SURFACE COMPLEXATION OF URANIUM(VI) IN SUBOXIC AQUIFERS IN THE PRESENCE OF URANIUM(IV) MINERALS

The mobility of naturally occurring U in aquifers is influenced by redox and sediment adsorption. Uranium exists primarily in two oxidation states in the environment, the mostly soluble oxidized U(VI), and mostly insoluble reduced U(IV). In natural systems microorganisms can rapidly scavenge oxidants driving reducing conditions resulting in the formation of sediment U in both the reduced and oxidized state. Oxidation of U(IV) species upon exposure to molecular oxygen or another oxidant can lead to the formation of U(VI) thus creating experimental artifacts. To date, previous studies have effectively quantified U(VI) sorption under oxic conditions and described the data using Surface Complexation Modeling (SCM). Here we present a method for SCM equilibrium adsorption experiments that accounts for differential U(VI) adsorption where U(IV) exists in sediment. Two intact sediment cores were collected where groundwater (GW) was oxic (Eh > +250 mV) and one core from where groundwater was suboxic (Eh < -350mV). Total sediment U measured by gamma spectrometry varied between 0.8 to 3.36mg kg^-1. X-ray absorption near edge spectroscopy (XANES) indicated sediment U in the oxic cores was more oxidized (85% ± 10%) than sediment samples from the suboxic core (50% ± 10%). Desorption of U(VI) by weak bicarbonate extraction indicated U(VI) adsorbed to sediment ranged from 11.3 to 158.8ug kg^-1. A series of equilibrium adsorption experiments revealed that oxidation of reduced U could influence adsorption. In suboxic samples (GW <0.05mg L^-1 Dissolved Oxygen (DO)), up to 365.5ug kg^-1 of U from sediment was released during equilibrium experiments versus anoxic controls. In an oxic sediment sample (GW DO >7.0mg L^-1), no increase in U(VI) was observed. Models generated using UCODE coupled to PHREEQC for equilibrium constant (Log K^c) optimization revealed that suboxic sediment samples fit with data generated under anoxic conditions; whereas, data generated from an oxic aquifer fit with data generated under oxic conditions. These results indicate that sediment redox influences the experimental outcome of equilibrium adsorption experiments conducted under oxic conditions. Thus where reduced U(IV) exists in sediment, utilization of an anoxic method to account for adsorption to natural sediments should be used.