Adsorption of Phosphate by Surface Precipitation on Lanthanum Carbonate Through In Situ Anion Substitution Reactions

Highlights

• Lanthanum carbonate was an effective adsorbent for phosphate under weak acidic conditions, with the maximum adsorption being 106.6 mg/g at pH 2.9.

• The presence of either inorganic anions or natural organic matter would inhibit the adsorption of PO₄³⁻ on La₂(CO₃)₃.

• Because of the longer adsorption path and weaker competition in the adsorption process, the influence of natural organic matter was weaker than that of inorganic anions

• The mechanism is the combined result of physisorption and chemisorption according to the characterization, in which LaPO₄ is formed when PO₄³⁻ is adsorbed on the surfaces of La₂(CO₃)₃

Introduction

Eutrophication has become one of the most pressing environmental issues that harm the quality of water. It produces undesirable color, taste, odor, and turbidity. Eutrophication reduces biodiversity, destroys aquatic habitats, and also poses significant public health risk (Wu et al., 2007; Gao et al., 2013; Su et al., 2013). Nutrient enrichment disturbs the natural ecological balance in lakes and rivers. One of the most important factors that drive the eutrophication in rivers and lakes is excess of phosphorus. Phosphate enters the environment not only through effluents of wastewater treatment plants (WWTPs) but also due to surface runoff of urban and agricultural areas. The development of a rapid and efficient method for the removal of PO₄³⁻ is a highly sensitive and very interesting topic for the scientific community.

Various techniques have been developed for the removal of PO₄³⁻, including chemical precipitation (Chouyyok et al., 2010), biological processes such as harvesting biomass (Yao et al., 2011), and adsorption (Pan et al., 2014). Adsorption is a promising method because it has many advantages, including efficiency, greater speed, adaptability, easy to operate, and does not pollute the environment. In general, the adsorption, and therefore removal capacity, is directly conditioned by the physical and chemical properties of the adsorbent. In this regard, there has been a great interest in the field of environmental engineering, in advancing the development of efficient and cost-effective adsorbents. Recently, some scientific studies have reported on the removal of PO₄³⁻ using different types of adsorbent, such as layered double hydroxide (Das et al., 2006; Chitrakar et al., 2010; Mandel et al., 2013), Fe–Mn binary oxide (Zhang et al., 2009), fly ashes (Chen et al., 2007), activated carbon fibers (Zhang et al., 2011; Liu et al., 2013), silica materials (Hamoudi and Belkacemi, 2013), ferrihydrite (Mallet et al., 2013), or goethite (Belelli et al., 2014). Once PO₄³⁻ is adsorbed, the complex is often removed from the suspension by flocculation, facilitated either by polymeric materials (amphoteric chitosan) or by alum (Sherman et al., 2000; Agbovi and Wilson, 2018). On the other hand, flocculants such as alum can cause toxic effects, especially when it is released during flocculation. A few adsorbents can maintain a maximum adsorption capacity of PO₄³⁻ under a broad range of pH, especially under acidic conditions (Lurling et al., 2014; Xie et al., 2014).

The lanthanum-based material has a great adsorption capacity and chemical stability. The lanthanum-based adsorbent contains the trivalent lanthanum ion (La³⁺) that has a strong affinity for PO₄³⁻ even at trace levels. Once La³⁺ is released, it can bind with PO₄³⁻ and generate an insoluble complex under acidic conditions, lanthanum-phosphate (La⁺³-PO₄³⁻), which is nonabsorbable (Samy et al., 2010; Yang et al., 2013). Among the new lanthanum-based adsorbents that have been developed for the removal of PO₄³⁻, NaLa(CO₃)₂/Fe₃O₄ (Hao, et al., 2019), La(OH)₃ (He et al., 2015), La³⁺/La(OH)₃ (Dong et al., 2017), and La-201 (Zhang et al., 2016) are worth noting. Using such materials, adsorption of PO₄³⁻ is favored over a wide range of pH, and the adsorption mechanism involves the electrical interaction and ligand-exchange between lanthanum and PO₄³⁻ (Hao et al., 2019). The main concerning of the adsorbent was related to two basic factors—how to control phosphorus in the waters and the safety of chemicals used for this processing. La₂(CO₃)₃ was developed in recent years, and since then, its application in the pharmaceutical industry has been extensive (Persy et al., 2006). La₂(CO₃)₃ contains La⁺³ and has a very strong binding capacity to PO₄³⁻. Furthermore, La₂(CO₃)₃ does not contain aluminum or calcium and does not contaminate the environment. Despite the lanthanum-based material was considered, by many, as a potent agent for the removal of PO₄³⁻ from water, few scientific articles have reported a systematic study concerning the removal of PO₄³⁻ by La₂(CO₃)₃. Therefore, the removal of phosphorus from water by La₂(CO₃)₃ was systematically studied in this study, and it provided an effective method for the removal of PO₄³⁻ in the aquatic environment.

