Application of Dithiocarbamate Chitosan Modiﬁed SBA-15 for Catalytic Reductive Removal of Vanadium(V)

: We have successfully synthesized dithiocarbamate chitosan modiﬁed SBA-15 (CS 2 C@SBA) composites, with promise in vanadium (V(V)) elimination. Among the three composites using different mass ratios of dithiocarbamate chitosan to SBA-15, CS 2 C@SBA − 3, which had the highest CS 2 substitution, showed the best performance on V(V) removal of which the maximum adsorption capacity could achieve 218.00 mg/g at pH 3.0. The adsorption kinetics were best ﬁtted with a pseudo − second order reaction model, suggesting a chemisorption mechanism. Meanwhile, the Langmuir model ﬁtted better with the adsorption isotherm, revealing a monolayer adsorption behavior. Through FTIR and XPS analysis, the functional group − SH was identiﬁed as dominating reduction sites on this composite, which reduced 73.1% of V(V) into V(IV) and V(III). The functional group − NH − was the main adsorption site for vanadium species. This reaction followed a catalytic reduction coupled adsorption mechanism reducing most of V(V) into less toxic vanadium species. Furthermore, CS 2 C@SBA − 3 showed great selectivity towards V(V) in the presence of various co − existing ions in synthetic wastewater and real water samples. Moreover, CS 2 C@SBA − 3 could retain a removal efﬁciency over 90% after ﬁve adsorption − desorption cycles. Based on the aforementioned results, we can conclude that CS 2 C@SBA − 3 has great potential to be applied in efﬁcient remediation of vanadium water − pollution.


Introduction
Vanadium pollution has become a worldwide problem with the extending process of industrialization, such as steelmaking, battery manufacturing, and medicinal processing [1][2][3]. Vanadium is a potential toxic contaminant that may cause pulmonary tumors and oxidative cell damage if it is over ingested [4]. There exist various valence states of vanadium including +2, +3, +4 and +5 [5,6]. The higher the valence state of vanadium is, the more toxicity and stability it has [7]. Over the past, a variety of methods including ion exchange, surface complexation and electrostatic interaction have been applied for the elimination of V(V). Anion exchange resins such as D201, D314 and 711 were employed for the removal of V(V), achieving a removal efficiency up to 99.0% [8]. Lactococcus raffinolactis was proved to have promising prospects in the elimination of V(V), offering microbial resources for the bioremediation of a V(V)−polluted environment [9]. Among aforementioned traditional technologies for the remediation of V(V), adsorption was the most widely used, due to its high removal efficiency and low cost. For example, surface modified pine bark was investigated with the maximum adsorption capacity of 35.00 mg/g with quaternary nitrogen groups on its surface as the dominant adsorption sites [10]. Kong  The physical properties of SBA-15 and CS 2 C@SBA−3 are shown in Table S1 and Figure S1. As the successful hybrid of CS 2 −chitosan, the Brunauer-Emmett-Teller (BET) surface areas of SBA-15 and CS 2 C@SBA−1, CS 2 C@SBA−2 and CS 2 C@SBA−3 were calculated as 632. 6, 121.4, 223.4 and 309.2 m 2 /g (Table S1), respectively. The BET surface area increased with the increasing mass ratio of CS 2 −chitosan to SBA-15. Furthermore, the pore width increased from 2.04 to 2.28 nm while the pore volume increased from 0.1637 to 0.1762 cm 3 /g, respectively (Table S1 and Figure S1). CS 2 C@SBA was mainly micro−mesopore structure (>2 nm). The X-ray diffraction (XRD) patterns revealed the ordered mesoporous structure of SBA-15 and CS 2 C@SBA−3 ( Figure S2). The peak at 23 • was assigned for the amorphous SiO 2. Typical peaks at 0.8 • (100), 1.6 • (110) and 1.8 • (200) indicated the mesopore structure of SBA-15 [31][32][33][34]. Furthermore, SEM images (Figure 2A-F) clearly showed the surface morphology of SBA-15, CS 2 −chitosan and CS 2 C@SBA−3 [35,36]. In the SEM image of CS 2 C@SBA−3, it can be found that SBA-15 was uniformly wrapped by CS 2 −chitosan ( Figure 2E,F). To sum up, all the analyses above from the different characterizations proved the coating of CS 2 −chitosan on the surface of SBA-15 [37]. TEM images ( Figure S3) of SBA-15 exhibited cylindrical silicate layer morphologies [21,35]. After reaction with CS 2 −chitosan, TEM images of CS 2 C@SBA−3 ( Figure S3) exhibited a core−shell structure which indicated the successful hybrid of CS 2 −chitosan and SBA-15 [21]. Besides, after the reaction with V(V), CS 2 −chitosan was still coated on the surface of SBA-15 ( Figure S4). The phenomena suggested a robust nature of CS 2 C@SBA−3 in the adsorption coupled catalytic reduction processes.  (Table 1). All the signals of O, N, C, S and Si were found. XPS spectra of CS2C@SBA−3 confirmed the existence of sulfur signals at the binding energies of 161.7 and 162.8 eV, which indicated the presence of −C=S and −CS− functional groups, respectively ( Figure 1D) [30]. According to these characterization results, we can conclude that the CS2chitosan was successfully fabricated on the surface of SBA−15.  The physical properties of SBA−15 and CS2C@SBA−3 are shown in Table S1 and Fig  ure S1. As the successful hybrid of CS2−chitosan, the Brunauer-Emmett-Teller (BET) sur face areas of SBA−15 and CS2C@SBA−1, CS2C@SBA−2 and CS2C@SBA−3 were calculated as 632.6, 121.4, 223.4 and 309.2 m 2 /g (Table S1), respectively. The BET surface area in creased with the increasing mass ratio of CS2−chitosan to SBA−15. Furthermore, the pore width increased from 2.04 to 2.28 nm while the pore volume increased from 0.1637 to 0.1762 cm 3 /g, respectively (Table S1 and Figure S1). CS2C@SBA was mainly micro−meso pore structure (>2 nm). The X−ray diffraction (XRD) patterns revealed the ordered meso porous structure of SBA−15 and CS2C@SBA−3 ( Figure S2). The peak at 23° was assigned for the amorphous SiO2. Typical peaks at 0.8° (100), 1.6° (110) and 1.8° (200) indicated the mesopore structure of SBA−15 [31][32][33][34]. Furthermore, SEM images ( Figure 2A-F) clearly showed the surface morphology of SBA−15, CS2−chitosan and CS2C@SBA−3 [35,36]. In the SEM image of CS2C@SBA−3, it can be found that SBA−15 was uniformly wrapped by CS2−chitosan ( Figure 2E,F). To sum up, all the analyses above from the different charac terizations proved the coating of CS2−chitosan on the surface of SBA−15 [37]. TEM im ages ( Figure S3) of SBA−15 exhibited cylindrical silicate layer morphologies [21,35]. Afte reaction with CS2−chitosan, TEM images of CS2C@SBA−3 ( Figure S3) exhibited a core−shel structure which indicated the successful hybrid of CS2−chitosan and SBA−15 [21]. Besides after the reaction with V(V), CS2−chitosan was still coated on the surface of SBA−15 ( Figure  S4). The phenomena suggested a robust nature of CS2C@SBA−3 in the adsorption coupled catalytic reduction processes.

