Progressive Sorption and Oxidation/Hydrolysis of Fe(II) Affects Cadmium Immobilization by Bacteria-Iron Oxide Composites

Abstract

Iron-bacteria composite suspensions were prepared by exposure of Anoxybacillus flavithermus cells to increasing concentrations of Fe2+ cations under different degrees of oxidation. Cd2+ immobilization was investigated during and after the synthesis of the iron-bacteria composites via scanning electron microscopy and isotherm sorption experiments conducted at varied ratios of total iron to bacteria (expressed by the variable ρ, mg Fe per g dry bacteria). At ρ less than about 20, precipitation of Fe(III) oxide was hindered, and for ρ up to about 50 Cd immobilization was decreased relative to iron-free control experiments, even for conditions where Fe(III) oxide was forming. For ρ above 50 to the maximum investigated value of 124, the Cd immobilization capacity of the iron-bacteria composite suspensions was partly recovered. Surface complexation models developed to describe the data indicate that 1) Fe2+ and Cd2+ cations compete with comparable affinities for the reactive sites on the bacterial cell walls, and 2) sorption of the progressively oxidizing iron can reduce the total concentration of bacterial surface sites available for metal adsorption by more than 40%, inferred to be due to masking or blocking of the binding sites by Fe(II) or Fe(III) monomers or oligomers or precipitated Fe(III) oxides. This study demonstrates that the immobilization of metal cations in bacteria-bearing settings cannot be examined independently of redox processes such as the oxidation, hydrolysis and precipitation of iron.