# Low-Cost Goethite Nanorods for As (III) and Se (VI) Removal from Water
## 3 Results and Discussion
### Adsorption Experiments of As(III) and Se(VI)
#### 3.2.1 Adsorbent Dose Optimization Study  
Figure 4 shows the dose study results and the corresponding Langmuir and Freundlich adsorption isotherms for both As(III) and Se(VI) uptake. Figure 4a,d depict the adsorbent dose data pertaining to As(III) and Se(VI) removal by the \(\alpha\)-\(FeOOH\) adsorbent, respectively. Of course, the adsorbent doses were plotted against the removal efficiency (%) at \(\sim\)500 ug/L of adsorbate concentrations. As expected, the results revealed a noticeable increase in the removal efficiency as the adsorbent (\(\alpha\)-\(FeOOH\)) dose also increased until a plateau was reached. This stability is presented in a horizontal line as an indication of complete adsorption, whatever the adsorbent dose is. Particularly, the results in the case of As(III) indicate that 0.33 g/L is the optimum dose of \(\alpha\)-\(FeOOH\) adsorbent to remove As(III) ions (\(\sim\)515 ug/L) below the WHO standards (10 ug/L). Results for Se(VI) suggest slightly inferior removal efficiencies and an adsorbent dose of \(\sim\)0.5 g/L of \(\alpha\)-\(FeOOH\) was sufficient to remove Se(VI) ions (502 ug/L) below the WHO standards (40 ug/L). In fact, among all other phases of selenium oxyanions, Se(VI) often shows slightly lower uptake [35]. Of course, complete adsorption of Se(VI) species and reaching the plateau could be launched at 1.5 g/L of \(\alpha\)-\(FeOOH\) adsorbent, as suggested by the results of Figure 4d. For 1 g/L of \(\alpha\)-\(FeOOH\) dose, 99.7% and 97% removal efficiency was observed for As(III) and Se(VI), respectively. This indicates stronger adsorption affinity between \(\alpha\)-\(FeOOH\) adsorbent surfaces for both As(III) and Se(VI) ions in an aqueous medium, and also using higher than the commonly observed concentrations of these polluting ions. The maximum adsorption capacities (\(Q_{max}\)) of \(\alpha\)-\(FeOOH\) adsorbent are determined from the Langmuir model and recorded in Table 1. Particularly, the \(Q_{max}\) is found at 8.253 mg/g for As(III), which is higher than most of the reported values [36; 37; 38]. However, for Se(VI), the maximum adsorption capacity was 4.753 mg/g, which is also higher than different other synthetic and natural adsorbents as tabulated by [PERSON] and [PERSON] [39]. These values correlate well with removal efficiencies observed in Figure 4a,d by using Equation (1). Of note, the very high adsorption capacity of \(\alpha\)-\(FeOOH\) is a distinct advantage in addition to its low cost, low toxicity, and present facile synthesis, which can make the present \(\alpha\)-\(FeOOH\) material one of the highest potential adsorbents explored, especially for the As(III) and also for Se(VI) removal. Another indication of good adsorption is the values of the equilibrium parameter (\(R_{L}\)) that are recorded in Table 1. The value of \(R_{L}\) is determined by Equation (5). \[R_{l}=\frac{1}{1+K_{l}C_{o}} \tag{5}\]  
where \(K_{L}\) and \(C_{o}\) are constants related to the Langmuir model. Calling back the adsorption parameters obtained from Langmuir adsorption isotherm plots, \(R_{L}\) values for As(III) and Se(VI) are found at \(R_{L}\) = 0.126 and \(R_{L}\) = 0.14, respectively. In general, adsorption is favored when \(R_{l}\) the value is in the 0-1 range. In the present analysis, \(R_{L}\) of both adsorbate oxyanions indicates a favorable monolayer adsorption process, but it also indicates strong adsorption for As(III) than Se(VI). These findings are also confirmed by the correlation coefficient, \(R^{2}\), from the plots of Figure 4b,e. In the same way, Freundlich isotherm is another precise adsorption model to describe the interaction mechanism. Of course, if the data fitted well with this isotherm, it indicates heterogeneous adsorption. It gives proper information on the nature of adsorption as well as specific adsorption capacity of the \(\alpha\)-\(FeOOH\). A linear logarithmic equation of Freundlich isotherm is described in Equation (6).