Acid precipitation coupled electrodialysis to improve separation of chloride and organics in pulping crystallization mother liquor☆

Zhaoyang Li,Rongzong Li,Zhaoxiang Zhong,Ming Zhou*,Min Chen,Weihong Xing*

State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 211816,China

Keywords:Crystallization mother liquor Acid precipitation Electrodialysis Chloride recovery

ABSTRACT Inefficient separation of inorganic salts and organic matters in crystallization mother liquor is still a problem to industrial wastewater treatment since the high salinity significantly impedes organic pollutant degradation by oxidation or incineration.In the study,acidification combined electrodialysis(ED)was attempted to effectively separate Cl-ions from organics in concentrate pulping wastewater.Membrane's rejection rate to total organic carbon(TOC)was 85%at wastewater intrinsic pH=9.8 and enhanced to 93%by acidifying it to pH=2 in ED process.Negative-charged alkaline organic compounds(mainly lignin)could be liberated from their sodium salt forms and coagulated in acidification pretreatment.Neutralization of the organic substances also made their electro-migration less effective under electric driving force and in particular improved separation efficiency of chloride and organics.After acid-ED coupled treatment(pH=2 and J=40 mA·cm-2)[TOC]remarkably reduced from 1.315 g·L-1 to 0.048 g·L-1 and[Cl-]accumulated to 130 g·L-1 in concentrate solution.Recovery rate of NaCl was 89%and the power consumption was 0.38 kW·h·kg-1 NaCl.Irreversible fouling was not caused as electric resistance of membrane pile maintained stably.In conclusion,acidic-ED is a practical option to treat salinity organic wastewater when current techniques including thermal evaporation and pressure-driven membrane separation present limitations.

1.Introduction

Zero discharge of industrial wastewater could be practiced by utilizing biological,physical and chemical treatments to realize clean water recycling and mineral resources recovery[1-3].In the treatment of pulping wastewater using membrane separation technology coupled with evaporative crystallization,the organic matters in the wastewater accumulate in the evaporator causing problems for evaporation efficiency and quality of salt.It has to regularly discharge a certain amount of the crystallization mother liquid to ensure the stability of evaporation process and salt purity.The discharged crystallization mother liquor contains ultra-high concentration of both inorganic salt and organic compounds,which is extremely difficult to handle for separation and degradation.

In the pulping and paper-making field,wastewater biodegradability is low and biological digestion approaches are not desirable in the case[4,5].The benzyl structure of lignin molecules could be toxic to many microorganisms[6].Meanwhile,the high salinity in water could deactivate growth and reproduction of the microorganisms[7].Hence,advanced oxidation processes(AOPs)as producing reactive hydroxyl radicals have been often applied in the case[8].It has fast reaction rate and strong oxidation capacity leading to wide applications to degrade organic pollutants in wastewater of high salinity[9,10].However,a single utilization of AOPs may cause high cost and possible toxic intermediates in the chain reactions.When there is high concentration of chloride in water the chloride would react with hydroxyl radicals and reduce degrading efficiency[11,12].In addition,in AOPs chloride radicals could form toxic chlorinated by-products as secondary pollution[13,14].

Incineration as a common approach to burn municipal and hazardous solid waste has been increasingly employed to burn the concentrated organic wastewater[15,16].There are still problems in wastewater sintering stage when it is of high salinity.The alkali metal elements(e.g.Na,K,Ca and Mg)could react with silica and form lowmelting-point eutectic alloy resulted in coking and slagging on the furnace wall[17,18].It causes instability in fluidization engineering and could severely corrode bed incinerator[19,20].On the other hand,it is worthwhile to recover the mineral salts from the concentrated wastewater when the co-existent organic components could be separated prior to evaporation crystallization[21].

