Diffusiophoresis contributes significantly to colloidal fouling in low salinity reverse osmosis systems

Diffusiophoresis is the migration of particles in salt gradients, and depends on the particle charge and the difference in diffusion coefficients of major ions making up the salt. This phenomenon is ubiquitous where salt gradients and charged particles exist and has been studied for several decades. However, this phenomenon has not been explored in RO membrane systems and could be relevant for low salinity feed solutions (such as brackish water or reclaimed wastewater), where significant salt gradients arise next to the membrane and where charged colloidal particles exist natively. Here we demonstrate through experiments and modeling that under low salinity feed conditions, diffusiophoresis contributes significantly to particle transport and cake formation. We further propose that particulate fouling due to diffusiophoresis will increase in a positive feedback loop over time due to cake enhanced concentration polarization.