Bioremediation: A Critical Review Chapter Abstracts

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Abstract
Bioremediation has developed from the laboratory to a fully commercialised technology over the last 30 years in many industrialised countries. However, the rate and extent of development has varied from country to country. This chapter examines the reasons for this differential development using examples particularly from the USA, Netherlands and the UK. Key factors in the development of bioremediation are the academic infrastructure, the regulatory regime and the commercial environment. The capability of the technology is still expanding. Approaches to treat a wider range of contaminants under an increasing number of geological and hydrogeological conditions are being developed, and suggestions are made for fruitful new areas of research. The final section discusses the balancing of the needs of researchers, practitioners and regulators. It draws some conclusions as to how these stakeholders may positively influence the development of future new technologies such as bioremediation.

Abstract
Successful bioremediation of soil contaminants relies on the management of soil microbial populations capable of catabolising the organic contaminants present. The role of soil microbiota in the biochemical conversion of organic contaminants has been realised for at least 50 years. In the last 20 years, significant research effort has developed a detailed process understanding of the ecological, biochemical and genetic basis of microbial contaminant degradation, with a view to enhancing microbial capabilities and thus designing more effective bioremediation processes. This chapter will critically review the various approaches taken with regard to the study of soil microbial contaminant degradation; specifically how the outcomes of such studies have been adopted by commercial bioremediation practitioners, in the optimization of bioremediation design. The chapter will also address the potential reasons why, in many cases, research outcomes have not been adopted in new bioremediation process design and suggest which microbiological research strategies/techniques have the potential to aid future developments in bioremediation technology.

Abstract
Until relatively recently, the design and engineering of biological treatment processes have been performed using methods that were more empirical than theoretical. This paper presents an alternate perspective on biological process design: one more grounded in theoretical ecology than in quasi-empirical engineering. Here we present four ecological frameworks that demonstrate how and why many current design practices are inconsistent with principles from basic ecology, which is unfortunate given that engineered biological processes are functionally complex ecosystems. Theories presented include resource ratio theory, predator-prey theory, non-linear dynamics and chaos theory, and theories from island biogeography. Each theory is first described and then discussed within the context of improving our understanding of how and why engineered biological processes do what they do. A new philosophy for biological process design is then presented which accounts better for basic ecological interactions that influence the performance and successful operation of such systems.

Abstract
Basic concepts from hydrogeology, geochemistry and microbiology are combined to form a general conceptual model for the behaviour of groundwater contamination plumes and the associated environmental risk. Biodegradation processes are described as stoichiometric electron-transfer reactions in order to quantify mass and electron balances that help assess and predict rates of biodegradation. Mass change in soluble reactants and products identifies which biodegradation processes occur, and at what rate. This includes processes at the plume fringe where soluble oxidants (O₂, NO₃^-, SO₄^2-) are mixed into the contaminant plume, and within the plume interior where Fe and Mn minerals can provide a large reservoir of oxidant capacity and where fermentation processes can occur.

Dissolved H₂ is a particularly important parameter for evaluating biodegradation processes. Approaches to interpreting H₂(aq) data are conflicting in the current literature but are conceptually powerful and provide tremendous potential for diagnostic and predictive application. Comparison of approaches within a mathematical framework for describing biodegradation kinetics identifies significant discrepancies in the underlying conceptual models upon which these approaches are based. There is a pressing need for a wider compilation of data to further test and develop these approaches.

A case study of a contaminant plume arising from a petrochemical plant demonstrates the application of mass and electron balance models. Results show that large uncertainties arise from 1) complex and unknown source histories at such sites (spatial scale, timing and magnitude of spills and infiltration) and 2) poorly quantified rates of hydrodynamic dispersion at the plume fringe. Electron balances, however, do offer the possibility to quantify the rates of specific biodegradation processes.

Abstract
Lighter isotopes of the same element form weaker bonds than heavier isotopes. Since their bonds are more readily broken, lighter isotopes react faster - an effect known as kinetic fractionation. Such fractionations occur during bacterial degradation of organic contaminants and the accumulated effect on contaminant molecules can be used to constrain the extent of degradation that has taken place. Dilution and sorption are not associated with significant isotopic effects, so this approach can distinguish biodegradation from changes in contaminant concentration due to other processes.

The most commonly used approach is to look for changes in the ¹³C/¹²C ratio of organic contaminant molecules during biodegradation. To be effective as an index for degradation, isotopic effects must be greater than any initial variability in the isotopic composition of the contaminant source. In general, large and aromatic molecules exhibit small fractionations relative to smaller molecules, limiting the usefulness of this approach for many types of contaminant. Reductive dehalogenation of halocarbons results in particularly large carbon isotopic fractionations and the technique is thus well suited to studies of halocarbon degradation. Alternatively, similar kinetic fractionations associated with bacterial consumption of electron acceptors such as sulphate (³⁴S/³²S) and nitrate (¹⁵N/¹⁴N) during biodegradation can be used to estimate the extent to which these species have been consumed. This then enables the role played by sulphate and nitrate reducing bacteria during biodegradation to be calculated. Care must be taken with sulphate reduction that the reduced sulphide product is not being recycled to sulphate by reoxidation. Isotopic compositions of oxygen in sulphate molecules can be used to investigate this possibility.