Electrodialysis(ED)process is an effective approach to separate charged ions with ion-exchange membranes under electrical driving force.For instance,ED has been applied to treat industrial wastewater including discharged water from power plant,electroplating,printing and dyeing and so forth[22-25].The ED process is also employed to recover chloride from salt manufacturer discharged water[26,27].By treating Kraft pulping stream,the monovalent selective ion-exchange membrane(IEM)was used to separate and recover chloride from divalent sulfate anions and organic pollutants[28].The ED process using homogeneous ion-exchange membranes was carried out to separate salts and organics in reverse-osmosis concentrate solution of coal processing[29].More than 99%of the inorganic salts(mainly sodium sulfate and sodium chloride)could be retrieved with a retention rate of organic compounds ca.85%-95%.It presents a promising solution to recover chloride from the wastewater in the paper making industry.Lignin and hemicellulose macromolecules in water could remarkably influence the separation efficiency and selectivity in electrodialysis,but adequate studies have not been made yet on separation of mineral salts and organics in high-salinity pulping wastewater[30,31].

Acid precipitation is a traditional method to retrieve the lignin from the Kraft black liquor.Sulfuric acid,as a strong acid,was usually selected in precipitation treatment for its operational feasibility and inexpensive cost[32-34].Acidification treatment in pulping streams could classically be practiced either with carbonization(using carbon dioxide)and sulfuric acid together or with sulfuric acid alone[34,35].It has been found that the yield of lignin was 93%-94% as the black liquors are being acidized to pH=4 using 83 wt% sulfuric acid solution[36].In one study on treatment of black liquor,90% of lignin and 90% of the total sodium were separated and recovered with a combination method of acid precipitation and electrodialysis[34].Accordingly,it is reasonable to pre-acidify the saline mother liquor in order to improve the recovery efficiency of mineral salts in the electrodialysis process.

In the work,separation and recovery of chloride from organics in crystallization mother liquor of pulping wastewater were studied.An integration approach of acidification-electrodialysis was proposed to first deposit the organic pollutants and then recover chloride from the saline wastewater in electro-membrane process.Acidification pH and temperature,membrane selectivity,current efficiency and ED stability have been investigated to enhance separation efficiency of chloride with the minimal power energy consumption.

2.Experimental

2.1.Membrane and materials

AMX anion exchange membrane and CMX cation exchange membrane were the homogeneous ion-exchange membranes purchased from Astom Corporation,Japan whose properties are given in Table 1.Ion exchange capacity(IEC)is measured in 0.5 mol·L-1NaCl solution at 25°C.

The dark pulping wastewater was discharged from a paper manufacturer and supplied by Nantong Nengda Water Company in China.Characterization on the wastewater is presented in Table 2.The wastewater was filtered with a membrane of average pore size 0.22 μm to remove solid particles before each experiment.

Sulfuric acid and sodium sulfate with analytical purification were purchased from Sinopharm Corporation,China and sodium hydroxide from Xilong Scientific Company,China.Deionized water was used in all the experiments.

2.2.Experimental procedure

2.2.1.Acid precipitation

The wasted crystallization mother liquor was first acidified prior to the electrodialysis stage.In each experiment acidification treatment was carried out to 200 ml wastewater in a beaker whose temperature was maintained in oil-bath.The beaker was covered with a film to prevent dehydration.Solution of 5 mol·L-1sulfuric acid was added to the mother liquor by titration as the mixture liquid was being stirred.Acidification pH adjusted to 0,1,2,3,4,5,6,7 and 8 was studied at constant temperature 30°C.And acidification temperature was then studied at 30°C,40°C,50°C,60°C,70°C and 80°C with pH maintained as 2.The acidified mother liquor was aged for 24 h at 30°C and then went through vacuum filtration to remove the precipitates.Colority and total organic carbon(TOC)of the pretreated liquor were examined.Repetition experiments were performed three times for each operational condition.

2.2.2.Electrodialysis

A schematic description of the electrodialysis experiment is presented in Fig.1.The EX-3BT membrane stack(Lanran Company,China)was operated using direct current under the voltage from 0 V to 30 V.The membrane pile consisted of 11 pieces of cation exchange membrane and 10 pieces of anion exchange membrane being inserted alternately.Active membrane surface area was 55 cm2and the distance between two membranes was 0.5 mm.At the beginning,700 ml acidified mother liquor was contained in dilute compartment and 700 ml deionized water in concentrate compartment.One liter of 30 g·L-1Na2SO4solution was used as electrolyte solution.

