Applications of Bioremediation and Phytoremediation

1. Introduction The decontamination of soil and water from pollutants using microorganisms (bioremediators) is known as bioremediation (1). There are essentially two approaches, described as in situ and ex situ. In situ methods are those in which the contaminated material is treated at the site, whereas when the material is physically removed to be treated elsewhere it is referred to as ex situ. Some technologies that are related to bioremediation include those of phytoremediation (2,3), and are outlined below. It is possible for bioremediation to occur under natural conditions, or it can be stimulated, e.g. by the application of fertilisers (biostimulation), and more recently it has been shown that through the addition of matched microbe strains to the medium, the effectiveness of the resident microbe population to decompose contaminants may be enhanced. It should not be imagined that every type of contaminant can be disposed of by means of microorganisms. Heavy metal contaminants, e.g. [Cd.sub. 2+] and [Pb.sup. 2+], tend to resist interception by microorganisms. In such cases, phytoremediation is useful because the toxins are bioaccumulated into the body of plants, above ground, which can then be harvested and removed. By measuring the oxidation reduction potential (redox) in soil and groundwater, along with pH, temperature, [O.sub. 2] tension, concentrations of electron acceptors and donors, and of decomposition products, such as C[O.sub. 2], a measure of the bioremediation process can be obtained. Table 1 shows different biological decomposition processes, the rates of which decrease in decreasing order of the redox potential (fastest at higher potentials), although the detail of the overall bioremediation process per se is only scantly indicated by such values. To gain insight over a larger area, sufficient measurements should be made on and around the contaminated site such that contours of equal redox potential can be drawn. It is further necessary to perform analyses to ascertain that the ultimate levels of the contaminating compounds (and their products of decomposition) are below regulatory limits. Bioremediation can be used in locations that cannot readily be treated other than by excavation, e.g. spillages of petrol or chlorinated solvents which may contaminate groundwater. This is usually a much cheaper approach than excavating material to be disposed of elsewhere, or through other ex situ strategies, and which reduces or eliminates the need for "pump and treat", which is often employed where clean groundwater has been contaminated. The process may be enhanced by the addition of appropriate oxidising or reducing amendment agents. There is scope too for the creation of genetically modified microorganisms that are specifically tailored for bioremediation (5), e.g. the most radioresistant organism known so far, the aptly named bacterium Deinococcus radiodurans has been modified to consume and digest toluene and mercury cations in the presence of high level nuclear waste (6). 2. Some applications of microbial biodegradation The move toward finding "green" ways to ameliorate many environmental woes, including dealing with polluted environments, has led to a rising focus towards microbial degradation. Such methods of bioremediation and biotransformation exploit the remarkable diversity of xenobiotic metabolism by microbes. Thus, an enormous range of polluting materials may be addressed, including hydrocarbons (e.g. from oil-spills), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), heterocyclics, pharmaceutical substances, pesticides, heavy metals (e.g. [Cd.sup.2+], [Pb.sup.2+], [Cu.sup.2+], [Zn.sup.2+]) and various radionuclides (e.g. [Cs.sup.+], [Sr.sup.2+]). Detailed genomic, metagenomic, proteomic, bioinformatic and other high-throughput analytical techniques, as applied to environmentally important microorganisms, have disclosed key features of critical biodegradative pathways and the ability of such organisms to adapt to changing environmental conditions and stress factors. …