| Lead(Pb) is a highly toxic heavy-metal to human health, which can not be degraded once the lead pollution existed. Facing the critical situation of lead pollution in China, the research on the purification treatment of wastewater containing lead is of great significance to the water resource safety. While the traditional methods for the treatment of lead pollution have the disadvantages, such as high energy-consumption, high cost on investment and maintenance. Capacitive deionization(CDI), a method for removal of ions from aqueous solutions, is one of the promising techniques due to its advantages of low energy-consumption, pollution-free, simple electrode regeneration process and low maintenance cost. However, Currently CDI is mainly applied in the field of deionization or salt removal for the water with high concentration of salt, and there is little literature on the removal of low concentration heavy-metal ions, for example lead ions. The removal mechanisms, process settings and influencing factors on the CDI removal of lead ions need to be further studied. Therefore, a systematic research on CDI technology for the lead ions removal is of important theoretical and socio-economic values.This paper systematically studied the carbon nanotubes(CNT) electrodes, plasma enhanced CNT electrodes, CNT and polypyrrole(PPy) composite electrodes capacitive deionization for the lead ions removal, involving electrode preparation and modification methods, process settings, dynamics and mechanisms of lead removal. The main works are as follows:The influence of carbon nanotubes’ pretreatment and CNT electrode’s preparation conditions on electrode’s electrochemical performance and its lead electrosorption capacity was discussed. The morphology, pore size distribution, electrochemical performance and wettability of CNT and CNT electrode were studied. The process conditions such as operation voltage and flow rate, water conditions such as p H, temperature, ions concentration, coexisting ions, as well as their effects on electrosorption performance were analyzed. The results show that: After KOH treatment, the CNT possesses larger specific surface area(SSA) and the CNT electrode shows enhanced electrochemical activity, i.e. 90% increase in the capacitance, and 21% improvement in the electrosorption quantity at the initial lead concentration of 10 mg/L. The comprehensive performance of electrode is optimal when electrode containing 10% adhesives. The CNT electrode, prepared from the mixture of CNT and adhesives, is with smaller SSA and pore volume than CNT, and a poor wettability. The experimental lead removal conditions of CNT electrode CDI were as follows: operation voltage of 450 m V, flow rate of 20 m L/min, temperature of 20 ℃, p H of 6. The removal process conforms to the first-order kinetics equation and abides by the Freundlich isotherm model. A dynamic process model for lead removal was established based on the electric double layer theory and the simulation results are consistent with the experimental outcome. Combining with the cyclic voltammetry and electrochemical impedance analysis, the dynamics process simulation indicates that the removal of lead ion by CNT electrode under set operation conditions is a typical electrical double layer adsorption.The CNT electrode was directly activated by air-plasma to improve the wettability, electrochemical performance as well as the SSA and electrosorption capacity for the electrode. The effects of plasma enhancement conditions on electrode performance were studied. A comparative analysis was conducted on the surface morphology, SSA, pore size distribution characteristics, electrochemical properties and wetting properties between the electrodes with and without plasma enhancement. The mechanism of plasma enhancing CNT electrode was deduced base on the plasma theory and measurements. It is found that, after plasma enhancement of 5 min, the oxygen-containing and nitrogen-containing groups increase significantly, while the F-containing groups decrease significantly on the surface of electrode, and the plasma enhanced electrode(P-CNT electrode) has a larger SSA and pore volume than CNT electrode. The surface of P-CNT electrode shows super-hydrophilic properties and enhanced electrochemical activity. The mass transfer resistance is reduced after plasma enhancement. Compared with CNT electrode, the capacitance of P-CNT electrode is increased by 47.4% and electrosorption quantity is increased by 58.6% at the initial lead concentration of 10 mg/L. The process and water conditions of P-CNT electrode for lead removal were studied, and the results show that the effects are the same as CNT electrode. The kinetics and electrosorption isotherms of P-CNT electrode capacitance deionization for lead removal were analyzed, and the electrosorption process conforms to second-order equation and abides by Freundlich adsorption isotherm model. Due to interactions of chelating and hydrogen bonding between the lead ions and oxygen-containing groups on the surface of P-CNT, which are intensified with the increase of lead concentration, the actual electrosorption quantity is evidently larger than the simulation results of dynamic process model at the high lead concentration.To further improve the capacitance and lead ions’ electrosorption capacity of CNT electrode, the conductive polymer polypyrrole(PPy) was combined with CNT to prepare CNT and PPy composite. In order to enhance the performance of composite, the CNT was firstly modified by air plasma(labeled as P-CNT), and then covalently bonded with PPy. The composite of plasma activated CNT and PPy is labeled as P-CNT-PPy. The effects of preparation conditions on the performance of P-CNT-PPy electrode were studied, and the preparation mechanism for the composite was analyzed. The influence of plasma treatment on the composite’s chemistry and physical properties, electrochemical performance and lead ions removal capacity was compared with the composite without plasma treatment. The process conditions, kinetics, electrosorption isotherms and mechanism of P-CNT-PPy composite electrode CDI for lead removal were analyzed. The preparation conditions for P-CNT-PPy composite were as follows: The time of plasma treatment is 20 min, the mass ratio of CNT and polypyrrole is 1:4, the HCl volume is 10 m L and the APS is added by two steps. With the plasma modified CNT, part of PPy forms covalent polymerization via pyrrolic nitrogen groups on P-CNT, thus the composite possesses the longer conjugated structure, improved electrical conductivity and thermal stability. Compared with CNT electrode, the capacitance of P-CNT-PPy electrode increases 5.7 times, and the lead removal capacity of P-CNT-PPy electrode increases 1.6 times under the following conditions: initial lead concentration of 50 mg/L, operation voltage of 450 m V. The experimental lead removal conditions of P-CNT-PPy electrode CDI were as follows: operation voltage of 600 m V, p H of 6. The removal process conforms to the second-order kinetics equation and abides by the Langmuir isotherm model. The removal of ions occurs in the pseudo capacitance existing on the P-CNT-PPy skeleton and the double electrical layer capacitance. |
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