Effect of sediment particle size on polycyclic aromatic hydrocarbon biodegradation: importance of the sediment-water interface.

Mechanisms for the effects of sediment on the biodegradation of organic compounds in the aquatic environment are not clear. In this research, effects of sediment characteristics on biodegradation kinetics of chrysene and benzo[a]pyrene were studied by inoculating polycyclic aromatic hydrocarbon (PAH)-degrading bacteria. Because water and PAHs can pass a polytetrafluoroethylene membrane yet bacteria and sediment cannot, a membrane experiment was performed to compare the biodegradation rates of PAHs in water and at the sediment-water interface, providing direct evidence that the PAH biodegradation rate is enhanced by the presence of sediment. Biodegradation of PAHs in water-sediment systems was fitted to zero-order kinetics; the order of biodegradation rate in water-sediment systems with different sediment was fine silt > clay > coarse silt. Biodegradation of PAHs in water-sediment systems occurred mainly at the sediment-water interface. According to membrane experiment results, when the biodegradation kinetics was fit to a zero-order equation, the maximum specific growth rates of bacteria (1/d) at the sediment-water interface were approximately three- to fourfold those in the water phase. Furthermore, the associated mechanisms regarding the effect of sediment characteristics were analyzed by investigating the process of bacterial growth and the distribution of bacteria and PAHs between water and sediment phases.