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Recovery of nickel from aqueous solutions by complexation-ultrafiltration process with sodium polyacrylate and polyethylenimine
Journal of Hazardous Materials 244–245 (2013) 472–477
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Recovery of nickel from aqueous solutions by complexation-ultrafiltration process with sodium polyacrylate and polyethylenimine Jiahui Shao a,∗ , Shu Qin a , Joshua Davidson b , Wenxi Li a , Yiliang He a , H. Susan Zhou b a b
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, United States
h i g h l i g h t s
Nickel removal rate of 99.5% could be reached using complexation-ultrafiltration process. The mechanism of adding salt on metal removal could be the compressing electric double layer. Diafiltration technology was effectively used to regenerate the polymer. Langmuir-type binding isotherm equation was effectively used to evaluate nickel bound to polymer.
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 28 September 2012 Accepted 1 October 2012 Available online 5 November 2012 Keywords: Nickel Complexation agent Complexation-ultrafiltration Diafiltration Langmuir-type binding isotherm
a b s t r a c t The recovery of nickel from aqueous dilute solutions by complexation-ultrafiltration process with sodium polyacrylate (PAAS) and polyethylenimine (PEI) was studied. Experiments were performed as a function of aqueous pH, polymer/Ni2+ ratio and background electrolyte concentration. At optimum experimental conditions, the nickel removal rate reaches 99.5% using PAAS and 93.0% using PEI as the complexation agent. The nickel removal rate was found to decrease as the adding salt NaCl concentration increases for both complexation agents. A series of experiments implied that the mechanism could be the compressing electric double layer other than the competitive complexation. Diafiltration technique was further performed to regenerate complexation agents and recover nickel. The nickel removal rates were found to be close to those obtained with the original PEI and PAAS. Finally, Langmuir-type binding isotherm equation was employed to evaluate the extent of nickel bound to PAAS and PEI. The overall results from the two-step process of complexation-UF and decomplexation-UF separation showed that it could be a promising method for nickel removal and recovery from aqueous solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Water is not only essential for life; it has become the primary workhorse of industries around the world as a working fluid, transport medium, heat transfer fluid, cleaning agent, etc. Unfortunately, this has often led to the degradation of water quality as harmful effluents are returned to the environment with various contaminants from these processes. One of the most startling groups of water contaminants are those of heavy metals due to their accumulation in biological systems and their toxicity even at relatively low concentrations [1–3]. Sources of heavy metal water contamination are varied and can be seen in every step of production from mining, purification and processing, metal finishing and electroplating, and even end use [4].
∗ Corresponding author. Tel.: +86 21 54745634; fax: +86 21 54740825. E-mail address: [email protected] (J. Shao). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.10.070
Industry currently treats heavy metal wastewater, such as electroplating wastewater, a major contributor to water contamination by a wide variety of heavy metal ions [5], via a lime-soda precipitation technique that, although effective, essentially shifts the problem to large volumes of sludge containing heavy metals [5,6]. Not only does this method not solve the problem of heavy metal pollution, electroplating industry also must deal with the loss of the useable metal which is becoming increasingly expensive due to a decrease in the quality of metal ores [7,8]. What is needed is an economical method, not only for the removal of heavy metals from waste water, but for the recovery of these metals. One separation technique that can meet the above requirement is complexationultrafiltration process, also called polymer enhanced ultrafiltration (PEUF) [4–12]. Complexation-ultrafiltration process is based on the principle that the polymers with larger molecular weights than molecular weight cut-off (MWCO) of UF membranes can bind heavy metals to form macromolecular complexes and be retained by UF
J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477
membranes. After this UF process, the almost metal-free permeate stream can be discharged directly or reused for specific purposes. The metal-rich rejected stream can then be decomplexed into metal ions and regenerated polymer by lowering pH. A second step of UF is then followed to regenerate polymer to be used again in the metal removal process. The metal ions can further be recovered by electrolysis or other suitable techniques [13,7]. High degrees of small solute removal are attained using a diafiltration (DF) mode in which the small solute is effectively washed through the membrane and away from the product by the continuous (or discontinuous) addition of new buffer [14]. The most common approach is to perform the diafiltration using a constant retentate volume, in which case the small solute concentration in the product solution can be evaluated from a simple mass balance as: Ci = exp (−ND Si ) Cio
(1)
where ND is the number of diavolumes, which is equal to the total collected filtrate volume divided by the constant retentate volume during the diafiltration process. Si is the small solute sieving coefficient (=1 − removal rate), defined as the ratio of the solute concentration in the filtrate solution to that in the retentate. This diafiltration technique has been successfully used to extract the maximum amount of metal ions and regenerate the polymer as pure as possible [9]. The past fifteen years have seen not a few papers on the use of PEUF to remove heavy metals from wastewaters [8,15–19]. Most of the publications investigated the influence of pH, concentration of metal ions and background electrolytes on efficiency of metal removal with various complexing agents [11,12]. The mechanism of background electrolytes on efficiency of metal removal was not well studied. The binding capacity or the mass of heavy metal removed per mass of polymer is also normally not included in the heavy metal removal studies. Further, few studies have investigated the technology used to decomplex metal and polymer to regenerate the polymer and recover metal [10,15]. And also the detailed diafiltration technique and theoretical calculation for the metal recovery were not provided. The objective of this study was to investigate the removal of nickel from aqueous solution by complexation-ultrafiltration with PAAS and PEI. Experiments were performed as a function of aqueous pH, polymer/Ni2+ ratio, and background electrolyte concentration with PES membranes. Mechanism of the effect of background electrolyte concentration on nickel removal was proposed. Diafiltration technique was utilized to regenerate PAAS and PEI and recover the nickel. Langmuir-type binding isotherm was tried to evaluate the extent of nickel bound to polymer. 