Flow-Electrode CDI Removes the Uncharged Ca-UO₂-CO₃ Ternary Complex from Brackish Potable Groundwater: Complex Dissociation, Transport, and Sorption

Unacceptably high uranium concentrations in decentralized and remote potable groundwater resources, especially those of high hardness (e.g ., high Ca²⁺, Mg²⁺, and CO₃^2- concentrations), are a common worldwide problem. The complexation of alkali earth metals, carbonate, and uranium(VI) results in the formation of thermodynamically stable ternary aqueous species that are predominantly neutrally charged (e.g ., Ca₂(UO₂)(CO₃)₃⁰). The removal of the uncharged (nonadsorbing) complexes is a problematic issue for many water treatment technologies. As such, we have evaluated the efficacy of a recently developed electrochemical technology, termed flow-electrode capacitive deionization (FCDI), to treat a synthetic groundwater, the composition of which is comparable to groundwater resources in the Northern Territory, Australia (and elsewhere worldwide). Theoretical calculations and time-resolved laser fluorescence spectroscopy analyses confirmed that Ca₂(UO₂)(CO₃)₃⁰ was the primary aqueous species followed by Ca(UO₂)(CO₃)₃^2- (at circumneutral pH values). Results under different operating conditions demonstrated that FCDI is versatile in reducing uranium concentrations to <10 μg L^-1 with low electrical consumption (e.g ., ∼0.1 kWh m^-3). It is concluded that the capability of FCDI to remove uranium under these common conditions depends on the dissociation kinetics of the Ca₂(UO₂)(CO₃)₃⁰ complex in the electrical field. The subsequent formation of the negatively charged Ca(UO₂)(CO₃)₃^2- species results in the efficient transport of uranium across the anion exchange membrane followed by immobilization on the positively charged flow (anode) electrode.