Molecular-level insights into the mitigation of magnesium-natural organic matter induced ultrafiltration membrane fouling by high-dose calcium based on DFT calculation

While magnesium cation (Mg2+) universally coexists with natural organic matter (NOM) in the water environment, influence of Mg2+ on NOM fouling in membrane filtration process is still unclear. This work was therefore performed to investigate effects of Mg2+ on NOM (sodium alginate (SA) as a model substance) fouling and role of Ca2+ in mitigating fouling from Mg2+ in the ultrafiltration (UF) water treatment process. Filtration tests showed two interesting fouling phenomena: (1) membrane fouling caused by combination of Mg2+ and SA maintained at a high value with the increased Mg2+ concentration; (2) the high fouling property of Mg2+ can be significantly improved by the prominent addition of calcium cation (Ca2+). It was found that changes of foulant morphology played essential roles through thermodynamic mechanisms represented by the Flory-Huggins lattice theory. Density functional theory (DFT) calculation showed that the combination of SA and Mg2+ tends to coordinate two terminal carboxyl groups in SA, beneficial to stretching alginate chains and forming a stable gel network at low doses. In addition, intramolecular coordination is difficult to occur between SA and Mg2+ due to the high hydration repulsion radius of Mg2+. Therefore, a dense and thick gel network remained even under high Mg2+concentration. Furthermore, due to the higher binding affinity of Ca2+ over Mg2+, high doses of Ca2+ trigger a transition of the stable SA-Mg2+ gel network to other configurations where flocculation and aggregation occur, thereby reducing the specific filtration resistance. The proposed thermodynamic mechanism satisfactorily explained the above interesting fouling behaviors, facilitating to development of new solutions to control membrane fouling.