Biosorption is the binding and concentration of heavy metals from aqueous solutions (even very dilute ones) by certain types of inactive, dead, microbial biomass⁶. Pioneering research on biosorption of heavy metals has led to the identification of a number of microbial biomass types³ that are extremely effective in concentrating metals.

Some types of biomass are waste byproducts of large-scale industrial fermentations (e.g., the mold Rhizopusor the bacterium Bacillus subtilis). Other metal-binding biomass types, such as certain abundant seaweeds (particularly brown algae, e.g., Sargassum, Ecklonia), can be readily harvested from the oceans.

These biomass types can accumulate in excess of 25% of their dry weight in deposited heavy metals: Pb, Cd, U, Cu, Zn, Cr and others. Research on biosorption is revealing that it is sometimes a complex phenomenon where the metallic species could be deposited in the solid biosorbent through various sorption processes, such as ion exchange, complexation, chelation, microprecipitation, etc.

Individuals with different backgrounds, from engineering to biochemistry, can make significant contributions to the understanding of biosorption. Interdisciplinary efforts are essential to exploit this technology commercially. A chemical engineering background is particularly useful for expanding the application of this technology in large-scale process industries.

Threats from the Environment

The greatest demand for metal sequestration today comes from the need to immobilize the metals released to the environment (or mobilized) by and partially lost through human technological activities. It has been established that dissolved metals (particularly heavy metals) escaping into the environment pose a serious health hazard¹¹. They accumulate in living tissues throughout the food chain (Figure 1), which has humans at its top, multiplying the danger. Thus, it is necessary to control emissions of heavy metals into the environment.

Am example of one method for prioritizing the recovery of ten metals is presented in Table 1. This may be simplistic, but it provides a useful direction by ranking metals into three general priority categories:

1. Environmental Risk (ER)
2. Reserve Depletion Rate (RDR)
3. Combination of ER and RDR.

Environmental risk assessment could be based on a number of different factors, which could also be weighted.

Table 1: Ranking of Risks Associated with Various Metals

Relative Priority	Environmental Risks	Reserve Depletion	Combined Factors
High	Cd	Cd	Cd
	Pb	Pb	Pb
	Hg	Hg	Hg
	---	Zn	Zn
Medium	Cr	---	---
	Co	Co	Co
	Cu	Cu	Cu
	Ni	Ni	Ni
	Zn	---	---
Low	Al	---	Al
	---	Cr	Cr
	Fe	Fe	Fe

The Need for Novel Technology

Conventional techniques to remove toxic metals and radionuclides, such as ion exchange and precipitation, lack specificity and are ineffective at low metal ion concentrations. The need for effective and economically viable technologies is driven by environmental pressures such a:

• Stricter regulations with regard to the metal discharges are being enforced, particularly in industrialized countries.
• Toxicology studies confirm the dangerous impacts of heavy metals.
• Current technologies for the removal of heavy metals from industrial effluents often create secondary problems with metal-bearing sludge.