Reaction heterogeneity in the bridging effect of divalent cations on polysaccharide fouling

Polysaccharide is an important foulant in membrane filtration. Nevertheless, the “bridging effect” of divalent cations on polysaccharide fouling is still not completely understood and thus is the main objective of this study. To reveal the basic underlying mechanism, a wide range of concentrations of Ca2+ and Mg2+ were analysed and alginate sodium was employed as model foulant to generate results. Results showed that the fouling propensities of alginate first increased and then decreased along with the increasing of the concentration of Ca2+. Ca2+ at high concentration acted like coagulant in alleviating alginate fouling. While the membrane fouling caused by alginate continuously aggravated with increasing Mg2+ level in feed water and surprisingly the specific filtration resistances caused by Mg2+ were much higher than those of Ca2+ at all ion concentrations. Transparent exopolymer particles (TEP) measurements revealed that both Ca2+ and Mg2+ increased the molecular interactions of alginate via “bridging effect”, while the binding of Ca2+ to alginate was much greater than that of Mg2+ which resulted with aforementioned fouling phenomenon. This study suggests that the roles of Ca2+ and Mg2+ in polysaccharide fouling are dramatically different and Mg2+ is more important than we thought.