Design Of Novel Bipolar Membrane Based Processes And Applications

Electrodialysis is a novel mass transfer separation technology based on ion exchange membranes,utilizing the charged groups in membranes to realize the selective separation of ions and charged substances.The function of ion exchange membranes can help with energy consumption reduction and pollution reduction in modern industries,which allows them to have broad application prospect in clean production and environmental protection.With the rapid development of materials science and technology,a variety of ion exchange membranes with different properties and functions have been developed,such as mono/multi-valent ion selective exchange membranes,bipolar membranes,etc.According to the kinds of ion exchange membranes used and the different design of membrane processes,electrodialysis can be divided into common electrodialysis(CED),selective electrodialysis(SED)and bipolar membrane electrodialysis(BMED).Bipolar membranes can dissociate water molecules into H+ and OH-under electric field,which makes it possible for BMED to recycle the waste salt from saline water and convert them into acid/base products without additional chemical reagents.However,the traditional applications of BMED still have problems like low integration and product purity.In recent years,the research and development of new bipolar membrane based processes have just started,and they have great application potentials.According to this,in order to expand the application scope of BMED and improve the process efficiency,the functions and advantages of bipolar membranes are deeply explored,the existing technologies are integrated and improved,and several novel bipolar membrane based electro-driven processes are proposed in this paper.The main research contents are as follows:1)BMED can realize the conversion of salt to acid and base,but it can't separate mixed salt.SED can separate mono/multi-valent ions,allow the migration of monovalent ions and retain multivalent ions.Combining these two technologies can simultaneously utilize the advantages of SED and BMED to recover monovalent ions from mixed salt solution and produce acid/base with high purity,by applying mono/multi-valent ion selective exchange membranes in BMED stack.The concentrated brine produced by reverse osmosis is not suitable to be discharged directly because its high concentration of sodium chloride and other multivalent ions,and difficult to recycle by traditional methods.Selectrodialysis with bipolar membranes(BMSED)can be used in its desalination and to produce high purity hydrochloric acid and sodium hydroxide from the sodium chloride recycled.The experimental results approved that this process can be used to treat concentrated brines.According to the composition of acid/base products,the selectivity for Na⁺/Ca²⁺ and Cl^-/SO₄^2-was 5-10 and 50-60 respectively.In a single unit operation,105 g/L concentrated brine could be desalinated and converted into acid/base products.The concentrations of sodium hydroxide and hydrochloric acid obtained were 2.2 mol/L and 1.9 mol/L respectively,and the purities were over 99.99%.2)The traditional treatments of dye wastewater and other organic pollutant wastewater all have some shortcomings.Among which,electrocoagulation uses metal ions dissociated from metal anode and hydroxide ions produced by cathode electrolysis to form flocculation to adsorb dye.This technology performs well but the problems like electrode passivation and consumption are difficult to solve.BMED can realize the controllable migration of metal ions,which is used to replace the metal ions obtained from sacrificial anode in electrocoagulation.Meanwhile,electroplating wastewater containing a large amount of heavy metal ions can also be used as feed solution to realize resource recycling.Here we propose the bipolar membrane based water splitting coagulation process,which utilizes the metal ions passing through the cation exchange membrane to react with the hydroxyl ions generated by the bipolar membranes to produce hydroxide flocs.The flocs can in-situ adsorb and settle the dyes in wastewater.The results showed that while treating 725 mg/L dye wastewater,the removal rate of dyes was higher than 98%,and the lowest energy consumption was 87 kWh/kg-dye.3)In the above-mentioned water splitting coagulation method,the migration rates of metal ions and hydroxyl ions into the base chamber are controlled by the current.Thus the hydroxides generated in the base chamber will have smaller particle size and larger specific surface area,and not tend to agglomerate.Dyes or other substances in the base chamber can act as surfactants,which can effectively avoid the agglomeration of hydroxide flocs and help to form nanomaterials with smaller particles and larger adsorption capacity.After the post-treatment process,the original porous structure of hydroxide flocs can be retained,so as to obtain metal oxide nanoparticles with better electrochemical performance.The products in the experiments were characterized as pure phase NiO,and uniform nanoparticles with morphology of about 20 nm size could be observed under SEM.The highest specific capacitance was 369 F/g(under 1 A/g),which was higher than 307 F/g of commercial nickel oxide nanoparticles.4)SED can separate mono/multi-valent ions,but the separation performance is not satisfied while the ions have similar hydration radius.In the process of extracting lithium from Salt Lake,it is necessary to selectively extract lithium from brine with high Mg/Li ratio,but the high concentration of magnesium ions will lead to low selectivity.Here we proposed an internal multistage enhanced BMSED process.Through the continuous use of selective ion exchange membranes,the monovalent and multivalent ions were multistage-separated,and the monovalent and multivalent ions were successively enriched at different rates in each chambers,so as to obtain high purity monovalent ions in the final stage.A mathematical model was established based on the experimental data,then the separation performance under different conditions could be simulated and estimated.Finally,while extracting lithium from real Salt Lake brine,the selectivity coefficient of lithium and magnesium reached 17000,and the purity of lithium was higher than 99.9%.