Effects of pH
The solution pH is one of the most important influence factors in the removal of heavy metal since it can change not only the surface charge properties of adsorbents but also the distribution of metal ions species [38,39]. As is acknowledged, the distribution of V(V) oxyanions exists in different forms at different solution pH values [40,41]. Taking those facts into consideration, we decided to determine the removal of V(V) by CS 2 C@SBA−1, CS 2 C@SBA−2 and CS 2 C@SBA−3 at a specific pH ranging from 2.0 to 10.0. Figure 3 showed the effects of pH value on the removal of V(V). CS 2 C@SBA−1, CS 2 C@SBA−2 and CS 2 C@SBA−3 achieved V(V) removal of 62.2%, 72.1% and 98.2% at pH 3.0, respectively ( Figure 3). CS 2 C@SBA−3 demonstrated the highest efficiency in the removal of V(V). Furthermore, the removal of V(V) can be strongly influenced by the mass ratio of SBA-15/CS 2 −chitosan. With the increase of SBA-15/CS 2 −chitosan mass ratio, the removal efficiency was remarkably enhanced since there were more functional groups grafted on the surface of the composites, which acted as reactive sites to remove V(V). However, the removal efficiency of V(V) by CS 2 C@SBA−3 decreased from 98.2% to 4.6% with the increase of solution pH from 2.0 to 10.0, while it decreased from 62.2% and 72.1% to 4.5% and 4.6% for CS 2 C@SBA−1 and CS 2 C@SBA−2, respectively ( Figure 3). Hence, the optimum pH value for the removal of V(V) was determined as 3.0. The pH zpc of CS 2 C@SBA−3 is 6.9 ( Figure 1B) which suggested a negative surface charge at pH over 6.9 and a positive surface charge at pH under 6.9. Vanadium anions were reported to mainly exist as H 2 V 10 O 28 4− , H 2 VO 4− and H 3 V 10 O 28 3− at pH 3.0 [2]. Therefore, positively charged CS 2 C@SBA−3 should have an electrostatic attraction with V(V) oxyanions at solution pH 3.0, favoring the adsorption of V(V) oxyanions. The analysis above suggests a strong co−relationship between pH value and the removal of V(V). An acidic environment favored the immobilization of V(V) by CS 2 C@SBA−3.

