The Mechanism of Metal Cation and Anion Biosorption

Abstract

The passive, not metabolically mediated, biosorption uptake of metals by (dead) biomass appears as a powerful tool for somewhat
selectively removing heavy metals from solution. Immobilization of dissolved toxic heavy metals and their physical removal
by biosorption in a water puricication process is not only technically feasible but it may prove to be economically quite
attractive. In order to effectively optimize such a process, the mechanisms involved in metal biosorption need to be well
understood and the metal speciation in aqueous solutions has to be taken into consideration as it plays an important role.

As phenomena of complexation, coordination, chelation, ion exchange, adsorption, inorganic microprecipitation may all be involved
in the overall metal uptake by biosorption, the configuration and state of the active binding site in the biomass have to
be well understood. The state and effectiveness of the binding site is, to a large degree, also affected by the environmental
conditions such as pH, temperature and ionic strength of the solution. Because of the multiparameter complexity of the sorption
system it is most useful to express the interdependence of the key parameters mathematically whereby the set of equations
could be organized into a model of the sytem that could be used for predicting its metal uptake performance under different
conditions. The elements and fundamentals of the approach are discussed and outlined in the chapter.

When the microprecipitation phenomenon and physical collection of insolubilized metal is excluded, extensive research results
indicate that ion exchange tends to be the dominant metal immobilization mechanism in biosorption. The fact that this phenomenon
is in most cases reversible offers an attractive possibility of effective wash-release of the deposited metal, resulting in
a highly concentrated regeneration solution suitable for some conventional metal recovery and a refreshed biosorbent material
ready for another metal uptake cycle. This feature undoubtedly reinforces the feasibility and competitiveness of the metal
biosorption process.

KeywordsBinding sites-Biosorption-Microorganisms (bacteria, fungi and algae)-Complexation-Hard and soft ions-Metal speciation in solution-Micro-precipitation