Determination of bioavailable Fe(III) content in column experiments with the reactive tracer phosphate

Different methods were compared to evaluate the oxidation capacity of ferric iron in column studies. The specific adsorption of the reactive tracer phosphate on the Fe(III) oxide surface was used as an alternative approach to determine the oxidation capacity utilizing the linear correlation between the long-term extent of Fe(III) reduction and the specific surface area of the oxide. Although a low crystalline form of ferric iron (two-line ferrihydrite) was used as electron acceptor and toluene as a carbon source, only 31 and 24% respectively of the total iron was reduced by Geobacter metallireducens in parallel experiments. The results of the phosphate tracer tests were in good agreement with the Fe(III) that was actually reduced and the microbially oxidized toluene. The oxidation capacity of ferric iron is therefore overestimated by the chemical extraction methods, which completely dissolve the ferrihydrite and neglect surface-dependent limitations.