Cosolubilization Of Mixed Polycyclic Aromatic Hydrocarbons (PAHs) In Surfactant Micelle Systems

Due to the still increasing demand and consumption of fossil fuels and the frequent oil spill accidents,contamination of environments with polycyclic aromatic hydrocarbons(PAHs)is posing ongoing threat to human and ecosystem.According to the bulletin of China‘s nationwide survey of soil pollution status released in April 2014,PAHs are the second largest group of organic contaminants determined.Surfactant-enhanced remediation(SER)has been assessed as an efficient remediation method that can facilitate the transport of PAHs from non-aqueous phase liquids/soils to the aqueous phase,thus increasing their availability for subsequent treatment such as biodegradation and advanced oxidation processes.In the real contaminated environments,PAHs exist in complex mixtures.The effect of interactions between mixed PAHs inside the surfactant micelle on SER efficiency remains to be studied.Furthermore,knowledge about how PAHs exist and coexist in surfactant micelle is yet limited.PAHs are divided into two groups: low-molecular-weight(LMW)formed by 2 and 3 benzene rings and high-molecular-weight(HMW)composed of ? 4 benzene rings.This research has investigated cosolubilization of mixed LMW PAHs or LMW-HMW PAHs in surfactant micelles based on the pollution characteristics of contaminated sites and tried to reveal how PAHs stay in the micelle using the approach of molecular dynamics simulations.The main findings of this research are summarized as follows:(1)Due to the universal existence of LMW PAHs in petroleum-polluted environments,cosolubilization of three LMW PAHs,naphthalene(NAP),phenanthrene(PHE)and fluorene(FLU),in pure Triton X-100(TX100),sodium dodecyl sulfate(SDS)and mixed TX100-SDS surfactant solutions were investigated.NAP/PHE and NAP/FLU,with significantly different hydrophobicities,showed synergistic effect,i.e.solubility of NAP and PHE in TX100 was increased by 20.8% and 38.5% when cosolubilized,respectively.Solubility of PAHs with higher log Kow gained larger enhancement.PHE/FLU with similar hydrophobicity exhibited inhibitive effect due to competition for the same solubilization sites,i.e.solubility of PHE and FLU in TX100 was decreased by 36.1% and 6.5% when cosolubilized,respectively.The cosolubilization trends were consistent in single and mixed surfactant systems,whereas the enhancing and suppressive extent,on the whole,were weakened and strengthened with increasing mole ratio of SDS in the mixed micelles because of reduction in micelle size.Therefore,a detailed survey of the composition of LMW PAHs in a petroleum-contaminated site should be done before remediation to accurately determine surfactant concentrations for SER.(2)Owing to the wide distribution of 3-ring and 4-ring PAHs in urban and coking plant soils,cosolubilization of an LMW PAH,PHE,and an HMW PAH,pyrene(PYR),in pure and mixed surfactant solutions was explored,and the effect of surfactant loss,soil composition,PAH properties and cosolubilization on codesorption of PHE and PYR from soils by SER was further examined.Cosolubilization of PHE and PYR in pure TX100,SDS and mixed TX100-SDS systems showed synergism in increasing each other‘s solubility due to that PAHs in micelle shell could reduce the micelle-water interfacial tension thus increasing solubilization spaces.Soil clay content has more significant impact on adsorption of surfactants onto soils than soil organic matter.Less surfactant loss and soil dissolved organic matter facilitated PHE removal.Yet this failed to occur on PYR due to its larger log Kow making it bound to the soil more strongly.The strengthened TX100 solubilization capacity towards PHE and PYR could increase the two PAHs‘ codesorption efficiency from soil,accompanied by a synergistic extent of 6-15%.However,synergism in codesorption was weaker than that observed in the cosolubilization system(15-18%),which may relate to surfactants‘ solubilization power,surfactant loss to soil and PAH bounding to soil.Occurrence of cosolubilization synergism in codesorption of PAH mixtures from soils suggests the significance of combining PAH-PAH interaction into evaluating SER.(3)Explicit molecular dynamics(MD)simulations were performed to investigate the solubilization characteristics of single NAP,PYR and PHE into an SDS micelle as a function of PAH concentration.The radial density profiles(RDPs)of the center of mass(COM)of PAHs with respect to micelle COM indicated that NAP,PYR as well as PHE could reside in the micelle shell and core regions in line with the experimental results,which may arise from the movement of the three PAHs inside the micelle.In addition,the three PAHs prefer to reside in the micelle shell,especially near the core-shell interface.This may be induced by the more available volume of micelle shell,the formation of hydrogen bonds between NAP and water and the larger molecular volume of PYR/PHE.Meanwhile,NAP,PYR and PHE can form sporadical clusters of 2 to 8 NAP/PYR/PHE molecules depending on the PAH concentration in the systems.By examining several micelle properties(size,shape,internal structure,SDS alkyl chain conformation and orientation,and micelle internal dynamics),the solubil-ization of single PAHs whatever their concentrations are exhibit little effect on the micelle structures and alkyl chain conformation.Simulation results revealed how PAHs distribute between micelle core and shell regions.(4)MD simulations of cosolubilization of NAP/PHE and PHE/PYR in an SDS micelle were further performed.The RDPs of the COMs of NAP,PYR and PHE with respect to micelle COM manifested that the three PAHs could coexist in both the micelle shell and core regions.Though under cosolubilization conditions,NAP,PYR and PHE all prefer to stay in the shell,the movement of the three PAHs inside the micelle along with their different probability to contact with SDS non-hydrogen atoms may contribute to their possibility to coexist in the micelle.Furthermore,in addition to form clusters with the same PAHs,NAP/PHE and PHE/PYR could also form mixed clusters composed of 2 to 14 molecules depending on the total PAH concentration in the mixed PAH simulation systems.The micelle radius of mixed PAH solubilization system was slightly increased to solubilize mixed PAHs compared with that of pure micelle.Yet the other micelle properties(shape,internal structure,SDS alkyl chain conformation and orientation,and micelle internal dynamics)were negligibly affected by the solubilization of mixed PAHs.Cosolubilization simulations confirm our assumption in the experiments that PAHs having different solubilization sites contribute to their coexistence in the surfactant solutions.