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

Accepted Manuscript 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 PII: DOI: Reference:

S1004-9541(19)30798-0 https://doi.org/10.1016/j.cjche.2019.07.002 CJCHE 1529

To appear in:

Chinese Journal of Chemical Engineering

Received date: Revised date: Accepted date:

27 May 2019 25 June 2019 1 July 2019

Please cite this article as: Z. Li, R. Li, Z. Zhong, et al., Acid precipitation coupled Electrodialysis to improve separation of chloride and organics in pulping crystallization mother liquor, Chinese Journal of Chemical Engineering, https://doi.org/10.1016/ j.cjche.2019.07.002

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ACCEPTED MANUSCRIPT 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

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Engineering Research Center for Special Separation Membrane, Nanjing Tech University, Nanjing 211816, China *

Corresponding author: E-mail: [email protected] (Ming Zhou);

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E-mail: [email protected] (Weihong Xing)

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Abstract: Inefficient separation of inorganic salts and organic matters in crystallization mother liquor is still a problem to industrial wastewater treatment since

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the high salinity significantly impedes organic pollutants 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.

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Membrane’s rejection rate to total organic carbon (TOC) was 85% at wastewater

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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

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driving force and in particular improved separation efficiency of chloride and organics. After acid-ED coupled treatment (pH = 2 and J = 40 mA/cm2) [TOC] remarkably reduced from 1.315 g/L to 0.048 g/L and [Cl-] accumulated to 130 g/L in concentrate solution. Recovery rate of NaCl was 89% and the power consumption was 0.38 kWh/kg·NaCl. Irreversible fouling was not caused as electric resistance of membrane pile maintained stably. In a conclusion, acidic-ED is a practical option treat salinity organic wastewater when current techniques including thermal evaporation and pressure-driven membrane separation present limitations.

ACCEPTED MANUSCRIPT Keywords: Crystallization mother liquor; Acid precipitation; Electrodialysis; Chloride recovery

1. Introduction Zero discharge of industrial wastewater could be practiced by utilizing biological,

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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

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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

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purity. The discharged crystallization mother liquor contains ultra-high concentration

for separation and degradation.

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of both inorganic salt and organic compounds, which is extremely difficult to handle

In the pulping and paper-making field, wastewater biodegradability is low and

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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,

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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

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pollutants in wastewater of 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

ACCEPTED MANUSCRIPT low-melting-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].

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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]. ED process is also

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employed to recover chloride from salt manufacturer discharged water [26, 27]. By treating Kraft pulping stream, the monovalent selective ion-exchange membrane (IEM)

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was used to separate and recover chloride from divalent sulfate anions and organic pollutants [28]. ED process using homogeneous ion-exchange membranes was carried

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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

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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

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paper making industry. Lignin and hemicelluloses 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].

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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 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 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 was separated and recovered with a combination method of acid precipitation and electrodialysis [34].

ACCEPTED MANUSCRIPT Accordingly, it is reasonable to pre-acidify the saline mother liquor in order to improve the recovery efficiency of mineral salts in electrodialysis process. In the work, separation and recovery of chloride from organics in crystallization mother liquor of pulping wastewater was studied. An integration approach of acidification-electrodialysis was proposed to first deposit the organic pollutants and

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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.

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2. Experimental

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2.1 Membrane and materials

AMX anion exchange membrane and CMX cation exchange membrane were the

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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 NaCl solution at 25 ºC.

AMX CMX

IEC (meq/g)

Specific area resistance (Ω·cm2)

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Membrane

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Table 1 Properties of the ion exchange membranes

1.4~1.7 1.5~1.8

Thickness (mm)

Stability (pH)

Temperature (◦C)

0.14 0.17

0~8 0~10

≤40 ≤40

2.4 3.0

The dark pulping wastewater was discharged from a paper manufacturer and supplied by Nantong Nengda Water Company in China. Characterization on the

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wastewater is presented in Table 2. The wastewater was filtered with membrane of average pore size 0.22 μm to remove solid particles before each experiment. Table 2 Characterization of the pulping wastewater Sample

pH

TOC (mg/L)

Conductivity (mS/cm)

Chloride (g/L)

Sulphate (g/L)

Wastewater

9.8

1315

665

210

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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

ACCEPTED MANUSCRIPT 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

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beaker was covered with a film to prevent dehydration. Solution of 5 mol/L sulfuric acid was added to the mother liquor by titration as the mixture liquid being stirred. Acidification pH adjusted to 0, 1, 2, 3, 4, 5, 6, 7 and 8 was studied at constant temperature 30 ºC. And acidification temperature was then studied at 30 ºC, 40 ºC, 50 C, 60 ºC, 70 ºC and 80 ºC with pH maintained as 2. The acidified mother liquor was

