Effects And Mechanisms Of The Removal Of Azo Dye Acid Orange 7 From Water By Red Mud Biochar

The development of efficient and safe water treatment materials and technologies are keys to refractory organic pollutants（ROPs）reduction and water environment safety.Iron-loaded biochar is a novel environmental functional material that combines the advantages of carbon and iron,retaining the adsorption properties of biochar,associate with high redox and catalytic properties of iron/iron oxides.In this study,red mud biochar（RMBC）with high adsorption and catalytic properties were synthesized from industrial waste red mud and agricultural waste shaddock peel to remove azo dye acid orange 7（AO7）from aqueous environment.The AO7removal effect,along with mechanism,of RMBC was investigated.The theoretical basis for the removal of AO7 by RMBC was provided.The main conclusions are as follows:（1）High iron loading RMBCs were successfully prepared by one-step co-pyrolysis.As compared to unmodified biochar,the physicochemical properties were optimized.As for the physical characteristics,mineral particles were observed on the surface of RMBC,the specific surface area and total pore capacity were improved,and stronger saturation magnetization was detected.With regards to chemical properties,the iron morphology on RMBC changed from oxidized to reduced state as pyrolysis temperature increased.Meanwhile,it was also observed that the surface iron and carbon were uniformly distributed,and Fe-OH functional groups containing graphitized structure were successfully introduced.（2）Adsorption and reductive degradation were the main mechanisms of RMBC for the efficient removal of AO7.The RMBC produced at 800℃with red mud:shaddock peel mass ratio of 1:1(RMBC800_1:1)exhibited the best capability for AO7removal（32.14 mg/g）.The higher surface area of RMBC800_1:1 and its greater affinity for AO7 led to the higher adsorption.The reduction forms of Fe in RMBC800_1:1provided electrons for breaking down the azo bond in AO7 molecules and result in degradation,which was further enhanced in acid conditions due to the participation of readily release of Fe（II）and the available H⁺in AO7 degradation.（3）RMBC boosted the removal efficiency of AO7 in the photo-Fenton system.The decolorization efficiency of RMBC800_1:1 for AO7 reached 100%within 10 min,and the mineralization efficiency reached 87.01%within 120 min.RMBC800_1:1 had photocatalytic activity,which can be excited by visible light to produce photogenerated electrons and holes.The Fe（II）reacted with H₂O₂ to form·OH,and photogenerated electrons reacted with dissolved oxygen to form O₂^·-and subsequently ¹O₂.¹O₂ was the main contributor to the photo-Fenton degradation of AO7,followed by·OH and O₂^·-.The photogenerated holes was also responsible for AO7 degradation.In addition,zero-valent iron and a low redox potential photogenerated electrons contributed to the Fe（II）/Fe（III）reduction cycle to enhance the formation of·OH.