Main Mechanisms of Metal Biosorption

Different metal-binding mechanisms have been postulated to be active in biosorption metal uptake such as chemisorption by ion-exchange, complexation, coordination, chelation; physical adsorption and microprecipitation (Volesky 2001). There are also possible oxidation-reduction reactions taking place in the biosorbent. Due to the complexity of biomaterials and biosorbents, it is also plausible that at least some of these mechanisms are acting simultaneously to varying extents depending on the biosorbent composition, surface properties and functional chemical groups, and the solution environment (Volesky 2001). Biomass materials offer several molecular groups that are known to offer ion exchange sites, carboxyl, sulphate, phosphate, and amine, could be the main ones (Volesky 2001).

Ion-exchange is an important concept in biosorption, because it explains many of the observations made during heavy metal uptake experiments (Davis et al. 2003). It should be pointed out that the term ion-exchange does not explicitly identify the binding mechanism, rather it is used here as an umbrella term to describe the experimental observations (Davis et al. 2003) . The precise binding mechanism(s) may range from physical (i.e. electrostatic or London-van der Waals forces) to chemical binding (i.e. ionic and covalent). The term sorption would refer to binding of a metal cation to a free site as opposed to one that was previously occupied by another cation. It is distinct from adsorption that defines binding in terms of a physical rather than chemical surface phenomenon. In the case of biosorption of heavy metals by biomass, the mechanisms can be viewed, in principle, as being extracellular, or occurring discretely at the cell wall. Intracellular sorption would normally imply bioaccumulation by a viable organism (Davis et al. 2003).

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