Jiefeng Pan ,Wei Zhang ,Huimin Ruan ,Jiangnan Shen ,*,Congjie Gao

1 Center for Membrane Separation and Water Science&Technology,Ocean College,Zhejiang University of Technology,Hangzhou 310014,China

2 Huzhou Institute of Collaborative Innovation Center for Membrane Separation and Water Treatment,Zhejiang University of Technology,1366 Hongfeng Road,Huzhou 313000,China

Keywords:Electrodialysis Separation of the mixed salts Monovalent selective AEMs Ion separation

ABSTRACT The industrial products or wastewater rich in the mixed salts(Cl-/SO4 2-)not only causes the environmental damage,but also induces waste of resource.In thisstudy,an EDstack w ith monovalent selective AEMs and conventional CEMs w as employed to separate the Cl-and SO4 2-from simulated w astew ater.The effect of current density and mass fraction percentage was investigated in order to optimize the experimental conditions during EDprocess.It w as found that at a concentration ratio between NaCl and Na2SO4 of 95/5(w t%/wt%)and a current density of 40 mA·cm-2,a current ef fi ciency of 72%,an energy consumption of 1.6 kW·h·kg-1 NaCl and a Cl-/SO4 2-concentration(67.5/3.5 g·L-1)were obtained.Hence,it isappropriate and effective to separate Cl-and SO4 2-by EDusing the monovalent selective AEMs.

1.Introduction

The industrial products or w astew ater which contains the mixed salts(Cl-/SO42-)are generated in variouschemical processes.For example,the wastew ater of many salt minesin Chinacontainssulfate besides the sodium chloride[1].Moreover,the salinity of oil fi eld w astew ater and coal chemical w astew ater reaches up to tens of thousands of mg·L-1,which also mainly contains mixed salts(Cl-/SO42-)[2].To improve the industrial product purity or minimize the industrial cost,the separation strategy betw een the Cl-and SO42-is designed.

Conventional separation techniques of the Cl-and SO42-are mainly based on the evaporation and crystallization processes.How ever,the complexity of the process and the higher corporate and investment cost associated w ith the common separation techniques limit their development.Recently,the new separation strategy for mixed salts based on the membrane materials,selectrodialysis(SED),has attracted theinterest of many researchers.SEDisanovel electrically driven membrane process,w hich can selectively separate mono/multi valent ions based on monovalent selective membrane operation.Zhang et al.[3]fi rstly proposed the concept of“Selectrodialysis”and investigated the fractionation of divalent ions betw een monovalent ions using a novel electrodialysis stack.Furthermore,Van der Bruggen et al.[4]also investigated the separation of monovalent and divalent ions from aqueous solution by electrodialysis,and the formula of separation ef fi ciency(S)w asproposed in thisliterature.Nie et al.[5]haverealized the separation of lithium and magnesium from the salt lake brine by electrodialysis w ith the monovalent selective membranes.Ghyselbrecht et al.[6]have separated and removed thedivalent anions/cationsfrom industrial saline w ater by EDw ith monovalent selective AEMs and CEMs.In addition,our previous w ork[7,8]also researched the separation of divalent ions from the reverse osmosis concentrate by electrodialysis with monovalent-selective membranes.All of w hich indicated that the Selectrodialysis is an effective and feasible strategy for the separation of mixed salts(mono-and multi-valent ions).

Electrodialysis has been w idely used in puri fi cation and separation of w asted liquid[9-14]due to some signi fi cant advantages[15-20],such as environmental-friendly,cost-effective,convenient operation,small covered area,etc.[21-28].How ever,conventional ED with ordinary ion-exchange membranes has obvious draw backs for the separation of mono-and multi-valent ions;therefore,the monovalent selective anion or cation exchange membranes w ere developed and assembled into ED and endow ed it w ith selective performance of mono-and muti-valent mixed salts.Asmentioned before,many studies have proved that the selectrodialysis has show n promising results for separating monovalent and multivalent ions.

Hence,the Selectrodialysis w ith monovalent selective AEMs w as selected to separate the chloride ions from sulfate ions for a mixed salt solution.And,an ED stack equipped w ith the monovalent selective AEMs(NEOSEPTA? ACS)and the conventional CEMs(NEOSEPTA?CMX)w as applied to separate the model mixed brine solution(NaCl and Na2SO4).Therefore,this study could provide a useful reference for the separation of NaCl and Na2SO4in the industry fi eld.Furthermore,the effect of current density and the ratio of mixed brine solution(NaCl/Na2SO4)w ere investigated to optimize the experimental operation.The current ef fi ciency,energy consumption and fi nal anions concentrations were also calculated during ED process.

Table 1 Commercial membranes and their properties①

2.Experimental

2.1.Materials

The commercial CEMs(NEOSEPT A?CMX)w ere obtained from ASTOM Co.,Japan,and commercial monovalent selective AEMs(NEOSEPTA?ACS)w ere purchased from ASTOM Co.,Japan.The CV/CC pow er supply(WYL 1703*2)was purchased from Hangzhou Siling Electrical Instrument Ltd.,China.Deionized w ater w as used in ED throughout,and the other reagents(Na2SO4and NaCl)are of analytical gradeand used asreceived.Theconventional propertiesof thecommercial membranes are listed in Table 1.

