Biosurfactant enhancement of microbial degradation of various structural classes of hydrocarbon in mixed waste systems

This study examines the effect of two rhamnolipid biosurfactants on the first-order biodegradation rate constant for a microbial consortium growing on a mixture of hydrocarbons representing four structural classes of hydrocarbons. The microbial biodegradation rate of hexadecane, dodecane, benzene, toluene, iso-octane, pristane (2,6,10,14 tetramethyl pentadecane), naphthalene, and phenanthrene in the presence and absence of a mixture of rhamnolipid biosurfactant was determined. A first-order biodegradation model was applied in these studies to better discern the differential solubilization and biodegradation rates for specific structural classes of hydrocarbons in hydrocarbon mixtures. The biodegradation rate was enhanced by the addition of biosurfactant levels above the critical micelle concentration for all hydrocarbon species except phenanthrene and naphthalene. The time required for complete removal of each of hydrocarbons from the Culture was shortened due to the presence of surfactant, indicating a clear pattern in their order of removal based upon their structural class. The rhamnolipid biosurfactants enhanced the rate of linear alkane biodegradation more than the biodegradation rate of the monoaromatics. The rate constants for hexadecane and dodecane increased by 111 and 76% to 4.7 and 0.3 mg/h, respectively, while those of benzene and toluene increased by 34 and 65% to 3.1 and 4.0 mg/hr, respectively. The branched alkane degradation rate constants were also increased by 71% for iso-octane and 39% for pristane. In contrast, the biodegradation rate for the polycyclic aromatic hydrocarbons (PAHs), naphthalene and phenanthrene, decreased by 25 and 27%, respectively. These were the only biodegradation rate decreases noted upon addition of biosurfactant. Biodegradation rates of all other hydrocarbon species were characterized by shorter times to total removal from the culture as well as increased removal rates. Hence, micellar solubilization affects the rate of hydrocarbon degradation for various hydrocarbons differently depending upon their ability to partition into the micellar core. In mixed waste systems, PAHs must compete with other substrates to partition into the micelle and a decreased rate of biodegradation may be observed due to exclusion, or very low levels of solubilization within the micelle. Surfactant solubilization and enhancement of biodegradation appears most important for insoluble hydrocarbons, and is significantly influenced by the size and structure of the hydrocarbon contaminant.