# Efficient Sulfate Adsorption on Modified Adsorbents Prepared from _Zea mays_ Stems
## 3 Results
### Adsorption Equilibrium  
The sulfate adsorption isotherms over B 1:1 and MC are represented in Figure 4, and the experimental data were adjusted with the Langmuir, Freundlich and Dubinin  
Figure 3: Sulfate adsorption kinetics for (**a**) B 1:1 and (**b**) MC.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model** & **Parameters** & **MC** & **B 1:1** \\ \hline \multirow{4}{*}{Pseudo-first order} & q\({}_{\text{e}}\) (mg/g) & 6.8739 & 14.801 \\  & k\({}_{1}\) (min\({}^{-1}\)) & 0.0258 & 0.018 \\  & SE & 0.19820 & 0.5168 \\  & R\({}^{2}\) & 0.9730 & 0.966 \\ \hline \multirow{4}{*}{Pseudo-second order} & q\({}_{\text{e}}\) (mg/g) & 7.4891 & 16.646 \\  & k\({}_{2}\) (g/mg-min) & 0.0045 & 0.0013 \\  & SE & 0.0764 & 0.4291 \\  & R\({}^{2}\) & 0.9975 & 0.988 \\ \hline \multirow{4}{*}{Elovich} & \(\alpha\) (mg/g.min) & 0.7856 & 0.976 \\  & \(\beta\) (g/mg) & 0.9199 & 0.976 \\ \cline{1-1}  & SE & 0.2935 & 0.2113 \\ \cline{1-1}  & R\({}^{2}\) & 0.9455 & 0.959 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Adjustment parameters of sulfate adsorption kinetics.  
Radushkevich equations (Table 6). The sulfate adsorption isotherm over B 1:1 rose very rapidly as concentrations increased (Figure 3a), indicating a high affinity of part of the bioadsorbent for the anion. When using MC, the adsorption capacity increased at lower sulfate concentrations. However, as the sulfate concentration in solution increased, the amount of sulfate adsorbed reached a limit value and decreased. The behavior of MC suggested that there is a strong affinity between sulfate ions and adsorbent, although it showed worse performance than B 1:1 ([PERSON] et al., 2018).  
According to the standard error (SE) and R2 values, [PERSON]'s model fits the data better when using MC. Since the E value is higher than 8 KJ/mol, it can be concluded that the process is controlled by chemisorption with strong interactions between the active centers and the anion ([PERSON] and [PERSON], 2018), as evidenced by the calculation of thermodynamic parameters (Table 3). Based on the fit to [PERSON]'s model, it can be assumed that the bioadsorbent presents a heterogeneous structure ([PERSON] and [PERSON], 2018). Using B 1:1, it was found that [PERSON]'s model describes the adsorption equilibrium, indicating that the process occurs in multilayers on the surface of the bioadsorbent due to its heterogeneity ([PERSON], 2018).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model** & **Parameter** & **B 1:1** & **MC** \\ \hline \multirow{4}{*}{[PERSON]} & q\({}_{\text{max}}\) (mg/g) & 29.514 & 19.779 \\  & K\({}_{\text{L}}\) (L/mg) & 9.06 \(\times\) 10\({}^{-5}\) & 5.16 \(\times\) 10\({}^{-5}\) \\  & SE & 0.2957 & 0.5165 \\  & R\({}^{2}\) & 0.945 & 0.7014 \\ \hline \multirow{4}{*}{Freundlich} & Kf (mg/g) & 0.221 & 0.0627 \\  & n & 0.965 & 0.9041 \\  & SE & 1.139 & 2.9381 \\  & R\({}^{2}\) & 0.946 & 0.7081 \\ \hline \multirow{4}{*}{Dubinin–Radushkevich} & q\({}_{\text{DR}}\) (mg/g) & 20.247 & 10.7359 \\  & K\({}_{\text{DR}}\) (mol\({}^{2}\)/kJ\({}^{2}\)) & 1.29 \(\times\) 10\({}^{-4}\) & 2.4493 \(\times\) 10\({}^{-4}\) \\ \cline{1-1}  & E (KJ/mol) & 62.257 & 45.1819 \\ \cline{1-1}  & SE & 2.4117 & 2.1699 \\ \cline{1-1}  & R\({}^{2}\) & 0.919 & 0.8465 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Adjustment parameters of sulfate adsorption isotherms.  
Figure 4: Isothermal sulfate adsorption on (**a**) B 1:1 and (**b**) MC.