Influence of dissolved organic matter on U(VI) removal at varying hydrogeochemical scenarios in ultrafiltration process

Abstract

Uranium (U) is a chemical and radioactive groundwater contaminant that needs to be regulated in drinking water to avoid health hazards. In this study, we have investigated the mechanisms underlying ultrafiltration (UF) process, a low energy-intensive technology, for the removal of uranium from contaminated groundwater, in the presence of dissolved organic matter (i.e. Humic Acid [HA]) under environmentally-relevant conditions representative of regional scenario of Punjab, India. Stirred cell UF experiments with aqueous solutions containing uranium were performed with five different UF membranes with molecular weight cut off (MWCO) ranging from 1 kDa to 30 kDa at a pH of 8.5 that represented ambient groundwater scenario. In the absence of HA, the U(VI) removal was highest for the UF membrane with the lowest MWCO (i.e. 1 kDa) and vice-versa. Further, the effect of various solution parameters viz. pH, concentration of HA, and salinity have been studied using three different UF membranes viz. 5 kDa, 10 kDa, and 30 kDa. Uranium rejection was found to be maximum at pH 5.5 with ca. 97 %, 94 %, and 87 % rejection for 5 kDa, 10 kDa, and 30 kDa membranes, respectively. Further, U(VI) speciation results of the hydrogeochemical model corroborated that the removal of U(VI) in the presence of HA was highly dependent on feed solution pH. Moreover, U(VI) removal increased significantly with an increase in HA concentration, indicating the dominant role of U(VI)-HA complexes. Further, it was observed that increasing the salinity levels to 100 mM in the feed solution (i.e. semi-brackish water scenario) decreased U(VI) rejection primarily due to the charge screening effect. Our results show that using the UF separation process, the World Health Organization’s drinking water guideline value of 30 µg of U L⁻¹ could be achieved in U(VI) contaminated groundwater that contains significant HA levels.