Novel forward osmosis process to effectively remove heavy metal ions

Novel forward osmosis process to effectively remove heavy metal ions

Journal of Membrane Science 467 (2014) 188–194 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 467 (2014) 188–194

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Novel forward osmosis process to effectively remove heavy metal ions Yue Cui a,b, Qingchun Ge b, Xiang-Yang Liu a,c, Tai-Shung Chung b,d,n a

Department of Chemistry, National University of Singapore, Singapore 117542, Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117542, Singapore Department of Physics, National University of Singapore, Singapore 117542, Singapore d Water Desalination & Reuse (WDR) Center, King Abdullah University of Science and Technology, 23955-6900 Saudi Arabia b c

art ic l e i nf o

a b s t r a c t

Article history: Received 21 February 2014 Received in revised form 7 May 2014 Accepted 19 May 2014 Available online 27 May 2014

In this study, a novel forward osmosis (FO) process for the removal of heavy metal ions from wastewater was demonstrated for the first time. The proposed FO process consists of a thin-film composite (TFC) FO membrane made from interfacial polymerization on a macrovoid-free polyimide support and a novel bulky hydroacid complex Na4[Co(C6H4O7)2]  2H2O (Na–Co–CA) as the draw solute to minimize the reverse solute flux. The removal of six heavy metal solutions, i.e., Na2Cr2O7, Na2HAsO4, Pb(NO3)2, CdCl2, CuSO4, Hg(NO3)2, were successfully demonstrated. Water fluxes around 11 L/m2/h (LMH) were harvested with heavy metals rejections of more than 99.5% when employing 1 M Na–Co–CA as the draw solution to process 2000 ppm(1 ppm¼ 1 mg/L) heavy metal solutions at room temperature. This FO performance outperforms most nanofiltration (NF) processes. In addition, the high rejections were maintained at 99.5% when a more concentrated draw solution (1.5 M) or feed solution (5000 ppm) was utilized. Furthermore, rejections greater than 99.7% were still achieved with an enhanced water flux of 16.5 LMH by operating the FO process at 60 1C. The impressive heavy metal rejections and satisfactory water flux under various conditions suggest great potential of the newly developed FO system for the treatment of heavy metal wastewater. & 2014 Elsevier B.V. All rights reserved.

Keywords: Removal of heavy metal Hydroacid complex draw solution Wastewater treatment Forward osmosis

1. Introduction Currently, heavy metal contamination has become a severe environmental issue because of the exponential increase in the use of heavy metal compounds in various industrial processes. Since heavy metals cannot be metabolized by the body or decomposed naturally, they tend to accumulate inside the body and cause severe body dysfunction [1]. Hence, removal of toxic heavy metal ions has become a top priority in wastewater treatment. To address this issue, several techniques have been applied, such as chemical precipitation, flotation, ion exchange, electrochemical deposition, adsorption, and membrane filtration [2]. Each technique above, however, suffers from certain drawbacks. The cost of chemical precipitation and flotation is usually high, while the disposal of extra sludge generated from these processes incurs extra cost [3]. Ion exchange cannot handle concentrated metal solutions due to fouling by organics and other solids in the wastewater. Moreover, ion exchange is non-selective and is highly sensitive to the pH of the solution [4,5]. Adsorption suffers from poor selectivity and slow regeneration [5–7], although it has the n Corresponding author at: Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117542. Tel.: þ 65 65166645; fax: þ65 67791936. E-mail address: [email protected] (T.-S. Chung).

http://dx.doi.org/10.1016/j.memsci.2014.05.034 0376-7388/& 2014 Elsevier B.V. All rights reserved.

