Decoupling Fe ⁰ Application and Bioaugmentation in Space and Time Enables Microbial Reductive Dechlorination of Trichloroethene to Ethene: Evidence from Soil Columns

Abstract

Fe ⁰ is a powerful chemical reductant with applications for remediation of chlorinated solvents, including tetrachloroethene and trichloroethene. Its utilization efficiency at contaminated sites is limited because most of the electrons from Fe ⁰ are channeled to the reduction of water to H ₂ rather than to the reduction of the contaminants. Coupling Fe ⁰ with H ₂-utilizing organohalide-respiring bacteria (i.e., Dehalococcoides mccartyi ) could enhance trichloroethene conversion to ethene while maximizing Fe ⁰ utilization efficiency. Columns packed with aquifer materials have been used to assess the efficacy of a treatment combining in space and time Fe ⁰ and a D. mccartyi-containing culture (bioaugmentation). To date, most column studies documented only partial conversion of the solvents to chlorinated byproducts, calling into question the feasibility of Fe ⁰ to promote complete microbial reductive dechlorination. In this study, we decoupled the application of Fe ⁰ in space and time from the addition of organic substrates and D. mccartyi-containing cultures. We used a column containing soil and Fe ⁰ (at 15 g L ^–1 in porewater) and fed it with groundwater as a proxy for an upstream Fe ⁰ injection zone dominated by abiotic reactions and biostimulated/bioaugmented soil columns (Bio-columns) as proxies for downstream microbiological zones. Results showed that Bio-columns receiving reduced groundwater from the Fe ⁰-column supported microbial reductive dechlorination, yielding up to 98% trichloroethene conversion to ethene. The microbial community in the Bio-columns established with Fe ⁰-reduced groundwater also sustained trichloroethene reduction to ethene (up to 100%) when challenged with aerobic groundwater. This study supports a conceptual model where decoupling the application of Fe ⁰ and biostimulation/bioaugmentation in space and/or time could augment microbial trichloroethene reductive dechlorination, particularly under oxic conditions.

Abstract

Decoupling the application of Fe ⁰ and bioaugmentation in space and/or time augments microbial reductive dechlorination and overall treatment of chlorinated ethenes in groundwater.