Enhanced biodegradation of pentachlorophenol in unsaturated soil using reversed field electrokinetics
Introduction
Application of an electric field to soil can instigate a number of chemical processes, collectively known as electrokinetic phenomena. These include electromigration, where charged species present in solution move towards the electrode of opposite charge, and electroosmosis, where a flow of water is generated, usually towards the cathode, in soils with charged surfaces (e.g. clays). The transport of both organic and inorganic contaminants in soils through application of electrokinetic phenomena has been studied extensively (Pamukcu and Wittle, 1992, Acar and Alshawabkeh, 1993, Probstein and Hicks, 1993, Khodadoust et al., 2006) although its application in the field to date has been limited (Lageman et al., 1989). Potential difficulties in remediating soils in this way include achieving relatively long distance transport through heterogeneous material, changes in contaminant state due to soil chemistry effects and remediation of insoluble or strongly sorbed contamination. Electrokinetics has also been used to introduce materials (e.g. limiting nutrients) into soils that can facilitate biodegradation of organic contaminants (Budhu et al., 1997, Acar et al., 1997, Thevanayagam and Rishindran, 1998), move contaminants towards a microbial treatment zone (Jackman et al., 2001) or can be combined with microbial oxidation or reduction to facilitate treatment of inorganics (Maini et al., 2000). Microorganisms as well as chemicals can also be moved by these processes, particularly by electroosmosis (Deflaun and Condee, 1997, Wick et al., 2004).
Electrokinetics has the potential to enhance bioremediation of organic contaminants through control and movement of both contaminant and bacteria, facilitating greater interaction and hence contaminant bioavailability (Wick et al., 2007). Bioavailability is a critical factor in bioremediation, as the sorption, sequestration and heterogeneous distribution of chemicals can lead to their persistence within soils (Reid et al., 2000). Electrokinetics may have the ability to overcome these factors through contaminant desorption and redistribution on both a micro- and a macro-scale. However, soil properties such as pH and moisture content have a significant effect on biodegradation and contaminant behaviour, and can be rapidly altered by an applied field (Acar and Alshawabkeh, 1993). They may also affect the health of the soil microbial community and its response to contamination. An electric current in liquid culture can have a detrimental effect on cell viability at high enough levels, for example Jackman et al. (1999) found a current density of 200 A m−2 inactivated acidophilic bacteria. However, lower current densities have little effect (e.g. Wick et al., 2004 [current density of 1.57 A m−2]) or may even stimulate cell activity (She et al., 2006). This latter occurrence was attributed to the formation of hydrogen and oxygen via water electrolysis. Jackman et al. (1999) found that the presence of solid particles offers protection from the current, even at high levels, whilst Lear et al., 2004, Lear et al., 2007 reported that a low electric current (3.14 A m−2) in soil detrimentally impacted communities only near the anode. This was largely attributed to changes in pH or contaminant distribution. No discernible effect of the electric current itself was observed.
Luo et al., 2005, Luo et al., 2007 and Fan et al. (2007) applied a direct current to phenol-contaminated soil, and by frequently changing the direction of the current produced an enhanced degradation rate. This minimized changes in soil physical properties, particularly pH, whilst generating contaminant movement. Similarly, Niqui-Arroyo et al., 2006, Niqui-Arroyo and Ortega-Calvo, 2007 found that pH control helped electrokinetics enhance polyaromatic hydrocarbon degradation, with electroosmosis improving contaminant availability and subsequent degradation.
The aim of the work described here was to investigate the effects of electrokinetics on degradation of a persistent contaminant, pentachlorophenol (PCP). It was hypothesised that small movements of contaminant or bacteria through application of a low direct current would increase contact between the two, for example by moving contaminants sequestered in micropores to a nearby area containing degrading bacteria. Using laboratory microcosms, the study examined the effects of electric current regime and subsequent pH and moisture content conditions on a clayey soil artificially contaminated with PCP, and inoculated with PCP-degrading bacteria (Sphingobium sp. UG30; Leung et al., 1997a, Leung et al., 1997b, Lear et al., 2007). PCP is an organic biocide and a U.S. EPA priority pollutant. It has several human health effects, including probable carcinogenicity (Proudfoot, 2003). It is a persistent environmental contaminant, toxic to many microorganisms (Chaudri et al., 2000) and biodegradation in natural situations is often restricted. Several microbial species are able to degrade the chemical (e.g. Leung et al., 1997a, Yang and Lee, 2008) and here a single degrading strain was employed in combination with a recalcitrant contaminant. Contaminant radiolabelling was used to enable detection of 14CO2 evolved through biodegradation (Niqui-Arroyo et al., 2006, Lear et al., 2007). Sequential chemical extractions provided indications of the relative availability of PCP remaining in the soil, and dehydrogenase activity was monitored as a proxy for degradative activity. It was found to be necessary to control physical changes to the soil (pH and moisture content) in order to prevent negative impacts on the microbial community.
Section snippets
Microcosm preparation
A silty clay soil was obtained from Wytham, Oxfordshire, UK (Table 1). Soil was prepared by air-drying before sieving past 2.0 mm. This was then rehydrated to a moisture content of 19% (w:w), thoroughly mixed and allowed to equilibrate overnight. Soil microcosms (dimensions 130 × 59 × 54 mm; 500 g moist soil) were prepared in plastic cartridges (Fig. 1) with compaction at 50 kPa as described by Lear et al. (2007).
Microcosm contamination and inoculation
The soil was contaminated with an aqueous pentachlorophenol (PCP, sodium salt,
Results
Summary plots of pH and moisture content are presented in Fig. 2 for electrokinetic treatments only, showing profiles at the start and end of each experiment. Points of note are the large pH decrease at the anode in experiment I and the significant increase (p < 0.02) in moisture content (electrokinetic treatment) in experiment II. It is expected that, in experiment III, pH near the electrodes varied (between acid and neutral at the anode and neutral and alkaline at the cathode) due to the
Discussion
The experimental results illustrate how the application of electrokinetics to an unsaturated soil can cause major changes to the soil properties, with subsequent impacts upon microbial activity and biodegradation. They also suggest that by controlling these properties biodegradation of PCP can be enhanced by electrokinetics.
Combustion of soil and recovery of the resulting 14CO2 was only performed in experiment I, due to limited availability of the analytical apparatus. Recovery was low (<7% of
Conclusions
In an unsaturated soil, electrokinetics was found to induce significant pH changes and, when pH was controlled, increases in moisture content. These were thought to hinder any electrokinetic enhancement of contaminant biodegradation. When both pH and moisture content were controlled using a regularly reversed electric field, the three sets of 14C recovery results (total soil extract, spatial distribution in soil and 14CO2) taken together indicate that the presence of the field had a positive
Acknowledgements
This work was funded through the Waste and Pollution Management Programme by the Engineering and Physical Sciences Research Council (EPSRC) and Natural Environment Research Council (NERC). Apparatus used was constructed by Mr. C. Waddup of the Department of Engineering Science, University of Oxford. The authors thank Dr. S. Jackman for considerable assistance and Dr. K. Semple and his laboratory (University of Lancaster, UK) for use of soil sample oxidation equipment.
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