pH-dependent reactive transport of uranium(VI) in unsaturated sand

Uranium contamination of subsurface environments was once thought to be an isolated occurrence, mostly at production sites. But recent evidence has shown that the presence of uranium in phosphate fertilizers has caused massive amounts of this element to be released worldwide. Concerns are related to uranium movement to groundwater supplies and its significant toxicological risks to human populations. Information is direly needed on how geochemical processes control uranium transport in the vadose zone. Laboratory experiments were performed to investigate the effects of the pH of the soil solution on the reactive transport of uranium(VI) in the vadose zone. The uranium solution was prepared by dilution of a 10−3 M stock solution of uranium perchlorate, (UO2(ClO4)2), with DI water. Two U(VI) solutions were prepared at concentrations of 2×10−6 M at pH 6 and 11 and were percolated under steady-state conditions through columns filled with sand. The convective-dispersion equation was used to analyze the tracer and uranium breakthrough curves resulting from the column experiments. Results indicated that U(VI) transport was significantly retarded at pH 6, with about 52% of the added U(VI) adsorbed to the quartz sand, likely in cationic forms such as UO2OH+ and UO2^{2+}. At pH 11, the U(VI) concentration increased gradually, suggesting the influence of mass transfer processes in U(VI) reactive transport.