Abstract
Co-contamination of the environment with toxic chlorinated organic and heavy metal pollutants is one of the major problems facing industrialized nations today. Heavy metals may inhibit biodegradation of chlorinated organics by interacting with enzymes directly involved in biodegradation or those involved in general metabolism. Predictions of metal toxicity effects on organic pollutant biodegradation in co-contaminated soil and water environments is difficult since heavy metals may be present in a variety of chemical and physical forms. Recent advances in bioremediation of co-contaminated environments have focussed on the use of metal-resistant bacteria (cell and gene bioaugmentation), treatment amendments, clay minerals and chelating agents to reduce bioavailable heavy metal concentrations. Phytoremediation has also shown promise as an emerging alternative clean-up technology for co-contaminated environments. However, despite various investigations, in both aerobic and anaerobic systems, demonstrating that metal toxicity hampers the biodegradation of the organic component, a paucity of information exists in this area of research. Therefore, in this review, we discuss the problems associated with the degradation of chlorinated organics in co-contaminated environments, owing to metal toxicity and shed light on possible improvement strategies for effective bioremediation of sites co-contaminated with chlorinated organic compounds and heavy metals.
Highlights
In tandem with rapid economic and industrial advancement, human activities have instigated widespread pollution of the natural global environment [1]
This review addresses: toxic metals and toxicity mechanisms to microorganisms; factors affecting the physical and chemical state of metals, which in turn influence metal speciation and bioavailability; measurement of bioavailable metal concentrations; influence of heavy metals on microbiological processes required for effective bioremediation; and improvement strategies aimed at increasing biodegradation in co-contaminated environments
A dearth of information exist on the impacts of heavy metals on the biodegradation of chlorinated organic pollutants, additional studies that incorporate a variety of benchmark chlorinated organic chemicals and various manipulations of environmental factors that affect metal speciation and bioavailability are needed
Summary
In tandem with rapid economic and industrial advancement, human activities have instigated widespread pollution of the natural global environment [1]. The physicochemical properties of these chlorinated compounds, as components of dense nonaqueous liquid phases (DNAPLs), make them difficult to remove once they have entered the subsurface and they remain among the most difficult contaminants to remediate in the environment [4] Due to their latent toxicity to both wildlife and humans, production and use of several chlorinated organic chemicals have been completely banned in many countries around the world. Co-contamination often causes a synergistic effect on cytotoxicity [8], and the two components often must be treated differently [9,10,11] Such concerns have heightened the need for novel and advanced bioremediation techniques to effectively remove organic pollutants from a variety of co-contaminated environmental media including water, sediments and soil [1,12]. This review addresses: toxic metals and toxicity mechanisms to microorganisms; factors affecting the physical and chemical state of metals, which in turn influence metal speciation and bioavailability; measurement of bioavailable metal concentrations; influence of heavy metals on microbiological processes required for effective bioremediation; and improvement strategies aimed at increasing biodegradation in co-contaminated environments
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