Removal and recovery of Ni2 + from electroplating rinse water using electrodeionization reversal

Removal and recovery of Ni2 + from electroplating rinse water using electrodeionization reversal

Desalination 348 (2014) 74–81 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Removal and re...

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Desalination 348 (2014) 74–81

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Removal and recovery of Ni2 + from electroplating rinse water using electrodeionization reversal Huixia Lu a, Yuzhen Wang a,b, Jianyou Wang a,⁎ a b

College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China Faculty of Printing and Packaging Engineering, Xi'an University of Technology, Xi'an 710048, China

H I G H L I G H T S • • • •

EDIR has been proposed for removal and recovery of Ni2 + from dilute solution. The duration of polarity reversal should not be greater than 4 h. Stepwise stream switching mode was employed to reduce Ni2 + loss. Ni2 + removal efficiency was 97% and the concentrating factor was 79.2.

a r t i c l e

i n f o

Article history: Received 19 February 2014 Received in revised form 22 May 2014 Accepted 13 June 2014 Available online 2 July 2014 Keywords: Electrodeionization reversal Polarity reversal period Nickel ions Electroplating rinse water

a b s t r a c t The removal and recovery of Ni2+ from simulated electroplating rinse water by electrodeionization reversal (EDIR), i.e., electrodeionization using periodic changes in polarity, was studied. Based on the EDIR characteristic curves, the appropriate applied stack voltage was determined to be 30 V. The influence of the polarity reversal period on EDIR performance was examined. In addition, a stepwise stream switching mode was proposed to reduce Ni2+ loss. The experimental results showed that the duration of polarity reversal should not be greater than 4 h to prevent the formation of metal hydroxide precipitates. Additionally, the EDIR process with a polarity reversal period of 4 h and stepwise stream switching mode exhibited good separation performance, removing 97.0% of Ni2 + from the feed solution and simultaneously recovering these ions into a concentrate stream with a high concentration of 3961 mg·L−1. The current efficiency was 32.6%, and the corresponding energy consumption to treat 1 m3 of water was 1.02 KW·h. Thus, EDIR has considerable potential for the recovery and reuse of heavy metal ions from electroplating rinse water. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Electroplating rinse water contains a substantial amount of heavy metals, which are major causes of water and soil pollution and detrimental to the health of organic bodies when dispersed in the environment [1,2]. With increasing awareness concerning the need for environmental protection and the consequent severity of legislation regarding the disposal of electroplating waste in China [3], the development of the electroplating industry is facing a tough challenge. In addition, electroplating rinse water has considerable potential for reuse if the heavy metals can be effectively removed and collected. Therefore, the removal and recovery of heavy metals from electroplating rinse water has received great attention in recent years. The conventional techniques for treating dilute solutions containing heavy metal ions are chemical precipitation and ion exchange [4]. ⁎ Corresponding author. Tel.: +86 22 66229536. E-mail address: [email protected] (J. Wang).

http://dx.doi.org/10.1016/j.desal.2014.06.014 0011-9164/© 2014 Elsevier B.V. All rights reserved.

However, chemical precipitation requires the addition of chemicals and disposal of the sludge that contains large amounts of heavy metals, which is often problematic. Ion exchange resins must be regenerated with a concentrated electrolyte, and it will produce a concentrated waste stream that causes secondary pollution [5,6]. One alternative technique for the treatment of dilute heavy metal wastewater is electrodeionization (EDI), which is an electrically driven process involving the use of ion-selective membranes and ion exchange resins. It has been suggested that EDI combines the advantages of electrodialysis and ion exchange techniques, the separation efficiency of which is not subject to the limitations of concentration polarization. In recent years, EDI has been widely used in the production of ultrapure water, and the advantages of this technique have also attracted researchers to explore its feasibility for treating dilute heavy metal wastewater. For example, dilute nickel solutions were extensively treated by Spoor's group [7,8], Dzyazko and Belyakov [9], Dzyazko [10] and by Taghdirian et al. [11]. Solutions containing copper [12,13], and chromium [14,15] were also investigated. Furthermore, EDI has been

