Uranium removal from mine drainage with zero-valent iron

Sammanfattning

Zero-valent iron is a widely used material for removal of heavy metals and other pollutants fromwater. This study evaluates different zero-valent iron materials and their ability to reduce Uconcentrations to less than 10 µg/L, a concentration that can be related to an existing dischargerequirement for a mine in Sweden.An experimental setup consisting of columns filled with different zero-valent iron materials wasused to study if it was possible to achieve a reduction in uranium concentrations. Waste rockleachate from LKAB’s Kiruna mine, with approximately 100 µg/L uranium, was pumped intoeach column in an upward flow direction. In total, ten columns were used in the study, withseven containing different types and amounts of zero-valent iron. The zero-valent iron columnsenabled an evaluation of the effect of different hydraulic residence times on uranium removaland loading capacity, defined as the maximum amount of uranium per mass zero-valent iron thatcan be removed before the zero-valent iron material needs to be changed.The study found that all zero-valent iron materials initially achieved a uranium reduction toconcentrations less than 0.02 µg/L. However, five of the columns which contained a smalleramount of zero-valent iron exhibited uranium breakthrough after one to five weeks depending onthe material. The lowest hydraulic residence time tested was 3 minutes which also gave the resultof concentrations under 0.02 µg/L. While it was not possible to determine the total loadingcapacity of the different iron materials over the period of this study, minimum loading capacitiesof 0.15 to 0.25 mg U / g zero-valent iron material were achieved over a six-week period. At the highest tested hydraulic residence time tested of around 20 hours, depending on thecolumn, nitrate reduction occurred and produced ammonium, where the ammonium-nitrogenconcentration in those zero-valent iron columns was equal to the inlet concentration of nitratenitrogen. The pH was also initially high in these zero-valent iron columns, between 11 – 12, butdecreased over time to a pH just under 9.This study shows that the application of zero-valent iron in the field to remove uranium for minedrainage should be possible. It needs to be considered which zero-valent iron material to usebased on grain-size, surface area and loading capacity.