Effects of pH
The solution pH is one of the most important influence factors in the rem heavy metal since it can change not only the surface charge properties of adsorbe also the distribution of metal ions species [38,39]. As is acknowledged, the distribu V(V) oxyanions exists in different forms at different solution pH values [40,41]. those facts into consideration, we decided to determine the removal of V CS2C@SBA−1, CS2C@SBA−2 and CS2C@SBA−3 at a specific pH ranging from 2.0 to Figure 3 showed the effects of pH value on the removal of V(V). CS2C@ CS2C@SBA−2 and CS2C@SBA−3 achieved V(V) removal of 62.2%, 72.1% and 98.2% 3.0, respectively ( Figure 3). CS2C@SBA−3 demonstrated the highest efficiency in moval of V(V). Furthermore, the removal of V(V) can be strongly influenced by th ratio of SBA−15/CS2−chitosan. With the increase of SBA−15/CS2−chitosan mass ra removal efficiency was remarkably enhanced since there were more functional grafted on the surface of the composites, which acted as reactive sites to remove However, the removal efficiency of V(V) by CS2C@SBA−3 decreased from 98.2% t with the increase of solution pH from 2.0 to 10.0, while it decreased from 62.2% and to 4.5% and 4.6% for CS2C@SBA−1 and CS2C@SBA−2, respectively ( Figure 3). Hen optimum pH value for the removal of V(V) was determined as 3.0. The pH CS2C@SBA−3 is 6.9 ( Figure 1B   To determine the effects of the dosage of CS 2 C@SBA−3, a series of different dosages were examined in a 100 ppm V(V) solution. As the dosage increased from 0.1 to 2.0 g/L, the removal efficiency of V(V) increased from 31.2% to 99.7% ( Figure 4). This might be attributed to the increase of the density of reactive sites on the surface. However, the removal efficiency of V(V) increased only slightly with increases in dosage from 1.0 to 1.5 and 2.0 g/L. The phenomena suggested that the removal of V(V) attained an equilibrium with the dosage of CS 2 C@SBA−3 higher than 1.0 g/L. Thus, the adsorption capacity decreased from 312.44 to 49.85 mg/g along with the increase of the dosage of CS 2 C@SBA−3 ( Figure 4). The moles ratio of V(V) to CS 2 C@SBA-15 is inadequate at higher adsorbent dosages, indicating that the reactive sites on the surface of CS 2 C@SBA−3 did not reach saturation point in those cases.
Catalysts 2022, 12,1469 with the dosage of CS2C@SBA−3 higher than 1.0 g/L. Thus, the adsorption cap creased from 312.44 to 49.85 mg/g along with the increase of the dosage of CS2C ( Figure 4). The moles ratio of V(V) to CS2C@SBA−15 is inadequate at higher a dosages, indicating that the reactive sites on the surface of CS2C@SBA−3 did n saturation point in those cases.  Table 2 show the kinetics of the elimination of V(V). The reaction time were investigated in a 100 ppm V(V) solution. Within the initial 5 m of V(V) was removed by CS2C@SBA−3 ( Figure 5A). The elimination rate was fa first 10 min. These results might be attributed to the abundance of reactive site surface of CS2C@SBA−3 which were quickly enriched with V(V). After 360 min of an equilibrium was reached, with 98.2% removal of V(V) due to the saturation of sites ( Figure 5A). Based on the data, the maximum adsorption capacity was calc 98.24 mg/g. The kinetics data were carefully fitted by both pseudo−first−order m pseudo−second−order models. The regression coefficient (R 2 ) values of pse ond−order model were higher (0.9999) compared with pseudo−first−order mo 0.9851) and Elovich model (R 2 = 0.8788) ( Figure 5B-D). Hence, the pseudo−secon kinetic model fitted better than the pseudo−first−order kinetic model and Elovic did. The phenomena indicated that the elimination of V(V) by CS2C@SBA−3 was p directed by the chemisorption [37,40,42,43].   Table 2 show the kinetics of the elimination of V(V). The effects of reaction time were investigated in a 100 ppm V(V) solution. Within the initial 5 min, 48.9% of V(V) was removed by CS 2 C@SBA−3 ( Figure 5A). The elimination rate was fast in the first 10 min. These results might be attributed to the abundance of reactive sites on the surface of CS 2 C@SBA−3 which were quickly enriched with V(V). After 360 min of reaction, an equilibrium was reached, with 98.2% removal of V(V) due to the saturation of reactive sites ( Figure 5A). Based on the data, the maximum adsorption capacity was calculated as 98.24 mg/g. The kinetics data were carefully fitted by both pseudo−first−order model and pseudo−second−order models. The regression coefficient (R 2 ) values of pseudo−second−order model were higher (0.9999) compared with pseudo−first−order model (R 2 = 0.9851) and Elovich model (R 2 = 0.8788) ( Figure 5B-D). Hence, the pseudo−second−order kinetic model fitted better than the pseudo−first−order kinetic model and Elovich model did. The phenomena indicated that the elimination of V(V) by CS 2 C@SBA−3 was primarily directed by the chemisorption [37,40,42,43]. Table 2. The parameters of kinetic and thermodynamics models for the adsorption of V(V) on CS 2 C@SBA−3.

Models Parameters
Pseudo−first−order model

Adsorption Isotherm
To identify the adsorption interaction and the type of adsorption, adsorption isotherms were investigated at a series of initial V(V) concentrations (10-500 ppm). Figure 6 and Table 2 show the isotherm results of V(V) removal. The V(V) removal efficiency decreased from 98. 6% to 43.6% while the adsorption capacity increased from 8.64 to 218.00

Adsorption Isotherm
To identify the adsorption interaction and the type of adsorption, adsorption isotherms were investigated at a series of initial V(V) concentrations (10-500 ppm). Figure 6 and Table 2 show the isotherm results of V(V) removal. The V(V) removal efficiency decreased from 98. 6% to 43.6% while the adsorption capacity increased from 8.64 to 218.00 mg/g along with the increase of V(V) concentration from 10 to 500 ppm ( Figure 6A). Two different isotherm models including Langmuir and Freundlich ones were used to fit the data [44,45]. From the parameters listed in Table 2, the Langmuir model fitted better than Freundlich models, since their correlation coefficient (R 2 ) were 0.9982 and 0.9218, respectively ( Figure 6B and Table 2). From the calculation results of the Langmuir model, Q m of CS 2 C@SBA−3 is 221.19 mg/g which is appropriately equal to the measured experimental value (218.00 mg/g) ( Table 2). The phenomena suggested that the adsorption of V(V) by CS 2 C@SBA−3 was based on the assumption of monolayer distribution of V(V) on the surface of CS 2 C@SBA−3 and no lateral interaction between V(V) and CS 2 C@SBA−3 [46,47]. As long as a molecule occupies the reactive site on CS 2 C@SBA−3, there is no adsorption that can take place at this site any more. value (218.00 mg/g) ( Table 2). The phenomena suggested that the adsorption of V(V) by CS2C@SBA−3 was based on the assumption of monolayer distribution of V(V) on the surface of CS2C@SBA−3 and no lateral interaction between V(V) and CS2C@SBA−3 [46,47]. As long as a molecule occupies the reactive site on CS2C@SBA−3, there is no adsorption that can take place at this site any more.

