The process of biosorption of heavy metal ions (Cr3+, Cd2+, Cu2+) by blue–green algae Spirulina sp. is discussed in
this paper. Spirulina sp. was found to be a very efficient biosorbent. The aim of the present study was to investigate
quantitatively the potential binding sites present at the surface of Spirulina sp., using both potentiometric titrations
and adsorption isotherms. The kinetic experiments showed that the process equilibrium was reached quickly, in less
than 5–10min. It was found that the equilibrium dependence between biosorption capacity and bulk metal ion concentration
could be described with Langmuir equation. This suggests that the mechanism of biosorption is rather chemisorption
than physical adsorption and was further confirmed by the low surface area associated with physical
adsorption and by the presence of cations that appeared in the solution after biosorption. The maximum contribution
of physical adsorption in the overall biosorption process was evaluated as 3.7%. It was proposed that functional groups
on the cell surface contributed to the binding of metal ions by a biosorbent via equilibrium reaction. Three functional
groups capable of cation exchange were identified on the cell surface. The biomass was described as weakly acidic ion
exchanger. Since deprotonation of each functional group depends on pH, the process of biosorption is strongly pHdependent.
This was confirmed in the biosorption experiments carried out at different pH. The contribution of functional
groups in the biosorption process was confirmed by chemical modification of the groups. Chemically blocked
groups did not show neither biosorption nor ion-exchange capabilities. It has been shown that growth conditions
can affect the metal adsorption properties of microalgae. The paper also discusses desorption characteristics of the biosorbent.
The criteria for desorption were high elution efficiency and preservation of biosorptive properties. Desorbent
that possessed these characteristics was nitric acid.