Fungi as potential bioremediation agents in soil contaminated with heavy or radioactive metals.

Origin and impact of heavy metal and radionuclide pollution Pollution of soil by heavy metals arises from a number of sources, including emissions from motor vehicle exhausts and from manufacturing industry, sewage sludge application, and acid mine drainage. Heavy metals such as zinc, cadmium and lead are toxic and may affect man directly through ingestion after passing through the food chain [l], or indirectly through their effect on other organisms [2,3]. Contamination of soil with radionuclides has occurred as a result of fall-out from nuclear weapons testing, and controlled and accidental discharges from nuclear power stations and nuclear fuel reprocessing plants. The risk to health presented by a radionuclide depends upon the quantity present, its decay products, the potential for entry of the radionuclide into biological systems, its half-life, and the type and energy of the radiation it emits. For example, the release of I3’Cs following the accident at the Chernobyl reactor was of concern because this isotope has a half-life of 30 years, emits high-energy y and radiation and is readily taken up into biological systems [4,5]. Remediation of land polluted with heavy or radioactive metals presents particular difficulties compared with remediation of contaminated water, or land contaminated with organic pollutants. Metals can be removed from aqueous solution either by conventional physicochemical techniques or by biological treatments, for example through bacterial reduction of metal species to form insoluble precipitates [6]. M’ icrobial remediation of soil contaminated by organic pollutants can be achieved through the catabolism of the pollutants to relatively harmless products such as carbon dioxide and water [7]. Heavy metals, unlike hydrocarbons, cannot be removed by degradation. Because soil is a solid matrix, it is not amenable to remediation by filtration or adsorption techniques. Physicochemical methods for the removal of metal pollution from soil include leaching and electroremediation [8,9]. In many cases these approaches are infeasible or uneconomic, especially where remediation in situ is required. The potential for the exploitation of fungi in remediation by leaching of metals from spoil heaps has already been recognized [lo]. A cheap and simple method for the remediation in situ of metal-contaminated soil, using fungi, would be of great benefit.