Solubilization and biodegradation of polycyclic aromatic hydrocarbon compounds in soil-water suspensions with surfactants

Bioremediation of soils and sediments contaminated with organic compounds may be a cost effective technology for environmental clean-up and aquifer restoration. Hydrophobic organic compounds HOCs), however, sorb strongly onto soil and sediment material, and the effectiveness of microbial treatment can be diminished by HOC phase partitioning and decreased substrate accessibility to microorganisms. Surfactant addition has been suggested as a technique for decreasing the interfacial tension and partitioning of the HOC with soil, and thereby increasing HOC mobility and bioavailability. However, this study indicates that nonionic surfactant solubilization of HOCs from soil may not be beneficial for the enhancement of soil bioremediation.; The solubilization and microbial degradation of phenanthrene, a three-ring polycyclic aromatic hydrocarbon (PAH) compound, was examined in various soil-water systems with commercially-available surface-active agents. This was accomplished by a series of batch tests using radiolabeled techniques. The purpose of surfactant addition was to assess the effect of surfactant solubilization of PAHs on their biodegradation.; For soil-water suspensions without surfactant approximately 50-60% of the phenanthrene was mineralized over the course of ten weeks. The addition of nonionic surface-active agents was observed not to be beneficial for microbial mineralization of phenanthrene in the soil-water systems, and for supra-CMC surfactant doses phenanthrene mineralization was completely inhibited for all the surfactants tested. Sub-CMC levels of surfactant in the soil-water systems generally did not have an inhibitory effect on phenanthrene mineralization, but neither did such doses serve to enhance the rate of degradation, which proceeded most rapidly in the absence of any surfactant.; Companion tests suggest that the supra-CMC inhibitory effect is not a toxicity phenomenon, per se, of the surfactant or micellized PAH. Chemical modeling of the phase partitioning of phenanthrene between water, soil, and micelles showed that micellization results in a decrease in the equilibrium free aqueous phenanthrene concentration insufficient to explain the microbial inhibition observed. Additional experiments demonstrated a relatively high exit-rate for phenanthrene from micelles. An assessment of the results from the various experiments suggest that the inhibitory effect is probably related to a reversible physiological surfactant micelle-bacteria interaction, possibly through partial complexing or release of membrane material without disrupting membrane lamellar structure. Additional work is required to assess surfactant/microbial membrane interactions and the implications for environmental engineering application. (Abstract shortened with permission of author.)...