Study On The Electrochemical Adsorption Mechanism For The Efficient Removal Of Heavy Metal Ions By Manganese Oxides

As the major components of water pollutants,heavy metal ions were accumulated in vivo and difficult to degrade,causing increasing risks to the ecological environment and human health.Traditionally,the heavy metal ions have been removed from aqueous solutions by different physical-chemical methods?chemical precipitation,membrane filtration,ion exchange,adsorption?.However,the application of most of these methods is limited by their high cost,complex operation or secondary pollution.Capacitive deionization,as a salt ion removal technology,has attracted enormous attention in recent years due to the advantages of low energy consumption,simple operation and environmental friendliness.When manganese oxide was used as pseudocapacitive electrode materials,the intercalation-deintercalation of ions occurs during the charge-discharge redox processes,which may accompanied by the adsorption and desorption of heavy metal ions.In this study,based on the redox processes accompanied by the intercalation-deintercalation of ions and adsorption capacity for heavy metal ions,manganese oxide was used as electrode materials to efficiently remove heavy metal ions from aqueous solutions by electrochemical methods?galvanostatic charge-discharge,constant cell voltage?.The influence of the crystal structure and chemical components of manganese oxides,initial pH of electrolyte,the loading of electrode materials,and the interaction between heavy metal ions on the electrochemical adsorption capacity for heavy metal ions was studied.The mechanism of electrochemical adsorption of heavy metal ions by manganese oxides and the adsorption sites of heavy metals on manganese oxides are preliminarily clarified.The main results are as follows:1.Birnessite was prepared using the reduction of potassium permanganate by concentrated hydrochloric acid in boiled solution.Birnessite-based electrode was used to remove Cd²⁺from aqueous solutions by galvanostatic charge-discharge.The Cd²⁺removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated.The results showed that the redox reactions of birnessite were usually accompanied by the intercalation-deintercalation of Cd²⁺.The dissolution-recrystallization reaction of birnessite was occurred and the Mn AOS decreased during the charge-discharge processes.Because of the incompletely reversible intercalation-deintercalation of Cd²⁺,the adsorption capacity increased with an increase in the cycles of charge-discharge and the maximum electrosorption capacity of birnessite for Cd²⁺was about 900.7 mg g^-1,which was significantly higher than the adsorption isotherm capacity of birnessite(125.8 mg g^-1).The electrosorption capacity of birnessite for Cd²⁺increased and then reached equilibrium with an increase in initial Cd²⁺concentration,and decreased with an increase in the loading of active birnessite.In the pH range of 3.0-6.0,the electrosorption capacity increased at first with an increase in pH and then reached equilibrium above pH 4.0.2.The cryptomelane?0.1Cry,0.5Cry,1.0Cry?was prepared by a microwave-assisted hydrothermal reaction of potassium permanganate and manganese sulfate in 0.1,0.5 and 1.0 mol L^-1 sulfuric acid,respectively.The particle size of obtained cryptomelane decreased and their BET specific surface area increased with the increase of sulfuric acid concentration.0.1Cry,0.5Cry and1.0Cry were used as electrode materials for Cd²⁺removal from aqueous solution by galvanostatic charge–discharge and the effects of the chemical components of cryptomelane and manganese oxide crystal structure on electrochemical adsorption were studied.The results showed that the maximum electrosorption capacity of 0.1Cry,0.5Cry and 1.0Cry for Cd²⁺was 98.2,95.8 and 192.0 mg g^-1,respectively,which were significantly higher than the corresponding isothermal adsorption capacity(4.9,7.6 and 12.2 mg g^-1).When the H⁺was exchanged from 1.0Cry cryptomelane tunnel structure by 0.5 mol L^-1 K₂SO₄,the electrosorption capacity of Cd²⁺decreased to 169.4 mg g^-1,which