The economic threshold for commercial selection of a biological process to replace a physico-chemical process for heavy metal removal from a waste stream as assessed by Macaskie (1991) is that a metal-loading capacity greater than 15% of the biomass dry wt must be demonstrated. Before the selection of any technology, it is imperative to note the hierarchy of hazardous waste management options: reduce; reuse; recycle.
The option of last resort is to treat and dispose of the waste in safe landfills, while minimizing the resultant volume, since disposal sites are few and space is precious not to mention expensive. A given bioremediation technology should be able to perform on a large scale in order for it to be commercially viable. The organism or biomaterial selected to accomplish the goal of removing or altering a heavy metal or metal ion rendering it less toxic must be very efficient in performing its intended function. The literature is rich with reports of studies attesting to the "potential" of a particular biomass or biomaterial to carry out bioremediation of metal-contaminated waste streams, but few have actually ventured beyond the laboratory bench scale. What is clear is the apparent dearth of genetic engineering reports in the literature. Classical genetic selection methods have proven useful, for e.g., for isolating a strain of S. cerevisiae out of 240 tested, capable of uptake of 3.2 mg chromium (Cr(VI)) per g dry wt of yeast cells (Liu et al. 2001). Genes which carry out certain functions in micro-organisms that provide them with resistance to heavy metals may be exploited for the development of new bioremediation technologies. Hunts for metal-resistant and metal ion-binding microorganisms appear to be very successful, but the investigators nearly always neglect to pursue the genetics behind their discovery. Silver recently reviewed the genetics of metal resistance by bacteria and noted several specific genes involved in metal uptake (Silver 1998). Many of these genes are located on plasmids and although not directly relevant to fungi and yeast they may have some characteristics in common with Eukaryotes. Ow (1996) has suggested that that employing Schizosaccharomyces pombe as a model system, and understanding of yeast metal tolerance genes may become clearer. Newly developed technologies can also be used to immobilize or encapsulate an isolated biotrap into gels where it can function in an in vitro manner if desirable. For example, gel-immobilized metal-binding proteins could then trap metal ions in a more efficient manner and perhaps be amenable to the reverse reaction, enabling the recovery of the metal ion, and providing a reusable cost-effective system.
Recovery of heavy metals from contaminated soils and sediments by in situ bioremediation, however, remains elusive. To mobilize metals in those environments would likely threaten local groundwater, and when necessary the technology in current use, excavation and soil washing, may remain the only viable but expensive alternative. However, under certain conditions it may be possible to introduce metal-binding organisms or nonliving metal biotraps into soils and sediments to enhance the stability of metal ions at contaminated sites. Likewise, it may be possible to even enhance the population of indigenous metal-binding bacteria at contaminated sites by providing to them fixed carbon in gaseous form, e.g. methane or other volatile organics, essentially the reverse in situ bioremediation process known as biosparging. For the immediate future then, new heavy metal bioremediation technologies may be limited to the development of industrial applications for meeting regulatory agency requirements for pretreating wastewaters for later discharge.
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