Removal of Heavy Metals from Electroplating Wastewater by Thin-Film

Article pubs.acs.org/IECR

Removal of Heavy Metals from Electroplating Wastewater by ThinFilm Composite Nanofiltration Hollow-Fiber Membranes Xiuzhen Wei,*,† Xin Kong,† Songxue Wang,† Hai Xiang,‡ Jiade Wang,† and Jinyuan Chen*,† †

College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China ‡ College of Bioengineering, Zhejiang Chinese Medical University, Hangzhou 310053, People’s Republic of China ABSTRACT: In this study, thin-film composite nanofiltration (NF) hollow-fiber membranes were used to remove heavy metals from actual electroplating wastewater. The effects of the operating pressure, feed temperature, and feed pH on the membrane performance for the treatment of electroplating wastewater were investigated. The rejection rates for chromium, copper, and nickel ions reached 95.76%, 95.33%, and 94.99%, respectively, at 0.4 MPa. With a rise in the feed temperature, the permeate flux increased while the rejection rates of heavy metals did not significantly change. It was evident that the feed pH greatly affected the permeate flux and heavy-metal rejection as well. In addition, all of the rejection rates of heavy metals by the membrane were over 94.8% throughout the electroplating wastewater concentration process. Also, the NF hollow-fiber membrane showed good stability in electroplating wastewater with a pH value of 2.31.

1. INTRODUCTION With the rapid development of the electroplating industry in recent years, electroplating wastewater pollution has attracted increasing attention. Electroplating wastewater contains heavy metals such as nickel, copper, cadmium, and chromium, which are detrimental to the health of living organisms because of the inability of these metals to biodegrade and their tendency to accumulate in the tissues of organisms when exposed in the environment. At the same time, however, if the heavy metals from electroplating wastewater can be effectively removed, both the filtered wastewater and heavy metals have a great possibility of being directly reused. Hence, removal of heavy metals from industrial wastewater such as that from the electroplating industry prior to its discharge is highly desired.1,2 Traditionally, conventional treatment techniques, such as chemical precipitation, coagulation−flocculation, flotation, adsorption, and ion exchange, have been employed to remove heavy metals from industrial wastewater.3−7 However, these methods for removal of heavy metals have their limitations and may contribute to other problems. For example, although chemical precipitation and coagulation−flocculation have been widely used to treat electroplating wastewater, their drawbacks, like a large amount of chemical consumption, sludge production, and extra handling cost for sludge disposal, are obvious. The adsorption method is also confronted with some problems, such as poor selectivity and slow regeneration. For the ionic exchange process, it is difficult to develop suitable ionexchanger resins that are available for all heavy-metal removal, and the capital cost is high. 4,8 Membrane separation technologies have been determined to be a feasible and promising option for heavy-metal removal because of their high efficiency and ease of operation. Ultrafiltration (UF) and reverse osmosis (RO) are now being increasingly used for removal of heavy metals from wastewater.9−12 However, nanofiltration (NF) is less investigated intensively than UF and RO for removal of heavy metals. © 2013 American Chemical Society

NF is a relatively new, pressure-driven membrane process, possessing separation characteristics between those of RO and UF.13 Compared with RO, NF has some outstanding advantages, such as a high permeability for monovalent ions and a low permeability for divalent ions, lower operating pressure, higher permeate flux, and lower energy consumption.14−16 Compared to UF, NF membranes have a lower molecular weight cutoff (MWCO) ranging from 200 to 1000 Da. Therefore, NF with an integrated UF process would be a promising technique for the treatment of electroplating wastewater.17 Presently, several researchers have conducted studies on removal of heavy metals from simulated wastewater by adding metal salts to distilled water.18−22 However, research concerning the use of NF membranes for the treatment of actual electroplating industry wastewater has only rarely been reported. For instance, Wang et al.23 studied the membrane performance for the treatment of electroplating rinse wastewater containing copper and chromium under different operating conditions using three types of commercial NF membranes: DL, DK, and NTR-7450. Boricha and Murthy24 fabricated new composite NF membranes by coating the chelating agent diethylenetriaminepentaacetic acid on a poly(ether sulfone) UF membrane substrate. These fabricated NF membranes were used for removal of zinc and iron from electroplating wastewater. However, note that the membranes used in the aforementioned studies concerning removal of heavy metals from simulated or actual electroplating wastewater were all thin-film composite NF flat-sheet membranes. Consideration that hollow-fiber membranes provide a considerably larger surface area per unit module volume, are mechanically self-supporting, and are easier to handle during Received: Revised: Accepted: Published: 17583