In this study, La₂(CO₃)₃ was synthesized and evaluated for its phosphate adsorption capacity. The effect of solution pH, adsorbent dosage, coexist inorganic ions, and natural organic matter on the removal of PO₄³⁻ by La₂(CO₃)₃ was investigated. The adsorption kinetics and isotherms were determined to compare their adsorption capacity and understand adsorption mechanisms. The X-ray photo-electron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were used to explore the mechanisms of adsorption.

Materials and Methods

Materials and Chemicals

La₂(CO₃)₃ was obtained from Guangfu Institute of Fine Chemicals, China. All other chemicals used (KH₂PO₄, Na₂SiO₄·9H₂O, NaCl, NaNO₃, Na₂CO₃, and Na₂SO₄) were of analytical grade. The measurements of pH were carried out using a PHS-3C pH-meter (Dapu instrumentation Corp., Ltd. Shanghai, China). All glassware used in experiments was carefully cleaned and rinsed with deionized water. The samples of natural organic matter (HA and FA) were collected from the soils of Jiufeng Mountain (Beijing) (Lin et al., 2011).

Batch Adsorption Experiments

La₂(CO₃)₃ was equilibrated with a suitable amount of PO₄³⁻ solution (10–100 mg L⁻¹) using magnetic stirring for 20 h. Once the adsorbent was recovered by centrifugation, the concentration of PO₄³⁻ in the supernatant was measured using the ammonium molybdate blue method. The adsorption amount of PO₄³⁻ was calculated based on the difference between the balance and total amount. The influence of temperature was evaluated by setting the concentration of PO₄³⁻ from 10 to 40 mg L⁻¹ and applying a temperature which ranged from 303.15 to 323.15 K. To determine the concentration effect, the weight from 20 to 100 mg was added to 50 ml of PO₄³⁻ solution (200 mg L⁻¹), and then the suspension was agitated on a shaker for 24 h.

Interference Study

To study the influence of coexisting inorganic anions (Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and SiO₃²⁻), 40 mg of La₂(CO₃)₃ was mixed with PO₄³⁻ solution (100 mg L⁻¹) and various competing ions. The effects of natural organic matter including HA and FA were also investigated, by combining 40 mg of La₂(CO₃)₃ with 100 mg L⁻¹ of PO₄³⁻ solution and HA and FA at the concentrations of 10, 30, or 50 mg L⁻¹. The mixture was shaken for 24 h, and the suspension was filtered through a 0.45-μm fiber membrane. The concentration of PO₄³⁻ in the solution was measured using the ammonium molybdate blue method.

pH_zpc Determination

The pH_zpc of La₂(CO₃)₃ was estimated according to the △pH method (Kinniburgh et al., 1975; Zhang et al., 2014), for which 50 mg of La₂(CO₃)₃ or PO₄³⁻-saturated La₂(CO₃)₃ was mixed with 50 ml of NaNO₃ (0.01 mol L⁻¹). These mixtures were shaken at room temperature for 20 h on an automatic shaker and then adjusted to various values of pH by additions of NaOH or HNO₃. After 60 min of equilibrium, pH was measured and defined as pH_(initial). Then, 1 g of NaNO₃ was added to each suspension. After shaking for 1 hour, the pH_(final) was measured, and the change in pH (△pH) was calculated as follows: pH_(final)-pH_(initial).

Characterization of the Adsorbent Before and After Adsorption

A JEOL JSM-6700F scanning electron microscope (SEM) was used to measure surface morphology. The energy dispersive spectrometer (EDS, Oxford X-MAX-20) associated with the SEM system and FTIR (Nicolet 6,700, United States) was utilized to examine chemical compounds on the surface and shape of the adsorbent before and after adsorption. Powder X-ray diffraction (XRD, Bruker D8 Advance, Germany) was also used to characterize the adsorbent. The chemical composition of La₂(CO₃)₃ after adsorption was determined by XPS (ESCALAB250 Thermo-VG Scientific, United States). The release of CO₃²⁻ and HCO₃⁻ from La₂(CO₃)_3, during adsorption was detected by acid–base titration.

