Formatted Title
Microbial Chain Elongation Supports Reductive Dechlorination of Chlorinated Ethenes: Outlook from a Study at a Superfund Site
Background/Objectives
Chlorinated solvents like trichloroethene (TCE) are legacy and toxic contaminants in groundwater. These chlorinated solvents are frequently treated via in situ reductive dechlorination, a H2-consuming metabolism for organohalide-respiring bacteria. H2 is provided by injecting fermentable substrates into the subsurface. However, the efficacy of this approach can vary depending on site-specific hydrogeology and the native microbial communities, making effective substrate delivery difficult. In situ bioremediation has been a major treatment approach for chlorinated ethenes in groundwater at a Superfund Site in the San Francisco Bay Area. While in situ bioremediation has had documented success at the Site, specific challenges have also been recorded. Specifically, substrates molasses, lactate, and emulsified vegetable oil are rapidly consumed and diverted towards non-dechlorinating, H2-consuming microbial communities, such as methanogens. Additionally, highly viscous substrates like emulsified vegetable oil and other complex commercial products have led to (bio)clogging at the Site, which further complicates effective substrate delivery. The current study details the first field-scale demonstration to use chain-elongation substrates (ethanol and acetate) to provide the H2 needed for in situ reductive dechlorination at a Superfund Site. Chain-elongation is a H2-producing microbial metabolism where short-chain carboxylates (e.g., acetate) are elongated to medium-chain carboxylates (e.g., butyrate) using reduced organic compounds (e.g., ethanol). Chain-elongation has been demonstrated as an effective approach to support complete reductive dechlorination of TCE in microcosm studies by using low-viscosity substrates ethanol and acetate while demonstrating potential to inhibit methanogenesis.
Approach/Activities
Groundwater push-pull tests were conducted at the study Site where chlorinated ethenes, including TCE, cis-1,2-dichloroethene, and vinyl chloride, are the major constituents of concern. Tests were divided into two stages: Stage 1, 3 weeks to assess biostimulation using chain-elongation substrates (acetate and ethanol), and Stage 2, 5 weeks to assess bioaugmentation using chain-elongation substrates and a chain-elongating culture (MAT-1) from Arizona State University. Bromide was used as a tracer to understand substrate delivery and the extent of dilution in groundwater. Concentrations of chlorinated ethenes, ethene/ethane, and methane were measured, as well as substrates ethanol and acetate and potential chain-elongation and fermentation products (C1-C6 carboxylates, butanol, and hexanol). Quantitative polymerase chain reaction was performed to quantify Dehalococcoides mccartyi, the model organism for complete reductive dechlorination, and methanogens in groundwater. The microbial community composition was characterized via high-throughput amplicon sequencing of chromosomal DNA.
Results/Lessons Learned
Addition of substrates acetate and ethanol and bioaugmentation with culture MAT-1 effectively stimulated reductive dechlorination of chlorinated ethenes to non-toxic ethene and ethane (Stage 2). It was concluded that bioaugmentation with MAT-1 is required to foster chain-elongation and subsequent reductive dechlorination in Site groundwater as chain elongation activity was minimal in Stage 1 when only biostimulation was employed. Substrates ethanol and acetate did not contribute to (bio)clogging of injection wells and the local subsurface. Bioaugmentation positively redirected the Site groundwater to chain-elongate acetate to butyrate and trace amounts of isovalerate for long-term H2 release. Propionate was also a major product during Stage 1 and 2, likely from conversion of ethanol, acetate, and CO2 via the succinate pathway and/or from reduction of acetate and/or CO2. Propionate is known to drive reductive dechlorination. Importantly, methanogenesis was not majorly stimulated, an outcome in line with a lab treatability study performed before the field study. Moving forward, greater substrate and culture loading is recommended for full-scale implementation at the Site due the site-specific hydrogeology and microbial community.