Magnesium-Induced Variation of Polyamide Membrane Behavior for the Treatment of Haloacetic Acids in Swimming Pool Waters

The co-presence of haloacetic acids (HAAs) and Mg²⁺ in swimming pool waters and the potential interaction among Mg²⁺, HAAs, and membranes prompted a systematic investigation of membrane performance affected by Mg²⁺. The rejection behavior of the tight NF90 was not significantly affected by Mg²⁺ due to the predominant role of size exclusion. For loose NF270, HAA rejection and water flux were reduced by 27% and 12%, respectively, after the addition of 5 mM Mg²⁺. The notable magnesium content in NF270 exposed in Mg²⁺ indicated the binding of Mg²⁺ to membranes. The interaction between Mg²⁺ and membranes resulted in a less negatively charged and hydrophilic surface and a smaller effective pore radius for membranes. The reduction of the effective pore size by Mg²⁺ was verified as a result of competing effects between “-COO^–-induced conformational change” and “cation-induced pore shielding”. The increased permeability for charged HAAs and NaCl in the presence of Mg²⁺ was mainly attributed to charge neutralization, while the reduced permeability for neutral surrogates and water was due to the reduced effective pore size and hydrophilicity. The underlying mechanism for resolving the interaction among different divalent cations, trace contaminants, and polyamide membranes may have significant implications for practical water purification by membrane technologies.