Elucidating pH-Dependent Silica Removal Mechanisms in Aluminum-Based Coagulation

Silica scaling is a major limiting factor to the treatment and valorization of saline brines as alternative water sources. While coagulation provides competitive silica removal capacity compared to ion exchange and adsorption, the incomplete knowledge of silica removal mechanisms hinders efforts to improve process efficiency. In this study, we employed experimental investigations and materials characterization to advance the molecular-level understanding of silica removal during electrocoagulation (EC) and chemical coagulation (CC). EC provided greater silica removal compared to CC for all coagulant types considered. For both EC and CC, silica removal capacity increased 2- to 5-fold with rising pH. Characterization of precipitates showed that the pH-dependent increase in silica removal capacity was driven by Al-O-Si bond formation and silicic acid polymerization. A novel titration method revealed that at pH ≤ 8.0, silica binding occurred via η-OH2+ activated sites on aluminum precipitate, whereas at pH 10.0, binding resulted from reactions among partially deprotonated η-OH sites, bound silica, and silanol groups. Silica removal mechanisms were validated using synthetic brackish groundwater, where synergistic interactions between aluminum and Ca2+/Mg2+ substantially increased silica removal. This work provides a mechanistic basis for the rational operation of aluminum-based coagulation to enhance silica removal from saline brines.