Effects of Competing Anions
In real wastewater or groundwater, there are abundant co−existing anions which may have negative impacts on the removal of V (V). The co−existing anions may compete with V(V) for the reactive sites on the surface of composites. Thus, to simulate the real groundwater, the effects of co−existing anions (60 ppm PO4 3− , 600 ppm SO4 2− , 400 ppm NO 3− , 400 ppm HCO 3− ) and 60 ppm Cu 2+ and ionic strength (0.01, 0.1, and 0.3 M) on the removal of V(V) were studied [11,48]. Figure 7A,B indicate the impacts of various of co−existing anions and ionic strength on the elimination of V(V) by CS2C@SBA−3. CS2C@SBA−3 showed a predominant selectivity for the uptake of V(V). Despite of the presence of the competing anions, over 98.0% of V(V) could be removed from the solution except for two cases in the presence of 600 ppm SO4 2− and 400 ppm HCO 3− . The removal efficiency rates of V(V) were 95.9% and 97.2% in the presence of SO4 2− and HCO 3− , respectively ( Figure 7A). This could be attributed to slight competition on reactive sites between V(V) and the two anions. Furthermore, the ionic strength demonstrated only slight impacts on the elimination of V(V) by CS2C@SBA−3 because V(V) removal efficiency could achieve over 98.0% in a wide range of NaCl concentrations from 10 to 300 ppm ( Figure 7B). All the aforementioned results suggest a promising performance of CS2C@SBA−3 in the presence of common anions in wastewater or groundwater.

Effects of Competing Anions
In real wastewater or groundwater, there are abundant co−existing anions which may have negative impacts on the removal of V (V). The co−existing anions may compete with V(V) for the reactive sites on the surface of composites. Thus, to simulate the real groundwater, the effects of co−existing anions (60 ppm PO 4 3− , 600 ppm SO 4 2− , 400 ppm NO 3− , 400 ppm HCO 3− ) and 60 ppm Cu 2+ and ionic strength (0.01, 0.1, and 0.3 M) on the removal of V(V) were studied [11,48]. Figure 7A,B indicate the impacts of various of co−existing anions and ionic strength on the elimination of V(V) by CS 2 C@SBA−3. CS 2 C@SBA−3 showed a predominant selectivity for the uptake of V(V). Despite of the presence of the competing anions, over 98.0% of V(V) could be removed from the solution except for two cases in the presence of 600 ppm SO 4 2− and 400 ppm HCO 3− . The removal efficiency rates of V(V) were 95.9% and 97.2% in the presence of SO 4 2− and HCO 3− , respectively ( Figure 7A). This could be attributed to slight competition on reactive sites between V(V) and the two anions. Furthermore, the ionic strength demonstrated only slight impacts on the elimination of V(V) by CS 2 C@SBA−3 because V(V) removal efficiency could achieve over 98.0% in a wide range of NaCl concentrations from 10 to 300 ppm ( Figure 7B). All the aforementioned results suggest a promising performance of CS 2 C@SBA−3 in the presence of common anions in wastewater or groundwater.
CS2C@SBA−3 is 221.19 mg/g which is appropriately equal to the measured experimental value (218.00 mg/g) ( Table 2). The phenomena suggested that the adsorption of V(V) by CS2C@SBA−3 was based on the assumption of monolayer distribution of V(V) on the surface of CS2C@SBA−3 and no lateral interaction between V(V) and CS2C@SBA−3 [46,47]. As long as a molecule occupies the reactive site on CS2C@SBA−3, there is no adsorption that can take place at this site any more.

Effects of Competing Anions
In real wastewater or groundwater, there are abundant co−existing anions which may have negative impacts on the removal of V (V). The co−existing anions may compete with V(V) for the reactive sites on the surface of composites. Thus, to simulate the real groundwater, the effects of co−existing anions (60 ppm PO4 3− , 600 ppm SO4 2− , 400 ppm NO 3− , 400 ppm HCO 3− ) and 60 ppm Cu 2+ and ionic strength (0.01, 0.1, and 0.3 M) on the removal of V(V) were studied [11,48]. Figure 7A,B indicate the impacts of various of co−existing anions and ionic strength on the elimination of V(V) by CS2C@SBA−3. CS2C@SBA−3 showed a predominant selectivity for the uptake of V(V). Despite of the presence of the competing anions, over 98.0% of V(V) could be removed from the solution except for two cases in the presence of 600 ppm SO4 2− and 400 ppm HCO 3− . The removal efficiency rates of V(V) were 95.9% and 97.2% in the presence of SO4 2− and HCO 3− , respectively ( Figure 7A). This could be attributed to slight competition on reactive sites between V(V) and the two anions. Furthermore, the ionic strength demonstrated only slight impacts on the elimination of V(V) by CS2C@SBA−3 because V(V) removal efficiency could achieve over 98.0% in a wide range of NaCl concentrations from 10 to 300 ppm ( Figure 7B). All the aforementioned results suggest a promising performance of CS2C@SBA−3 in the presence of common anions in wastewater or groundwater.