2. Experimental 2.1. Materials The membranes used were polyethersulfone with molecular weight cut-off (MWCO) of 30 kDa from Shanghai Nuclear Research Institute of Chinese Academy of Sciences (SNRICAS), China. Sodium polyacrylate (PAAS) with molecular weight of 3 × 107 Da and polyethylenimine (PEI) with molecular weight of 5 × 105 Da were used as the complexation agents. The filtration of two polymers with 30 kDa membranes indicates that not any of these two polymers passes through the membrane and gets into the permeate solution. Therefore, polymers purchased were used as-received. Nickel sulfate (NiSO4 ·6H2 O) was used to make the solution of Ni2+ . Distilled water was used to prepare the solutions. The pH value was adjusted using NaOH or HCl as needed. The background electrolyte of the solution was adjusted by adding appropriate NaCl. Prior to
473
ultrafiltration experiments, the solution containing Ni2+ and one polymer at certain pH was gently agitated in the stirrer (Hengfeng Instrument, Jintan, China) at speed of 200 rpm for 2 h to ensure equilibrium binding. All the chemicals used in this study were from Sinopharm Chemical Reagent Corp. (SCRC), China, otherwise being noted. 2.2. Experimental methods 2.2.1. Ultrafiltration process UF experiments were performed in a dead-end 70 mm diameter stirred cell with effective volume of 350 mL (SNRICAS, China), connected to a nitrogen-pressurized solution reservoir. Each membrane was flushed with approximately 100 L/m2 of DI water prior to remove any agents. The stirred cell and reservoir were then filled with the well mixed polymer–nickel solution at 100 kPa. The water flux was measured by timed collection, with filtrate samples collected periodically for subsequent analysis. The first 10 cm3 of the permeate was discarded. The permeate flux and solute rejection were determined by analyzing 1 cm3 permeate solution. The average nickel concentration of three permeate samples was used to calculate the removal rate for nickel. The schematic diagram of ultrafiltration experiment is shown in Fig. 1. Retention (R) is calculated as follows to show the removal of the nickel:
R=
1−
Cp Cf
× 100%
(2)
where Cp and Cf are the concentrations (mg/L) of nickel in the permeate and feed solution, respectively. Loading ratio (L) is calculated as follows to indicate the concentration ratio of polymer (Cpolymer ) to metal ion. L=
Cpolymer Cf
(3)
2.2.2. Diafiltration experiment The pH of the concentrated solution with polymer–nickel complex was first acidified to 2 by adding H2 SO4 and then was transferred to the dead-end stirred cell (same as that used for UF experiments) for diafiltration experiments to recover polymer and nickel. The stirred cell was connected to a solution reservoir containing H2 SO4 buffer (without any polymer or nickel). The applied pressure is 100 kPa. Filtrate samples were collected as a function of time for subsequent determination of the nickel concentrations. 2.2.3. Equilibrium binding isotherm The binding interactions between polymer and nickel were evaluated using the same stirred cell as ultrafiltration experiments which allowed polymer-free samples to be collected through the UF membrane. A polymer–nickel mixture was added into the stirred cell with a 30 kDa MWCO polyethersulfone membrane. The magnetic stirrer of the ultrafiltration unit was set at 200 rpm. The device was then nitrogen pressed to P = 100 kPa, and small samples were periodically withdrawn through the membrane for off-line analysis of the unbounded Ni2+ concentration. Data were also obtained with pure Ni2+ to evaluate the sieving coefficient of free Ni2+ through the same type of membrane used in the experiment. 2.2.4. Concentration analysis Nickel concentration was analyzed by UV/Vis spectrophotometer (Shanghai MAPADA instruments Co. Ltd.) with dimethylglyoxime as chromatic agent. The detailed procedure of this method is described in Chinese National Standard Methods for Water and Waste water Monitoring and Analysis method GB11910-89 [20].
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Fig. 1. Schematic diagram of ultrafiltration experiment.
Concentrations of Na+ were determined by ion chromatography (Metrohm Ltd.)
100 L=0 L=2
3.1. Effect of pH on nickel removal Fig. 2 shows the effect of pH on the nickel removal at different PAAS/Ni2+ ratios. The nickel removal rates were all smaller than 4% at pH values less than 8.0 in the absence of PAAS, indicating that the comparatively large pores in the UF membrane probably reject or absorb very small amount of nickel. At pH 9.0, the nickel removal rate increased sharply to 81% due to hydroxide precipitation. The nickel removal rate was observed to increase as the value of pH increases for all the PAAS/Ni2+ ratios and reaches a plateau after pH 6.0 for loading ratio of 5 and 10. At high PAAS/Ni2+ ratio, there are more available complexing sites to bind nickel due to the high PAAS concentration. While at low PAAS/Ni2+ ratio, the available complexation sites are not enough to bind all nickel. The large amounts of free metal ions exist and pass through the membrane, causing the nickel removal not efficient. Fig. 3 shows the effect of pH on the nickel removal at different PEI/Ni2+ ratios. Similarly, the nickel removal rates were all smaller than 4.0% at pH values less than 8.0 in the absence of PEI. Compared with the ultrafiltration without PEI complexation, the nickel removal rate increases dramatically as the value of pH increases for all the PEI/Ni2+ ratios and reaches an almost plateau after pH 6.0 for loading ratio of 5 and 10.
Removal Rate (%)
80
3. Results and discussion
L=3 L=4
60
L=5 L=10
40
20
0
1
0
2
3
4
5
6
7
8
9
10
pH Fig. 3. Effect of pH on nickel removal at different PEI/Ni2+ ratios.
