Unravelling pH-dependent formation and deposition mechanisms of Si–Al complex scale in reverse osmosis systems

Reverse osmosis (RO) is a leading technology for wastewater/water treatment and resource recovery. The formation of Si–Al complex scale significantly impairs the performance of RO systems. Due to the diverse forms of silica and aluminum species, the scaling process involves both homogeneous and heterogeneous mechanisms, presenting substantial challenges in understanding Si–Al scale formation. Herein, we investigated the scaling mechanism of Si–Al complexes in RO systems under acidic, neutral, and alkaline conditions, focusing on Si–Al interactions in solution and their deposition behavior on membrane surfaces. We found that Si–Al interactions in solution did not directly determine their fouling behavior on RO membranes. In acidic conditions, silicic acid and aluminum species (i.e., Al 3+ ) remained stable in solution but underwent heterogenous polymerization on the membrane surface. The electrostatic shielding and the bridging effects of Al 3+ facilitated the formation of a dense and thick Si–Al complex layer. Under neutral and alkaline conditions, the Si–Al complex primarily underwent homogenous polymerization before depositing on the membrane surface, due to the adsorption of silicic acid by polymeric aluminum hydroxide. The electrostatic repulsion between the Si–Al complex and the membrane resulted in minimal scaling. This work offers comprehensive insights into the pH-dependent scaling behavior of Si–Al complexes in RO processes, providing a theoretical foundation for developing effective scaling control strategies.