Dilute solution,concentrate solution and electrolyte solution were circulating in each compartment independently in the batch experiment.For the first 30 min it was diffusion stage and no voltage was supplied.Concentration-driven permeation of ions from mother liquor side to deionized water side increased the conductivity of the system.Meanwhile,elimination of gas bubbles in solution during diffusion could also improve conductivity[37].After 30 min voltage was supplied to the membrane stack at current density 40 mA·cm-2.Water samples were taken and analyzed every 30 min.Temperature of solution in each compartment was first warmed up due to the input electricity power.As thermal exchange reaches equilibrium the solution temperature was kept at（30±1)°C with circulative cooling water.

In the test on process stability,the acidified mother liquor(pH=2)was treated under the same operational conditions in the electrodialysis process.Each batch experiment lasted for 270 min.The membrane stack was not washed in the continuous tests with 5 repetition experiments in order to investigate membrane fouling phenomenon.

Table 1 Properties of the ion exchange membranes

Table 2 Characterization of the pulping wastewater

2.3.Analytical methods

The pH of solutions was measured with a pH meter(PHS-25,Inesa,China).The raw mother liquor was diluted with pH-adjusted solution and Zeta potentials of the acidified mother liquor were measured by a Zetasizer(Nano-ZS90,Malvern,UK).Concentration of Cl-andanions in water was determined with an ion chromatograph(ICS-2000,Dionex,USA).Total organic carbon(TOC)was analyzed with a multi N/C 3100 instrument(Jena,Germany).Colority of water sample was measured with a UV/vis spectrophotometer(DR3900,HACH,USA).Scanning electron microscopy(HitachiS-4800,Japan)was applied to characterize the microscopic structure of the ion-exchange membrane before and after electrodialysis experiments.

2.4.Data analysis

The TOC removal rate(Racid)of the mother liquor in the acid precipitation process was calculated by Eq.(1)as follows:

where[TOC]0and[TOC]1are the initial concentration and instant concentration of the acidified mother liquor,respectively.

The decolorization rate(D)of the mother liquor in the acid precipitation process was calculated by Eq.(2)as follows:

where Color0and Color1are the initial chroma and instant colority of the acidified mother liquor,respectively.

In the electrodialysis process the selectivity(S)of ion exchange membrane to Cl-ions overions was calculated by Eq.(3)as follows:

where ΔMCl-is mol of Cl-ions transferred through the membrane at interval time,andis mol ofions transferred through the membrane at interval time.

In the electrodialysis process using the acidified mother liquor,the TOC rejection(RED)by the ion exchange membrane was calculated by the following Eq.(4):

where[TOC]0and[TOC]trepresent the TOC concentration in the dilute side before and after electrodialysis,respectively.V0and Vtrepresent the volume of the solution in the dilute side before and after electrodialysis,respectively.

Energy consumption(E)and average current efficiency(η)in the electrodialysis process were calculated using Eqs.(5)and(6),respectively:

where E is the energy consumption(kWh·kg-1NaCl)and was calculated as extrapolating the results for 1 kg NaCl in the concentrate side,t is the running time(h),U is the operating voltage(V),I is the operating current(A),and Ctand Vtare the concentration and volume of chloride solution in the concentrate compartment at constant time.

where η is the average current efficiency(%),N is the number of pairs of cation-and anion-exchange membranes in the stack(N=10 in the work),Q is the electricity consumption Q=It(C),and F is the Faraday constant,96485 C·mol-1.

3.Results and Discussion

3.1.Acidification removal of organic substances

Organic pollutants in the pulping wastewater were mainly derived from lignin-structured and hydrocarbon compounds.The organic matters largely exist in the form of lignin sodium salt(R-ONa)with the high content of inorganic salt(NaCl)in water[28].Acidification pretreatment was aimed to deposit and separate organic compounds from the mother liquor before electrodialysis treatment.Liberation of phenolic groups in their sodium forms(Ar-OH,pKa of 9-11)took place at pH ca.8 and liberation of aliphatic acids(R-COOH,pKa 3-5)could be made at pH 2-3[34].The general reactions are given as examples in Eqs.(7)and(8).