advantages of low-cost and rapid adsorption owing to the various low-cost adsorbents recently developed. In the past decades, membrane technology has been proven as a feasible option in wastewater treatment due to its high rejection to contaminants, such as organic compounds or dye molecules, and low fabrication cost [8]. Nanofiltration (NF) is an effective method for wastewater treatment and heavy metal removal [9,10] by means of size exclusion and Donnan exclusion [11,12]. However, NF membranes suffer from high fouling tendency, which resulted in reduced productivity and extra operational cost. Besides, insufficient rejection of heavy metal ions in NF processes in water treatment leads to extra cost on further purification [13]. Forward osmosis (FO) is an emerging technology for water reuse and desalination. The driving force of this process is the osmotic pressure difference across the semi-permeable membrane [14–16]. Water transports across the membrane from the feed solution to the draw solution, while unwanted compounds are rejected by the membrane. Although with such unique characteristics, the current FO technology is far from perfect. For example, the choice of FO membranes is limited [17] and the draw solute needs to be regenerated using processes such as membrane distillation (MD), NF, ultrafiltration and others [18,19], which may require extra energy. However, in comparison to pressure driven processes, FO offers the advantages of no or low operation pressure, low fouling propensity and easy cleaning [20–22]. Since

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most FO membranes are tailored to reject monovalent ions from seawater, it could theoretically offer very high rejections to heavy metal ions which are multivalent in nature. Thus, the FO process could be a promising option for heavy metal removal. To effectively remove heavy metal ions, a thin film composite (TFC) membrane was fabricated in this study by synthesizing a polyamide layer via interfacial polymerization upon a Matrimid substrate because this type of membranes has characteristics of high flux and salt rejection [23,24]. Fig. S1 shows the membrane morphologies. In addition, a novel cobaltic hydroacid complex was synthesized as the draw solute because it has a metal(s) center bonding to hydroacid ligands, as shown in Fig. S1. As a result, it can not only produce an extremely high osmotic pressure due to the formation of multi- ionic species in water but also minimize reverse solute flux with its expanded structure [25]. Besides, this draw solute can be readily regenerated using MD or NF membranes due to its large molecular weight and bulky structure [25]. Therefore, we aim to explore the efficiency of the proposed FO system for the removal of heavy metal ions under various draw solution and feed solution concentrations as well as different operating temperatures. The encouraging results from this study suggest that this novel FO system may have great potential for heavy metal ion removal in wastewater treatment.

2. Materials and methods 2.1. Materials The commercially available polyimide polymer, Matrimids 5218 (Vantico Inc.) was utilized to fabricate the membrane substrate. The solvent N-methyl-2-pyrrolodinone (NMP, 499.5%) and the non-solvent polyethylene glycol 400 (PEG 400, Mw ¼ 400 g/mol) were purchased from Merck. M-phenylenediamine (MPD, 499%) and trimesoylchloride (TMC, 4 98%) were ordered from Sigma-aldrich and used as the monomers for the interfacial polymerization reaction. Co(NO3)2  6H2O (99%) and citric acid (CA) (99%) were purchased from Sigma-Aldrich for the synthesis of draw solute. Ethanol (99%), obtained from Acros Organics, was utilized to crystallize the draw solute. The commercial TFC NF membrane NE2540-70 obtained from Woongjin Chemical Co. was used for performance comparison with the lab-fabricated FO TFC membrane. Six heavy metal salts, i.e., chromate (Na2Cr2O7), arsenic (Na2HAsO4), lead (Pb(NO3)2), cadmium (CdCl2), copper (CuSO4), and mercury (Hg(NO3)2), purchased from Sigma-aldrich, were used to determine the solute transport properties. The deionized (DI) water was produced by a Milli-Q ultrapure water system (Millipore, USA). All chemicals were used as received. 2.2. Fabrication of the macrovoid-free Matrimid substrate The flat sheet membranes were prepared by a solution casting process, followed by the non-solvent induced phase inversion with a dope formulation of Matrimid/polyethylene glycol 400 (PEG 400)/ N-methyl-2-pyrrolidinone (NMP) at the weight ratio of 18/16/66. A detailed description of the fabrication process for flat sheet membranes has been documented in the Supporting information (SI), or elsewhere [17,24,26]. 2.3. Interfacial polymerization of thin-film-composite (TFC) membranes The formation of a thin polyamide layer on top of Matrimid substrates was achieved by an interfacial polymerization reaction between MPD in the aqueous phase and TMC in the organic phase. The detailed preparation steps are disclosed in the SI.

