Amino-Thiol Bifunctional Polysilsesquioxane/Carbon Nanotubes Magnetic Composites as Adsorbents for Hg(II) Removal

Amino-thiol bifunctional polysilsesquioxane/carbon nanotubes (PSQ/CNTs) magnetic composites were prepared by sol-gel method with two types of functional siloxanes coating on carboxyl CNTs simultaneously. The composites were served as efficient adsorbents for removing Hg(II) in aqueous solution and the adsorption properties were investigated systematically. The optimal pH of bifunctional composites for Hg(II) removal is at pH 4.5. The thermodynamic fitting curves are more consistent with the Langmuir model and the adsorption capacities of the bifunctional composites for Hg(II) varied from 1.63 to 1.94 mmol g−1 at 25°C according to the Langmuir model. The kinetics curves are more fitted to the pseudo-second-order model and the composites could selectively adsorb Hg(II) in a series of binary metal ions solution. The elution regeneration tests showed that the adsorption rate could still reach 78% after repeat cycle three times. It is expected that the bifunctional PSQ/CNTs magnetic composites can be potentially applied to remove low concentration Hg(II) from waste water.


INTRODUCTION
Environmental problems caused by harmful pollutants have become serious threats to the survival of human beings and other living things (Hsu et al., 2021;Wang et al., 2021), and water pollution is particularly prominent among them. Common water pollutants include inorganic pollutants like heavy metal ions, acid, alkali and salt, and other organic pollutants (Mudasir et al., 2020, Wang et al., 2020aZhangal and Zhang, 2020) stem from metallurgy, chemical fiber, papermaking, printing and dyeing, and other industrial wastewaters (Khan et al., 2021;Verma and Balomajumder, 2020). Hg(II), Ni(II), Pb(II), and other heavy metal ions enter human body through the enrichment of the food chain and harm human health ultimately (Guo et al., 2020;Fu et al., 2021;Ge and Du, 2020), especially the strongest toxic mercury. Adsorption is more attractive in removing metal ions than chemical precipitation, sedimentation, ion exchange, filtration, and other traditional means Liu et al., 2019;Zhang B. et al., 2019), thanks to its simple operation, less secondary pollution, high adsorption efficiency, low cost, and other advantages (Zhao et al., 2012;Wang et al., 2017;Yang et al., 2019). Therefore, it is urgent to find a suitable adsorption material to treat Hg(II) in wastewater.
Carbon nanotubes (CNTs) are one novel nano-adsorbent used in water pollution treatment over the past decades (Alimohammady et al., 2017;Ahmadi et al., 2019). Compared to zeolite, kaolinite, chitosan, biopolymer, and other traditional adsorption materials (Zhang M. et al., 2019;Fu et al., 2021;Khan et al., 2021), CNTs have more excellent aspect ratio, specific surface area, and unique one-dimensional structure (Sone et al., 2008;Aliyu, 2019), and all these structural features provide convenient conditions for metal ions adhering. Inevitably, the wide application of CNTs in adsorption field is also subjected to its own limitations (Basheer et al., 2020;Samareh and Siochi, 2017), such as poor dispersion, inferior dissolution, and skimp active sites. Hence, it has a great significance to modify CNTs with efficient functional molecules to maximize advantages in adsorption. Abbasi et al. (2021) prepared a novel calcined CuAl-layered double hydroxides/carbon nanotubes/ polyvinylidene fluoride composites via a hydrothermal, casting, and calcination, and the composites show favorable adsorption properties to carminic acid. Alimohammady et al. (2017) reported a novel nano-adsorbent synthesized by the carboxylic multi-walled carbon nanotubes (MWCNTs-COOH) and 3aminopyrazole, and the composite MWCNTs-f can remove 83.7% Cd(II) from aqueous solution at the optimum conditions. AlOmar et al. (2016) synthesized six deep eutectic solvents (DESs) systems based on choline chloride and six different hydrogen bond donors, and then the DESs were used to functionalize CNTs for adsorbing lead ions. Plentiful literatures indicating modified CNTs showed excellent adsorption performances in removing hazardous pollutants, include heavy metal ions, inorganic acid, and organics. Nonetheless, the exploration of modifying CNTs is still in progress, and researchers are working to find other substitutable molecules to prepared potential nano-adsorbents.
Polysilsesquioxane (PSQ) is a kind of organic-inorganic hybrid material with Si-O inorganic bond backbone and organic groups in side chains (El-Nahhalal et al., 2007;Kierys et al., 2018). Due to the special composition and structure, PSQ has many unique and excellent properties (Tang et al., 2013;Rathnayake et al., 2021), for instance, the organic groups connected with Si can be connected with other groups through different chemical reactions . In addition, PSQ has distinct corrosion resistance, thermal stability, chemical reactivity Liu et al., 2021), and can be used in semiconductor materials, catalysis, adsorption, and other fields (Wu et al., 2018;Park et al., 2020;Kong et al., 2021). PSQ as an adsorbent commonly has high adsorption capacity and adsorption rate Wang et al., 2020b). Niu et al. (2014) prepared a thiol-functionalized PSQ used for adsorbing Hg(II) and Mn(II) from aqueous solution. Wang et al. (2017) prepared two types of fibrous adsorbents by coating thiol-and amino-functionalized PSQ on poly (p-phenylenetherephthal amide) fibers, and evaluated the adsorption properties of Hg(II).
In our previous work , the bifunctional PSQ/ CNTs magnetic composites were prepared by sol-gel method with carboxylic carbon nanotubes (CNTs-COOH), 3aminopropyl-trimethoxysilane (APTMS), and 3mercaptopropyl-trimethoxysilane (MPTMS), at the same time, magnetic Fe 3 O 4 was embedded into composites to improve the separation speed from aqueous solution (Yuan et al., 2016;Zhang S. et al., 2020). The composites were applied to recycle Au(III) from wastewater and the results indicated that the bifunctional PSQ/CNTs magnetic composites have excellent performances in adsorbing Au(III).
In the present work, we target investigating the adsorption abilities of the composites we prepared previously in treating low concentration Hg(II). Consequently, a complete set of adsorption experiments were operated and analyzed, mainly contain static adsorption, optimal pH, adsorption isotherms and kinetics, adsorption mechanism, selectivity, and recycle.

