Uranium rejection with nanofiltration membranes and the influence of environmentally relevant mono- and divalent cations at various pH†

Nanofiltration (NF) can be used as a low-energy pressure-driven membrane treatment process with potential applications in mitigating uranium contamination from groundwater. Uranium can interact with groundwater minerals which can influence NF uranium rejection. This study used two commercially available membranes (NF90 and NF270) to remove uranyl complexes in the presence of environmentally relevant cations (Na⁺, Mg²⁺, and Ca²⁺). The analysis includes extensive membrane characterization, calculating NF treatment performance, investigating uranium adsorption to the functionalized polyamide top layer of the membrane, and determining membrane selectivity. Under batch experiments, using environmentally relevant ion concentrations, we measured uranium rejection rates for the NF90 between 58–99% and NF270 between 4–98%. The mechanisms of low uranium rejection are not only explained by steric hindrance but also by the reduction of the Donnan exclusion mechanism, which originates from the decrease in membrane charge density caused by the addition of mono- and divalent ions. Additionally, exclusion mechanisms were observed to be directly influenced by solution pH, which governs the variation in uranyl complexation type and membrane charge. Calcium has a complexation affinity to uranium with broad implications in uranyl-complex molecular weight, valance, and molecular shape, all of which can influence water treatment efficiency. Lastly, both membranes were evaluated based on their membrane selectivity, the ratio of cation fluxes to uranium(VI) ion flux. Ideal membrane selectivity occurred at pH 7. Na⁺ to uranium(VI) ion ratio was 190 for NF90 and 100 for NF270. The results of this study advance the understanding of using NF membranes for groundwater uranium removal.