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 25 degrees C with regard to the influence of contact time, initial mercury concentration, solution pH, salinity and presence of several divalent cations. The acid-base 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)2, HgOHCl and Hg(OH)2, to the algal surface sites.

A previous model, developed to describe the thermodynamic properties of associating electrolytes within the binding mean spherical approximation (BIMSA), is modified on the basis of a better representation of the pair. The ions are represented as charged hard spheres interacting through a sticky point potential, leading to the creation of pairs. In this version of the model, the species constituting the pair are allowed to have charges and sizes different from those of the free ions. New expressions for the osmotic and activity coefficients are proposed that account for these features. The equations were applied to fit the parameters in the model to osmotic data for a variety of aqueous solutions of inorganic acids, and for a base, NaOH, at 25°C up to the highest concentrations for which data are available. In all cases, accurate representations could be obtained up to very high concentrations, such as 28 mol kg -1 for nitric acid, 45 mol kg -1 for hydrochloric acid, and 29 mol kg -1 for NaOH. The case of sulfuric acid could be treated consistently in the concentration range 0-27.7 mol kg -1 by considering the equilibrium between sulfate and bisulfate ions. In all cases, the new model represents an improvement over the previous BIMSA model.

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We examined the variation with ionic strength (I, adjusted with KCl, KNO(3), KBr, NaCl or NaClO(4)) of the formal potential (E(const)) for glass electrodes exhibiting a Nernstian response (i.e. E(cell)=E(const)-slog[H(+)]). For this purpose, we investigated the different factors included in the formal potential, so we obtained reported values for the liquid junction potential as a function of ionic strength and determined the logarithm of the activity coefficient for the proton in various saline media, using Pitzer equations.

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