvecioglu</surname>
<given-names>&#xd6;.</given-names>
</name>
<name>
<surname>Derin</surname>
<given-names>B.</given-names>
</name>
<name>
<surname>Y&#xfc;cel</surname>
<given-names>O.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>Carbothermal Recovery of Zinc from Brass Ash</article-title>. <source>Mineral. Process. Extractive Metall.</source> <volume>112</volume> (<issue>2</issue>), <fpage>95</fpage>&#x2013;<lpage>101</lpage>. <pub-id pub-id-type="doi">10.1179/037195503225002790</pub-id> </citation>
</ref>
<ref id="B13">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Kim</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Choong</surname>
<given-names>C. E.</given-names>
</name>
<name>
<surname>Hyun</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Park</surname>
<given-names>C. M.</given-names>
</name>
<name>
<surname>Lee</surname>
<given-names>G.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Mechanism of Simultaneous Removal of Aluminum and Fluoride from Aqueous Solution by La/Mg/Si-Activated Carbon</article-title>. <source>Chemosphere</source> <volume>253</volume>, <fpage>126580</fpage>. <pub-id pub-id-type="doi">10.1016/j.chemosphere.2020.126580</pub-id> </citation>
</ref>
<ref id="B14">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Kong</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Tian</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Pang</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Huang</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>N.</given-names>
</name>
<etal/>
</person-group> (<year>2020</year>). <article-title>Needle-like Mg-La Bimetal Oxide Nanocomposites Derived from Periclase and Lanthanum for Cost-Effective Phosphate and Fluoride Removal: Characterization, Performance and Mechanism</article-title>. <source>Chem. Eng. J.</source> <volume>382</volume>, <fpage>122963</fpage>. <pub-id pub-id-type="doi">10.1016/j.cej.2019.122963</pub-id> </citation>
</ref>
<ref id="B15">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Li</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhou</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Yang</surname>
<given-names>T.</given-names>
</name>
<name>
<surname>Chen</surname>
<given-names>L.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>Thermodynamics of Fluoride Removal from Waste Acid in Metallurgy Plant</article-title>. <source>J.&#x20;Cent. South. Univ. (Sci. Tech.</source> <volume>44</volume> (<issue>9</issue>), <fpage>3580</fpage>. <comment>in Chinese</comment>. </citation>
</ref>
<ref id="B16">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Lin</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Pei</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Lei</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Liu</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Xia</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Xie</surname>
<given-names>F.</given-names>
</name>
</person-group> (<year>2018</year>). <article-title>Trace Muscovite Dissolution Separation from Vein Quartz by Elevated Temperature and Pressure Acid Leaching Using Sulphuric Acid and Ammonia Chloride Solutions</article-title>. <source>Physicochem. Probl. Mi.</source> <volume>54</volume>, <fpage>448</fpage>. <pub-id pub-id-type="doi">10.5277/ppmp1839</pub-id> </citation>
</ref>
<ref id="B17">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Liu</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Yin</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Liu</surname>
<given-names>W.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Hu</surname>
<given-names>Q.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Treatment of Aluminum and Fluoride during Hydrochloric Acid Leaching of Lepidolite</article-title>. <source>Hydrometallurgy</source> <volume>191</volume>, <fpage>105222</fpage>. <pub-id pub-id-type="doi">10.1016/j.hydromet.2019.105222</pub-id> </citation>
</ref>
<ref id="B18">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Liu</surname>
<given-names>S.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Practical Study on the Advanced Defluorination Process Optimization of Copper Smelting Wastewater</article-title>. <source>Indust. Water Treat.</source> <volume>41</volume> (<issue>1</issue>), <fpage>118</fpage>. <comment>in Chinese</comment>. <pub-id pub-id-type="doi">10.1016/j.jwpe.2021.101998</pub-id> </citation>
