Dominant Mechanisms of Uranium(VI)-phosphate Interactions in Subsurface Environments PDF Download

Author: Vrajesh Sanat Mehta
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 191

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Book Description
Anthropogenic activities associated with the production of nuclear materials have resulted in uranium contaminated soil and groundwater. The carcinogenic and toxic effects of uranium contamination pose a significant risk to the environment and human health. Phosphate addition to uranium-contaminated subsurface environments has been proposed as a strategy for in situ remediation. Addition of phosphate amendments can result in uranium sequestration in its oxidized +VI state without sustaining reducing conditions as is needed for in situ immobilization via chemical or biological reduction of U(VI) to less soluble U(IV) species. Phosphate addition can be used as a stand-alone process or as a complementary process to bioremediation-based methods, especially for sites with naturally oxic conditions. Although recent studies have reported phosphate-induced precipitation of U(VI)-phosphates in laboratory and field-scale tests, the fundamental mechanisms controlling U(VI) immobilization are not well known. Hence understanding the mechanisms at the microscopic and molecular levels is imperative to successfully designing and implementing phosphate-based in situ uranium immobilization. Interactions with phosphate can result in uranium immobilization through various processes. This study investigated the dominant mechanisms of U(VI)-phosphate reactions using an integrated approach of aqueous phase and solid phase characterization techniques. Batch experiments were performed to study the effect of pH and co-solutes (dissolved inorganic carbon (DIC), Na+ and Ca2+) on the products and solubility of uranium(VI) precipitated with phosphate. The results suggested that in the absence of co-solute cations, chernikovite [H3O(UO2)(PO4)·3H2O] precipitated despite uranyl orthophosphate [(UO2)3(PO4)2·4H2O] being thermodynamically more favorable under certain conditions. The presence of Na+ as a co-solute led to the precipitation of sodium autunite [Na2(UO2)2(PO4)2], and the dissolved U(VI) concentrations were generally in agreement with equilibrium predictions of sodium autunite solubility. In the calcium-containing systems, the observed concentrations were below the predicted solubility of autunite [Ca(UO2)2(PO4)2]. Consequently, specific batch studies were conducted to investigate the dependence of U(VI) uptake mechanisms on the starting forms of calcium and phosphate at concentrations relevant to field sites. Depending on the experimental conditions, uranium uptake occurred through adsorption on calcium-phosphate solids, precipitation of autunite, or incorporation into a calcium-phosphate solid. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis using structural model fittings and linear combination fitting allowed quantification of the contribution of each uranium uptake mechanism mentioned above. Following the batch experiments with simple systems, the effect of phosphate amendment on uranium immobilization was evaluated for sediments obtained from a field site in Rifle, Colorado using batch sorption studies and column experiments. Batch sorption studies showed that phosphate addition increased the U(VI) adsorption, however the net uranium uptake was limited due to the dominance of the aqueous speciation by Ca-U(VI)-carbonate complexes. Column experiments were performed under conditions that simulated the subsurface environment at the Rifle site. Remobilization experiments showed increased retention of uranium when phosphate was present in uranium-free influent. The response of dissolved uranium concentrations to stopped-flow events and the comparison of experimental data with a simple reactive transport model indicated that uranium transport was controlled by non-equilibrium processes. Intraparticle diffusion is thought to be acting as the rate-limiting process. Sequential extractions and laser induced fluorescence spectroscopy (LIFS) analysis indicated that adsorption was the dominant mode of uranium immobilization. When uranium and phosphate were added concurrently to columns packed with sediments, significant uptake of uranium continued as long as phosphate was present in the influent. Even when phosphate was removed from the influent, the columns retained significant amounts (~ 67 %) of the accumulated uranium. Sequential extractions showed that the uranium accumulated transformed into less easily extractable (i.e., more immobile) species with the relative amounts of accumulated uranium extracted in the acetic acid and hot acid digestion step being highest for the column that was treated with phosphate for the longest duration. The uranium retained in the sediments after the phosphate was removed from the influent was primarily in a form that could be extracted with acetic acid and ammonium acetate. The extraction results, aqueous phase analysis and LIFS analysis showed that uranium uptake occurred through multiple processes. For select conditions, EXAFS analysis was used to quantify the contribution of uranium uptake which confirmed that uranium uptake occurred through a combination of precipitation and adsorption. The information gained from this research project improved our understanding of U(VI)-phosphate reactions that can be used to identify and manipulate the conditions that lead to the greatest decreases in U(VI) mobility. The results illustrate that precipitation of uranyl-phosphates is not the only means of in situ uranium remediation and that a wide range of uranium immobilization mechanisms can control uranium mobility following phosphate addition. Although phosphate addition led to significant retardation of uranium release and also resulted in increased net uptake of uranium for conditions of the Rifle site, phosphate amendments could be more beneficial at sites with lower pH and dissolved inorganic carbon concentrations.