Materials and Methods
All chemicals and reagents were of analytical grade and used as received unless otherwise stated. CNTs-COOH was purchased from Times nano, Chengdu Organic Chemicals Co., Ltd., China. APTMS was purchased from Shanghai Macklin Biochemical Co., Ltd., China. MPTMS was purchased from Qufu Wanda Chemical Co., Ltd., China. Mercuric nitrate (Hg(NO 3 ) 2 ·1/2H 2 O) were purchased from Sinopharm Chemical Reagent Co., Ltd., China.
Bifunctional PSQ/CNTs magnetic composites were synthesized by sol-gel method. The first step is magnetization; prepared magnetic Fe 3 O 4 was embedded in the original material to give CNTs-COOH@Fe 3 O 4 , and then CNTs-COOH@Fe 3 O 4 was coupled with APTMS via amidation to give the intermediate CNTs-APTMS@Fe 3 O 4 with siloxane introduced on the surface. In the final step, CNTs-APTMS@Fe 3 O 4 was decorated with variable proportions of APTMS and MPTMS to give the bifunctional PSQ/CNTs magnetic composites. The reaction steps as shown in Figure 1 and the monofunctional PSQ/ CNTs magnetic composites were also prepared in the same way for comparison. The specific experimental methods and dosages were described in Ref. Xu et al. (2021). The structures and surface morphologies of PSQ/CNTs magnetic composites were confirmed by FT-IR, SEM, VSM, XRD, XPS, and BET analysis, and all the characterization analyses were also detailed explicated in Ref. Xu et al. (2021).

