Structural Regulation Enhanced Heterogeneous Fenton Catalytic Degradation Of Bisphenol A On Fe-based Biochar

With the development of the economy and the prosperity of industry,more and more pollutants are discharged into the environment.Among them,are organic pollutants with the characteristics of higher toxicity,difficult degradation,and bioaccumulation,which seriously threaten human health and ecological environment balance.Bisphenol A（BPA）is a kind of organic pollutant with the characteristics of the least decomposition rate or degradation.Long-term exposure to BPA can cause serious harm to the human body,leading to various diseases,and even death.In recent years,to the efficient removal of BPA from water has become a hot topic.Advanced oxidation techniques can rapidly decompose BPA without producing harmful byproducts.For instance,Fenton catalytic oxidation is the simplest and most commonly used method.However,the traditional Fenton oxidation process has a low degradation efficiency of organic matter.Fe³⁺and OH^-ions combine to produce a large amount of iron mud and the increase in pH causes a lower conversion rate of Fe³⁺/Fe²⁺.The heterogeneous Fenton reaction does not produce iron mud in the process of catalytic degradation of organic matter and is less affected by pH,which expands the application conditions of catalyst.However,the catalytic efficiency of heterogeneous Fenton greatly depends on the structure of the catalyst.Different crystal structures have greater differences in catalytic performance thus Fe³⁺/Fe²⁺cycling efficiency of heterogeneous Fenton is much slower than the traditional Fenton oxidation.Besides,the agglomeration of iron oxide nanoparticles which is easy to occur in the preparation process would reduce the utilization rate of iron oxide nanoparticles.Biochar can be used as a good carrier of nano-oxide and iron oxide nanoparticles can be evenly dispersed on the surface of carbon materials.Currently,it has been suggested that natural organic acids such as ascorbic acid can be used as a reducing agent to promote Fe³⁺/Fe²⁺cycling during Fenton reaction.However,the promotion effect of different organic acids on Fe-based catalysts is not clear.The objective of this study was to investigate the catalytic degradation of BPA by different Fe-based biochar catalysts prepared with different precursors and the enhancement effect of different natural organic acids on Fe-based biochar catalysis.The main contents of this paper are as follows:1.In this paper,we impregnated ginkgo biloba biomass with different iron salts and pyrolyzed it at 600 ^oC for 2 h to obtain Fe-based biochar catalysts.The structures of several Fe-based biochars prepared by different precursors were analyzed by thermo-gravimetric analysis（TG）,field emission scanning electron microscopy（SEM）,Brunauer-Emmett-Teller（BET）surface area analysis,X-ray diffraction（XRD）,and X-ray photoelectron spectroscopy（XPS）.The results showed that the structures of Fe-based biochar catalysts were quite different.Firstly,because the biomass used in the preparation of biochar were all ginkgo leaves,the TG curves of several Fe-based biochar catalysts were consistent,and the differences in their TG rate curves indicated that different crystal oxides were derived during the preparation process.In addition,the SEM results showed that the surfaces of several Fe-based biochar were loaded with nano-oxide crystal particles.Combined with XRD results,with ferric chloride（FeCl₃）and ferrous chloride（FeCl₂）as precursors of ginkgo biloba pyrolysis got impregnated Fe-based biochar FeCl₃@BC and FeCl₂@BC loaded with the oxide of Fe₃O₄,and the specific surface area(about 10 m²·g^-1)was 4%of the original biochar(about 250 m²·g^-1).The surface crystal oxides of Fe NO₃@BC with ferric nitrate（Fe NO₃）as precursors wereγ-Fe OOH andγ-Fe₂O₃,while the surface of Fe SO₄@BC and Fe₂（SO₄）₃@BC with ferric sulfate（Fe SO₄@BC）and ferrous sulfate（Fe₂（SO₄）₃@BC）as precursors wereγ-Fe OOH,α-Fe₂O₃ andγ-Fe₂O₃ which had the highest specific surface area(about 200 m²·g^-1)in several kinds of Fe-based biochar.The O 1s and Fe 2p results（XPS）confirmed the presence of distinct iron（hydrogen）oxides on the surfaces of several Fe-based biochar catalysts.2.Subsequently,in this study,different Fe-based biochar catalysts prepared with different precursors were used to decompose BPA in a Fenton-like catalytic system.The effects of different Fe-based biochar catalysts on BPA were investigated,as well as the effects of pH,initial concentration of BPA,the dosage of catalyst,temperature,co-existing ions,and trapping agents on the catalytic process.The enhancement of degradation of BPA by organic acids was also studied.The results showed that FeCl₃@BC,with