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2.4. Draw solute (Na4[Co(C6H4O7)2]) synthesis Co(NO3)3  6H2O (40 mmol, 11.64 g) and citric acid (80 mmol, 15.36 g) were added in DI water (150 mL) and stirred overnight at 50 1C. Then NaOH (1.0 M) was added drop wise to adjust the pH till neutral. After reaction, 300 mL cold ethanol was added to the solution and the precipitates were harvested with the aid of centrifuge; this step was repeated three times. The resultant pink solid Na4[Co(C6H4O7)2]  2H2O (referred to as Na–Co–CA) was dried under vacuum (yield 495%) and preserved in a desiccator for further FO tests. The osmotic pressure of the Na–Co–CA aqueous solution was measured using a model 3250 osmometer (Advanced Instruments, Inc.) and its kinematic viscosity, ηk (mm2/s), at different temperatures and concentrations is calculated as following:

ηk ¼ kt

ð1Þ

where t (s) is the elution time of the cobaltic complex aqueous solution measured by an AVS 360 inherent viscosity meter, and k is the constant which equals to 0.003211 mm2/s2. 2.5. Water reclamation through forward osmosis FO experiments were conducted on a lab-scale FO unit [17,25]. The volumetric flows of both draw solution (100 mL Na–Co–CA aqueous solution) and feed solution (400 mL DI water) were 0.2 L/min. They flowed co-currently through the FO cell and were circulated in the setup. The water flux (Jw, L m  2 h  1, LMH) was calculated as JW ¼

Δm 1 Δt Am

ð2Þ

where Am is the effective area of the FO cell, which is 4 cm2; Δm (g) is the absolute weight loss in the feed solution side, and Δt (h) is the test duration, which is 0.5 h. During each test, masses of the draw solution were recorded before and after testing. After each test, a certain amount of Na–Co–CA, which is calculated based on the equation listed below, was added into the draw solution to maintain the concentration. madd ¼ ðmaf ter  mini Þ=ρwater  C draw

ð3Þ

where mafter (g) and mini (g) are the masses of the draw solution after and before testing, respectively. ρwater is the water density, which equals to 1 g/cm3, and Cdraw (g/L) is the initial concentration of the draw solution. Six types of heavy metal aqueous solutions were prepared by dissolving the salts, i.e., Na2Cr2O7, Na2HAsO4, Pb(NO3)2, CdCl2, CuSO4, and Hg(NO3)2 in ultrapure water, respectively, and used as the model wastewater without further pH adjustments. The concentrations of the waste water solutions were 1000, 2000, and 5000 ppm (1 ppm¼ 1 mg/L), respectively. The solute rejection R (%) was defined as the percentage of feed solutes that were retained by the membrane. It was calculated as R ¼ 1

C d  V d =V p Cf

ð4Þ

where Cd (ppm) is the heavy metal concentration in the draw solution at the end of each FO test, Vd (L) is the final volume of draw solution, Vp (L) is the volume of the permeate water, and Cf (ppm) is the heavy metal concentration in the feed solution. Cd (ppm) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). In the FO process, since the permeate is diluted by the bulk draw solution, there is a maximum reliable rejection based on the detection limitation of ICP-OES. In this study, the draw solution was recrystallized with ethanol and heat digested to evaporate the ethanol so that the sample was concentrated to guarantee the maximum reliable rejection is

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where ΔCt (g/L) and V (L) are the changes of salt concentration and feed solution volume, respectively. The reverse salt flux was only measured when pure water was used as the feed solution. All experimental tests were conducted under the FO mode (i.e., the selective layer faces to the feed solution) unless otherwise specified.

and temperatures were evaluated and presented in Fig. 1. The osmotic pressure is proportional to the draw solution concentration. The higher the draw solution concentration, the higher the driving force is for the FO process. In addition, an increase in cobaltic complex concentration leads to a drastic increase in kinematic viscosity. However, the kinematic viscosity declines significantly with increasing temperature, especially for highly concentrated draw solutions. As a result, the difference in kinematic viscosity among different concentrations of cobaltic complex solutions becomes smaller when the temperature reaches 60 1C. These characteristics favor the use of this draw solute at high concentrations and moderate temperatures for water reuse and draw solute regeneration by means of hybrid processes such as the integrated FO-membrane distillation (FO–MD) system [28,29].

