Rhamnolipid Fermentative Production And Removal Of Lead And Cadmium In Contaminated Soil

Rhamnolipid(RL) is an efficient anionic biosurfactants which bears the capacity of emulsifying, foaming, anti-bacteria, anti-virus, and anti-mycoplasma. To date, rhamnolipid is widely used in fine chemicals, oil recovery, environmental protection, soil remediation and other fields, and has further application potential. In this paper, fatty acids addition and semi-solid fermentation technology were used to produce rhamnolipid by Pseudomonas aeruginosa AB93006 with glycerol as the carbon source. Also, soil remediation applications of rhamnolipid to remove Pb and Cd in contaminated soils were investigated preliminarily.Firstly, effect of fatty acid addition on rhamnolipid production was investigated. Results showed that addition of different fatty acids had significant impact on rhamnolipid production. Among them, addition of myristic acid could improve the yield of rhamnolipid, and addition of palmitic acid and stearic acid had little impact on rhamnolipid production. However, addition of lauric acid and decanoate inhibited rhamnolipid biosynthesis. The optimized medium compositions are as follows(g·L-1): myristic acid 8, glycerol 50, NaNO3 6. Under this condition, the rhamnolipid yield reached 13.21 g·L-1.Transcription analysis of related fatty acid metabolism genes at different fermentation times were conducted by real-time PCR(RT-PCR). Transcription levels of β-oxidation related genes(fadD1、fadA), glycolysis-related genes(algC、rmlB) and rhamnolipid biosynthesis gene(rhlA) increased to some extent after myristic acid addtion. In addition, the effect of myristic acid addition on the structure, components and properties was studied. The proportion of single rhamnolipid component increased, emulsifying ability improved, and the critical micelle concentration(CMC) reduced to 63.45 mg·L-1.Secondly, in order to control the bubble problem, a semi-solid state fermentation(SSF) was tested for rhamnolipid production using rapeseed and wheat bran as a solid matrix. The optimal conditions for semi-solid fermentation by single factor experiment and response surfaces were as follows: the proportion of rapeseed meal and wheat bran matrix 1.24:1(w/w), inoculation amount 10%(v/v), glycerol 45.60 g·L-1. Under this condition, the rhamnolipid production reached 19.16 g·L-1. According to structural analysis by MALDI-TOF MS, the proportion of single rhamnolipid from SSF increased as compared with submerged fermentation, and correspondingly the emulsifying capacity increased. Also the CMC decreased to 64.98 mg·L-1.Finally, the rhamnolipid was applied for removal of heavy metal Pb、Cd in contaiminated soil. The soil polluted by Pb and Cd was treated with rhamnolipid solutions from SSF and the ability to remove Pb and Cd in the soil under different conditions was evaluated. The optimal treating conditions were as follows: concentration of rhamnolipid 3%(w/v), leaching time 6 d, the highest removal rate of Pb 39.27%, and the highest removal rate for Cd 70.28%. The heavy metal binding form in the contaminated soil was determined for Pb and Cd. As a result, the exchangeable form of Pb and Cd was easy to remove, the carbonate-bound form and Fe-Mn oxide-bound form were followed by, and the organic-bound form and residual form were hard to remove.