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

Abstract

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 (CDE) was used to analyze the tracer and uranium breakthrough curves resulting from the column experiments. The program CXTFIT was used to estimate the transport parameters of equilibrium and nonequilibrium (i.e., two-site and mobile-immobile) models applied to the experimental data. Comparison of U(VI) breakthrough behavior at pH 6 with that of a nonreactive tracer indicated that U(VI) transport was significantly retarded, and about 52 % of the added U(VI) adsorbed to the quartz sand, likely in the cationic forms UO2OH+ and UO 2 2 + . The adsorption was reversible upon the addition of deionized water. At pH 11, the U(VI) breakthrough curve increased gradually and reached a plateau value C/C 0 oscillating between 72 and 82 %. Upon reaction, Si was released from the dissolution of quartz sand, which allowed the possible transport of U(VI) following precipitation of a U(VI) containing solid, such as uranyl-silicate minerals, or sorption of U(VI) onto silica colloids. Two-site and mobile-immobile (MIM) models suggested an influence of either rate-limited mass transfer processes or immobile/mobile water partitioning in U(VI) reactive transport. The reactive transport of U(VI) governed by adsorption-desorption processes, precipitation, and complexation reactions in which kinetic behaviors are controlled by pH, solution chemistry, and heterogeneous flow regime impacts the mobility of U(VI). The column transport experiments indicated that under geochemical conditions and vadoze zone processes (preferential flow) that favor the mobility of U(VI), dissolved- and colloidal-phase associations of U(VI) may be transported rapidly and in high concentrations from the soil surface to the groundwater.