H. Lu et al. / Desalination 348 (2014) 74–81

demonstrated to be a promising technique for the removal and recovery of heavy metal ions from dilute industrial wastewater. However, the operational stability and separation performance will deteriorate when metal hydroxide precipitates form on the surfaces of resins and membranes during the EDI process. Therefore, current research should focus on preventing the formation of heavy metal hydroxide precipitates and on improving separation performance during the EDI process for treating dilute solutions containing heavy metal ions. In our previous work, some measures were taken to inhibit scale formation of heavy metal hydroxides, such as improving the cell configuration, establishing electrode protective compartments, and controlling the applied stack voltage and pH of the dilute and concentrate streams [16–18]. In addition to these measures, periodic polarity reversal has been widely proven to be an effective method for preventing or minimizing scale formation in electromembrane processes [19–23]. In particular, the electrodeionization reversal (EDIR) method, which utilizes periodic polarity changes during EDI operation, has been developed in recent years. For example, Lee et al. investigated the feasibility of using the EDIR system in the water softening process and found that both resistance and power consumption decreased during the EDIR operation [24,25]. However, a study on the EDIR process for treating wastewater containing heavy metal ions has rarely been reported in the literature. To investigate the prevention of scale formation by polarity reversal, the EDIR process was applied for the removal and recovery of Ni2+ from simulated electroplating rinse water in the present study. The Ni2+ removal characteristics were examined through lab-scale experiments, and the applied stack voltage was optimized. Moreover, the influence of the polarity reversal period on the performance of the EDIR process was thoroughly analyzed, and the optimum polarity reversal period was then determined accordingly. Finally, EDIR with the stepwise stream switching mode was employed to reduce Ni2 + loss, and the resulting performance was evaluated.

2. Experimental 2.1. Materials

2.2. Experimental setup The inner configuration of the applied EDIR stack with 4 cell pairs in two hydraulic stages is shown in Fig. 1. Two protective compartments were placed close to the corresponding cathode and anode compartments to decrease the reduction of Ni2+ on the cathode and to prevent the transport of produced OH− or H+ ions from the electrodes to the adjacent concentrate or dilute compartment. Mixed ion-exchange resins were packed in both the dilute and concentrate compartments. The volumetric ratio of resins was 6:4 (cation to anion). Both the anode and the cathode were prepared with titanium coated with ruthenium. The effective area of each membrane was 158.4 cm2 (22 × 7.2 cm), and the thickness of each dilute or concentrate compartment was 0.3 cm. 2.3. Operation of EDIR systems A flow diagram of the EDIR setup is presented in Fig. 2. This setup consists of an EDIR stack, three separate liquid lines, a power supplier, a measuring system and auxiliary components such as pumps, tanks, flow meters and pressure gauges. The dilute stream was passed once through the dilute compartments, while the concentrate stream was operated with partial recirculation. The electrolyte effluent was reclaimed to tank 3 and then fed into the stack after degassing. The dilute feed was divided into two steams: one as dilute influent and the other as make-up water of concentrate influent. The concentrate effluent was also separated into two parts: one as a circulating concentrate stream and the other as concentrated product water. To maintain a constant liquid volume in the concentrate tank, the flow rate of makeup water from the dilute stream should be equal to that of the concentrated product water. During operation, as the polarity of the electrodes was reversed periodically, the ion movement direction reversed and the dilute and concentrate streams were interchanged with each other, and thus, the open and closed positions of valves 8-1,8-2,8-3,8-4 and 8-5,86,8-7,8-8 were also interchanged accordingly. The initial solutions in tanks 1 and 2 were prepared by dissolving NiSO4 · 6H2O in ultrapure water to achieve a desired Ni2+ concentration of 50 mg⋅L−1, and then these solutions were further acidified with H2SO4 to attain a pH of 3.0. The 3000 mg⋅L− 1 Na2SO4 solution was used as electrolyte rinse water to reduce the resistances of the electrode compartments. The flow rates of the dilute, concentrate and electrolyte streams were maintained constant at 20 L⋅h− 1, 15 L⋅h− 1 and 10 L⋅h− 1, respectively. The concentrate make-up influent and

dilute efflurnt

dilute influent concentrate influent C A

C

concentrate effluent A

C

A

C

A

C

A C

-

+

Table 1 Characteristics of the ion exchange resins. Resin

D072

D296

Type Matrix Functional group Total exchange capacity (mmol⋅mL−1) Water content (%) Size (mm)

Strong-acid Styrene \SO− 3 ≥1.4 50–60 0.71–0.90

Strong-base Styrene \N+(CH3)3 ≥1.1 50–55 0.71–0.90

C-cation exchange membrane A-anion exchange membrane

-cation exchange resin -anion exchange resin

Fig. 1. Cell configuration of the EDIR.