Results and Discussion

Characterization of La₂(CO₃)₃ Prior to and After Adsorbtion of Phosphorus

The FTIR spectrums of La₂(CO₃)₃ prior to and after adsorption of PO₄³⁻ are shown in Figure 1. Compared with the spectrum of La₂(CO₃)₃, some vibration peaks corresponding to CO₃²⁻ at 1,420, 878, and 713 cm⁻¹ almost disappeared, while other peaks at 1,054, 616, and 542 cm⁻¹ were observed after the adsorption of PO₄³⁻. Furthermore, the peak at 1,054 cm⁻¹ was assigned to the asymmetric stretching vibration of P-O of the PO₄³⁻ group, and the peaks at 616 and 542 cm⁻¹ were assigned to the bending vibration of O-P-O (Li et al., 2014; Wang et al., 2016), which indicated that the adsorption mechanism of PO₄³⁻ on the surfaces of La₂(CO₃)₃ included a ligand exchange process.

FIGURE 1

FIGURE 1. FTIR spectrums of La₂(CO₃)₃ and La₂(CO₃)₃/PO₄³⁻.

The typical SEM images of La₂(CO₃)₃ prior to and after adsorption of PO₄³⁻ are shown in Figure 2. La₂(CO₃)₃ exhibited a more regular surface and better particle dispersion after the adsorption of PO₄³⁻. The adsorption of PO₄³⁻ on La₂(CO₃)₃ was confirmed by energy-dispersive spectroscopy analysis. As shown in Figure 2D, the characteristic peaks of P appeared in the spectra of La₂(CO₃)₃ after the adsorption of PO₄³⁻, which indicated that PO₄³⁻ was successfully adsorbed on the surfaces of La₂(CO₃)₃. Meanwhile, the characteristic peaks of C decreased significantly after the adsorption of PO₄³⁻ on La₂(CO₃)₃, evidencing that CO₃²⁻ was mostly replaced by PO₄³⁻ on the surfaces of La₂(CO₃)₃ (Huang et al., 2007).

FIGURE 2

FIGURE 2. SEM/EDS images of La₂(CO₃)₃ (A,B) and La₂(CO₃)₃/PO₄³⁻ (C,D).

The analysis of the powder X-ray diffractograms of the La₂(CO₃)₃ prior to and after adsorption of PO₄³⁻ was illustrated in Fig. S1, in which the XRD standard diffraction card was also presented. La₂(CO₃)₃ used as an adsorbent in this study can be indexed as La₂(CO₃)₃·8H₂O (JCPS card NO.25-1,400) from the XRD patterns. The XRD pattern of La₂(CO₃)₃ after the adsorption of PO₄³⁻ was closely matched to LaPO₄·0.5H₂O (JCPS card NO.46-1,439). These results demonstrated that a new substance was generated after PO₄³⁻ being adsorbed on the surfaces of La₂(CO₃)₃.

Effect of Solution pH

The pH can affect not only charges on the surfaces of La₂(CO₃)₃ but also dissociation and solubility of the adsorbent, which would influence the adsorption of PO₄³⁻ on La₂(CO₃)₃ (Yang et al., 2013). The effect of solution pH in the range of 1.0–8.0 is shown in Figure 3A. The adsorption amount increased sharply when the pH changed from 1.0 to 3.0 and then decreased slowly with an increase in pH. The maximum adsorption amount of PO₄³⁻ was obtained at pH 3.0 (101.6 mg g⁻¹), which is 15.6-fold greater than that at pH 1.1. It means that La₂(CO₃)₃ was an efficient adsorbent for the removal of PO₄³⁻ under acidic conditions. This is similar to the results of other research studies. Many researchers have studied the influence of the pH value on PO₄³⁻ adsorption capacity and found that the lanthanide adsorbent has a higher PO₄³⁻ removal efficiency only at a lower pH value (Lu et al., 2021). There have been many studies on the removal of PO₄³⁻ by various materials, including binary and ternary compounds, for e.g., ZrO₂, red-mud, La–Cu, Fe–Zr, Fe–Mn, and Fe–Al–Mn (Lǚ et al., 2013; Zhang et al., 2009; Liu et al., 2008; Huang et al., 2008; Zhao et al., 2014; Long et al., 2011). The adsorption amount of PO₄³⁻ on these materials was different and significantly affected by the solution pH (Table 1). The adsorption amount of binary materials (La–Cu) and ternary materials (Fe–Al–Mn) was significantly higher than that of other materials, and the adsorption capacity of La₂(CO₃)₃ was much greater than that of other materials. Meanwhile, the composition and synthesis method of La₂(CO₃)₃ was simple and better when applied to removal of PO₄³⁻.

