Geochemical Controls during the Biodegradation of Petroleum Hydrocarbons in Soils

The microbial transformation of Fe (III) to Fe (II) can be coupled to the oxidation and reduction of organic contaminants in sub-oxic to anoxic environments. A multidisciplinary approach was adopted in this study to investigate the biogeochemical influences on the degradation of toluene (a representative of the class of aromatic hydrocarbons collectively known as BTEX) using experimental analogues of subsurface soil environments under predominantly iron-reducing conditions. The removal of toluene over the period of incubation indicated the soil-water mixture supported the degradation of toluene under predominantly iron-reducing conditions. Chemical sequential extractions showed the removal of toluene in the active mesocosms induced an increase in carbonate-bound iron fractions from 196.1 ± 11.4 mg/kg to 5,252.1 ± 291.8 mg/kg and a decrease in the reducible iron fraction from 2,504.4 ± 1,445.9 mg/kg to 375.6 ± 20.8 mg/kg. Analysis of the soil-water mixture showed slight shifts in the pH of the control and active mesocosms at the start of the experiments however these shifts occurred to a lesser degree over the remainder of the incubation period. Further experiments analysed the degree of influence of differing soil matrices and extraneous sources of iron (hematite, goethite, magnetite, ferrihydrite and lepidocrocite) on toluene removal. With the exception of the lepidocrocite-amended mesocosms, all of the iron-amended mesocosms were shown to have supported toluene removal. The presence of hematite, goethite and magnetite did not produce a significant change in the pH or total iron concentrations of the soil-water mixture. However the presence of ferrihydrite in the ferrihydrite-amended mesocosms induced a decrease in pH to slightly acid values ranging between pH 6.5 at the start of the experiments and 5.2 at the end of the experiments. The lepidocrocite-amended mesocosms induced a change to slightly alkaline values ranging between pH 8.4 and 8.8 during the period of incubation. All of the soil-amended mesocosms supported the removal of toluene in the soil-water mixture. The mesocosms containing soils with a greater percentage clay fraction removed higher amounts of toluene, possibly an indication that the bulk of this removal was sorption-induced and not microbially-mediated. An experimental approach based on the standard stable carbon isotope analytical method made it possible to determine the source of carbon in the incubated mesocosm material. The application of the mixed effects model approach to analyse the repeatedly measured experimental data demonstrated the possibility of producing predictive models for toluene removal in soil.