electrolyte tank

Commercial heterogeneous cation- and anion-exchange membranes (Hangzhou, China) were used for all of the experimental setups. The cation- and anion-exchange membranes containing sulfonic acid and quaternary ammonium groups, respectively, were specifically manufactured with low water permeability and salt diffusion coefficients. The characteristic properties of these ion exchange membranes were reported in our previous work [26]. In this study, cationexchange resins and anion-exchange resins, which were macroporous and strongly acidic and basic types of resins and named D072 and D296 (Chemical Plant of Nankai University, China), respectively, were used as the ion-exchange fillers. Characteristics of the resins are listed in Table 1. Before each experiment, the membranes and resins were first immersed in a 0.5 mol·L−1 NiSO4 solution for 48 h and then transferred to a 1 mol·L− 1 NiSO4 solution for at least 72 h to thoroughly translate the functional groups into Ni2+ and SO2− forms. 4

75

H. Lu et al. / Desalination 348 (2014) 74–81 8-3 8-7

8-5 8-2

9

8-8 8-6 6

9

7 6

6

10

1 3 6

6

1

1

1

Stack current (A)

8-4

1.20

Stack current

1.05

Stack resistance

42 40 38

0.90 36 0.75 34 0.60 32 0.45

2

1

4

30

5

0.30 1-feed tank; 2-concentrate tank; 3-electrolyte tank; 4-concentrate effluent tank; 5-dilute effluent tank; 6-flow meter; 7-REDI stack; 8-1,8-2,8-3,8-4,8-5,8-6,8-7,8-8üvalve; 9-conductivity meter; 10-pH meter; 11-pump

10

The conductivity and pH of the dilute and concentrate streams were measured using on-line meters. Samples were collected at the outlets of the dilute and concentrate product streams at regular time intervals, and the Ni2+ concentrations were analyzed with the flame atomic absorption method using a spectrophotometer (TAS-996, Beijing Purkinje General Instrument Co. Ltd., China). The SEM (Scanning Electron Microscopy, Hitachi model S-3400 N, Japan) photograph was used to confirm surface condition of used resins. The stack current, the conductivity, the pH and Ni2+ concentration data as a function of time were subjected to a repeated measure analysis of variance (P b 0.05 as probability level for acceptance) using SPSS 17.0. 3. Results and discussion 3.1. Optimization of stack voltage The applied stack voltage was first increased to 10 V and then increased step-wise. However, the electrode polarity was not changed in this experiment to evaluate the status of precipitation. The current– voltage and resistance–voltage characteristic curves are shown in Fig. 3. It can be observed that the stack resistance first increased with the operation voltage before reaching a peak of 38 Ω, and then the stack resistance suddenly decreased after the applied voltage exceeded 22.5 V. Finally, the stack resistance decreased to attain a steady level in the range of 36–37 Ω, which is consistent with our previous experiments [27]. The results indicated that as the applied voltage increased, the stack resistance in the dilute compartment also increased due to concentration polarization. The stack voltage reached the highest value when ions at the membrane surface facing the dilute solution were fully depleted and the boundary layer resistance increased drastically. As no ions were available for transporting electrical current, water splitting occurred at the membrane–solution interface, which resulted in a further drastic decrease in the stack resistance. To ensure good separation performance, a certain degree of water splitting should occur in the dilute compartment, and considering energy consumption,

35

40

28

the voltage should be as low as possible. Therefore, the applied voltage was chosen to be 30 V for the EDIR operation. 3.2. The duration of polarity reversal Although a higher concentrating factor can be achieved by increasing the polarity reversal duration, the over-length interval will also cause the precipitation of metal hydroxides. Therefore, to determine the longest reversal interval for optimal performance, a planned experiment was performed without polarity reversal under a constant stack voltage of 30 V, and the variations in stack current and resistance versus operation time are shown in Fig. 4. At first 4 h operation, the stack current increased at a regular rate due to the increasing ion concentration in the concentrate compartments. And then the stack current gradually decreased from 0.81 A to 0.52 A while the stack resistance remarkably increased from 36.9 Ω to 57.6 Ω, which might result from both concentration polarization and the precipitation of metal hydroxides on the surfaces of the resins and membranes in the dilute compartments. To verify our speculation, the stack was disassembled after the experiment to observe the status of Ni(OH)2 precipitation. As expected, some green precipitates appeared on the surface of the cation-exchange membrane facing the dilute solution, which indicated that water splitting occurred at the interfaces between the cation-exchange membrane and the solution in the dilute compartment. The hydrogen ions could transfer from the dilute compartment to the concentrate compartment under the

Stack current (A)

2.4. Analysis

20 25 30 Voltage (V)

Fig. 3. I–V and R–V characteristics curves of the EDIR process.