Static Adsorption
For measuring the adsorption properties of the composites to different metal ions, 10 mg composite was added into 40 ml 100 mg L −1 Hg(NO 3 ) 2 ·1/2H 2 O, AgNO 3 , Pb(NO 3 ) 2 , Cu(NO 3 ) 2 ·3H 2 O, and Ni(NO 3 ) 2 ·6H 2 O, respectively. At the same time, the unmodified CNTs-COOH@Fe 3 O 4 and the intermediate CNTs-APTMS@Fe 3 O 4 were also measured in the same way. Then the mixtures were shaken at 25°C for 24 h. The initial and equilibrium concentrations of metal ions were measured by atomic absorption spectrometry (AAS). The adsorption capacity was calculated by the equation as follows: where q e (mmol g −1 ) is the equilibrium adsorption capacity; C 0 and C e (mmol L −1 ) represent the initial and equilibrium concentration of metal ion, respectively; V (L) is the volume of solution; and W (g) is the weight of the composite adsorbent.

Optimal pH
The effect of pH on the uptake of composites for Hg(II) was performed by the following method: 10 mg composite was added into 40 ml 100 mg L −1 Hg(II) solution, the different solution pH values were adjusted with dilute HNO 3 and NaOH aqueous, after that the mixtures shaken at 25°C for 24 h. The initial and equilibrium concentrations were measured by AAS and the adsorption capacities were calculated by Equation 1 as well.

Adsorption Isotherms
A total of 10 mg composite was added into 40 ml Hg(NO 3 ) 2 ·1/ 2H 2 O solution with different initial concentrations, adjusted to optimal pH, and the mixtures were then shaken at 25°C for 24 h. Simultaneously, the same method was used to determine the adsorption capacities of the composites at 15 and 35°C and calculated with Equation 1.

Adsorption Kinetics
A total of 10 mg composite was added into 100 ml 50 mg L −1 Hg(II) solution, adjusted to optimal pH, and then shaken at 25°C. The concentrations of Hg(II) were determined at regular time intervals and calculated by the following equation as follows: where q e (mmol g −1 ) is the equilibrium adsorption capacity; C 0 and C t (mmol L −1 ) represent the initial and time t concentration of metal ion, respectively; V t (L) is the volume of solution at time t; and W (g) is the weight of the composite adsorbent.

Adsorption Selectivity
A total of 10 mg composite was added into 40 ml solution with binary metal ions in the same concentrations (100 mg L −1 ) and shaken at 25°C for 24 h. The concentrations of the two metal ions were determined by AAS, respectively, calculated with Equation 1,  and then the adsorption selective coefficient (α) was defined as formula (3): α The adsorption capacity of Hg (II) on adsorbent The adsorption capacity of coexisting metal ion on adsorbent (3)

Adsorption Mechanism
A total of 10 mg composite was added into the Hg(II) solution under the optimal conditions and shaken thermostatically for 24 h. It was filtered and then the adsorbent with metal ions anchored was placed in the oven at 60°C until completely dry. The mechanism of adsorption process was inferred by analyzing the changes of the elements in XPS spectra before and after adsorption.

Elution Regeneration
Different concentrations of eluents were prepared according to the acid environment that Hg(II) located. A total of 10 mg adsorbent with Hg(II) anchored was added into 40 ml eluent (0.1 mol L −1 HNO 3 , 1, 2, 3, 4, and 5% thiourea in 0.1 mol L −1 HNO 3 , respectively). It was then shaken at 25°C and desorption for 24 h, and the metal ion concentrations of eluents after desorption were measured by AAS, to determine the best eluent. Repeat adsorption-desorption three times, and judge the regeneration performances of the composites.

