flocculation and nanofiltration

Journal of Membrane Science 469 (2014) 306–315

Contents lists available at ScienceDirect

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

Treatment of highly concentrated wastewater containing multiple synthetic dyes by a combined process of coagulation/flocculation and nanofiltration Can-Zeng Liang, Shi-Peng Sun n, Fu-Yun Li, Yee-Kang Ong, Tai-Shung Chung n Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

art ic l e i nf o

a b s t r a c t

Article history: Received 11 April 2014 Received in revised form 26 June 2014 Accepted 29 June 2014 Available online 8 July 2014

The treatments of dyes (acid, basic and reactive dyes) wastewater were studied by applying individual coagulation/flocculation (CF) and nanofiltration (NF) processes as well as their combination (referred as CF–NF). For the treatment of highly concentrated multiple dyes wastewater (MDW, 1000 ppm), polyaluminum chloride (PAC) and polydiallyldimethyl ammonium chloride (PDDA) were found to be the most effective coagulant and flocculant, respectively. The CF process can achieve about 90% of dye removal at the optimal dosage of PAC/PDDA¼400/200 ppm, and the MDW with pH 4 3 is favorable for the CF treatment. A positively charged NF hollow fiber membrane was fabricated and used for NF treatment. It is able to remove almost 100% dyes with a permeate flux of about 1.0 L m  2 h  1 under an operating pressure of 1 bar. The combination of CF and NF can complement each other's strengths and overcome their individual limitations. The NF treatment can completely remove the strong color left in CF treated dye solutions, while the efficiency of coagulant/flocculant is improved by treating NF concentrated streams and subsequently results in much less sludge. In addition, membrane fouling is abated and NF permeate flux is increased by applying the CF process as a pretreatment. Thus, the combination of CF–NF improves the overall performance for the dyes wastewater treatment. & 2014 Elsevier B.V. All rights reserved.

Keywords: Multiple dye wastewater Coagulation/flocculation Nanofiltration Hollow fiber membranes

1. Introduction A dye molecule consists of two components; namely, the dye chromophore and the dye auxochrome. When the dye molecule is exposed to light, the chromophore structure which includes double bonds (CQC) oscillates to absorb light and generates visible color [1,2]. By estimation, more than 100,000 synthetic dyes and over 700,000 t of dyestuff are produced annually [3,4]. The color that appears in industrial wastewater effluents caused by residual dyes is esthetically undesirable and harmful to the environments and the ecosystems. Even a very low concentration of dyes can generate strong color [5]. The dye wastewater is generated during dye-related activities such as dye production, textile dyeing, leather tanning and paper production, etc [6]. Particularly in the textile industry, about 10–15% dyes are lost in dyeing processes, and typically 200–350 m3 wastewater are generated to produce one ton finished product [7]. It is becoming more difficult to directly discharge this kind of wastewater

n

Corresponding authors. Tel.: þ 65 6516 6645; fax: þ65 6779 1936. E-mail addresses: [email protected] (S.-P. Sun), [email protected] (T.-S. Chung). http://dx.doi.org/10.1016/j.memsci.2014.06.057 0376-7388/& 2014 Elsevier B.V. All rights reserved.

because the legislations related to environments are becoming more stringent in many countries [8]. Generally, there are two decolorization methods [9]; namely, destruction of dye molecules and separation of dyes from water. To destruct or transform the dyes, conventional processes are usually applied, such as chemical oxidation, photo-catalysis and biodegradation [10]. However, the destruction methods is found to be inadequate and require extensive energy to break down the dye molecules, most of which are stable to light, oxidizing agents and microbiological degradation [1,6,11]. The separation methods include adsorption, coagulation/flocculation (CF) and membrane separation [9]. Adsorption of dyes on powder activated carbon is popular and effective. However, the activated carbon is not cheap, and the adsorption performance is reduced sharply after regeneration or reactivation, which also results in a 10–15% loss of the sorbent [3,7,12]. Coagulation/flocculation is widely used for dyes removal due to its low capital cost and simple operation [12–14]. Coagulation of dye solutions is a process of destabilizing the dye solution systems to form agglomerates or flocs. Flocculation is the process of destabilizing suspended particle systems and bridging the aggregated flocs to form larger agglomerates that settle down under gravity [15,16]. Charge neutralization is regarded as a prerequisite condition in most of coagulation processes [17]. The CF process generates sludge and but sometimes it is ineffective

