Enhanced adsorption of zinc is associated with aging and lysis of bacterial cells in batch incubations

Abstract

Bacteria can immobilize significant quantities of trace metals through surface complexation reactions. However, bacterial cell lysis is an integral part of the development process, and the extent to which this process affects adsorbed metals has not been properly investigated. In order to evaluate the effects of cell lysis on metal fixation, bacterial suspensions containing approximately 10 ppm Zn in 0.01 M NaNO(3) were monitored over an one-month period for adsorbed Zn, pH, cell concentration, dissolved organic carbon, NH(3) and dissolved amino acids. Cell concentration decreased with time, in parallel with an increase in dissolved organic carbon. Zn adsorption decreased with time for suspensions with near-neutral (5.5-7.0) initial pH values, consistent with the reduction in cell concentration and/or formation of metal-ligand complexes in solution, with lysis products acting as ligands. However, Zn adsorption increased with time for suspensions with initially low pH (<or=5), and was accompanied by an upward shift in suspension pH. Surface complexation modelling suggests that enhanced adsorption of Zn is predominantly due to the increase in pH, with ternary surface complexation at pH values below the pK(a) of the carboxyl surface sites. The increase in pH is due to production of ammonia, and/or proton buffering by the amphoteric cytoplasmic compounds. The observed changes may have implications for understanding metal sequestration during remineralisation of organic matter in nature.