ADSORPTION AND DESORPTION HYSTERESIS OF PAHS IN SURFACE SEDIMENT

The overall objective of this research is to investigate the desorption reversibility of two PAHs (naphthalene and phenanthrene) from sediment materials. Batch adsorption and desorption experiments were designed to minimize the contribution of experimental artifacts. The sorbent was sediment obtained from Johnson Ranch (Lula, OK) that contained 92% sand, 6% silt, 2% clay, and 0.27% organic matter. Batch adsorption and desorption experiments with radiolabeled chemicals were conducted at various pH levels and ionic strengths, solute concentrations ranging over several orders of magnitude, and equilibrium times up to five months. These model pollutants appear to be readily adsorbed by sediment, but a portion of the bound pollutants resists desorption. The adsorption is rapid and predictable using published K-OC and K-OW relationships. From 30 to 50% of the adsorbed amount resists desorption. The resistant fraction can be desorbed in water only at rates one to three orders of magnitude slower than previous observations or theories. It is hypothesized that some multistep chemical or mechanical effects promote soil aggregation and/or entrapment of pollutants in the soil organic matter during adsorption.