Doctoral Dissertations

Ick Tae Yeom

Date of Award

12-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Mriganka M. Ghosh

Committee Members

Chris Cox, Kevin Robinson, Bruce Robinson, Gary Sayler, Michael Essington

Abstract

What controls PAH biodegradation in tar-contaminated soils? What role must surfactants play in facilitating bioremediation in such soils? The present research sought answers to these key questions. Thermodynamic equilibria and kinetics of surfactant-PAНinteractions in tar-contaminated soils are considered with some rigor in parts two and three of this report. These are brought to fore in part four to gain insight into the limitations and benefits of surfactant-mediated bioremediation of tar-contaminated soils. Five nonionic polyoxyethylene (POE) surfactants and a tar-contaminated soil from a manufactured gas plant (MGP) site were used in this study.Equilibrium distribution of four PAHs,phenanthrene, anthracene, pyrene, and benzo(a)pyrene among soil, water, and micelle phases was determined for given soil-to-surfactant ratios. Up to 30% of Soxhlet-extractable PAHs could be solubilized in 16 days in completely stirred batch reactors at surfactant loadings of 0.055 g/g soil. Longer periods were required to reach equilibrium at higher surfactant dosages. Surfactants with higher hydrophile-lipophile balance (HLB)produced greater solubilization. Raoult's law satisfactorily described the partitioning of constituent PAHs between the weathered tar and the water. An equilibrium model was developed to predict the solubilization of constituent PAHs for given properties of soil and surfactant. Prediction was satisfactory at relatively low surfactant dosages (<5.5 g/L). Themodel overestimated solubilization at higher surfactant dosages. Presumably,thermodynamic equilibrium could not be reached during the duration (380 hours) of the experiments because of mass transfer limitations.

To determine the maximum rates of PAH release achievable under such kinetic limitations, experiments were conducted in the presence of Tenax resin in which organic solutes are known to partition nearly completely. With Tenax in water most constituent PAHs failed to attain equilibrium during the 55-day experiments. However, the release of PAHs significantly increased upon the addition of surfactants. It is postulated that surfactants promote PAH release by two mechanisms: 1) they significantly increase the solubilities of PAHs in micellar solution, and, in so doing they increase the concentration gradient at the tar-water interface; 2) they penetrate and swell the polymeric tar phase and thereby increase matrix diffusivities of PAHs. A modified radial diffusion model was used along with a parametric sensitive analysis to evaluate the relative importance of these mechanisms in surfactant washing of contaminated soils. The matrix diffusivity increased by up to two orders of magnitude in soils washed with 5.5 g/L surfactant solutions. It was concluded that surfactants promote PAH release from contaminated soils mainly by increasing the matrix diffusivities of contaminants; increase in solubility by micellar solubilization plays only a secondary role. Further, the diffusivity enhancement is related to the structure of the surfactant. Surfactants with smaller ethoxylate (polar) head groups and linear alkyl chains performed much better than those with larger head groups and/or branched hydrophobic chains.

Biodegradation of soil-bound PAHs in MGP soil was investigated with and without surfactants. On occasion, soils artificially contaminated with a single PAH were used to develop baseline information on biodegradation employing substrate-specific microbial strains. While surfactants did not affect mineralization of PAHs in soils contaminated for a short period (2 days), they clearly did so in soils contaminated for longer periods (10 months). Biodegradation of PAHs in the MGP soil appeared to be limited by the slow release of PAHs from soil. With no surfactant added, 40-45% of phenanthrene was degraded in 98 days while biodegradation of PAHs with more than three aromatic rings was insignificant. Biodegradation of high ring PAHs increased somewhat at supra-CMCdosages. At dosages of 2.5 g/L or higher, both Triton X-100 and Brij 35 resulted in 70-80% degradation of phenanthrene in 98 days. However, the surfactants themselves were also degraded in the process. In fact, all surfactants except Triton X-100 were completely degraded in 25 days. Biodegradation of PAHs was highest in soil samples washed with the least biodegradable surfactant.

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