Treatment Of Electroplating Wastewater By Electrochemical Process

Wastewater fromelectroplating industry is one of the main sources of heavy metalwastewater. Electroplating wastewater contains a large number of heavy metal ions, cyanide andorganic pollutants like nitrilotriacetic acid, citric acid, organic phosphoric acid, EDTA, ascomplexing agent. For wastewater from an electroplating factory, in this work,electro-coagulation and H₂O₂oxidation/electro-coagulation are used for treating anelectroplating wastewater containing heavy metals, cyanide, and organic contaminants. Effect ofcurrent density, pH and H₂O₂dosage on the removal of heavy metals, cyanide and organics wasinvestigated as well as their removal process and mechanism.The electroplating wastewaterr contained cyanide, heavy metal ions and organic pollutants,the mounts of cyanide, Cu, Ni, Cr, Fe, Zn and chemical oxygen demand （COD） were70,315,64,13,12,148and335mg/L, respectively. The electro-coagulation was an effective method forremoving total cyanide and heavy metals from electroplating wastewater; the removalefficiencies of total cyanide and heavy metal increased with the current density. At a currentdensity of10mA/cm2, the residual total cyanide, Cu²⁺, Ni²⁺, Cr6+and Zn²⁺were23.0,25.0,4.5,0.2and0.2mg/L, respectively. Removal of COD by electrocoagulation process was limited.Addition of H₂O₂to the electrocoagulation system, removal efficiency of the total cyanide andheavy metal and was increased; With a H₂O₂dose of3ml L-1, the residual total cyanide, Cu²⁺,Ni²⁺, Cr6+, Zn²⁺and COD were0.2,2.0,3.0,1.5,0.1and220mg/L, respectively.Furthermore, the waswater was treated with H₂O₂oxidation for destroying complex andcyanide, and then heavy metal ions released were removed by electro-coagulationprocessing.The initial pH and hydrogen peroxide dosage had obvious effect on the total cyanideremoval in the H₂O₂oxidation of the wastewater under acidic conditions. At pH4.0, the removalefficiency of total cyanide reached the biggest, the residual total cyanide were20.2mg/L; H₂O₂dosage affected the oxidation oxidation efficiency of total cyanide significantly. In theH₂O₂-electrocoagulation process for the wastewater, At a current density of20mA/cm2, theresidual total cyanide, Cu²⁺, Ni²⁺and COD were0.2,1.5,0.5and200mg/L, respectively.Removal of COD by electrocoagulation process was limited. Addition of H₂O₂to theelectrocoagulation system, With a H₂O₂dose of3ml L-1, the residual total cyanide, Cu²⁺, Ni²⁺, and COD were0.2,2.0,1.5and65.0mg/L, respectively.Furthermore, electro-coagulation treatment of copper cyanide complex in aqueous solutionwas performed. It was observed that the removal efficiency of total cyanide and copper ionincreased with the increase of pH and current density; At pH4.0, the residual total cyanide andCu²⁺were33.5and24.3mg/L, respectively; At a current density of10mA/cm2, the residualtotal cyanide and Cu²⁺were40.7and33.5mg/L, respectively. The ratio of copper to cyanide wasimportant for the removal of the total cyanide and copper ions. A ratio of1:3.1was selected, theresidual total cyanide and Cu²⁺were44.6and34.6mg/L, respectively. Addition of H₂O₂to theelectrocoagulation system, as H₂O₂dose was3ml L-1, the residual total cyanide were0.2mg/L.The wastewater contained three species copper cyanide complexes:[Cu（CN）2]-,[Cu（CN）3]2-and[Cu（CN）4]3-. Integration of coagulation, adsorption and coprecipitation process to removecontaminants in electro-coagulation system, the cyanide and organic pollutants were removed byH₂O₂and hydroxyl radical oxidation generated simultaneously, and then the complex wasdestoyed and the heavy metals were removed furtherly.