Integrating Oxidation By Hydroxyl Radicals With Reduction By Fe(Ⅱ) For Highly Efficient Removal Of Cr(Ⅲ)-carboxyl Complexes

Industrial wastewaters such as tanning and electroplating effluent contain a large amount of Cr（Ⅲ）,thereby posing a great threat to the ecosystem.Cr（Ⅲ）is mainly present in the form of stable complexes with carboxyl ligands,and it is hard to achieve satisfactory removal of Cr（Ⅲ）complexes by traditional techniques such as precipitation and coagulation.Due to the high stability of Cr（Ⅲ）complexes and extremely low complexation/decomplexation rate,the methods based on metal replacement are unsuitable for removing Cr（Ⅲ）complexes.Although advanced oxidation processes（AOPs）were effective to degrade organic ligands,the substantial generation of toxic Cr（Ⅵ）compromising their application in Cr（Ⅲ）contamination control.Overall,there was still a severe lack of efficient and safe treatment strategies for Cr（Ⅲ）complexes.In this study,a new strategy based on·OH oxidation and the subsequent Fe（Ⅱ）reduction for the efficient removal of Cr（Ⅲ）complex was proposed.In brief,Cr（Ⅲ）was converted to Cr（Ⅵ）by·OH radicals accompanying the release of organic ligands,followed by the reduction of Cr（Ⅵ）into Cr（Ⅲ）by the newly formed Fe（Ⅱ）.Efficient removal of the free Cr（Ⅲ）ions was achieved by the subsequent chemical precipitation.To verify the effectiveness of the combined process,this paper mainly carried out the following aspects:（1）UV/Fe（Ⅲ）-NaOH system was constructed to remove Cr（Ⅲ）-carboxyl complex.Cr（Ⅲ）was converted to Cr（Ⅵ）by the·OH radicals generated by UV/Fe（Ⅲ）at pH 3.0.Meanwhile,the organic ligands were released and then re-complexed with Fe（Ⅲ）.Under UV irradiation,Fe（Ⅲ）complexes were decomposed through ligand-metal charge transfer process（LMCT）,producing massive Fe（Ⅱ）.Then,the Cr（Ⅵ）was reduced to Cr（Ⅲ）by the generated Fe（Ⅱ）and removed by the following alkaline precipitation.In this process,the concentration of Cr（Ⅲ）-citrate complex decreased from 10.4 mg Cr/L to 0.36 mg Cr/L,accompanying with more than 60%TOC removal and negligible formation of Cr（Ⅵ）.UV/Fe（Ⅲ）-NaOH also exhibited satisfactory removal efficiency when applied in removal of Cr（Ⅲ）complexes with other ligands such as EDTA,tartrate,oxalate and acetate,as well as the authentic tanning wastewater.（2）To overcome the deficiency that UV/Fe（Ⅲ）was only operated in acidic conditions,FeS₂/H₂O₂-NaOH system was developed for deep removal of Cr（Ⅲ）-organic complex.In this process,the·OH radicals generated by FeS₂/H₂O₂were employed for converting Cr（Ⅲ）to Cr（Ⅵ）and simultaneously releasing the carboxyl ligands.Cr（Ⅵ）was then reduced to Cr（Ⅲ）by the dissolved Fe（Ⅱ）as well as the added FeS₂.Since a large amount of H⁺was generated for acidification during the reaction between FeS₂ and H₂O₂,FeS₂/H₂O₂ was capable of removing Cr（Ⅲ）complex efficiently in a wide pH range（3.0-9.0）.The evolution of H₂O₂,Fe,Cr,TOC and pH were monitored and fitted by the reaction kinetic data.Other Cr（Ⅲ）complexes like Cr（Ⅲ）-EDTA,Cr（Ⅲ）-tartrate,Cr（Ⅲ）-oxalate could be also effectively eliminated by FeS₂/H₂O₂ system.Less than 0.3 mg/L Cr was observed in the effluents.When dealing with the authentic chromium wastewater,the total Cr concentration could be reduced to<1.5 mg/L with negligible Cr（Ⅵ）generation.（3）Pyrite/H₂O₂ system was used in a column reactor to evaluate its practical potential for the removal of Cr（Ⅲ）-carboxyl complex.Under alkaline conditions（pH8.0）,the pyrite-packed column has better removal performance than the zero valent iron（ZⅥ）packed ones,and the operating life was prolonged by more than 200 times.The concentrations of the total Cr and total Fe in the solution of the packed column were decreased gradually along with the flow direction.After chemical precipitation,the total Cr and total Fe concentration in the effluent were reduced to<0.5mg/L.The former parts of the packed column were dominated by the oxidative degradation of Cr（Ⅲ）complex,whereas the latter parts played a key role in reducing Cr（Ⅵ）into Cr（Ⅲ）and decomposing the residual H₂O₂.H⁺was gradually released during the reaction,resulting in a gradual drop of the initial pH from 8.0 to 3.0-3.5.A series of characterization methods were adopted to further clarify the role of pyrite in each section along the column.Scanning electron microscopy（SEM）and energy dispersion spectrum（EDS）results showed that the corrosion degree of pyrite decreased along with the flow direction.The analysis of XPS and XANES on the surface of FeS₂demonstrated that Fe（Ⅱ）was the main Fe speciation in the column,while considerable amount of Fe（Ⅲ）（7.6%-17.5%）still existed on the surface of pyrite.XRD analysis suggested that the crystalline structure of FeS₂ remained unchanged.