Rhamnolipid Functioned Migration And Transformation Of 17α-ethinylestradiol In River Water/sediment System And Its Bioavailability Study

Recently, as emerging contaminants, pharmaceuticals and personal care products (PPCPs) were widely detected in the aqueous environment. Most of PPCPs have moderate hydrophobic properties. They are tended to adsorb and deposit onto sediments and soils. It thus leads them long-term persistent. Biosurfactants, taking place of chemical synthetic surfactants, are usually applied to change migration and transformation of the hydrophobic toxic contaminants and/or enhance their biodegradation of them, which are important parts of the modern bioremediation technology.Rhamnolipids are among the best known biosurfactants and are usually produced as multicomponent mixtures of homologues. However, the predominance of individual rhamnolipidic homologue in each bioremediation part, and the relationship with their molecular components for such biosurfactant mixtures had not been clearly understood. Because of the moderate hydrophobic property, the environmental chemical behaviors of PPCPs are differentiated from that of the complete hydrophobic hydrocarbons. It thus may lead to different functional performances for rhamnolipids. Accordingly, in this study, molecular structures and micellization properties of the purified rhamnolipidic homologue fractions were characterized. The most estrogenic synthetic 17α-ethinylestradiol (EE2) was chose as typical PPCPs contaminant. Based on these, the rhamnolipidic functioned performances for EE2 including micellar solubilization, sorption and desorption, and enhanced biodegradation were systematically investigated step by step within different environmental medium systems. Finally, EE2 migration and transformation mechanisms in modeled sediment column affected by rhamnolipids were explored by mixable displacement experiment. Research results were as follows:(1) Two representative rhamnolipidic fractions, RL-F1 and RL-F2, were seperated and purified from the secretions of Pseudomonas aeruginosa mutant strain MIC-N146. They were characterized as mono- and di-rhamnlipids through HPLC-MS and FTIR analyses. The critical micelle concentration of RL-F1 and RL-F2 were 0.11 and 0.07 mM, respectively. DLS and TEM measurements indicated that, the micelle size of RL-F2 was relatively bigger than RL-F1. Size distribution for RL-F2 micelles showed multi-dispersity, while monodisperse mode was observed for RL-F1. It also found that aggregate shape transition was tended to accompany with increase of the RL-F2 bulk concentration.(2) Solubilization equilibrium and solubilization kenitics experiments were carried out as batch tests. RL-F2 had a higher maximum solubilizaiton capability of EE2, but observed smaller mass transfer rate than RL-F1. Organic solubilization brought about size increase of the micelles. It was found that the increase of micelle size and size dispersity for RL-F2 aggregates were relatively larger after EE2 solubilzation. Combination of the moderate hydrophobic EE2 and rhamnolipidic micelle were not caused by a single interaction mode. Once solubilization, EE2 molecules located in the core-shell interface having moderate polar microevironment, as well as the hydrophobic core part. According to model analysis of the micelle/solubilizate composite structure, the type of rhamnolipid and aggregated geometry has a great impact on the incorporation of solubilizate into surfactant aggregates.(3) EE2 distribution within sediment-water sorption and desorption systems effected by rhamnolipids were investigated and compared. When freshly-treated sorbate was used, in general, RL-F2 was observed to inhibit EE2 desorption from sedment. In contrast, the coexistence of RL-F1 produced considerable net enhancement for EE2 sorption. When aged sorbate was used, both rhamnolipids could facilitate mobilization of EE2. But the desorption efficiency functioned by RL-F2 was generally larger than that of RL-F1. It was consistent with its higher solubilization capacity for EE2. Experimental data was further estimated by the conceptual model that considered the sorbed rhamnolipids and aqueous micelles for organic partitioning. It demonstrated strongly rhamnolipid type being a most important influencing factor towards HOC distribution.(4) The effect of rhamnolipids on the bioavailability of EE2 in sediment/water system was tested. Addition of rhamnolipids was observed to not change the process of microbial transformation of EE2. The enhancement extend of EE2 biodegradation functioned by RL-F1 was much larger than that by RL-F2. After comparing the actual degradation-rate enhancing coefficients with the theoretical ones, it could be inferred that the crude rhamnolipids functioned effect for HOC's bioavailability is not a simple addition action process played by each homologue fraction. The functional effect of individual component will be suppressed within a mixed system. The contribution of rhamnolipid homologue to EE2 biodegradation enhancement was not positively proportional to its corresponding solubilization and mobilization efficacy. When in the present of rhamnolipid, there were diverse paths for HOC uptake by microorganisms. The total bioavailability of the rhamnolipid micelle after HOC solubilizing played a major role for biodegradation.(5) When with higher pore water velocity, RL-F2 functioned EE2 breakthrough rate was about 83.3%, while biotransformation part covered only 13.6%. It led to a marked decrease in the final residual rate. When the pore water velocity was lower, EE2 was largely retained within the sediment column due to enhanced sorption. Most of the sorbed EE2 went through biodegradation process. The biodegradation rate was enhanced as high as 75.3%. It also led to an efficient removal of HOC.Taken together, rhamnolipid-homologue functioned performance present high specificity and inconsistency when within different environmental medium and effect process. In actual applications, one should carry out an overall consideration in terms of the remediation target, biosurfactant homologue composition and each homologue performance. Monorhamnolipid showed good sorption enhancement of EE2. It also could increase the EE2 bioavailability in a large extend. This provides an important technique for HOC removal within complex environment solid system, i.e. a combined technique based on monorhamnolipid functioned HOC immobilization and enhanced biodegradation. The results of this study provide useful information for controlled transport of PPCPs and bioremediation enhancement technology. It also enriches the research content for bioremediation technology.