The influence of Fe(III) and Mn(IV) on the biotransformation of hydrocarbons in groundwater

According to the thermodynamic ladder, microorganisms in groundwater use electron acceptors consistently – for transformation of pollution from the pollution plume edge to its core. However, some researchers come to the conclusion that only methanogenic biotransformation of pollution or reduction of Fe(III) and Mn(IV) from the solid phase can occur in the plume, and due to the kinetic factor microorganisms use electron acceptors from the aqueous phase (O2, NO3- и SO42-) only on the edge of the pollution plume. The purpose of the research was to determine whether microorganisms use Fe(III) and Mn(IV) as acceptors of electrons for hydrocarbons transformation in groundwater in the northern part of the Middle Heilongjiang-Amur River basin aquifer. In the study area, both lenses of petroleum-hydrocarbons (non-aqueous phase liquids) on the surface of groundwater (up to 2.5 m) and high concentrations of dissolved hydrocarbons (up to 1000 mg/l) are noted. Microbiological processes were assessed in situ by the method of geochemical indicators. The most active biogeochemical processes occurred during the spring-summer rise of groundwater level. The seasonal increasing of level led to the entry of Fe(III) and Mn(IV) into the pollution plume and activation of the microbiological processes of its reduction. Microorganisms mostly use electron acceptors from the solid phase – Fe(III) and Mn(IV), but not NO3, SO42 from the aqueous phase. This is confirmed by the close correlation of HCO3- formation and that of Fe(II) and Mn(II) in groundwater (r2 up to 0.93). This says that for the groundwater self-purification the kinetic factor rather than thermodynamic one is decisive; and microorganisms use electron acceptors that are currently available. As a result of microbiological pollution destruction, the content of Fe(II) in groundwater increased up to 100 mg/l, Mn (II) – up to 16 mg/l, which exceeds the natural background 4 and 8 times, respectively. This was also because the regional geochemical background of the study area (Amur River basin) forms Fe and Mn.