# 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)
#### 3.2.2 Adsorption Kinetics
This is experimental evidence for the effective adsorption of As(III) by \(\alpha\)-\(FeOOH\) NRs. Similarly, the pseudo-second-order adsorption model of Se(VI) is fitted well and plotted in Figure 5f. The calculated results are tabulated in Table 2, which imply that \(Q_{e}\) = 7.04 and \(K_{2}\) = 0.01173. In this case, the value of \(Q_{e}\) is relatively higher than Qmax calculated by Langmuir isotherm in Figure 4e. In this way, the kinetics of As(III) and Se(VI) adsorption on \(\alpha\)-\(FeOOH\) NRs surfaces follow pseudo-second-order sorption isotherm, which indicates the chemical nature of adsorption [40]. For Se(VI), the adsorption mechanism fits well with Langmuir isotherms, indicating a chemical interaction. These findings are consistent with the theoretical finding that assumes a mixture of inner- and outer-shell surfaces on \(\alpha\)-\(FeOOH\)[35; 41]. Figure 5: (**a,d**) kinetics study data with removal (%) as a function of contact time for \(\alpha\)-\(FeOOH\) adsorbent at high As(III) and Se(VI) concentrations; (**b,e**) Pseudo 1 st order; and (**c,f**) Pseudo 2 nd order for As(III) and Se(VI), respectively. Figure 6a displays the results of As(III) and Se(VI) adsorption into the surface of \(\alpha\)-_FeOOH_ NRs with variable pH values. Of note, the experiments were performed at \(\sim\)500 ug/L adsorbate dilution, 0.25 g/L \(\alpha\)-_FeOOH_ dose, and 6 h contact time. Apparently, the results demonstrated that adsorption is highly influenced by pH changes. Besides, the results confirmed that the migration of As(III) oxyanions into the negatively charged \(\alpha\)-_FeOOH_ surfaces is higher and slightly flatter than Se(VI), but selenium adsorption, in general, shows the minimum pH deviation especially at acidic solutions. Of course, the above finding is useful for practical considerations. Apparently, the results demonstrated that adsorption is highly influenced by pH changes. Besides, the As(III) adsorption is indeed greater than the selenium uptake; however, the swings in ion removal caused by pH variations seem to be similar for both compounds (\(\sim\)50% change between the highest and lowest removal values at different solution pH) [42]. Of course, the above finding is useful for practical considerations. Figure 6b shows the reuse capability experiment results, which reveal that \(\alpha\)-_FeOOH_ can be used as an adsorbent for more than five cycles effectively. The experiments considered the amount of adsorbent loss due to the washing and filtration processes in each cycle. The results showed that, in the first cycle, the weight loss of adsorbent was 8.32% and it reduced until it reached 3.87% after the fifth cycle. Therefore, the weight of the powder was measured immediately after each cycle and the quantity of treated water is modified to be equivalent to the weight of the obtained powder after calculating the losses in each reuse. The results indicate a negligible amount of As(III) and Se(VI) ions after each cycle with a slight increase as the number of cycles increases. All in all, the regeneration results implied that the \(\alpha\)-_FeOOH_ nanorods are an efficient adsorbent for real applications. The capability of several times efficient regeneration enhances the economic feasibility to fabricate real adsorbents based on \(\alpha\)-_FeOOH_ NRs. Furthermore, easy processing and low-cost iron precursors support its cost-effective production. Recalling the results of zeta potential experiments from Figure 2e, which suggested that PZC was depicted at pH \(\sim\)5.2, the ultimate As(III) removal (\(\sim\)92%) was also observed at pH \(\sim\)5.2 as shown in Figure 6. At pH greater than PZC (pH 5-8) and due to the negatively charged goethite surfaces, the attraction forces get reduced and then lower adsorption of As(III) ions. However, removal is