Preparation Of High Iron Red Mud Based Heterogeneous Catalyst And Its Mechanism Investigation In Advanced Oxidation Reaction

Red mud（RM）is a kind of strongly alkaline iron-rich solid waste residue produced in the process of alumina extraction from bauxite.The iron content of Bayer high-iron RM exceeds 30%.In general,every 1 ton of alumina produced results in the manufacture of 1.0-1.8 tons of RM.With the expansion of the aluminum industry and the reduction of aluminum grade,the production of RM has continued to increase.Currently,more than 4.0 billion tons of RM are estimated to be stored on land worldwide with an annual growth rate of approximately 120 million tons.The large amount of accumulated RM poses a serious threat to the ecological environment and human health,and red mud with high alkalinity（p H=10-13）also causes serious pollution to groundwater,land and air.Therefore,considering the sustainable development of alumina industry,both safe disposal and resource utilization of RM have become the key issues in alumina industry.In this paper,based on Bayer high-iron RM,a variety of new RM based heterogeneous catalysts were prepared by simple methods,The catalytic mechanism and pollutant degradation mechanism of as-prepared catalysts in advanced oxidation reactions,such as photo-Fenton system and persulfate system,were studied in depth.The main research contents and conclusions are as follows:（1）Using high-iron red mud（RM）as main starting material,a simple method of mechanochemical synthesis（MCS）was introduced as a green approach to synthesize heterogeneous RM based layered double hydroxide（Fe₂O₃/Zn-Al LDH）,which could be used as a new catalyst for photo-Fenton reaction.The optimum preparation conditions of Fe₂O₃/Zn-Al LDH were as follows:mass ratio of Zn（NO₃）₂·6H₂O to RM 2:1,dry milling time 6 h,H₂O dosage 2 m L,ball-to-powder mass ratio 50:1 and milling speed 250 rpm.The effects of the preparation conditions on the crystal structure and catalytic activity of Fe₂O₃/Zn-Al LDH were analyzed.The Fe₂O₃/Zn-Al LDH was characterized by XRD,FT-IR,TG,XPS,SEM,（HR）TEM.The characterization results showed the composite had a crystallized hydrotalcite-like structure,and the crystalline phases in the optimum Fe₂O₃/Zn-Al LDH were Fe₂O₃and Zn-Al LDH.A hetero-interfaces between Fe₂O₃and Zn-Al LDH existed in the as-synthesized Fe₂O₃/Zn-Al LDH composite.Furthermore,the possible mechanism for Fe₂O₃/Zn-Al LDH formation in the MCS process was proposed.Overall,our results provide a systematic understanding of the preparation of LDH composite through MCS using RM as main material,and our findings help to develop green technology for reusing RM.（2）The catalytic activity of Fe₂O₃/Zn-Al LDH composite（1）in photo-Fenton reaction has been studied in detail.The Fe₂O₃/Zn-Al LDH composite exhibited significantly enhanced catalytic activity in photo-Fenton system.The apparent rate constant of Fe₂O₃/Zn-Al LDH in photo-Fenton reaction was about 4 times higher than that of pure Fe₂O₃/Zn-Al LDH,and was about 10 times higher than that of the red mud,which was the main material for synthesized Fe₂O₃/Zn-Al LDH.It was found that besides the increased surface area（45.64 m³/g）and enhanced pore volume of Fe₂O₃/Zn-Al LDH composite,the suitable concentration of oxygen vacancy could account for the significantly enhanced catalytic activity of Fe₂O₃/Zn-Al LDH.The oxygen vacancy has favored the visible light absorption and the efficient charge separation and transfer of Fe₂O₃/Zn-Al LDH.Moreover,the density functional theory（DFT）calculations were used to explore the electronic structures of as-prepared samples,and the photocatalytic activity of Fe₂O₃/Zn-Al LDH was also analyzed on the results of electronic structures.The Fe₂O₃/Zn-Al LDH composite held excellent settleability,good reusability and stability in use.Finally,the possible mechanisms involved in the photo-Fenton system catalyzed by Fe₂O₃/Zn-Al LDH have been proposed.（3）A oxygen vacancies enriched RM based mixed metal oxide（M-RM/MMO）derived from Fe₂O₃/Zn-Al LDH（1）was applied as an efficient peroxydisulfate（PDS）activator to realize the degradation of ciprofloxacin（CIP）under visible light（VL）irradiation（i.e.,M-RM/MMO/PDS/VL system）.The optimal composite of M-RM/MMO-700 exhibited good performance,and approximately 87.4%of CIP could be degraded at 60 min in the M-RM/MMO-700/PDS/VL system.The result indicated the abundant oxygen vacancies in M-RM/MMO-700 could improve light absorption and promote electron-hole separation,both of which were favorable for the activation of PDS,resulting in promoting the generation of active species in M-RM/MMO-700/PDS/VL system.Quenching experiments and electron paramagnetic resonance technology indicated that?O₂^-,¹O₂and h⁺were the main active species for the degradation of CIP.The M-RM/MMO catalyst possessed excellent reusability,stability and magnetic behavior in use.The degradation pathways of CIP were deduced based on DFT calculation and LC-MS analysis.Based on the quantitative structure-activity relationship（QSAR）,the ecotoxicity of the degradation intermediates was analyzed in detail.This work provided a new insight for future designing of red mud-based LDHs-derived MMO heterogeneous catalyst with abundant oxygen vacancies to efficiently treat antibiotics wastewater via the photocatalysis activation of PDS.