Study On Membrane Electrochemical Properties And Fouling Behaviors In Reverse Electrodialysis

In order to alleviate the energy crisis and reduce the environmental pollution,it is urgent to develop renewable energy and sustainable development resources.Salinity gradient power(SGP)is a kind of renewable energy generated by mixing two different salinity of solutions.Reverse electrodialysis(RED)can be used to generate energy from SGP,nevertheless the low efficiency of energy generation and serious membrane fouling limit the practical application of RED.Furthermore,the evaluation method for electrochemical properties of ion exchange membrane and RED stack was deficient.To solve the above problems,the impact factors and mechanism on electrochemical properties of ion exchange membranes and RED stack were investigated by optimized evaluation system of electrochemical properties,which provided theoretical support for improvement of RED performance and economic benefit.Ion exchange membrane,the core of RED technology,formed three sublayers when immersed in solution: the individual membrane,the electrical double layer(EDL)and the diffusion boundary layer(DBL).The EDL and DBL are considered as the interfacial layers of membranes,and have significant influences on membrane transport properties.The electrochemical properties of ion exchange membrane systems(IEMs)include the membrane resistance,the resistance and capacitance of EDL and DBL.Direct current method(DC)and Electrochemical impedance spectroscopy(EIS)can be used to measure the resistance.In contrast to DC,EIS shows a strong potential for quantitative evaluation of each sublayers of IEMs.The relationships between two methods are that the overall resistance of IEMs determined by DC equaled to the sum of resistance of pure membrane,EDL and DBL,and the overall conductivity of IEMs from DC measurement had a linear relationship with th e pure membrane resistance from EIS measurement.The EDL and DBL in the equivalent circuit for EIS fitting should be represented as a resistor and a constant phase element(CPE)in parallel.A simple and convenient formula(? = (?)for DBL thickness calculation was derived in this thesis,where Rd and Cd are resistance and capacitance of DBL,respectively.Comparing to the DBL thickness measured from Chronopotentiometry and Linear sweep voltammetry,the data from EIS was minimum and more close to the actual value.On the basis of EIS evaluation system and equivalent circuit,the effects of membrane material,physicochemical properties of electrolyte solution as well as operation conditions(flow rate,temperature)on electrochemical properties of IEMs were further studied.Analysis of experimental data revealed that the membrane material,electrolyte solution concentrations(0.01~0.5 mol/L),temperature(10~40 ?)and p H(2~11)affected the electrochemical properties of pure membrane and its interfacial layer,whereas the flow rate only affected the DBL electrochemical properties.The EDL showed CPE behavior which was attributed to t he surface heterogeneity and the CPE exponent n can be used for evaluating the degree of the surface heterogeneity.It is found that the resistance and the capacitance of EDL were consistent with the concentration dependence of the Debye equation.The temperature affected the electrochemical properties of IEMs by changing the ion transfer rate in the solution.The solution p H can not only influence the membrane swelling degree but also influence the dissociation degree of fixed charge groups in anion exchange membrane,thus influence the electrochemical properties of membrane and its interfacial layers.The DBL thickness was proportional to the square root of the flow rate.The changes of electrochemical properties in membrane and interfacial layers during fouling processes by Bovine serum albumin(BSA)and Sodium alginate(SA)were investigated by EIS.SEM,FT-IR,contact angle,Zeta potential and so on were also used to analyze the physicochemical properties and electrochemical properties of membrane surface.The equivalent circuit for EIS data fitting was optimized that an additional section of a resistor and a CPE in parallel was added to represent the gel layer formed on the membrane surface by contaminants.Based on the data analysis,the fouling mechanism was determined: firstly,the contaminants with negative charges adsorbed on the fixed charge groups on the membrane surface,and then a loose and porous gel layer was formed on the membrane surface,finally the gel layer was compacted.In the fouling process,the resistance of membrane and gel layer both increased and the increase rates were different in different stages.The capacitance of fouling layer decreased to approach zero because of the dielectric constant changes in fouling layer.Surface hydrophilicity,ion exchange capacity and Zeta potential of membrane after fouling were reduced,so the resistance and capacitance of EDL were affected.The DBL thickness significantly increased after fouling.The effects of configurations and operating conditions on the power output of the RED stack were studied.It showed that the configurations and operating conditions have significant influences on resistance and power generation efficiency of RED stack.The optimal configuration of RED stack contained 07-465/49 spacer and ion exchange membrane Type II.The optimal operating conditions were that the concentration rate of high concentration solution/low concentration solution was 30,and the flow velocity rate of high concentration solution/low concentration solution was 1.4.