3.1.1.Effect of acidification pH

Negatively-charged alkaline organic compounds could be liberated from their sodium salt forms and coagulated in acidification pretreatment.Coagulation and precipitation of organic matters induced by protonation in acidification treatment are illustrated in Fig.2a.Reduction on chromaticity and TOC of wastewater was effectively improved as pH decreases from 8 to 2 at constant temperature 30°C(Fig.2b).In particular,a remarkable enhancement on organic pollutant removal was found at adjusting pH from 5 to 2.At lower pH about 2-3 small-mass organic molecules could be further released from salt forms and coagulated.With even stronger acidification to pH less than 2 precipitation removal of organic substances changed slightly.

The Zeta potential of the mother liquor was measured as 1.95 mV,0.60 mV,-1.02 mV,-4.44 mV,-7.95 mV and-20.70 mV at pH=0,2,4,6,8,and 10 respectively.It is found that the raw mother liquor was of negative Zeta potential at intrinsic pH=9.8 due to the contained alkaline organic substances.The untreated wastewater was stable with a relatively large ionic strength(i.e.an absolute value of Zeta potential 20 mV).It became less stable because of organic compound neutralization in acidified mother liquor of pH 8-4 when the absolute Zeta value became smaller.Aggregation of compounds occurred at low pH 4-0 as positive Zeta potential was measured due to protonation.The reduction rates of TOC and chromaticity increased to 27%and 86%respectively at pH=2.Acid precipitation treatment was carried out at pH=2 in the following experiments.

3.1.2.Effect of acidification temperature

Temperature is another key factor thermodynamically affecting agglomeration and deposition of organic matters in acidified mother liquor.Working temperature in acidification pretreatment was studied from 30°C to 80°C at constant effluent pH=2.Decolorization rate decreased slightly from 84%to 81%and TOC removal rate declined from 24%to 6%when heating the liquor from 30°C to 80°C(Fig.3).More frequent thermal motion and opening of unsaturation bonds could lead to higher solubility at higher temperature.The acidification temperature was accordingly determined at 30°C in the following experiments for high precipitation efficiency and low power consumption.

Fig.2.(a)Illustration on acid precipitation process and(b)pH effect on Zeta potential,decolorization and TOC removal in acidification treatment.

Fig.3.Temperature effect on decolorization and TOC removal in acidification treatment.

3.2.Electrodialysis separation of chloride

3.2.1.Membrane stack voltage

The electrodialysis process was operated with constant current density and variable membrane stack voltage proportional to the electrical resistance.There was an evident voltage drop on the membrane pile in the first 80 min as seen in Fig.4.The voltage declined sharply by 40%being attributed to a large resistance made by large concentrate difference at the initial time.The voltage stabilized after the first 80 min and rebounded slightly after 130 min with the continuous transportation of ions from the feed compartment to the concentrate compartment.Concentration difference of ions decreased and then became larger oppositely in the two compartments.Hence,the stack resistance(or the voltage)has risen gradually when the ions were being concentrated in the counter compartment to the feed side.

Fig.4.The membrane stack voltage in electrodialysis.

In the first 80 min effluent temperature was discovered with an elevation by 20-30°C because of the electric power input.Raised temperature improved ion diffusivity and reduced the electric resistance(i.e.membrane stack voltage).Then the temperature was maintaining at 30°C as thermal exchange equilibrium was reached later on.Acidification treatment weakly affected the voltage drop since the effluent viscosity did not change much.It should be noted that irreversible membrane fouling was not induced for all the experiments which would be discussed in Section 3.2.4.

3.2.2.ED separation selectivity and efficiency

The objective of the work is to recover chloride from sulfate and organics in the discharged pulping wastewater.Separation selectivity of Cl-andions was first studied in the ED process.Concentrations of[Cl-]and[]in the feed cell(i.e.desalting cell)and the concentrate cell were measured as seen in Fig.5.Change of both ions'concentration in the two compartments was in compensation.A slight increase of[]in the desalting cell was caused by the addition of H2SO4solution in acidification treatment to pH=2.