3.2. Effect of loading ratio on nickel removal The pH value is one of the most important parameters determining the binding capacity of Ni2+ with both PAAS and PEI. The nickel removal experiments at different pH values and different PAAS/Ni2+ ratios show that pH 8.0 is the appropriate choice for the complexation-ultrafiltration process to remove nickel from aqueous solutions. Fig. 4 shows the effect of PAAS/Ni2+ ratio on nickel removal at pH 8.0. It was observed that the nickel removal increased as the PAAS/Ni2+ ratio increases and reaches a plateau at 97.8% after
100
100 L=0 L=2
80
L=3
Removal Rate (%)
Removal Rate (%)
80
L=4
60
L=5 L=10
40
20
60
40
20
0 0
1
2
3
4
5
6
7
8
9
pH Fig. 2. Effect of pH on nickel removal at different PAAS/Ni2+ ratios.
10
0
0
2
4
6
8
10
L Fig. 4. Effect of PAAS/Ni2+ ratio on nickel removal at pH 8.0.
12
J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477 Table 1 Competitive mechanism verification test of adding NaCl.
100
Feed solution
Na+ conc. in feed (mg/L)
Na+ conc. in permeate (mg/L)
I
NaCl only
11.5 ± 0.3
11.6 ± 0.3
II
NaCl + PAAS NaCl + PEI
11.7 ± 0.4 11.5 ± 0.4
11.3 ± 0.4 11.8 ± 0.3
III
NaCl + PAAS + Ni2+ NaCl + PEI + Ni2+
11.4 ± 0.4 11.8 ± 0.5
11.7 ± 0.2 11.4 ± 0.3
Removal Rate (%)
80
60
40
20
0
0
2
4
6 L
8
10
12
Fig. 5. Effect of PEI/Ni2+ ratio on nickel removal at pH 7.0.
PAAS/Ni2+ ratio of 5. Considering from both the nickel removal rate and the economical cost of PAAS, the optimum PAAS/Ni2+ ratio was chosen to be 5 for further complexation-ultrafiltration process for the nickel removal using PAAS as the complexation agent. Similarly, the nickel removal experiments at different pH values and different PEI/Ni2+ ratios show that pH 7.0 is the appropriate choice for the complexation-ultrafiltration process to remove nickel from aqueous solutions. Fig. 5 shows the effect of PEI/Ni2+ ratio on nickel removal at pH 7.0. It is observed that the nickel removal increases as the PEI/Ni2+ ratio increases and reaches a plateau at 93.5% after PEI/Ni2+ ratio of 5. Considering from both the nickel removal rate and the economical cost of PEI, the optimum PEI/Ni2+ ratio was chosen to be 5 for further complexationultrafiltration process for the nickel removal using PEI as the complexation polymer. 3.3. Effect of ionic strength on nickel removal Fig. 6 shows the effect of adding salt concentration on nickel removal. Ultrafiltration experiments at all the different ionic strengths (0, 10 mM, 50 mM, 100 mM, 200 mM and 500 mM) were conducted with the applied pressure of 0.1 MPa, polymer/Ni2+ ratio of 5 and pH value of 8 for PAAS and 7 for PEI. The nickel removal rate decreases sharply to 62.8% for PEI after the ionic strength greater than 100 mM, while the removal rate is at more than 99.5% without any NaCl added. Even more effect of ionic strength on nickel removal was observed for PAAS. The nickel removal rate decreased
PAAS PEI
80
to only 13.8% with solution ionic strength of 500 mM. This phenomenon was also observed by Barron-Zambrano et al. [7] and Verbych et al. [22]. One possible mechanism to explain the effect of background electrolytes on nickel removal rate is that increasing the salt concentration of the solution probably leads to compression of the electric double layer, thus to reduction in the binding between ions and polymer. As a result, the unbound nickel in the solution passes through the membrane, leading to lower removal rate. The other possible explanation is from the aspect of competitive complexation [8]. The adding background electrolyte of Na+ might compete with the metal to bind with the polymer, which causes less metal binding with the polymer than without any extra background electrolytes. This further leads more metal passing through the membrane and lower metal removal rate. The following sets of experiments were performed to validate the competition of the monovalent ion Na+ with polyvalent one Ni2+ to bind polymer. One set of experiments were performed with only adding salt NaCl in feed solution. The concentration of Na+ about 11.5 mg/L was used, which corresponds to the solution ionic strength of 500 mM. Second set of experiments were performed with NaCl and polymer. Third set of experiments were conducted with NaCl, polymer and nickel. The concentrations of Na+ were analyzed in the feed solution and in the permeate in three sets of experiments and results are shown in Table 1. Results show that the concentrations of Na+ are basically same in the permeate side in three sets of experiments and also same as those in the feed solution. This indicates that the competitive complexation mechanism should not be counted as one possible reason to explain the decreasing nickel removal behavior as the adding electrolyte concentration increases. The almost certain explanation of decreasing nickel removal behavior as the adding electrolyte concentration increases is the compression of electric double layer. 3.4. Flux and nickel retention during UF process Fig. 7 shows the nickel retention and the normalized filtrate flux ratio (J/J0 ) during PAAS enhanced UF process for 720 min (12 h), J/J0 is the ratio of filtrate flux during the filtration process over the filtrate flux at the beginning of the filtration. It is observed that the flux during 720 min filtration process declined less than 5%, which indicated that the membrane fouling is not severe at all during the whole filtration process. The high removal rate for nickel was also maintained during the whole filtration process. Using PEI as complexation agent, the similar results for flux and nickel retention change during 12 h filtration process were observed and results are shown in Fig. 8.
100
Removal Rate (%)
475
60
40
20
3.5. Diafiltration experiment to regenerate polymer 0 0
100
200
300
400
500
600
Ionic Strength (mM) Fig. 6. Effect of ionic strength on nickel removal with PAAS and PEI as the complexation agents.
Diafiltration experiments were conducted after decomplexation of nickel and polymer, and the results are shown in Fig. 9 for PAAS and Fig. 10 for PEI, with the filled symbols representing the nickel concentrations in the retentate solution calculated from the measured filtrate concentrations using a simple material
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J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477
100
Solute Concentration, C i /Ci,o
100
80 Removal Rate 70
J/J o
Removal Rate (%)
90
J/Jo
60
10
Experimental data using PEI
1 Theoretical data based on Eq.(1)
50 40 0
0.1 100
200
300
400
500
600
700
800
0
1
Fig. 7. Effect of concentration time on the permeate flux and the nickel removal with PAAS as the complexation agent.
