Biosorption is a term that describes the broad range of processes by which biomass removes metals (and other substances) from solution, yet it can also be used in a stricter sense to describe uptake by dead (detritus) or living biomass by purely physico-chemical processes such as adsorption or ion exchange (White et al. 1995). Metabolic processes inherent in living biomass may contribute to the uptake mechanism. Ideally a biosorbent has the ability to be recycled and the sorbed metal ions recovered for reuse or safer disposal. Choice of a suitable biotrap is at times made easier if certain genetic and biochemical characteristics of an organism are known. That fungi and yeast can serve as biotraps for heavy metals has been the subject of a great deal of research as evidenced by several prior reviews on the subject (Blackwell et al. 1995; Kapoor and Viraraghavan 1995; 1997).
Predicting the effectiveness of a biotrap requires that some of its chemical composition be understood. For example, chitin and chitosan are well known metal-ion adsorbers due to the presence of both carboxyl and amine groups which make up these biopolymers (Ashkenazy et al. 1997; Cuero 1996; Fourest and Roux 1992; Juang et al. 1999). In fungi, the cell walls present a multilamillar architecture where up to 90% of the dry weight consists of amino- or nonamino-polysaccharides (Farkas 1980). The fungal cell wall is in essence a two-phase system consisting of a fibrous chitin-based skeletal framework embedded within an amorphous polysaccharide matrix (Griffin 1994). Yeast and higher fungi such as the deuteromycetes Trichoderma viride have cell walls composed primarily of chitin and glucan polymers while the lower fungi such as Rhizopus arrhizus have cell walls of chitosan and chitin (Morley and Gadd 1995). Although chitosan does bind metal ions, the more highly polymerized and cross linked chitin performs better as a metal biotrap. However, Crusberg et al. (1994) reported that chitin, derived from a marine invertebrate source, was capable at best of binding 14.7 mg Cu2+/g biotrap at pH 4.0, with a binding constant of 27 mM, and concluded that this system was not likely to be developed commercially. There were better alternatives.
There is virtually no standardization in the literature for reporting details of metal-binding experiments with biotraps. Concentrations of metal ions are reported in several variations of mass per unit volume (mg/l, parts per million or ppm, and mg/l, parts per billion or ppb), as well as in moles per unit volume, millimolar (mmol/l) and micromolar (mml/l). For example, copper ion (Cu2+) at 63 mg/l (ppm) may also be expressed as 1 mM but a uranium concentration of 1 mM would be almost four times larger in terms of mass per unit volume, at 238 mg/l. In fact, Cu2+ is often chosen as the model heavy metal in initial studies on a potential biosorbent because there are many quite sensitive analytical techniques available for its analysis. Most regulatory agencies mandate permissible metal ion concentrations in wastewater discharges in terms of mass per unit volume, e.g., ppm or ppb (parts per billion or mg/l). Other issues that should be properly addressed by investigators, but are often omitted in scientific reports, are effects of pH of an experiment on metal ion solubility, and ionic strength effects on equilibrium constants. For example, Cu2+ at 10 mg/l precipitates (as Cu(OH)2) when the pH rises above 6.3, and precipitates at 1 mg/l when the pH rises above 6.8, based on a Ksp of 5 X 10"20 (13*, 3*, 24). Ionic strength may affect rate constants which comprise an equilibrium constant according to Debey-Huckle theory. Even buffers used to maintain pH during uptake/binding studies should be chosen to minimize their chelating potential for the metal ions under consideration.
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