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aged for 24 hours at 30 ºC and then went through vacuum filtration to remove the

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precipitates. Colority and total organic carbon (TOC) of the pretreated liquor were examined. Repetition experiments were made for three times for each operational

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condition. 2.2.2 Electrodialysis

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Schematic description of electrodialysis experiment is presented in Fig. 1. The EX-3BT membrane stack (Lanran Company, China) was operated using direct current

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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 alternatively. Active membrane surface area was 55 cm2 and the distance between two membranes was 0.5 mm. At the beginning, 700 ml acidified mother liquor was

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contained in dilute compartment and 700 ml deionized water in concentrate compartment. One liter of 30 g/L Na2SO4 solution was used as electrolyte solution.

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Fig. 1 Schematic diagram of the electrodialysis system and the membrane stack configuration

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Dilute solution, concentrate solution and electrolyte solution were circulating in each compartment independently in the batch experiment. For the first 30 minutes it

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was diffusion stage and no voltage was supplied. Concentration-driven permeation of ions from mother liquor side to deionized water side increased conductivity of system.

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Meanwhile, elimination of gas bubbles in solution during diffusion could also improve conductivity [37]. After 30 minutes voltage was supplied to the membrane

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stack at current density 40 mA/cm2. Water samples were taken and analyzed every 30 minutes. Temperature of solution in each compartment was first warmed up due to the

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input electricity power. As reaching the thermal exchange equilibrium the solution temperature was kept at 30 ± 1 ºC with circulative cooling water. In the test on process stability, the acidified mother liquor (pH = 2) was treated under the same operational conditions in electrodialysis process. Each batch

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experiment lasted for 270 minutes. The membrane stack was not washed in the continuous tests over 5 times repetition experiments in order to investigate membrane fouling phenomenon. 2.3 Analytical methods The pH of solutions was measured with 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- and SO42- anions in water was determined with ion chromatograph (ICS-2000, Dionex, USA). Total organic carbon (TOC) was analyzed

ACCEPTED MANUSCRIPT with multi N/C 3100 instrument (Jena, Germany). Colority of water sample was measured with UV/vis spectrophotometer (DR3900, HACH, USA). Scanning electron microscopy (HitachiS-4800, Japan) was applied to characterize the microscopic structure of ion-exchange membrane before and after electrodialysis experiments. 2.4 Data analysis

calculated by Eq. (1) as follow: [𝑇𝑂𝐶]0 − [𝑇𝑂𝐶]1 𝑅𝑎𝑐𝑖𝑑 = × 100% [𝑇𝑂𝐶]0

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The TOC removal rate (Racid) of mother liquor in acid precipitation process was

(1)

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

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The decolorization rate (D) of mother liquor in acid precipitation process was

(2)

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calculated by Eq. (2) as follow: 𝐶𝑜𝑙𝑜𝑟0 − 𝐶𝑜𝑙𝑜𝑟1 𝐷= × 100% 𝐶𝑜𝑙𝑜𝑟0

where Color0 and Color1 is the initial chroma and instant colority of the acidified mother liquor, respectively.

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In electrodialysis process the selectivity (S) of ion exchange membrane to Cl-

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ions over SO42- ions was calculated by Eq. (3) as follow: 𝑆=

∆𝑀𝐶𝑙 − ∆𝑀𝑆𝑂42−

(3)

where ∆𝑀𝐶𝑙 − is mol of Cl- ions transferred through membrane at interval time,

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∆𝑀𝑆𝑂42− is mol of SO42- ions transferred through membrane at interval time. In electrodialysis process using the acidified mother liquor, the TOC rejection (RED) by the ion exchange membrane was calculated by the following Eq. (4): 𝑅𝐸𝐷 =

[𝑇𝑂𝐶]0 × 𝑉0 − [𝑇𝑂𝐶]𝑡 × 𝑉𝑡 × 100% [𝑇𝑂𝐶]0 × 𝑉0

(4)

where [TOC]0 and [TOC]t represent the TOC concentration in the dilute side before and after electrodialysis, respectively. V0 and Vt represent 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

ACCEPTED MANUSCRIPT process was calculated using the Eq. (5) and Eq. (6), respectively: 𝑡