（4）High-iron red mud（RM）was activated in a ball mill and applied as an effective activator of peroxymonosulfate（PMS）for tetracycline hydrochloride（TC-HCl）degradation.Compared with that of unmilled RM（69.69%）,the TC-HCl decomposition ratios of ball-milled RM（BM-RM）（72.2%～92.0%）were all improved in the presence of PMS.Systematic characterization suggested that ball milling could optimize the physicochemical properties of RM,such as increased surface area,increased oxygen vacancies（OVs）,enhanced electrical conductivity and increased exposure of Fe（II）sites,all of which could effectively enhance the degradation of TC-HCl by activated persulfate from BM-RM.The quenching experiments and EPR technique revealed that ¹O₂and SO₄·^-contributed dominantly to the TC-HCl degradation.LC/MS analysis combined with DFT calculation revealed that the degradation pathways of TC-HCl were driven by hydroxylation,N-demethylation and dehydration in BM-RM/PMS system.Based on QSAR prediction using the T.E.S.T.software,the toxicity of almost all intermediates was significantly reduced.An obvious inhibition effect on TC-HCl was occurred in the presence of Cl^-,whereas the presences of NO₃^-and SO₄^2-had little effect.However,HCO₃^-improved TC-HCl removal efficiency.BM-RM had a wide working p H range（p H=3-11）and showed good stability and reusability in use.Overall,this work not only offers a simple and promising approach to improve the catalytic activity of RM,but also opens new insights into the ball-milled RM as an effective PMS activator for wastewater treatment.（5）Using high-iron red mud（RM）as a host material to grow silver phosphate（Ag₃PO₄）,a new heterogeneous Ag₃PO₄/RM（APRM）composite was successfully prepared through a facile in-situ deposition method.The characterization results demonstrated Ag₃PO₄nanoparticles uniformly deposited on RM to form P-O-Fe bonds,and a semiconductor heterostructure was constructed between the two nanocrystalline components of Ag₃PO₄and Fe₂O₃.The optimal composite of APRM-110 exhibited a remarkable photocatalytic activity for the degradation of tetracycline hydrochloride（TC-HCl）in photo-Fenton reaction.The enhanced photocatalytic performance and stability could be attributed to the S-scheme charge transfer in APRM-110,which accelerated the photo-generated carrier separation and then effectively restrained the photocorrosion of APRM-110.The APRM-110 composite held good reusability,stability and excellent settleability.The main intermediates were identified and the degradation pathway of TC-HCl in photo-Fenton reaction catalyzed by APRM-110 was proposed.The aquatic toxicity of TC-HCl solution could be effectively weakened by the photo-Fenton catalyzed by APRM-110.This work has designed a cost-efficiency and stable RM-based Ag₃PO₄composite,and as a highly effective catalyst for TC-HCl degradation in photo-Fenton reaction.（6）High-iron red mud（RM）was used as a potential carrier for supporting Co₃O₄quantum dots（QDs）.A novel heterogeneous catalyst of RM based Co₃O₄QDs（Co₃O₄/RM）was prepared via a simple method and used for the activation of peroxymonosulfate（PMS）under visible light（Vis）to degrade levofloxacin（LVF）.Characterization results demonstrated that in-situ growth of Co₃O₄QDs on the surface of RM could increase the surface area,reduce the aggregation,enhance visible light response and accelerate the separation of photogenerated electron-hole pairs,all of which effectively improved the catalytic activity of Co₃O₄/RM composite.Approximately 95.3%of LVF（10 mg/L）was decomposed within 20 min in the PMS/Vis system catalyzed by the optimal Co₃O₄/RM-1200 composite.Both SO₄·^-and h⁺generated in Co₃O₄/RM-1200/PMS/Vis system were responsible for the rapid LVF degradation,especially the SO₄·^-was the dominate active specie.Based on LC-MS technology and DFT calculation,the reasonable degradation pathways of LVF in Co₃O₄/RM-1200/PMS/Vis system were proposed.The ecotoxicity of degradation intermediates was evaluated according to the Toxicity Estimation Software Tool（T.E.S.T.）software.In addition,critical influencing factors on the degradation of LVF in Co₃O₄/RM-1200/PMS/Vis system were investigated.Exhilaratingly,it was found that the Co₃O₄QDs were deposited homogeneously on the surface of RM with the Fe-O-Co bond between Co₃O₄component and Fe₂O₃component,which not only prevented the leaching of metal ions but also contributed to the superior stability and reusability.This work suggested that RM could be used as an novel and cost-effective support for the development of durable red mud-based QDs catalyst for the activation of PMS and the antibiotic wastewater treatment.