Micelle Behaviors Of Binary Mixed Biosurfactants And Their Applications In Remediation Of Hydrophobic Organics Contaminated Sites

Surfactant enhanced remediation (SEM) has been developed to be a promisingremediation method for the removal of hydrophobic organic compounds (HOCs) fromgroundwater and soil. Recently, the use of a wide range of surfactants in SEMremediation technology are synthetic surfactants. Biosurfactants have gained growinginterest due to their distinct advantages over their synthetic counterparts, such asspecificity, biodegradability, nontoxicity, and a broad range of structures. Mixedsurfactant systems often present superior properties to those with a single surfactantcomponent, such as better efficiency in decreasing surface/interfacial tensions, lowercritical micelle concentration (CMC), higher solubilizing power, and expandedcolloidal stability. Therefore, mixed surfactant systems, especially biosurfactantmixed systems, have good prospects of application for the remediation ofhydrophobic organics contaminated site. However, few recent studies havehighlighted the micelle structure and properties of biosurfactants, especially mixedbiosurfactants. The limited number of works involving these aspects has prevented adeeper knowledge and understanding of biosurfactants application insurfactant-enhanced remediation of hydrophobic organics contaminated. Therefore, itis difficult to guide the application of biosurfactants and mixed bisurfactant forremediation of hydrophobic organics contaminated site.In this study, sophorolipid production strain with high yields was first screenedand main physicochemical properties were determined. Micellar and surfaceproperties of sophorolipid and rhamnolipid mixed systems were studied bysteady-state fluorescence spectroscopy, surface tension measurements, dynamic lightscattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). Then,sophorolipids and rhamnolipids mixed systems were investigated for their application in synergism enhanced remediation of polycyclic aromatic hydrocarbons (PAHs)contaminated soil. Finally, these two types of glycolipid biosurfactants and sorbitolderivants nonionic surfactants were used to develop high efficient and low toxic oilspill dispersants. The main results were summarized as follows:1. A sophorolipid production strain with high yields was screened. The structures andcompositions of the biosurfactant produced by the strain grown on different carbonsources were analysed and physicochemical properties of the biosurfactant were alsostudied intensively. Fermentation medium compositions were optimized for theproduction of sophorolipid.(1) A surfactant producing strain, named as O-13-1, was screened from oilywastewater and identified as Starmerella bombicola based on18S rDNA sequence.The results of TLC, FT-IR and HPLC-MS/MS showed that the biosurfactant producedby strain O-13-1was sophorolipids mixture. Using different media based on glucoseand either oleic acid, rapeseed oil, cottonseed oil, and frying waste oil, thesophorolipids produced mainly existed in the lactone form with the hydroxyl-fattyacids of (ω-1) hydroxyoleic acid (C18:1), such as17-L-([2’-O-β-D-glucopyranosyl-β-D-glucopyranosyl]oxy)-octadecenoic acid-1’,4"-lactone-6’,6"-diacetate.(2) Sophorolipid had high surface activity, such as low surface tension and CMC.Meanwhile, the heat resistance, temperature resistance, and acid and alkali resistanceof sophorolipid were high. The minimum surface tension and CMC of the producedsophorolipids in aqueous solution were found to be37.0mN/m and30mg/L,respectively. Unhindered surface activity of the sophorolipids was found at widerange of pHs (2-10), temperature (heating for2h) and salt concentrations (0-20%).The CMC of lactonic sophorolipid increased with the adding of aliphatic alcohol butdecreased with the adding of NaCl.(3) The orthogonal experiment was used to optimize fermentation mediumcompositions for sophorolipid-producing strain and the compositions were: glucose60g/L, oleic acid60mL/L, yeast extract8g/L, and urea2g/L. The yields ofsophorolipid production were low when using low-cost fermentative substrates, suchas glucose syrup, molasses, cottonseed oil and frying waste oil. Less expensive sophorolipids were obtained by using combinations of glucose and oleic acid/rapeseedoil, and the cost of carbon source per mass yield of sophorolipid production were11040and11425CNY/t. Therefore, glucose, oleic acid, and rapeseed oil can be usedas preferred carbon source for sophorolipids production by O-13-1strain.2. Micellar and surface properties of sophorolipids and rhamnolipids mixed systemswere studied by a variety of technologies.(1) CMC of lactonic sophorolipid (LS) and dirhamnolipid (R2) mixed systemswere determined by steady-state fluorescence spectroscopy. Interaction parameter (β)and free energy were calculated according to Rubingh’s model. In the LS/R2mixedsystems, the experimental CMC values were always lower than that calculatedaccording to Clint’ ideal mixing model (CMCideal). The deviation of experimentalCMC from CMCsidealhad a maximum at the mole fraction of LS0.1, and the CMC ofLS/R2mixed system decreased by61.7%and18.9%compared to single LS and R2,respectively. More negative values of β and thermodynamic parameter (ΔGmicand GE)of mixed systems were found when αLSwere between0.025and0.3. These resultsindicate that strong synergistic interactions between LS and R2occurred in mixedmicelles and the mixed micelles formation is spontaneous process when a smallamount of LS in the LS/R2systems.