Heavy Metals Removal From High Salts Smelting Wastewater By Electrocoagulation:Optimization Of Parameters And Mechanisms

Smelting industry is one of the high emission,high polluting,and high toxicity industries in nonferrous metal field.The wastewater produced in its productive process is very complicated,in which there are various kinds of heavy metals(Pb,Zn,Cu,Cd²⁺,Mn,As and Hg)and high concentration of electrolytes,such as F^–,Cl^–,SO₄^2–,Ca²⁺and Mg²⁺.Recently,many technologies were employed in the treatment of smelting wastewaters.Electrocoagulation（EC）is one of the biggest concerns for heavy metal removal.In this work,the treatment of synthetic and real smelting wastewater contains high concentration of heavy metals,mainly including zinc,cadmium,and manganese,were investigated in various parameters.Energy consumption,sludge production,and the removal mechanism also were evaluated.At the first stage,various parameters of electrocoagulation in the treatment of synthetic wastewater are investigated and the results are as follows:（i）The optimal parameters of PC are T_Fe:Al 25:5 s,f 5000Hz and r 40%.（ii）The optimal electrode material and conbination in DC–EC is Fe–Fe.（iii）Heavy metals removal efficiency of DC–EC is much better than PC–EC when j≥8 mA/cm².（iv）Better EC performance is obtained with the increase of current density from 4 to 10 mA/cm²,although ENC,ELC and m_s are increased.Thus,8 mA/cm² is recommened.（v）Based on the quality of real smelting wastewater,the optimal initial pH of wastewater is 7.（vi）A intense competitive adsorption between Zn²⁺and Cd²⁺is observed,and higher initial C_Zn will lead to poor removal of Cd²⁺.（vii）Single Cl^–solution contributes to the removal of Mn²⁺,and the presence of SO₄^2–(both SO₄^2–and Cl^–+SO₄^2–)favors the removal of Cd²⁺.（viii）High concentration of Ca²⁺and Mg²⁺might accelerates the passivation of electrodes and lead to poor EC performance,practically in the presence of SO₄^2–.Interestingly,the addition of Cl–can weaken the passivation and the optimal R_C:S is 2.5:50.（viv）The real smelting wastewater is treaded at optimal parameters,after 60 min EC,the removal efficiency of Zn²⁺,Cd²⁺and Mn²⁺are99.52%,66.67%and 99.82%,respectively,while ENC,ELC and m_s are 2.6 kWh/m³,0.66kg/m³ and 2.15 kg/m³.At the second stage,enhancing the removal of Cd²⁺by adjusting the ratio of chloride and sulfate（Zeta potential）is studied.（i）In both single Cl^–and SO₄^2–solutions,the removal efficiency of Cd²⁺are increased with anions concentration.However,the coagulation performance in Cl^–solution is far inferior to SO₄^2–solution.（ii）The Zeta potential is increased with time,and lower Zeta potential is obtained at higher Cl^–concentration.The addition of SO₄^2–will lead to the decrease of Zeta potential（negative charged flocs）.（iii）According to XRD and SEM–EDS analyses,lepidocrocite and magnetite are the main production in EC process in the presence of SO₄^2–,it can be attributed to the generation of layered double hydroxides（LDHs）.At the third stage,the effect of oxidation and reduction conditions on EC performance is investigated.The addition of Na₂SO₃ shows a better removal efficiency of Cd²⁺than H₂O₂ solution.The results of XRD,Raman and FT–IR analyses show that iron hydroxides transform to lepidocrocite and goethite in H₂O₂ solution,but goethite is obtained in Na₂SO₃ solution.In brief,the mechanism of electrocoagulation in oxi–redcution condition are as follow:（i）The addition of H₂O₂ and Na₂SO₃ lead to oxidation and reduction solution environment and controls the state of Fe（II）/Fe（III）.（ii）In EC process,there are two main forms of iron hydroxides（flocs）in solution.The addition of Na₂SO₃ increases the ratio of Fe（II）/Fe（III）hydroxides（5.5:1）,the generation of stable layered double hydroxides（LDHs）,which has high adsorption capacity on heavy metals.