The Elimination of V(V) in Real Waters
In order to evaluate the practical potential of CS 2 C@SBA−3, the elimination of V(V) was tested in three types of real water samples including lake, pond and tap waters. At pH 3.0, the removal efficiency of V(V) from lake, pond and tap waters was 99.5%, 99.8% and 99.3% respectively ( Figure 7C). The results indicated that CS 2 C@SBA−3 was highly efficient in real water bodies. However, V(V) removal efficiency by CS 2 C@SBA−3 decreased to 77.5%, 75.2% and 75.4% in lake, pond and tap waters, respectively, at a local pH of 8.0 ( Figure 7C). Hence, we can conclude that pH value has a significant impact on the removal of V(V), which was consistent with our previous results in Section 2.2.1. The dramatic decrease of V(V) removal may be attributed to the electrical repulsion between negatively charged CS 2 C@SBA−3 and V(V) oxyanions at pH 8.0. The results suggest that CS 2 C@SBA−3 can be successfully employed for the remediation of V(V) in real wastewater with appropriate adjustment of pH value.  Figure 8A-E and Figure S4 show the structure and morphology of CS 2 C@SBA−3 after the reaction. Comparing with Figure 2E,F, SBA-15 was still coated with CS 2 −chitosan uniformly and there was nearly no obvious change on the morphology after treatment with 100 ppm V(V) for 24 h. Table 3, Figures S5 and S6 show the results of EDS elemental mapping of CS 2 C@SBA−3 before and after the reaction. The results are consistent with the results of elemental analysis which can be considered as solid evidence supporting the successful complexation of the composites (Tables 1 and 3). Before the reaction, no signal of V was detected ( Figure 8F). After treatment with large amounts of vanadium ions, CS 2 C@SBA-15 was enriched with V which accounted for 12.2 wt% of the composite ( Figure 8G and Table 3). The results once again indicated that large amounts of V(V) were adsorbed on the surface of CS 2 C@SBA−3.

The Elimination of V(V) in Real Waters
In order to evaluate the practical potential of CS2C@SBA−3, the elimination of V(V) was tested in three types of real water samples including lake, pond and tap waters. At pH 3.0, the removal efficiency of V(V) from lake, pond and tap waters was 99.5%, 99.8% and 99.3% respectively ( Figure 7C). The results indicated that CS2C@SBA−3 was highly efficient in real water bodies. However, V(V) removal efficiency by CS2C@SBA−3 decreased to 77.5%, 75.2% and 75.4% in lake, pond and tap waters, respectively, at a local pH of 8.0 ( Figure 7C). Hence, we can conclude that pH value has a significant impact on the removal of V(V), which was consistent with our previous results in Section 2.2.1. The dramatic decrease of V(V) removal may be attributed to the electrical repulsion between negatively charged CS2C@SBA−3 and V(V) oxyanions at pH 8.0. The results suggest that CS2C@SBA−3 can be successfully employed for the remediation of V(V) in real wastewater with appropriate adjustment of pH value.

