Metal ion binding by biological surfaces: voltammetric assessment in the presence of bacteria

Voltammetric techniques (differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV)) were evaluated for their capability to distinguish, without prior separation of the solid phase (e.g. filtration, centrifugation), between dissolved and particulate concentrations of Zn(II), Pb(II) and Cu(II), and to measure the extent of binding of these metals to the surface of a bacterium (Klebsiella pneumonia, formaldehyde treated). From titration curves of bacterial cell suspensions with metals the specific adsorption of metals was determined and quantified in terms of average surface complex formation constants and differential equilibrium functions. The following stability sequence for surface complexes was found: Cu2+ greater than Pb2+ greater than Zn2+ much greater than Ca2+. Simultaneous analytical determination permitted the measurement of both the binding of Cu(II) to the cell surface, and the binding to the solute exudate ligands. The affinity of the metal ions for the functional groups of the cell surface is strongly pH-dependent, and, at a given pH, decreases with increasing metal loading of the bacterial surfaces. This indicates that metal ions bind first to the highest affinity surface ligands and subsequently to those of lesser activity. Copper(II) appears to form stronger surface complexes with the high affinity ligands of the bacterial surface than with the functional groups of hydrous oxides.