Static Adsorption
The adsorption properties of different materials for metal ions are shown in Figure 2A. As you can see from the figure, the PSQ/CNTs magnetic composites showed excellent adsorption capacities for Hg(II) rather than Ag(I), Pb(II), Cu(II), and Ni(II). Compared with pristine CNTs-COOH@Fe 3 O 4 and intermediate CNTs-APTMS@ Fe 3 O 4 , PSQ/CNTs magnetic composites displayed more remarkable adsorption performance to Hg(II), and the adsorption capacities increased 0.3-0.9 mmol g −1 to different extent. Further, the bifunctional PSQ/CNTs magnetic composites expressed relatively higher adsorption capacities than monofunctional composites, especially the composite A@Fe 3 O 4 . As a whole, the bifunctional PSQ/ CNTs magnetic composites showed superior adsorption abilities to Hg(II) and the systematic exploration would be further studied in the following. Figure 2B shows the effect of solution pH values on different composites for adsorbing Hg(II); the experimental pH values varied from 1.0 to 4.5. Obviously, the adsorption capacities of composites to Hg(II) were greatly affected by the solution pH values. To be specific, the monofunctional composite A@Fe 3 O 4 showed consistent change with pH variation, and this can be interpreted as H + tends to compete with Hg 2+ to form -NH 3 + at relatively low pH values (Wang et al., 2017;Wang et al., 2019),  the adsorbent surface is positively charged and electrostatic repulsion occurs with Hg 2+ , thus the adsorption capacity is low at highly acidic solution. With the growth of pH value, the concentration of H + decreases and the protonation of -NH 2 weakens, and thus the adsorption amount of composite for Hg(II) increased spontaneously. For M@Fe 3 O 4 , the effect of solution pH was mainly divided into two parts, and the adsorption capacity reached the maximum at pH 4.5. For bifunctional composites, the adsorption capacities greatly affected by the thiol group before pH 1.5 and mainly affected by the amino group after pH 1.5, and finally reached the maximum at pH 4.5.

Adsorption Isotherms
The adsorption isotherms of the composites for Hg(II) were carried out with initial concentration at 50, 75, 100, 125, and 150 mg L −1 at pH 4.5 and different temperatures. The adsorption capacities variation of PSQ/CNTs magnetic composites with initial concentration are shown in Figure 3A, and the bifunctional composites can basically achieve complete adsorption at relatively low initial concentrations; the monofunctional composites have less adsorption, especially the composite A@Fe 3 O 4 . As the initial concentrations increase, the equilibrium adsorption capacity of all composites increased.
The equilibrium adsorption isotherm plays an important part in investigating the adsorption mechanism and adsorption capacity Hu et al., 2021). Four types of adsorption model, Langmuir, Freundlich, Tempkin, and Hill models, were used to fit the adsorption process to further explore the thermodynamic adsorption for Hg(II). The Langmuir model deems that adsorption occurs on a monolayer of uniform surface with no interaction between the adsorbents (Zhao et al., 2019;Ghodsi et al., 2021); differently, the Freundlich mode deems that adsorption occurs on multilayers of heterogeneous surfaces. The Temkin model reflects that the reaction between the adsorbate and the adsorbent linearly reduces the heat of adsorption. The Hill model showed the relation of different species on the homogeneous surfaces and assumed that one adsorption site can capture n 2 ions. Where n 2 > 1 said positive cooperativity, n 2 1 proved non-cooperative and n 2 < 1 showed negative cooperativity between the binding (Saadi et al., 2015). The equations of the four models are as follows, respectively: C e q e C e q max + 1 q max K L (4) q e n 2 N M C n2 e C n2 e + C n2 where C e (mmol L −1 ) is the equilibrium solution concentration; q e (mmol g −1 ) is the equilibrium adsorption capacity; q max (mmol g −1 ) is the maximum adsorption capacity; K L is the Langmuir constant; and K F and n are the Freundlich constants related to adsorption capacity and strength, respectively. K m was the constant of the Temkin model and B T was related to the heat of adsorption. n 2 , N M, and C 1/2 were the number of the adsorbed ions at each site, the density of the receptor sites, and semi saturated concentration, respectively. The fitting of the composite 2A-M@Fe 3 O 4 at different temperatures to the four models are shown in Figures  3C-F, respectively, and the related parameters of all bifunctional PSQ/CNTs magnetic composites are calculated and shown in Tables 1, 2. From the fitting curves in figure and parameters in the table, we can find that the fitted related coefficient R L 2 > R F 2 > R T 2 > R H 2 , that is, the Langmuir model is more suitable to describe the thermodynamic process of composites to Hg(II) adsorption, and thus, the adsorption occurs on a monolayer with no interaction.
Further, thermodynamic parameters of the bifunctional PSQ/ CNTs magnetic composites to Hg(II) were also calculated by Equations 8, 9, including Gibbs free energy (ΔG), entropy change (ΔS), and enthalpy change (ΔH) (Niu et al., 2014): where K L is the Langmuir constant, R is the gas constant (R 8.314 J mol −1 K −1 ), and T (K) is the experimental temperature.
Plotting ln K L against 1/T to give fitting straight-lines of bifunctional PSQ/CNTs magnetic composites in Figure 3B, the slope and intercept are -ΔH/R and ΔS/R, respectively. The adsorption thermodynamic parameters ΔG, ΔS, and ΔH were calculated in Table 3, in which the values are ΔG < 0, ΔS > 0, and ΔH > 0, indicating that the adsorption process of bifunctional PSQ/CNTs magnetic composites to Hg(II) is a spontaneous, increased confusion and endothermic reaction process. That is to say, the whole adsorption process is a spontaneous behavior and we can promote the metal ions adhered on the surface of composites to enhance adsorption capacities by increasing temperatures. Compared with the adsorbents of the same type reported in the literature listed in Table 4, the adsorbents prepared in this paper show superior adsorption performance.

