Effect of pH on the selective electrodialytic recovery of acidogenic-derived volatile fatty acids

• Higher pH enhances the fractionation degree for VFA separation with PC200D. • It is recommended to use feed solutions at a pH of approximately 7 with PC200D. • The greater fractionation with PC200D is due to tertiary amines in its fixed groups. • For Fumasep, higher feed pH slightly increased the VFA fractionation. • The Ralex membrane showed minimal sensitivity to changes in the initial feed pH. Electrodialysis can be utilized to separate acidogenic-derived volatile fatty acids (VFA) used in the production of biobased and biodegradable polyhydroxyalkanoates (PHA). All VFAs are commonly recovered simultaneously in the receiver solution, although their selective recovery could benefit the fermentation process. The effect of membrane type, electric voltage and time on this selective recovery has been studied, but the impact of solution pH — an essential factor in organic acidś separation — remains still not sufficiently explored. Herein, the effect of the initial feed pH on the fractionation degree of four VFAs commonly present in fermented solutions — namely acetic, propionic, butyric, and valeric acid — was systematically evaluated. Three distinct anion-exchange membranes were tested: Ralex AMH-PES, Fumasep FAS-PET-130, and PC200D. It was found that pH significantly influenced VFA separation with the PC200D membrane, particularly enhancing the transfer of smaller organic anions, such as acetate and propionate, while the percent extraction (PE%) of larger anions, such as butyrate and valerate, remained practically unchanged as the pH increased. Thus, the higher the pH, the greater the fractionation degree achieved. For the electrodialysis operated with a feed solution at pH 7, ratios of (PE Ac + PE Prop )/(PE But + PE Val ) and PE Ac /PE Val of 1.6 and 2.0, respectively, were obtained. This may be related to the presence of tertiary amines in the PC200D’s fixed groups, which enhanced water dissociation occurrence. The transfer of VFAs through the Ralex and Fumasep membranes was less affected by pH as they contain only quaternary ammonium as their fixed charged groups.