2.6. Nanofiltration performance of the commercial and lab-fabricated TFC membranes

3.2. Comparison of draw solutes

99.90%. The rejection higher than this value is expressed as 499.90%. The reverse salt flux (Js, g m  2 h  1, gMH) of the draw solution was calculated from the conductivity increment in the feed solution, as Js ¼

ΔC t V 1 Δt Am

ð5Þ

Water permeability, A (L m  2 h  1 bar  1), and solute rejection, R (%), of the commercial and lab-fabricated TFC membranes were determined by testing the membranes under a trans-membrane pressure, ΔP, of 5.0 bar in dead-end cells at room temperature, as described in our previous studies [26,27] The feed solutions were six different heavy metal solutions at 2000 ppm. The concentration of heavy metal solution in the feed (Cf) and permeate (Cp) were determined by conductivity measurements with a conductivity meter (Metrohm, Swiss). The water permeability was calculated as JW ¼

Δm 1 Δt Am ΔP

ð6Þ

where Δm (g) is the absolute weight loss in the feed solution side, and Δt (h) is the test duration, Am is the effective area of the testing cell, ΔP (bar) is the applied trans-membrane pressure. The salt rejection R was calculated from:   Cp  100% ð7Þ R ¼ 1 Cf

An ideal draw solution for the FO process should have a high water but a low reverse flux. Thus, the performance of 0.5 M NaCl and 0.5 M Na–Co–CA as draw solutes was tested and compared. As shown in Table 1, the cobaltic complex (Na–Co–CA) outperforms NaCl under both FO and pressure retarded osmosis (PRO, the selective layer facing the draw solution) modes. The cobaltic complex produces higher water fluxes but much lower reverse solute fluxes under both modes. Under the PRO mode, the water flux obtained from 0.5 M Na–Co–CA is almost 80% higher than that from 0.5 M NaCl. Moreover, the reverse solute flux of the former is much lower than the latter. Although the water flux difference between these two draw solutes under the FO mode is not as big as that in the PRO mode due to the more serious internal concentration polarization (ICP) in the FO mode [30–32], the performance of Na–Co–CA still surpasses NaCl. In addition, the reverse solute flux of the former is much lower than that of the latter under the FO mode. Thus, using Na–Co–CA as the draw solute not only produces a higher water flux but also lowers the

Table 1 Comparison of different draw solutions.

3. Results and discussion

0.5 M NaCl

3.1. Intrinsic properties of the Na–Co–CA complex Since the intrinsic properties of the cobaltic complex draw solution have significant effects on the FO performance, its osmotic pressure and viscosity under different concentrations

PRO FO

0.5 M Na–Co–CA

Water flux (LMH)

Reverse solute flux (gMH)

Water flux (LMH)

Reverse solute flux (gMH)

12.3 70.58 9.4 70.79

3.22 7 0.21 2.30 7 0.52

22.28 7 0.42 13.577 0.28

1.047 0.03 0.92 7 0.06

70

Kinematic viscosity (mm2/s)

Osmotic pressure (atm)

50 40 30 20 10 0 0.0

Pure water 0.5M 0.75M 1M 1.5M

5

60

4 3 2 1 0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Draw solution concentration (M)

1.6

20

25

30

35

40

45

50

55

Temperature ( )

Fig. 1. (a) Osmotic pressure and (b) kinematic viscosity as a function of draw solution concentration.

