Effect Of Rhamnolipids On The Degradation Of Hydrophobic Organic Compounds

Hydrophobic organic compounds are widely present in the environment and endangering the ecosystem and human health. Phenolic compounds are a special class of hydrophobic organic compounds. On one hand, they have large octanol/water partition coefficient, on the other hand have a relatively large solubility in aqueous solution. Phenolic pollutants are widely found in nature and difficult to be degraded because of their toxicity to microorganisms. Biosurfactants play important roles in the enhancing the biodegradation of hydrophobic organic pollutants because of their amphiphilic structural characteristics. However, it lacks of investigation on the effects of biosurfactants on the degradation of phenolic compounds. In this paper, the influences of biosurfactants on cell surface properties and enzyme characteristics and the association with the degradation of phenol were studied, and rhamnolipids were used as the typical representative of biosurfactants. The degradation of hexadecane was also analyzed in comparison.The influences of rhamnolipid (RL) and Triton X-100on cell surface hydrophobicity and charge properties of Penicillium simplicissimum were studied firstly. It was found that both surfactants significantly enhanced cell surface hydrophobicity of P. simplicissimum. The role of surfactants was related closely to its own concentrations. Cell surface hydrophobicities of P. simplicissimum which was pre-treated by0.005%RL,0.05%RL,0.05%Triton X-100and0.2%Triton X-100were1.7,2.0,3.0and2.4-folds of those of intact biomass, respectively. The pre-treatments of both surfactants enhanced the cell surface zeta potential of P. simplicissimum. It was also found that both surfactants changed the element component on the cell surface, which may be one important reason for the changed cell surface hydrophobicity and charge properties.It was also found that monorhamnolipids can change the cell surface hydrophobicity of Candida tropicalis. Cell surface hydrophobicity increased with the concentration of rhamnolipids from0to19mg/L. However, the increase of the concentration did not cause significant changes in cell surface hydrophobicity as the rhamnolipids concentration was higher than19mg/L. Rhamnolipids also have an impact on the cell surface charge. This effect was not obvious when the rhamnolipids concentration was lower than38mg/L. However, the zeta potential was increased obviously with the rhamnolipid concentration higher than38mg/L. It was also found that rhamnolipids could change the FTIR spectra of cell surface, suggesting that the biosurfactants caused the chemical structure changes on the cell surface of C. tropicalis, which may be another important reason for the biosurfactants to change the cell surface hydrophobicity and charge properties.The pre-treatments of rhamnolipids and Triton X-100increased the adsorption of phenol by P. simplicissimum. The pseudo-second-order model and Freundlich equation were more suitable to describe the adsorption process of phenol than the pseudo-first-order model and Langmuir equation in this experiment, respectively. The increased adsorption capabilities may be due to the changed cell surface hydrophobicity and zeta potential.It was found that monorhamnolipids at concentrations of11.4,19and38mg/L increased the degradation of hexadecane by C. tropicalis, and19mg/L was the optimal concentration. The enhancement may be due to the pre-solubilization of hexadecane by rhamnolipids and the changed cell surface hydrophobicity and charge properties. However,114mg/L monorhamnolipids inhibited the degradation of hexadecane, which may be due to the reduced bioavailability of hexadecane as it moved into the biosurfactant micelle cores. Monorhamnolipids was not toxic to C. tropicalis and degraded as carbon sources. However, hexadecane promoted the growth of C. tropicalis better than equal mass concentration of monorhamnolipids, suggesting that rhamnolipids was not the prior carbon source in the fermentation system. The results showed that the rhamnolipids has potential applications in the bioremediation of petroleum hydrocarbon pollutions.The influence of monorhamnolipids on the degradation of aqueous phenol by C. tropicalis was studied, which was compared to that of Tween80. Phenol was toxic to C tropicalis, but it was still degraded by this strain. Monorhamnolipids was degraded by C. tropicalis in the fermentation process, while the concentrations of Tween80did not change. The addition of surfactant reduced the toxicity of phenol to cells and enhanced cell growth and phenol degradation. The higher the concentration of surfactants, the more obvious this effects. The results suggested the potential application of these two kinds of surfactants in the bioremediation of phenols pollutions. The influences of rhamnolipids on the degration of hexadecane and phenol were compared in this study. Due to the different characteristics of hexadecane and phenol, the effects of rhamnolipids on their degradation were different. But the changed cell surface properties were the common reasons for the enhancement on their degradation.Finally, the effect of dirhamnolipids on the removal of aqueous phenol by laccase was studied. It was found that the biosurfactant was effective in promoting the removal of various concentrations of phenol. Especially during the first24h, the removal of phenol enhanced by3.0CMC dirhamnolipids was4.3-6.4folds of that in the control when the phenol concentration was400mg/L. The increased removal rate reduced the laccase consumption and reaction time in the treatment of phenolic wastewater. In addition, dirhamnolipids led to the complete removal (>98%) of phenol at the initial concentrations of50-400mg/L. The removal of phenol was also enhanced by dirhamnolipids within a certain pH and temperature ranges. These results indicated that the potential application of rhamnolipids in the removal of phenols catalyzed by laccase.