Removal Of Cr(Ⅵ) And As(Ⅲ) From Water By Photocatalytic Oxidation/Reduction Of Polysulfide Heterojunction Based On Carbon Skeleton

With the rapid economic and social development,the energy crisis and environmental pollution problems have become increasingly serious.The application of poly sulfide heterojunctions in the fields of environmental protection,energy storage,optics and catalysis has attracted widespread attention because of their high chemical stability,abundant resources and easy preparation methods.In this thesis,we investigated the preparation of poly sulfide heteroj unctions based on carbon skeleton,and explored the morphology,composition,optical properties and photocatalytic oxidation/reduction of Cr(Ⅵ)and As(Ⅲ)in water.The main studies and results of this paper are as follows:(1)In2S3-X2S3(X=Bi:Sb)was constructed based on metal-organic backbone MOFs as precursors,and then combined with TFPT-COFs with C=N bonds to prepare In2S3-X2S3(X=Bi;Sb)@TFPT-COFs composites.SEM and TEM results showed that the Sb3+-doped composites were layered tubular nanostructures and the Bi3+-doped composites were layered rod-like nanostructures.XRD analysis showed that the surface modification of In2S3 by Bi3+or Sb3+did not affect its crystal structure.The presence of C=N bonds in FT-IR analysis proved that the TFPT-COFs combined with In2S3-X2S3(X=Bi;Sb)by chemical bonding rather than simple physical mixing,and further illustrated the importance of surface amino functionalization of In2S3-X2S3(X=Bi;Sb).The XPS results illustrated the valence states of Sb(Bi),In,S,C,N and O in the composites as well as the chemical environment.DRS,PL as well as EIS analysis indicated that the synergistic effect of the system in the composites enhanced the generation and separation of photogenerated carriers and accelerated the efficiency of charge transfer.The photocatalytic experimental results indicated that In2S3-Sb2S3@TFPT-COFs reduced the degradation of 30 mg/L Cr(Ⅵ)in only 5 min,and the interfacial effect of In2S3-X2S3(X=Bi;Sb)and TFPT-COFs can promote the photocatalytic reduction of Cr(Ⅵ).Therefore,the as-prepared composites can be used as a novel photocatalytic platform for the efficient degradation of Cr(Ⅵ).(2)The biochar/SnS2 composite with-C=Sn-S bonds was designed using biochar as the template interface.SEM and TEM results showed that SnS2 was successfully modified in the biochar and the interface was clear and intact.BET and contact angle analysis showed that the biochar/SnS2-0.25 composite has a specific surface area of about 293.1 m2·g-1 and a contact angle of 19.2°.FT-IR combined with XPS demonstrated the presence of-C=Sn-S in the composite system.The DRS,PL and EIS analysis showed that the-C=Sn-S bonds facilitated the expansion of the photo-response range and effectively suppressed the complexation of photogenerated carriers,while it can reduce the charge transfer resistance.The experimental results of photocatalytic oxidation of As(Ⅲ)illustrated that the removal of 7500 μg/L As(Ⅲ)can be achieved within 55 min in the system of biochar/SnS2-0.25 coexisting with CaSO4,and the conversion of As(Ⅲ)to As(Ⅴ)and the arsenic precipitation of As(Ⅴ)by Ca3(AsO4)2 can be achieved simultaneously,which indicated that due to the polarization and conjugation effects of-C=Sn-S bonds polarization and conjugation effect,the biochar/SnS2 composite can achieve the photocatalytic conversion of As(Ⅲ)to As(Ⅴ)more effectively.Meanwhile,CaSO4·nH2O(n=0,0.5,2)can rapidly convert AsO43-to Ca3(AsO4)2 precipitation to remove arsenic species from aqueous solution.In addition,the electron transfer pathway of arsenic in the photocatalytic oxidation system was proposed based on the Mott-Schottky(MS)diagram of SnS2 and graphitic carbon.Based on the electron paramagnetic resonance(EPR)results,which showed that·O2-and h+were the main active substances in the photo-oxidation of As(Ⅲ)system.This work presented a potential non-toxic treatment method for arsenic systems.(3)The carbon-based catalyst biochar/SnS2/phosphotungstic acid was successfully constructed using Magnoliaceae root as the carbon source,doped with SnS2 and modified with heteropolyacid.XRD results indicated that the introduction of SnS2 can activate carbon atoms and disrupt the electronic inertness of disordered biochar(002)planes.DRS as well as PL analysis indicated that the synergistic interaction between the Keggin unit of phosphotungstic acid and biochar/SnS2 can suppress the recombination of photogenerated e--h+carriers.The adsorption and photocatalysis experiments results showed that the efficiency of removing As(Ⅲ)by biochar/SnS2/phosphotungstic acid(biochar/SnS2/PTA)systems was 1.5 times that of biochar/SnS2 systems,and the concentration of total arsenic in the biochar/SnS2/PTA composite system gradually decreased during the photocatalysis process.The formation of As-POMs can simultaneously realize As(Ⅲ)photooxidation and As(Ⅴ)coprecipitation.The synergistic effect of biochar/SnS2/phosphotungstic acid and W2O3(AsO4)2(As(Ⅴ)-POMs)co-precipitation can achieve efficient removal of arsenic.It further indicated that the activation of carbon atoms in biochar was an important way to realize the reuse of waste woody biomass resources.Specifically,the composites achieved the conversion of S atoms at the interface of biochar into SO4·-radicals to enhance the As(Ⅲ)photooxidation performance.This work can provide a valuable platform for expanding the practical application value and prospects of biochar,which is beneficial to the sustainable development of ecological environment.