Adsorption Kinetics
Adsorption kinetics is a common method to characterize the adsorption efficiency and indicate the adsorption type of solute in the adsorption process (Yang et al., 2014;Wei et al., 2021). Figure 4A is the adsorption rate of all prepared PSQ/CNTs magnetic composites for Hg(II) at 25°C, and the adsorption rate fell in the following order: Obviously, the whole adsorption process for Hg(II) consists of two distinct parts, the first hour is the rapid adsorption stage to give the main adsorption capacity, the second stage is relatively smooth and slow to give the final equilibrium adsorption capacity. As can be seen from the figure, the adsorption process reached adsorption equilibrium in about 11-13 h.
For better exploring the adsorption process of PSQ/CNTs magnetic composites to Hg(II), the kinetics data were fitted into the pseudo-first-order and pseudo-second-order models Wang L. et al., 2020) as follows, respectively: ln q e − q t lnq e − k 1 t (10) t q t 1 k 2 q 2 e + t q e where q e and q t (mmol g −1 ) are the adsorption capacity for Hg(II) at equilibrium and time t (h), respectively; k 1 (h −1 ) and k 2 (g mmol −1 h −1 ) are the rate constant of pseudo-first-order and pseudo-second-order models. Table 5 show the fitting results and the specific kinetic parameters of bifunctional PSQ/CNTs magnetic composites to the two models. As can be seen from the fitting lines and data, the pseudo-second-order model has higher correlation coefficient than the pseudofirst-order model (R 2 2 > R 1 2 ). That is, the pseudo-secondorder model is more suitable to describe the adsorption process for Hg(II), thus, the limiting step of adsorption rate may be chemical adsorption involving valency forces through the sharing or exchange of electrons between PSQ/ CNTs and Hg(II) (Qu et al., 2009).

Adsorption Selectivity
The adsorption selectivity is an important factor to evaluate the adsorption properties of adsorbents (Mudasir et al., 2020;Wang et al., 2020a), thus, the adsorption selectivity of the representative composite A-2M@Fe 3 O 4 for Hg(II) with common coexisting metal ions was carried out. The experiments were operated at optimal pH (pH 4.5) and the results in Table 6 suggested that the composite tend to adsorb Hg(II) rather than coexisting metal ions, especially ions Pb(II), Ni(II), Cu(II), and Cd(II). The selectivity coefficient is relatively low when Ag(I) coexists with Hg(II), and this can be interpreted by the hard-soft acidbase theory (HSAB). Hg(II) and Ag(I) are classified as soft ions and can form high affinity with functional groups containing nitrogen and sulfur, thus the coexistence of Ag(I) has a great influence on the adsorption of Hg(II). As a whole, the bifunctional PSQ/CNTs magnetic composites can potentially adsorb and separate Hg(II) in coexisting metal ions system.

