## 3 Results and Discussion
### pH Profile Studies  
Figure 3 shows the adsorption of As(III) and As(V) over the pH range from 2 to 6 at room temperature. The binding of the As(III) and As(V) was almost pH-independent across the studied range. A slight decrease in the binding can be observed in Figure 3 with As(III) and As(V) binding at pH 5. However, even though the As(III) binding does increase again at pH 6, it was still lower than pH 4. Similarly, the binding of the As(V) was lower at pH 6 than at pH 4. Thus, pH 4 was chosen as the optimal binding pH for further reactions. The binding of both As(III) and As(V) has been shown to be pH-dependent when binding to different materials [45; 46; 47; 48]. For example, the binding of As(III) to TiO\({}_{2}\) nanoparticles has shown pH independence from pH 2 through pH 5, and has been observed to increase after pH 5 then decrease from pH 6 to 10, whereas the binding of As(V) to TiO\({}_{2}\) nanoparticle has been shown to increase almost linearly from pH 3 through 6.5 and subsequently decrease from pH 6.5 to pH 10 [45]. [PERSON] et al. investigated the binding of Arsenate from aqueous solution using TiO\({}_{2}\) nanoparticles, which showed the highest binding was observed at a pH of 2 and a linear decrease in binding was observed up to pH 8 where no binding was observed [46], whereas [PERSON] et al. investigated the binding of As(III) and As(V) to high surface area Fe\({}_{3}\)O\({}_{4}\) nanoparticles and showed both ions had high binding at pH 2 [47]. However, the binding decreased with increasing the removal of As(III) using ZnO nanoparticles. After calcining the particles at 500 \({}^{\circ}\)C for 3 h in a limited air supply, the pH study showed a decrease in binding with increasing pH from 2 to 10 [48]. The pH results in the current study are consistent with those reported in the literature for arsenic binding to other materials [30; 49; 50; 51; 52; 53; 54; 55; 56]. In addition, pH independence of the binding of As has been observed with the binding of As(III) and As(V) by Fe\({}_{7}\)S\({}_{8}\) nanoparticles, MnFe\({}_{2}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\), in the pH range of 2-6 [30; 40], whereas Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) have shown pH-independent binding at higher pHs 6 through 10 [41]. The pH-independent behavior of the binding has been shown to be related to the surface charge of the nanoparticles and the arsenic species in solution. In the literature, the point of zero charge for ZnO has been determined to be at pH 9.4, and the isoelectric point has been determined to be at pH 6.4 [57]. Below the PZC pH 9.2, the nanomaterials have a positive charge, while above the PZC, the surface of the nanomaterial will become negatively charged. Thus at a pH below the PZC, the nanoparticles will have a positive charge and attract the arsenic ions. Above the PZE, the nanoparticles will repulse the As ions. Alternatively, the pH-independent binding behavior may be due to the relatively low concentration of As and the high number of active sites on the nanomaterial surface. A high number of surface binding sites and a low concentration of ions in solution would ensure the complete binding of the As(III) and As(V) to the nanomaterial surface and would not show a strong dependence on pH. Furthermore, the lack of surfactant used in the synthesis of the nanoparticles may have generated nanoparticles with a highly active clean surface for As binding. In the present study, a nanomaterial that showed low pH dependency has been synthesized  
Figure 2: SEM of as synthesized ZnO nanoparticles from 30 mM ZnNO\({}_{3}\) solution.  
without the requirement of surfactants or calcination at high temperature. The lack of calcination and surfactant results in the generation of a more cost-effective adsorbent for the removal of As(III) and As(V) from aqueous solution. In addition, the low dependence on pH makes the nanomaterial possibly effective in the treatment of industrial waste as well as drinking water.