Ammonium-functionalized mesoporous silica MCM-41 for phosphate removal from aqueous solutions

The aim of this study was to investigate the removal of phosphate (P) using ammonium-functionalized MCM-41 (A-MCM-41). Batch experiments were performed with A-MCM-41 under various conditions including the adsorbent dose, reaction time, initial P concentration, and solution pH. The results showed that the phosphate removal in the MCM-41 (initial P conc. = 2 mgP L^{− 1}) was negligible with the percent removal of ≤1.0% at the adsorbent dosages between 0.5 and 2.0 g L^{− 1}. In the case of A-MCM-41, the percent removal increased from 73.6 to 100% as the adsorbent dose increased from 0.5 to 2.0 g L^{− 1}. This indicated an enhancement of the phosphate removal due to the surface modification of the MCM-41 through the functionalization of the ammonium group (NH₃ ⁺). The results also demonstrated that the phosphate removal by the MCM-41 was minimal throughout the acidic and alkaline pH conditions (3.1% at pH 3.5; ≤1.0% at pH 5.4–10.7), whereas the phosphate removal by the A-MCM-41 was effective throughout the acidic and neutral pH values (94.7–97.6% at pH 3.5–7.4). In highly alkaline conditions, however, the phosphate removal by the A-MCM-41 was greatly reduced (3.4% at pH 10.7) due to the competition of the hydroxyl groups (OH⁻) with phosphate ions for sorption sites. The results indicated that the pseudo-second-order model was most suitable for describing the kinetic data with the parameter values of q_e = 1.86 mgP g^{− 1} and k₂ = 183.98 g mgP^{− 1} h^{− 1}. The Redlich–Peterson isotherm fits well with the equilibrium data, with the parameter values of K_R/a_R = 7.90 mgP g^{− 1} and g = 0.83.