Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 82:1004–1011 (2007) Effect of an emulsifying surfactant on diesel degradation by cultures exhibiting inducible cell surface hydrophobicity Gita Mohanty and Suparna Mukherji ∗ Centre for Environmental Science and Engineering (CESE), Indian Institute of Technology (IIT) Bombay, Powai, Mumbai 400076, India Abstract BACKGROUND: Accumulation of petroleum hydrocarbons in the environment is often attributed to inherently slow degradation rates. This study was conducted to determine if diesel biodegradation by cultures exhibiting inducible cell surface hydrophobicity could be enhanced by addition of a chemical surfactant promoting emulsification. Experiments were designed to determine quantitatively the enhancement in diesel degradation caused by Triton X-100 for two bacterial cultures, Exiguobacterium aurantiacum and Burkholderia cepacia. RESULTS: In the presence of Triton X-100 at twice the critical micelle concentration (CMC) the extent of degradation in whole diesel in 5 days was 1.6–1.8 times that observed over 15 days with no surfactant. The enhancement in overall degradation of diesel range resolved peaks (DRRP, representing n-alkanes and branched alkanes) was 1.76 and 1.26 times for E. aurantiacum and B. cepacia cultures respectively. Although an enhancement in decay rate was observed for all n-alkane components, it was most significant for C19, C25 and pristane. CONCLUSION: For cultures exhibiting direct interfacial uptake, the beneficial effect of an emulsifying surfactant found here is in contrast to earlier studies reporting a detrimental effect due to detachment of cells from the oil/water interface. Surfactant addition may have significant potential in enhancing degradation of the more recalcitrant components.  2007 Society of Chemical Industry Keywords: Triton X-100; NAPL biodegradation; maximum decay rate; pristane INTRODUCTION Bioremediation of oil-polluted environments is inher- ently slow owing to bioavailability limitations. 1 Chem- ical surfactants can potentially reduce the timescale of hydrocarbon bioremediation by enhancing pseudo- solubilisation within surfactant micelles or by pro- moting emulsification. However, while some studies have found that hydrophobic contaminants partitioned into the hydrophobic core of surfactant micelles are bioavailable, 2,3 other investigators have reported a strong inhibition in biodegradation of polynuclear aromatic hydrocarbons (PAHs) in the presence of surfactants above their critical micelle concentration (CMC). 4 In a recent study, preferential uptake of the hydrophobic part of Tween series surfactants by a PAH-degrading strain causing destabilisation of micelles and release of PAHs from the micellar core was found to enhance phenanthrene bioavailability. 5 For cultures that produce extracellular biosurfactants, the effect of addition of chemical surfactant (ionic or non-ionic) has been shown to be negligible. 6 Bardi et al . 7 reported that enhancement in degradation in the presence of surfactants is often negligible for aliphatic compared with aromatic hydrocarbons. Thus the impact of chemical surfactants on oil/hydrocarbon biodegradation may potentially depend on multiple factors, e.g. the chemical nature of the surfactant (ionic or non-ionic) and its hydrophile/lipophile bal- ance (HLB) number, the concentration of surfactant applied with respect to its CMC, the impact of the surfactant on micro-organisms (growth-supporting or toxic), the nature of hydrocarbon contaminants (aliphatic or aromatic) and the type, physiological state and inherent uptake mechanism of micro-organisms. Emulsification causes interfacial area enhancement, which can potentially enhance biodegradation rates significantly. However, studies with multiple surfac- tants (Brij 30, Tergitol series surfactants and others) have illustrated that the degree of enhancement in oil surface area, i.e. the dispersing ability of chemical sur- factants, has no direct effect on oil utilisation rates by micro-organisms. 8 In emulsified crude oil the bacterial oxidation rate of n-alkanes is significantly affected by the physiological state of the bacterium, i.e. its surface hydrophobicity and inherent ability for synthesis of surface-active emulsifying agents. For Rhodococcus sp. ∗ Correspondence to: Suparna Mukherji, CESE, IIT Bombay, Powai, Mumbai 400076, India E-mail: mitras@iitb.ac.in Contract/grant sponsor: Department of Biotechnology, Government of India, New Delhi (Received 2 March 2007; revised version received 18 April 2007; accepted 20 May 2007) Published online 6 July 2007; DOI: 10.1002/jctb.1753  2007 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2007/$30.00