Degradation Of Perfluoroalkyl Sbstances And Total Pertroleum Hydrocarbons In Soil By Dual-frequency Ultrasound Coupling Persulfate

Soil contamination caused by total petroleum hydrocarbons(TPH)is a significant issue in China.Unfortunately,the extinguishment of petroleum fires and regular fire drills also result in a huge release of aqueous film forming foams(AFFF),leading to perfluoroalkyl substances(PFAS)contamination environmentlly.Hence,some oil industries face the co-contamination of TPH and PFAS.Among numerous techniques for soil remediation,Na2S2O8 is increasingly used due to its lower cost,longer lifetime,higher stability,production of benign end products,etc.However,the Na2S2O8 application still face difficulty in pollutants desorption from soil and limited diffusion of Na2S2O8,resulting in low degradation efficiency.To improve the degradation,this study introduced ultrasound into soil and constructed an ultrasound coupling Na2S2O8(ultrasound/Na2S2O8)system towards synergic and effective PFAS and TPH degradation.To accelerate the degradation,dual-frequency ultrasound was combined with persulfate(dual-frequency ultrasound/Na2S2O8)to synergistically degrade PFAS and TPH.This research aimed at investigating the(1)synergic effectiveness of dual-frequency ultrasound/Na2S2O8,(2)effects of potential influencing factors on degradation efficiency,and(3)the degradation pathways and mechanisms.Although ultrasound removed 84%of PFOA and 49%of TPH from the soil,respectively,79%of PFOA and 43%of TPH were transferred into the supernatant.The optimal removal efficiencies of PFOA and TPH obtained by the response surface method were 84.32%and 48.04%,respectively.The corresponding ultrasound power were 500 W and 130 W,reaction time was 120 min and water-soilratio(w/w)was 5.The optimized degradation efficiencies of TPH and PFOA were 5.01%and 5.27%,respectively.The corresponding ultrasound power were 500 W and 96 W,and the reaction time and water-soil ratio were both 120 min and 5,respectively.The electron spin resonance(ESR)results showed that the main radical produced by the ultrasound was ·OH.The scanning electron microscopy(SEM)analysis indicated that the ultrasound could break soil aggregates and enhance the organic desorption.The ultrasound/Na2S2O8 system achieved effective degradation of TPH(82%),increasing the content of TPH,Cl-,HCO3-and HA inhibited the degradation.The PFAS degradation in this system was still negligible,although the highest removal was 85%,80%of the PFOA was transferred into the supernatant.Interestingly,the combination of dual-frequency ultrasound and Na2S2O8 significantly improved the PFAS degradation from the supernatant with the defluorination percentage of 100%,86.9%and 46.5%for perfluorooctanoic acid(PFOA),perfluorooctane sulfonate(PFOS)and 1H,1H,2H,2H-perfluorooctanesulfonic acid(6:2 FTS),repectively.This system effectively degraded simulated PFOA soil(76.7-91.3%),real PFAS soil(totally 71%for 14 types of PFAS)and real TPH soil(88.9%)with negligible residue in the supernatant.Similar to the ultrasound/Na2S2O8,higher power,acidic conditions and high concentration of Na2S2O8(not excessive)benefited the degradation in dual-frequency ultrasound/Na2S2O8.Neither the ultrasound nor the temperature alone was responsible for the activation of Na2S2O8,but the heat generated by ultrasound played a crucial role in effectively activating Na2S2O8.Even the dual-frequency ultrasound is much stronger than single-frequency one,it was not enough to activate Na2S2O8 at room temperature to achieve effective degradation.The kinetics investigation indicated that the TPH degradation rate in ultrasound/Na2S2O8 system could be expressed as r=1.4 L·g-1·min-1[TPH]0[Na2S2O8]1.While the degradation rate for dual-frequency ultrasound/Na2S2O8 was more than 25 times of that for ultrasound/Na2S2O8,inidacting that the combination of dual-frequency ultrasound and Na2S2O8 was not simply addition with each other,but with synergistic effect of degradation.The quenching and ESR tests indicated that,for the single or dual-frequency ultrasound/Na2S2O8,although the main radicals were SO4·-and OH,SO4·-was the predominant contributor to the synergic degradation.However,the generation of SO4·-and OH for dual-frequency ultrasound/Na2S2O8 was 1.6 times and 2.5 times as much,respectively,as for ultrasound/Na2S2O8.The degradation intermediates suggested a degradation mechanism of a loss of sequential CF2 units from PFOA and its intermediates.Finally,the SEM images and the specific surface area verified that the dual-frequency ultrasound could break the soil aggregates more effectively than the single-frequency one,enhancing the organics desorption and the oxidants diffusion.Overall,the dual-frequency ultrasound/Na2S2O8 synergistically enhanced the cavitation and pyrolysis,the generation of SO4·-and OH,the organics desorption and oxidant diffusion,which significantly improved the PFAS defluorination and TPH degradation in soil.This study has great significance on the research and application of soil remediation and the development of ultrasound/persulfate process.