90 Removal Rate J/Jo
J/Jo
Removal Rate (%)
100
80
70
60
0
100
200
300
400
500
600
700
800
Time (min) Fig. 8. Effect of concentration time on the permeate flux and the nickel removal with PEI as the complexation agent.
balance. The solid line is the model calculation based on Eq. (1), with the observed sieving coefficients for nickel determined from the average value calculated direct measurements of the filtrate and bulk concentrations at both the beginning and end of the diafiltration giving Si = 0.95 for PAAS and Si = 0.89 for PEI. The linearity of the data demonstrates that the sieving coefficient for nickel remains constant throughout the diafiltration. This indicates that nickel adsorption on the membrane surface or within the membrane pores has minimal effect on the observed behavior. As the diavolume increased, the concentration of nickel in the
Solute Concentration, C i /Ci,o
100
10
1
5
3.6. Equilibrium binding isotherm Previous study [21] proposed that a simple multsite Langmuirtype binding isotherm of the form might effectively explain the binding data: C bound =
nKeq Cpolymer Cfree 1 + Keq Cfree
and
Theoretical data based on Eq.(1)
Cfree =
3
4
5
Number of Diavolumes, ND Fig. 9. Relative concentration of Ni2+ as a function of diavolume in diafiltration process (PAAS).
(4)
where n is the number of moles of Ni2+ bound per mole of polymer, Keq is the equilibrium binding constant, Cbound is the concentration of bound metal in the solution, Cfree is the concentration of unbound (that is free) metal in solution and Cpolymer is the total concentration of polymer in solution. Dividing Cbound of Eq. (4), the following equation was obtained.
Experimental data using PAAS
2
4
stirred cell decreased and the recovery for the polymer increased. The Ni2+ concentration in the stirred-cell decreased to only 2.7% and 4.4% of its initial concentration for PAAS and PEI, respectively, when the diavolume was 4. The regenerated PAAS and PEI from the diafiltration experiments were used again to repeat the complexation-UF process, using the optimum experimental conditions for PAAS and PEI, respectively. The nickel removal rate with recycled PAAS was found to be 92.7% compared to 99.5% of the fresh one. The nickel removal rate of 85.3% with recycled PEI was obtained compared to 93.0% of the refresh one. The removal rates for both recycled polymers are similar to the values for the new polymers. This confirms that the diafiltration technology after acidification of polymer and nickel complex is effective to recover polymer to be used again. The permeate with concentrated nickel can then be recovered by electrodeposition, which was not included in this study. The overall results from the two-step process of complexation-UF and decomplexation-UF separation showed that it could be a promising method for nickel removal and recovery from aqueous solutions.
Cfree 1 1 = · Cfree + Cbound nCpolymer nK eq Cpolymer
0
3
Fig. 10. Relative concentration of Ni2+ as a function of diavolume in diafiltration process (PEI).
1
0.1
2
Number of Diavolumes, N D
Time (min)
(5)
Cp So
where the observed sieving coefficient for free Ni2+ (So ), defined as the ratio of the solute concentration in the permeate to that in the bulk solution, was evaluated prior to performing the binding experiment using the same membrane and the same concentration of pure Ni2+ that was added to the mixed solution. The data are linear when plotting the ratio Cfree /Cbound as a function of Cfree . The
J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477
of nickel bound to polymers. Results indicate that two-step process of complexation-UF and decomplexation-UF separation could be a promising method for nickel removal and recovery from aqueous solutions.
5.0
Conc. of Free Ni/Bounded Ni
477
4.0
3.0
Acknowledgements
2.0
This research was supported by the National Water Body Pollution Control and Reclamation Project (2008ZX07106-2 and 2009ZX07211-002).
1.0
References 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Conc. of Free Ni Ion (mM) Fig. 11. Linearized form of the binding isotherm of Ni2+ at a PEI concentration of 4 mg/L. Table 2 Numbers of nickel bound per polymer unit at different pH values. pH
n for PAAS n for PEI
3
4
5
6
7
8
0.01 0.03
0.03 0.09
0.22 0.77
0.39 1.19
0.40 1.57
0.50 1.81
binding parameters of n and Keq then can be obtained from the slope of 1/(nCpolymer ) and intercept of 1/(nKeq Cpolymer ) (Fig. 11). By counting the unit number of 318,998 per mole PAAS and 997 per mole PEI, the number of nickel ions bound per polymer unit n is obtained. The best fit values of n for both PAAS and PEI at different pH values are summarized in Table 2. Results showed that n increased as the solution pH increased, which indicated that each polymer unit could bind more ions as pH increased. The large increase of n after pH 5.0 was observed for two polymers compared to n values at lower pH values than 5.0. The values of n at pH values larger than 5.0 are comparatively close. This is consistent with the nickel removal rates observed in Figs. 2 and 3 that the nickel removal rates increased dramatically after pH 5. The simple Langmuir-type binding isotherm used in this study was effectively used to explain the results for nickel removal rates at different pH values. 4. Conclusions Complexation-ultrafiltration process with sodium polyacrylate (PAAS) and polyethylenimine (PEI) was used to remove nickel from aqueous dilute solutions. Solution pH was found to be the major factor that determines the removal rate of nickel using complexation-ultrafiltration process. At optimum experimental conditions, the nickel removal rate reaches 99.5% using PAAS and 93.5% using PEI as the complexation agent. The background electrolyte concentration is also found to have a significant effect on the nickel rejection. A series of experiments implied that the mechanism could be compressing electric double layer other than the competitive complexation. Diafiltration technology after the acidification of polymer–nickel complex was further proved to be effective to recover the polymer and metal. Langmuir-type binding isotherm equation was found to be effective to evaluate the extent
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Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Recovery of nickel from aqueous solutions by complexation-ultrafiltration process with sodium polyacrylate and polyethylenimine Jiahui Shao a,∗ , Shu Qin a , Joshua Davidson b , Wenxi Li a , Yiliang He a , H. Susan Zhou b a b
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, United States
h i g h l i g h t s
Nickel removal rate of 99.5% could be reached using complexation-ultrafiltration process. The mechanism of adding salt on metal removal could be the compressing electric double layer. Diafiltration technology was effectively used to regenerate the polymer. Langmuir-type binding isotherm equation was effectively used to evaluate nickel bound to polymer.
