# 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
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Pseudo-first-order Isotherm**} & \multicolumn{4}{c}{**Pseudo-second-order Isotherm**} \\ \hline
**Parameter** & **As(III)** & **Se(VI)** & **Parameter** & **As(III)** & **Se(VI)** \\ \hline \(R^{2}\) & 0.798 & 0.75 & \(R^{2}\) & 0.9485 & 0.987 \\ \(q_{e}\) & 1.48 & 29.42 & \(q_{e}\) & 8.302 & 7.04 \\ \(K_{1}\) & 0.005 & 0.0021 & \(K_{2}\) & 0.0235 & 0.012 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A representation of kinetic parameters of pseudo-first-order and pseudo-second-order models of the \(\alpha\)-_FeOOH_ adsorbent for the removal of As(III) and Se(VI) from water. Figure 6: (a) pH adsorption study and (b) five cycles regeneration experiments of As(III) and Se(VI) ions on the surface of \(\alpha\)-_FeOOH_ NRs. significantly reduced for the alkaline range 8-12. This large reduction is attributed to the presence of larger concentrations of O-H ions, which compete for arsenic ions to occupy the adsorption sites. Besides, well-fitting to the pseudo-second-order model and Langmuir isotherm suggests chemisorption interaction. Similarly, Se(VI) shows the highest removal efficiency of \(\sim\)75% at neutral pH 7 but it shows better sorption properties in acidic solution than alkaline solutions. The low removal percentage in pH study could be ascribed to the use of lower \(\alpha\)-FeOOH doses (0.25 g/L). To conclude, the results implied that the adsorption attraction at an alkaline solution (pH \(>\) 7) is unfavorable for As(III) and Se(VI), for the reasons explained above. Similarly, at higher acidic solutions (pH \(<\) 3), the removal efficiency of As(III) and Se(VI) on \(\alpha\)-FeOOH NRs surfaces are also unfavorable. As a conclusion, the removal of As(III) and Se(VI) on \(\alpha\)-FeOOH surfaces are preferred in the pH range of 5-8. This pH range is consistent with the acceptable pH range (5.5-8) of the drinking water. Unlike selenite (Se(IV)), which is extensively studied in the literature, the mechanism of adsorption of selenate (Se(VI)) to the surface of goethite is still unclear [35; 43; 44; 45]. An earlier study by [PERSON] et al. [45], inferred that the adsorption affinity of Se(VI) is much less than selenite anions at all pH values. Furthermore, they found that selenite adsorbs more strongly than selenate at all conditions, plausibly due to the high mobility of selenate. Few studies have investigated the adsorption mechanism of selenate into goethite. However, their conditions were limited to the near-neutral pH [44]. The effect of pH on the adsorption of selenate to the goethite adsorbent has been well studied in the literature [39; 41; 46; 47; 48]. Unlike selenite (Se(IV)), sorption that forms an inner-sphere surface complex selenate behaves in a different way. The interaction mechanism varies with the pH fluctuation and ion strength. In other words, selenate sorption on goethite forms inner-sphere complexes at acidic pH and outer-sphere complexes at alkaline pH. This variation in adsorption with pH could be attributed to the surface charge variation with pH and the predominance of the HSeO\({}_{4}\)- selenium species [39]. To confirm the ionic sorption of As(III) and Se(VI) to the surface of goethite nanorods and to prove their structure after sorption experiments, X-ray diffractograms post to the sorption experiments were conducted. Figure 7 depicts the diffractograms of regenerated goethite nanopowder after five reuse cycles. The peaks indicate that the goethite phase kept unchanged despite the varied intensities and peak shifts toward the right angle in the case of As(III) sample. The observed shift in both cases could be attributed to the semiconducting properties of arsenic and selenium ions. This phenomenon was also observed for selenium in lepidocrocite [49].