</ref>
<ref id="B19">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Martins</surname>
<given-names>J.&#x20;M. A.</given-names>
</name>
<name>
<surname>Dutra</surname>
<given-names>A. J.&#x20;B.</given-names>
</name>
<name>
<surname>Mansur</surname>
<given-names>M. B.</given-names>
</name>
<name>
<surname>Guimar&#xe3;es</surname>
<given-names>A. S.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Comparison of Oxidative Roasting and Alkaline Leaching for Removing Chloride and Fluoride from Brass Ashes</article-title>. <source>Hydrometallurgy</source> <volume>202</volume>, <fpage>105619</fpage>. <pub-id pub-id-type="doi">10.1016/j.hydromet.2021.105619</pub-id> </citation>
</ref>
<ref id="B20">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Millar</surname>
<given-names>G. J.</given-names>
</name>
<name>
<surname>Couperthwaite</surname>
<given-names>S. J.</given-names>
</name>
<name>
<surname>Wellner</surname>
<given-names>D. B.</given-names>
</name>
<name>
<surname>Macfarlane</surname>
<given-names>D. C.</given-names>
</name>
<name>
<surname>Dalzell</surname>
<given-names>S. A.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Removal of Fluoride Ions from Solution by Chelating Resin with Imino-Diacetate Functionality</article-title>. <source>J.&#x20;Water Process Eng.</source> <volume>20</volume>, <fpage>113</fpage>&#x2013;<lpage>122</lpage>. <pub-id pub-id-type="doi">10.1016/j.jwpe.2017.10.004</pub-id> </citation>
</ref>
<ref id="B21">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Nagaraj</surname>
<given-names>A.</given-names>
</name>
<name>
<surname>Sadasivuni</surname>
<given-names>K. K.</given-names>
</name>
<name>
<surname>Rajan</surname>
<given-names>M.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Investigation of Lanthanum Impregnated Cellulose, Derived from Biomass, as an Adsorbent for the Removal of Fluoride from Drinking Water</article-title>. <source>Carbohydr. Polym.</source> <volume>176</volume>, <fpage>402</fpage>&#x2013;<lpage>410</lpage>. <pub-id pub-id-type="doi">10.1016/j.carbpol.2017.08.089</pub-id> </citation>
</ref>
<ref id="B22">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>O&#x27;Keefe</surname>
<given-names>T. J.</given-names>
</name>
<name>
<surname>Han</surname>
<given-names>J.</given-names>
</name>
</person-group> (<year>1992</year>). <article-title>Electrochemical Evaluation of Adherence of Zinc to Aluminium Cathodes</article-title>. <source>Surf. Coating Technol.</source> <volume>53</volume> (<issue>3</issue>), <fpage>231</fpage>. <pub-id pub-id-type="doi">10.1016/0257-8972(92)90381-J</pub-id> </citation>
</ref>
<ref id="B23">
<citation citation-type="book">
<person-group person-group-type="author">
<name>
<surname>Puigdomenech</surname>
<given-names>I.</given-names>
</name>
</person-group> (<year>2004</year>). <source>Make Equilibrium Diagrams Using Sophisticated Algorithms (MEDUSA), Inorganic Chemistry</source>. <publisher-loc>Stockholm, Sweden</publisher-loc>: <publisher-name>Royal Institute of Technology</publisher-name>. </citation>
</ref>
<ref id="B24">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ravuru</surname>
<given-names>S. S.</given-names>
</name>
<name>
<surname>Jana</surname>
<given-names>A.</given-names>
</name>
<name>
<surname>De</surname>
<given-names>S.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Performance Modeling of Layered Double Hydroxide Incorporated Mixed Matrix Beads for Fluoride Removal from Contaminated Groundwater with the Scale up Study</article-title>. <source>Separat. Purif. Technol.</source> <volume>277</volume>, <fpage>119631</fpage>. <pub-id pub-id-type="doi">10.1016/j.seppur.2021.119631</pub-id> </citation>
</ref>
<ref id="B25">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Sadhu</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Bhattacharya</surname>
<given-names>P.</given-names>
</name>
<name>
<surname>Vithanage</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Padmaja Sudhakar</surname>
<given-names>P.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Adsorptive Removal of Fluoride Using Biochar - A Potential Application in Drinking Water Treatment</article-title>. <source>Separat. Purif. Technol.</source> <volume>278</volume>, <fpage>119106</fpage>. <pub-id pub-id-type="doi">10.1016/j.seppur.2021.119106</pub-id> </citation>
</ref>