Fig. 2. Schematic flow diagram of the EDIR system.

concentrate product streams both had a flow rate of 0.24 L⋅h−1. The effective volume of the concentrate tank was maintained at 1 L. All experiments were performed in potentiostatic mode by using an adjustable power supply, and the duration of the process was 10 h.

15

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

65 Stack current

Stack resistance

60 55 50 45 40 35 1

2

3

4

5 6 7 Time (h)

8

30 9 10 11

Fig. 4. Variation of stack current and resistance versus operation time.

Stack resistance (Ω)

8-1

Stack resistance (Ω)

76

H. Lu et al. / Desalination 348 (2014) 74–81

1h

a

3h

4h

4000 3000 2000 1000 0 0

3500

100

b

200

1h

3000

300 400 Time (min)

3h

500

600

500

600

4h

2500 2000 1500 1000 500 0 0

6.5 pH of dilute stream

In this experiment, the simulated nickel-electroplating rinse water was treated using the EDIR process with polarity reversal periods ranging from 1 to 4 h. The cathode and anode were exchanged with each other at regular intervals, and the influent and effluent of the dilute and concentrate streams were switched synchronously. The influences of different reversal periods on the stack current were investigated, and the results are shown in Fig. 5. As shown in Fig. 5, the current instantly increased at the moment the electrode polarities were reversed, and then the current declined to a relatively steady value over the course of a few minutes. This result was mainly due to the previous concentrate stream entering the new dilute stream as the polarities reversed, thus causing the instantaneous ion concentration in the dilute compartments to be ten times greater than that of the period prior to reversal. Furthermore, a high concentration of ions would transfer to the new concentrate compartments under the effect of the electric field, which thus led to the increase in the stack current. Subsequently, the stack current sharply decreased and then dropped to the normal level as the ion concentration in the dilute compartments rapidly decreased. Furthermore, it is also shown that the longer the reversal period, the greater the instantaneous current would increase at the moment after polarity reversal because higher ion concentrations would occur in the concentrate stream with a longer reversal period. Therefore, longer polarity reversal periods are favorable for Ni2+ recovery without Ni(OH)2 precipitation. Water quality of the dilute effluent streams as a function of operation time is plotted in Fig. 6., Similar to current, the conductivity and Ni2+ concentration in the dilute stream instantly increased at the moment after polarity reversal, and then it decreased to a relatively steady value over the course of a few minutes (Fig. 6a and b). This results were because the previous concentrate stream entering the new dilute stream as the polarities reversed, and some Ni2 + ions were brought out from the dilute compartments by the dilute effluent before being transferred into the concentrate compartments, which thus led to the instantaneous increase in conductivity and Ni2+ concentration in the dilute effluent stream. Furthermore, the lowest Ni2+ concentration of approximately 1.5 mg⋅L−1 was achieved with the reversal period of 4 h, whereas that of 1 h was 2.9 mg⋅L− 1. The pH of dilute effluent increased from 3.00 to 5.42 in the first 60 min for the three different

Conductivity of dilute stream (μS.cm -1)

3.3. Influences of the polarity reversal period on the process performances

5000

Ni2+ concentration dilute strream (mg.L-1)

effect of the electrical field, whereas the hydroxide ions would still remain in the dilute compartment, resulting in further Ni(OH)2 precipitation. Based on the experimental results, it was suggested that the duration of polarity reversal should not exceed 4 h.

77

6.0

100

c

200 300 400 Time (min)

1h

3h

4h

5.5 5.0 4.5 4.0 3.5 3.0 2.5

Stack current (A)

3.0

2.0 1h 3h

2.5

1.5

0 60 120 180 240 300 360 420 480 540 600

Time (min)

4h

2.0

Fig. 6. (a) Variation of conductivity of the dilute effluent versus operation time. (b) Variation of Ni2+ concentration in the dilute effluent versus operation time. (c) Variation of pH of the dilute effluent versus operation time.

