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The ability of Cystoseira baccata algal biomass to remove Hg(II) from aqueous
solutions is investigated. The mercury biosorption process is studied through batch
experiments at 25ºC with regard to the influence of contact time, initial mercury
concentration, solution pH, salinity and presence of several divalent cations. The acidbase
properties of the alga are also studied, since they are related to the affinity for
heavy metals. The studies of the pH effect on the metal uptake evidence a sharp
increasing sorption up to a pH value around 7.0, which can be ascribed to changes both
in the inorganic Hg(II) speciation and in the dissociation state of the acid algal sites.
The sorption isotherms at constant pH show uptake values as high as 178 mg.g
-1 (at pH
4.5) and 329 mg.g
-1 (at pH 6.0). The studies of the salinity influence on the Hg(II)
sorption capacity of the alga exhibit two opposite effects depending on the electrolyte
added; an increase in concentration of nitrate salts (NaNO3, KNO3) slightly enhances
the metal uptake, on the contrary, the addition of NaCl salt leads to a drop in the
sorption. The addition of different divalent cations to the mercury solution, namely
Ca2+, Mg2+, Zn2+, Cd2+, Pb2+ and Cu2+, reveals that their effect on the uptake process is
negligible. Finally, the equilibrium sorption results are compared with predictions obtained from the application of a simple competitive chemical model, which involves a
discrete proton binding constant and three additional constants for the binding of the
main neutral inorganic Hg(II) complexes, Hg(Cl) HgOHCl and Hg(OH)2, to the algal
surface sites.
This work was funded by the projects BQU2002-02133 (from the Ministerio de Ciencia y Tecnología of Spain) and PGDIT02TAM10302PR (from the Xunta de Galicia). |