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 28 September 2012 Accepted 1 October 2012 Available online 5 November 2012 Keywords: Nickel Complexation agent Complexation-ultrafiltration Diafiltration Langmuir-type binding isotherm
a b s t r a c t The recovery of nickel from aqueous dilute solutions by complexation-ultrafiltration process with sodium polyacrylate (PAAS) and polyethylenimine (PEI) was studied. Experiments were performed as a function of aqueous pH, polymer/Ni2+ ratio and background electrolyte concentration. At optimum experimental conditions, the nickel removal rate reaches 99.5% using PAAS and 93.0% using PEI as the complexation agent. The nickel removal rate was found to decrease as the adding salt NaCl concentration increases for both complexation agents. A series of experiments implied that the mechanism could be the compressing electric double layer other than the competitive complexation. Diafiltration technique was further performed to regenerate complexation agents and recover nickel. The nickel removal rates were found to be close to those obtained with the original PEI and PAAS. Finally, Langmuir-type binding isotherm equation was employed to evaluate the extent of nickel bound to PAAS and PEI. The overall results from the two-step process of complexation-UF and decomplexation-UF separation showed that it could be a promising method for nickel removal and recovery from aqueous solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Water is not only essential for life; it has become the primary workhorse of industries around the world as a working fluid, transport medium, heat transfer fluid, cleaning agent, etc. Unfortunately, this has often led to the degradation of water quality as harmful effluents are returned to the environment with various contaminants from these processes. One of the most startling groups of water contaminants are those of heavy metals due to their accumulation in biological systems and their toxicity even at relatively low concentrations [1–3]. Sources of heavy metal water contamination are varied and can be seen in every step of production from mining, purification and processing, metal finishing and electroplating, and even end use [4].
∗ Corresponding author. Tel.: +86 21 54745634; fax: +86 21 54740825. E-mail address: [email protected] (J. Shao). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.10.070
Industry currently treats heavy metal wastewater, such as electroplating wastewater, a major contributor to water contamination by a wide variety of heavy metal ions [5], via a lime-soda precipitation technique that, although effective, essentially shifts the problem to large volumes of sludge containing heavy metals [5,6]. Not only does this method not solve the problem of heavy metal pollution, electroplating industry also must deal with the loss of the useable metal which is becoming increasingly expensive due to a decrease in the quality of metal ores [7,8]. What is needed is an economical method, not only for the removal of heavy metals from waste water, but for the recovery of these metals. One separation technique that can meet the above requirement is complexationultrafiltration process, also called polymer enhanced ultrafiltration (PEUF) [4–12]. Complexation-ultrafiltration process is based on the principle that the polymers with larger molecular weights than molecular weight cut-off (MWCO) of UF membranes can bind heavy metals to form macromolecular complexes and be retained by UF
J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477
membranes. After this UF process, the almost metal-free permeate stream can be discharged directly or reused for specific purposes. The metal-rich rejected stream can then be decomplexed into metal ions and regenerated polymer by lowering pH. A second step of UF is then followed to regenerate polymer to be used again in the metal removal process. The metal ions can further be recovered by electrolysis or other suitable techniques [13,7]. High degrees of small solute removal are attained using a diafiltration (DF) mode in which the small solute is effectively washed through the membrane and away from the product by the continuous (or discontinuous) addition of new buffer [14]. The most common approach is to perform the diafiltration using a constant retentate volume, in which case the small solute concentration in the product solution can be evaluated from a simple mass balance as: Ci = exp (−ND Si ) Cio
(1)
where ND is the number of diavolumes, which is equal to the total collected filtrate volume divided by the constant retentate volume during the diafiltration process. Si is the small solute sieving coefficient (=1 − removal rate), defined as the ratio of the solute concentration in the filtrate solution to that in the retentate. This diafiltration technique has been successfully used to extract the maximum amount of metal ions and regenerate the polymer as pure as possible [9]. The past fifteen years have seen not a few papers on the use of PEUF to remove heavy metals from wastewaters [8,15–19]. Most of the publications investigated the influence of pH, concentration of metal ions and background electrolytes on efficiency of metal removal with various complexing agents [11,12]. The mechanism of background electrolytes on efficiency of metal removal was not well studied. The binding capacity or the mass of heavy metal removed per mass of polymer is also normally not included in the heavy metal removal studies. Further, few studies have investigated the technology used to decomplex metal and polymer to regenerate the polymer and recover metal [10,15]. And also the detailed diafiltration technique and theoretical calculation for the metal recovery were not provided. The objective of this study was to investigate the removal of nickel from aqueous solution by complexation-ultrafiltration with PAAS and PEI. Experiments were performed as a function of aqueous pH, polymer/Ni2+ ratio, and background electrolyte concentration with PES membranes. Mechanism of the effect of background electrolyte concentration on nickel removal was proposed. Diafiltration technique was utilized to regenerate PAAS and PEI and recover the nickel. Langmuir-type binding isotherm was tried to evaluate the extent of nickel bound to polymer. 2. Experimental 2.1. Materials The membranes used were polyethersulfone with molecular weight cut-off (MWCO) of 30 kDa from Shanghai Nuclear Research Institute of Chinese Academy of Sciences (SNRICAS), China. Sodium polyacrylate (PAAS) with molecular weight of 3 × 107 Da and polyethylenimine (PEI) with molecular weight of 5 × 105 Da were used as the complexation agents. The filtration of two polymers with 30 kDa membranes indicates that not any of these two polymers passes through the membrane and gets into the permeate solution. Therefore, polymers purchased were used as-received. Nickel sulfate (NiSO4 ·6H2 O) was used to make the solution of Ni2+ . Distilled water was used to prepare the solutions. The pH value was adjusted using NaOH or HCl as needed. The background electrolyte of the solution was adjusted by adding appropriate NaCl. Prior to
