## 3 Results and Discussion
### Evaluation of the As(V) Removal Capacity of the Ion-Exchange Fabric  
The removal capacity of As(V) by the ion-exchange fabric (IEF) was evaluated. The influence of the solution pH on the As(V) removal capacity was tested, and the removal of As(V) by the IEF was affected by the solution pH (Figure 4a). The removal capacity of As(V) was less than 1 mg/g at solution pH 2 and increased sharply and reached a plateau at solution pH 4. Next, the As(V) removal capacity decreased gradually with increasing solution pH from 4 to 12 and dropped to 0.44 mg/g at solution pH 12. The pH dependence of As(V) removal by the IEF is attributed to the number of protonated sites available for anion exchange. At low solution pH, the amine groups on the fabric surface protonate to form NH\({}_{4}\)\({}^{+}\), and the surface is subsequently more positively charged [15; 16]. However, due to the speciation of As(V), un-dissociated H\({}_{3}\)AsO\({}_{4}\) was the dominant form at solution pH 2 and the removal capacity by the IEF became very low [13]. The optimum adsorption capacity at solution pH 4 was due to the large number of protonated sites on the fabric surface and the anionic form of As(V) (H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\)). As the pH increased, As(V) removal decreased due to a decrease in protonated sites. At solution pH 12, almost none of the amine groups were protonated, thus the removal capacity of As(V) by the IEF became negligible. The effect of contact time on the As(V) removal capacity was evaluated (Figure 4b). The As(V) adsorption capacity of the IEF rapidly increased and reached equilibrium within 120 min, with the rapid adsorption characteristic of As(V) being due to the easily accessible sites on the fabric surface [20]. A further increase in the contact time only had a minor effect on the As(V) adsorption capacity. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**pH**} \\  & **8** & **6** & **4** & **2** \\ \hline As\({}_{2}\)O\({}_{5}\)(s) & \(-\)27.57 & \(-\)21.40 & \(-\)17.31 & \(-\)14.22 \\ Bianchite & \(-\)4.85 & \(-\)4.77 & \(-\)4.77 & \(-\)5.08 \\ Goslarite & \(-\)4.60 & \(-\)4.53 & \(-\)4.53 & \(-\)4.83 \\ Mirabilite & \(-\)8.57 & \(-\)8.57 & \(-\)8.57 & \(-\)8.87 \\ Thenardite & \(-\)10.01 & \(-\)10.00 & \(-\)10.00 & \(-\)10.30 \\ Zincite & 1.43 & \(-\)2.50 & \(-\)6.50 & \(-\)10.53 \\ Zincosite & \(-\)10.54 & \(-\)10.47 & \(-\)10.47 & \(-\)10.77 \\ Zn metal & \(-\)36.53 & \(-\)36.05 & \(-\)38.87 & na.\({}^{\dagger}\) \\ Zn(OH)\({}_{2}\) (am) & 0.18 & \(-\)3.74 & \(-\)7.74 & \(-\)11.78 \\ Zn(OH)\({}_{2}\) (beta) & 0.90 & \(-\)3.02 & \(-\)7.02 & \(-\)11.06 \\ Zn(OH)\({}_{2}\) (delta) & 0.81 & \(-\)3.11 & \(-\)7.11 & \(-\)11.15 \\ Zn(OH)\({}_{2}\) (epsilon) & 1.12 & \(-\)2.80 & \(-\)6.80 & \(-\)10.84 \\ Zn(OH)\({}_{2}\) (gamma) & 0.92 & \(-\)3.00 & \(-\)7.00 & \(-\)11.04 \\ Zn\({}_{2}\)(OH)\({}_{2}\)SO\({}_{4}\)(s) & \(-\)1.46 & \(-\)5.31 & \(-\)9.30 & \(-\)13.64 \\ Zn\({}_{3}\)(AsO\({}_{4}\))\({}_{2}\)\({}^{-}\)\({}_{2}\)\({}_{2}\)SiO(s) & 3.21 & \(-\)2.39 & \(-\)10.30 & \(-\)19.32 \\ Zn\({}_{3}\)(OSO\({}_{4}\))\({}_{2}\)(s) & \(-\)19.48 & \(-\)23.26 & \(-\)27.26 & \(-\)31.90 \\ Zn\({}_{4}\)(OH)\({}_{6}\)SO\({}_{4}\)(s) & 2.96 & \(-\)8.74 & \(-\)20.74 & \(-\)33.15 \\ ZnSO\({}_{4}\)\({}_{1}\)H\({}_{2}\)O(s) & \(-\)5.98 & \(-\)5.90 & \(-\)5.90 & \(-\)6.20 \\ \hline \hline \end{tabular} \({}^{\dagger}\) n.a. = not available.  
\end{table} 
Table 1: Saturation index of As and Zn calculated via Visual MINTEQ 3.1 according to solution pH. The effect of initial As(V) concentration on the removal capacity was also examined (Figure 4c). The amount of As(V) adsorbed on the IEF increased rapidly to 12.22 mg/g with an initial As(V) concentration of 200 mg/L, and the As(V) uptake further increased to 13.29 mg/g with an initial As(V) concentration of 500 mg/L. This increment is due to the higher mass-transfer driving force from the solution to the ion-exchange sites of the fabric at the higher initial concentration [20].