Effects of a nonionic surfactant on biodegradation of phenanthrene and hexadecane in soil

The influence of a nonionic (alcohol ethoxylate) surfactant (Witconol SN70) on biodegradation of phenanthrene and hexadecane (nonaqueous-phase liquid) in soil was studied in batch and transport systems. Simultaneous enhanceme nt of phenanthrene and hexadecane degradation was noted at surfactant doses resulting in aqueous-phase surfactant concentrations below the critical mi celle concentration (CMC). Conversely, degradation rates of both compounds declined to essentially zero at supra-CMC doses, suggesting that distinct m echanisms of inhibition and enhancement were operating depending on the eff ective surfactant concentration (i.e., accounting for surfactant sorption, log K-D = 2.2 L/kg). Surfactant doses resulting in enhanced degradation cor related with enhanced gross microbial activity as determined using total CO 2 evolution rates. Supra-CMC doses that resulted in inhibited degradation d id not suppress gross microbial activity. Furthermore, measurements of phen anthrene solubilization and surface tension indicated that phenanthrene was solubilized at supra-CMC levels of surfactant. Mechanisms of inhibition of phenanthrene and hexadecane degradation at supra-CMC surfactant concentrat ions may include changes in interfacial chemistry and subsequent mass trans fer processes due to sorbed surfactant, reduced bioavailability of micelle- bound phenanthrene and hexadecane, or inhibition of specific members of the microbial community responsible for hydrophobic organic compound degradati on.