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Recent Discoveries and the Ultimate Fate of Organic Contaminants
Citation:
WILSON, J. T., C. J. ADAIR, M. Ferrey, AND Y. T. He. Recent Discoveries and the Ultimate Fate of Organic Contaminants. Presented at The REMTEC Conference, Atlanta, GA, March 03 - 05, 2009.
Impact/Purpose:
To show that from an engineering design perspective, degradation of chlorinated solvents such as PCE and TCE through these abiotic reactions is beneficial because the minerals, once formed, tend to be stable and persist through time. In contrast, biotransformation processes are limited by the supply of substrate, and cease when the supply of substrate is exhausted.
Description:
With very few exceptions, the common organic contaminants in soils, sediments, and ground water can be transformed or entirely degraded by oxidation or reduction reactions that are either carried through direct involvement with microorganisms, or indirectly through abiotic reactions with the metabolic end products of microbial activity. Often contaminated ground water is anaerobic, either as a consequence of the microbial metabolism of organic contaminants such as the BTEX compounds in petroleum, or through the metabolism of substrates intentionally added to ground water as part of active in situ bioremediation of chlorinated solvents such as TCE. In either case, organic compounds are available to support iron or sulfate reduction. Biological iron reduction can produce magnetite, and concomitant iron reduction and sulfate reduction can produce iron (II) sulfide, pyrite or sulfate green rust. These minerals may accumulate in aquifer sediments as a consequence of biological degradation of organic substrates. In some aquifers, magnetite is a natural component of the sediment. From a risk management perspective, degradation of chlorinated solvents such as PCE and TCE through abiotic reactions with magnetite, iron (II) sulfide, pyrite or sulfate green rust is beneficial because a major fraction of the contaminate is destroyed through a pathway that leads through acetylene (which is benign), and only a minor fraction is transformed through a pathway that leads through cis-DCE to vinyl chloride (which is just as hazardous as the original contaminants). From an engineering design perspective, degradation of chlorinated solvents such as PCE and TCE through these abiotic reactions is beneficial because the minerals, once formed, tend to be stable and persist through time. In contrast, biotransformation processes are limited by the supply of substrate, and cease when the supply of substrate is exhausted. Concentrations of natural magnetic minerals in aquifer sediment with a magnetic susceptibility of 1 x 10-6 m3 kg-1 (on the order of 10,000 mg/kg magnetic minerals such as magnetite) can sustain pseudo first order rate constants for abiotic transformation of TCE or cis-DCE that are near 1.0 per year. Similarly, concentrations of iron (II) sulfide near 10mg/kg to 30 mg/kg can rate for abiotic transformation of TCE or cis-DCE that are near1.0 per year.