Insights on the Mechanism of Uranium Removal via Nanofiltration in Alkalinity and Salinity Dominated Groundwater Systems.

Groundwater quality of Punjab, India, is a matter of huge concern due to the presence of various toxic contaminants (e.g., uranium, selenium, and arsenic) from either geogenic or anthropogenic origins. In this study, we have evaluated the potential of nanofiltration for uranium removal from contaminated groundwater under environmentally relevant conditions. The influence of membrane material properties for uranium retention capabilities in two different nanofiltration membranes, viz., NF2 and NF500, was explored. Aqueous speciation modeling of uranium (U) representing the geochemical conditions of groundwater at U-contaminated region of Punjab, India, was performed to identify the dominant uranyl species. Laboratory experiments for uranium removal by both nanofiltration membranes were conducted using synthetic groundwater. The pure water flux of NF2 and NF500 was 24 ± 0.62 L·m²·h⁻¹·bar⁻¹ and 5.4 ± 0.21 L·m²·h⁻¹·bar⁻¹, respectively, wherein the former was 5 times higher than the latter despite NF2 having lesser molecular weight cutoff (MWCO). The uranium removal was highly dependent on the aqueous speciation influenced primarily by solution pH. Uranium rejection was maximum at neutral pH in both the membranes, with 87% and 78% rejection for NF2 and NF500, respectively. Uranium uptake by the membranes was highest at pH 7.0 with 77.10% and 44.6% for NF2 and NF500, respectively. Membrane transport parameters, i.e., solute flux and solute permeability, indicated the prevalence of size exclusion and/or charge-based interactions operative over a range of pHs. The uptake of uranium onto the membrane was minimal at alkaline pH with the least flux decline (ca. 6.25% in NF2) that indicated low membrane fouling. Overall, our results indicate that both World Health Organization and Atomic Energy Regulatory Board (AERB) drinking water guideline values (i.e., 30 µgL⁻¹ and 60 µgL⁻¹) could be achieved via nanofiltration technology as U-affected regions of Punjab have the ambient groundwater pH in neutral-alkaline range. [ABSTRACT FROM AUTHOR]