Fatty acids fouling on forward osmosis membrane: impact of pH

Abstract

The fouling propensity of the fatty acids is investigated during the operation of the osmotically driven membrane process. Experiments were performed at various pH values ranged from 4.0 to 9.0 in the presence of the octanoic acid as the model fatty acids. Results demonstrated that the fatty acid fouling was significantly dependent on the changes of pH. In all tested pH values, permeate flux was sharply decreased during the initial coverage of fatty acids on the membrane surface, then slowly decreased as the fatty acid layer became developed. At pH 9.0, fouling propensity was less than those of pH values around 4.9 (pK(a) of octanoic acid) due to the protonation of the carboxylic group of octanoic acid molecules, which resulted in electrostatic repulsion between octanoic acid molecules and membrane surfaces. The measurement of contact angle and FTIR spectra also supports that more octanoic acid molecules are adsorbed on FO membrane surfaces at the low pH than at the high pH.