Study On The Selective Removal/extraction Of Metal Ions From Water By Capacitive Desalination Using PBA-based Electrode Material

At present,water shortage is increasing,and compared with other traditional desalination technologies,capacitive desalination technology（CDI）has attracted wide attention in the field of seawater desalination because of its advantages of environmental friendliness and low energy consumption.Compared with the low salt adsorption capacity exhibited by CDI electrodes composed of traditional carbon-based materials,intercalation materials exhibit higher desalination capacity by inserting cations into the lattice through redox reactions.Also,these electrode materials present great advantages for heavy metal ion removal and lithium extraction from brine applications.In this thesis,Prussian blue-like materials were used to prepare electrodes and assembled into a rocking chair capacitive desalination device（RCDI）to investigate their desalination and metal ion selective removal/extraction performance,respectively.The main research contents and results are as follows:Structural/Compositional tailoring of nickel ferrocyanide（NiHCF）can significantly change its desalination performance.Coating carbon on NiHCF can effectively improve the long-term cycling stability of the synthesized NiHCF@C electrode,but the carbon film partially blocked the ion transport pores in the PBA framework and reduced its salt adsorption capacity.Co-coating carbon and nitrogen on NiHCF adversely affects the desalination ability and cycling stability of NiHCF@N/C due to the side reaction between pristine NiHCF and the nitrogen precursor during the synthesis process.However,the highly crystalline NiHCF cubes（NiHCF-HQ）obtained by chelator-assisted co-precipitation showed significantly enhanced desalting ability but poor cycling stability compared to the conventional NiHCF.However,NiHCF nanoskeletons obtained by HCl etching of highly crystalline NiHCF cubes（NiHCF-NF）not only had the highest desalination capacity(121.38 mg·g^-1 in 500 mg·L^-1 Na Cl solution at 1.2 V)but also had good cycling stability among these studied electrodes.In addition,these NiHCF electrodes showed higher selectivity for Na⁺than for Ca²⁺and Mg²⁺ions,NiHCF-NF electrodes showed selectivity of Na⁺over divalent Ca²⁺and Mg²⁺ions in the range of 3.35-13.13.Overall,NiHCF-NF with skeletal structure and high crystallinity has great potential for seawater desalination,especially for preferential removal of Na⁺ions from brine.Prussian blue materials with cubic structure,nano-core-shell structure and nano-frame structure were synthesized by composition/structure control strategy,and Prussian blue/carbon cloth-like composite electrodes were prepared by carbon cloth loading.Compared with the cubic structure and nano-core-shell structure,the Cu_0.6Ni_0.4HCF-NF@s CC and Cu Mn HCF-NF@s CC electrodes with nano-frame structure showed the highest Pb²⁺removal ability(SAC of Cu Mn HCF-NF@s CC electrode reached 194.6 mg·g^-1 at 1.2V).This indicates that the nano-frame structured Prussian blue-like material is more favorable for Pb²⁺removal.By comparing the selectivity of the two electrodes for Pb²⁺,the results showed that the Cu_0.6Ni_0.4HCF-NF@s CC electrode still achieved a selectivity of 1.7 for Pb²⁺even in the presence of a high concentration of Na⁺at 100 mmol,exhibiting excellent Pb²⁺selectivity.In the mixed solution of Ca²⁺and Mg²⁺divalent ions,the Cu_0.6Ni_0.4HCF-NF@s CC electrode also exhibited excellent Pb²⁺selectivity,and the selective adsorption factor of Pb²⁺to Mg²⁺reached 8.53 in equimolar solutions of Pb²⁺and Mg²⁺(Pb²⁺,Mg²⁺=10 mmol).The PBA/LIS composite electrode was obtained by compounding PBA with lithium ion sieve,and its Li ion adsorption performance on simulated Li-containing wastewater and brine from Dongtai Genel Lake was investigated by RCDI system.The results showed that NiHCF@λ-Mn O₂ and NiHCF@δ-Mn O₂ with NiHCF core（δ）λ-Mn O₂ core structure had stronger lithium extraction capacity compared withα-Mn O₂@NiHCF withα-Mn O₂ core NiHCF shell structure.The lithium extraction capacities were 30.22,125.93 and 82.99 mg·g^-1,respectively.The experimental results of cycling stability show that after 40 cycles,the NiHf@λ-Mn O₂ electrode still reaches 78.4%of the initial desalting capacity,showing good cycling stability.The results of selective Li⁺extraction experiments showed that excellent Li⁺selectivity for synthetic brine（maximum adsorption capacity of 2.28 mmol）and Dongtai Jinel Salt Lake brine（maximum adsorption capacity of 13.9 mmol）,Li⁺selectivity for bivalent Mg²⁺ions reached 5.37 and 20.96,respectively.Therefore,Nihcf-@λ-Mn O₂ can be used to extract Li⁺from actual brine.