Anaerobic biodegradation of polycyclic aromatic hydrocarbons in dredged material

This report examines the anaerobic biodegradation potentials of 16 EPA priority Polycyclic Aromatic Hydrocarbon (PAH) compounds in dredged sediments from Milwaukee, Wisconsin, under several redox environments. Batch biodegradation tests were conducted in bioslurry reactors containing sediment samples and amendments to enhance certain redox conditions. Tests relied on PAH biodegradation by native bacteria utilizing background nutrients in the sediment. Unamended and abiotic (poisoned) controls were also run.;The amount of degradation of 16 PAH compounds found in the sediment, ranging from naphthalene to dibenzo(ah )anthracene, was measured. It was observed that all 16 PAH compounds biodegraded to some extent in the unamended reactors. Enhanced biodegradation occurred in reactors amended with sulfate, and to a lesser extent, nitrate and iron(III) citrate.;A general trend was observed with more disappearance of lower molecular weight (MW) PAHs than higher MW PAHs. Degradation rates were directly related to the number of heterocyclic rings and the MW of the PAH compound. Lower MW PAHs with two- to three-ring structures degraded much faster than PAHs with four rings, which, in turn, degraded faster than higher MW PAHs with five and six rings.;Higher concentrations tended to degrade faster than lower concentrations. Degradation rates generally followed first order decay. For example, the three-ring compound acenaphthene (MW = 154) degraded with a first order K value of --0.15 day--1, while benzo(a)pyrene (five rings, MW = 252) degraded with a first order K value of --0.05 day--1.;The iron(III) phosphate amended reactors clearly inhibited PAH biodegradation compared to the unamended reactors. The methanogenic reactors inhibited the biodegradation of four, five and six ring compounds.;Relatively high background sulfur (2450 mg/kg) and soluble sulfate (as high as 525 mg/L of SO42--) levels in this sediment may have dominated the intended amendments. In the unamended reactors, a biologically mediated increase in soluble sulfate levels occurred. Addition of nitrate caused even greater increases in sulfate levels. No comparable increases were noted in the abiotic reactors. High sulfate levels may have been inhibitory to methanogenic bacteria. Indications are that sulfate-reducing bacteria native to the sediment played a significant role in effecting PAH biodegradation.