∫ 𝑈𝐼𝑑𝑡 𝐸= 0 1000𝐶𝑡 𝑉𝑡

(5)

where E is the energy consumption (kWh/kg·NaCl) and was calculated as extrapolating the results for 1 kg NaCl in the concentrate side, t the running time (h),

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U the operating voltage (V), I the operating current (A), Ct and Vt is the concentration and volume of chloride solution in the concentrate compartment at constant time. 𝜂=

𝐶𝑡 𝑉𝑡 𝐹 × 100% 𝑁𝑄

(6)

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where η is the average current efficiency (%), N the number of pairs of cation- and anion-exchange membranes in the stack (N = 10 in the work), Q the electricity

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consumption Q = It (C), F is the Faraday constant 96485 C/mol.

3. Results and discussion

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3.1 Acidification removal of organic substances

Organic pollutants in the pulping wastewater were mainly derived from

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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)

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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].

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The general reactions are given as examples in Equation (7) and (8). R − COO− Na+ + +H2 SO4 → R − COOH + Na2 SO4

(7)

Ar − O− Na+ + H2 SO4 → Ar − OH + Na2 SO4

(8)

3.1.1 Effect of acidification pH Negative-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 is illustrated in Fig. 2a. Reduction on chromaticity and TOC of wastewater was

ACCEPTED MANUSCRIPT effectively improved as decreasing pH from 8 to 2 at constant temperature 30 ºC (Fig. 2b). In particular, a remarkable enhancement on organic pollutants 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

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slightly. The Zeta potential of 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

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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

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became less stable because of organic compounds neutralization in acidified mother liquor of pH 8 ~ 4 when the absolute Zeta value became smaller. Aggregation of

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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%

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respectively at pH = 2. Acid precipitation treatment was determined to carry out at pH

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= 2 in the following experiments.

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

3.1.2 Effect of acidification temperature

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Temperature is another key factor thermodynamically affecting agglomeration and deposition of organic matters in acidified mother liquor. Working temperature in

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acidification pretreatment was studied from 30 ºC to 80 ºC at constant effluent pH = 2. Decolorization rate decreased slightly from 84% to 81% and TOC removal rate declined from 24% to 6 % as heating the liquor from 30 ºC to 80 ºC (Fig. 3). More

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frequent thermal motion and opening of unsaturation bonds could lead to higher solubility at higher temperature. The acidification temperature was accordingly

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determined at 30 ºC in the following experiments for high precipitation efficiency and

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low power consumption.

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

3.2 Electrodialysis separation of chloride 3.2.1 Membrane stack voltage Electrodialysis process was operated with constant current density and variable

ACCEPTED MANUSCRIPT 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 since 80 min and rebounded slightly after 130 min with the continuous transportation of ions from the

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feed compartment to the concentrate compartment. Concentration difference of ions minored and then became larger oppositely in the two compartments. Hence, the stack resistance (or the voltage) has risen gradually when the ions being concentrated in counter compartment to the feed side.

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In the first 80 min effluent temperature was discovered with an elevation by 20 30 ºC because of the electric power input. Raised temperature improved ion diffusivity

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and reduced the electric resistance (i.e. membrane stack voltage). Then the temperature was maintaining at 30 ºC as thermal exchange equilibrium was reached

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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

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did not induced for all the experiments which would be discussed in the section 3.2.4.

Fig. 4 The membrane stack voltage in electrodialysis

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- and SO42- ions was first studied in ED process. Concentrations of [Cl-] and [SO42-] in the feed cell (i.e. desalting cell) and the concentrate cell were measured as seen in Fig. 5. Change of the

ACCEPTED MANUSCRIPT both ions concentration in the two compartments was in compensation. A slight increase of [SO42-] in the desalting cell was caused by addition of H2SO4 solution in acidification treatment to pH = 2. 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- and SO42- ions

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were very slow at the diffusion stage. When the voltage supplied Cl- ions started to electro-migrate fast through the anion exchange membrane resulting a notable decrease in desalting cell and increase in concentrate cell. Electrical-driven transfer of the divalent sulfate ions was less efficient than that of the monovalent chloride ions.

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Chloride solution was eventually collected from the concentrate cell of [Cl-] = 130 g/L and [SO42-] = 0.8 g/L at 270 min. As a result, the recovery rate of chloride was up to

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89% using the acid-electrodialysis method. In ED process the chloride solution was inevitably diluted by water flux mainly caused by osmotic pressure and

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electro-osmosis (i.e. water molecules entrained in solvation shell of ions) [38].