SEM−EDS analysis was conducted to investigate the morphology and elemental composition of CS2C@SBA−3 before and after the remediation of V(V). Figures 8A-E and S4
show the structure and morphology of CS2C@SBA−3 after the reaction. Comparing with Figure 2E,F, SBA−15 was still coated with CS2−chitosan uniformly and there was nearly no obvious change on the morphology after treatment with 100 ppm V(V) for 24 h. Table  3, Figures S5 and S6 show the results of EDS elemental mapping of CS2C@SBA−3 before and after the reaction. The results are consistent with the results of elemental analysis which can be considered as solid evidence supporting the successful complexation of the composites (Tables 1 and 3). Before the reaction, no signal of V was detected ( Figure 8F). After treatment with large amounts of vanadium ions, CS2C@SBA−15 was enriched with V which accounted for 12.2 wt% of the composite ( Figure 8G and Table 3). The results once again indicated that large amounts of V(V) were adsorbed on the surface of CS2C@SBA−3.   FTIR and XPS measurement was further conducted to uncover the deeper mechanisms at work. Figure 9A-F reveals the results of FTIR spectra and XPS spectra of CS 2 C@SBA−3 before and after the reaction with V(V). The bands stretching at 3454, 2924, 2854, 1632 and 1465 cm −1 represents the presence of −OH/−NH 2 /−SH, −CH 3 , −CH 2 , −CNH, and −C=S, respectively, on CS 2 C@SBA−3 after the reaction ( Figure 9A) [49][50][51]. Figure 9B, Figures S7 and S8 show the XPS survey spectra of CS 2 C@SBA−3 before and after the reaction. Comparing these two XPS results, there was a distinct signal of V 2p at 516.8 eV which could prove the adsorption of V(V) on the surface of CS 2 C@SBA−3 ( Figure 9B) [39]. To illustrate the mechanism of the reduction of V(V) by CS 2 C@SBA−3, XPS spectra of V 2p, V 2p 1/2 and V 2p 2/3 are provided in Figure 9C, D and E, respectively. There were three peaks at binding energies of 524.0, 516.8, and 513.3 eV, which occupied 26.9%, 66.9% and 6.6% of peak area, respectively ( Figure 9C). The three peaks in XPS spectra of V 2p were characteristic of V 2 O 5 , V(IV), and V(III) respectively [52,53]. The results indicated that 73.1% of V(V) was reduced to V(IV) and V(III) on the surface of CS 2 C@SBA−3 ( Figure 9C-E).  FTIR and XPS measurement was further conducted to uncover the deeper mechanisms at work. Figure 9A-F reveals the results of FTIR spectra and XPS spectra of CS2C@SBA−3 before and after the reaction with V(V). The bands stretching at 3454, 2924, 2854, 1632 and 1465 cm −1 represents the presence of −OH/−NH2/−SH, −CH3, −CH2, −CNH, and −C=S, respectively, on CS2C@SBA−3 after the reaction ( Figure 9A) [49][50][51]. Figures 9B,  S7 and S8 show the XPS survey spectra of CS2C@SBA−3 before and after the reaction. Comparing these two XPS results, there was a distinct signal of V 2p at 516.8 eV which could prove the adsorption of V(V) on the surface of CS2C@SBA−3 ( Figure 9B) [39]. To illustrate the mechanism of the reduction of V(V) by CS2C@SBA−3, XPS spectra of V 2p, V 2p1/2 and V 2p2/3 are provided in Figure 9C, 9D and 9E, respectively. There were three peaks at binding energies of 524.0, 516.8, and 513.3 eV, which occupied 26.9%, 66.9% and 6.6% of peak area, respectively ( Figure 9C). The three peaks in XPS spectra of V 2p were characteristic of V2O5, V(IV), and V(III) respectively [52,53]. The results indicated that 73.1% of V(V) was reduced to V(IV) and V(III) on the surface of CS2C@SBA−3 ( Figure 9C-E). The successful synthesis of CS2C@SBA−3 introduced organic functional groups including −SH, −OH, −NH2 to the composite which can enable the adsorption coupled reduction of V(V). Figures 1D and 9F demonstrate the results of the XPS spectra of S 2p before and after the elimination of V(V), respectively. Before the reaction with V(V), S 2p  Figure 9F) [21]. The weak peak at 540 cm −1 in FTIR spectra also confirmed the formation of disulfide bonds ( Figure 9A) [49,54]. The results suggested that thiol functional groups on the surface of CS 2 C@SBA−3 donated electrons to V(V) for its reduction. Meanwhile, a new peak appeared at 162.2 eV (38.5%) accounting for the protonated −C=SH + or −SH 2 + due to the doping of H + on dithiocarbamate ( Figure 9F). The formation of protonated sulphur species −C=SH + or −SH 2 + would foster the electrostatic attraction between the residual V(V) anions and CS 2 C@SBA−3, facilitating the nearly total elimination of V(V).
To identify other factors which might support the elimination of V(V), N 1s and O 1s XPS spectra were de−convoluted. Figure 10A-D show the XPS spectra of N 1s and O 1s before and after the elimination of V(V). Before the reaction with V(V), N 1s XPS peaks appearing at binding energies of 402.4, 400.2 and 399.3 eV could be associated to N−SiO 2 , −NH 2 and −NH−, respectively ( Figure 10A) [55]. After the reaction with V(V), the corresponding binding energies of N 1s peaks changed to 401.5, 399.4 and 398.7 eV, respectively ( Figure 10B). The relative amounts of N−SiO 2 and −NH 2 increased from 4.4% and 15.0% to 41.5% and 55.3%, respectively. In contrast to the two functional groups, the relative amount of −NH− dramatically decreased from 80.6% to 3.2% after the elimination of V(V). The results suggest that large amounts of V(V) occupied −NH−. We could conclude that −NH− constituted the dominant adsorption site for V(V) among the nitrogen functional groups. Furthermore, O 1s XPS spectra located at binding energies of 533.4, 532.6 and 531.8 eV corresponded to C−O−C, −CH 2 OH and −C−O−Si, respectively ( Figure 10C,D) [56].
The new peak at 529.8 eV indicated the presence of V 2 O 5, as illustrated in Figure 9D [53]. After the elimination of V(V), the relative density of −OH decreased from 57.6% to 51.5%. The decrease of the density of -OH was most likely attributable to the formation of hydrogen bonds between the V species and hydroxyl groups. As discussed above, the reaction between the CS 2 C@SBA−3 and V(V) species is a catalytic reduction process in which most of V(V) (73.1%) is reduced into V(IV) and V(III) species which are less toxic. A small amount of V(V) was transformed into V(V) oxide, V 2 O 5 . Meanwhile, the abundance of functional groups −NH− enhanced the adsorption between vanadium species and CS 2 C@SBA−3. The functional group −SH is the dominant reactive site for reducing V(V) and was oxidized into disulfide bonds (−S−S−) in this catalytic reduction process.
XPS peaks at 163.1 and 161.8 eV could be attributed to the sulphur species of CS2−chitosan including −C=S and −CS− respectively ( Figure 1D). Their distribution molar percentages were 51.2% and 48.8%. After the reaction with V(V), a new S 2p XPS peak appeared at 163.2 eV (46.3%) which was attributed to the oxidation of −SH to disulfide bonds (−S−S−) ( Figure 9F) [21]. The weak peak at 540 cm −1 in FTIR spectra also confirmed the formation of disulfide bonds ( Figure 9A) [49,54]. The results suggested that thiol functional groups on the surface of CS2C@SBA−3 donated electrons to V(V) for its reduction. Meanwhile, a new peak appeared at 162.2 eV (38.5%) accounting for the protonated −C=SH + or −SH2 + due to the doping of H + on dithiocarbamate ( Figure 9F). The formation of protonated sulphur species −C=SH + or −SH2 + would foster the electrostatic attraction between the residual V(V) anions and CS2C@SBA−3, facilitating the nearly total elimination of V(V).
To identify other factors which might support the elimination of V(V), N1s and O1s XPS spectra were de−convoluted. Figure 10A-D show the XPS spectra of N 1s and O 1s before and after the elimination of V(V). Before the reaction with V(V), N 1s XPS peaks appearing at binding energies of 402.4, 400.2 and 399.3 eV could be associated to N−SiO2, −NH2 and −NH−, respectively ( Figure 10A) [55]. After the reaction with V(V), the corresponding binding energies of N 1s peaks changed to 401.5, 399.4 and 398.7 eV, respectively ( Figure 10B). The relative amounts of N−SiO2 and −NH2 increased from 4.4% and 15.0% to 41.5% and 55.3%, respectively. In contrast to the two functional groups, the relative amount of −NH− dramatically decreased from 80.6% to 3.2% after the elimination of V(V). The results suggest that large amounts of V(V) occupied −NH−. We could conclude that −NH− constituted the dominant adsorption site for V(V) among the nitrogen functional groups. Furthermore, O 1s XPS spectra located at binding energies of 533.4, 532.6 and 531.8 eV corresponded to C−O−C, −CH2OH and −C−O−Si, respectively ( Figure 10C,D) [56].
The new peak at 529.8 eV indicated the presence of V2O5, as illustrated in Figure 9D [53]. After the elimination of V(V), the relative density of −OH decreased from 57.6% to 51.5%. The decrease of the density of -OH was most likely attributable to the formation of hydrogen bonds between the V species and hydroxyl groups. As discussed above, the reaction between the CS2C@SBA−3 and V(V) species is a catalytic reduction process in which most of V(V) (73.1%) is reduced into V(IV) and V(III) species which are less toxic. A small amount of V(V) was transformed into V(V) oxide, V2O5. Meanwhile, the abundance of functional groups −NH− enhanced the adsorption between vanadium species and CS2C@SBA−3. The functional group −SH is the dominant reactive site for reducing V(V) and was oxidized into disulfide bonds (−S−S−) in this catalytic reduction process.

