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Open Access Research

Role of bicarbonate as a pH buffer and electron sink in microbial dechlorination of chloroethenes

Anca G Delgado12, Prathap Parameswaran1, Devyn Fajardo-Williams1, Rolf U Halden13 and Rosa Krajmalnik-Brown13*

Author Affiliations

1 Swette Center for Environmental Biotechnology, Biodesign Institute, Arizona State University, PO Box 875001, Tempe, AZ 85287-5701, USA

2 School of Life Sciences, Arizona State University, Tempe, USA

3 Ira A Fulton Schools of Engineering, Arizona State University, Tempe, USA

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Microbial Cell Factories 2012, 11:128  doi:10.1186/1475-2859-11-128

Published: 13 September 2012

Abstract

Background

Buffering to achieve pH control is crucial for successful trichloroethene (TCE) anaerobic bioremediation. Bicarbonate (HCO3) is the natural buffer in groundwater and the buffer of choice in the laboratory and at contaminated sites undergoing biological treatment with organohalide respiring microorganisms. However, HCO3 also serves as the electron acceptor for hydrogenotrophic methanogens and hydrogenotrophic homoacetogens, two microbial groups competing with organohalide respirers for hydrogen (H2). We studied the effect of HCO3 as a buffering agent and the effect of HCO3-consuming reactions in a range of concentrations (2.5-30 mM) with an initial pH of 7.5 in H2-fed TCE reductively dechlorinating communities containing Dehalococcoides, hydrogenotrophic methanogens, and hydrogenotrophic homoacetogens.

Results

Rate differences in TCE dechlorination were observed as a result of added varying HCO3 concentrations due to H2-fed electrons channeled towards methanogenesis and homoacetogenesis and pH increases (up to 8.7) from biological HCO3 consumption. Significantly faster dechlorination rates were noted at all HCO3 concentrations tested when the pH buffering was improved by providing 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as an additional buffer. Electron balances and quantitative PCR revealed that methanogenesis was the main electron sink when the initial HCO3 concentrations were 2.5 and 5 mM, while homoacetogenesis was the dominant process and sink when 10 and 30 mM HCO3 were provided initially.

Conclusions

Our study reveals that HCO3 is an important variable for bioremediation of chloroethenes as it has a prominent role as an electron acceptor for methanogenesis and homoacetogenesis. It also illustrates the changes in rates and extent of reductive dechlorination resulting from the combined effect of electron donor competition stimulated by HCO3 and the changes in pH exerted by methanogens and homoacetogens.

Keywords:
Acetogen; Alkalinity; Bicarbonate competition; Dehalococcoides; pH range; Trichloroethylene