# Effect of Hydrogen Ion Presence in Adsorbent and Solution to Enhance Phosphate Adsorption
## 3 Results and Discussion
### Solution pH Effect  
The effect of pH on phosphate adsorption was investigated at various initial pH ranging from 3 to 9. The removal efficiency and pH of the final solution are shown at the left and the right y-axis in Figure 3a, respectively. It should be noted that the initial pH of the solution was not fixed although the initial pH of the solution was adjusted to the initial pH at desired times during the experiment. At pH 3, the removal efficiency is only \(\sim\)35% regardless of the GACs. With an increase in the pH, there is a difference, as described. For the GAC, removal efficiency of 70% is achieved at the acid condition. Since the initial pH is 7, the removal efficiency rapidly decreases with the increase of pH, finally being \(\sim\)20% at an initial pH of 9.  
Figure 3: _Cont_.  
Figure 2: Zeta potential as a function of pH for GAC and GACA.  
The removal efficiency for the GACA is to be 65-70% at a pH of less than 7; this was comparable to that of the GAC, and \(\sim\)55% efficiency can be reached, being reduced only by \(\sim\)15% even at a pH of 9. Therefore, it could be observed that significant and mild phosphate removal efficiency occurred for the GAC and the GACA at base conditions. This could be explained by the effect of the solution pH. Based on the final pH of the solution during the experiment described in Figure 3 at the first y-axis, for the GAC, the initial pH of 4 and 5 is finally increased to 5.5 and 6.0, respectively, although the solution pH was adjusted, while the initial and final pH do not exhibit large differences at the base condition. For the GACA, the phenomena are reversed--the initial pH of 8 and 9 is finally decreased to 6.5 and 7.5, respectively. As a result, the optimized pH on the phosphate removal for the GAC or GACA can be determined at weak acid conditions, showing comparable values as those in Table S1, and the higher removal efficiency for the GACA at the base condition was due to the lower solution pH effect, which will be mentioned in Section 3.5.  
The dotted line is the fraction of phosphate species (right y-axis) in Figure 3b with phosphate uptake (left y-axis). At a lower pH than 6.5 and a higher pH than pH 8, the species of H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) and HPO\({}_{4}\)\({}^{2-}\) are dominated by over 80%, respectively. Ideally, when electrostatic force governs the interaction between the surface of a solid and the ions in solution, the two negative charges of the phosphate at pH 8 can interact more with the positively charged surface of the solid at the base condition. However, the contrary result is due to (1) the pH\({}_{\text{pzc}}\) of the adsorbent and (2) the real solution pH. According to Figure 2, the pH\({}_{\text{pzc}}\) and its values are 4.5 and 6.0 for the GAC and GACA, respectively. Therefore, electrostatic force results in increased phosphate adsorption for the GACA. However, at a pH of 3, the highest positive \(\xi\), is revealed because the phosphate species of H\({}_{3}\)POH\({}_{4}\) exists, and the removal efficiency is obtained at a pH of less than 4, which ideally attempts to interact with only a few H\({}_{3}\)PO\({}_{4}\).