July 25, 2013 November 1, 2013 November 20, 2013 November 20, 2013 dx.doi.org/10.1021/ie402387u | Ind. Eng. Chem. Res. 2013, 52, 17583−17590

Industrial & Engineering Chemistry Research

Article

module fabrication and system operation compared to flat-sheet membranes,25 there has been a growing interest to develop thin-film composite NF hollow-fiber membranes for the treatment of wastewater in recent years. Although hollowfiber membranes possess an advantage over membranes in the flat-sheet configuration, limited studies have been conducted on the membrane performance for removal of heavy metals from wastewater using thin-film composite NF hollow-fiber membranes. For example, Chung et al.8,26 fabricated amphoteric poly(benzimidazole) (PBI) NF hollow-fiber membranes through phase inversion, and the membranes were used to reject heavy metals of chromium and copper. The results showed that the heavy metals could be effectively removed by PBI NF hollow-fiber membranes from their aqueous solutions. However, to our knowledge, there is no literature available on the study of the thin-film composite NF hollow-fiber membrane performance for removal of heavy metals from actual electroplating industry wastewater. Here, thin-film composite NF hollow-fiber membranes, prepared in the laboratory, were employed to treat actual electroplating wastewater. The performance of NF hollow-fiber membranes for treating actual electroplating wastewater was investigated by varying the following parameters: operating pressure, feed temperature, and feed pH. In addition, an electroplating wastewater concentration process was performed in the experiment, and the stability of NF hollow-fiber membranes in electroplating wastewater was also investigated.

main aim of the present work is to remove chromium, copper, and nickel from actual electroplating wastewater through the NF hollow-fiber membranes in the paper. 2.2. NF Hollow-Fiber Membranes. Thin-film composite NF hollow-fiber membranes were prepared by an interfacial polymerization technique. PIP and TMC were used as reactive monomers to form the polyamide film on the inner surface of the hollow-fiber membrane. To begin with, the aqueous solution of PIP (2.0 w/v %) with 1.0 w/v % Na3PO4 as the acid acceptor was extruded into the lumen side of hollow-fiber membranes and allowed to remain for 10 min, and then excess aqueous solution was drained off the soaked surface and airdried using nitrogen. Subsequently, the organic-phase solution with 0.5 wt % TMC in n-hexane was pumped through the lumen side of hollow-fiber membranes in the same way for 50 s to allow the interfacial polymerization reaction to take place, which resulted in the formation of a polyamide active skin layer on the inner surface of the PS/PES supporting membrane. After removal of excess organic solution, the hollow-fiber membranes were cured for approximately 10 min in an oven at 70 °C for further polymerization. Finally, the prepared NF hollow-fiber membranes were rinsed with DI water for 30 min and stored in a 1.0 wt % NaHSO3 solution for further use. In this work, the NF hollow-fiber membranes, prepared in the laboratory and characterized previously, were further used to remove heavy metals from actual electroplating wastewater.27 The detailed characteristics of NF hollow-fiber membranes are presented in Table 2. 2.3. Experimental Apparatus. The NF membrane performance experiments were conducted through a laboratory-scale cross-flow filtration apparatus at a constant flow rate of 1.1 L/min in a batch circular mode. Both the permeate and retentate were recycled back to the feed tank in order to keep a constant concentration. In the experiments, each NF membrane module contained eight hollow fibers with 0.8 cm inner diameter and approximately 14.0 cm length, resulting in an effective area of approximately 23 cm2. The feed solution was pumped into the lumen side of the hollow fibers, while the permeate solution exited from the shell side. Prior to NF experiments, all NF hollow-fiber membranes were prepressurized under 0.5 MPa for 1 h with DI water to make sure the membranes were in the steady state. 2.4. Membrane Performance. A flat-sheet UF membrane with a MWCO of 50000 Da was adopted to pretreat electroplating wastewater before treatment using NF hollowfiber membranes. The application of an UF pretreatment could effectively remove most particles in the wastewater and reduce fouling of the NF membrane, resulting in a longer membrane life and the ability to operate the NF membrane at higher permeate flux.28 After pretreatment, the concentrations of chromium, copper, and nickel ions in the UF permeate slightly dropped to 121.23, 56.55, and 142.23 mg/L, respectively. Also, the pH value of the wastewater altered from 2.20 to 2.31. The permeate solution of the UF pretreatment was used as the feed solution of the NF performance experiments. The performance of the NF hollow-fiber membranes in terms of the permeate flux and heavy-metal rejection rates for the treatment of actual electroplating wastewater was studied under different operation conditions. The electroplating wastewater concentration process was also performed. The concentration refers to continuous removal of water from wastewater, while the heavy metals were entrapped and cycled