60

65

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amount of draw solute flowing back to the feed solution. This performance superiority may imply the following advantages of using this cobaltic complex such as (1) significantly reduce the replenishment cost of the draw solute, (2) minimize the increase of the total dissolved solids (TDS) in the feed solution, and (3) improve the efficiency of wastewater treatment. 3.3. Comparison of heavy metal ion removal between FO and NF processes To compare the removal efficiency of heavy metal ions between NF and FO, a commercial TFC NF membrane and a lab-fabricated TFC membrane were firstly tested under the NF process for heavy metal removal. Then the results were compared with the labfabricated TFC membrane tested under the FO process. Table 2 summarizes the results. The solute rejection under the NF process varies with the type of heavy metal ions. The commercial TFC NF membrane exhibits poor rejections to chromic and cadmium ions, but moderate rejection to cupric ions. The causes of these unimpressive rejections may be due to excessive convective transport, poor size exclusion or ineffective charge repulsion [9,33,34]. In contrast, the lab-fabricated TFC membrane shows much better rejections to heavy metal ions, which are higher than 50% to all metal ions. This may be attributed to the fact that the labfabricated TFC membrane has a denser selective layer and a smaller pore size than the commercial one. Besides, the labfabricated TFC membrane has the highest rejection to cupric ions of 96.1%, but exhibits a relatively low rejection of 51.3% to mercury ions. Although the lab-fabricated TFC membrane has significant improvements on heavy metal rejection under the NF process, the rejections are still not high enough for practical wastewater Table 2 Comparison of heavy metal removal efficiency in FO and NF processes. Solute rejection NF (%)

Na2Cr2O7 Na2HAsO4 Pb(NO3)2 CdCl2 CuSO4 Hg(NO3)2

FO (%)

Commercial NF membrane

Lab fabricated TFC membrane

Lab fabricated TFC membrane

8.93 7 1.34 60.69 7 1.04 24.727 3.36 12.377 2.19 83.217 3.14 28.95 7 3.94

81.50 7 3.60 93.86 7 0.49 57.46 7 4.06 75.89 7 1.05 96.107 1.88 51.34 7 3.05

99.87 7 0.18 99.747 0.04 99.377 0.43 499.90 7 0 99.78 7 0.03 99.777 0.16

Testing conditions: 2000 ppm feed solution; Room temperature; NF tests were under 5 bar; For FO tests, 1 M Na–Co–CA was applied as the draw solution.

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treatment. Moreover, even the most advanced NF membranes reported in the open literature do not have sufficient rejections to heavy metal ions, especially at high heavy metal concentrations [34–38]. As a result, an extra post-treatment on the permeate such as multi-stage NF or reverse osmosis (RO) process is needed, which would incur additional costs. In addition, a pH adjustment is often carried out in NF processes in order to enhance charge repulsion and obtain satisfactory rejections [9,33,34,37]. It will not only add extra costs, but also be likely to damage the membrane. Besides, the pH adjustment for each ion separately is not practical in waste water treatment. In addition, so far most NF membranes have poor rejections to mercury ions, especially under high concentrations [5]. On the contrary, the lab-fabricated TFC membrane tested under the FO process outperforms itself and the commercial NF membrane under the NF process in terms of heavy metal rejection. The solute rejections for all six heavy metal ions operated under the FO process are all higher than 99%, and most of them are higher than 99.7%. Most importantly, an outstanding rejection to mercury ions is achieved under the FO process, which is always low in a normal NF process [5]. The high rejections to heavy metal ions are also beneficial to the regeneration of the draw solution where clean water is also produced. The high rejections to heavy metal ions under FO could be attributed to many factors. Since no pressure is applied in the FO process, the effect of convective flow on metal ion transport is insignificant. Compared to the NF membrane, the dominant mechanism for heavy metal transport across the TFC FO membrane is the solution–diffusion mechanism. Since diffusivity decreases with increasing hydrated radius, the heavy metal ions with larger radii of hydration can be rejected easily. In addition, due to the presence of highly concentrated bulky draw solute ions, the Donnan equilibrium effect [39] may retard ionic permeation rates of the feed ions across the active layer in some cases [40,41]. For example, when 2000 ppm (i.e., low concentration) NaHAsO4 and 1 M (i.e., high concentration) Na–Co–CA are used as feed and draw solutions respectively, the transport of HAsO4 will be retarded due to a few reasons. Firstly, given that the bulky draw solute anion is difficult to diffuse to the feed side, the ionexchange between this anion and HAsO4 ion is negligible. Besides, the sodium ion concentration in the draw solution is much higher than that in the feed solution. Consequently, the sodium cation is less likely to diffuse from the feed side to the draw side. As a result, few HAsO4 will diffuse to the draw side to maintain the electroneutrality. Hence, the high rejection to HAsO4 can be expected under such circumstance. Thus, the Donnan equilibrium effect may also contribute to the high rejections under the FO process. In summary, the experimental results clearly suggest that the proposed FO process is superior to NF in terms of heavy metal ion