Using isotopic studies to gauge reaction progress during biodegradation can give unique insights into the extent of biodegradation and the mechanisms by which it occurs. Whilst not applicable in all cases, changes in ¹³C/¹²C ratio of pollutant molecules provides a direct measure of biodegradation and can be a powerful tool in assessing the fate of subsurface contaminants.

Abstract
The discipline of ecotoxicology has long been considered as being only relevant in water testing. Well developed protocols, international collaborative programmes and the adoption of key optimised assays have underpinned this strategy. By contrast terrestrial assessments of hazard and risk are still dominated by chemical tests that fail to relate pollutant doses to biological receptors. Even the term "bioavailability" is often misused in site assessment. Here the basis of soil ecotoxicity testing is compared with traditional and novel environmental chemical methods. Through specific examples it can be seen that to develop accurate site assessment strategies (fundamental to accurate risk assessment and effective remediation strategies) complementary ecotoxicity data and chemical analysis represent the most viable approach. In the future, site assessment procedures must become more integrated and combine the key elements of biology, chemistry, hydrogeology and process modelling if we are to develop sustainable solutions to land contamination issues.

Abstract
The persistence of organic pollutants in the environment and their resistance to biodegradation is a widely recognised phenomenon. An important factor in determining persistence is bioavailability. The reduced bioavailability of such compounds with time is known as 'ageing' and has been related to their hydrophobic nature and their interaction with soil/sediment organic matter (SOM). Theories have been proposed that relate adsorption/desorption behaviour, hysteresis effects, and the ageing of hydrophobic organic compounds (HOCs) in soils and sediments to the composition and molecular structure of SOM. In addition to naturally occurring SOM, combustion residues (e.g.soot) have been implicated in the sorption process for certain HOCs, in particular polycyclic aromatic hydrocarbons. Assessment of the risks associated with HOCs, their bioavailability and strategies for their bioremediation require an ability to predict their behaviour. To accomplish this it is necessary to have knowledge of processes such as HOC partition, diffusion coefficients and sorption in soils. However, the nature of the systems involved means that patterns of sorption are often complex and experimental determination of thermodynamic parameters is difficult and time consuming. Provided with satisfactory model structures, computational chemical techniques may offer insights into mechanisms of sorption and provide estimates of relevant thermodynamic parameters.

Abstract
There is an increasing interest in the science and application of permeable reactive barriers (PRBs) for the in situ treatment of groundwater contaminants. Although reasonably immature as a bioremediation technology, there are numerous configurations and an increasing array of possible treatment options that are being evaluated in laboratory and field experiments. Here bio-reactive PRBs are reviewed, for a range of contaminants such as petroleum hydrocarbons, chlorinated solvents, nutrients and metals. Some recent research on innovative delivery systems for sustaining altered subsurface geochemistry conducive to biodegradation processes is highlighted, along with design issues for PRBs. Innovation in the delivery mechanisms and systems is claimed to be key to the long-term cost effectiveness of bio-reactive PRB systems.

Abstract
Over the past few decades the use of microorganisms for the attenuation of contaminated environments has attracted significant attention from both the public and private sectors. Many bioremediative technologies utilizing aerobic bacteria are currently in place. However, the application of anaerobic microorganisms to contaminant treatment has remained underutilized. Recent developments in the understanding of anaerobic microbial metabolism and in the isolation of novel anaerobic microorganisms has indicated the true potential of these systems with a resultant renewed interest in their application to the removal of both organic and inorganic contaminants in situ.

Abstract
Inoculation of microbial specialists into polluted sites to enhance the rate and quality of bioremediation is a highly controversial subject. This review highlights some of the underlying principles. The rationale behind the addition of inoculants is that it is the catabolic potential of the microbial community present at the site that limits the rate of biodegradation. The required catabolic potential may be missing locally because of the history of the site, or it may be incomplete in all microbial communities studied so far, resulting in accumulation of dead end products or toxic intermediates. In such instances inoculation with natural microorganisms harbouring the catabolic trait in question or with genetically optimized bacteria which overcome the metabolic limitations of the indigenous microbial community is a rational strategy to improve bioremediation. Examples for successful inoculation treatments including recalcitrant xenobiotics, especially highly chlorinated aromatic compounds, and mixtures of chlorinated and methylated compounds are used in this chapter. Misrouting of intermediates from ortho- and meta-cleavage pathways for the degradation of substituted aromatics is a well-studied example in which a collapse of community metabolism could be prevented by inoculation with genetically engineered microorganisms. Principles of inoculation, factors influencing the survival of inoculants, especially of a well studied genetically engineered strain, and new strategies to design more predictable inoculants are discussed.

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