<ref id="B26">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Samadi</surname>
<given-names>M. T.</given-names>
</name>
<name>
<surname>Zarrabi</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Sepehr</surname>
<given-names>M. N.</given-names>
</name>
<name>
<surname>Ramhormozi</surname>
<given-names>S. M.</given-names>
</name>
<name>
<surname>Azizian</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Amrane</surname>
<given-names>A.</given-names>
</name>
</person-group> (<year>2014</year>). <article-title>Removal of Fluoride Ions by Ion Exchange Resin: Kinetic and Equilibrium Studies</article-title>. <source>Environ. Eng. Manage. J.</source> <volume>13</volume>, <fpage>205</fpage>. <pub-id pub-id-type="doi">10.1016/j.ecolecon.2013.11.002</pub-id> </citation>
</ref>
<ref id="B27">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Sandoval</surname>
<given-names>M. A.</given-names>
</name>
<name>
<surname>Fuentes</surname>
<given-names>R.</given-names>
</name>
<name>
<surname>Nava</surname>
<given-names>J.&#x20;L.</given-names>
</name>
<name>
<surname>Core&#xf1;o</surname>
<given-names>O.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Hern&#xe1;ndez</surname>
<given-names>J.&#x20;H.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Simultaneous Removal of Fluoride and Arsenic from Groundwater by Electrocoagulation Using a Filter-Press Flow Reactor with a Three-Cell Stack</article-title>. <source>Separat. Purif. Technol.</source> <volume>208</volume>, <fpage>208</fpage>&#x2013;<lpage>216</lpage>. <pub-id pub-id-type="doi">10.1016/j.seppur.2018.02.018</pub-id> </citation>
</ref>
<ref id="B28">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Turner</surname>
<given-names>B. D.</given-names>
</name>
<name>
<surname>Binning</surname>
<given-names>P.</given-names>
</name>
<name>
<surname>Stipp</surname>
<given-names>S. L. S.</given-names>
</name>
</person-group> (<year>2005</year>). <article-title>Fluoride Removal by Calcite: Evidence for Fluorite Precipitation and Surface Adsorption</article-title>. <source>Environ. Sci. Technol.</source> <volume>39</volume> (<issue>24</issue>), <fpage>9561</fpage>&#x2013;<lpage>9568</lpage>. <pub-id pub-id-type="doi">10.1021/es0505090</pub-id> </citation>
</ref>
<ref id="B29">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wajima</surname>
<given-names>T.</given-names>
</name>
<name>
<surname>Umeta</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Narita</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>Sugawara</surname>
<given-names>K.</given-names>
</name>
</person-group> (<year>2009</year>). <article-title>Adsorption Behavior of Fluoride Ions Using a Titanium Hydroxide-Derived Adsorbent</article-title>. <source>Desalination</source> <volume>249</volume>, <fpage>323</fpage>&#x2013;<lpage>330</lpage>. <pub-id pub-id-type="doi">10.1016/j.desal.2009.06.038</pub-id> </citation>
</ref>
<ref id="B30">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wan</surname>
<given-names>K.</given-names>
</name>
<name>
<surname>Huang</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Yan</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Ma</surname>
<given-names>B.</given-names>
</name>
<name>
<surname>Huang</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Luo</surname>
<given-names>Z.</given-names>
</name>
<etal/>
</person-group> (<year>2021</year>). <article-title>Removal of Fluoride from Industrial Wastewater by Using Different Adsorbents: A Review</article-title>. <source>Sci. Total Environ.</source> <volume>773</volume>, <fpage>145535</fpage>. <pub-id pub-id-type="doi">10.1016/j.scitotenv.2021.145535</pub-id> </citation>
</ref>
<ref id="B31">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Wu</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Tang</surname>
<given-names>D.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>G.</given-names>
</name>
</person-group> (<year>2018</year>). <article-title>Simultaneous and Efficient Removal of Fluoride and Phosphate by Fe-La Composite: Adsorption Kinetics and Mechanism</article-title>. <source>J.&#x20;Alloys Compd.</source> <volume>753</volume>, <fpage>422</fpage>&#x2013;<lpage>432</lpage>. <pub-id pub-id-type="doi">10.1016/j.jallcom.2018.04.177</pub-id> </citation>
</ref>
<ref id="B32">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wei</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Jiang</surname>
<given-names>F.</given-names>