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Fig. 5 (a) Concentration Cl- ions, (b) concentration of SO42- ions and (c) membrane selectivity of Cl-/SO42- in electrodialysis process

In electrodialysis process of current density 40 mA/cm2 and pH = 2 the

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permselectivity of Cl-/SO42- ions as a function of time is given in Fig. 5 (c). It should be reminded that there was no electric field supplied as driving force in the first 30

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min. An extraordinary high selectivity on Cl-/ SO42- at the diffusion stage was caused by the high initial [Cl-] about 6 folded of [SO42-] in the feed effluent. Permselectivity of Cl-/SO42- declined to ca. 800 when the voltage started to apply since 30 min.

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Electro-migration of Cl- and SO42- ions were both effectively accelerated under the

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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.

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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 particles transfer through membrane by different driving force. In the first 30 min before starting voltage in ED process permeation rate of organic matters by diffusion is presented in Fig. 6. 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.

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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

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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

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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 ED process with various

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pH conditions. [TOC] in acidified liquor could be reduced by 10% - 30% by acidification pretreatment as discussed previously. In general, [TOC] in the dilute

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solution was 1.5 folded of that in the feedstock after 270 min ED operation mainly due to dehydration regardless to pH change. For instance, [TOC] in the concentrate solution eliminated as low as to 48 mg/L and rejection rate to TOC was 93% in at pH = 2. Neutralization of the alkaline organic compounds at low pH was the main reason

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for improvement on membrane rejection to TOC since the electric field as the driving force became less effective to the neutralized particles.

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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

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In the end of electrodialysis process the concentration of inorganic salts increased and the concentration of organics decreased in the concentrate cell as

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schematically presented in Fig. 8. Most of the organics were trapped in desalting cell by the ion exchange membrane and chloride ions were accelerated to enter the

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concentrate cell. The obtained solution from concentrate cell is so called chloride

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solution ready for crystallization.

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

3.2.3 Average current efficiency and energy consumption Average current efficiency is calculated as the number of electro-immigrated ions over the number of current supplied electrons. Power consumption is calculated

ACCEPTED MANUSCRIPT 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 kWh/kg NaCl in ED process with J = 40 mA/cm2 at 30 ºC. In the literature of seawater desalination and salt concentration from brine by electrodialysis the energy consumption is generally between 0.1 ~ 0.2 kWh/kg·NaCl[39]. The energy

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consumption of the work (0.38 kWh/kg·NaCl) 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 electrodialysis process (Fig. 9). It indicated that partial

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removal of organic substances did not affect electro-migration of inorganic ions. Initial high concentration of organic compounds has determined the viscosity and

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resistance of the mother liquor. The large concentration difference between desalting cell and concentrate cell resulted in great osmotic pressure. Convection flow of

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ED

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chloride ions could also reduce the current efficiency in the system.

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Fig. 9 Comparison on current efficiency and power consumption in electrodialysis at different pH at constant current density 40 mA/cm2 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 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 to 48 mg/L in the chloride solution. Stability of membrane and process is investigated to evaluate application to treat

ACCEPTED MANUSCRIPT 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 performing repeatedly throughout continuous operation without any cleaning for 1350 min. As seen in Fig.

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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. And there was no cleaning operation between each repetition experiment so that salt solution would present in the concentrate side for the continuous tests

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resulted in 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

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repetition tests. Irreversible fouling was not caused on membrane surface according to the constant current efficiency. SEM surface images of the ion-exchange membranes

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(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

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membrane surface maintained and no contaminant layer was formed. As a conclusion, the acid-electrodialysis method was proven with good stability for chloride recovery

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from saline pulping wastewater.

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/cm2, running time of 270 min and pH = 2).

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Fig. 11 SEM images of ion-exchange membranes before and after electrodialysis experiments

4. Conclusion

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A feasible method was proposed to recover chloride in pulping wastewater by combining acid removal of organic substances and further electrodialysis separation

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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

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exchange membranes’ rejection to organic matters in ED process due to neutralization. TOC concentration was minimized to 48 mg/L with a removal rate up to 93% after

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acidic-ED treatment (pH = 2) working with current density 40 mA/cm2 at 30 ºC. Recovery rate of chloride was 89% from the pulping wastewater and purity of crystallized NaCl was upgraded. Yet acidification has insignificant effect on current efficiency and power consumption in the ED process. Antifouling property of the

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homogeneous ion exchange membranes have 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.

Acknowledgements Financial support from the Prospective Joint Research Project of Jiangsu Province (BY2014005-06), the Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM) is gratefully acknowledged.

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