Introduction

For well over a century, metal-contaminated industrial wastewaters have been released into the environment by industry, agriculture, sewage treatment, and mining operations worldwide. Since the WWII era, the nuclear fuel cycle has contributed an additional and unique waste burden of uranium and other radioactive metals. Metal ions, unlike most organic chemicals, can persist in the environment indefinitely, posing threats to organisms which are exposed to them (Volesky and Holan 1995). Governmental control of such discharges has only been energetically regulated in the past two or three decades. Many toxic inorganic chemicals have, over time, accumulated in soils, sediments, and impoundments throughout the world. Metal-bearing liquid wastes may be of known and predictable composition if generated by a single industry, e.g., electroplating wastewaters, or in other cases may be a heterogeneous mix of many dissolved metal ions and organic compounds at various pH values and ionic strengths, with colloidal and particulate matter present as well. Governments are now regulating this problem by mandating preventative actions and forcing industries and laboratories and other waste generators to intercept toxic metals before they are discharged. Most heavy metal containing wastewaters are treated using remediation technologies that have been borrowed primarily from the unit operations of the chemical industry which rely on a mixture of physical and chemical processes (Table 1) to render the metal ion contaminants less toxic or more easily handled. Unfortunately the chemical form of the converted metal (e.g., a gelatinous precipitate) is itself often in need of careful and expensive disposal, and conventional treatment becomes less efficient and more expensive when metal ion concentrations fall into the 1-10 mg/l range. Table 2 provides a listing of discharge limits of metal finishing wastewaters in the United States, and as such represents goals that must be met by any new technology or existing technologies. Discharge limits for municipal wastewater treatment plants in the United States are much stricter than those listed in Table 2. Influent levels of Cu2+ in wastewaters arriving at municipal sewage treatment plants range from 100 to 250 mg/l, but effluent levels of 6-25 mg/l are expected to be attained under new U.S. Environmental Protection Agency guidelines (Amer 1998). Regulations governing aqueous metal discharges in the United Kingdom have been reviewed by Forster and Wase (1997) who also discussed the toxic biological effects of several of the important heavy metals. A genuine need now exists for new and certainly more cost-effective technologies to replace or supplement the physico-chemical approaches currently in use for removing metal contamination at existing sites and for preventing future contamination of natural waters by heavy metals. There is hope that biotechnology may provide new insights to solving these problems (Crusberg et al. 1994; 1996; Gretsky 1994).

Biotechnology has been successfully exploited as a remedy for many types of discharges of organic wastes and for in situ bioremediation of sediments at contaminated sites and in fact is the preferred remedy for many instances (Alexander 1999). There is hope that some of the physiological processes and genetic adaptations that protect organisms against toxic metals and other inorganic contaminants can be identified and exploited for the removal and recovery of those metals from aqueous waste streams (Crusberg et al. 1991; 1996; Hartley et al. 1997; Nies 1999; Ow 1996; 1997). Systems which use renewable biomass to extract metal ions from solutions may be an environmentally friendly alternative to physico-chemical

*Current affiliation: Cornell University, Ithaca, New York, USA

Table 1 Physicochemical processes for heavy metal wastewater treatment

Process

Detrimental factors

Membrane separation

Expensive, durability of membranes

Liquid-liquid extraction

Limited applications

Carbon adsorption

Expensive adsorbent, requires

regeneration

Ion exchange

Expensive adsorbent, requires

regeneration

Electrolytic treatment

Limited applications

Precipitation

Forms sludges, often very

gelatinous

Coagulation/flocculation

Forms hydrated sludges

Chemical reduction

Limited applications

Flotation

Forms hydrated materials

Vitrification

Very expensive, landfill required

Evaporation

Energy intensive

Crystallization

Time dependent, landfill required

processes and will be considered for their ability to serve as biotraps for metals (chiefly as ions). This discussion will center on heterotrophic microorganisms, primarily yeast and fungi as possible candidates for development as heavy metal biosorbents or "biotraps." Biotraps in this review are defined as any organism (living or nonliving) or component of an organism, which can alter the form of, or bind with a toxic metal or metal ion allowing its removal and recovery from a waste stream, or rendering it harmless.

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