Fig.5.(a)Concentration of Cl-ions,(b)concentration of ions and(c)membrane selectivity of Cl-/in electrodialysis process.

The first 30 min was the initial stabilization stage when there was no voltage supplied and only salt diffusion took place.The permeation rates of Cl-andions were very slow at the diffusion stage.When the voltage was supplied Cl-ions started to electro-migrate fast through the anion exchange membrane resulting in a notable decrease in desalting cell and increase in concentrate cell.Electricity-driven transfer of the divalent sulfate ions was less efficient than that of the monovalent chloride ions.Chloride solution was eventually collected from the concentrate cell of[Cl-]=130 g·L-1and[]=0.8 g·L-1at 270 min.As a result,the recovery rate of chloride was up to 89%using the acidelectrodialysis method.In the ED process the chloride solution was inevitably diluted by water flux mainly caused by osmotic pressure and electro-osmosis(i.e.water molecules entrained in solvation shell of ions)[38].

In the electrodialysis process of current density 40 mA·cm-2and pH=2 the permselectivity of Cl-/ions as a function of time is given in Fig.5(c).It should be kept in mind that there was no electric field supplied as driving force in the first 30 min.An extraordinary high selectivity on Cl-/at the diffusion stage was caused by the high initial[Cl-]about 6-fold of[]in the feed effluent.Permselectivity of Cl-/declined to ca.800 when the voltage was started to be applied after the first 30 min.Electro-migration of Cl-andions was effectively accelerated under the electric filed and permeation efficiency was less distinct.Along with operational duration membrane selectivity decreased gradually since the concentrate gradient difference became smaller.Ultrahigh[Cl-]in the mother liquor was the important characteristic to recover chloride in the work.

Separation of Cl-ions and organics was also studied in the acidified electrodialysis method.Acidification pretreatment not only partially removed some organic substances by precipitation but also neutralized the charges.It affected particle transfer through membrane by different driving forces.Fig.6 shows the permeation rate by diffusion of organic matters in the first 30 min before voltage was applied in ED process.TOC diffusive permeation from desalting cell to concentrate cell was better eliminated when acidification pH was lower.The permeation rate was the lowest(1.85%)for acidified effluent to pH=2.It improved separation efficiency between Cl-ions and organics.

Fig.6.Concentration diffusion of organics of electrodialysis process without voltage in diffusion stage of the first 30 min.

Two ideal streams were attempted after electrodialysis as“dilute solution”free of chloride and“concentrate solution”rich in chloride but free of organic components.The photos of mother liquor,dilute solution(in desalting cell)and concentrate solution(in concentrate cell)were presented in Fig.7.And the concentration of TOC in each solution was analyzed and compared after 270 min in the ED process with various pH conditions.[TOC]in acidified liquor could be reduced by 10%-30%by acidification pretreatment as discussed previously.In general,[TOC]in the dilute solution was 1.5-fold of that in the feedstock after 270 min ED operation mainly due to dehydration regardless of pH change.For instance,[TOC]in the concentrate solution decreased to as low as 48 mg·L-1and rejection rate to TOC was 93%at pH=2.Neutralization of the alkaline organic compounds at low pH was the main reason for improvement on membrane rejection to TOC since the electric field as the driving force became less effective to the neutralized particles.

Fig.7.TOC concentration in the mother liquors,dilute solution and concentrate solution in electrodialysis with different pH conditions and TOC rejection by the IEM.

In the end of the electrodialysis process the concentration of inorganic salts increased and the concentration of organics decreased in the concentrate cell as schematically presented in Fig.8.Most of the organics were trapped in the desalting cell by the ion exchange membrane and chloride ions were accelerated to enter the concentrate cell.The obtained solution from the concentrate cell is so called chloride solution ready for crystallization.

3.2.3.Average current efficiency and energy consumption

Fig.8.Schematic diagram on composition change in desalting cell and concentrate cell during electrodialysis process.