showed that H⁺in cryptomelane tunnel contributed to the Cd²⁺electrosorption.The electrochemical adsorption of todorokite and pyrolusite for Cd²⁺was further studied to investigate the effects of tunnel size on the electrochemical adsorption performance of manganese oxides.The results showed that the maximum electrochemical adsorption capacity of todorokite and pyrolusite was about 44.8 and 13.5 mg g^-1,respectively.The electrochemical adsorption capacity of different tunnel manganese oxides for Cd²⁺followed the order of cryptomelane>todorokite>pyrolusite.3.Birnessite was prepared using the reduction of potassium permanganate by concentrated hydrochloric acid in boiled solution.Birnessite was used to electrochemically adsorb heavy metals in single solutions of 0.5 mmol L^-1 Cd²⁺,Zn²⁺,Pb²⁺,Ni²⁺,and the mixed solutions by galvanostatic charge-discharge.In order to further clarify the effect of the interaction between these heavy metal ions on removal efficiency,50 cycles of galvanostatic charge-discharge tests were carried out in0.5 mmol L^-1 Cd²⁺/Zn²⁺,Cd²⁺/Pb²⁺,Cd²⁺/Ni²⁺,and Zn²⁺/Ni²⁺solution,respectively.The influence of different potential windows on the removal of heavy metals was also investigated.The results showed that the individual removal efficiency of Cd²⁺,Zn²⁺,Pb²⁺and Ni²⁺was 41.8%,37.2%,85.6%and 31.3%,respectively.When there were mixed heavy metal ions in electrolyte,the removal efficiency of Pb²⁺was 83.9%which was closed to its individual removal efficiency and the removal efficiency of Cd²⁺,Zn²⁺and Ni²⁺decreased to 19.9%,11.6%and 5.4%,respectively.These results indicated that the sorption capacity of birnessite for Pb²⁺was not remarkably affected by other heavy metal ions,and Pb²⁺could occupy the active sorption site of birnessite,impeding the adsorption of Cd²⁺,Zn²⁺and Ni²⁺on birnessite.Individual and simultaneous removal efficiency of birnessite for heavy metal ions during charge-discharge process was significantly higher than that during the open circuit process.In the mixing solution of Cd²⁺/Zn²⁺,Cd²⁺/Pb²⁺,Cd²⁺/Ni²⁺,and Zn²⁺/Ni²⁺,it can be found that Cd²⁺has an inhibitory effect on Zn²⁺and Ni²⁺removal efficiency,Zn²⁺and Ni²⁺reciprocally inhibited the electrochemical adsorption,and Zn²⁺had a stronger inhibitory effect than that of Ni²⁺.The electrochemical removal capacity of birnessite for heavy metal ions followed the order of Pb²⁺>Cd²⁺>Zn²⁺>Ni²⁺.The removal efficiency of Pb²⁺increased with an increase in potential window,but that of Ni²⁺,Cd²⁺and Zn²⁺was less affected by potential window.4.Birnessite was prepared by a microwave-assisted hydrothermal reaction of potassium permanganate and manganese sulfate in 0.1 mol L^-1 potassium sulfate.When birnessite was used as both positive and negative electrode materials,different cell voltages?0-1.2 V?were applied to remove Cd²⁺and AsO₄^3-from the mixed solution of 0.5 mmol L^-1 Cd²⁺and AsO₄^3-.The individual and simultaneous removal efficiency and mechanism of Cd²⁺and AsO₄^3-were studied,and the effect of concentration of Cd²⁺and Na⁺on the removal of AsO₄^3-was analyzed.The results showed that the removal efficiency of Cd²⁺and As increased at first and then decreased with the increasing cell voltage in the mixing solution of 0.5 mmol L^-1 Cd²⁺and AsO₄^3-,and reached the maximum values of 65.3%and 48.5%,respectively at the cell voltage of 0.6 V.The simultaneous removal efficiency of As was significantly higher than individual removal efficiency.When cell voltage was applied at 0.3 and 0.6 V,the Mn₃?AsO₄?₂·4H₂O precipitate was generated on the negative electrode.The removal efficiency of As increased with the increase of Cd²⁺concentration in the mixing solution of Cd²⁺and AsO₄^3-and the maximum removal efficiency of As reached 66.1%.The change of Na⁺concentration had little effect on As removal.The removal capacity of birnessite electrode for Cd²⁺and As increased at first and then reached equilibrium with the increase of Cd²⁺and AsO₄^3-concentrations in the mixed solution.The highest removal capacity of birnessite for Cd²⁺and As respectively were 2132.0 and 1996.0 mmol kg^-1 at the cell voltage of 0.6 V.