2. EXPERIMENTAL SECTION 2.1. Materials and Wastewater. Polysulfone/poly(ether sulfone) (PS/PES) hollow-fiber UF supporting membranes (MWCO = 30000 Da) with a pure water flux of approximately 152 L/m2·h at 0.1 MPa were provided by the Development Center of Water Treatment Technology, SOA, Hangzhou, China. Trimesoyl chloride (TMC; purity >99.0%) was purchased from Qingdao Benzo, China, and used as the reactive monomer of the organic phase with n-hexane (Shanghai Lingfeng, China) as the organic solvent. The aqueous-phase solution consisted of piperazine (PIP; purity >99.0%, Aladdin, China) used as the reactive monomer and sodium phosphate (Na3PO4; Sigma-Aldrich) used as the acid acceptor. All other chemicals used in the experiments were of analytical purity grade with no further purification. Deionized (DI) water (pH ≈6.8) used in all experiments was treated with RO membranes. The wastewater used in the experiment was obtained from a local actual electroplating plant. The composition of raw wastewater is shown in Table 1. From the table, we can see that electroplating wastewater has relatively higher concentrations of chromium, copper, and nickel ions than other cations. So, the Table 1. Composition of Raw Electroplating Wastewater item pH TOC nickel (Ni2+) chromium (Cr3+/ Cr6+) copper (Cu2+) sodium potassium

concentration (mg/L)

item

concentration (mg/L)

2.20 90.45 146.65 123.50

magnesium calcium zinc chloride

19.94 76.85 14.75 943.76

57.81 653.79 105.74

sulfate nitrate nitrite

971.01 64.77 38.53 17584

dx.doi.org/10.1021/ie402387u | Ind. Eng. Chem. Res. 2013, 52, 17583−17590


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Table 2. Characteristics of NF Hollow-Fiber Membranes Used in the Experiments characteristic

MWCO (Da)

IEP

pH tolerance

MgSO4 rejection (%)

pure water flux (L/m2·h)

NF hollow-fiber membrane

520

6.6

2−11

96.2

47.5

back to the feed tank until it was concentrated to a certain volume. The stability of the NF hollow-fiber membrane in electroplating wastewater was investigated through the immersion experiment. The NF membrane was immersed in the electroplating wastewater after the UF pretreatment with a pH value of 2.31. After 5, 10, 20, and 40 days of immersion, this NF membrane was taken out and used to treat electroplating wastewater with a pH value of 2.31. During experiments of the membrane performance, the operating pressure was consistently kept at 0.4 MPa and the feed temperature was maintained at 25.0 °C except for special illustration. Also, the effect of the feed pH on the membrane performance was studied with the help of hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M) aqueous solutions. All of the samples were collected after 1 h of filtration except for the electroplating wastewater concentration process. In order to ensure reproducibility of the results, all experiments were carried out in duplicate. The results presented are an average of two identical experiments, and the standard deviations were less than 5% . 2.5. Analytical Methods. The permeate flux was determined by direct measurement of the permeate flow in terms of liter per square meter per hour (L/m2·h). The observed rejection rate was calculated by the relation

Figure 1. Effects of the operating pressure on the pure water and permeate fluxes as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane.

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

where Cp and Cf (mg/L) are the concentrations of heavy metals in the permeate and feed, respectively. The concentrations of metal ions of both the feed and permeate were analyzed by means of a flame-type atomic absorption spectrometer (AAnalyst 800, PerkinElmer, USA) following standard procedures.29 The analytical lines used were 357.9, 324.8, and 232.0 nm for chromium, copper, and nickel, respectively. Anion concentrations were measured by an ionic chromatograph (ICS-900, Dionex, USA). Total organic carbon (TOC) was determined by a TOC analyzer (TOC-V CPN, Shimadzu, Japan). Measurements of the pH values were carried out using a pH-meter of PHS-3D (Shanghai Precision Instrument Co., Ltd., China). A field-emission scanning electron microscope (Sirion-100, FEI, Netherlands) was used to observe the surface morphologies of the NF hollow-fiber membrane before and after fouling.