Recycling Application
As recyclability is one of the most important factors to judge the practical potential of these findings, a series of recycling application experiments were conducted. Figure 11 shows the removal efficiency of V(V) in five consecutive adsorption-desorption cycles. The removal efficiency of V(V) by CS 2 C@SAB−3 decreased from 98.2% to 90.8% from the first to the fifth cycle of application. The results suggested that a V(V) removal up to 90.0% could be achieved after five adsorption−desorption cycles. Thus, CS 2 C@SAB−3 can be regarded as a promising composite for the elimination of V(V) via catalytic reduction coupled adsorption process with satisfying reusability and good practical potential.

Recycling Application
As recyclability is one of the most important factors to judge the practical p of these findings, a series of recycling application experiments were conducted. F shows the removal efficiency of V(V) in five consecutive adsorption-desorptio The removal efficiency of V(V) by CS2C@SAB−3 decreased from 98.2% to 90.8% first to the fifth cycle of application. The results suggested that a V(V) removal up could be achieved after five adsorption−desorption cycles. Thus, CS2C@SAB−3 ca garded as a promising composite for the elimination of V(V) via catalytic reduct pled adsorption process with satisfying reusability and good practical potential.

Preparation of SBA−15, CS2−Chitosan and CS2C@SBA
SBA−15 and modified dithiocarbamate chitosan (CS2−Chitosan) was syn based on the methods in previous literature [49]. Details can be found in Texts S respectively. CS2C@SBA was synthesized by the following method. Initially CS2−chitosan was dissolved in 100 mL acetic acid solution (1.2% v/v). Then, differ ages of SBA−15 were added (mass ratios SBA−15: CS2−chitosan = 1:2, 1:1 and 2:1). ing to these mass ratios, CS2C@SBA composites are named as CS2C@SBA−1, CS2C and [57,58] CS2C@SBA−3, respectively. Afterwards, the above mixture was stirre min at 313 K, followed by the addition of 0.50 mL glutaraldehyde (50.0% v/v) and stirred for 2 h. After that, the pH of the mixture was adjusted to 9.0 by adding 0.1M and stirred for 30 min. Finally, the solid suspension was filtered, washed sever with distilled water and dried at 333 K overnight.

Preparation of SBA-15, CS 2 −Chitosan and CS 2 C@SBA
SBA-15 and modified dithiocarbamate chitosan (CS 2 -Chitosan) was synthesized based on the methods in previous literature [49]. Details can be found in Texts S1 and S2 respectively. CS 2 C@SBA was synthesized by the following method. Initially, 0.60 g CS 2 −chitosan was dissolved in 100 mL acetic acid solution (1.2% v/v). Then, different dosages of SBA-15 were added (mass ratios SBA-15: CS 2 −chitosan = 1:2, 1:1 and 2:1). According to these mass ratios, CS 2 C@SBA composites are named as CS 2 C@SBA−1, CS 2 C@SBA−2 and [57,58] CS 2 C@SBA−3, respectively. Afterwards, the above mixture was stirred for 30 min at 313 K, followed by the addition of 0.50 mL glutaraldehyde (50.0% v/v) and further stirred for 2 h. After that, the pH of the mixture was adjusted to 9.0 by adding 0.1M NaOH, and stirred for 30 min. Finally, the solid suspension was filtered, washed several times with distilled water and dried at 333 K overnight.

Characterizations
Fourier Transform Infrared Spectra (FTIR) was measured by Nicolet 5700 spectrometer through the KBr pellet method over a wavelength range from 400 to 4000 cm −1 under a 4 cm −1 resolution. The Scanning Electron Microscopy (SEM) was performed on SEM Virion 200 combined with an electron dispersive X-ray (EDS) analyzer. The Transmission Electron Microscopy (TEM) was conducted on JEM−2100. The elemental composition (C, H, N and S wt%) of the materials was analyzed by Vario MICRO cube elemental analyzer (German). The Brunauer-Emmett-Teller (BET) pore properties of materials were measured via Belsorp−minill. Before the N 2 adsorption−desorption measurement, the composite was dried and degassed at 373 K under vacuum for 24 h. The X-ray diffraction (XRD, MiniFlex600, Japan) was conducted with radiation of Cu Kα and scanning rate of 0.2 • /min from 0.6 • to 2.5 • and 5 • /min from 10 • to 80 • . Zeta potentials of the materials were measured by the Zetasizer Nano series at different pH (2.0-9.0). Firstly, 150 mg of composite was added into a plastic conical flask containing 65 mL DI water. Then the flask was mechanically shaken for 24 h. The resultant suspension was collected for zeta potentials determination in 0.01 mM NaCl solution. The pH was adjusted with 0.01 mM NaOH and HCl. The point of zero charge (pH ZPC ) was calculated by the zeta potentials under different pHs. The X-ray photoelectron spectroscopy (XPS) equipped with Al/Mg Kα X-ray source with 30 eV pass energy in 0.5 eV step over an area of 650 mm × 650 mm to the samples was carried out (ESCALAB 250Xi) at vacuum of 10 −9 Torr, and the results were corrected by a reference of the C 1s peak from adventitious carbon at 284.8 eV and fitted with the Shirley method of background subtraction on XPSpeak4.1 Software.