Adsorption Mechanism
The binding energy changes of elements in XPS scan before and after adsorption were analyzed to infer the adsorption mechanism for Hg(II). Figure 5A shows the wide scan XPS spectra of bifunctional PSQ/CNTs magnetic composites after Hg(II) adsorption, the binding energy peaks of Hg 4f and Hg 4d are clearly displayed in the curves, and the peak of Hg 4p is easily found as well. The binding energy peaks of Hg 4f at 99.8 and 103.6 eV in Figure 5D are vested to Hg 4f 5/2 and Hg 4f 7/2 (Guo et al., 2020;Xia et al., 2021), respectively. Further, the main high-resolution peaks of Hg 4f 7/2 contains two different valence state peaks Hg 2+ and Hg 0 (Wang et al., 2020b), which located at 99.6 and 100.5 eV, respectively, indicating that the redox reaction may occur in the process of adsorbing Hg(II). The element binding energy changes of the composite A-M@ Fe 3 O 4 before and after adsorption are displayed in Figures 5B,C. In the spectra of N 1s , the binding energy shifts from 397.3 to 397.5 eV for -NH 2 and the relative peak area increased, the binding energy shifts from 398.1 to 398.4 eV for -CONH-, the binding energy shifts from 399.2 to 400.1 eV for the protonated -NH 3 + . The changes of binding energy and peak area indicate that chelation or ion exchange may occur during the process of adsorbing Hg(II). Similarly, in the spectra of S 2p , the binding energy of -SH shifts from 162.6 to 161.8 eV, indicating that chelation or ion exchange may be carried out. Meanwhile, the binding energy of oxidized S appeared at around 166.5 eV, which can be inferred that redox reaction may took place between -SH and Hg(II), and corresponds to the reduction of Hg 2+ . In short, chelation or ion exchange may primarily occurred in the whole adsorption process, and certainly, limited redox reaction took place as well.

Elution Regeneration
The composite A-M@Fe 3 O 4 was selected to explore the elution and regeneration performance for Hg(II), the desorption rate of A-M@Fe 3 O 4 in a series of 0.1 mol L −1 HCl solution with different concentrations of thiourea eluents shown in Figure 6. Obviously, the desorption rate reached maximum in 0.1 mol L −1 HCl solution with 3% thiourea eluents; thus, the following elution experiments were taking place in it and the adsorption rate varied with regeneration times shown in Figure 6 inset. The adsorption rate gradually decreased with the increasing of regeneration frequency and the adsorption rate still achieved 78% after repeat elution-regeneration three times. That is to say, the bifunctional PSQ/CNTs magnetic composites have favorable regenerability and can be expected to be a kind of potential economic adsorbent.

CONCLUSION
Amino-thiol bifunctional PSQ/CNTs magnetic composites were successfully prepared by amidation and condensation between CNTs and two types of siloxane. The systematic adsorption experiments showed that the composites have favorable adsorption properties for low concentration Hg(II) removal. Specifically, the static adsorption indicates that the bifunctional PSQ/CNTs magnetic composites have superior adsorption capacities than monofunctional composites, pristine material CNTs-COOH@Fe 3 O 4 , and common intermediate CNTs-APTMS@Fe 3 O 4 , particularly, the adsorption capacities increase 0.3-0.9 mmol g −1 to different extent compared to unfunctional materials. The thermodynamic curves are more consistent with the Langmuir model and the kinetics curves are fitted to the pseudo-second-order model well. The maximal adsorption capacities of the bifunctional composites for Hg(II) varied from 1.63 to 1.94 mmol g −1 at 25°C according to the Langmuir model, and the kinetics indicate that the ratedetermining step of the adsorption process may be controlled by chemical reactions. Selectivity experiments declare that the composites tend to adsorb Hg(II) rather than a bunch of other coexisting metal ions and the adsorption rate could still reach 78% after repeating the cycle three times in elution regeneration tests. Although the adsorption capacity is a little lower compared to Au(III), the bifunctional PSQ/CNTs magnetic composite is still anticipated to be an adsorbent with potential practical value in adsorbing low concentration harmful metal ions Hg(II) from waste water.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.