Figure 2. Effects of the feed temperature on the permeate flux as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane.

3. RESULTS AND DISCUSSION 3.1. Effects of the Operating Pressure on the NF Membrane Performance. The effects of the operating pressure on the pure water and permeate fluxes as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane were investigated with electroplating wastewater. The experiment was carried out under operating pressures varying from 0.1 to 0.6 MPa, and the results are presented in Figure 1. According to the curves in Figure 1, we can clearly see that both the pure water and permeate fluxes for the treatment of electroplating wastewater increased linearly with an increase in the operating pressure, which indicates that

Figure 3. Effects of the feed pH on the permeate flux as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane.

an increase in the operating pressure enhanced the driving force and overcame membrane resistance. Moreover, the difference between the pure water flux and the corresponding permeate flux for the treatment of electroplating wastewater also tended to increase with the operating pressure because concentration 17585



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main reason for low rejection rate. As the operating pressure increases, convective transport becomes more important compared to diffusive transport, which makes it possible to decrease the concentration of solute in the permeate, causing an increase in solute rejection rates. However, concentration polarization will also increase with increasing operating pressure, which results in a decrease in solute rejection rates by a decrease in the charge effect.24,31,32 The resulting increased heavy-metal rejection rates with increased operating pressure show that increased convective transport dominates the solute rejection behavior at all operating pressures. In addition, it can be seen that the rejection rates of different heavy metals by the membrane at a given pressure follow the sequence of chromium ions > copper ions > nickel ions, which results from the following: (1) The isoelectric point (IEP) of the NF hollowfiber membrane is approximately 6.6, and the feed pH value is 2.31. Therefore, the membrane surface exhibits a positive charge. According to the Donnan exclusion principle, heavy metals with a high valence charge will be rejected more effectively because of the stronger electrostatic repulsion exhibited by the membrane. Consequently, chromium ions will be rejected more efficiently than other heavy-metal ions. (2) For cations with the same valence charge, such as copper and nickel ions, the rejection sequence may be determined by the diffusion coefficients. The diffusion coefficients of copper and nickel in water at 25 °C are 0.78 × 10−9 and 1.32 × 10−9 m2/s,20,33 respectively. Because the diffusion coefficient of nickel is higher than that of copper, more nickel ions diffuse across the membrane, which results in a lower rejection rate. Comparable experimental results have been reported by other

Figure 4. Changes in the permeate flux versus operating time under a continuous concentration process for the NF hollow-fiber membrane.

polarization is negligible under lower operating pressures but not at higher pressures. When the operating pressure increases, the higher concentration polarization leads to an increase in the osmotic pressure and consequently to a decrease in the effective pressure, resulting in a decreased permeate flux.30 The influence of the operating pressure on the chromium, copper, and nickel rejection rates from electroplating wastewater is also shown in Figure 1. It can be seen that all of the rejection rate curves present the same trend in variation. With an increase in the operating pressure, all heavy-metal rejection rates increased gradually, which can be explained by the following. At low pressure, a high diffusive transport of solutes through the membrane compared to convective transport is the

Figure 5. Changes in the (a) chromium, (b) copper, and (c) nickel ion concentrations in the retentate and permeate and rejection rates versus operating time under a continuous concentration process for the NF hollow-fiber membrane. 17586