(2) In view of the fermentation products of sophorolipids and rhamnolipids arealways their homologous mixtures. Micellar and surface properties of sophorolipidshomologs (SLs) and rhamnolipids homologs (RLs) mixed systems were studied bysurface tension measurement. Results showed that the experimental CMC values areobviously lower than the CMCidealvalues in the SLs/RLs mixed system. β was morenegative when a small amount of SLs was added to RLs solution, indicating that SLsin the SLs/RLs systems promote stronger interaction between SLs and RLs. It is ingood agreement with the results of LS/R2mixed systems. The mole fraction of SLs inthe surface (xSLsσ) and mixed micelle (xSLs) increased with the increased of SLs in thesolution (αSLs) and were always higher than αSLs, indicating that both the surfaceadsorption and mixed micelles are dominated by SLs, and the higher contribution ofSLs to synergism. (3) Steady-state fluorescence spectroscopy, dynamic light scattering (DLS), andcryogenic transmission electron microscopy (cryo-TEM) results showed that the Naggvalues of LS/R2mixed systems were small but micelle sizes were large. The Naggvalues were from3to6and monodisperse micelle diameters were in the range of5-20nm when the total concentrations of LS and R2were in the range of0.3-0.8mmol/L. Meanwhile, the large size micelle aggregations were found. The micellediameters of LS/R2mixed systems had a maximum as the mole fraction of LS (αLS) inthe LS/R2systems was0.7, and according micelle diameter was about160nm. BothLS and R2consisted dimeric glucose and long-chain aliphatic, enabled the moleculeto cover large space. The large headgroups and long hydrophobic chain of glycolipidsprevented surfactant aggregation. Consequently, the mixed LS/R2systems formedlarge and incompact vesicular micelles aggregations.3. Facile solubilization and soil washing of polycyclic aromatic hydrocarbons (PAHs)by sophorolipids and rhamnolipids mixed systems (SLs/RLs) were investigated. Thethe effects that might affect solubilization and desorption efficiency, such as pH,salinity, and heavy metal were also investigated.(1) The mixed SLs/RLs systems were more effective for the solubilization anddesorption of PAHs than single SLs and RLs. The molar solubilization ratio (MSR) ofphenanthrene (Phen) and pyrene (Py) had maximums at the mole fractions of SLs was0.7. The MSR of Phen for SLs/RLs mixed system (αSLs=0.7) were0.274. It is1.6and8.6times as much as that for single SLs and RLs, respectively. For Py, the MSR was0.079and the corresponding multiples were1.80and5.3, respectively. The desorptionpercentage of Phen and Py for SLs/RLs system (αSLs=0.7) had a maximum of71.5%and59.3%with the total glycolipid concentration of4.2mmol/L, respectively. It is39.7%and35.7%higher than SLs, and about33.2%and32.7%higher than RLs,respectively.(2) The solubilizing capabilities of SLs/RLs mixed system (αSLs=0.7) increasedwith the increase of pH from5.5to8.0and had maximum when the solution had pHvalues between5.5and6.0. The effect of salinity on the desorption of SLs/RLssystem depended on the concentrations of glycolipid and the difference of salinity. For SLs/RLs mixed system, the desorption of Phen and Py from soil were promotedby the adding of NaCl with the concentration of0.01mol/L when the total glycolipidconcentrations were more than4mmol/L. However, when the concentration of NaClincreased to0.05mol/L, the desorption of Phen and Py from soil were inhibited forboth SLs/RLs mixed system and single SLs and RLs. Meanwhile, the salt resistanceof SLs/RLs mixed system was higher than single SLs and RLs. Therefore, mixedSLs/RLs systems improved salt tolerance of SLs and RLs. Heavy metal (Cadmium)has no significant effect on the desorption of PAHs from soil for single SLs, RLs, andmixed SLs/RLs system.4. SLs/RLs mixed systems were used to prepare oil spill dispersant. High efficient,low toxic, and environmentally friendly oil spill dispersants were developed. Thesuitable application environmental conditions were identified.(1) To develop more efficient and less toxic oil spill dispersant of concentratedtype, two kinds of glycolipid biosurfactants (sophorolipid and rhamnolipid) and twokinds of sorbitol derivant nonionic surfactant were chosen in this study. Oneoptimized dispersant formulation SRTG-16was identified by uniform design methods.The formulation composition was as follows: sophorolipids/rhamnolipids/sorbitolderivant1/sorbitol derivant2/solvent:6.80/0.83/3.28/39.10/50. The dispersioneffectiveness (DE) of the dispersant was higher than single surfactant. The dispersanthad the ten minute’ DE of46.30%for QHD32-6crude oil.(2) The dispersant had higher DE when it was used in suitable environmentalconditions. DE of dispersant kept high at the dispersant-to-oil ratio from1:10to1:25.The maximun DE of56.65%was found at5℃. pH and salinity had no significantinfluence on DE of dispersant. It can be concluded that the dispersant is suitable forremediation of oil spill not only in fresh water but also in seawater.(3) Danio rerio (freshwater fish) and Tridentiger trigonocephalus (saltwater fish)were used to study the acute toxicity of dispersant SRTG-16. Results showed thatlethality rate for Danio rerio at the end of48hours was0%, and for Tridentigertrigonocephalus, lethality rate was also0%at the end of24hours. It is far better thanthe demand of national standard (GB18181.1-2000). The results showed that the dispersant had lower toxicity.(4) For the dispersant, the ratio of BOD/COD was33.8%, meeting the demand ofnational standard (GB18181.1-2000), in which this value is greater than30%. Theresult showed that the oil spill dispersant was biodegradable.The results above showed that the dispersant obtained could be used in oil spillresponse operations under appropriate conditions.