## 3 Results and Discussion
### Adsorption Isotherms  
The adsorption of both As(III) and As(V) ions to the ZnO nanoparticles was observed to follow the Langmuir isotherm. The linear form of the Langmuir isotherm equation is given below:  
\[\frac{1}{q_{e}}=\frac{1}{q_{m}}+\frac{1}{K_{a}q_{m}C_{e}} \tag{2}\]  
where q\({}_{e}\) is defined as the removal capacity at any concentration, q\({}_{m}\) is the maximum binding capacity, C\({}_{e}\) is the equilibrium concentration, and K\({}_{a}\) is the Langmuir adsorption constant. Figure 7 shows the Langmuir isotherm plots for the binding of both As(III) and As(V) to the ZnO nanomaterial at 4, 22, and 45 \({}^{\circ}\)C. The isotherm plots had correlation coefficients (R\({}^{2}\)) of 0.99 or better. Table 4 shows the binding capacity of ZnO nanoparticles for the As(III) and As(V) ions at 4, 22, and 45 \({}^{\circ}\)C. Table 4 shows the binding capacity of ZnO nanoparticles for the As(III) and As(V) ions at 4, 22, and 45 \({}^{\circ}\)C. Table 4 shows the binding capacity of ZnO nanoparticles for the As(III) and As(V) ions at 4, 22, and 45 \({}^{\circ}\)C. As can be seen in Table 4, the highest binding capacities for As(III) and As(V) were 5.8 mg/g and 14.68 mg/g, respectively, and were observed at 4 \({}^{\circ}\)C. At the highest tested temperature, 45 \({}^{\circ}\)C, the ZnO showed the lowest binding capacities for both As(III) and As(V) than at 4.4 and 12.1 mg/g. The lower binding capacities are more than likely due to the thermodynamics of the reaction, as discussed below, the binding reaction was exothermic. The lower binding of the As(III) is typical for nanomaterials; in general, As(V) is more readily adsorbed than As(III) [36; 69; 70]. Different studies have shown increased removal of As(III) based on the oxidation of As(III) to As(V) and subsequent adsorption to nanomaterials. Furthermore, it has also been observed that metal oxides generally have a higher capacity to remove As(V) than As(III) [27]. Other metal oxide nanomaterial-based adsorbents have shown high binding capacities for both As(III) and As(V). For example, Mn\({}_{3}\)O\({}_{4}\) has shown binding capacities in the range of 10-11.5 mg/g [50]. [PERSON] et al. showed MnFe\({}_{2}\)O\({}_{4}\) nanomaterials had binding capacities of 0.7 and 2.1 mg/g for As(III) and As(V) at pH 4, respectively. [30]. [PERSON] et al. showed that As(III) to Fe\({}_{2}\)O\({}_{3}\), and Fe\({}_{3}\)O\({}_{4}\) had binding capacities of 1.3 and 8.5 mg/g within 1 h of contact at pH above 7. [PERSON] et al. further showed As(V) had binding capacities of 4.6 and 6.7 mg/g to Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) nanomaterials, respectively [41]. TiO\({}_{2}\) has shown binding capacities close to 30 mg/g, which has been shown to be dependent on the TiO\({}_{2}\) phase present [56]. ZnO nanoparticles coated with acetate have shown a binding capacity of 25.9 mg/g [71], while ZnO embedded in aluminosilicate has been shown a binding capacity of 123.94 mg/g [72]. ZnO nanorods have been found to have a binding capacity up to 52.63 mg/g at pH 7 [73]. The variation in binding capacities may be due to the specific isotherm model used to determine the binding capacities. For example, [PERSON] et al. used a Langmuir isotherm on the ZnO nanorods, and the binding capacity changed dramatically from 52.62 mg/g with the Temkin isotherm model to 0.00165 mg/g or 1.65 mg/g The binding capacity results of the current study fall within the range of binding capacities reported in the literature for metal oxide-based nanomaterials. In fact, the binding capacities for the current study using ZnO nanomaterials are on the higher end for metal oxide nanoparticles and especially for non-surface modified nanoparticles. The high binding capacities indicate the ZnO NPs would result in a material that potentially has long lifetime. A long lifetime, due to the binding capacity in conjunction with the cost-effective synthesis could reduce the cost of water treatment.