| Water pollution caused by dyeing and finishing wastewater has raised increasing attention around the world.Dyeing and finishing wastewater is characterized by high water consumption and high colority,Generally,dyeing and finishing wastewater is toxic heavy metal ions and refractory organic pollutants.Antimony and chromium are typical and regulated pollutants in dyeing and finishing wastewater,and their toxicity is closely related to morphology and forms.For example,the toxicity of trivalent antimony(Sb(?))is around 10 times higher than that of pentavalent antimony(Sb(?)),and hexavalent chromium(Cr(?))is 50-100 times more toxic than trivalent chromium(Cr(?)).In addition,aniline is the metabolic products of azo dye in the biological treatment of wastewater,owning carcinogenic and mutagenicity.In the latest amended standards for pollutants discharge of textile industry,aniline,Cr and Sb were all included in the regulated list,which are limited at 1.0mg·L-1,0.5mg·L-1,0.1mg·L-1,respectively.Tightening government legislation is forcing textile industry to treat the effluent to a high standard.Recently,due to the low economic cost and the realization of waste recycling,biochar as a new environmental functional material shows remarkable effects in advanced oxidation and adsorption.In the present study,the sludge from secondary sedimentation tank was used as pyrolysis feedstock to prepare sludge-derived biochar,and then used in the simultaneous removal of aniline,hexavalent chromium and trivalent antimony.The main results are as follows:(1)Removal efficiency and characterization of sludge-derived biocharBy comparing different feedstocks derived biochar,it was determined that sludge-derived biochar had excellent effect in aniline,Cr(?)and Sb(?)removal.FESEM,TEM,EDS,BET and metal composition analysis showed that sludge-derived biochar possessed larger specific surface area and enrichment of iron element.FTIR,XRD and magnetic analysis confirmed the presence of zero-valent iron on the biochar surface.(2)Influence factors on the removal of aniline,Cr(?)and Sb(?)by sludge-derived biochar and removal stabilityThe effects of pyrolysis temperature,dosage and initial pH on the removal effect were investigated.In particular,the removal efficiency of aniline under moderate and alkaline conditions was higher than that under acidic conditions,where the removal efficiency could reach over 90%within 45 min.Under acid condition within 2 h,the removal efficiency of Cr(VI)and Sb(III)achieved 100%and 82.0%,respectively.The optimal conditions of aniline degradation were finally determined:the pyrolysis temperature of 800?,the biochar dosage of 8 g/L and the neutral initial pH.For Cr(?)and Sb(?),the optimal conditions were as follows:the pyrolysis temperature of 800?,the biochar dosage of 8 g/L and the initial pH of 3.0.The regeneration experiment was conducted to evaluate the stability of the sludge-derived biochar.After ultrasonic and chemical cleaning(pickling),the biochar still exhibited excellent adsorption/degradation performance,meaning the well stability of sludge-derived biochar.(3)Mechanism of aniline,Cr(?)and Sb(?)removal by sludge-derived biocharEPR characterization and quenching experiments showed that the degradation of aniline was mainly attributed to the reactive oxygen free radicals of of 1O2?O2-·and HO·.The system also generated H2O2 by itself without additional reagents.Meanwhile,no obvious intermediate products were detected by GC-MS during the reaction,while the TOC removal efficiency reached 90%,indicating that most aniline was directly removed by mineralization,and the adsorption of sludge-derived biochar only had a negligible contribution for aniline removal.For Cr(?)and Sb(?),the removal mechanism was confirmed by XPS,AFS characterization and a variety of control experiments.Firstly,Cr(?)is reduced by ZVI,some reductive functional groups(such as C-C/C=C and C-O)on biochar and O2-·produced.Then,the generated water-soluble Cr(?)ions gradually reacted with OH+to form the water-insoluble Cr(OH)3 or the precipitation mixed with CrxFe(1-x)(OH)3.Finally,the precipitation can be removed from the water through the adsorption of biochar.Sb(?)was firstly oxidized to low-toxic Sb(V)by the iron oxide on the biochar,and then removed by the adsorption of the biochar.(4)Simultaneous removal of three specific pollutants in synthetic wastewater and actual wastewaterIn the system with three pollutants,biochar could achieve simultaneous removal of aniline,Cr(?)and Sb(?)efficiently.Within 120 min,the removal efficiencies of aniline,Cr(?)and Sb(?)could reach 98.3%,97.7%and 82.4%,respectively.Notably,due to the potential interaction between organic compounds and heavy metals,there is a slight promotion or inhibition effect on the removals of three specific pollutant.Among them,the removal efficienciy of aniline and antimony in mixed system was higher than that of single system,but for chromium,it was slightly lower than that of single system.The optimal conditions for simultaneous removal of the three pollutants in the mixed system were determined:the pyrolysis temperature of 800?,biochar dosage of 8 g/L and initial pH of 3.Under this condition,the removal rate of aniline reached over 90%within 60 min,and Cr(?)and Sb(?)approached 100%within 30 min.In actual wastewater,aniline removal efficiency could reach 75%within 1 h.However,Cr(?)and Sb(?)had no significant removal due to the interference of complex components in actual wastewater.The related mechanism needs to be further studied.In conclusion,the sludge-derived biochar prepared in this research provided theoretical support for the reduction control of characteristic pollutants(aniline,hexavalent chromium and trivalent antimony)in dyeing and finishing wastewater. |
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