Introduction

Microorganisms, including fungi, are known to accumulate metals from their external environment and the possibility of using fungi as a means of treating metal/radionuclide-containing effluents is well recognized (Siegel, Galun & Siegel, 1990; Gadd, 1993). However, to date, there are no commercial systems in operation which specifically use fungi as a basis for a metal treatment system. This is despite the fact that certain fungal species, under optimal conditions, are as effective as ion exchange resins in the removal of metals from solution (Tsezos & Volesky, 1981). As yet, the development of this potential from scientific curiosity to commercial fact remains to be demonstrated.

The mechanisms of microbial metal uptake may be either independent of, or dependent on, cell metabolism (Huang, Huang & Morehart, 1990; Avery & Tobin, 1992). Metal uptake which is independent of cell metabolism will be referred to as biosorption in this work. It is generally regarded that biosorptive metal uptake mechanisms would be more appropriate for use in a metal treatment system (Kuyucak, 1990). This is because environmental conditions in most effluents may be too toxic for microbial growth. Biosorption, in many cases, accounts for most of the metal accumulated by the cell, and can represent 10–20% or more of the cell dry weight (Luef, Prey & Kubicek, 1991; Gadd, 1993). The process occurs by either physical or chemical means and usually involves surface interactions of metals with microbial cell walls or excreted cell products.