1.5 1.0 0.5 0.0 0

100

200 300 400 Time (min)

500

Fig. 5. Variation of stack current versus operation time.

600

polarity reversal periods (Fig. 6c). This is because the H+ ions that existed in the initial dilute feed solutions would migrate from dilute to concentrate compartments under applied voltage, and thus the pH of dilute effluent increased before polarity reversal. However, pH of the dilute effluent decreased at the moment after polarity reversal. This behavior could be due to the H+ ions in the previous concentrate stream entering the new dilute stream resulting in a fast pH decrease. In addition, after each polarity reversal, there showed a somewhat lower pH

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H. Lu et al. / Desalination 348 (2014) 74–81

value of dilute effluent. Indeed, with more frequent polarity changing, the lower the pH value was. It started at a value of 3.00 and ended at values 3.22, 4.57 and 4.68 for reversal periods of 1 h, 3 h and 4 h, respectively. The evolution of pH and Ni2+ concentration as a function of time in the concentrate effluent for the three different polarity reversal periods are plotted in Figs. 7. Unlike the pH of dilute effluent, the pH of concentrate effluent increased at the moment after polarity reversal due to the previous dilute stream entering the new concentrate stream as the polarities reversed, and the pH of concentrate effluent decreased in time with the migration and accumulation of H+ ions (Fig. 7a). Meanwhile, it can also be observed that the Ni2+ concentration in the concentrate effluent decreased at the moment after polarity reversal due to the dilution by the previous dilute solution (Fig. 7b). Ni2+ concentration in the concentrate effluent increased as the polarity reversal period increased from 1 h to 4 h. The final Ni2+ concentrations in the concentrate effluent were 2896 ± 26.91, 3313 ± 23.06 and 3487 ± 36.52 mg⋅L−1 for polarity reversal periods of 1, 3 and 4 h, respectively. After the above experiments were completed, the EDIR stacks were disassembled again for checking the precipitation status, and no colored precipitates were observed on either the resin surface or the membrane surface. Additionally, to verify whether the Ni(OH)2 precipitate forms at interstices between the mixed resins, SEM (Scanning Electron

pH of concentrate stream

3.5

a 1h

Fig. 8. SEM images for used resins: (a) 35× magnification and (b) 15,000× magnification.

3500

Microscopy) was applied. As shown in SEM of Fig. 8(a) and (b), no precipitate appeared at interstices between the mixed resins and surfaces of used resins, which further validated the safety of 4 h reversal. It is clear that the best separation performance was achieved with the longest polarity reversal period of 4 h. The ion exchange membranes and resins were thoroughly translated into Ni2+ and SO2− forms before they were used. Additionally, no sig4 nificant water splitting occurred at the membrane–solution interfaces in this experiment. Therefore, mass balance of Ni2+ for the system by hourly basis was given in Fig. 9. Due to partial circulation of the concentrate stream, Ni2+ concentration in concentrate product stream would reach a steady value at steady state, and it can be calculated by the mass balance of Ni2+ as follow:

3000

Nin ¼ Nout :

2.0 1.5

-1

100

200 300 400 Time (min)

500

600

4000

b

1h

3h

4h

ð1Þ

2500

Qd’ , cdin

2000

Qd, cdin

1500 1000 500 0 0

concentrate compartment

dilute compartment

2+

4h

2.5

0

Ni concentration in concentrate stream (mg.L )

3h

3.0

Qc’ , ccout

Qd, cdout

100

200 300 400 Time (min)

500

600

Fig. 7. (a) Variation of pH of the concentrate effluent versus operation time. (b) Variation of Ni2+ concentration in the concentrate effluent versus operation time.

concentrate circulation tank

Fig. 9. Mass balance of Ni2+ ions for the EDIR process.

