Influential Factors Of Surfactant-enhanced Remediation Of PAHs Contaminated Soils And Its Mechanisms

With the rapid development of coking and petrochemical industry, incomplete combustion of organic matter in coal and petroleum has increased, thus a large number of polycyclic aromatic hydrocarbons(PAHs) are released into the soil environment. PAHs are sorbed and deposited to soils, which cause heavy pollution of PAHs in soils. How to remediate PAHs contaminated soils efficiently is a focus of ecological remediation. For surfactant micelles show good performance on solubilizing PAHs and removing it from soils, surfactant enhanced remediation(SER) has become a fast and efficient way to remove PAHs from contaminated soils. Especially, duo to the mixtures of anionic and nonionic surfactants forming mixed micelles to reduce sorption of surfactant to soils, it became a hot spot of SER. However, The effects of salt, clay minerals and carbon nanomaterials on anionic and nonionic surfactant and SER are still unclear. Therefore, with a forcus to improve SER of PAHs contaminated soils, this paper investigated effects and mechanisms of salt, clay minerals and carbon nanomaterials on PAHs contaminated soil remediation, the sorption of surfactant to clay minerals and carbon nanomaterials and its sorption model, the distribution of PAHs between clay minerals, carbon nanomaterials and surfactant solution, systematically study of how to get efficient soil remediation in the presence of the above three factors. The main results of the paper are as follows:(1) Salts promote solubilization and desorption of pyrene by mixed anionic–nonionic surfactants. Salt reduced critical micelle concentration of mixed surfactant, and increased the mass fraction of SDBS in micelles(XSDBS), and more surfactant monomer generated micelles. Molar solubilization ratio(MSR), micelle-water partition coefficient(Km) and shift coefficient R increased significantly in the saline system. Therefore saline mixed anionic–nonionic surfactants have higher solubility of pyrene. The standard free energy change ΔGS0 was significantly reduced, resulting in an easier solubilization of pyrene. Sorption of single and mixed surfactant TX-100/SDBS to soils was inhibited by salt. Therefore, the higher concentration of surfactant in the solution, the higher desorption effectiveness of pyrene. Mechanism of enhanced solubilization and desorption of pyrene include that the salt reduced critical micelle concentration of anionic–nonionic surfactants, and thus, larger numbers of mixed micelles with stronger interaction and more stable structures were achieved.(2) Clay mineral composition of soils have a significant influence on the effectiveness of surfactant washing of pyrene-contaminated soil by anionic–nonionic surfactants. Compared with mixed anionic–nonionic surfactants, 100% TX-100 was most effective in desorbing pyrene from kaolin–rich red soils in southern China, while 100% SDBS was most effective in desorbing pyrene from montmorillonite–rich soils. Pyrene desorption from montmorillonite and montmorillonite–rich soil remained nearly unchanged during the washing time, indicating that pyrene is difficult to desorb from montmorillonite-rich soils; meanwhile the removal of pyrene from kaolin and kaolin–rich soil increased with time and was well fitted by a first-order two-compartment model and a simple first-order model, respectively. In addition, Increasing temperature and shaking speed favor removal of pyrene from soil and clay minerals. in comparison with SDBS, TX-100 have a higher solubility of pyrene and lower sorption amount of surfactant to kaolin-rich soils; SDBS have a lower sorption amount to montmorillonite-rich soils, which is the main mechanism that single surfactant have higher soil washing efficiency than mixed surfactant. TX-100 strongly sorbed to interlayers of 2:1 montmorillonite and SDBS was deposited with Ca2+ and Mg2+ which is main reason for single surfactant show lower sorption amount in comparation with mixed surfactant. When selecting surfactant for SER of PAHs contaminated soils, it is necessary to thoroughly identify the composition of soil clay minerals in order to choose the best surfactant for SER.(3) Carbon nanomaterials have influences on the performance of surfactant enhaced remediation. Phenanthrene had low concentrations in SDBS solution and is more easily sorbed to carbon nanomaterials with a decrease of desorption efficiency; while phenanthrene was more easily to solubilize in TX-100 solution and had low sorption amount on carbon nanotubes and graphene. Therefore TX-100 is more suitable for soil remediation in the presence of carbon nanotubes. Compared with carbon nanotubes, the loss of phenanthrene sorbed to graphene is lower in phenanthrene-carbon nanomaterials-surfactant system. When the surfactant concentration is above the critical desorption concentration, solubility of phenanthrene in watercarbon nanomaterial-surfactant system improved linearly. Compared with graphene, carbon nanotubes have higher carbon content, higher specific surface area, higher sorption amounts of phenanthrene, surfactant TX-100 and SDBS. The maximum sorption amount of phenanthrene to carbon nanotubes is 120mg/g, to that of graphene was 104mg/g. In addition, sorption of phenanthrene to carbon nanotubes and graphene fits in Freundlich model. Sorption of TX-100 and SDBS to graphene and carbon nanotubes showed that carbon nanotubes(TX-100 833mg/g, SDBS 850mg/g) had a higher sorption amount of surfactant than graphene(TX-100 562mg/g, SDBS 661mg/g). Sorption of TX-100 to carbon nanotubes and graphene fits linear model, while sorption of SDBS fits Freundlich model. Therefore, compared with graphene, carbon nanotubes have more influence on surfactant enhanced remediation. This study could present scientific evidence for SER of PAHs contaminated soils containing carbon nanomaterials.(4) The results showed that slurry reactor is efficient for pyrene contaminated soil remediation and desorption equilibrium of pyrene was quickly reached in 1h. Increases of temperature, stirring speed and aeration rate promoted removal of pyrene from soils. Optimum conditions of soil washing by TX-100 is at surfactant concentration of 5000mg/L, stirring speed of 200 rpm, temperature of 45°C, aeration rate of 0.75 vvm, slurry concentration of 20%. Desorption of pyrene in contaminated soil fits the first-order mass transfer with singleequilibrium model(R23 99%). Factors of surfactant concentration, temperature of desorption, stirring rate, aeration rate and slurry concentration have significant influences on the efficiency of soil washing of kaolin-rich soil. The result could provide useful technical references for engineering remediation of PAHs contaminated soils.