15 14 100.0

Solute rejection (%)

Water flux (LMH)

13 12 11 10 9 8

Cr Cd

7

As Cu

Pb Hg

99.6

99.2

As Cu

Cr Cd

Pb Hg

98.8

6 5 0.4

0.6

0.8

1.0

1.2

1.4

Draw solution concentration (M)

1.6

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Draw solution concentration (M)

Fig. 2. (a) Water flux and (b) solute rejection as a function of draw solution concentration. Feed solution concentration: 2000 ppm, room temperature.

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rejection. The FO process may have potential to compete with and replace NF in heavy meal removal. 3.4. Influences of the draw solution concentration To speed up the wastewater treatment in the FO process, concentrated draw solutions are utilized in this study. Fig. 2 shows the effect of draw solution concentration on water flux and solute rejection. The water flux increases with the draw solution concentration due to the increase in osmotic pressure. When 0.5 M Na–Co– CA is used as the draw solution, the water flux is around 9 LMH. A flux increment of more than 40% is observed when a draw solution concentration of 1.5 M is used. However, the enhancement of water flux is not proportional to the increase of draw solution concentration, due to the ICP effect in the sublayer. In other words, the dilution effect of draw solution in the porous sublayer decreases the effective driving force across the membrane and lowers the water flux [14–16,30–32,42–45]. This phenomenon is further intensified by the increased viscosity of the hydroacid complex at higher concentrations. As a consequence, the water flux increases slowly with an increase in complex concentration. Fig. 2(b) shows the heavy metal ion rejection as a function of draw solution concentration. The rejections to all heavy metal ions can reach as high as 99.5% or above with various draw solution concentrations. Although the rejection for each metal ion does not change much, a slightly increasing trend in solute rejection can be observed in spite of some exceptions. When 1.5 M Na–Co–CA is used as the draw solution, the rejections to all the heavy metal ions are satisfactorily higher than 99.7%. Theoretically, metal ions are transported through the membrane by the solution–diffusion mechanism [40,41,46,47]. Since the water flux increases with an increase in draw solution concentration while the metal ion

concentration across the FO membrane remains almost the same, a higher solute rejection can be expected. Therefore, employing a draw solution with a higher concentration is a potential option to enhance the FO performance for heavy metal ion removal. 3.5. Influence of the feed solution concentration To investigate the extent to which the feed solution can be concentrated while still sustaining a high rejection, the water fluxes and solute rejections to six different heavy metal ions as a function of feed concentration using 1 M cobaltic complex as the draw solution were explored and the results were presented in Fig. 3. A water flux of around 11.5 LMH is obtained with a feed concentration of 1000 ppm. By increasing the feed concentration to 5000 ppm, a minor drop of water flux to 10.5 LMH is observed due to a slight decrease in osmotic pressure difference between the draw and feed solution. Fig. 3(b) presents the metal ion rejection as a function of feed concentration. At a feed concentration of 1000 ppm, the rejections are higher than 99.8% for all six heavy metal ions. The rejections drop slightly but remain as high as 99.6% or above when the feed concentration is increased to 2000 ppm or even 5000 ppm. Theoretically, the increased feed concentration would enhance the diffusion of heavy metal ions, and the diffusion flux is proportional to the ion concentration difference. However, an increase in feed concentration would increase the feed osmotic pressure and reduce the overall driving force for water transport. As a result, there is an insignificant reduction in the solute rejection. However, the rejections to all six heavy metal ions remain impressively high. The excellent rejections imply that the proposed FO process is able to treat heavy metal concentrations greater than 5000 ppm. Given that the heavy metal concentration in industrial plants is often in the range of hundreds

13

100.4

11

10 9 8

As Cu

Cr Cd

7

Pb Hg

Solute Rejection (%)

Water flux (LMH)

12

6

100.0 99.6 99.2

As Cu

Cr Cd

98.8

Pb Hg

5 1000

2000

3000

4000

5000

1000

2000

3000

4000

5000

Feed solution concentration (ppm)

Feed solution concentration (ppm)

Fig. 3. (a) Water flux and (b) solute rejection as a function of feed solution concentration. Draw solution: 1 M Na–Co–CA, room temperature.

