Effects of background cations on the fouling of polyethersulfone membranes by natural organic matter: Experimental and molecular modeling study

Adsorptive fouling of a polyethersulfone (PES) membrane by natural organic matter (NOM) in the presence of common metal cations was investigated with both experimental and computational molecular modeling techniques. NOM, calcium, magnesium and silicon were identified as important foulants through chemical analysis of two Midwestern surface waters. A model feed solution mimicking the lake waters and containing only NOM (Suwannee River NOM) and Ca²⁺ resulted in a fouling pattern similar to the surface waters; Mg²⁺ and Na⁺ caused much lower fouling at the same ionic strength as the calcium solution. Molecular modeling of the model solution allowed detailed probing of the fouling process. This work suggests that divalent ions (Ca²⁺ and Mg²⁺) may cause membrane fouling not by forming "ionic bridges" between the negatively charged functional groups on membrane surface and the negatively charged functional groups of NOM, but by promoting the aggregation of NOM molecules in solution. The carboxyl groups of NOM strongly associate with the divalent ions, while the sulfonyl groups in the polyethersulfone do not. Although Ca²⁺ and Mg²⁺ are both coordinated to the NOM carboxyl groups predominantly by outer-sphere-type complexation, Ca²⁺ associates with the carboxyl groups more strongly than Mg²⁺ due to the looser second hydration shell structure of Ca²⁺. The stronger Ca²⁺-NOM complexation is also manifested by the decreased mobility (diffusion coefficients) of the Ca²⁺ bound to the NOM.