H. Lu et al. / Desalination 348 (2014) 74–81

0

Nout ¼ Q d  cdout þ Q c  ccout

ð3Þ

0

where Q c is the flow rate of the concentrate product stream in L⋅h−1; cdout and ccout are the Ni2 + concentrations in the dilute effluent and concentrate product stream in mg⋅L−1, respectively.

ccout ¼

  0 Q d þ Q d  cdin −Q d  cdout Q

ð4Þ

0

c

For polarity reversal periods of 4 h, the average Ni2+ concentration in the dilute effluent was 1.5 mg⋅L−1. According to given Ni2+ concentrations and flow rates, the Ni2+ concentration in concentrate product stream at steady state could be calculated using Eq. (4):

ccout ¼

ð20 þ 0:24Þ  50−20  1:5 ‐1 ¼ 4091:7 mg  L : 0:24

ð5Þ

The concentrating factor, CF, can be calculated using Eq. (6): cc o u t : cd i n

ð6Þ

-1

It is easy to determine that the theoretical Ni2+ concentration in the concentrate product effluent could reach 4091.7 mg⋅L−1, and the corresponding theoretical concentrating factor could reach 81.8 under stable operation. However, the concentrating factors for different polarity reversal periods after 10 h of operation were all lower than the theoretical value of 81.8 (as shown in Fig. 10). The final concentrating factors for

The stepwise stream switching mode was proposed to overcome the shortcoming of Ni2+ loss in the simultaneous switching mode, in which the dilute and concentrate streams were switched step-by-step. The new operation mode was carried out as follow: at the moment when the electrode polarities were reversed, valves 8-5 and 8-6 were opened while simultaneously closing valves 8-1 and 8-2 to switch the influent streams; then, valve 8-7 was opened while simultaneously closing valve 8-3. However, valves 8-4 and 8-8 were still kept open and closed, respectively, to make the dilute effluent still discharge into the concentrate tank after the polarities were reversed. After several minutes of operation lag, valves 8-4 and 8-8 were simultaneously closed and opened, respectively, to finish the stream switching; namely, the concentrate stream was switched with a lag for several minutes. In that case, the dilute stream containing a high concentration of Ni2+ would discharge into the concentrate tank within several minutes after the electrode polarity was changed. Another experiment with the stepwise stream switching mode, in which the concentrate stream was switched with a lag of 3 min, was operated under the same conditions as the simultaneous stream switching mode for comparison of separation performance. The results are shown in Fig. 11. After 10 h of operation, the Ni2+ concentration in the concentrate effluent reached a maximum of 3961 mg⋅L−1, which is approximately 14% higher than that value achieved with the EDIR process

90

Concentration factor

80 70

18.9%

14.8%

29.2%

60 50 40 30 20

0

2+

10 1h

3h

4 h theoretical Polarity reversal period

Fig. 10. Concentrating factors for different polarity reversal periods.

Ni

CF ¼

3.4. Influence of the stream switching mode on EDIR

4000 3500

50

3000 2500

stepwise simultaneous

40

2000

8

1500

6

1000

4

500

2 0 0

0 2

4

6

Time ( h )

8

10

concentration in concentrate stream (mg.L-1)

0

Where Qd and Q d are the flow rates of the dilute influent and concentrate make-up influent in L⋅h−1; cdin is the Ni2 + concentration in the dilute influent in mg⋅L−1.

2+

ð2Þ

Ni

0

Nin ¼ Q d  cdin þ Q d  cdin :

polarity reversal periods of 1, 3 and 4 h were 57.9 ± 0.78, 66.3 ± 0.86 and 69.7 ± 0.81, respectively, and these values were 29.2%, 18.9% and 14.8% lower than the theoretical values, respectively. The main reason for the “Ni2 + loss” was associated with the practical operation mode of the EDIR process, in which the stream switching was performed simultaneously with the polarity reversal. Because the Ni2+ concentration in the new dilute compartments, which were previously concentrate compartments, still remained at a high level for a period of time after polarity reversal, an amount of time would be required for the dilute concentration to reach a normal value. During this period of abnormal dilute concentration, many Ni2+ ions would be discharged into the dilute effluent tank as the streams and polarities were synchronously changed. To prevent the problem of Ni2 + loss, the stream switching mode needs to be improved.

concentration in dilute stream (mg.L )

Where Nin is the amount of Ni2+ introduced to the system in unit time in mg; Nout is the amount of Ni2+ discharged from the system in unit time in mg.

79

Fig. 11. Variation of Ni2+concentration in the dilute and concentrate effluents versus operation time.

80

H. Lu et al. / Desalination 348 (2014) 74–81

Table 2 Performances of the EDIR process with a polarity reversal period of 4 h and stepwise stream switching mode.

ðUIt Þ

E S ¼ ðC

.

