Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation

A commercial polyamide reverse osmosis (RO) membrane was surface-modified with polydopamine deposited from buffered, aqueous dopamine solution at ambient conditions. The influence of various modification conditions (i.e., dopamine solution concentration, polydopamine deposition time, and initial pH of Tris–HCl buffer) on pure water flux, flux during filtration of an oil/water emulsion, and NaCl rejection was investigated. Dead-end filtration results showed decreased pure water flux with increasing dopamine solution concentration and polydopamine deposition time. Membranes modified at a pH of 5 exhibited no change in pure water flux or flux during fouling experiments compared to the native membranes, suggesting that polydopamine was not deposited under such acidic coating conditions. All polydopamine-modified membranes, except those coated at a pH of 5, had higher fluxes when filtering an oil/water emulsion than that of unmodified membranes. NaCl rejection values in all membranes were within the manufacturer's specification. The increased flux when filtering an oil/water emulsion was not sensitive to dopamine concentration, coating time greater than 60 min, or alkaline buffer pH value. Short deposition times slightly reduced the fouling resistance of coated membranes, and membranes modified at acidic pH values showed no improvement in fouling.