Enhanced biodegradation of diesel oil by newly synthesized biosurfactants and classified microorganisms

Abstract

Five mycolic acids [2-alkyl-3-hydroxyfatty acid: $R-1C^*H(OH)C^*HR_2COOH]$ were synthesized using acyl chlorides with different lengths of alkyl chain (total carbon numbers of mycolic acids, 12, 16, 20, 24, 36). The mycolic acids were synthesized in three steps: (1) dimerization of acyl chloride into alkyl ketene dimer, (2) selective reduction of C=C to C─C by hydrogenation, and (3) b-lactone ring cleavage under alkaline conditions. The relationship between the chemical structure of mycolic acids and surface active properties was determined. The critical micelle concentrations (CMC’s) of $C_{12-}$, $C_{16-}$, and $C_{20-}$ mycolic acid were $2.2 x 10^{-4}$, $1.36 x 10^{-4}$, and R7.4 x 10^{-5}$ M, respectively. As the carbon number increased, surface tension at CMC value was also lower; the values for $C_{12-}$, $C_{16-}$, and $C_{20-}$ mycolic acid were 46.54, 43.59, and 41.57 dynes/cm, respectively. Emulsifying activities of mycolic acids were determined for n-tetradecane, n-hexadecane, cyclohexane, and diesel oil. Dynamics of diesel oil biodegradation were investigated at 1,000 to 20,000 ppm as the initial substrate concentrations using Gordonia sp. and Rhodococcus sp. isolated from wastewater previously. Following the gas chromatogram profiles of diesel-degradation, diesel oil with concentration of up to 10,000 ppm was efficiently degraded by these strains. At the 15,000 and 20,000 ppm concentrations, however, the degradation was not effective by the strains. Then, the biodegradation enhancement of diesel oi1 (15,000 and 20,000 ppm) by the synthetic mycolic acid biosurfactant was investigated. At inoculated cultures, the degradation of diesel oil was stimulated by the biosurfactant. For comparison, diesel oil degradation in batch incubations was measured by addition of rhamnolipid and other surfactants. The synthetic mycolic acid stimulated the degradation to a greater extent than any other surfactant tested. In addition, a Gram-positive, non-motile...