| Wastewater containing heavy metals discharged with industrial production activities has become a serious environmental problem.Wastewater containing heavy metals mainly comes from electroplating,non-ferrous metal smelting,tanning,and other industries.Among which,chromium(Cr)is a relatively common heavy metal.Ethylene diamine tetraacetic acid(EDTA)is an important chemical agent in electroplating and tanning industries,it can complex with heavy metal ions to form stable metal-organic complexes,such as Cr-EDTA.Due to the higher chemical affinity between Cr and EDTA,Cr-EDTA is more difficult to remove from water than inorganic Cr.which is also the difficulty of treating heavy metal wastewater nowadays.To this end,it is necessary to develop a method for efficient decomplexation of Cr-EDTA and removal of Cr.Advanced oxidation processes(AOPs)is one of the important techniques for the decomplexation.Among them,peroxymonosulfate(PMS)has attracted much attention due to its strong oxidizing capacity.With the decomplexation of Cr-EDTA,the released CrⅢ will be eventually oxidized to CrⅥ due to the non-selectivity of radicals.This not only increases the toxicity of Cr,but also improves the difficulty of Cr removal.Electrochemical technology can efficiently activate the PMS,and realize the electroreduction of heavy metals.Therefore,a cathode-driven PMS/Fe-C3N4/E process with three-dimensional electrode composite(a synergistic system of PMS,Fe-C3N4 and electric field)was proposed for efficient treatment of Cr-EDTA-containing wastewater.The three-dimensional particle electrode dispersed in the system can effectively activate PMS and facilitate the decomplexation of Cr-EDTA;and the in situ generated CrⅥ can be reduced to CrⅢ through electrochemical reduction.Finally,the CrⅢ will be removed by precipitation method.This paper mainly carries out the following research work:Firstly,the Fe-C3N4 nanomaterial was synthesized by a self-assembly,adsorption,and polymerization method,and the Fe-C3N4 was used as a three-dimensional particle electrode in the system.The successful synthesis of Fe-C3N4 nanomaterial was confirmed through SEM,XRD,FTIR,XPS,and other characterized techniques.The Fe-C3N4 presented a flake-like structure with numerous pores.Fe was homogeneously dispersed and supported on the surface of C3N4,and no nanoparticles were observed.Meanwhile,Fe-C3N4 maintained a tri-s-triazine structures ring structure and had lots of pyrrole N components on the surface,which is beneficial for the adsorption of PMS,thereby improving the activation efficiency of PMS.Secondly,in this study,the mechanism of the PMS/Fe-C3N4/E process for the treatment of Cr-EDTA was deeply explored.The results show that with the synergistic effect of electric field and Fe-C3N4,PMS was activated to generate four main active species,SO4·-,O2·-,·OH and 1O2.Among which,1O2 played the most important role in the decomplexation process of Cr-EDTA.After the decomplexation,the released CrⅢwould be further oxidized to CrⅥ by reactive oxygen species,and FeⅡ on the surface of Fe-C3N4 after the reaction would convert to FeⅢ due to the activation of PMS.Due to the electrochemical reduction on the cathode,CrⅥ and FeⅢ were reduced to CrⅢ and FeⅡ,which was beneficial to the removal of Cr and improves the catalytic capacity of Fe-C3N4.In addition,the theoretical calculations and liquid chromatography quadrupole time-of-flight mass spectrometry were also carried out to explore the reaction active sites and degradation intermediates.The decomplexation pathway of CrEDTA was proposed through the DFT results and the detection of nine major degradation intermediates.Finally,this study systematically explored the effects of material dosage,PMS dosage,and current intensity on the removal of total Cr from Cr-EDTA,and therefore the optimal process parameters was determined.When the initial pH of the solution was 4,the material dosage was 0.8 g/L,the PMS dosage was 10 mM,and the current intensity was 5 mA/cm2,the removal percentage of total Cr could reach 91.5%after 60 min,and the residual CrⅥ was less than 0.5 mg/L,which is in line with the effluent of Chinese electroplating wastewater water quality standards.In addition,the initial pH of the solution and some common co-existing metal ions had negative effects on the treatment of Cr-EDTA.The cycle experiment shows that the treatment effect of the PMS/Fe-C3N4/E process remained basically unchanged during the first three cycles,and decreased to a certain extent after the fifth cycle.Overall,the PMS/Fe-C3N4/E process has a potential in the practical treatment of industrial wastewater containing CrEDTA,and also provides insight for the treatment of other metal organic complex contained wastewater. |
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