Calcium-mediated polysaccharide gel formation and breakage: Impact on membrane foulant hydraulic properties

Abstract

The solution phase interaction between the polysaccharide alginate and calcium at various concentrations is examined in this paper. The observed gelation behavior and resulting hydraulic properties of the fouling layer deposited on flat sheet membranes can be directly related to the initial calcium concentration (in bulk solution). Particular attention is given to the effect of calcium concentration, with changes in calcium binding behavior of alginate in bulk solution shown to be closely related to the resistance of the cake layer formed on the membranes. At low calcium concentrations, Ca2+ ions interact strongly with guluronate (G) block functional groups forming a highly interconnected gel assemblage. This resulted in the fouling layer deposited on the membrane surface exhibiting high specific resistance despite being highly porous. Further increase in calcium concentration leads to enhanced calcium binding and significant aggregation of alginate in suspension with concomitant breakdown in the gel structure of the material accumulated on the membrane. The increased calcium concentration resulted in a cake that was characterized by a lower porosity and higher solid fraction and, most importantly, a markedly lower specific resistance to the passage of water. The materials properties of the alginate layers formed on flat sheet membranes were also determined under various pressures, with empirical expressions describing the hydraulic conductivity and compressibility of fouling layers developed as a function of trans-membrane pressure (TMP) for a range of calcium concentrations. The resulting materials properties expressions could be applied for the prediction of the filtration behavior of an alginate-rich solution for a range of calcium concentrations under constant flux (and thus time varying TMP) conditions.