Average current efficiency is calculated as the number of electroimmigrated ions over the number of current supplied electrons.Power consumption is calculated as the used electric energy to recover unit weight of NaCl.For acidified mother liquor(pH=2)the average current efficiency was 71%and energy consumption was 0.38 kW·h·kg-1NaCl in the ED process with J=40 mA·cm-2at 30°C.In the literature of seawater desalination and salt concentration from brine by electrodialysis the energy consumption is generally between 0.1 and 0.2 kW·h·kg-1NaCl[39].The energy consumption of the work(0.38 kW·h·kg-1NaCl)was slightly larger mainly because the treated crystallization mother liquor contained a large amount of organic matter which increased electrical resistance.Different acidification pH insignificantly influenced the power efficiency in the electrodialysis process(Fig.9).It indicated that partial removal of organic substances did not affect electro-migration of inorganic ions.The initial high concentration of organic compounds has determined the viscosity and resistance of the mother liquor.The large concentration difference between the desalting cell and concentrate cell resulted in great osmotic pressure.Convection flow of chloride ions could also reduce the current efficiency in the system.

Fig.9.Comparison on current efficiency and power consumption in electrodialysis at different pH at constant current density 40 mA·cm-2 within 270 min.

3.2.4.Stability of electrodialysis process

It can be concluded that a pre-acidification treatment on the crystallization mother liquor of pulping wastewater has a weak effect on chloride removal,current efficiency or energy consumption in electrodialysis.However,it could significantly improve the ion exchange membrane's rejection to organic components and eventually has reduced[TOC]from 1315 mg·L-1to 48 mg·L-1in the chloride solution.

Stability of membrane and process is investigated to evaluate application to treat the crystallization mother liquor in industry.Repetition batch experiments were made over 5 times and the results on current efficiency and membrane stack resistance are given in Fig.10.Average current efficiency maintained stably ca.70%in all the repeated tests.The electric resistance of membrane stack was present repeatedly throughout continuous operation without any cleaning for 1350 min.As seen in Fig.10(b)the initial resistance of each recycling experiment was relatively large because the concentrate compartment was at first filled with deionized water of low conductivity.No cleaning was operated between each repetition experiment leading to accumulation of salt in the concentrate compartment and the increased conductivity of water.This is the reason why the initial resistance was the highest in the first test in comparison to the continuous four repetition tests.Irreversible fouling was not caused by membrane surface according to the constant current efficiency.SEM surface images of the ion-exchange membranes(AMX and CMX)are compared before and after electrodialysis experiments as displayed in Fig.11.It is consistent to find that morphology and structure of membrane surface maintained and no contaminant layer was formed.As a conclusion,the acidelectrodialysis method was proven with good stability for chloride recovery from saline pulping wastewater.

4.Conclusions

A feasible method was proposed to recover chloride in pulping wastewater by combining acid removal of organic substances and further electrodialysis separation of chloride from organic matters and sulfate ions.It is found that pre-acidification could not only remove some organics by coagulation but also improve the ion exchange membranes'rejection to organic matters in the ED process due to neutralization.TOC concentration was minimized to 48 mg·L-1with a removal rate up to 93%after acidic-ED treatment(pH=2)working with current density 40 mA·cm-2at 30 °C.Recovery rate of chloride was 89% from the pulping wastewater and purity of crystallized NaCl was upgraded.Yet acidification has an insignificant effect on current efficiency and power consumption in the ED process.The antifouling property of the homogeneous ion exchange membranes has been proven in the coupled treatment method with repetition tests lasting for 1350 min.Therefore,the acid precipitation-electrodialysis combination is a practical approach to treat industrial mother liquor so that recover of mineral resources and further degradation treatment could be possible.

Acknowledgments

The Jiangsu National Synergistic Innovation Center for Advanced Materials(SICAM)is gratefully acknowledged.

Fig.10.In the electrodialysis process,the change of average current efficiency and membrane stack resistance for the repeated experiments of 5 batches(the current density of 40 mA·cm-2,running time of 270 min and pH=2).