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FORMATION PROCESSES AND CONSEQUENCES OF REACTIVE AND NON-REACTIVE MINERAL PRECIPITATES IN PERMEABLE REACTIVE BARRIERS
Citation:
Wilkin*, R T., C. Su, AND R W. Puls*. FORMATION PROCESSES AND CONSEQUENCES OF REACTIVE AND NON-REACTIVE MINERAL PRECIPITATES IN PERMEABLE REACTIVE BARRIERS. Presented at Int'l. Symposium on Permeable Reactive Barriers, Belfast, Northern Ireland, IRELAND, March 14 - 16, 2004.
Impact/Purpose:
To inform the public.
Description:
Mineral precipitates in zero-valent iron PRBs can be classified by formation processes into three groups: 1) those that result from changes in chemical conditions (i.e., change in pH, e.g., calcite); 2) those that are a consequence of microbial activity (i.e., sulfate reduction, e.g., mackinawite); and, 3) those that are the result of iron metal instability and corrosion (e.g., magnetite). The presentation will explore these mineral formation processes and consequences with respect to the hydraulic and reactive longevity of PRB systems.
The formation of mineral precipitates in PRBs can impact system performance through time. As minerals precipitate in iron walls they occupy volume and therefore reduce porosity and permeability of the reactive zone. In this way the hydraulic performance of PRBs (e.g., residence time, capture zone) could degrade through time as the effective porosity of the iron wall approaches or exceeds that in the adjacent aquifer. For example, preferential mineral accumulation in regions of a PRB resulting from higher inputs of dissolved solutes may lead to increases in ground-water residence times. However, adjacent regions of the reactive barrier may experience greater throughput and decreased residence times, potentially leading to contaminant breakthrough. A second largely unexplored effect of mineral precipitation relates to the reactivity of materials placed in PRBs. As mineral precipitates accumulate on iron surfaces it is expected that electron transfer between metal and contaminant species, critical for the degradation of chlorinated organic compounds, becomes less efficient. Until recently, the formation of mineral precipitates has been viewed generally as a process that limits the long-term performance of reactive barriers for ground-water cleanup. Yet some corrosion products that deposit on the surfaces of iron particles may also contribute to the overall treatment effectiveness of reactive barriers for both organic and inorganic contaminants (e.g., Butler and Hayes, 2000; Lee and Batchelor, 2002; Furukawa et al., 2002). For example, iron sulfides, magnetite, and green rust minerals can chemically transform chlorinated organic compounds. Several iron- and sulfur-bearing mineral species have been investigated in laboratory experiments and reaction processes at the mineral-water interface appear to promote the rapid degradation of a variety of chlorinated compounds, including perchloroethylene (PCE) and trichloroethylene (TCE). Abiotic reaction conditions favor transformation of PCE and TCE by dichloroelimination rather than by sequential hydrogenolysis; consequently, this pathway is desirable in that the production of comparatively more toxic daughter products is circumvented.