Mechanochemical Degradation And Resource Disposal Of Perfluorooctanoic Acid

Perfluorooctanoic acid（PFOA）has been widely used in various industry applications such as the aviation,transportation,electronicand household applications as cookwares and appliances due to the excellent thermaland chemical resistance and unique surface activity.The diverse applications of PFOA lead to its massive and continuous release into the natural environment.PFOA has been determinedworldwidein both the environmental media and organisms.Due to its toxicity and potential carcinogenicity,selecting the appropriate remedial strategies of PFOA has become one of the hotspots in the field of environmental protection.Because C-F bondswere extremely strong,PFOA was usually chemical and thermal stable,and became one of the organic pollutants which weremost diffcult to be destructed.Mechanochemistry was regarded as the most promising non-incineration technology for the treatments of the persistent organic pollutants solid waste.However,the researches on the disposal of PFOA solid waste werestill limited at present.The object of this dissertation is committed to study the resource utilizationand innocuous disposal processof PFOA by mechanochemical（MC）methods.The main points of this thesis were summarized as follows:In Chapter 1,the basic environmental and chemical properties of PFOA as well as the chemical disposal methods were summarized systematically.The mechanochemical methods which were applied to destruct the organic pollutants,such as the organic bromide and perfluorochemicals were also reviewed.Meanwhile,thekey points that needed to be solved in the MC treatments of PFOA solid waste were pointed out.In Chapter 2,the required pretreatment and analytical methodswere established for identifying and quantitating residual PFOA,F^–and degradation intermediates during the MC treatments.As for the pretreatments of milled samples,ethanol was used as the extractant for extracting the residual PFOA.Utilizing the HPLC system with a CAD detector and C8 reverse phase column,a method was developed to quantify the PFOA concentration.The qualitation and quantification of the polyfluoro olefins products can be achieved by using Agilent 7890B GC–FID system with the Rtx-502.2 capillary column.In Chapter 3,an alumina-mediated solid-state MC method was developed to simultaneously degrade PFOA and synthesize 1H-perfluorohept-ene（1H-1-PFHp）,which is a valuable organofluorine block.A 2.5-h MC treatment resulted in a nearly complete removal（99.4%）of PFOA and a high yield（92.5%）of 1H-1-PFHp.In this transformation,the surface hydroxyl groups on the alumina are critical for anchoring the PFOA molecules during the defluorination reaction,and the milling process promotes the dehydration of the alumina to produce reactive Lewis acid sites for activating the C-F bonds.The combined actions between the mechanocaloric effect and the catalytic effect of alumina drive PFOA to overcome a high reaction barrier for the breakage of C-F bonds.This catalytic MC process shows promising applications in the green treatment of PFOA solid wastes.In Chapter 4,a novel method was developed for efficiently degrading powdery PFOA by using a MC process with alumina and potassium persulfate（PS）as co-milling reagents.When both alumina and PS were used as the co-milling agents,the MC method yielded complete degradation of PFOA with a defluorination efficiency of 100%and a mineralization efficacy of 97.6%in 2 h.The enhanced degradation of PFOA was attributed to the synergistic effect between alumina and PS.During the MC treatment,alumina strongly anchored both PFOA and PS.Meanwhile,both the mechanocaloric effect arising from ball milling and the released free electrons from the alumina milling could activate PS to produce SO₄^·-,which then readily reacted with the surface hydroxyl groups on alumina and converted into·OH radicals.The·OH as the major contributor together with SO₄^·-as a minor contributor caused the deep degradation of PFOA.The degradation of PFOA was initiated by the decarboxylation due to the COO^--Al coordination,and then the addition of·OH induced the stepwise defluorination of the CF₂groups until a complete defluorination realized.In Chapter 5,an efficient MC treatment system was proposed for the reductive degradation of PFOA with powdery Al as milling reagent.Powdery Al was the most efficient reagent for the MC degradation of PFOA among the zero valent metals,such as Fe,Zn and Mg.Under the conditions of 350 rpm,mass ratio of milling ball to the reaction mixtures100:1,the molar ratio of Al and PFOA 100:1,the completed degradation of PFOA was achieved.The removal of PFOA by powdery Al can be attributed to the following main mechanisms:PFOA adsorbed and coordinated to surface coated Al₂O₃layer in a monodentate mode,which made the F atoms of PFOA closed to the reductant.When ball mill proceeded,the oxide layer was crushed thereafter the electron can transfer from the electron donator to the nearby F sites in PFOA molecular.Meanwhile,the PFOA was also decarbonated by the effect of instant high temperature,which the decarboxylation and reductive defluorination was achieved.Furthermore,the exposed Al can react with O₂ and H₂O in atmosphere,new oxide layer was therefore generated in situ and bound PFOA continuously,which further promoted the deep degradation PFOA.The carbon backbone of PFOA will not break during the reaction.In Chapter 6,the main conclusions of the dissertation were summarized,the follow-up researches were prospected,and the application prospects of the three disposal methods of PFOA solid wastes in this dissertation were also proposed.