Process And Mechanism Of Simultaneous Removal Of Zn/Cu/Fe From Cyanide Wastewater By Photo/Electro Catalytic Oxidation Based On Nano-TiO₂

The conventional cyanidation method for gold extraction produces a large amount of cyanide-containing wastewater,and there is an urgent need to explore efficient and green process methods to remove free cyanide and heavy metals complexed with cyanide from cyanide-containing wastewater.Titanium dioxide has the strongest catalytic activity among the common semiconductor catalysts and has strong oxidation capacity,stable physicochemical properties,non-toxic and low price,and has been at the core of research in the field of environmental purification.In this paper,real gold mine wastewater was treated as the object of catalytic degradation of cyanide-containing wastewater by several different nano-TiO₂-based catalytic oxidation processes.The main research contents and results are as follows:（1）In the first part of the study,a mesoporous Si O₂ with tunable pore structure was prepared as a carrier by changing the ratios of surfactants and inorganic salts,and two TiO₂loading methods,solid-phase dispersion and sol-gel method,were compared,and characterized as well as adsorption-photocatalytic experiments were performed.The samples were characterized by scanning electron microscopy（SEM）,energy dispersive X-ray spectroscopy（EDS）,nitrogen adsorption desorption test,X-ray diffraction analysis（XRD）,ultraviolet visible light absorption spectroscopy（UV-Vis DRS）,infrared spectroscopy（IR）,thermogravimetric analysis（TGA）,and X-ray photoelectron spectroscopy（XPS）.Catalytic experiments with different TiO₂ contents,different catalyst dosages and different initial concentrations were carried out;adsorption kinetics,adsorption thermodynamics models were established;free radical quenching experiments were performed to reveal the influence of the synergistic effect of adsorption and catalysis of the materials on the degradation of cyanide-containing wastewater.The experiments proved that the gel-sol method was more effective,and the kinetic and thermodynamic analysis of dark adsorption process was performed.The results indicated that the dark adsorption kinetics conformed to the pseudo second order,and the thermodynamics indicated that it was an endothermic process.The experimental data stated clearly that the degradation efficiency of total cyanide reached 95.98%and the removal efficiencies of copper and zinc reached 92.70%and 98.69%under the conditions of dark adsorption for0.5 h and photocatalysis for 4.0 h,respectively.The cyanide was chemisorbed on the material surface.XPS,XRD analysis indicated that cyanide was finally decomposed into NO₃^-and CO₂;copper and zinc were removed in the form of Cu O,Cu₂O,Zn O.（2）In the second part of the study,we prepared Si O₂ with a three-dimensional ordered interconnected pore structure by gravity self-assembly of ps hard templates,and then synthesized macroporous Si O₂/TiO₂ composites by using sol-gel method to load TiO₂.It was used as a catalyst for the degradation of cyanide-containing wastewater.The prepared materials were characterized by SEM,EDS,XRD,TGA,and nitrogen adsorption-desorption tests.The effects of TiO₂ loading,catalyst dosage,adsorption time and photocatalytic time on the catalytic effect were investigated.The experimental results demonstrated that the total cyanide degradation rate,Cu,Fe and Zn removal rate reached98.79%,99.10%,100%and 92.26%for 0.35 g catalyst addition,1.0 h adsorption and 4.0h photocatalysis,respectively.the reduction process of Cu²⁺can get electrons and improve the photocatalytic efficiency.（3）In the third part of the study,TiO₂/graphite anodes were prepared by the lifting impregnation method.A synergistic optical and electrical catalytic reaction system was constructed,with TiO₂/graphite electrode as the anode,graphite electrode of the same size as the cathode,and diluted cyanide-containing wastewater as the electrolyte.The TiO₂/graphite electrode was characterized by X-ray diffraction spectroscopy（XRD）,scanning electron microscopy（SEM）,nitrogen adsorption-desorption,ultraviolet-visible diffuse reflectance spectroscopy（UV-Vis DRS）,Raman spectroscopy（Raman）and photoluminescence spectroscopy（PL）.Photoelectrocatalytic experiments were conducted to investigate the effects of various condition factors on the effect of treating cyanide-containing wastewater;the effect of the applied electric field on the electron transfer of TiO₂ was investigated by electrochemical tests to discuss the photoelectric synergistic catalytic effect;the mechanism of free cyanogen and Cu/Zn removal from cyanide-containing wastewater by photoelectrocatalytic oxidation was investigated.The experimental results show the degradation efficiency of total cyanide and removal efficiencies of copper and zinc reached 96.90%,99.76%and 90.56%,respectively,under the optimum conditions of 2.5 V external bias voltage and 4.0 h with air and lighting.An external electric field can significantly improve the migration and separation of photo-generated electrons（e^–）and holes（h⁺）,thereby increasing the photocatalytic efficiency.Simultaneously,the surface potential of semiconductor is lower than the body under the action of bias voltage and leads to energy band bending.XPS analysis indicates that cyanide is finally degraded into carbon oxide and nitrogen.Cu and Zn are mainly recovered as elementary substance.