Differences in the Effect of Mn(2+) on the Reverse Osmosis Membrane Fouling Caused by Different Types of Organic Matter: Experimental and Density Functional Theory Evidence

Landfill leachate from some sites contains a high concentration of Mn(2+), which may cause reverse osmosis (RO) membrane fouling during RO treatment. In this study, the effect of Mn(2+) on RO membrane fouling caused by typical organic pollutants (humic acid (HA), protein (BSA), and sodium alginate (SA)) was systematically investigated, and it was found that Mn(2+) exacerbates RO membrane fouling caused by HA, SA, and HBS (mixture of HA + BSA + SA). When the Mn(2+) concentration was 0.5 mM and 0.05 mM separately, the membrane fouling caused by HA and SA began to become significant. On the other hand, with for HBS fouling only, the water flux decreased significantly by about 21.7% and further decreased with an increasing Mn(2+) concentration. However, Mn(2+) has no direct effect on BSA. The effect degrees to which Mn(2+) affected RO membrane fouling can be expressed as follows: HBS > SA > HA > BSA. The density functional theory (DFT) calculations also gave the same results. In modeling the reaction of the complexation of Mn(2+) with the carboxyl group in these four types of organic matter, BSA has the highest energy (−55.7 kJ/mol), which predicts that BSA binding to Mn(2+) is the most unstable compared to other organic matter. The BSA carboxylate group also has the largest bond length (2.538–2.574 Å) with Mn(2+) and the weakest interaction force, which provides a theoretical basis for controlling RO membrane fouling exacerbated by Mn(2+).