473
ultrafiltration experiments, the solution containing Ni2+ and one polymer at certain pH was gently agitated in the stirrer (Hengfeng Instrument, Jintan, China) at speed of 200 rpm for 2 h to ensure equilibrium binding. All the chemicals used in this study were from Sinopharm Chemical Reagent Corp. (SCRC), China, otherwise being noted. 2.2. Experimental methods 2.2.1. Ultrafiltration process UF experiments were performed in a dead-end 70 mm diameter stirred cell with effective volume of 350 mL (SNRICAS, China), connected to a nitrogen-pressurized solution reservoir. Each membrane was flushed with approximately 100 L/m2 of DI water prior to remove any agents. The stirred cell and reservoir were then filled with the well mixed polymer–nickel solution at 100 kPa. The water flux was measured by timed collection, with filtrate samples collected periodically for subsequent analysis. The first 10 cm3 of the permeate was discarded. The permeate flux and solute rejection were determined by analyzing 1 cm3 permeate solution. The average nickel concentration of three permeate samples was used to calculate the removal rate for nickel. The schematic diagram of ultrafiltration experiment is shown in Fig. 1. Retention (R) is calculated as follows to show the removal of the nickel:
R=
1−
Cp Cf
× 100%
(2)
where Cp and Cf are the concentrations (mg/L) of nickel in the permeate and feed solution, respectively. Loading ratio (L) is calculated as follows to indicate the concentration ratio of polymer (Cpolymer ) to metal ion. L=
Cpolymer Cf
(3)
2.2.2. Diafiltration experiment The pH of the concentrated solution with polymer–nickel complex was first acidified to 2 by adding H2 SO4 and then was transferred to the dead-end stirred cell (same as that used for UF experiments) for diafiltration experiments to recover polymer and nickel. The stirred cell was connected to a solution reservoir containing H2 SO4 buffer (without any polymer or nickel). The applied pressure is 100 kPa. Filtrate samples were collected as a function of time for subsequent determination of the nickel concentrations. 2.2.3. Equilibrium binding isotherm The binding interactions between polymer and nickel were evaluated using the same stirred cell as ultrafiltration experiments which allowed polymer-free samples to be collected through the UF membrane. A polymer–nickel mixture was added into the stirred cell with a 30 kDa MWCO polyethersulfone membrane. The magnetic stirrer of the ultrafiltration unit was set at 200 rpm. The device was then nitrogen pressed to P = 100 kPa, and small samples were periodically withdrawn through the membrane for off-line analysis of the unbounded Ni2+ concentration. Data were also obtained with pure Ni2+ to evaluate the sieving coefficient of free Ni2+ through the same type of membrane used in the experiment. 2.2.4. Concentration analysis Nickel concentration was analyzed by UV/Vis spectrophotometer (Shanghai MAPADA instruments Co. Ltd.) with dimethylglyoxime as chromatic agent. The detailed procedure of this method is described in Chinese National Standard Methods for Water and Waste water Monitoring and Analysis method GB11910-89 [20].
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Fig. 1. Schematic diagram of ultrafiltration experiment.
Concentrations of Na+ were determined by ion chromatography (Metrohm Ltd.)
100 L=0 L=2
3.1. Effect of pH on nickel removal Fig. 2 shows the effect of pH on the nickel removal at different PAAS/Ni2+ ratios. The nickel removal rates were all smaller than 4% at pH values less than 8.0 in the absence of PAAS, indicating that the comparatively large pores in the UF membrane probably reject or absorb very small amount of nickel. At pH 9.0, the nickel removal rate increased sharply to 81% due to hydroxide precipitation. The nickel removal rate was observed to increase as the value of pH increases for all the PAAS/Ni2+ ratios and reaches a plateau after pH 6.0 for loading ratio of 5 and 10. At high PAAS/Ni2+ ratio, there are more available complexing sites to bind nickel due to the high PAAS concentration. While at low PAAS/Ni2+ ratio, the available complexation sites are not enough to bind all nickel. The large amounts of free metal ions exist and pass through the membrane, causing the nickel removal not efficient. Fig. 3 shows the effect of pH on the nickel removal at different PEI/Ni2+ ratios. Similarly, the nickel removal rates were all smaller than 4.0% at pH values less than 8.0 in the absence of PEI. Compared with the ultrafiltration without PEI complexation, the nickel removal rate increases dramatically as the value of pH increases for all the PEI/Ni2+ ratios and reaches an almost plateau after pH 6.0 for loading ratio of 5 and 10.
Removal Rate (%)
80
3. Results and discussion
L=3 L=4
60
L=5 L=10
40
20
0
1
0
2
3
4
5
6
7
8
9
10
pH Fig. 3. Effect of pH on nickel removal at different PEI/Ni2+ ratios.
3.2. Effect of loading ratio on nickel removal The pH value is one of the most important parameters determining the binding capacity of Ni2+ with both PAAS and PEI. The nickel removal experiments at different pH values and different PAAS/Ni2+ ratios show that pH 8.0 is the appropriate choice for the complexation-ultrafiltration process to remove nickel from aqueous solutions. Fig. 4 shows the effect of PAAS/Ni2+ ratio on nickel removal at pH 8.0. It was observed that the nickel removal increased as the PAAS/Ni2+ ratio increases and reaches a plateau at 97.8% after
100
100 L=0 L=2
80
L=3
Removal Rate (%)
Removal Rate (%)
80
L=4
60
L=5 L=10
40
20
60
40
20
0 0
1
2
3
4
5
6
7
8
9
pH Fig. 2. Effect of pH on nickel removal at different PAAS/Ni2+ ratios.
