Electrosorption Of Cd²⁺in Aqueous Solution Using CNT/MnO₂ Composite Electrodes

Cadmium?Cd?,as one kind of heavy metal,is harmful to human body.In periods of excess intake,it may cause human liver,lung,kidney,or other organ diseases and even cancer indicated by research.Therefore,research and development of effective and economical methods to treat and control Cd-containing wastewater has great economic,environmental and social significance.Common approaches to treat Cd²⁺-containing wastewater are oxidation,membrane separation,adsorption,precipitation,and ion-exchange resin.Although these methods can effectively reduce concentration of Cd ions,some limitations exist in practical applications,such as difficult recycle,high price and secondary pollution.Capacitive deionization,also known as electrosorption,is a new type of water treatment technology based on electric double layer capacitance theory.it doesn't cause secondary pollution and has low energy consumption and maintenance costs.Currently,capacitive deionization has been widely used in desalination of seawater and high-concentration brine.However,there are relatively few researches on mechanism of heavy metals adsorption removal.Consequently,study of mechanism and influencing factors of heavy metal ions removal by capacitive deionization have important theoretical significance.This study focuses on preparation of carbon nanotube?CNT?unit electrode material and CNT/MnO₂ composite electrode material,experimental conditions of equipment installation and electro-adsorption,kinetics and adsorption mechanism for Cd²⁺removal by capacitance.In research,CNT/MnO₂ composites were prepared from CNTs cladded by MnO₂ using redox reaction,and CNTs and CNT/MnO2 electrodes were made by coating method.Solution recirculating flowed through electrode by peristaltic pump.In this study,scanning electron microscopy?SEM,Hitachi S-4800,Japan?was used to observe surface morphologies of samples before and after compound ed;electrochemical performance of electrodes was measured by cyclic voltammetry?CV?using an electrochemical workstation produced in Shanghai;XPS?X-ray Photoelectron Spectrometer,Kratos Amicus?from the United Kingdom was used to characterize components contained on the surface of electrodes;infrared spectrum of sample was analysed by Fourier transform infrared spectrometer?FTIR,Spectrum One,USA?;specific surface area of material was measured by low-temperature nitrogen adsorption-desorption technology,and adsorption performance of electrodes was explored.Results showed that compared with CNTs,CNT/MnO₂ composites had significantly shorter carbon nanotubes coated by MnO₂;pores were reduced;surface become rough;pecific capacitance was increased by 1.13 times;OH and C=C functional groups increased;O content increased from 1.89%to 33.15%;and Mn content was 12.38%.It proved that MnO₂ was successfully loaded onto CNTs;specific surface area increased;and equilibrium adsorption capacity of CNT/MnO ₂composite electrode for Cd²⁺increased by 6.125 times.Influence factors on electrosorption efficiency of CNT/MnO₂ composite electrodes,such as different CNT/MnO₂ electrode ratios,electrode voltage,solution pH,initial Cd²⁺concentration,flow rate and electrode spacing,of electrosorption experiments were investigated in this research.When ratio of CNT/MnO₂ is 1:4,effect of electro-adsorption is the best.Electrode adsorption capacity of Cd²⁺increased with voltage across electrode,pH and initial concentration of solution,and decreased with electrode space.Circulating flow rate affected speed of electro-adsorption.Adsorption kinetics was used to study adsorption mechanism of Cd²⁺on CNT electrode and CNT/MnO₂ composite electrode.Results showed that adsorption process of CNT/MnO₂ electrode and CNT composite electrode followed pseudo-second-order kinetic model and pseudo-first-order kinetic model respectively.