FIGURE 3

FIGURE 3. Effect of solution pH on the adsorption of PO₄³⁻ on La₂(CO₃)₃ (A). Zero-point charge (pH_zpc) of La₂(CO₃)₃ and La₂(CO₃)₃/PO₄³⁻ (B).

TABLE 1

TABLE 1. Comparison of the maximum adsorption amount of phosphate on various adsorbents.

The isoelectric points of La₂(CO₃)₃ prior to and after adsorption were 2.1 and 6.5, respectively (Figure 3B), indicated that PO₄³⁻ neutralized the positive charge on the surface of La₂(CO₃)₃ and caused an increase in the isoelectric point. These findings revealed that in the surface of La₂(CO₃)₃, there was a change from carbonate to PO₄³⁻ when the pH values were 2.0–6.0. The existence of PO₄³⁻ species mainly in the form of H₂PO₄⁻ is explained because the adsorbent gathered more positive charges on the surface under acidic conditions, which caused strong adsorption of PO₄^3- on the surface of La₂(CO₃)₃ by electrostatic attraction. Meanwhile, La³⁺ and CO₃²⁻ in La₂(CO₃)₃ were dissociated in weak acid solutions and could be replaced by H₂PO₄⁻ under acidic conditions (Haghseresht et al., 2009; Xie et al., 2014). When the pH was greater than 6.0, the adsorption amount of PO₄³⁻ on La₂(CO₃)₃ continue to decrease presumably due to the competition for the adsorption sites between PO₄³⁻ and other coexisting anions such as CO₃²⁻ or OH⁻.

Effect of Adsorbent Dosage

The effect of dosage on the adsorption efficiency of PO₄³⁻ is shown in Figure 4. The adsorption efficiency varied from 38.4 to 94.6% for the range of concentrations of 20–80 mg 50 ml⁻¹, which suggested that the adsorption efficiency of PO₄³⁻ by La₂(CO₃)₃ was directly proportional to the dose of La₂(CO₃)₃. This is because there were more adsorption sites available for PO₄^3- as the dosage increased. When the concentration of La₂(CO₃)₃ was higher than 80 mg 50 ml⁻¹, it had a negligible effect on the adsorption efficiency of PO₄³⁻.

FIGURE 4

FIGURE 4. Effect of La₂(CO₃)₃ dosage on the adsorption amount of PO₄³⁻. Initial concentration of PO₄³⁻: 200 mg L⁻¹, temperature: 298.15 K, and pH: 6.8.

Adsorption Kinetics

Figure 5A demonstrates the effect of time on the adsorption amount of PO₄³⁻ on the surfaces of La₂(CO₃)₃. The adsorption amount of PO₄³⁻ increased rapidly during the first 50 min, probably due to a fast exchange of CO₃²⁻ and PO₄³⁻ on the surface of La₂(CO₃)₃ and occupancy of the sites by PO₄³⁻. After that, the adsorption amount of PO₄³ increased slowly over time. This implied mass transfer of CO₃²⁻ and PO₄³⁻ and subsequent exchange, predominant during the adsorption process. There was no significant change in the adsorption amount of PO₄³⁻ on the surface of La₂(CO₃)₃ after 24 h.

FIGURE 5

FIGURE 5. Effect of adsorption time on the adsorption amount of PO₄³⁻ on La₂(CO₃)₃ (A). Pseudo-first-order kinetic curve of adsorption of PO₄³⁻ on La₂(CO₃)₃ (B). Initial concentration of PO₄³⁻: 100 mg L⁻¹, temperature: 298.15 K, and pH = 6.8.

A quantitative approach to determine adsorption is feasible using a kinetic model. The equation for pseudo-first-order kinetic was introduced by Lagergren (Eq. 1), which is used for the prediction of the physisorption of the adsorbate onto the adsorbent in a given system.

