The Suitability of Bioremediation
Bioremediation as a biotechnological intervention for cleaning up the residual effects of previous human activities on a site, typically relies on the inherent abilities and characteristics of indigenous bacteria, fungi or plant species. In the present discussion, the emphasis will concentrate on the contribution made by the first two types of organism. The use of plants, including bioaccumulation, phytoextraction, phytostabilisation and rhizofiltration, all of which are sometimes collectively known as phytoremediation. Thus, the biological mechanisms underlying the relevant processes are biosorp-tion, demethylation, methylation, metal-organic complexation or chelation, ligand degradation or oxidation. Microbes capable of utilising a variety of carbon sources and degrading a number of typical contaminants, to a greater or lesser extent, are commonly found in soils. By enhancing and optimising conditions for them, they can be encouraged to do what they do naturally, but more swiftly and/or efficiently. This is the basis of the majority of bioremediation and proceeds by means of one of the three following general routes.
Mineralisation, in which the contaminant is taken up by microbe species,used as a food source and metabolised, thereby being removed and destroyed. Incomplete, or staged, decomposition is also possible, resulting in the generation and possible accumulation of intermediate byproducts, which may themselves be further treated by other micro-organisms.
Cometabolism, in which the contaminant is again taken up by microbes butthis time is not used as food, being metabolised alongside the organism’s food into a less hazardous chemical. Subsequently, this may in turn be mineralised by other microbial species.
Immobilisation, which refers to the removal of contaminants, typically met-als, by means of adsorption or bioaccumulation by various micro-organism or plant species.
Unsurprisingly, given the expressly biological systems involved, bioremedi-ation is most suited to organic chemicals, but it can also be effective in the treatment of certain inorganic substances and some unexpected ones at that. Met-als and radionuclides are good examples of this. Though, obviously, not directly biodegradable themselves, under certain circumstances their speciation can be changed which may ultimately lead to their becoming either more mobile and
accessible or less so. The net result produced in either case can, under the right conditions, be a very effective functional remediation. A list of typical contam-inants suitable for bioremediation would include the likes of crude oil and its derivatives, some varieties of fungicides and herbicides, hydrocarbons, glycols, phenols, surfactants and even explosives.
Developments in bioprocessing continually redefine the definitive catalogue of what may, and may not, be treated and many chemicals once thought ‘impossible’ are now routinely dealt with biologically. Table 5.1 reflects the current state of the art, though this is clearly subject to change as new approaches are refined.
As a result, it should be obvious that a large number of opportunities exist for which the application of remediating biotechnologies could have potential relevance. Even so, there are a number of factors which affect their use, which will be considered before moving on to discuss practical treatment issues themselves.
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