10
0
0
2
4
6
8
10
L Fig. 4. Effect of PAAS/Ni2+ ratio on nickel removal at pH 8.0.
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J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477 Table 1 Competitive mechanism verification test of adding NaCl.
100
Feed solution
Na+ conc. in feed (mg/L)
Na+ conc. in permeate (mg/L)
I
NaCl only
11.5 ± 0.3
11.6 ± 0.3
II
NaCl + PAAS NaCl + PEI
11.7 ± 0.4 11.5 ± 0.4
11.3 ± 0.4 11.8 ± 0.3
III
NaCl + PAAS + Ni2+ NaCl + PEI + Ni2+
11.4 ± 0.4 11.8 ± 0.5
11.7 ± 0.2 11.4 ± 0.3
Removal Rate (%)
80
60
40
20
0
0
2
4
6 L
8
10
12
Fig. 5. Effect of PEI/Ni2+ ratio on nickel removal at pH 7.0.
PAAS/Ni2+ ratio of 5. Considering from both the nickel removal rate and the economical cost of PAAS, the optimum PAAS/Ni2+ ratio was chosen to be 5 for further complexation-ultrafiltration process for the nickel removal using PAAS as the complexation agent. Similarly, the nickel removal experiments at different pH values and different PEI/Ni2+ ratios show that pH 7.0 is the appropriate choice for the complexation-ultrafiltration process to remove nickel from aqueous solutions. Fig. 5 shows the effect of PEI/Ni2+ ratio on nickel removal at pH 7.0. It is observed that the nickel removal increases as the PEI/Ni2+ ratio increases and reaches a plateau at 93.5% after PEI/Ni2+ ratio of 5. Considering from both the nickel removal rate and the economical cost of PEI, the optimum PEI/Ni2+ ratio was chosen to be 5 for further complexationultrafiltration process for the nickel removal using PEI as the complexation polymer. 3.3. Effect of ionic strength on nickel removal Fig. 6 shows the effect of adding salt concentration on nickel removal. Ultrafiltration experiments at all the different ionic strengths (0, 10 mM, 50 mM, 100 mM, 200 mM and 500 mM) were conducted with the applied pressure of 0.1 MPa, polymer/Ni2+ ratio of 5 and pH value of 8 for PAAS and 7 for PEI. The nickel removal rate decreases sharply to 62.8% for PEI after the ionic strength greater than 100 mM, while the removal rate is at more than 99.5% without any NaCl added. Even more effect of ionic strength on nickel removal was observed for PAAS. The nickel removal rate decreased
PAAS PEI
80
to only 13.8% with solution ionic strength of 500 mM. This phenomenon was also observed by Barron-Zambrano et al. [7] and Verbych et al. [22]. One possible mechanism to explain the effect of background electrolytes on nickel removal rate is that increasing the salt concentration of the solution probably leads to compression of the electric double layer, thus to reduction in the binding between ions and polymer. As a result, the unbound nickel in the solution passes through the membrane, leading to lower removal rate. The other possible explanation is from the aspect of competitive complexation [8]. The adding background electrolyte of Na+ might compete with the metal to bind with the polymer, which causes less metal binding with the polymer than without any extra background electrolytes. This further leads more metal passing through the membrane and lower metal removal rate. The following sets of experiments were performed to validate the competition of the monovalent ion Na+ with polyvalent one Ni2+ to bind polymer. One set of experiments were performed with only adding salt NaCl in feed solution. The concentration of Na+ about 11.5 mg/L was used, which corresponds to the solution ionic strength of 500 mM. Second set of experiments were performed with NaCl and polymer. Third set of experiments were conducted with NaCl, polymer and nickel. The concentrations of Na+ were analyzed in the feed solution and in the permeate in three sets of experiments and results are shown in Table 1. Results show that the concentrations of Na+ are basically same in the permeate side in three sets of experiments and also same as those in the feed solution. This indicates that the competitive complexation mechanism should not be counted as one possible reason to explain the decreasing nickel removal behavior as the adding electrolyte concentration increases. The almost certain explanation of decreasing nickel removal behavior as the adding electrolyte concentration increases is the compression of electric double layer. 3.4. Flux and nickel retention during UF process Fig. 7 shows the nickel retention and the normalized filtrate flux ratio (J/J0 ) during PAAS enhanced UF process for 720 min (12 h), J/J0 is the ratio of filtrate flux during the filtration process over the filtrate flux at the beginning of the filtration. It is observed that the flux during 720 min filtration process declined less than 5%, which indicated that the membrane fouling is not severe at all during the whole filtration process. The high removal rate for nickel was also maintained during the whole filtration process. Using PEI as complexation agent, the similar results for flux and nickel retention change during 12 h filtration process were observed and results are shown in Fig. 8.
100
Removal Rate (%)
475
60
40
20
3.5. Diafiltration experiment to regenerate polymer 0 0
100
200
300
400
500
600
Ionic Strength (mM) Fig. 6. Effect of ionic strength on nickel removal with PAAS and PEI as the complexation agents.
Diafiltration experiments were conducted after decomplexation of nickel and polymer, and the results are shown in Fig. 9 for PAAS and Fig. 10 for PEI, with the filled symbols representing the nickel concentrations in the retentate solution calculated from the measured filtrate concentrations using a simple material
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100
Solute Concentration, C i /Ci,o
100
80 Removal Rate 70
J/J o
Removal Rate (%)
90
J/Jo
60
10
Experimental data using PEI
1 Theoretical data based on Eq.(1)
50 40 0
0.1 100
200
300
400
500
600
700
800
0
1
Fig. 7. Effect of concentration time on the permeate flux and the nickel removal with PAAS as the complexation agent.