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

for some soluble dyes. In fact, it is always challenging in selecting appropriate coagulant/flocculant due to the large number of existing and emerging dyes that have complex structures [7]. Another important separation method is the pressure-driven membrane technology including ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [1]. Ultrafiltration has been successfully used for separating high molecular weight and insoluble dyes from water. However, UF is not able to remove those water-soluble dyes with low molecular weights [1,18]. Although efficient dye removal can be achieved by RO, the high pressure required in the RO process hinders its wide applications to treat dye wastewater. NF is positioned in between UF and RO [9], the nominal molecular weight cutoff (MWCO) of NF membranes is in the range of 100 to 1000 Da and with the pore size of about 0.5–2.0 nm [19,20]. NF was introduced in the early 1980s and has become popular due to its low operating pressures and relatively low capital and operating costs [20]. Currently most NF membranes are negatively charged flatsheet composite membranes, which may have high fouling tendency because most foulants are positive charged. In recent years, there is a growing interest to fabricate hydrophilic and positively charged NF membranes which are less prone to organic fouling [19–30]. Compared to the sophisticated module fabrication process for flat-sheet or spiral-wound membranes, it is convenient to fabricate hollow fiber membrane modules, which also provide a high surface area per unit volume and thus to reduce manufacturing costs [31]. NF hollow fiber membranes have attracted much attention due to its superiorities to the conventional flat-sheet NF membranes. Recently, several efforts have been made on the development and application of NF hollow fiber membranes for removing dyes from water. For instance, Sun et al. employed both crosslinking and interfacial polymerization to coat a positively charged polyethyleneimine (PEI) layer on the outer surface of a negatively charged polyamideimide (PAI) hollow fiber substrate to produce a double-repulsive NF membrane that can remove 99.8% of the positively charged Safranin O and 98.8% of the negatively charged Orange II sodium salt [19,21]. Shao et al. developed a thinfilm composite NF hollow fiber membrane by interfacial polymerization on the inner surface of polyetherimide (PEI) hollow fiber supports for removal of Safranin O and Aniline blue and achieved higher than 90% dye rejections [25]. Wei et al. fabricated a NF hollow fiber membrane through interfacial polymerization on a polysulfone/polyethersulfone supporting membrane for the treatment of reactive brilliant blue X-BR and acid red B dye solutions and obtained above 99.9% dye rejections [26]. Zheng et al. developed positively charged TFC hollow fiber NF membranes via the dip-coating method on polypropylene hollow fiber microfiltration membranes. The resultant NF membrane can achieve dye rejections

307

of 99.8%, 99.8% and 99.2% for Brilliant green, Victoria blue B and Crystal violet, respectively [27]. However, the drawbacks of membrane technologies including NF are the flux decline caused by membrane fouling and the generation of concentrated streams [32]. To minimize the flux reduction, one approach is to implement a right pretreatment process [33], the other approach is to produce fouling-resistant membranes [34]. The generation of concentrated streams is an intrinsic issue for membrane separation processes since membranes only achieve separation rather than destruction or transformation. For the dye-containing wastewater treatment, the concentrated stream is usually an unwanted by-product. It must be further treated before discharge [35,36]. To overcome the aforementioned problems, a combination of various separation methods is necessary to achieve a high dye removal and high separation efficiency. To our best knowledge, there is no study on treatment of synthetic dye wastewater containing multiple dyes of different classes by a combined process of CF and NF with a positively charged NF hollow fiber membrane. In this work, reactive, acid and basic dyes were chosen because of their popularity in textile and dye industries as well as environmental concerns [6,7,12,37]. The representative dye concentrations in dye wastewater streams are in the range of 0.05–0.1 g/L [38]. For this study, aqueous dye solutions with a dye concentration of 1000 ppm (1.0 g/L) were prepared to simulate the highly concentrated dye house waste effluent and the concentrated stream after membrane filtration. In order to maximize the efficacy of the CF process and minimize the generation of sludge, proper CF formulations were carefully evaluated, screened and selected. To reduce membrane fouling, hydrophilic and positively charged NF hollow fiber membranes were made in our laboratory for this study. The CF, NF and the combination of CF–NF were studied to remove dye(s) from synthetic dye wastewaters, which consist of either single dye or multiple dyes. The objectives of this work are to demonstrate that both CF and NF techniques can remove dyes from wastewater effectively and the combination of CF–NF is able to improve the overall performance. This could lead to a new approach for the dye wastewater treatment by a hybrid system.

