Remediation Of Phenanthrene Contaminated Soil By Sodium Dodecyl Sulfate Washing Coupled With Advanced Oxidation Processes

Polycyclic aromatic hydrocarbons（PAHs）are formed by two or more unsubstituted benzene rings sharing carbon atoms.There are hundreds of PAHs,most of which have strong toxicity and carcinogenicity.In fact,90%of the global PAHs pollution is in the soil.However,traditional physical or biological remediation technologies have low efficiency and incomplete effects.In this study,surfactant（sodium dodecyl sulfate as an example）washing coupled with advanced UV-based oxidation processes were used to investigate the remediation of phenanthrene（PHE）contaminated soil.The concentration of sodium dodecyl sulfate（SDS）,washing time and the water-soil ratio are proportional to the PHE removal.Using 20 g/L SDS with a water-to-soil ratio of 20:1,the highest PHE washing removal efficiency of 92.9%can be achieved after 24 hours treatment.However,when the surfactant concentration is too high,a large amount of foaming in the soil will block the pores and affect the soil ecology.Moreover,soil washing only transfers PHE from soil to liquid phase.Further treatment of the effluent is required to reduce the risk of secondary pollution.Larger dosage of effluent（larger water-soil ratio）will increase the load of the secondary treatment.Soil washing reached equilibrium in 8-12 h,subsequent extended washing time had less effect on PHE removal.In order to further degrade PHE while recovering surfactants,advanced oxidation process were used to treat the effluent.Both the UV alone and UV/PDS systems can effectively degrade the PHE in the effluent（the degradation rate is higher than 98.2%）,and the SDS recovery rate is above 95.2%.Through radical quenching experiments and EPR analysis,the ¹O₂ was the main oxidizing species in UV alone system,while ¹O₂ as well^·OH and SO₄^·-were responsible for PHE removal in UV/PDS system.PHE can be used as a photosensitizer to generate ¹O₂ under UV irradiation,and directly attack PHE in SDS micelles or vesicles.The active radicals generated by UV/PDS attack PHE during the change of the molecular structure of the surfactant.The selective degradation of PHE in both systems can be performed under a wide range of p H.The higher the concentration of surfactant,the stronger the inhibition of PHE degradation.PHE degradation is not affected by its initial concentration,that is,PHE degradation meets the characteristics of zero-order reaction in UV and UV/PDS systems.The acute toxicities of the simulated soil effluent before and after UV or UV/PDS treatment were evaluated by activated sludge inhibition tests.The UV irradiation process may produced more toxic intermediates than UV/PDS process.The possible intermediates of PHE degradation were identified by liquid chromatography-mass spectrometry（LC-MS）and the corresponding degradation pathways in both systems were proposed.The recovered SDS could be directly used in soil washing without surfactant supplement and nearly the same washing efficiencies were obtained during three cycles.Based on the strong selectivity of the UV alone and UV/PDS systems for PHE degradation,the simulated mud treatment was used to remediate the PHE contaminated soil.UV and PDS alone systems have limited PHE degradation ability,while UV/PDS system shows a good advantage.The addition of 8 m M PDS in UV system,after 2 h of treatment,the degradation of PHE can reach 83.9%.However,when SDS was added to the mud oxidation system as a solubilizer,the removal efficiency of PHE under PDS and UV alone systems were improved.This is because the SDS can compatibilizing PHE,and the PHE in the liquid phase is more easily for degradated.SDS was added to UV/PDS system,PHE degradation efficiency was suppressed.This is because more active oxygen species were generated,and the SDS was partially degraded during the reaction process.Desorption and mass transfer of PHE may inhibit by the hydrophobic group produced by SDS degradation,thus,PHE degradation was inhibited.