IN-SITU CHEMICAL STABILIZATION OF METALS AND RADIONUCLIDES THROUGH ENHANCED ANAEROBIC REDUCTIVE PRECIPITATION

摘要

The objective of this NETL sponsored bench-scale study was to demonstrate the efficacy of enhanced anaerobic reductive precipitation (EARP) technology for precipitating uranium using samples from contaminated groundwater at the Fernald Closure Project (FCP) in Cincinnati, Ohio. EARP enhances the natural biological reactions in the groundwater through addition of food grade substrates (typically molasses) to drive the oxidative-reductive potential of the groundwater to a lower, more reduced state, thereby precipitating uranium from solution. In order for this in-situ technology to be successful in the long term, the precipitated uranium must not be re-dissolved at an unacceptable rate once groundwater geochemical conditions return to their pretreatment, aerobic state. The approach for this study is based on the premise that redissolution of precipitated uranium will be slowed by several mechanisms including the presence of iron sulfide precipitates and coatings, and sorption onto fresh iron oxides. A bench-scale study of the technology was performed using columns packed with site soil and subjected to a continuous flow of uranium-contaminated site groundwater (476 {micro}g/L). The ''treated'' column received a steady stream of dilute food grade molasses injected into the contaminated influent. Upon attainment of a consistently reducing environment and demonstrated removal of uranium, an iron sulfate amendment was added along with the molasses in the influent solution. After a month long period of iron addition, the treatments were halted, and uncontaminated, aerobic, unamended water was introduced to the treated column to assess rebound of uranium concentrations. In the first two months of treatment, the uranium concentration in the treated column decreased to the clean-up level (30 {micro}g/L) or below, and remained there for the remainder of the treatment period. A brief period of resolubilization of uranium was observed as the treated column returned to aerobic conditions, but the concentration later returned to below the clean-up level. Speciation analysis was conducted on soil collected from the treated column after rebound testing. The experimental results show that: (a) The mass of uranium resolubilized in more than four months of column testing was much lower than the amount precipitated. (b) The majority of the uranium was precipitated in the first few inches of the treated column. The majority of the uranium precipitated was associated with iron oxides or in other immobile/sequestered phases. It is important to contrast this result with the results reported by Bryan (2003) who shows that most of the uranium associated with contaminated aquifer solids at Fernald under the existing natural attenuation/pump and treat with reinjection conditions is carbonate bound. Carbonate bound forms are traditionally seen as fairly mobile, but may not be under a calcite/dolomite saturated condition. Fernald is currently conducting further studies to investigate the mobility of the carbonate bound forms. (c) Though reoxidation concentrations from the bench-scale column exceeded 30 {micro}g/L for a time, they later returned to below this value. Effluent concentrations from the treated column are expected to over predict full-scale concentrations for reasons discussed in depth in the text. Finally, these results must be viewed in light of the site's ongoing pump-and-treat with reinjection system. There is reason to believe that although the pump-and-treat technology is currently effectively controlling the uranium plume and reducing the groundwater concentration, it may not be able to reach the treatment standard of 30 {micro}g/L within an economical operating lifetime and then maintain that concentration without rebound. This study suggests that Enhanced Anaerobic Reductive Precipitation can change the speciation and thus reduce the mobility of uranium at the site and expedite closure.