2.2.Experimental process of the selectrodialysis

The lab-scale Selectrodialysis stack is assembled w ith arranging CEMs and monovalent selective AEMs alternately w ith an effective area of 189 cm-2(length×w idth,21 cm×9 cm).The spacers w ith a thickness of 0.7 mm w ere used betw een membranes alternately for partitioning off membranes.The fi ve repeating units w ere used in this Selectrodialysis stack.All the membranes w ere fastened betw een tw o piecesof electrode plates inlaid w ith a titanium coated w ith ruthenium.Electrode plates were connected with a CV/CCpower supply.

The cathode and anode chambers w ere interconnected w ith 3%sodium sulfate circulating into a1000 ml measuring cylinder.The dilute chamber circulated the mixed salts(Cl-/SO42-)solution and the deionized w ater as initial solution used in concentrate chamber.And each chamber connected w ith storage tanks(1000 ml).The solution w ascirculated betw een EDstack chamber and storage tanksw ith aconstant fl ow rate of 40 L·h-1.The initial solution volume of three circulating solution w as 500 ml.The 30 min of eliminating the visible bubble w asused in all the EDprocess.Thecon fi guration and schematic diagram of the Selectrodialysis stack were show n in Fig.1.In particular,the ED stack w as operated in batch mode.

Aconductivity meter wasused for measuring solution conductivity(S220 type,Mettler-Toledo,Sw itzerland).And the concentrations of mono-/multi-anions w ere monitored by an ion chromatography(792Basic IC,Metrohm,Sw itzerland).The voltage drop cross the ED stack can beread directly by thepow er supply under aconstant current.

The current ef fi ciency can be calculated by following Formula(1):

w here C0and Ctw ere the NaCl concentration in the concentrate chamber of SEDstack at time 0 and t,respectively.Vtw as the circulated solution volume in the concentrate chamber at time t,F represented Faraday's constant,N and I w ere the number of repeating units of the stack(N=5)and the electrical current.

The energy consumption can be calculated by following Formula(2):

w here U and I were the voltage drop and the applied current,Mbis the molar mass of NaCl,Vtw as the circulated solution volume in the concentrate chamber at time t,and Ctw as the NaCl concentration in the concentrate chamber of SEDstack at time t.

Fig.1.Con fi guration of the Selectrodialysis stack w ith CEMs and monovalent selective AEMs.

3.Results and Discussion

3.1.Change of voltage drop by using different current density

As aforementioned,a Selective electrodialysis system w as chosen to treat mixed brine with monovalent selective ion-exchange membranes(see Table 1).Current density is an important operational parameter in ED.Meanw hile,the mass ratios of NaCl to Na2SO4(95%/5%,90%/10%,80%/20%)w ere conducted in this w ork,according to the mixed salts(Cl-/SO42-)ratio of the industrial products and w astew ater(around 90%/10%)[1,2].Because it is closely related to the current ef fi ciency and energy consumption,current density should be controlled below the limiting current density in order to avoid the phenomenon of w ater dissociation at the surface of ion-exchange membrane[29].Figs.2-4 show the in fl uence of current density(30-50 m A·cm-2)on the separation of Cl-and SO42-w ith different mass fractions.

Fig.2.Voltage drop w ith thecurrent density of 30-50 m A·cm-2,concentration ratio NaCl/Na2SO4 is 95/5(w/w),initial concentration is 200 g·L-1.

Fig.3.Voltage drop w ith the current density of 30-50 mA·cm-2,the concentration ratio NaCl/Na2SO4 is 90/10(w/w),initial concentration is 200 g·L-1.

Fig.4.Voltage drop with the current density of 30-50 m A·cm-2,the concentration ratio NaCl/Na2SO4 is 80/20(w/w),initial concentration is 200 g·L-1.

Fig.5.Variation of conductivity by using EDwith different current densities,the massratio of NaCl to Na2SO4 is 95%/5%,initial concentration is 200 g·L-1,D-Dilute chamber,CConcentrate chamber.

Fig.6.Variation of conductivity by using EDwith different current densities,the massratio of NaCl to Na2SO4 is 90%/10%,initial concentration is 200 g·L-1,D-Dilute chamber,CConcentrate chamber.

For each fi gure,the voltage drop of the ED stack show s a“U”shape as a function of time.This behavior can be explained as follow s:in theinitial stage,theionsfrom thefeed compartment migrateto theconcentrate compartment due to the constant electrical fi eld,w hich causes the decreased electrical resistance of membrane stack.However,in the intermediate stage,the electrical resistance of membrane stack maintains a stable state for several minutes.In the fi nal stage,the electrical resistance of membranes stack w ould be sharply increased due to the low ions concentration of the feed chamber.In addition,Figs.2-4 also indicate that the increment of voltage drop w ith the applied current density rises.The reason is explained simply:according to Ohm's law(U=IR),w hen the electric current increases,the voltage increases correspondingly.Meanw hile,the ability of ions that permeate the IEMs w as intensi fi ed w ith the rapid increment of conductivity in the concentrate compartment as seen in Fig.4.

