Al₂O₃-mediated Mechanochemical Reductive Degradation Of C8-perfluorinated Compounds

Perfluorooctanoic acid（PFOA）and perfluorooctane sulfonate（PFOS）,as two typical C8 perfluorinated compounds（C8-PFCs）,are widely used in industrial production and daily life for their superb superficial property.However some researches indicate these C8-PFCs have potential biotoxicity and environmental persistence and bioaccumulation.As a result,they were listed as persistent organic pollutants（POPs）successively.Incineration is a typical method for treating organic soild waste which is reluctant to degradate.But it will produce toxic and harmful gas such as fluocarbon and HF during calcination of perfluorinated compounds.Mechanochemical（MC）approach is a promising method for destruct solid contaminant.Thinking about C–F bond is crucial to oxide and defluorinate but to reductive destruct,the object of this dissertation is committed to study reductive mechanochemical disposal of C8-PFCs and the main result is as follow:（1）The established sample pretreatment and analytic method for monitioring degradation and defluorination of maternal contaminant and produced organic product during ball-milling process,provide strong analytic guarantee on study MC reductive degradation of C8-PFCs.The PFOA and PFOS was extracted by alkalescent solution and the concentration was quantified by HPLC equipped with charged aerosol detector.Using high concentration of alkalescent solution to extrat F^–adsorbed on Al₂O₃ and quantified by ion selective electrode.The relative correction factor of organic product was calculated and modified by experiment,and it’s was adsorbed by acetone and quantified by gas chromagraphy（GC）equipped with flame ionization detector using fluorobenzene（FB）as internal standard.（2）γ-Al₂O₃ was heated at 100¹200°C for 3 h to change it’s surface property for studying the influence on MC reductive degradation of PFOA.And result shows that the main product of degradation of PFOA was 1-H-1-PFHp and F^–by only using Al₂O₃ as comilling agent.As the calcination temperature of Al₂O₃ increase,degradation rate and yield of 1-H-1-PFHp decrease,but defluorination rate increase.By analyzing interaction between griding agent and substrate and electron generation during MC process,it found that the difference(r_e-/r_{decarboxylation})of electron generation rate and decarboxylation of PFOA on dynamics has positive correlation with defluorination efficiency.Calcination hasn’t change ability of generating electrons during ball-milling with Al₂O₃(0.37⁰.40mmol g^–1),and amount of generated electrons was lower than PFOA needed theoretically(2.79 mmol g^–1),thus causing partial defluorination.On the other hand,calcination will affect adsorption model of carboxylate radica of PFOA on Al₂O₃ and decarboxylation ability.When pretreated temperature T≤700°C,acid-base neutralization reaction between OH on the surface of Al₂O₃ and carboxyl group of PFOA happen,and in-situ form Al³⁺-COO^–monooxy bridged inner layer complex,which is favorable for MC decarboxylation;when T=700°C,OH group on surface of Al₂O₃ disappeared,and PFOA forms a double-toothed outer layer complex by hydrogen bonding of COOH and Al₂O₃surface adsorption water,which is not favorable for activation of COOH and leaving.Thus,decarboxylation of PFOA become slow,inducing r_e-/r_{decarboxylation} higher than system of T≤700°C and defluorination rate increase.Adding zero-valent metal Al into Al₂O₃ treated at 1200°C increase electron generation and defluorination of PFOA;when r_e-/r_{decarboxylation}≥30,a complete defluorination of PFOA can be realized.（3）A new system was established on MC reductive degradation of PFOS usingα-Al₂O₃ and iron as comilling agent.Result shows:in the condition ofα-Al₂O₃ and zero-valent iron as co-grinding agent（molar ratio 1:1）was reacted for 75 min at a ball-to-batch ratio of 50:1 and 350 rpm,the degradation rate and defluorination rate of PFOS were 100%and 90%respectively,much higher than Al₂O₃（93.7%and 51%）and Fe（27.5%and 13.5%）alone.Compared to other zero-valent metals such as aluminum and zinc,zero-valent iron has the advantage of being cheap and not producing H₂S.Solid samples before and after the reaction were analyzed by FT-IR,XPS F1s and C1s,as well as TOC and Raman to further demonstrate that PFOS was reductively degradated and product was amorphous carbon and F^–.When Al₂O₃ is used as the grinding agent alone,surface OH group was favorable for adsorping PFOS and desulfurization and degradation of PFOS,but the ability of electron generation was insufficient resulting in defluorination rate was lower than degradation rate;although electron generation rate was high when use Fe alone,adsorption of PFOS was poor inducing degradation and defluorination of PFOS was slow;when combined Al₂O₃ and Fe,the PFOS was adsorped on the surface of Al₂O₃,and Fe nearby transfer electron to adsorped PFOS rapidly making defluorination and desulfurization happen at same time,thus,PFOS was rapid degradation and deep defluorination.In addition,the combined system also has a certain degradation effect on PFOA and F-53B.