Cadmium and zinc interactions with a Gram-positive soil bacterium : from variable charging behavior of the cell wall to bioavailability of heavy metals in soils

A detailed study is presented on the cadmium and zinc sorption to both isolated cell walls and intact, living cells of the Gram-positive soil bacterium <em>Rhodococcus erythropolis</em> A177. Acid/base titrations were performed on isolated cell wall material to characterize the type and amount of reactive sites on the cell wall. The proton binding was described with a three modal Langmuir-Freundlich equation, combined with a Donnan model to correct for the electrostatic interactions. Cadmium and zinc sorption to the isolated cell walls was reduced with increasing proton or calcium concentration. During the metal ion sorption, desorption of protons and calcium ions was observed. Calculations showed, that at high coverage with bivalent ions, charge reversal takes place. On the basis of the charging behavior, a competitive binding model, the NICA equation, was selected to describe the sorption data. The model is used to predict the sorption of cadmium and zinc to intact, living cells of the bacterium. The trends of sorption studies with intact cells at two exposure times suggest competitive interactions, not only for the adsorption of the metal ions to the cell wall, but also for the uptake into the cell. The impact of this competitive binding is reflected in the different levels of toxicity experienced for different pH conditions. A good correlation was found between cell wall adsorption and toxicity, indicating that for this organism, total body burden is a good indicator for potential effects, resulting from exposure to heavy metals. In combination with model descriptions of cadmium binding to a sandy and a clay soil, the influence of pH and calcium concentration on sorption to the bacterium when present in soil is predicted. Results show, that the impact of pH and calcium concentration is of the same order as the impact of soil type. It is quite obvious, that bioavailability is not only determined by the free metal ion concentration in solution, but that other parameters like the pH and the concentration of competing ions in solution play an important role as well.