# Arsenic Removal from Water by Adsorption onto Iron Oxide/Nano-Porous Carbon Magnetic Composite
## 3 Results and Discussion
### Adsorption Isotherm  
Adsorption isotherm indicates the distribution of adsorbate molecules between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state [45]. Mathematically, it is a curve that expresses the variation in the amount of adsorbate adsorbed by the adsorbent with its pressure or concentration at a constant temperature. Langmuir and Freundlich adsorption isotherms are  
Figure 4: Effect of contact time on the percentage of As (III) adsorption. Error bars represent the standard error from triplicate experiments.  
Figure 3: Effect of adsorbent dosage on the percentage of As (III) adsorption. Error bars represent the standard error from triplicate experiments.  
commonly applied in solid/liquid system. The Langmuir describes the saturated monolayer adsorption on the homogenous surface, and there is no significant interaction among adsorbed species [46]. Freundlich adsorption isotherm describes the non-ideal adsorption on heterogeneous surface sites with different adsorption potential [47]. Equation (7) represents the linear form of the Langmuir adsorption model.  
\[\frac{\mathrm{C_{e}}}{\mathrm{q_{e}}}=\frac{1}{\mathrm{bq_{m}}}+\frac{\mathrm{ C_{e}}}{\mathrm{q_{m}}} \tag{7}\]  
where \(\mathrm{q_{m}}\) (mg/g) is the maximum Langmuir adsorption capacity and b (L/mg) is the Langmuir constant related to free energy. The Langmuir constants \(\mathrm{q_{m}}\) and b were calculated from the slope and intercept of the Langmuir plot.  
Equation (8) represents the linear form of the Freundlich adsorption model.  
\[\mathrm{logq_{e}}=\log\mathrm{K}+\ \frac{1}{\mathrm{n}}\log\mathrm{C_{e}} \tag{8}\]  
where K (mg/g) is the Freundlich constant indicating adsorption capacity, n (L/mg) is the adsorption intensity that is the measure of the change in the affinity of the adsorbate with the change in adsorption density. The Freundlich constants K and n were calculated from the slope and intercept of the Freundlich isotherm plot.  
A Langmuir adsorption isotherm plot of \(\mathrm{C_{e}/q_{e}}\) versus \(\mathrm{C_{e}}\) for As adsorption is presented in Figure 5. Similarly, Freundlich adsorption isotherm for As adsorption plot is presented in Figure 6. In both graphs, the standard error bars are shown. The values of Langmuir and Freundlich parameters are given in Table 1. The applicability of the isotherm equation was compared by judging coefficients of determination (R\({}^{2}\)) of Langmuir and Freundlich isotherm plots. The value of the coefficient of determination (R\({}^{2}\)) of 0.991 for Langmuir isotherm was found to be higher as compared to Freundlich isotherm (R\({}^{2}\)) of 0.929. This suggested that adsorption isotherm data better fitted the Langmuir isotherm. It means the number of active sites on the composite surface was limited, and a monolayer of As (III) was formed over the homogenous composite surface. The values of 1/n lied between 0 and 1, indicating the adsorption process was favorable.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multirow{2}{*}{**Adsorbent**} & \multicolumn{3}{c}{**Langmuir Isotherm**} & \multicolumn{3}{c}{**Freundlich Isotherm**} \\ \cline{3-8}  & & **qm (mg/g)** & **b (L/mg)** & **R\({}^{2}\)** & **K (mg/g)** & **1/n** & **R\({}^{2}\)** \\ \hline As (III) & AC composite & 6.69 & 0.491 & 0.991 & 2.293 & 0.353 & 0.929 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Langmuir and Freundlich parameters for the adsorption of As (III) onto the composite adsorbent.  
Figure 5: Langmuir isotherm for the adsorption of As (III) onto the composite with error bars.