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Evaluating the Long-Term Stability of Metals Precipitated In-Situ
Citation:
Horst, J. F., R. T. WILKIN, J. Gillow, AND S. S. Suthersan. Evaluating the Long-Term Stability of Metals Precipitated In-Situ. Presented at The 7th International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 24 - 27, 2010.
Impact/Purpose:
review the key concepts underpinning the long-term stability of metals immobilized through a variety of in-situ remedial techniques (not just sulfide precipitation), and consider methods of evaluating that stability.
Description:
Because metals (including metals and metalloids) cannot be destroyed, unlike organic contaminants, in-situ approaches for their removal from groundwater necessarily involves fixation/immobilization in the solid aquifer matrix. Consequently, the success of precipitation based in-situ treatment approaches is dependent upon the following: • Kinetics: The ability to support meaningful rates of precipitation is a minimum threshold that must be met. If the rates are too slow, the approach will have limited usefulness. • Initial Solubility: The ability to achieve the required cleanup standards for groundwater during treatment is another minimum threshold consideration. This is determined by the solubility of the precipitated solid phases as they form and mature in the treatment zone. • Durability/Permanence: The long-term stability of the precipitated solids is a critical consideration. Changes in the geochemical environment (pH, ionic strength, or stimulation of oxidation/reduction reactions) can affect this stability. Thus, the concept of “meeting the standards” with respect to in situ metals precipitation extends beyond numeric cleanup goals and invokes a standard of care that considers not only short-term solubility achieved during active remediation, but the range of factors that might erode/compromise the stability of the precipitated solids over the long-term. For example, the solubility of metals precipitated as sulfides is suppressed during treatment under anaerobic conditions with the presence of reactive sulfide. Post treatment, sulfides tend to be slightly more soluble due to the absence of reactive sulfide and potential re-oxidation in the presence of oxygen as aerobic conditions are restored. However, long-term stability may be achieved by incorporating the targeted compound in a matrix of other precipitates formed through the treatment process. In the short term, this can include the precipitates of other more abundant metals (e.g., iron) that can preferentially scavenge oxygen. Longer term, this might transition to passivation within a matrix of more stable mineral phases, such that rates of rebound dissolution are sufficiently suppressed to maintain dissolved concentrations below remedial targets. This paper will review the key concepts underpinning the long-term stability of metals immobilized through a variety of in-situ remedial techniques (not just sulfide precipitation), and consider methods of evaluating that stability. Data sets from the literature and from field sites that highlight these concepts will be presented.