Dependence of initial silica scaling on the surface physicochemical properties of reverse osmosis membranes during bench-scale brackish water desalination

Abstract

Silica scaling of reverse osmosis membranes in brackish water desalination is less understood than hardness scaling due to the complex silica behaviors at the membrane/water interface. In this study, -COOH, -SO3H, -NH2 and -OH functional groups were introduced onto polyamide membranes to create distinct surface physicochemical properties. The resulting membranes were further studied under similar scaling conditions to yield temporal flux loss data that were empirically interpreted by a logistic growth model. The scaled membranes were also characterized by complementary analytical techniques. It was found that permeate flux loss was strongly correlated to the initial silica layer formed by direct interaction between reactive silanol (Si-OH) and reciprocal groups on the membrane surface, rather than the entire scaling layer. Importantly, membrane surface properties dictated the initial silica layer formation through three possible mechanisms, i.e., electrostatic repulsion, competitive adsorption, and interfacial energy change. Of these, electrostatic repulsion was identified as the primary one. Therefore, by modifying the membrane surface properties, the three aforementioned mechanisms may be enhanced to favor the formation of a loose, disordered initial silica scaling layer. Accordingly, membrane flux loss may be mitigated. This finding provided important insights into the design heuristics of scaling-resistant reverse osmosis membrane for brackish water desalination.