0 −C t ÞL

.10

3

ð10Þ

6

Items

Units

Values

Final Ni2+ concentration of dilute effluent Final Ni2+ concentration of concentrate effluent Removal efficiency Concentrating factor Current efficiency Energy consumption per ton water Specific energy consumption for Ni2+ removal

mg·L−1 mg·L−1 %

1.5 3961 97 79.2 32.6 1.02 21.03

% KW·h·m−3 KW·h·kg−1

operated in the simultaneous stream switching mode. The corresponding concentrating factor was 79.2, which was also very close to the theoretical value of 81.8. In addition, the stepwise stream switching mode had little negative effect on the dilute effluent concentration, which was still maintained at approximately 1.5 mg⋅L−1. These results suggest that the application of the stream switching lag mode could be a promising technique for reducing Ni2+ loss during the EDIR process. 3.5. Performance of the EDIR process The EDIR process performance can be evaluated in terms of the concentrating factor (CF), removal efficiency (RE, %), current efficiency (CE, %) and the energy consumption (EC, KW·h·m− 3) or the specific energy consumption for removal of per unit nickel (ES, KW·h·kg−1), defined, respectively, by Eqs. (7)–(10) [28,29].

RE ¼

cdin− cdout  100% cdin

ð7Þ

where cdin and cdout are the Ni2+ concentrations in the dilute influent and effluent in mg⋅L−1, respectively.

CE ¼

Q d  z  ðcdin −cdout Þ  F  100% NI

ð8Þ

where cdin and cdout are the Ni2+ concentrations in the dilute influent and effluent in mol⋅L−1, respectively; Qd is the flow rate of the dilute influent in L⋅s−1; z is the valency of the ion; F is the Faraday constant (96,500 A·s·mol− 1); N is the number of cell pairs; and I is the stack current (A).

EC ¼

UIt L

ð9Þ

where U is the applied voltage (V); I is the stack current (A); t is the time used (h) and L is the volume of dilute feed water (m3).

10

where C 0 is the initial Ni 2 + concentration of dilute feed water (mg⋅L−1); Ct is the final Ni2+ concentration of dilute effluent stream (mg⋅L−1) and L is the volume of dilute feed water (L). The CF, RE, CE,EC and ES were calculated for an EDIR process employing a polarity reversal period of 4 h and stepwise stream switching mode, and these values are listed in Table 2. The Ni2+ concentration reached a maximum of 3961 mg⋅L− 1, and the corresponding concentrating factor was 79.2. The removal efficiency of Ni2 + was 97%, and the final Ni2 + concentration in the dilute effluent could reach 1.5 mg · L− 1. The current efficiency was 32.6%, which appears to be in good agreement with the prior observations of Feng et al. [30] and Semmens et al. [31]. Feng et al. worked on the removal of heavy metal ions from electroplating effluent with an improved EDI stack configuration. They reported current efficiency of 33–38%, removal efficiency of at least 99.8%. However, in their works, the concentrating factor was only 22–36, far lower than ours. Semmens et al. investigated the ability of continuous electrodeionization (CEDI) to be used in a copper electroplating line to recover copper sulfate and purified water from rinse water. With a similar initial Cu2 + concentration of 50 mg⋅L− 1 and pH of 3.0, current efficiencies of 20% to 30% were observed for Cu2 +. Additionally, the energy consumption for treating 1 m3 feed water using the EDIR process was 1.02 KW·h and the corresponding specific energy consumption was 21.03 KW·h·kg−1. To evaluate the performance of the EDIR treatment process described above, a comparative study is also presented in terms of initial metal concentration (mg·L− 1), metal removal efficiency and energy consumption. The results of comparison are shown in Table 3. Although it has a relative meaning due to the different testing conditions, this comparison is useful to further understand the overall performance of EDIR treatment process. It is evident from the table that specific energy consumption varies considerably, depending on the initial metal concentration of the treated wastewater, the process employed, the amount and composition of impurities, as well as the extent of purification. EDIR process has achieved 97% of Ni2+ removal efficiency and 21.03 kW·h·kg−1 of specific energy consumption. The results are comparable to that of electrocoagulation [32], NF/RO and flotation process [33]. However, the specific energy consumption is lower than that of Dzyazko and Belyakov [9], which also employed EDI for Ni2+ removal from a dilute nickel solution with a similar initial concentration of 58.7 mg⋅L− 1. Obviously, electrochemical precipitation [34] and membrane electrolysis [35] are comparatively uneconomical for removal heavy metal ions from electroplating rinse waters. In general, the EDIR results showed the feasibility of the EDIR system for use in electroplating rinse water treating process with high concentrating factor, high removal efficiency and low energy consumption.