$\ln \left[q_e-q_t\right]=\ln q_e-kt,(1)$

where q_e is the amount of the adsorbate at equilibrium (mg g⁻¹), q_t is the amount of the adsorbate (mg g⁻¹) at time t (min), and K (min⁻¹) is the rate constant for the pseudo-first-order sorption.

This equation can also be expressed by the following alternative equation:

$q_t=q_e^{\left(1-{\exp }^{-kt}\right)}.(2)$

The kinetic curve of pseudo-first-order corresponding to the adsorption of PO₄³⁻ on La₂(CO₃)₃ is shown in Figure 5B. The q_e from the nonlinear optimization was 32.08 mg g⁻¹, the rate constant of the pseudo-first-order reaction was 6.3 × 10⁻³, and the correlation coefficient (r) of the formula was 0.95. These results indicated the existence of a reversible interaction between PO₄³⁻ and La₂(CO₃)₃.

Adsorption Thermodynamics

Figure 6 displays the adsorption amount of PO₄³⁻ on La₂(CO₃)₃ at 303, 313, and 323 K. The adsorption amount of PO₄³⁻ increased gradually with the increase of temperature, significantly affecting the adsorption of PO₄³⁻ on the surfaces of La₂(CO₃)₃, since more carbonate ions could be dissociated from the surfaces of the adsorbent. The exchange rate of ions between PO₄³⁻ and CO₃²⁻ also increased, which could accelerate the adsorption reaction. The increase of the adsorption amount indicated that the adsorption of PO₄³⁻ on the surfaces of La₂(CO₃)₃ was an endothermic process (Mezenner and Bensmaili, 2009).

FIGURE 6

FIGURE 6. Effect of temperature on the adsorption amount of PO₄³⁻ on La₂(CO₃)₃, pH: 6.8.

Effect of Coexisting Inorganic Ions

Inorganic ions such as Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and SiO₃²⁻ are ubiquitous in environmental water. The influence of such anions on the adsorption of PO₄³⁻ is shown in Figure 7A. Compared with the control, all anions had a negative effect on the adsorption efficiency of PO₄³⁻. In particular, the coexistence of CO₃²⁻ and SiO₃²⁻ reduced the adsorption efficiency of PO₄³⁻ from 54.1 to 14.2%–9.5 and 7.6%, respectively, when the concentration of CO₃²⁻ and SiO₃²⁻ increased from 1.0 to 10 mmol L⁻¹. The previous studies have also examined the effect of ions on adsorption efficiency of phosphorus of other materials, such as La-porous carbon composites (Koilraj and Sasaki, 2017). The result was consistent with this study, and the adsorption capacity was disturbed by 20 mM CO₃²⁻ but not reduced in the presence 20 mM Cl⁻ and 20 mM SO₄²⁻. To further study the mechanism involved, the change of pH was measured after adsorption in the presence of CO₃²⁻ and SiO₃²⁻. The pH rose when the concentration was increased for CO₃²⁻ (pH = 7.1–10.2) and SiO₃²⁻ (pH = 7.7–11.9). This then entails a strong interfering effect on the adsorption of PO₄³⁻ due to CO₃²⁻ and SiO₃²⁻. On the one hand, the rose of pH would cause a decrease of positive charges on the surfaces of La₂(CO₃)₃, weakening the electrostatic attraction between PO₄³⁻ and La₂(CO₃)₃. On the other hand, the increase of anions could lead to stronger competitive adsorption with PO₄³⁻, resulting in a decrease of adsorption amount of PO₄³⁻.

FIGURE 7

FIGURE 7. Effects of Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and SiO₃²⁻ (A) and natural organic matter (B) on adsorption of PO₄³⁻ on La₂(CO₃)₃.

Effect of Natural Organic Matter

HA and FA are the most important components in the natural organic matter, being ubiquitous in the aquatic environment. They are complex mixtures of heterogenous compounds with a negative charge, originated from the decomposition of plant and animal residues (Valencia et al., 2012; Mcintyre and Guéguen, 2013). The existence of HA and FA in environmental waters may cause interference in the adsorption of PO₄³⁻ on La₂(CO₃)₃ through a competitive process. As shown in Figure 7B, with no coexisting ions in solution as a blank controller (no co-ion), both HA and FA had a weaker effect, as interference on the adsorption of PO₄³⁻, than that observed for the anion. This is because the molecular weight of natural organic matter was larger than that of anion and requires a longer adsorption path. HA and FA were less competitive to PO₄³⁻ than inorganic anion, and FA, with a small molecular weight, had more influence on the adsorption than HA. Since HA and FA can be combined with PO₄³⁻ in aqueous solution, the complex of La₂(CO₃)₃/HA (or FA) can still be combined with PO₄³⁻, so the change in the adsorption amount of PO₄³⁻ was not perceivable in the presence of HA and FA.