Adsorption Experiments
The stock solution of V(V) (1000 ppm) was prepared by dissolving NaVO 3  In all experiments, the residual concentration of total V was measured by Inductively Coupled Plasma − Atomic Emission Spectroscopy (ICP−AES, Agilent 5110). Adsorption capacity (q t ) (mg/g) and percentage removal (%) were calculated by the following equations [41,59,60].
where C 0 and C t are, respectively, the initial concentration and equilibrium concentration of total V (ppm); V is the solution volume (mL) while m is the adsorbent amount (g). The results of the adsorption kinetic data were fitted by two models, pseudo−first−order, pseudo−second−order and Elovich models, as indicated below [10,61,62].
where q e and q t are, respectively, the adsorbed capacity of V(V) on CS 2 C@SBA−3 at equilibrium time and time t. k 1 (1/min) and k 2 (g/mg·min) are, respectively, the adsorption rate constants for pseudo−first−order and pseudo−second−order models. α (mg·min/g) is the initial rate constant and β (mg/g) is the desorption constant for Elovich model; To determine the adsorption isotherm, Langmuir and Freundlich models are fitted to calculate the adsorption data, as indicated as the following equations [63]: ln q e = ln K f + 1 n ln C e where q e and q m are, respectively, the adsorption capacity at equilibrium and the maximum adsorption capacity of V(V) by CS 2 C@SBA−3. C e (ppm) is the concentration of V(V) at equilibrium. K L (L/mg) and K f ((mg/g)/(mg/L) 1/n ) are, respectively, the Langmuir and Freundlich constants.

Regeneration and Recycle Experiments
To explore the recyclability of the material, the desorption was performed by using 25 mL NaOH solution (0.50 M) as the eluent at 298 K with 1 g/L CS 2 C@SBA−3, and then using 0.01 M Na 2 S 2 O 3 solution to regenerate the oxidized disulfide bonds. The regenerated CS 2 C@SBA−3 was employed to eliminate V(V) again, followed by the regeneration procedures. The experiment was repeated for five cycles. During the recycle experiments, the removal of V(V) was measured.

Effects of Groundwater Impurities
To investigate the effects of co−existing anions (60 ppm PO 4 3− , 600 ppm SO 4 2− , 400 ppm NO 3 − , 400 ppm HCO 3 − ) and 60 ppm Cu 2+ , based on the typical concentration of common ions in ground water, 1 g/L CS 2 C@SBA−3 was tested in 100 ppm V(V) solution at pH 3.0 for 24 h at 298 K. The effects of ionic strength were studied in 100 ppm V(V) solution at pH 3.0 for 24 h at 298 K with varying NaCl concentrations (10, 100 and 300 mM)

V(V) Elimination Assays in Spiked Real Water
The applicability of the adsorbent was also tested in spiked real water samples (10 ppm V(V)) including lake water, pond water and tap water collected from Wuhan at pH 3.0 and at their local pH 8.0. The physical and chemical properties of these samples are listed in Table S2.

Conclusions
In this study, dithiocarbamate chitosan modified SBA-15 (CS 2 C@SBA) was successfully synthesized via a facile and robust procedure. Among the three CS 2 C@SBA composites, CS 2 C@SBA−3 which embraced the highest mass ratio of dithiocarbamate chitosan showed the most efficient performance in the elimination of V(V). At 298 K, a V(V) removal efficiency of 98.2% was achieved by 1 g/L CS 2 C@SBA−3 in a solution containing 100 ppm of V(V). In that case, the adsorption capacity by CS 2 C@SBA−3 was 218.00 mg/g. The adsorption kinetics fitted better with pseudo−second−order model (R 2 = 0.9999), indicating that the elimination of V(V) by CS 2 C@SBA−3 was primarily triggered by chemisorption. The adsorption isotherm followed the Langmuir model (R 2 = 0.9981), revealing the monolayer adsorption on the surface of CS 2 C@SBA−3. FTIR, SEM−EDS and XPS characterization results demonstrated that the functional group −SH played a vital role, acting as a proton−coupled electron donor to reduce 73.5% V(V) into V(IV) and V(III) and transformed residual V(V) into V(V) oxide V 2 O 5 . Via the reduction of high−valent vanadium, its toxicity was reduced significantly. Furthermore, CS 2 C@SBA−3 displayed high selectivity towards V(V) elimination in the presence of various co−existing ions in synthetic and spiked real water samples which could be attributed to the abundant −NH− on its surface.
Hence, we can conclude that CS 2 C@SBA−3 is a promising composite to eliminate V(V) via an adsorption coupled catalytic reduction mechanism, with good practical potentials. CS 2 C@SBA−3 can be applied in the reductive elimination of high−valent heavy metals in efficient remediation of water pollution.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal12111469/s1, Text S1: Details of synthesis procedures of SBA-15; Text S2: Details of synthesis procedures of CS 2 −chitosan; Figure S1: Nitrogen sorption isotherms of SBA-15 and CS 2 C@SBA−3; Figure S2: The XRD patterns of preparation composite; Figure S3: TEM images of SBA-15 and CS 2 C@SBA−3; Figure S4: SEM images of CS 2 C@SBA−3 after reaction with V(V); Figure S5: EDS elemental mapping images of CS 2 C@SBA−3 before reaction with V(V); Figure S6: EDS elemental mapping images of CS 2 C@SBA−3 after reaction with V(V); Figure  S7: The XPS survey spectra of CS 2 C@SBA−3 before reaction; Figure S8: The XPS survey spectra of CS 2 C@SBA−3 after reaction; Table S1: Porous properties of SBA-15 and CS 2 C@SBA−3; Table S2: Physico-chemical properties of water samples used in this study.