Selenate adsorption to composites of Escherichia coli and iron oxide during the addition, oxidation, and hydrolysis of Fe(II)

• Selenate adsorption onto bacteria–iron oxide composites.
• Selenate was found to only adsorb to iron oxides.
• Non-additive adsorption as iron oxides aged.
• Models suggested masking of surface sites by bacteria and minerals, and mineral aggregation over time.

Adsorption onto iron oxide and bacterial surfaces can affect the fate of many dissolved ions, but currently there is a poor understanding of ion adsorption onto mixtures of these two sorbents. This study used chemical analyses of the aqueous and solid phases and electron microscopy to observe the adsorption of dissolved selenate (SeO42 −) onto composites of iron oxide and Escherichia coli during the addition, oxidation, hydrolysis of Fe(II)aq and precipitation of Fe(III)-oxide. No SeO42 − adsorption onto E. coli was observed under the conditions of this study ([SeO4]total = 3 ppm, biomass concentration 0.11–0.44 dry g/l, ionic strength = 0.01 M, pH 3–8, reaction time 0–24 h). SeO42 − adsorption onto abiotic and biotic iron oxides decreased with time which was attributed to blockage of iron oxide surface sites by other mineral particles. Adsorption onto bacteria–iron oxide composites was significantly reduced compared to the end-member systems. Surface complexation models (SCMs) were developed to fit the experimental data and suggested that the reduction in SeO42 − sorption by the composites was due to masking of iron oxide surface sites by both bacteria and other iron oxides and also via interactions with the bacterial supernatant. This study has shown that SeO42 − adsorption in bacteria-bearing systems cannot be evaluated without considering redox processes like Fe(II)aq oxidation, hydrolysis and precipitation.