Biodegradation of three nonionic surfactants, Tergitol 15-S-X (X = 7, 9 and 12). and their effects on the biodegradation of phenanthrene by marine bacteria. Neptunomonas naphthovorans, were studied. The experimental outcomes could be fit well with the first-order biodegradation kinetics model. It was observed that the biodegradability of these surfactants decreased with an increase in the chain length of the hydrophilic moiety of the surfactant. When surfactant concentrations initially present were less than 250 mg carbon/L, biodegradability of Tergitol 15-S-X surfactants is around 0.3. Reduced biodegradability of Tergitol 15-S-7 and Tergitol 15-S-9 was observed when their concentrations initially present were increased to 322 and 371 mg carbon/L, respectively. In general, biodegradation of phenanthrene was enhanced with increasing solubilization of phenanthrene by these surfactants. However, with the same initial concentration of phenanthrene, biodegradability of phenanthrene was found to decrease with an increase in surfactant concentration. For these three surfactants, more than 80% of the phenanthrene was degraded when surfactant concentrations initially present were 200 mg/L. However, less than 30% of phenanthrene could be degraded, if initial surfactant concentrations were increased to 1000 mg/L. Interestingly, the concurrent biodegradation of the surfactants reduced their effective concentrations for micelle formation and. hence, contribute to the higher bioavailability of phenanthrene by gradually releasing phenanthrene molecules into the aqueous phase. (C) 2008 Elsevier B.V. All rights reserved.