Mechanism For Rhamnolipid Biosurfactant To Improve The Microenvironment Of Compost Matrix And Its Application In Composting

In this study, the bacteria which produce biosurfactant were isolated from compost community. The biosurfactant rhamnolipid was produced by an isolate, Pseudomonas aeruginosa, and its purification and characterization were carried out. The functions for rhamnolipid to improve the microenvironment for degradation of organic granules were investigated. Based on these functions, it was further applied to the composting to determine its effect on enhancement of composting efficiency. Finally its potential effect to enhance removal of the petroleum hydrocarbon and metal from composting was primarily investigated.Firstly isolation of biosurfactant producers from composting was carried. Result indicated Bacillus subtilis and Pseudomonas aeruginosa are the important bacteria that produce biosurfactant in composting, in which the Pseudomonas aeruginosa has stronger capability of producing the biosurfactant product in liquid culture with glucose as the sole carbon source. The consequential separation and purification of the product, rhamnolipid, from the culture was performed and HPLC-MS was applied to the purified rhamnolipid for its characterization. Monorhamnolipid and dirhamnolipid with respectively one and two rhamnosyl group in the molecular structure were found to be major components.Next the potential functions of rhamnolipid in composting were investigated experimentally. Based on the analysis on physico-chemical properties and microbial effects presented by literatures, the rhamnolipid was assumed to have effect on water distribution and preservation, bacterial mobility and transportation as well as on organic matter degradation manner in compost granule matrix. The self-degradation of rhamnolipid in composting was also studied. The result showed that rhamnolipid attenuated water evaporation from the matrix which may favor preservation of microbial activity for longer time. It also enhanced water fluidity in the matrix and accelerated its downflow to deep layer of the matrix, which may potentially optimize distribution of added water in the composting. Rhamnolipid at certain concentrations could weaken adsorption of Pseudomonas aeruginosa cells on the granules and may potentially enhance transportation of the bacteria to deep layers or pores in compost matrix, that is, it may favor distribution of local or added bacteria. The organic granule degradation experiment showed that it even enhanced dispersion of organic matter to the aqueous phase in the matrix and meantime it enhanced microbial growth and degradation of organic matter. The SEM detection indicated that interference of rhamnolipid caused translocation of the degradation reaction. The experiment of rhamnolipid self-degradation showed that rhamnolipid could be degraded by compost microorganisms, but not preferentially to other readily degradable substrates. These experimental results indicate that biosurfactant have the potential to improve the microenvironment of compost and enhance composting effect, and it will not cause secondary pollution.Based on the result of the mechanism study, the rhamnolipid was finally applied to composting of municipal solid waste (laboratory scale) to consolidate its functions. First degradation of readily degradable vegetable substrate in the compost material in the presence of rhamnolipid was investigated and the result showed that rhamnolipid did accelerate the degradation to some extent. Next rhamnolipid was introduced to the composting containing petroleum to see the effect of rhamnolipid on degradation of both the compost organic components and the hydrophobic petroleum. As a result, the biosurfactant not only enhanced the composting efficiency by shortening the total time for stabilization of the compost under middle temperature, but also dramatically strengthened degradation of the petroleum hydrocarbons. This result is in consistence with the conclusion that biosurfactant is able to enhance petroleum degradation in soils. Heavy metals in compost is an important factor that influence the quality of compost product, and the compost humic acid has crucial impact on migration of the heavy metals. We finally investigated the effect of rhamnolipid interference on adsorption of Pb2+ by humic acid from the compost material of late composting stage. The result showed that rhamnolipid possibly changed the style of Pb2+ adsorption onto the humic acid and it weakened the adsorption when the rhamnolipid concentration is high enough.In the whole study, production and purification of the biosurfactant rhamnolipid as well as investigation on some of its physico-chemical properties were performed. More significantly its potential functions to improve the microenvironment of organic granule substrate and its application in composting were investigated, and some achievements were obtained. It is a new field for biosurfactant application and may be meaningful to the concerned researchers.