Effects of -COOH and -NH2 on adsorptive polysaccharide fouling under varying pH conditions: Contributing factors and underlying mechanisms

The effect of -COOH and -NH2 functional groups on adsorptive polysaccharide fouling of a membrane surface was investigated at different pH values. Sodium alginate (SA: rich in -COOH) and chitosan (CTS: rich in -NH2) were selected as typical polysaccharides adsorbed onto the surface of polyvinylidene fluoride (PVDF) and PVDF modified using graphene oxide (GO-PVDF). The adsorption behavior and structure of the fouling layer were determined using quartz crystal microbalance and dissipation (QCM-D) and atomic force microscopy (AFM). The results showed that in the pH range of 4-6, the adherence of polysaccharide fouling and its reversibility were dependent on functional groups. When the organics were rich in -COOH, increase in the pH reduced their deposition on the membrane surface, and alleviated adsorptive fouling and its irreversibility. For the -NH2 functional group, the increase in pH led to more severe polysaccharide fouling due to the decreased degree of protonation, and the resulting fouling maintained high irreversibility. Modification using GO alleviated the adsorptive fouling of these two polysaccharides on PVDF; however, the extent of alleviation was dependent on the abundance of functional groups on the polysaccharides.