Aqueous Phase Biodegradation Kinetics of 10 PAH Compounds

Biodegradation kinetics were individually studied for 10 polycyclic aromatic hydrocarbons (PAHs): naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 2-ethylnaphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene. Experiments consisted of aerobic, continuously mixed, batch aqueous systems inoculated with a PAH-degrading consortium. The design of the experiments and the mathematical interpretation of the data isolated the kinetics of biotransformation. Other physical-chemical processes that may affect bioavailability were eliminated or independently measured. Model parameters were estimated using the maximum likelihood method for a bivariate data set of substrate and biomass concentration over time. For four of the PAHs, naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 2-ethyl naphthalene, there was sufficient growth to estimate biomass yield coefficients. For two of the PAHs, naphthalene and 1-methylnaphthalene, Monod parameters q_max and K _s were estimated. For the remaining compounds, only the first-order rate coefficients were estimated. The consortium was able to degrade all the PAHs except acenaphthene. The observed biodegradation rate coefficients did not correlate with molecular weight or any other chemical property. The biomass-normalized first-order biodegradation rate coefficients spanned a little less than two orders of magnitude, from 0.0255 to 1.11 h^-1 (mg protein/L)^-1. This is much less variation than is typically observed for PAHs in soils and sediments. This suggests that in field samples differences in biodegradation rates are not governed by differences in the kinetics of biotransformation. Rather, the differences in biodegradation rates in the field are primarily governed by the physical-chemical processes that control bioavailability, which differ significantly for different PAHs.