# Efficient Sulfate Adsorption on Modified Adsorbents Prepared from _Zea mays_ Stems
## 3 Results
### Effect of Temperature  
The zero pH loading point (pH\({}_{\text{PZC}}\)) was established to determine the ideal pH for anion adsorption, as the exposed surface of a bioadsorbent is protonated at pH values below the pH\({}_{\text{PZC}}\)[21]. The pH\({}_{\text{PZC}}\) of the B 1:1 was 5.17 and that of the MC was 6. Therefore, to guarantee the protonation of the surface of the biomaterial, the working pH of 4 was selected for use in the adsorption tests using the B 1:1 and the MC.  
The bioadsorption tests were performed at pH 4, initial concentration of 100 mg/L and adsorbent dose of 2 g/L, varying the temperature between 25 and 45 \({}^{\circ}\)C. Figure 2 shows the effects of temperature on sulfate adsorption capacity of B 1:1 and MC. The results showed that a temperature of 25 \({}^{\circ}\)C resulted in the best behavior of the adsorbents in the removal of the anion, and B 1:1 presented the best performance. Consequently, as the temperature increased, the adsorption capacity of the adsorbents decreased, which could be due to the exothermic nature of the process, as similarly reported for the removal of sulfates on quaternized cellulose [25].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Model** & **Equation** & **Parameters** \\ \hline Langmuir & q\({}_{\text{e}}=\frac{\text{q}_{\text{max}}\text{K}_{\text{t}}\text{C}_{\text{e}}}{1+ \text{K}_{\text{t}}\text{C}_{\text{e}}}\) & (7) & C\({}_{\text{e}}\) (mg/L): concentration of adsorbate in the equilibrium \\  & q\({}_{\text{e}}=\frac{\text{q}_{\text{max}}\text{K}_{\text{t}}\text{C}_{\text{e}}}{1+ \text{K}_{\text{t}}\text{C}_{\text{e}}}\) & (7) & q\({}_{\text{e}}\) (mg/g) amount of adsorbate adsorbed per gram of adsorbent at equilibrium Q\({}_{\text{max}}\) (mg/g): coverage capacity in the monolayer \\  & & KL: Langmuir isothermal constant \\ \hline Freundlich & q\({}_{\text{e}}=\text{K}_{\text{F}}\text{C}_{\text{e}}^{1/2}\) & (8) & q\({}_{\text{e}}\) (mg/g): amount of adsorbate adsorbed per gram of adsorbent at equilibrium \\  & & K\({}_{\text{F}}\) (L/g): [PERSON]’s isothermal constant, C\({}_{\text{e}}\) (mg/L): equilibrium \\  & & adsorbate concentration \\ \hline Dubinin–Radushkevich & q\({}_{\text{e}}=\text{q}_{\text{DR}}\text{*e}^{-\text{K}_{\text{DR}}\text{ }\text{The thermodynamic parameters, namely \(\Delta\)H\({}^{\circ}\), \(\Delta\)S\({}^{\circ}\) and \(\Delta\)G\({}^{\circ}\), were calculated using [PERSON]'s method, and the results are summarized in Table 4. The negative values of \(\Delta\)H\({}^{\circ}\) indicate that the process is exothermic, as shown by the results presented in Figure 1, which demonstrates that the application of energy to the system is not required for the reactions to occur [26]. The value of \(\Delta\)H\({}^{\circ}\) was higher for B 1:1, which explains the greater exothermic character of the reactions on the biochar. This would explain the decrease of the capacity of adsorption of B 1:1 as the temperature increased up to 45 \({}^{\circ}\)C. The decrease of the adsorption capacity as the temperature increased could result from the weakening of the interaction of forces between the adsorbate and the adsorbent due to the increase in the energy of the system [27].  
The positive value of \(\Delta\)G\({}^{\circ}\) shows that the system is not spontaneous, and the increase in its value indicates that the system loses spontaneity as the temperature increases, indicating that agitation is necessary [16]. The negative entropy value indicates the associative process of sulfate adsorption over B 1:1 and MC, which shows that the existing particles in the solution were smaller after the adsorption process, demonstrating that the process takes place by chemisorption [28].