Removal of chromate anions by micellar-enhanced ultrafiltration using cationic surfactants

Micellar-enhanced ultrafiltration (MEUF) of chromate anions (CrO42–) from aqueous streams has been studied at 30°C using twice cationic surfactants (cetyltrimethylammonium bromide and cetylpyridinium chloride). The solution is processed by ultrafiltration, using a membrane with pore sizes small enough to block the passage of the micelles and adsorbed ions. Rejection coefficients higher than 99% are reached in optimal conditions of pressure, feed concentration in cationic surfactant, and percent filtered volume. The rejection rate depends on the ionic strength and pH. The increasing of ionic strength decreases the retention of chromate ions and the permeate surfactant concentration. As long as the NaCl feed concentration is less than or equal to 100 mM, more than 88% of hexavalent chromium are retained and surfactant leakage was reduced.

Removal of chromate ions from water by MEUF using cetylpyridinium chloride and cetyltrimethylammonium bromide was investigated. Hexavalent chromium was rejected at surfactant concentrations where theoretically no micelles exist. The observed rejection of Cr(VI) in the vicinity of the cmc of the surfactant is due to the presence of concentration polarisation and membrane charge effects. Concentration polarisation results in the formation of micelles near the membrane surface below the surfactant cmc. For all surfactant/chromate systems, studied at surfactant concentration above the cmc, rejection reached 99.5%.