Description of the variation of retention versus pH in nanofiltration of organic acids

Abstract

Nanofiltration can be used to recover chemicals, like VFAs, produced by the fermentation of biomass. Solution pH is a key parameter for fermentation as well as for recovery using nanofiltration. Current work reports an experimental investigation carried out with single and mixed solutions that contain acetic, propionic, and butyric acids, at various concentrations and pHs. It is shown that retention of VFAs increases with solution pH following an S-shaped curve related to the VFA dissociation. Retentions of VFAs decrease with the concentration, the influence being negligible at low pH values and more significant at high pH values. A simple model is proposed to calculate VFAs retentions versus pH for different concentrations and a given filtration flux, considering the dissociation of VFAs and the retention of dissociated and undissociated solutes. Calculated retentions can fit well with the experimental ones, on condition to consider solutes pKa values slightly higher than theoretical ones, up to 1.24 pH units, or a variation of the membrane charge with pH, corresponding to an effective membrane pKa between 5.44 and 5.80. Both assumptions are supported by previous investigations.