Sediment control of facilitated transport and enhanced desorption

Laboratory column experiments examined the facilitated transport and enhanc ed desorption of benz(a)anthracene [B(a)A] by dissolved natural organic mat ter (OM) in sediments of low organic carbon content. The two-component expe riments examining OM-sediment interaction and B(a)A-sediment interaction we re modeled to determine the value of the linear rate constants describing t ransfer of B(a)A and OM between water and sediment. It was found that a two -rate approach better simulated B(a)A breakthrough and elution in the sedim ent relative to a one-rate approach. In contrast, OM-sediment interaction w as well-simulated with a one-rate approach due to low OM sorption by sedime nt. The three-component experiments examining facilitated transport and enh anced desorption of B(a)A by dissolved OM, showed rapid linear reversible B (a)A-OM interaction. The value, within a factor of 2, of the equilibrium di stribution constant for benz(a)anthracene distribution between water and OM was similar to 1E6 for soil humic acid and similar to 1E5 for Suwannee Riv er humic acid. Simulations of the three-component experiments based on the equilibrium distribution constants for B(a)A-OM interaction and the rate co nstants determined from the two-component experiments were performed to det ermine whether rate constants differed in the two-component versus three-co mponent systems. The simulations captured the major features of the facilit ated transport and enhanced desorption data; however, discrepancies indicat ed that either the two-rate model for solute-sediment interaction was inapp ropriate, or that B(a)A transfer from sediment to dissolved OM was altered in the three-component system relative to the two-component system.