Petroleum hydrocarbons are organic pollutants of major concern due to their wide distribution, persistence, complex composition, and toxicity. They can bioaccumulate in food chains where they disrupt biochemical or physiological activities and can affect genetic integrity of many organisms, resulting in carcinogenesis, mutagenesis and impairment of reproductive capacity. Polycyclic aromatic hydrocarbons (PAHs) have been recognized as priority pollutants due to their carcinogenic, mutagenic and teratogenic properties. Bioremediation, which utilizes the metabolic versatility of microorganisms such as bacteria and fungi to degrade or detoxify hazardous wastes into harmless substances has been recognized as a sustainable, economic, environmentally friendly and versatile alternative for the remediation of many contaminated environments; however its effectiveness is limited by low bioavailability of nonaquous phase and soil-bound PAHs and petroleum hydrocarbons due to their low aqueous solubility, high hydrophobicity and strong sorption to soil. The purpose of this study was to investigate the PAHs and petroleum hydrocarbons bioavailability and subsequent biodegradation enhancement potential of biosurfactants.
Biosurfactants have steadily gained increased significance in environmental applications such as bioremediation dueto several advantages over surfactants of chemical origin, such as biodegradability, environmental compatibility, low toxicity, high selectivity and specific activity at extreme temperature, pH and salinity. A series of experiments was designed to investigate the bioavailability and subsequent biodegradation enhancement potential of the biosurfactants produced by the bacterial strains Bacillus subtilis CN2, Ochrobactrum intermedium CN3, Paenibacillus dendritiformis CN5 and Bacillus cereus SPL_4 in liquid culture and soil microcosms with PAH-enriched microbial consortium from chronically contaminated sites. The biosurfactants exhibited a high level of thermal stability, tolerance to extreme levels of salinity and a positive effect for increasing pH. They were identified after Fourier Transform Infrared (FT-IR) spectrometry, Thin Layer Chromatography (TLC) and Liquid Chromatography/Tandem Mass Spectrometry (LC MS/MS) analyses. The biosurfactants physicochemical characterization displayed vast structural diversity and potent surface active properties of surface tension reduction and emulsion formation with a range of hydrocarbons. The lipopeptide biosurfactants produced by CN3 and CN2 enhanced degradations of used motor oil and petroleum sludge in liquid culture. In a shake flask pyrene degradation study, lipopeptide supplementations at 600 and 300 mg L-1 enhanced pyrene degradation to 83.5% and 67% respectively in 24 days compared to 16% degradation in its absence. However, degradation of pyrene was reduced to 57% as the lipopeptide supplementation was raised to 900 mg L 1. This demonstrates that the biodegradation of pyrene was found to increase with an increase in the lipopeptide concentration up to a threshold level.
In a soil bioremedial study, microcosms supplemented with 0.2 and 0.6% (w/w) lipopeptide, 51.2% of 4-ring and 55% of 5- and 6-ring PAHs, 64.1% of 4-ring and 79% of 5- and 6-ring PAHs were removed respectively, compared to, 29% of 4-ring and 25.5% of 5- and 6-ring PAHs removal in the surfactant free control after 64 days of incubation. However, there was no statistically significant change in the degradation rates of low molecular weight PAHs in surfactant amended and surfactant free controls. The degradation of 5 and 6 ring PAHs was significantly enhanced (p < 0.05) in the higher surfactant dosage compared to the lower dosage. The results of this work demonstrated that the use of biosurfactants is a viable option to reduce clean-up time and for effective remediation of soil and aqueous media contaminated with polycyclic aromatic and petroleum hydrocarbons. The study demonstrated potential applications of microbial surfactants and provided an insight for further investigation of their large scale production for commercial applications.