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

Abstract

AbstractThe 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 enhancement of phenanthrene and hexadecane degradation was noted at surfactant doses resulting in aqueous‐phase surfactant concentrations below the critical micelle concentration (CMC). Conversely, degradation rates of both compounds declined to essentially zero at supra‐CMC doses, suggesting that distinct mechanisms of inhibition and enhancement were operating depending on the effective surfactant concentration (i.e., accounting for surfactant sorption, log KD = 2.2 L/kg). Surfactant doses resulting in enhanced degradation correlated with enhanced gross microbial activity as determined using total CO2 evolution rates. Supra‐CMC doses that resulted in inhibited degradation did not suppress gross microbial activity. Furthermore, measurements of phenanthrene 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 concentrations may include changes in interfacial chemistry and subsequent mass transfer 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 degradation.