Abstract
The aim of this work is to present a critical review on slurry bioreactors (SB) and their application to bioremediation of soils and sediments polluted with recalcitrant and toxic compounds. The scope of the review encompasses the following subjects: (i) process fundamentals of SB and analysis of advantages and disadvantages; (ii) the most recent applications of SB to laboratory scale and commercial scale soil bioremediation, with a focus on pesticides, explosives, polynuclear aromatic hydrocarbons, and chlorinated organic pollutants; (iii) trends on the use of surfactants to improve availability of contaminants and supplementation with degradable carbon sources to enhance cometabolism of pollutants; (iv) recent findings on the utilization of electron acceptors other than oxygen; (v) bioaugmentation and advances made on characterization of microbial communities of SB; (vi) developments on ecotoxicity assays aimed at evaluating bioremediation efficiency of the process.From this review it can be concluded that SB is an effective ad situ and ex situ technology that can be used for bioremediation of problematic sites, such as those characterized by soils with high contents of clay and organic matter, by pollutants that are recalcitrant, toxic, and display hysteretic behavior, or when bioremediation should be accomplished in short times under the pressure and monitoring of environmental agencies and regulators. SB technology allows for the convenient manipulation and control of several environmental parameters that could lead to enhanced and faster treatment of polluted soils: nutrient N, P and organic carbon source (biostimulation), inocula (bioaugmentation), increased availability of pollutants by use of surfactants or inducing biosurfactant production inside the SB, etc. An interesting emerging area is the use of SB with simultaneous electron acceptors, which has demonstrated its usefulness for the bioremediation of soils polluted with hydrocarbons and some organochlorinated compounds. Characterization studies of microbial communities of SB are still in the early stages, in spite of their significance for improving reactor operation and design optimization.We have identified the following niches of research needs for SB in the near and mid term future, inter alia: (i) application of SB with sequential and simultaneous electron acceptors to soils polluted with contaminants other than hydrocarbons (i.e., pesticides, explosives, etc.), (ii) evaluation of the technical feasibility of triphasic SB that use innocuous solvents to help desorbing pollutants strongly attached to soils, and in turn, to enhance their biodegradation, (iii) gaining deeper insight of microbial communities present in SB with the intensified application of molecular biology tools such as PCR-DGGE, PCR-TGGE, ARDRA, etc., (iv) development of more representative ecotoxicological assays to better assess the effectiveness of a given bioremediation process.
Highlights
Bioremediation is an alternative that offers the possibility to destroy toxic pollutant using natural biological activity
We have identified the following niches of research needs for slurry bioreactors (SB) in the near and mid term future, inter alia: (i) application of SB with sequential and simultaneous electron acceptors to soils polluted with contaminants other than hydrocarbons, (ii) evaluation of the technical feasibility of triphasic SB that use innocuous solvents to help desorbing pollutants strongly attached to soils, and in turn, to enhance their biodegradation, (iii) gaining deeper insight of microbial communities present in SB with the intensified application of molecular biology tools such as polymerase chain reaction (PCR)-DGGE, PCR-TGGE, ARDRA, etc., (iv) development of more representative ecotoxicological assays to better assess the effectiveness of a given bioremediation process
SB is an ad situ and ex situ technology that can be used for bioremediation of problematic sites, such as those characterized by soils with high contents of clay and organic matter, contaminated with pollutants that are recalcitrant, toxic, and display hysteretic behavior, or when bioremediation should be accomplished in relatively short times under strong pressure and monitoring of environmental agencies and regulators
Summary
Bioremediation is an alternative that offers the possibility to destroy toxic pollutant using natural biological activity. Bioremediation is the use of living organisms, primarily microorganisms, to degrade the environmental contaminants into less toxic forms. It uses naturally occurring bacteria and fungi or plants to degrade or detoxify substances hazardous to human health and/or the environment [1]. Contaminant compounds are transformed by living organisms through reactions that take place as a part of their metabolic processes. When microorganisms are imported to a contaminated site to enhance degradation we have a process known as bioaugmentation [2]. Bioremediation techniques are typically more economical than thermal and physico-chemical remediation such as incineration [3,4]
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