Fig.7.Variation of conductivity by using EDw ith different current densities,the massratio of NaCl to Na2SO4 is 80%/20%,initial concentration is 200 g·L-1,D-Dilute chamber,CConcentrate chamber.

3.2.Effect of different current densities

The separation of Cl-and SO42-at different current densities is show n in Figs.5-7.For each curve,the conductivity of the concentrate compartment increases rapidly with time,meanwhile,the conductivity of feed compartment decreases sharply over time.On the other hand,when the current density increasesfrom 30 to 50 mA·cm-2,the change of conductivity of membrane stacks is obvious,thus the operation time w ould be shorter.The reason is that the strong electrical fi eld can greatly prompt the ion migration through the membranes.Besides,the ultimate conductivity of feed compartment decreased below 10 m S·cm-1,as show n in Figs.5-7 respectively,and the obtained brine can be used as industrial w ater or municipal w ater after posttreatment.

In order to evaluate the in fl uence of the current ef fi ciency,current density and energy consumption w ere calculated under different current densities and mass fraction percentage.As show n in Fig.8,it can be found that the energy consumption increases w ith current density whereas the current ef fi ciency slightly increases asthe current density risesfrom 30 to 50 m A·cm-2.On theother hand,thecurrent effi ciency and energy consumption are affected by mass fraction percentage during EDprocess(Fig.8).Summarizing,massratio of NaCl Na2SO4is 95%/5%,the current ef fi ciency and energy consumption achieve a favorable outcome.Then,the current density w as optimized to be 40 m A·cm-2in order to achieve the better combination of values for the current ef fi ciency (72%) and the energy consumption(1.6 kW·h·kg-1NaCl).

The energy consumption increased w ith the increment of the current density,mainly due to thefact that alargepart of thetotal electrical energy is consumed to overcome the electrical resistance w hen the applied current density increases[30].On the other hand,current ef fi ciency basically suffers the effect of the follow ing element:current density and NaCl concentration in the concentrate compartment.With the increment of current density,the concentration of Cl-rapidly increases than SO4-2.The reason is that the selective CEMs have a higher speci fi c permselectivity for Cl-ions under the higher current density as seen in Fig.9.

Fig.8.Current ef fi ciency and energy consumption w ith different current densities and mass fraction percentage,initial concentration is 200 g·L- 1.

Fig.9.Variation of Cl-and SO42-concentration with time at 30,40 and 50 m A·cm-2;the mass ratio of NaCl to Na2SO4 is 95%/5%and initial concentration is 200 g·L-1.

3.3.The effect of current density on mixed salt separation

The separation performance of ion-exchange membranes can be infl uenced by the applied current density.Therefore,the variation of Cland SO42-concentration w ith time is show n in Fig.9 at the current density of 30,40 and 50 m A·cm-2,respectively.For each fi gure,the Cl-and SO42-concentrations increased w ith time elapsed.Meanw hile,when the current density increases from 30 to 50 mA·cm-2,the Cl-and SO42-concentration increased faster due to the strong electric fi eld applied,w hich resulted in short operation time(Fig.9).In conclusion,the current density w as optimized to be 40 m A·cm-2,considering the observed values for the current ef fi ciency,energy consumption and Cl-/SO42-concentration.Furthermore,the Cland SO42-concentrations are 67.5 g·L-1,3.5 g·L-1,respectively,at the current density of 40 m A·cm-2.The purity of Cl-is almost up to 95%.Table 2 summarizes published data for separation of mixed anions by SED.From thistable,it can be seen that considering the current ef fi ciency and purity for the evaluation of SEDperformance,the calculated current ef fi ciency(72%)and purity(95%)in this w ork are advantageous compared to the other reported results.

4.Conclusions

In this w ork,an Selectrodialysis stack assembled w ith monovalent selective AEMs(NEOSEPTA?ACS)and conventional cation-exchange membranes(NEOSEPTA?CMX)w as applied to achieve the ef fi cient separation of mixed brine solution(NaCl and Na2SO4).The current density and mass fraction percentage w ere investigated in order to optimize the experiment conditions during EDprocess.The results indicate that the current density of 40 m A·cm-2is suitable since the current ef fi ciency(72%),energy consumption(1.6 kW·h·kg-1NaCl)and Cl-/SO42-concentration(67.5/3.5 g·L-1)are appropriate.Besides,mass concentation ratio of NaCl/Na2SO4is fi xed betw een 95%/5%and 80%/20%values,the current ef fi ciency and energy consumption are feasible and effective for separation of mixed salts(Cl-/SO42-).

Table 2 Comparison of published results for the brine treatment by SED