The effect of anaerobic biodegradation on the phase partitioning of a multicomponent dense non-aqueous phase liquid

Abstract

Many former petrochemical sites are contaminated with highly toxic benzene, toluene, ethylbenzene, xylenes (BTEX) and polycyclic aromatic hydrocarbons (PAHs). Through batch experiments, the potential for anaerobic natural attenuation and source zone biostimulation was evaluated for a former manufactured gas plant in Amersfoort (The Netherlands). Previous research found indications for increased degradation in batches with pure phase tar stimulated with additional electron acceptors. However, these same batches had the highest aqueous phase benzene concentrations. Our research suggests that tar in the batches acts as a hydrophobic matrix, initially adsorbing component from the aqueous phase. However, as the matrix is degraded by anaerobic biodegradation it releases components back to the aqueous phase. To prove this hypothesis, hydrocarbons were spiked to batches with and without a non-aqueous phase liquid (NAPL). The results of these experiments found that NAPLs sorb aqueous phase hydrocarbons and the effects are most pronounced for components with a logKow> 3.5. Furthermore, benzene spiked to batches with pure phase tar from site rapidly partitioned to the tar and equilibrium concentrations were measured after as little as 5 minutes. In the biologically stimulated batches, we propose a critical mass of tar is degraded and a new equilibrium state is established with higher-than-sterile aqueous benzene concentrations. This is a result of the selective degradation of tar components and the loss of storage capacity within the remaining tar. To evaluate the relative effectiveness of different electron acceptors on tar degradation, a method was developed to conduct a mass balance of tar components in each of the gas, aqueous and tar phases. Ten mobile aromatic and nineteen PAHs ranging from two to six rings were quantified in the aqueous phase in an initial test of the method. Degradation in batches results in substantially higher-than-sterile aqueous concentrations of benzene and PAHs larger than fluorene. This research highlights a potential risk of source zone enhanced bioremediation. Stimulating batches results in an overall mass loss of tar but a substantial release of tar components to the aqueous phase, which could result in increasing the size of contaminant plume zones in-situ.