Table 3 Comparison of the performance among treatment technologies for the removal of heavy metals. Type of treatment

Metal species

Initial metal concentration (mg·L−1)

Specific energy consumption (kW·h·kg−1)

Removal efficiency (%)

EDIR Electrocoagulation

Ni2+ Cu2+ Cr3+ Ni2+ Cu2+ Cu2+ Ni2+ Ni2+ Ni2+

50 45 44.5 394 50 63.5 58.7 40000 2000

21.03 20.83

97 100 100 100 95–99 28–41 NA 80–85 90%

NF/RO and flotation EDI EDI Electrochemical precipitation Membrane electrolysis NA: not available.

21.05–22.22 24.25–30.39 35.44 100.88 2333

References

[32]

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H. Lu et al. / Desalination 348 (2014) 74–81

4. Conclusions The improved EDIR process was successfully used for the removal and recovery of Ni2+ from simulated electroplating rinse water. The influence of the operating parameters, such as the applied stack voltage, polarity reversal period and the stream switching mode, on the separation performance was thoroughly investigated. For stable and reliable operation, the proper stack voltage should be determined experimentally according to the characteristic curves. Additionally, the results showed that the duration of polarity reversal should not exceed 4 h to prevent the formation of Ni(OH)2 precipitates. A high removal efficiency of 97% and a concentrating factor of 79.2 could be synchronously achieved in only one unit of the EDIR process. The stepwise stream switching mode, in which the concentrate stream is switched with a short delay of 3 min over the dilute stream, is effective for reducing Ni2+ loss. It was clearly shown in this study that the improved EDIR system can be successfully used in the treatment of electroplating rinse water with good stability and without metal hydroxide precipitate formation. Acknowledgment The authors gratefully acknowledge the support for this work by Tianjin Natural Science Foundation (12JCQNJC05100), Tianjin S&T Research Project for Ocean Industry (KJXH2011-07) and National Marine Research Special Funds for Public Welfare Projects of China (201405008-06). References [1] A. Mahmood, R.N. Malik, Human health risk assessment of heavy metals via consumption of contaminated vegetables collected from different irrigation sources in Lahore, Pakistan, Arab. J. Chem. 7 (1) (2014) 91–99. [2] P. Chaudhuri, B. Nath, G. Birch, Accumulation of trace metals in grey mangrove Avicennia marina fine nutritive roots: the role of rhizosphere processes, Mar. Pollut. Bull. 79 (1-2) (2014) 284–292. [3] MEPC, AQSIQ, Emission standard of pollutants for electroplating, China Environmental Science Press, Beijing, 2008. [4] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manag. 92 (3) (2011) 407–418. [5] T.A. Kurniawan, G. Chan, W.H. Lo, S. Babel, Physico-chemical treatment techniques for wastewater laden with heavy metals, Chem. Eng. J. 118 (1–2) (2006) 83–98. [6] N.H. Shaidan, U. Eldemerdash, S. Awad, Removal of Ni(II) ions from aqueous solutions using fixed-bed ion exchange column technique, J. Taiwan Inst. Chem. Eng. 43 (1) (2012) 40–45. [7] P.B. Spoor, L. Koene, W.R. Ter Veen, L.J.J. Janssen, Continuous deionization of a dilute nickel solution, Chem. Eng. J. 85 (2–3) (2002) 127–135. [8] P.B. Spoor, L. Grabovska, L. Koene, L.J.J. Janssen, W.R. Ter Veen, Pilot scale deionisation of a galvanic nickel solution using a hybrid ion-exchange/electrodialysis system, Chem. Eng. J. 89 (1–3) (2002) 193–202. [9] Yu.S. Dzyazko, V.N. Belyakov, Purification of a diluted nickel solution containing nickel combining ion exchange and electrodialysis, Desalination 162 (2004) 179–189. [10] Yu.S. Dzyazko, Purification of a diluted solution containing nickel using electrodeionization, Desalination 198 (2006) 47–55. [11] H.R. Taghdirian, A. Moheb, M. Mehdipourghazi, Selective separation of Ni(II)/Co(II) ions from dilute aqueous solutions using continuous electrodeionization in the presence of EDTA, J. Membr. Sci. 362 (1–2) (2010) 68–75.

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