ARTICLE; AGRICULTURE AND ENVIRONMENTAL BIOTECHNOLOGY Desorption kinetics of polycyclic aromatic hydrocarbons (PAHs) from contaminated soil and the effect of biosurfactant supplementation on the rapidly desorbing fractions Fisseha Andualem Bezza and Evans Martin Nkhalambayausi-Chirwa* Environmental Engineering Group, Department of Chemical Engineering, University of Pretoria, Pretoria, Republic of South Africa (Received 5 November 2014; accepted 9 March 2015) There are often two phases in the desorption of polycyclic aromatic hydrocarbons (PAHs): an initial phase of rapid desorption and a subsequent phase of much slower release. By assessing the rapidly desorbing fraction of PAHs, a direct measure of the microbially degradable component of PAH contamination can be obtained and achievable bioremediation performances can be predicted. In this study, microbial biosurfactant produced by a Pseudomonas aeruginosa strain, identified as a lipopeptide by attenuated total reflectance Fourier transform infrared spectroscopy, was investigated for its efficacy in enhancing PAH desorption and mobilization in a spiked soil system. The desorption of pyrene and phenanthrene from the artificially spiked soil was enhanced 3.54.0 times at 700 mg L ¡1 lipopeptide amendment than at 150 mg L ¡1 amendment or in the unamended soil. The amount desorbed was generally in direct proportion to the amount of lipopeptide present. Mathematical modelling using a first-order two-compartment model was applied to simulate the process of desorption from the soil in the presence of different concentrations of lipopeptide and to predict the effect of the biosurfactant on the rapidly desorbing fraction. With the increase of supplementation of lipopeptide from 150 to 700 mg L ¡1 , the rapidly desorbing fraction, which is generally considered to be the bioavailable fraction, increased from 18% to 73% and from 6% to 51% for phenanthrene and pyrene, respectively. This shows the potential application of the biosurfactant in increasing the bioavailable fraction and enhancing the bioremediation of PAH contaminated media. Keywords: biosurfactant; desorption; PAH; two-compartment model Introduction Polycyclic aromatic hydrocarbons (PAHs) are considered hazardous for human health due to their known or sus- pected genotoxic, mutagenic and carcinogenic potential. [1] They are ubiquitous pollutants and are generated mainly from anthropogenic activities such as the burning of fossil fuels, the use of wood preservatives such as creo- sote and the generation of wastes from coal gasification plants and other industrial activities.[2,3] As PAHs are highly hydrophobic, they interact strongly with organic matter in the soil, which is a major pool for hydrophobic contaminants.[4,5] PAHs are persistent organic pollutants, which is mainly due to their molecular stability and hydrophobic- ity.[6,7] Bioremediation is generally considered as a promising option for the complete removal and destruc- tion of contaminants.[7,8] However, bioremediation can be limited by the bioavailability of soil-bound PAHs due to their low aqueous solubility, high hydrophobicity and strong sorption to soil, which is exacerbated by the long ageing of contaminants in field-contaminated soils.[9] As a consequence, the bioremediation of PAHs in soilwater systems depends strongly on their desorption rates from the soil surface and the subsequent incorporation of the pollutant into the bulk aqueous phase,[10] since it is the aqueous phase where most microorganisms take PAHs from.[11] One method to enhance the PAH desorption rate into the aqueous phase is to add surfactants. Surfactants are known to improve the efficiency of desorption and bio- availability of hydrophobic organic compounds (HOCs) through enhancing their solubility in aqueous systems. [1214] It has been suggested that the underlying mecha- nisms of surfactant-enhanced removal of PAHs from soil include two steps: mobilization and solubilization.[15,16] The mobilization mechanism occurs at concentrations below the surfactant critical micelle concentration (CMC).[17] Phenomena associated with this mechanism include reduction of surface and interfacial tension, reduc- tion of capillary force, wettability and reduction of contact angle.[16] In turn, above the surfactant CMC, solubiliza- tion takes place, i.e., incorporation of these molecules into a micelle (for review see [18]). Surfactants have been found to enhance microbial remediation of PAH-contami- nated soils.[19,20] In recent years, microbially produced biosurfactants have found a new area of application in environmental remediation processes. Biosurfactants possess distinct *Corresponding author. Email: evans.chirwa@up.ac.za Ó 2015 The Author(s). Published by Taylor & Francis. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unre- stricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Biotechnology & Biotechnological Equipment, 2015 Vol. 29, No. 4, 680688, http://dx.doi.org/10.1080/13102818.2015.1028444