FeCl₃ as the precursor,had the highest catalytic degradation efficiency of BPA.A significant effect on the catalytic degradation of BPA contributed by pH.When pH increased from 3 to 9,the degradation rate of BPA decreased from 100%to 18.5%.BPA in the initial concentration increased from 10 mg·L^-1 to 100 mg·L^-1,and the degradation rate of the k value decreased to 2.63 from 5.76·min^-1,while BPA was still decomposed completely within 10 min.When the catalyst dosage increased linearly,the surface oxide active site as well as the degradation efficiency of BPA increased exponentially.The experiment of temperature revealed that catalytic degradation of BPA could be carried out thermodynamically spontaneously.When there were ions such as bicarbonate HCO₃^-which can affect the pH of the system,the degradation efficiency of BPA decreased due to the increase of the pH.As the capture agent of·OH,the addition of methanol to the system suppressed the catalytic degradation of BPA,while joining benzoquinone and furfuryl,the capture agent of super oxygen free radical（·O₂^-）and singlet oxygen（¹O₂）respectively,did not significantly decreased the degradation of BPA.BPA was decomposed within 5 min,indicated that·OH was the dominant reactive oxygen species in the process of catalytic.Fe₂（SO₄）₃@BC as a heterogeneous Fenton catalyst could not effectively degrade BPA,but the addition of natural organic acids such as acetic acid,lactic acid,oxalic acid,citric acid,and fulvic acid significantly improved the degradation efficiency of BPA.The degradation of BPA reach 95%within 10 min,and significantly expanded its application pH.3.The concentration of H₂O₂,concentration of iron,surface oxygen species of Fe-based catalysts change before and after reaction,·OH signal strength,and the degradation products of BPA during the reaction was determined by ultra performance liquid chromatography（UPLC）,ultraviolet-visible spectrophotometer（UV）,XPS,electron paramagnetic resonance（EPR）and high-resolution ion mobility liquid chromatography-mass spectrometry（HRIMLC-MS）,respectively.Results showed that only the H₂O₂ concentration in FeCl₃@BC system among the Fe-based catalysts decreases gradually,and EPR results showed strong·OH signals,which indicated H₂O₂ was gradually broken down to produce·OH.After the reaction,the concentration of iron in the solution was lower than the homogeneous Fenton reaction concentration,and the changes of Fe³⁺and Fe²⁺contents on the surface of different Fe-based biochar catalysts before and after the reaction were obtained.The mechanism of the FeCl₃@BC system with the highest BPA degradation was the Fe₃O₄ on the surface of FeCl₃@BC could effectively promote the conversion between Fe³⁺and Fe²⁺,thus effectively decomposing H₂O₂ and producing plenty of·OH.The increase of degradation of BPA after the addition of organic acids in the Fe₂（SO₄）₃@BC system attributed to the dissolution of iron from the surface of the catalyst.Due to the complexation and reduction of organic acids,the Fe³⁺/Fe²⁺cycle of the organic acid enhanced system was significantly promoted,thus improving the degradation efficiency of BPA.The results of HRIMLC-MS showed that four intermediates appeared in the degradation process of BPA,namely C₁₅H₁₆O₃,C₁₅H₁₆O₄,C₉H₁₂O₃ and C₉H₁₀O₄.Based on these intermediates,BPA degradation was as follows:firstly,·OH replaced a hydrogen atom on the benzene ring of BPA,then continue to replace another benzene in a symmetric position;under the constant·OH attack,two benzenes rings joint fracture C₉H₁₂O₃ and C₉H₁₀O₄ with C-C bond break and last·OH attacks made benzene ring open,the resulting short-chain carboxylic acids were eventually mineralized into H₂O and CO₂.In summary,Fe-based biochar prepared from biomass pyrolysis of Ginkgo biloba leaves with different precursors had different structural characteristics.Among them,FeCl₃@BC obtained with ferric chloride as precursors were rich in incomplete crystal development of Fe₃O₄,which had a good catalytic degradation efficiency of BPA type Fenton.Its high catalytic degradation efficiency was due to the surface of iron dissolution that improved the Fe³⁺/Fe²⁺cycle,the system continues to produce a large number of·OH.FeCl₃@BC could completely decompose BPA in solution within one minute,which was a good heterogeneous Fenton catalyst.The addition of natural organic acids significantly improved the catalytic degradation efficiency of BPA over Fe-based biochar catalysts,whose essence was to promote Fe³⁺/Fe²⁺cycling through reduction.During the BPA degradation,four kinds of intermediate were generated.Sustained·OH attack made the benzene ring open and produced short chain carboxylic acids,which were eventually turned into H₂O and CO₂.