Modeling manganese removal in chelating polymer-assisted membrane separation systems for water treatment

The removal of manganese from groundwater, using water-soluble chelating polymers such as polyacrylic acid (PAA) in combination with ultrafiltration (UF), was investigated. The effects of the solution pH and polymer dosages on the manganese removal were evaluated, and the removal efficiency was modeled considering the relevant chemical equilibria. In the absence of polymer, manganese removal with UF membranes alone was negligible at acidic pH values, but the removal increased substantially when polyacrylic acid (PAA) was added to the feed solution. The increase can be attributed to the formation of Mn2+–PAA chelates which are rejected by the membranes. A mathematical model was developed to explain this phenomenon based on chemical equilibria, including complex formation and precipitation. The chelation number (i.e., the number of carboxyl groups in the PAA binding to a single metal ion) and the equilibrium constants for metal–PAA chelation reactions were obtained by fitting experimental data at acidic pH in single-divalent metal systems. The model was able to predict Mn removal in chelation/UF systems at various pH levels and polymer dosages, and to account for the competitive interactions of PAA with the target (Mn2+) and background species (Ca2+, Mg2+) in multi-component systems. The predicted Mn removal efficiency was most sensitive to the chelation number.