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hollow-fiber membrane. The results (Figure 2) clearly show that the permeate flux increased with a rise in the feed temperature. As the feed temperature was increased from 25 to 45 °C, the permeate flux increased nearly linearly from approximately 35.4 to 57.2 L/m2·h. The viscosity of the aqueous solution decreased and the solvent diffusion coefficient increased with an increase in the feed temperature, which led to an increase in the permeate flux. In addition, the average pore size of the active skin layer increased slightly when the feed temperature rose, which also favored a rise in the permeate flux.35 However, the rejection rates of chromium, copper, and nickel ions did not change significantly with an increase in the feed temperature throughout the process, which can be explained by the following. Variation of the temperature has a great effect on the adsorption of heavy metals by the membrane. With an increase in the feed temperature, more heavy-metal ions will be adsorbed on the membrane surface and then diffuse across the membrane, resulting in a decrease in the rejection rates, while the increased permeate flux with an increase in the feed temperature leads to a decrease in the concentration of heavy metals in the permeate, causing an increase in the rejection rates.36 The counteracting contributions of the above processes result in an irregular fluctuation in the heavy-metal rejection rates. 3.3. Effects of the Feed pH on the NF Membrane Performance. The effects of the feed pH on the permeate flux as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane were also tested with electroplating wastewater. The permeate flux and heavy-metal rejection rates under different feed pH values ranging from 2.3 to 9.0 are presented in Figure 3. It can be seen that the permeate flux gradually increases with increasing pH, while the pH is less than 6.0. A similar trend was also reported by Gherasim et al. and Freger et al.22,36 According to the study by Childress and Elimelech,37 the peak flux was observed near IEP at a pH value of 6.6 for the NF membrane, which can be explained by the mechanisms of membrane pore-size modification and the electroviscous effect. The membrane pore size is reduced when the membrane is charged because the charged groups adopt an extended chain configuration because of their mutual electrostatic repulsion. Therefore, from pH 2.3 to 6, the pore size of the membrane gradually expands because of decreasing positive charge, and as a consequence, the permeate flux increases. The electroviscous effect is a physical phenomenon that occurs when an electrolyte solution passes through charged pores. At low pore surface charge, the permeating solution appears to exhibit a reduced viscosity when its flow rate is compared with the flow at high pore surface charge. Thus, the membrane exhibits the highest permeate flux when the membrane is uncharged at the IEP. After the permeate flux reaches its maximum value at a pH of 6.0, it slowly starts to decline with a continued rise in the pH. This decrease in the permeate flux can be attributed to the formation of insoluble metal hydroxides between heavy metals and hydroxyl ions at high pH values, leading to serious membrane fouling due to adsorption of insoluble metal hydroxides onto the membrane pores. Moreover, the osmotic pressure near the membrane surface increases because of severe concentration polarization. With an increase in the osmotic pressure, the net transmembrane pressure tends to decrease at constant operating pressure, which results in a decrease of the permeate flux.23,37

Figure 6. FE-SEM images of (a) a new NF hollow-fiber membrane (×30K) and (b) a fouled NF hollow fiber membrane (×30K) after the electroplating wastewater concentration experiment.

Figure 7. Changes in the permeate flux as well as in the chromium, copper, and nickel rejection rates versus immersion days for the NF hollow-fiber membrane.

researchers.20,24,34 Here, it was found that, under an operating pressure of 0.4 MPa, the membrane rejection rates for chromium, copper, and nickel ions were 95.76%, 95.33%, and 94.99%, respectively. 3.2. Effects of the Feed Temperature on the NF Membrane Performance. The influence of the feed temperature was tested on the permeate flux as well as on the chromium, copper, and nickel rejection rates for the NF 17587



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to heavy metals passing through the membrane and thereby led to an increase in the heavy-metal rejection rate.23 After the electroplating wastewater concentration process, the fouled NF hollow-fiber membrane was observed by fieldemission scanning electron microscopy (FE-SEM), as shown in Figure 6. Compared to the new membrane shown in this figure, a boundary layer has formed on the fouled NF hollow-fiber membrane’s inner surface because of the deposition of heavy metals onto the membrane surface, which caused a decrease in the permeate flux and an increase in the heavy-metal rejection rate. 3.5. Stability of the NF Membrane in Electroplating Wastewater. After immersion in electroplating wastewater with a pH value of 2.31, the influence of the immersion time on the permeate flux as well as on the chromium, copper, and nickel rejection rates for the NF hollow-fiber membrane were investigated with electroplating wastewater. Changes in the permeate flux and heavy-metal rejection rates for the membrane are presented in Figure 7. From this figure, we can clearly observe that the permeate flux and rejection rates of heavy metals did not change significantly as the immersion time increased. It should be noted that the membrane rejection rates for chromium, copper, and nickel were still higher than 95%, even when the immersion time was up to 40 days. The results indicated that the NF hollow-fiber membrane could stay stable in electroplating wastewater with a pH value of 2.31.

