ARTICLE pubs.acs.org/est
Sequential Reductive and Oxidative Biodegradation of Chloroethenes Stimulated in a Coupled Bioelectro-Process Svenja T. Lohner,†,§ Dirk Becker,‡ Klaus-Michael Mangold,‡ and Andreas Tiehm†,* † ‡
Water Technology Center, Department of Environmental Biotechnology, Karlsruher Strasse 84, 76139 Karlsruhe, Germany DECHEMA e.V., Karl-Winnacker-Institut, Electrochemistry Group, Theodor-Heuss-Allee 25, 60486 Frankfurt, Germany
bS Supporting Information ABSTRACT: This article for the first time demonstrates successful application of electrochemical processes to stimulate sequential reductive/oxidative microbial degradation of perchloroethene (PCE) in mineral medium and in contaminated groundwater. In a flow-through column system, hydrogen generation at the cathode supported reductive dechlorination of PCE to cis-dichloroethene (cDCE), vinyl chloride (VC), and ethene (ETH). Electrolytically generated oxygen at the anode allowed subsequent oxidative degradation of the lower chlorinated metabolites. Aerobic cometabolic degradation of cDCE proved to be the bottleneck for complete metabolite elimination. Total removal of chloroethenes was demonstrated for a PCE load of approximately 1.5 μmol/d. In mineral medium, long-term operation with stainless steel electrodes was demonstrated for more than 300 days. In contaminated groundwater, corrosion of the stainless steel anode occurred, whereas DSA (dimensionally stable anodes) proved to be stable. Precipitation of calcareous deposits was observed at the cathode, resulting in a higher voltage demand and reduced dechlorination activity. With DSA and groundwater from a contaminated site, complete degradation of chloroethenes in groundwater was obtained for two months thus demonstrating the feasibility of the sequential bioelectro-approach for field application.
’ INTRODUCTION In situ bioremediation is to date the most efficient strategy to transform groundwater contaminants such as perchloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cDCE), and vinyl chloride (VC) into harmless end products. Dehalococcoides species are capable of reductively dechlorinating PCE via TCE, cDCE, and VC to ethene,1,2 and other species can utilize cDCE and VC in aerobic oxidative degradation pathways.39 During oxidative degradation, oxygen is needed as an electron acceptor whereas reductive dechlorination requires electrons that have to be provided by an electron donor such as hydrogen. Specific stimulation of reductive dechlorination is challenging due to competing hydrogen consuming processes such as sulfate reduction or methanogenesis. Hence, it is desirable to use an electron donor that releases hydrogen with constant rates at low concentration as it has been shown that the hydrogen threshold for reductive dechlorination is much lower compared to competing reactions.10,11 Most current engineered bioremediation concepts involve the subsurface injection of an easily fermentable organic substrate or hydrogen releasing compounds (HRC) that provide hydrogen in excess stoichiometric amounts to achieve complete reductive dechlorination.12,13 However, the concept of a sequential reductive and oxidative degradation represents an attractive alternative because PCE and TCE are preferably r 2011 American Chemical Society
transformed by reductive dechlorination, whereas oxidative degradation for their less chlorinated metabolites, cDCE and VC, has been demonstrated.14,15 The implementation of a subsequent oxidative step could overcome the often observed accumulation of less chlorinated metabolites such as VC at contaminated sites, as transformation of cDCE or VC has been shown to be the rate-limiting step in reductive dechlorination.2 The coupling of electrochemical or electrokinetic processes to biological metabolism offers the possibility to significantly stimulate microbial pollutant degradation. Interest in bioelectricsystems has been growing over the last years.16,17 Some recent examples are the stimulation of reductive dechlorination of chloroethenes18,19 and perchlorate20 in bioelectrical reactors operated with synthetic mineral medium. Also, electrolytic methanogenic methanotrophic coupling for cometabolic aerobic PCE bioremediation has been shown.21 Recently, we described the use of water electrolysis to produce hydrogen and oxygen for stimulation of microbial reductive PCE dechlorination and oxidative VC biodegradation in two soil Received: July 28, 2010 Accepted: June 16, 2011 Revised: June 16, 2011 Published: June 16, 2011 6491
dx.doi.org/10.1021/es200801r | Environ. Sci. Technol. 2011, 45, 6491–6497
Environmental Science & Technology
ARTICLE
Table 1. Mineral Medium and Groundwater Composition from Different Contaminated Field Sites (KarlsruheKillisfeld, Frankenthal and Lahr)a parameter
Figure 1. Conceptual model of the sequential bioelectro-remediation of chlorinated ethenes. Reductive microbial dechlorination is stimulated by electrochemical hydrogen production at the cathode and subsequent aerobic biodegradation of the lower chlorinated metabolites with oxygen produced at the anode.
columns hydraulically separated by a bipolar membrane and operated with synthetic mineral medium.22 This bioelectroprocess was demonstrated to be controllable by adjusting the electric current, which was shown to correlate with PCE dechlorination activity and aerobic VC degradation, respectively. The goal of this research was to apply the bioelectro-process to stimulate sequential reductive and oxidative degradation of chloroethenes according to the schematic diagram in Figure 1. Without a hydraulic barrier between the two columns, the cathode electrolysis products and metabolites formed by reductive PCE dechlorination were allowed to enter the anode column for subsequent aerobic degradation. Effective application of this sequential concept would significantly contribute to alleviate the problematic accumulation of cDCE and VC in the field. The columns were operated with (i) mineral medium and (ii) groundwater from a contaminated site containing approximately 80 mg/L chloride (Table 1). The successful demonstration of electrolytically stimulated biodegradation of chloroethenes in real groundwater, as presented in this study for the first time, shows that this approach has potential for field application.
’ MATERIALS AND METHODS Sequential Column Setup and Operation. For the bioelectro-experiments, a two column setup was used as previously described.22 Briefly, two 20 cm columns were connected by a glass tube and filled with model soil (DORSILIT 9S silica sand). The anode and cathode were inserted into the corresponding columns and consisted of 10 cm2 stainless steel (16% Cr, 10% Ni) meshes. For experiments in groundwater, in addition dimensionally stable anodes (DSA), titanium electrodes with mixed oxide coating (De Nora, coating type DN201, Rodenbach, Germany), were used. Current densities varying from 0.03 to 0.05 mA/cm2 were applied throughout the experiments. Oxygenfree mineral medium or groundwater with spiked PCE was pumped through the columns. In the experiment with mineral medium, the flow rate and influent concentration were varied between 0.24 and 0.5 L/d and 6.51.0 mg/L respectively to investigate the dependence of PCE load (approximately 201.5 μmol/d) on the degradation efficiency. Samples were taken
MM GW(K) GW(F) GW(L)
Perchloroethene (spiked)
16.5 0.53
Conductivity (25 C°)
8040
1240
Unit μg/L
1520
738.0
μS/cm
pH
7.2
7.1
7.1
7.1
Acid capacity up to pH 4.2
n.d
10.0
6.6
6.1
mmol/L
Hardness Calcium
0.7 1.0
5.2 168
5.8 175
3.1 103
mmol/L mg/L
Magnesium
16.6
24.1
33.6
12.1
mg/L
Ammonium
61.4
8.9
0.18
0.21
mg/L
Iron
0.2
3.9
0.21
4.34
mg/L
Manganese
0.02
0.7
0.8
0.3
mg/L
Chloride
0.9
83.5
117
41.9
mg/L
Nitrate
