The Effect of Desorbability on Biodegradation

of Pyrene in a Natural Soil

Sangchul Hwang, Ph. D. Candidate (hwang@uakron.edu)

and

Teresa J. Cutright, Assistant Professor (tcutright@uakron.edu)

Department of Civil Engineering

The University of Akron

Akron, OH 44325-3905

Abstract

Sorption and desorption of spiked pyrene by a natural silty-sand soil (11.1% gravel, 58.9% sand, 20% silt, and 18% clay) was investigated to gain a better understanding of biodegradability of high molecular weight PAHs. To achieve the goal, both batch kinetic and isotherm sorption/desorption studies were performed. Pyrene was dissolved in hexane and spiked to make the concentrations (3000 ppb, 5000 ppb, 7000 ppb, 10000 ppb and 15000 ppb) in the glass centrifuge reactors. Pyrene concentration in the solution phase was detected by fluorescence spectrometry (l_ex = 335 nm and l_em = 368 nm). The sorption isotherm study was conducted using a 24-hr contact time that was obtained in the sorption kinetic study. The data was fit to both the linear model and the Freundlich model, which indicated 0.7779 mg/kg(L/mg) of K_p (R² = 0.93) and 2.068 mg/kg(L/mg)ⁿ of K_f with n = 0.798 (R² = 0.97), respectively. The high binding affinity of pyrene to the soil was attributed to relatively high percentages of the expandable clay minerals (in this case, vermiculite), which provided pyrene with internal sorption sites.

After sorption analysis, the amount decanted was replaced with the fresh background solution for the desorption studies. Desorption kinetic study revealed that desorption equilibrium was attained after a 6-day contact time. The sequential desorption isotherm study with the 6-day equilibrium time showed that the desorbed amount was increased slowly and by only a small amount even with second 6-day sequential desorption. For example, after the second 6-day sequential desorption step, the normalized desorbed amounts were 0.551 % and 2.506 % for the spiked concentrations of 3000 ppb and 10000 ppb, respectively. It was more difficult to remove pyrene from the solid phase at the lower spiked concentration. This was attributed to a phenomenon in which the small amount of pyrene at the lower spiked concentration found the sorption sites that had the higher bonding energy. To enhance desorption of pyrene, the amount decanted was refilled with a natural soil leachate rich in the dissolved organic matter (DOM). As expected, the normalized desorbed amounts were increased to 1.141 % and 2.579 % for the spiked concentrations of 3000 ppb and 10000 ppb, respectively. The increase was greater in the lower spiked concentration than in the higher spiked concentration. The subsequent bioavailability of pyrene is currently being investigated.