Influence of soil colloids on sorption and bioavailability of naphthalene

Batch experiments were conducted to determine the adsorption, desorption and mineralization rates of naphthalene on soil colloids. Initial kinetic studies were performed to measure the sorption rate of naphthalene on soil colloids. Sorption occurred at an initial fast rate followed by a longer slow rate. Initial fast rate was 20.0 µα naphthalene/g colloids/hr and later slower rate was 0.06 µα naphthalene/g colloids/hr, respectively. It was observed that the system was stable up to a period of 175 hours and apparent equilibrium was reached after 72 hours of contact. The total loss from the system (volatilization loss and sorption loss on the teflon tube walls) were high and were mainly responsible for the unstability of the system. Once the apparent equilibrium was established, adsorption isotherm experiments were conducted using the same colloid concentration and varying concentrations of naphthalene. Aqueous and colloid-bound naphthalene were directly measured and the isotherm obtained was linear and followed the Freundlich model. The partition coefficient (K_p;, = 557 cm³;/g) and the organic carbon normalized partition coefficient (K_oc = 5461 cm³;/g) in this naphthalene/colloid system are within the range of values found in the literature for soil/sediment systems. Desorption experiments were performed once apparent equilibrium was achieved using deionized distilled (DD) water and various organic solvents. Desorption rates followed an initial fast and a later slow pattern. Initial fast rates were measured as 7.6 µg naphthalene/g colloids/hr using an organic solvent, and 3.1 µg naphthalene/g colloids/hr using DD water, respectively. The corresponding slower desorption rates were measured as 0.093 and 0.058 µg naphthalene/g colloids/hr. With approximately 50% headspace in the tubes, the corresponding rates were 2.5 and 0.061 µg naphthalene/g colloids/hr with distilled water. Further experiments were conducted to determine the mineralization rate of colloidal bound naphthalene. Mineralization also had a two fold rate pattern. Mineralization rates for colloid-bound naphthalene were determined as 0.5 and 0.0046 µg naphthalene/g colloids/hr for the respective fast and slower phases. In comparing desorption rates with mineralization rates, it was observed that the initial desorption rate was much higher than the corresponding mineralization rate. In the later time periods, desorption slowed down, but the rate was still higher than the corresponding mineralization rate. It was concluded that microbial metabolism of naphthalene was limiting for the set of conditions in which the experiments were performed.

Keywords: PAH, naphthalene, soil colloids, microorganisms, sorption, isotherm, desorption, bioavailability.