19 100.0 15 13 11 9

As Cu

Cr Cd

7

Pb Hg

Solute Rejection (%)

Water Flux (LMH)

17 99.8 99.6 99.4 99.2 99.0

Cr

As

Pb

Cd

Cu

Hg

98.8 98.6

5 20

30

40

50

Temperature ( )

60

20

30

40

50

60

Temperature ( )

Fig. 4. (a) Water flux and (b) solute rejection as a function of temperature. Draw solution: 1 M Na–Co–CA; feed solution concentration: 2000 ppm.

Y. Cui et al. / Journal of Membrane Science 467 (2014) 188–194

ppm [2–6,8], the proposed FO system is able to maintain a high rejection even when the recovery of the feed solution reaches 90%. 3.6. Influence of the draw solution temperature According to the Van’t Hoff equation [18, 25], the osmotic pressure of a solution is proportional to its temperature. Thus, operating a FO process at elevated temperatures is preferred in terms of water flux. In addition, the FO unit may integrate with a MD unit for draw solution recovery and clean water production [14,18,19,28,29,48,49] and the MD unit is often operated at moderate temperatures to facilitate water vapor transport across the MD membrane, it is therefore important to study the heavy metal removal at elevated temperatures. Besides, the viscosity of the cobaltic complex solution decreases while its diffusivity increases with increasing temperature. These two factors may reduce the ICP effect in the porous sublayer and favor a higher water flux [50–52]. Consistent with our hypotheses, Fig. 4(a) displays a significant increase in water flux with increasing operating temperature. The water flux jumps about 50% from around 11 LMH to around 16.5 LMH as the draw solution temperature increases from room temperature to 60 1C. The heavy metal rejections at various temperatures are depicted in Fig. 4(b). The rejections to all six heavy metal ions ameliorate with increasing temperature. Rejections higher than 99.7% can be achieved at 60 1C, as a result of the 50% flux increment. Clearly, operating the novel FO process at high temperatures is encouraged.

4. Conclusion In this study, we have demonstrated the removal of heavy metal ions from wastewater by means of a novel forward osmosis process successfully. In the proposed FO process, we used a TFC FO membrane as the separating barrier and a novel hydroacid complex Na–Co–CA as the draw solute. The following conclusions can be drawn: 1. The proposed FO process shows great potential to remove heavy metal ions, i.e., Cr2O27  , HAsO24  , Pb2 þ , Cd2 þ , Cu2 þ , and Hg2 þ , from wastewater. Most of the rejections to them were higher than 99.5% while the water fluxes were around 11 LMH when a 1 M draw solution was utilized and the heavy metal salts concentration was 2000 ppm. The rejections obtained from FO process are higher than most of NF processes. 2. A higher concentration of draw solutions enhances the removal of heavy metal ions. When a 1.5 M draw solution was employed, the water flux increased to around 13 LMH and the rejection was promoted to 99.7%. 3. The proposed FO process maintains high rejections under high concentrations of heavy metal ions. Even when 5000 ppm feed solution was used, the rejections were maintained at 99.5%. 4. The rejections to heavy metal ions ameliorate with increasing temperature. Rejections greater than 99.7% can be achieved with an enhanced water flux of 16.5 LMH by operating the FO process at 60 1C. 5. High separation efficiency in terms of flux and rejection can be achieved for heavy metal removal by using the novel FO system consisting of the TFC FO membrane and the Na–Co–CA draw solute at high concentrations and elevated temperatures.

Acknowledgment This research is supported by the National Research FoundationPrime Minister's office, Republic of Singapore under its Competitive

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Research Program entitled “Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (Grant numbers R-279-000-336-281 and R-278-000339-281). Thanks are also due to Dr. Zhengzhong Zhou, Dr. Sui Zhang, Mr. Gang Han and Miss Xiuzhu Fu for their suggestions and help on this work.

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