90 Removal Rate J/Jo
J/Jo
Removal Rate (%)
100
80
70
60
0
100
200
300
400
500
600
700
800
Time (min) Fig. 8. Effect of concentration time on the permeate flux and the nickel removal with PEI as the complexation agent.
balance. The solid line is the model calculation based on Eq. (1), with the observed sieving coefficients for nickel determined from the average value calculated direct measurements of the filtrate and bulk concentrations at both the beginning and end of the diafiltration giving Si = 0.95 for PAAS and Si = 0.89 for PEI. The linearity of the data demonstrates that the sieving coefficient for nickel remains constant throughout the diafiltration. This indicates that nickel adsorption on the membrane surface or within the membrane pores has minimal effect on the observed behavior. As the diavolume increased, the concentration of nickel in the
Solute Concentration, C i /Ci,o
100
10
1
5
3.6. Equilibrium binding isotherm Previous study [21] proposed that a simple multsite Langmuirtype binding isotherm of the form might effectively explain the binding data: C bound =
nKeq Cpolymer Cfree 1 + Keq Cfree
and
Theoretical data based on Eq.(1)
Cfree =
3
4
5
Number of Diavolumes, ND Fig. 9. Relative concentration of Ni2+ as a function of diavolume in diafiltration process (PAAS).
(4)
where n is the number of moles of Ni2+ bound per mole of polymer, Keq is the equilibrium binding constant, Cbound is the concentration of bound metal in the solution, Cfree is the concentration of unbound (that is free) metal in solution and Cpolymer is the total concentration of polymer in solution. Dividing Cbound of Eq. (4), the following equation was obtained.
Experimental data using PAAS
2
4
stirred cell decreased and the recovery for the polymer increased. The Ni2+ concentration in the stirred-cell decreased to only 2.7% and 4.4% of its initial concentration for PAAS and PEI, respectively, when the diavolume was 4. The regenerated PAAS and PEI from the diafiltration experiments were used again to repeat the complexation-UF process, using the optimum experimental conditions for PAAS and PEI, respectively. The nickel removal rate with recycled PAAS was found to be 92.7% compared to 99.5% of the fresh one. The nickel removal rate of 85.3% with recycled PEI was obtained compared to 93.0% of the refresh one. The removal rates for both recycled polymers are similar to the values for the new polymers. This confirms that the diafiltration technology after acidification of polymer and nickel complex is effective to recover polymer to be used again. The permeate with concentrated nickel can then be recovered by electrodeposition, which was not included in this study. The overall results from the two-step process of complexation-UF and decomplexation-UF separation showed that it could be a promising method for nickel removal and recovery from aqueous solutions.
Cfree 1 1 = · Cfree + Cbound nCpolymer nK eq Cpolymer
0
3
Fig. 10. Relative concentration of Ni2+ as a function of diavolume in diafiltration process (PEI).
1
0.1
2
Number of Diavolumes, N D
Time (min)
(5)
Cp So
where the observed sieving coefficient for free Ni2+ (So ), defined as the ratio of the solute concentration in the permeate to that in the bulk solution, was evaluated prior to performing the binding experiment using the same membrane and the same concentration of pure Ni2+ that was added to the mixed solution. The data are linear when plotting the ratio Cfree /Cbound as a function of Cfree . The
J. Shao et al. / Journal of Hazardous Materials 244–245 (2013) 472–477
of nickel bound to polymers. Results indicate that two-step process of complexation-UF and decomplexation-UF separation could be a promising method for nickel removal and recovery from aqueous solutions.
5.0
Conc. of Free Ni/Bounded Ni
477
4.0
3.0
Acknowledgements
2.0
This research was supported by the National Water Body Pollution Control and Reclamation Project (2008ZX07106-2 and 2009ZX07211-002).
1.0
References 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Conc. of Free Ni Ion (mM) Fig. 11. Linearized form of the binding isotherm of Ni2+ at a PEI concentration of 4 mg/L. Table 2 Numbers of nickel bound per polymer unit at different pH values. pH
n for PAAS n for PEI
3
4
5
6
7
8
0.01 0.03
0.03 0.09
0.22 0.77
0.39 1.19
0.40 1.57
0.50 1.81
binding parameters of n and Keq then can be obtained from the slope of 1/(nCpolymer ) and intercept of 1/(nKeq Cpolymer ) (Fig. 11). By counting the unit number of 318,998 per mole PAAS and 997 per mole PEI, the number of nickel ions bound per polymer unit n is obtained. The best fit values of n for both PAAS and PEI at different pH values are summarized in Table 2. Results showed that n increased as the solution pH increased, which indicated that each polymer unit could bind more ions as pH increased. The large increase of n after pH 5.0 was observed for two polymers compared to n values at lower pH values than 5.0. The values of n at pH values larger than 5.0 are comparatively close. This is consistent with the nickel removal rates observed in Figs. 2 and 3 that the nickel removal rates increased dramatically after pH 5. The simple Langmuir-type binding isotherm used in this study was effectively used to explain the results for nickel removal rates at different pH values. 4. Conclusions Complexation-ultrafiltration process with sodium polyacrylate (PAAS) and polyethylenimine (PEI) was used to remove nickel from aqueous dilute solutions. Solution pH was found to be the major factor that determines the removal rate of nickel using complexation-ultrafiltration process. At optimum experimental conditions, the nickel removal rate reaches 99.5% using PAAS and 93.5% using PEI as the complexation agent. The background electrolyte concentration is also found to have a significant effect on the nickel rejection. A series of experiments implied that the mechanism could be compressing electric double layer other than the competitive complexation. Diafiltration technology after the acidification of polymer–nickel complex was further proved to be effective to recover the polymer and metal. Langmuir-type binding isotherm equation was found to be effective to evaluate the extent
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