Effect of Na+ on organic fouling depends on Na+ concentration and the property of the foulants

Adsorption behaviors of typical organics and the impact of structural variation on the fouling effect on a polyvinylidene fluoride membrane surface under various Na+ concentrations were compared. The results suggest that the impact of Na+ on the organics is based on their charge and structure. For the long-chain molecule sodium alginate, adding Na+ leads to an initial decrease but a subsequent regaining of the interaction force with the membrane, reflected by an inflection of relative irreversibility at a Na+ concentration of 10 mM. The lower charged Glucan presented such alterations at a higher Na+ concentration of 100 mM. As for the substances with more compact and plate-shaped structures such as humic acid and fulvic acid, the force fluctuated with the Na+ concentration, and the inflection of the relative irreversibility appeared at higher Na+ concentrations along the decreasing charge. Combined with the record on the size change in these organics, relevant mechanisms regarding the variation in fouling behavior were further analyzed. For polysaccharides, Na+ plays a role in changing the internal structure of the molecules. For relatively smaller organics, the effect of charge shielding plays a predominant role. For bovine serum albumin, the effect of Na+ concentration was nonsignificant.