Role of Reverse Divalent Cation Diffusion in Forward Osmosis Biofouling

We investigated the role of reverse divalent cation diffusion in forward osmosis (FO) biofouling. FO biofouling by Pseudomonas aeruginosa was simulated using pristine and chlorine-treated thin-film composite polyamide membranes with either MgCl₂ or CaCl₂ draw solution. We related FO biofouling behavior - water flux decline, biofilm architecture, and biofilm composition - to reverse cation diffusion. Experimental results demonstrated that reverse calcium diffusion led to significantly more severe water flux decline in comparison with reverse magnesium permeation. Unlike magnesium, reverse calcium permeation dramatically altered the biofilm architecture and composition, where extracellular polymeric substances (EPS) formed a thicker, denser, and more stable biofilm. We propose that FO biofouling was enhanced by complexation of calcium ions to bacterial EPS. This hypothesis was confirmed by dynamic and static light scattering measurements using extracted bacterial EPS with the addition of either MgCl₂ or CaCl₂ solution. We observed a dramatic increase in the hydrodynamic radius of bacterial EPS with the addition of CaCl₂, but no change was observed after addition of MgCl₂. Static light scattering revealed that the radius of gyration of bacterial EPS with addition of CaCl₂ was 20 times larger than that with the addition of MgCl₂. These observations were further confirmed by transmission electron microscopy imaging, where bacterial EPS in the presence of calcium ions was globular, while that with magnesium ions was rod-shaped.