# Arsenic (V) Removal by an Adsorbent Material Derived from Acid Mine Drainage Sludge
## 3 Results and Discussion
### Effect of pH  
The pH is a critical variable in environmental studies as it influences both the surface chemistry of the adsorbent and speciation of the adsorbate [32]. The adsorption of As(V) onto MIRESORBTM and GFH was interrogated over a pH range of 3 to 10, which are shown in Figure 5a. The adsorption capacity of both adsorbents decreased with increasing pH and ranged from 6.3-56.8 mg/g for GFH adsorbents and from 27.6-77.8 mg/g for MIRESORBTM adsorbents for pH conditions from 3-10. Overall, the MIRESORBTM materials had higher adsorption capacities over all of the tested pH conditions. The pHp\({}_{\text{P2c}}\) of both adsorbents are presented in Figure 5b. The pHp\({}_{\text{P2c}}\) of the MIRESORBTM and GFH materials were 8.7 and 6.2, respectively. Thus, the MIRESORBTM materials would be positively charged over a wider pH range (up to pH of 8.7) versus GFH (up to pH of 6.2).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & \multicolumn{3}{c}{**Langmuir Isotherm**} & \multicolumn{3}{c}{**Freundlich Isotherm**} \\ \cline{2-7}  & **q\({}_{m}\)** & **K\({}_{\text{L}}\)** & **R\({}^{2}\)** & **K\({}_{\text{F}}\)** & **n (1/n)** & **R\({}^{2}\)** \\  & **(mg/g)** & **(L/mg)** & & **([mg/g]. [mg/L]\({}^{1/n}\))** & **n (1/n)** & **R\({}^{2}\)** \\ \hline MIRESORBTM & 50.38 & 0.38 & 0.91 & 19.04 & 3.68 (0.27) & 0.98 \\ GFH & 29.07 & 0.33 & 0.94 & 12.00 & 4.35 (0.23) & 0.95 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Isotherm parameters for As(V) adsorption to MIRESORBTM and GFH materials.  
Figure 4: Arsenic isotherm materials. (**a**) MIRESORBTM; (**b**) GFH. The initial As(V) concentration was 50 mg/L, the adsorbent dose was 0.01–10 g/L, the pH was 6.6, and the contact time was 48 h.  
The solution pH influences the arsenic adsorption by influence adsorbent surface charge and speciation of arsenic ions. The valence of As(V) species changes from \(-1\) to \(-3\) as pH increases (pK = 2.2, 6.98, and 11.6), due to the deprotonation of species such as H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\), HAsO\({}_{4}\)\({}^{2-}\), and AsO\({}_{4}\)\({}^{3-}\). Simultaneously, the surface charge of the adsorbent becomes negative under high pH conditions due to deprotonation of the adsorbent surface. In iron oxides, the FeOH\({}^{+}\) groups predominate at pH conditions lower than the pH\({}_{\rm pzc}\) of iron oxide, and FeO\({}^{-}\) groups predominate at pH conditions higher than this pH\({}_{\rm pzc}\)[33]. Under high pH conditions, arsenic adsorption to iron oxides is inhibited by electric repulsions or competitions with other anions, such as hydroxyl ions [29; 32; 33]. Thus, the quantity of adsorbed As(V) is maximal under acidic pH conditions, which agrees well with experimental results. Adsorption under alkaline pH conditions could originate from coordination complex adsorption [34].  
Since the MIRESORB\({}^{\rm TM}\) materials would be positively charged over a wider pH range (up to pH of 8.7), therefore, the MIRESORB\({}^{\rm TM}\) materials would be expected to have greater As(V) adsorption performance over a wide range of pH.