Preparation,mechanical Activation And Photo-fenton Catalytic Performance Of Bentonite Based Bismuth Ferrites

Dye wastewater contains many potential carcinogens,which are toxic and mutagenic.Dye wastewater is difficult to be removed by traditional wastewater treatment system due to its high chromaticity and alkalinity,as well as poor biodegradability.The advanced oxidation processes,which can produce oxidizing OH,have been proven to be an efficient method for the degradation of dye wastewater.Among them,Fenton oxidation has been significantly developed due to its good effect.However,the application of homogeneous Fenton method is limited due to the difficult recovery and reuse of iron ions,as well as the production of iron sludge pollution.In order to overcome these shortcomings,many scholars have supported iron ions,iron oxides and other iron-containing catalytic active components on the carrier to synthesize heterogeneous Fenton catalysts which are insoluble in water.Bismuth ferrites are considered to be an effective visible-light-driven photocatalyst due to their narrow band gap,and the iron element existing in them also makes it possible to use them as heterogeneous photo-Fenton catalysts under visible light.Using cheap and accessible bentonite as the carrier,bismuth ferrites as iron-containing catalytic active component,a bentonite based bismuth ferrites heterogeneous photo-Fenton catalyst under visible light was prepared by sol-gel method.Using the catalyst to degrade Rhodamine B,the corresponding removal ratio of Rhodamine B was the main evaluation index to determine the optimum preparation parameters of bentonite based bismuth ferrites.Finally,the optimum catalyst,bentonite based BiFeO₃-Bi₂Fe₄O₉,was obtained.The internal causes for its high catalytic performance were analyzed by XRD,UV-Vis DRS,BET physical adsorption,TEM,SEM,EDS and PL characterizations.Meanwhile,its photo-Fenton catalytic properties were further investigated and relevant mechanism was proposed.Furthermore,mechanical activation was used to activate the bentonite based BiFeO₃-Bi₂Fe₄O₉,the optimum parameters for this mechanical activation process were determined,and the photo-Fenton catalytic properties of the bentonite based BiFeO₃-Bi₂Fe₄O₉ activated under the optimum mechanical activation parameters were studied.The key internal factors of improving catalytic performance by mechanical activation were described by XRD,UV-Vis DRS,TEM and BET physical adsorption characterizations.The optimum conditions for the synthesis of bentonite based bismuth ferrites catalyst by sol-gel method were:MFe/MBi=1:1,MFe/mbent=5 mmol·g-1,calcination temperature 500 ?,calcination time 2 h.The crystalline phase compositions of bismuth ferrites in the catalyst were affected by MFe/MBi,MFe/mbent,calcination temperature and calcination time.Under low MFe/MBi,MFe/mbent and calcination temperature,as well as short calcination time,Bi24Fe2O₃9 was more easily to be formed.Under high MFe/mbent,BiFeO₃ was more easily to be formed.Due to the volatility of bismuth,the high calcination temperature was not conducive to the formation of bismuth ferrites.In addition,the main crystalline phases of the bentonite based bismuth ferrites prepared under optimum conditions were BiFeO₃ and Bi₂Fe₄O₉.Therefore,the optimum catalyst was labeled as bentonite based BiFeO₃-Bi₂Fe₄O₉.The bentonite based BiFeO₃-Bi₂Fe₄O₉ showed the strong light absorption in both visible and ultraviolet light regions.Moreover,the largest specific surface area and macroporous volume were also the reasons for its highest catalytic activity among all bentonite based bismuth ferrites catalysts.The catalytic properties of bentonite based BiFeO₃-Bi₂Fe₄O₉ and the photo-Fenton mechanisms catalyzed by the catalyst under visible light were studied.The results showed that the catalyst was composed of many small particles,showing a fluffy aggregate with fish-scale surface,the functional components BiFeO₃ and Bi₂Fe₄O₉ were successfully supported on the catalyst in the nanometer scale.The interfacial charge transfer between BiFeO₃ and Bi₂Fe₄O₉ in the catalyst could effectively inhibit the recombination of e-/h+ pairs,and the introduction of Bi₂Fe₄O₉ into BiFeO₃ could effectively improve the photocatalytic activity of BiFeO₃.The catalyst had an excellent reusability and a long-term stability in the photo-Fenton process,and can almost retain its original catalytic activity after four recycling times.The total iron ion mass concentration in the reaction solution was lower than 0.4 mg L'1 at the end of each recycling experiment.By analyzing the photo-Fenton mechanisms catalyzed by the catalyst under visible light,a Z-scheme mechanism between BiFeO₃ and Bi₂Fe₄O₉ was proposed.The optimum activation conditions in the mechanical activation process of bentonite based BiFeO₃-Bi₂Fe₄O₉ were determined as follows:filling ratio of grinding medium 30%,combination mode of 30 mL stainless steel ball?D=10 mm?,activation frequency 20 Hz,activation time 30 min,Rbtp=30:0.5 mL·g-1.Under visible light irradiation,the bentonite based BiFeO₃-Bi₂Fe₄O₉ activated by optimum mechanical activation conditions had an excellent reusability and a long-term stability in the photo-Fenton process,and could almost retain its original catalytic activity after four recycling times.The total iron ion mass concentration in the reaction solution was lower than 0.2 mg·L-1 at the end of each recycling experiment..When Rhodamine B was almost completely removed,the reaction time used by activated bentonite based BiFeO₃-Bi₂Fe₄O₉ was shortened by half compared with that used by the catalyst without mechanical activation.The change of five activation conditions in the mechanical activation process could cause the crystalline phase transformation of bismuth ferrites in the catalyst.Under the five optimum activation values,the activated composite catalysts not only had strong light absorption ability to ultraviolet light and visible light,but also had small particle size and thin particle structure,which were also the reasons for their high photo-Fenton catalytic activity.The mechanical activation of bentonite based BiFeO₃-Bi₂Fe₄O₉ at too high frequency or for too long time would lead to the excessive powderization of the catalyst,resulting in the severe damage of the pore structure inside the catalyst,the collapse of pore,the recuction of pore volume,and the significant decline in the adsorption and catalytic performance.Additionally,large pore volume,specific surface area and total pore volume were also the reasons for the high photo-Fenton catalytic activity of activated composite catalysts under these optimum mechanical activation values.