Comparison of availability of copper(II) complexes with organic ligands to bacterial cells and to chitin

Bacterial cells or chitin were exposed to solutions with 100 mu M total but only 5 mu M free copper, due to the presence of a proper concentration of proline, lysine, cysteine, or ethylenediamine tetraacetate (EDTA). The influence of the nature and concentration of the particles and soluble ligands, on the sorption and on the desorption of the copper, at pH 6.50 and 25.0 degrees C, was investigated. The metal sorbed by the particles and that left in the solution were measured by atomic absorption spectrometry, after different periods of contact between particles and solution. The interpretation of the results was based on the copper(II) speciation calculated through equilibrium approaches applied to homogenous or heterogeneous systems. A significant fraction of copper bound to the organic ligands was displaced to the bacteria or chitin, and the extent of chemical reaction depended on the nature of both the soluble (or leaving) ligands and sites on the particle surface (or entering ligands), as expected by the equilibrium theory. But with chitin, the uptake of copper in the presence of cysteine or EDTA was higher than expected, which may be due to the adsorption of the soluble copper complexes on the particle surface. In consequence of a competition between soluble and particulate ligands (cells or chitin), the free copper(II) concentration decreased in the solution, even in the presence of very strong chelators. The results indicate that copper availability is not a simple function of the initial free copper concentration in the solution. Desorption of the previously fixed copper, originated by free soluble ligands indicated that the sorption of copper was ''quasireversible'' for both particles, though a larger dismissal of the equilibrium position occurred for the cells, probably due to their biological activity. Both the bacteria and chitin were able to fix metal initially bound to an organic ligand continuously for periods longer than 30 min, the kinetics of uptake varying with the nature of both the leaving and entering ligands. Therefore, long time-scale techniques, namely batch processes, warrant studies of metal availability in natural systems.