Figure 3 also shows variation of the chromium, copper, and nickel rejection rates at different feed pH values for the NF hollow-fiber membrane. From the figure, we can observe that all rejection rates of heavy metals have an initial tendency to decrease with rising pH. The decreases in the rejection rates can be mainly attributed to decreases in the electrostatic repulsive interactions between the heavy-metal cations and the positively charged membrane surface. Additionally, it is wellknown that a precipitate of metal hydroxides can be formed from a supersaturated solution once the ion product exceeds the solubility product. The rejection rate of chromium ions reaches its minimum at a pH value of 4.0, while both copper and nickel ions exhibit the lowest rejection rates at a pH value of 5.0. These results can be attributed to differences in the solubility products of heavy-metal ions and their varying ion concentrations in electroplating wastewater. The rejection rates of heavy metals begin to significantly increase with increasing pH once the pH value has passed the minimum rejection point. This is due to the transformation of some ions into insoluble metal hydroxides, which can be easily rejected by the NF membranes. 3.4. Electroplating Wastewater Concentration. The performance of the NF hollow-fiber membrane in the electroplating wastewater concentration process was studied with 1 L of electroplating wastewater during 12 h of operation. The changes in the permeate flux versus operating time under a continuous concentration process are shown in Figure 4. It can be seen that the permeate flux of the membrane decreases with continued operating time throughout the concentration process. The continual decrease of the permeate flux was mainly due to the increase in the solution viscosity and to heavy-metal deposition onto the membrane surface with increasing solution concentration, which led to more membrane fouling and severe concentration polarization. Resistance against water flux through the membrane increased because of the boundary layer over the membrane surface formed by heavy metals. Moreover, increasing osmotic pressure due to concentration polarization also led to a decrease in the effective driving force through the membrane.38,39 Parts a−c of Figure 5 show the changes in the chromium, copper, and nickel ion concentrations in the retentate and permeate and rejection rates versus operating time under a continuous concentration process, respectively. As shown in this figure, all heavy-metal concentrations in the retentate and permeate share the same trend in variation. Over the course of operation, the heavy-metal concentrations in the retentate and permeate gradually increased. During the concentration process, the concentration of chromium ions in the retentate ranged from 121.23 to 692.87 mg/L, while the concentration of copper ions ranged from 56.55 to 320.30 mg/L and the concentration of nickel ions ranged from 142.23 to 801.02 mg/ L. The concentrations of chromium, copper, and nickel ions increased by 5.72, 5.66, and 5.63 times compared with their original feed concentrations, respectively. In spite of the increase in the retentate concentrations, the heavy-metal concentrations in the permeate only slightly increased with continued operation. It was also observed that the rejection rates of heavy metals exhibited a tendency to increase over the course of operation. Also, all rejection rates of heavy metals by the membrane were over 94.8% throughout the concentration process. This result is likely due to membrane fouling resulting from the adsorption of heavy metals onto the membrane surface. This membrane fouling provided additional resistance

4. CONCLUSIONS In this work, the performance of thin-film composite NF hollow-fiber membranes for removal of heavy metals from actual electroplating wastewater was studied. To begin with, the NF hollow-fiber membranes were used to treat electroplating industry wastewater under different operating conditions. The permeate flux and heavy-metal rejection rates of the membrane increased with increasing operating pressure. At 0.4 MPa, the rejection rates for chromium, copper, and nickel ions were 95.76%, 95.33%, and 94.99%, respectively. With an increase in the feed temperature, the permeate flux increased while the rejection rates of heavy metals did not significantly change. It was also observed that the feed pH had noticeable effects on the permeate flux and rejection rates of heavy metals. In the electroplating wastewater concentration process, heavy-metal rejection rates tended to increase with prolonged operation. The membrane rejection rates for all heavy metals were over 94.8% throughout the concentration process. Finally, the concentrations of chromium, copper, and nickel ions in the retentate were 5.72, 5.66, and 5.63 times compared with their original feed concentrations. In addition, the NF hollow-fiber membrane showed good stability in electroplating wastewater with a pH value of 2.31.

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The authors declare no competing financial interest. 17588



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ACKNOWLEDGMENTS Financial support from the Ministry of Science & Technology of China (Grant 2011BAE07b09), State Administration of Traditional Chinese Medicine of Zhejiang Province (Grant 2013ZQ006), and Zhejiang Provincial Department of Environmental Protection (Grants 2012B013 and 2013A029) is gratefully acknowledged. The authors also greatly thank the Natural Science Foundation from Zhejiang Province (Grant LY13H280009).

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