</name>
<name>
<surname>Fan</surname>
<given-names>G.</given-names>
</name>
<name>
<surname>Chen</surname>
<given-names>Y.</given-names>
</name>
</person-group> (<year>2010</year>). <article-title>Speciation Analysis of the Fluoride in the Smelting Flue Gas of Copper and Nickel Metallurgy</article-title>. <source>J.&#x20;Tsinghua Sci. Technol.</source> <volume>50</volume> (<issue>12</issue>), <fpage>1925</fpage>&#x2013;<lpage>1929</lpage>. <comment>in Chinese</comment>. <pub-id pub-id-type="doi">10.1016/S1872-5813(11)60001-7</pub-id> </citation>
</ref>
<ref id="B33">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yan</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Tu</surname>
<given-names>H.</given-names>
</name>
<name>
<surname>Chan</surname>
<given-names>T.</given-names>
</name>
<name>
<surname>Jing</surname>
<given-names>C.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Mechanistic Study of Simultaneous Arsenic and Fluoride Removal Using Granular TiO2-La Adsorbent</article-title>. <source>Chem. Eng. J.</source> <volume>313</volume>, <fpage>983</fpage>&#x2013;<lpage>992</lpage>. <pub-id pub-id-type="doi">10.1016/j.cej.2016.10.142</pub-id> </citation>
</ref>
<ref id="B34">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname>
<given-names>K.</given-names>
</name>
<name>
<surname>Li</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Tian</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Peng</surname>
<given-names>K.</given-names>
</name>
<name>
<surname>Lai</surname>
<given-names>Y.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Removal of Fluoride Ions from ZnSO<sub>4</sub> Electrolyte by Amorphous Porous Al<sub>2</sub>O<sub>3</sub> Microfiber Clusters: Adsorption Performance and Mechanism</article-title>. <source>Hydrometallurgy</source> <volume>197</volume>, <fpage>05455</fpage>. <pub-id pub-id-type="doi">10.1016/j.hydromet.2020.105455</pub-id> </citation>
</ref>
<ref id="B35">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhong</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Wang</surname>
<given-names>R.</given-names>
</name>
<name>
<surname>Xu</surname>
<given-names>Z.</given-names>
</name>
<name>
<surname>Jiang</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Lv</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Lai</surname>
<given-names>Y.</given-names>
</name>
<etal/>
</person-group> (<year>2017</year>). <article-title>Influence of Mn 2&#x2b; on the Performance of Pb-Ag Anodes in Fluoride/chloride-Containing H 2 SO 4 Solutions</article-title>. <source>Hydrometallurgy</source> <volume>174</volume>, <fpage>195</fpage>&#x2013;<lpage>201</lpage>. <pub-id pub-id-type="doi">10.1016/j.hydromet.2017.10.014</pub-id> </citation>
</ref>
<ref id="B36">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhong</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Yu</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Jiang</surname>
<given-names>L.</given-names>
</name>
<name>
<surname>Lv</surname>
<given-names>X.</given-names>
</name>
<name>
<surname>Liu</surname>
<given-names>F.</given-names>
</name>
<name>
<surname>Lai</surname>
<given-names>Y.</given-names>
</name>
<etal/>
</person-group> (<year>2015</year>). <article-title>Influence of Fluoride Ion on the Performance of Pb-Ag Anode during Long-Term Galvanostatic Electrolysis</article-title>. <source>Jom</source> <volume>67</volume> (<issue>9</issue>), <fpage>2022</fpage>&#x2013;<lpage>2027</lpage>. <pub-id pub-id-type="doi">10.1007/s11837-015-1550-1</pub-id> </citation>
</ref>
<ref id="B37">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhou</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhu</surname>
<given-names>W.</given-names>
</name>
<name>
<surname>Yu</surname>
<given-names>J.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>H.</given-names>
</name>
<name>
<surname>Zhang</surname>
<given-names>Y.</given-names>
</name>
<name>
<surname>Lin</surname>
<given-names>X.</given-names>
</name>
<etal/>
</person-group> (<year>2018</year>). <article-title>Highly Selective and Efficient Removal of Fluoride from Ground Water by Layered Al-Zr-La Tri-metal Hydroxide</article-title>. <source>Appl. Surf. Sci.</source> <volume>435</volume>, <fpage>920</fpage>&#x2013;<lpage>927</lpage>. <pub-id pub-id-type="doi">10.1016/j.apsusc.2017.11.108</pub-id> </citation>
</ref>
</ref-list>
</back>
</article>