Review of Abiotic Degradation of Chlorinated Solvents by Reactive Iron Minerals in Aquifers

Abstract

Abiotic degradation of chlorinated solvents by reactive iron minerals such as iron sulfides, magnetite, green rust, and other Fe(II)‐containing minerals has been observed in both laboratory and field studies. These reactive iron minerals form under iron‐ and sulfate‐reducing conditions which are commonly found in permeable reactive barriers (PRBs), enhanced reductive dechlorination (ERD) treatment locations, landfills, and aquifers that are chemically reducing. The objective of this review is to synthesize current understanding of abiotic degradation of chlorinated solvents by reactive iron minerals, with special focus on how abiotic processes relate to groundwater remediation. Degradation of chlorinated solvents by reactive minerals can proceed through reductive elimination, hydrogenolysis, dehydrohalogenation, and hydrolysis reactions. Degradation products of abiotic reactions depend on degradation pathways and parent compounds. Some degradation products (e.g., acetylene) have the potential to serve as a signature product for demonstrating abiotic reactions. Laboratory and field studies show that various minerals have a range of reactivity toward chlorinated solvents. A general trend of mineral reactivity for degradation of chlorinated solvents can be approximated as follows: disordered FeS > FeS > Fe(0) > FeS2 > sorbed Fe2+ > green rust = magnetite > biotite = vermiculite. Reaction kinetics are also influenced by factors such as pH, natural organic matter (NOM), coexisting metal ions, and sulfide concentration in the system. In practice, abiotic reactions can be engineered to stimulate reactive mineral formation for groundwater remediation. Under appropriate site geochemical conditions, abiotic reactions can occur naturally, and can be incorporated into remedial strategies such as monitored natural attenuation.

Environmental implications and applications of engineered nanoscale magnetite and its hybrid nanocomposites: A review of recent literature

“…Thus, abiotic degradation by biogenic minerals may be long lasting if reducing conditions are maintained. This relationship can be used to design "in situ permeable reactive barriers" systems [398].…”

“…Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils through Fenton-like oxidation catalyzed by magnetite has been reported [398]. PAHs degradation is improved in soils pre-treated with availability-enhancement agents such as ethanol or cyclodextrin.…”

“…Another study found the rate constants for cis-DCE degradation in the order of GR > Fe(OH)2 > mackinawite = magnetite [397]. More recently, a general trend of mineral reactivity toward chlorinated solvent degradation has been approximated as follows: disordered FeS > FeS > Fe 0 > FeS2 > sorbed Fe 2+ > GR = magnetite > biotite = vermiculite [398]. Compared to Fe 0 , iron sulfides, and GR, magnetite is less reactive in dechlorination; however, magnetite is more resistant to oxidation than the other minerals and its role in dechloration can be significant if magnetite is present in large quantities in aquifers.…”

Effects of Sulfate Reduction on Trichloroethene Dechlorination by Dehalococcoides-Containing Microbial Communities

“…At remediation sites, sulfate reduction can overlap with iron reduction, which can lead to precipitated iron sulfide. The biogenic iron sulfide may reduce part of the sulfide toxicity and also perform abiotic chloroethene degradation when electron donor is in excess (43,44). The overall TCE remediation may benefit from the resulting iron sulfide formation.…”

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

“…by magnetite) initiates with a reduction and leads, after different steps including hydrolysis, to formate, CO and CO 2 as final products. CT "thiolytic reduction", after an initial reduction, produces CS 2 as final product by thiolytic substitution of dechlorinated intermediates, being mediated either by Fe(II)-sulfides or by bacteria (Field and SierraAlvarez, 2004;Penny et al, 2010;Koenig et al, 2012;He et al, 2015). CT reduction by Pseudomonas stutzeri also produces CO 2 as the final product, with transient accumulation of toxic phosgene and thiophosgene (Lewis and Crawford, 1995).…”