Physical–Chemical Treatment of Groundwater Contaminated by Leachate from Surface Disposal of Uranium Tailings

Leaching of trace elements including heavy metals, nonmetals, and radionuclides from surface impoundments of tailings generated during uranium mining and milling often leads to groundwater contamination. This paper reports results from coagulation and membrane filtration experiments designed to evaluate the capabilities of these processes to remove molybdenum, selenium, uranium, radium, thorium, and other mono- and divalent ions from contaminated groundwater in a shallow unconfined aquifer influenced by uranium tailings. A 10 mg ${\mathrm{Fe}}^{3+}/L$ dose at pH 4 and 10 was found to be very effective for removing radium and thorium that were associated with particles in the raw water. Molybdenum and uranium removals by coagulation are consistent with surface complexation and electrostatic interactions between major coagulant and contaminant complexes in solution. Poor selenium removal suggests that selenate [Se(VI)] was the dominant species in the surficial groundwater. Permeation coefficients (intrinsic transport parameters) for dissolved ions and complexes across three new generation thin-film composite nanofiltration and reverse osmosis membranes evaluated obeyed Gaussian distributions with ionic charge having a mean value of zero. Thus, dissolved solute rejection from brackish multicomponent solutions appears to be only a function of the magnitude of ionic charge and not its sign (positive or negative). Solute permeation coefficients decreased in a power-law fashion with increasing product of molecular weight and absolute ion charge suggesting that both properties determine their removal by nanofiltration and reverse osmosis membranes. Because nanofiltration and reverse osmosis were found to be highly effective for removing a variety of ionic solutes they can be employed in pump and treat operations for groundwater purification prior to reinjection.