TEMPO-mediated oxidation as surface modification for cellulosic ultrafiltration membranes: Enhancement of ion rejection and permeability

Abstract

In membrane technology, rejection of dissolved salts often requires the use of nanofiltration or reverse osmosis membranes due to their sufficiently dense structure and favourable charge effects. In this work, the concept of charge-based salt rejection is extended to the more open ultrafiltration class. Surface charge of a commercial regenerated cellulose acetate ultrafiltration membrane is enhanced with TEMPO-mediated oxidation. With the modification, a membrane whose properties lie between the traditional nanofiltration and ultrafiltration classes is obtained. Moreover, a purely charge-based salt rejection mechanism is introduced to an ultrafiltration membrane.Increasing hypochlorite exposures are found to induce successively increasing surface carboxylate densities, negative zeta potentials, permeabilities and molecular weight cut-off values. In the low exposure region, the treatment mostly affects surface charge, whereas high exposures begin to open the membrane pores. Ultimately, the pH 7 zeta potential is increased from −30 up to −100 mV, and 500 ppm Na2SO4 rejections increase from an initial 4% to 36%. At the same time, pure water permeability is doubled, and cut-off value increased from 2.8 to 8.0 kDa. The reactions causing the change in membrane performance seem to take place almost exclusively on the membrane surface, while the bulk material appears unchanged.