Study On The Mechanism And Control For The Electrochemically-driven Adsorption Of Heavy Metal Ions On Birnessite

Heavy metal ions,the important chemical components in the wastewaters released from mining,extractive metallurgy and chemical manufacturing,increasingly threaten the ecosystem,agricultural production and human health.As one type of transition metal oxides,manganese oxides are widely used for heavy metal ion removal due to their characteristics of having low point of zero charge?PZC?,abundant resources and being environmentally friendly.The adsorption capacities of manganese oxides for heavy metal ions are strongly affected by their properties including crystal structure,chemical composition and micromorphology.The chemical composition and micromorphology of manganese oxides can be adjusted by electrochemically controlled redox reactions.However,little is known about the effect of electrochemical redox reactions of manganese oxides on their heavy metal ion adsorption capacities.In this work,birnessite was used as adsorption materials.The cyclic electrochemical redox mechanism of birnessite was investigated by multi-cycle galvanostatic charge-discharge.The key influencing factors of electrochemical specific capacitance?i.e.,the adsorption and desorption capacities of ions in solution on birnessite electrode?were also studied.The electrochemical adsorption of ZnMn²⁺ were conducted in a three-electrode system by multi-cycle galvanostatic charge-discharge based on the electrochemically-driven cyclic redox of birnessite.The differences in adsorption capacity were evaluated in symmetric electrode and three-electrode systems.The electrochemical mechanism of heavy metal removal was further investigated by the adsorption of ZnMn²⁺ and NiMn²⁺ on birnessite using constant potential electrolysis,and the effect of potential on adsorption capacity were also studied.Finally,the electrochemical redox reactions of birnessite was used for arsenic?As?removal from the real wastewater.The main experiments and results are listed as follows.1.Birnessite was facilely prepared by adding dropwise NH₂OH·HCl to KMnO₄ solution under ambient temperature and pressure.The effects of chemical composition and specific surface area on their electrochemical performance were studied.The results showed that the Mn AOS decreased and the corresponding pore size and specific surface area of birnessite increased with increasing NH₂OH·HCl concentration.The synthesized nanostructured birnessite showed the highest specific capacitance of 245 F g^-1 at a current density of 0.1 A g^-1 within a potential range of 0–0.9 V,and excellent cycle stability with a capacitance retention rate of 92% after 3000 cycles at a current density of 1.0 A g^-1.The present work implies that specific capacitance is mainly affected by specific surface area and pore volume.2.Nanostructured birnessite with a high specific surface area(117.6 m² g^-1)was readily obtained through hydrothermal reaction of KMn O₄ and ?-cyclodextrin under microwave irradiation.The electrochemical behaviors of ZnMn²⁺ on birnessite electrode were studied by cyclic voltammetry.The redox of birnessite in ZnMn²⁺-containing solution was controlled by multi-cycle galvanostatic charge-discharge.The effects of electrolyte p H and birnessite mass on ZnMn²⁺ removal capacity were also investigated.The results indicated that ZnMn²⁺ removal capacity increased and decreased with increasing p H and birnessite mass,respectively.The highest ZnMn²⁺ removal capacity reached 530.0 mg g^-1,which was remarkably higher than the adsorption isotherm capacity(56.1 mg g^-1).The significant improvement of electrochemical removal capacity could be attributed to the nanostructure and the not fully reversible redox reaction of the birnessite.The result of X-ray absorption fine structure?XAFS?spectra indicated that ZnMn²⁺ was adsorbed above/below the vacancies and was inserted into the interlayer of birnessite,leading to the transformation of birnessite to Zn-buserite and hetaerolite during the charge-discharge process.3.Birnessite nanosheets were synthesized from the photochemical reaction of Mn^Mn2+aq and nitrate under solar irradiation,and ZnMn²⁺ was adsorbed using the as-obtained birnessite nanosheets by electrochemically controlled redox.The effects of current density and electrochemical techniques?symmetric electrode and three-electrode systems?on ZnMn²⁺ adsorption capacity were also investigated.The results showed that the maximum ZnMn²⁺ adsorption capacity of the birnessite in the presence of electrochemical redox reactions could reach 383.2 mg g^-1 and 442.6 mg g^-1 in the symmetric electrode and three-electrode system,respectively;however,the MnMn²⁺ release capacity in the three-electrode system was higher than that in the symmetric electrode system.With increasing current density from 0.1 to 0.5 A g^-1,the ZnMn²⁺ adsorption capacity decreased from 82.0 to 29.0 mg g^-1 in 200 mg L^-1 ZnMn²⁺ solution because that the diffusion of ZnMn²⁺ limited the rate of electrochemical reactions.4.Birnessite and birnessite/carbon nanotubes?CNTs?nanocomposites were synthesized through a microwave-assisted hydrothermal reaction and were used as active materials for ZnMn²⁺ and NiMn²⁺ removal from aqueous solution by constant potential electrolysis.The effects of operation potential and introduction of CNTs on ZnMn²⁺ and NiMn²⁺ removal capacities and regeneration performance were further investigated.The results demonstrated a significant enhancement of electrochemical removal capacities for ZnMn²⁺ and NiMn²⁺ by the pseudocapacitive properties of birnessite and the introduction of CNTs.The ZnMn²⁺ and NiMn²⁺ removal capacities of birnessite electrode increased first and then decreased with decreasing potential from 0.2 to-0.2 V?vs.SCE?,and the highest removal capacities for ZnMn²⁺ and NiMn²⁺ respectively reached 89.5 and 96.6 mg g^-1 when the potential was controlled at 0 V.The nanocomposite showed higher removal capacities(155.6 mg g^-1 for ZnMn²⁺ and 158.4 mg g^-1 for NiMn²⁺ when the relative content of manganese oxide was 45.6%)and a better cycling stability(about 90% and 88% of the initial ZnMn²⁺ and NiMn²⁺ removal capacities were retained after 5 cycles)than birnessite electrode.5.Birnessite was used for electrochemical adsorption of arsenic?As?from mining wastewater at a constant cell voltage in a symmetric electrode system,and the effect of cell voltage and the continuous use?without desorption?performance of birnessite electrode were also evaluated.At 1.2 V for 24 h,the concentrations of total As?As?T??and As???in wastewater decreased from 3808.7 to 73.7 ?g L^-1 and 682.8 to 21.4 ?g L^-1,respectively.The As?T?removal ratio increased with increasing cell voltage and reached 98.1% at 1.2 V,which was higher than that at open circuit?84.1%?.The MnMn²⁺ concentration also significantly decreased in wastewater during As adsorption.The high potential of birnessite anode and the generation of H₂O₂ on cathode facilitated As???oxidation,and the electrochemical redox reactions of birnessite contributed to the enhancement of As?T?removal.The application of cell voltage reversal could improve the utilization rate of birnessite electrodes by dissolution-recrystallization during continuous use process,and the As?T?removal ratio was increased from 73.5% to 85.1% after five cycles of voltage alteration.The present work provides some facile methods for birnessite preparation,clarifies the adsorption mechanism and modification method of birnessite for heavy metal ions in the presence of electrochemical redox reactions and expands the application of manganese oxides in the treatment of heavy metal contaminated water.