| Climate change and population growth have led to an increasing shortage of freshwater resources around the world.Due to these environmental problems,growing attention has paid to sewage discharge.Specifically,to meet the increasingly strict policy requirements,reusing power plant wastewater is the only way for sustainable development of enterprises.Near-zero wastewater discharge from power plants is mainly focused on desulfurization wastewater,and a promising method to treat desulfurization wastewater is Capacitive Deionization(CDI).It is advanced in high operational flexibility(flexible control of effluent quality according to water quality requirements),large adsorption capacity,no dosing and high degree of automation,and broad application prospects.Therefore,the paper proposes a technical route for desalination of desulfurization wastewater by using CDI.Taking the cations and anions mainly contained in the desulfurization wastewater as the removal targets,a new asymmetric CDI device and TiO2/CNTs composites were designed from the perspective of structural innovation and material innovation,respectively,to target the removal of the target ions.After that,a new"pool-type" CDI device was designed to investigate the relationship between the optimal operating conditions and the adsorption performance of various ions in the desulfurization wastewater.In addition,a two-dimensional concentration mass transfer model was developed to simulate and predict the multi-physics dynamic behavior of salt ions in CDI.Finally,a small-scale CDI was designed for treating actual desulfurization wastewater to further validate the actual effect of CDI desalination.The main research results are as follows:(1)The research of structural:Two new CDI devices were designed.One was an asymmetric CDI device for targeted removal of representative cations from desulfurization wastewater.The other one was a "pool type" structure,which was mainly designed for a large amount of FGD wastewater treatment,high salt content,and some drawbacks of the original CDI devices.Experimental studies on desalination were carried out using these two devices to obtain the optimal operating conditions.The internal flow processes were also simulated with COMSOL software.The newly designed CDI device achieved the expected desalination effect and had good desalination stability.(2)The research of materials:The CDI anode was modified with TiO2/CNTs composite material,and the representative anion-chloride ions in desulfurization wastewater were removed by catalytic synergistic adsorption.The TiO2/CNTs composites were characterized and found to have excellent physicochemical properties.Through desalination experiments,the ratio and operating conditions of TiO2/CNTs composites were optimized.By comparison,it was found that the maximum electroabsorption capacity of the modified TiO2/CNTs composite(6.5 mg/g)was much higher than that of pure CNTs(3 mg/g),which proved its excellent electroabsorption capacity.(3)Mechanistic analysis:The kinetic analysis and isotherm adsorption model analysis of the CDI device were carried out,showing that the experimental results of the CDI device are more consistent with the pseudo first-order model.Besides,the Freundlich and RedlichPeterson models are in better agreement with the experimental results.In addition,the adsorption patterns of the anions and cations mainly contained in the desulfurization wastewater were explored,and the sequential order of CDI adsorption ions was obtained:Mg2+>Na+>Ca2+>K+and Cl->CO32->NO3->SO42->HCO3-.Based on the previous model experience,a new two-dimensional CDI concentration mass transfer model was developed.It is divided into two directions:the flow direction and the vertical flow direction.The electric field migration and mass transfer diffusion are considered,which means that the ion transport process and concentration distribution inside the CDI can be analyzed.The results show that this two-dimensional concentration mass transfer model matches well with the actual results and can simulate the mass transfer process of CDI well.The twodimensional model simulation achieves the analysis and prediction of the multi-physics field kinetic behavior(ion diffusion and electromigration)of salt ions in CDI,which provides a basis for the optimization design of the internal device structure and concentration mass transfer study of CDI.(4)The research of small-scale equipment:According to the characteristics of desulfurization wastewater from power plants,small-scale equipment was carried out to establish a coupled wastewater treatment system integrating microfiltration and CDI.Commissioning and performance testing of CDI desalination system were carried out to investigate the effects of different water quality/quantity and working conditions on desalination effect and to obtain the optimal working parameters of the equipment.The results show that the stable desalination rate and water production rate of the equipment could bothreach more than 60%.This paper has achieved periodical results in CDI device structure,adsorption materials,operation conditions,desalination and adsorption mechanism,and internal simulation and emulation of CDI.Based on the experimental results and theoretical analysis,the experimental research in small-scale equipment has been carried out.Besides,a unique technical route of desulfurization wastewater treatment has been formed to solve some of the key technical problems in desulfurization wastewater treatment by CDI.Moreover,the obtained experimental data has provided basic data support for desulfurization wastewater treatment in the industrial application of CDI as well as valuable references for the development of CDI technology and engineering practice. |
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