# Batch and Column Adsorption of Phosphorus by Modified Montmorillonite
## 3 Results and Discussion
### The Batch Adsorption of Phosphorus by Al\({}_{13}\)-O-MMt  
#### 3.2.1 Effect of pH on Phosphorus Adsorption by Al\({}_{13}\)-O-MMt  
Figure 3 shows that the adsorption efficiency of Al\({}_{13}\)-O-MMt is higher than MMt in the pH range of 6-9. Phosphate is a form of phosphorus in aqueous solution, which has a negative charge. Modifiers occupying the negatively charged sites of MMt can reduce the electrostatic repulsion between the phosphate and adsorbent, resulting in an increase in the adsorption efficiency of Al\({}_{13}\)-O-MMt. In addition, the Lewis acidity sites from the Keggin Al\({}_{13}\) complex can attract substances with a negative charge [55; 56; 57]. Therefore, the adsorption efficiency of phosphorus onto Al\({}_{13}\)-O-MMt is better than that onto MMt. Figure 3 indicates that the pH has an effect on the efficiency of the adsorption of phosphorus onto both Al\({}_{13}\)-O-MMt and MMt. The efficiency of the adsorption of phosphorus onto Al\({}_{13}\)-O-MMt increased from pH 6 to 8. This is because the concentration of OH\({}^{-}\) increases with the increase in pH in the range of 6 to 8, causing HPO\({}_{4}\)\({}^{-}\) to transform into PO\({}_{4}\)\({}^{2-}\) and PO\({}_{4}\)\({}^{2-}\) can be more easily adsorbed by Keggin Al\({}_{13}\)[58]. The decrease in adsorption efficiency with a further increase in pH could be explained that the increase in negative charges on the Al\({}_{13}\)-O-MMt surface can result in strengthening electrostatic repulsion between adsorbent and phosphorus. In addition, the high concentration of OH\({}^{-}\) competes with PO\({}_{4}\)\({}^{2-}\), contributing to a decrease in the adsorption of phosphorus [16]. For the adsorption of phosphorus onto MMt, the adsorption efficiency of phosphorus decreased with increasing pH (Figure 3). This could be explained by, similar to the explanation for the adsorption of phosphorus onto Al\({}_{13}\)-O-MMt, the increasing pH changing the surface charge of MMt to make it more negative, thereby increasing the electrostatic repulsion between adsorption sites and phosphorus. Figure 2: The morphology of MMt (**A**) and Al\({}_{13}\)-O-MMt (**B**) characterized by SEM. 2.2 Effect of Initial Concentration of Phosphorus and Adsorption Temperature on Phosphorus Adsorption by Al\({}_{13}\)-O-MMt  
Since eight concentration gradients were used in this experiment and the amount of phosphorus adsorbed increased as the initial concentration of phosphorus increased, the increase in phosphorus ions could have led the reaction towards adsorption until all the adsorption sites were filled. An increase in phosphorus in the solution further promoted the entry of phosphorus into the interlayer of the adsorbent. Consequently, the total amount of phosphorus adsorbed increased with increasing phosphorus ions in the solution (Figure 4A). However, the adsorption efficiency decreased with the increasing initial concentration of phosphorus, as with the continuing increase in phosphorus in solution, additional phosphorus would not be able to enter the adsorption sites that were already filled [59]. In particular, the adsorption efficiency was higher than 95% at 25 \({}^{\circ}\)C, suggesting that the adsorption efficiency was almost stable with the increasing concentration of phosphorus at 25 \({}^{\circ}\)C. However, the adsorption efficiencies decreased as the concentration of phosphorus in the solution increased in the cases of 35 \({}^{\circ}\)C and 45 \({}^{\circ}\)C (Figure 4B). Figure 4: Three temperature gradients were measured to determine the amount adsorbed (**A**) and adsorption efficiency (**B**) of Al\({}_{13}\)-O-MMt in different initial concentrations of phosphorus. Figure 3: Adsorption efficiency (%) of MMt and Al\({}_{13}\)-O-MMt for phosphorus at different pH values. As illustrated in Figure 4, the phosphorus adsorption efficiency of Al\({}_{13}\)-O-MMt decreased as the temperature increased from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, revealing that phosphorus adsorption was spontaneously exothermic.