The Production Of Rhamnolipids And Their Application On Remediation Of Polychlorinated Biphenyls-contaminated Soils

On the basis of summarizing the related references home and abroad, a great deal of experiments had been done to study the production of rhamnolipid biosurfactant and the application of rhamnolipids in the remediation of polychlorinated biphenyls(PCBs)-contaminated soil.The optimized conditions for fermentation medium and the production regulation of rhamnolipids(RL) produced by pseudomonas aeruginosa AB93066 were as follows. The composition of the medium wasρ(yeast)=0.2g/L,ρ(bean oil)=120g/L,ρ(NaNO₃)=6.5g/L,ρ(KH₂PO₄)= 1.0 g/L,ρ(Na₂HPO₄·12H₂O)=1.0g/L,ρ(MgSO₄·7H₂O)=0.1g/L andρ(FeSO₄·7 H₂O)=0.2g/L, pH=6.0. The time range for harvesting RL was between 156h and 168h after fermentation. Scale-up production experiment indicated that RL yield was 56g/L and more under the optimal conditions. Surface tension measurements showed that the extracted RL could decrease the surface tension of deionized water to 29.01mN/m. Further, dirhamnolipid(R1) and monorhamnolipid(R²) were separated and identified from the extracted RL using liquid chromatography/mass spectrometry. The critical micelle concentration (CMC) of R1 and R2 were 0.03mmol/L and 0.04mmol/L, respectively. A new procedure of pretreatment/acid precipitation/ freeze -dryness for rhamnolipid extraction was determined by orthogonally designed experiments. That RL extracted through the new procedures was purer, the yield was 8.8g/L higher and the CMC was 5mg/L lower than that extracted through the conventional procedures. Large amount of harmful solvents such as chloroform and methanol were needed in the conventional procedures and no such toxic solvents were used in the new procedures for the RL extraction.Those surfactant micelles played a main role in solubilization of PCBs by RL. The apparent aqueous solubility of PCBs in RL solution decreased with increasing chlorination of the PCB molecule. RL was more effective for solubilization of PCBs than three nonionic chemical surfactants, such as POE(6), POE(10) and Brij35. Micelle-phase/aqueous -phase partitioning coefficient (K_mc) of PCBs in RL solution increased with decreasing intrinsic solubility of PCBs in pure water.The relationship between Octanol/water partitioning coefficient(KOW) and Kmc was described by linear repression analysis as following: logKmc=0.48logKOW+3.08. The apparent aqueous solubility of individual congener in PCB mixture in RL solution was much lower than the solubility when individual congener was the sole contaminant in RL solution. Na+,Mg²⁺,Ca²⁺ within optimal concentrations range could obviously enhance the apparent aqueous solubility of 2,2',4,4'CB in RL solution, but Ca²⁺ greater than 0.2mM was ineffective. Inorganic anion( such as Cl-, NO3-, SO42-)had little affection on the solubilization of 2,2',4,4'CB by RL. PCBs were synergistically solubilized by RL and nonionic surfactant. The synergistic power of the mixed surfactants for PCBs was positively relative to hydrophile-lipophile balance(HLB) of nonionic surfactant. Impact of pH on the solubilization of 2,2',4,4'CB by RL was relative to the concentration of RL. At a RL concentration of 300mg/L, the aqueous solubility of 2,2',4,4'CB in RL solution decreased when the pH changed from 5.5 to 8.0. At a RL concentration of 2 000mg/L, the aqueous solubility of 2,2',4,4'CB had little change when the pH changed from 5.5 to 7.0, but the aqueous solubility increased with pH increasing from 7.0 to 9.0.Rhamnolipids obviously enhanced PCBs'desorption from soils when their concentrations exceeded CMC. Rhamnolipid R2 with lower HLB was more effective for PCBs'desorption than rhamnolipid R1. The desorption percentage of PCBs from laboratory contaminated soil was higher than that from field contaminated soil. The greater the TOC of soils, the lower the desorption precentage of PCBs. The desorption percentage of PCBs increased as the increase of desorption duration, wash times, pH (above 7), and 48h of desorption duration, and 3 repeat washings were the most effective for the soil washing under the test conditions. Inorganic cationic, such as Na⁺, K⁺, Mg²⁺, Ca²⁺ within optimal concentrations range could obviously enhance desorption of PCBs from soil, and Ca²⁺ greater than 0.25mM or Mg²⁺ greater than 0.8mM was ineffective for increasing desorption of PCBs from soil. The Langmuir isotherm appeared to better describe binding of RL to the soils in the study. The column-washing experiment results showed: 60% of the sorbed PCBs were removed from laboratory contaminated soil using RL solution, but only 20% of the sorbed PCBs were removed from field contaminated soil. PCBs from laboratory contaminated soil were synergistically desorbed by RL and nonionic surfactant. The desorption of PCBs from laboratory contaminated soil was mainly attributed to solubilization of PCBs by RL, but the desorption of PCBs from field contaminated soil was mainly attributed to mobilization of PCBs by RL. Increasing the concentration of binary mixed-surfactants (RL-POE(6)) leaded to increasing precentage of PCBs washed from field contaminated soil.Pseudomonas strain LB400, a PCB specific microorganism, was able to grow rapidly in three culture media with RL, POE(6) or biphenyl as carbon source. The culture media with biphenyl present in solution yielded the highest cell density, while the cell density in culture media containing RL was ranked the second. Biodehradation rate of PCBs in growing cell system was greater than that in resting cell system. Biodegradation rate of PCBs with RL as carbon source was less than that with biphenyl as carbon source, but in growing cell it was very close to that with biphenyl as carbon source. In resting cell system, P.LB400 was not able to fully utilizing RL as carbon source, so RL had negative impact on Biodegradation of PCBs. In growing cell system, P.LB400 was able to fully utilizing RL as carbon source and the cell density of P.LB400 rapidly increased with increasing the concentration of RL, so RL obviously promoted Biodegradation of PCBs. Biodegradation rate of PCBs in the soil wash fluid containing surfactants mixed by RL and POE(6) was a little less than that in the soil wash fluid only containing RL. Precedent photolysis was able to improve Biodegradation of PCBs that remained in the soil wash fluid. A combination of UV pre-irradiation and biological treatment was beneficial to increasing degradation rate of PCBs. The products of photolysis did not inhibit biodegradation of the remaining PCBs.