A comparison of the roles of Ca2+ and Mg2+ on membrane fouling with humic acid: Are there any differences or similarities?

To further unravel the differences and similarities of the effect mechanisms of Mg2+ and Ca2+ on ultrafiltration membrane fouling with humic acid (HA), under a wide range of Mg2+ and Ca2+ contents, the adhesion forces of HA with both HA and membrane, HA cake layer structure, and fouling experiments were compared systematically. The results indicate that at lower Ca2+ or Mg2+ concentrations, membrane fouling is enhanced by the presence of Ca2+ or Mg2+, in agreement with the Derjaguin-Landau-Verwey-Overbeek theory. However, membrane fouling in the presence of Ca2+ was more serious than with Mg2+, mainly because the interaction of HA with the membrane and other HA molecules was much stronger in the Ca2+ environment. The stronger interaction promoted the accumulation of HA on the membrane surface and resulted in a denser fouling layer, which caused a more serious membrane fouling. In contrast, at much higher Ca2+ or Mg2+ concentrations, membrane fouling was mitigated significantly, but the mitigation mechanisms for both ions were different. For Ca2+, the aggregation of HA molecules was responsible for the decrease in membrane fouling, whereas for Mg2+, hydration repulsion forces prevented membrane fouling. Moreover, the aggregated HA in a Ca2+ environment seemed to mitigate membrane fouling to a greater extent, compared to the hydration forces with Mg2+ present.