Analysis of the Mechanism in the Adsorption of PO₄³⁻

As shown in Figure 8, the adsorption amount of PO₄³⁻ increased with the concentration of PO₄³⁻, reaching a maximum adsorption amount at 150 mg L⁻¹. When PO₄³⁻ concentration exceeded 150 mg L⁻¹, the adsorption amount decreased. The corresponding total molar volume of bicarbonate in the solution after adsorption is also shown in Figure 8, which rose when the concentration of PO₄³⁻ increased and attained equilibrium at variable concentration. In addition, the change of the adsorption amount of PO₄³⁻ (the initial concentration of PO₄³⁻ was 100–200 mg L⁻¹) could be due to the coexistence of anions such as OH⁻. Therefore, PO₄³⁻ in the solution would react with La₂(CO₃)₃, and HCO₃⁻ would be released.

FIGURE 8

FIGURE 8. Influence of the initial concentration of PO₄³⁻ on adsorption amount of PO₄³⁻ (A). Total molar volume of bicarbonate in the solution after adsorption (B).

The XPS spectra of La₂(CO₃)₃ after the adsorption of PO₄³⁻ is shown in Fig. S2(a). The content of C, O, La, and P at the surface of La₂(CO₃)₃ after adsorption were 13.7, 57.4, 18.0, and 10.8%, respectively. Fig. S2(b) and Fig. S2(c) show the fitted spectras corresponding to La_3d and P_2p, respectively. The peak at 835.10 eV corresponded to La_3d5/2 of LaPO₄ (Jorgensen et al., 2002). The binding energy of P 2p was 133.89 and 132.89 eV, which contributed to LaPO₄ (Ivanova et al., 1996) and HPO₄²⁻ (Kurmaev et al., 1996), respectively. The XPS spectrum indicated that the adsorption of PO₄³⁻ on the surfaces of La₂(CO₃)₃ might be the result of ion exchange between PO₄³⁻ and carbonate, according to the aforementioned results. Based on a previous study, the adsorption process of PO₄³⁻ on the surfaces of La₂(CO₃)₃ could be mainly explained in terms of ion exchange (chemisorptions). A tentative adsorption mechanism is presented in Fig. S3, when HPO₄²⁻ reacted with La₂(CO₃)₃, and HCO₃²⁻ was released in the solution.

Conclusion

This study has enabled the exploration of the adsorption process of PO₄³⁻ on the surfaces of La₂(CO₃)₃ and its characterization in aqueous solution. It has been evidenced that the pH had a greater impact in the adsorption of PO₄³⁻ on La₂(CO₃)₃, and the adsorbent have an excellent adsorption ability under acidic conditions (pH = 2.0–6.0). On the other hand, the presence of either inorganic anions or natural organic matter would inhibit the adsorption of PO₄³⁻ on La₂(CO₃)₃. The influences of both CO₃²⁻ and Si₂O₃²⁻ were far higher than those of other anions. Because of the longer adsorption path and weaker competition in the adsorption process, the influence of natural organic matter was weaker than that of inorganic anions. The mechanism involved in the adsorption of PO₄³⁻ on La₂(CO₃)₃ is the combined result of physisorption and chemisorption according to the characterization, in which LaPO₄ is formed when PO₄³⁻ is adsorbed on the surfaces of La₂(CO₃)₃. All these results prove that La₂(CO₃)₃ has a large adsorption capacity and useful for the removal of PO₄³⁻ from water.

Data Availability Statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author Contributions

SZ, ZT, and FX: conceptualization, methodology, and software. SZ and ZT: data curation and writing-original draft preparation. MF and TZ: visualization and investigation. ZT and FX: supervision. MF and TZ: software and validation. SZ, ZT, and JPG: writing-reviewing and editing.

Funding

This research was financially supported by the National Science Foundation of China (21777001, 21107001, and 42077349) and Key Research and Development Project of Anhui Province, China (202004i07020006).

Conflict of Interest

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.

Publisher’s Note

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.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2022.858258/full#supplementary-material

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