Surface area effects on the reduction of UVI in the presence of synthetic montmorillonite

Abstract

The redox transformations that affect the environmental mobility of metal or radionuclide contaminants typically take place in the presence of mineral or biological surfaces. Adsorption can alter the speciation and free energy of a dissolved ion and can thus change the occurrence, rate, or products of redox reactions relative to those expected in homogenous solution. Here, we investigated the effect of SYn-1, a redox-inactive synthetic montomorillonite clay mineral, on the reduction of UVI by dihydroanthraquinone-2,6 disulfonate (AH2QDS, the reduced form of the soluble electron shuttle AQDS). We varied the surface:U ratio in a circumneutral bicarbonate solution and measured the valence and atomic coordination of U in the solids using X-ray absorption spectroscopy (XANES and EXAFS). The spectra show that UIV was the predominant product both in the presence or absence of the clay mineral, indicating that adsorption of UVI to clay mineral surface sites does not affect its ability to be reduced by AH2QDS. In the absence of the clay mineral and at low-to-intermediate surface:U ratios the predominant UIV product was nanoparticulate uraninite, UO2. At the highest surface:U ratio tested (100g/L clay mineral and 50μMU) we observe a decrease in the proportion of uraninite down to 50%, with the remaining UIV present as adsorbed, non-uraninite species. These results are similar to previous findings with magnetite and rutile, where adsorbed UIV were the predominant species below a specific surface coverage and U in excess of this coverage was precipitated as uraninite. The threshold coverage determined here for the SYn-1 clay mineral is 10-100× lower than that determined for magnetite and rutile, suggesting that clay mineral surfaces may be less important than metal oxides for stabilizing non-uraninite UIV species in natural sediments.