2. Experimental 2.1. Chemicals and materials 7 inorganic and 4 organic chemicals, listed in Table 1, were chosen and used as coagulants and flocculants in this study. 5 dyes (2 acid dyes, 2 reactive dyes and 1 basic dye, purchased from

Table 1 Characteristics of coagulants and flocculants used in this work. Name of coagulant/flocculant

Code name

Molecular formula

Molecular weight (g/mol)

Purity

Polyaluminum chloride Aluminum sulfate-octadecahydrate Aluminum potassium sulfate-dodecahydrate Iron(III) chloride-anhydrous Iron(III) sulfate-pentahydrate Calcium oxide Magnesium chloride-anhydrous Cationic polyacrylamide Anionic polyacrylamide Polydiallyldimethyl ammonium chloride (cationic, dissolved in water) Cyanoguanidine

PAC AS APS IC IS CO MC CPAM APAM PDDA CYGU

Aln(OH)mCl(3n  m) (0o mo 3n) Al2(SO4)3  18H2O AlK(SO4)2  12H2O FeCl3 Fe2(SO4)3  5H2O CaO MgCl2 a (CH2–CH–CONH2)m a (CH2–CH–CONH2)m (C8H16ClN)n NH2C(¼NH)NHCN

Z 115b 666 474 162 490 56 95 800–1000 million 800–2500 million 200,000–350,000 84

Z30% (as Al2O3) Extra pure 99.5% 98% 97% Z98 % pure Z90% Z90% 20 wt% Z99 %

a b

Obtained from the specification of the products. Calculated by assuming m¼n ¼1 for PAC.

308

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

Aldrich-Sigma (Singapore)) were selected for CF and NF studies. Their characteristics are compiled in Tables 2 and 3. For simplicity and easy reading, these coagulants, flocculants and dyes will be referred in the text as their corresponding code names shown in Tables 1 and 2 thereafter unless state otherwise. Deionized water (DI water, Milli-Q water) was used to prepare the dye wastewater during the experiments. Sodium hydroxide (NaOH, 1.0 M) and hydrochloric acid (HCl, 1.0 M) were applied to adjust the solution pH. 2.2. The preparation of the positively charged NF hollow fiber membrane The membrane used in this work is a recently developed positively charged NF hollow fiber membrane, which was prepared by cross-linking hyperbranched PEI on the outer surface of a PAI hollow fiber [19]. The PAI hollow fiber was dry-jet wet-spun from a dope formulation of PAI/ethylene glycol (EG)/N-methyl-2pyrrolidinone (NMP) ¼21.5/13.5/65.0 wt% using a dope flow rate of 6.0 ml/min, bore fluid flow rate of 3.6 ml/min, air gap of 5.0 cm, and a take up speed of 50 m/min. The resultant PAI hollow fiber was cross-linked by 0.5 wt% PEI (60,000 g/mol, 50 wt% aqueous solution, Acro, USA) that was dissolved in a 1:1 mixture of Table 2 Characteristics of dyes tested in this work. Name of dye

Code name

Type of dye

Molecular formula

Molecular weight (g/mol)

Indigo carmine Remazol brilliant blue R Brilliant blue R Reactive black 5

INCA RBBR

C16H8N2Na2O8S2 C22H16N2Na2O11S3

466 627

BB-R RB-5

C45H44N3NaO7S2 C26H21N5Na4O19S6

826 992

Alcian blue 8GX

AB-8

Acid dye Reactive dye Acid dye Reactive dye Basic dye

C56H68Cl4CuN16S4

1299

isopropanol (IPA) and water. The cross-linking process was conducted at 70 1C for 60 min. A full description about this NF membrane can be found elsewhere [19]. 2.3. Coagulation/flocculation experiments The 1000 ppm synthetic single dye wastewater (SDW) was prepared by dissolving one dye of 1000 mg in 1.0 L DI water. SDW for each of the aforementioned 5 dyes was therefore prepared. Synthetic multiple dye wastewater (MDW) is a mixture of 5 SDWs in an equivalent volume ratio. The total dye concentration of MDW is 1000 ppm and each individual dye concentration is 200 ppm. The other dye concentrations were diluted from the 1000 ppm stock dye solutions according to working conditions. The jar-test technique was adopted to carry out coagulation/flocculation experiments, where an appropriate amount of coagulant and/or flocculant was added to a 100 g dye solution in a 250 ml glass beaker and stirred at a speed of 200 rpm for 1 min via using an agitator (Heidolph, RZR 2041) equipped with a plastic stirring blade. After 1-h settling, the supernatant solution was carefully poured out and filtered through Whatman #1 (diameter¼47 mm, pore size ¼11 μm) and Whatman #42 (diameter ¼47 mm, pore size¼2.5 μm) filter papers by using a vacuum filtration apparatus (Synthware glass filtration apparatus, 47 mm fritted disc, I L flask, KNF/Germany diaphragm vacuum pump). The filtrates were then kept for further analyses. 2.4. NF experiments The NF experiments were performed in a lab-scale NF setup [39]. Each membrane module contains 40 pieces of hollow fiber membranes, which have inner and outer diameters of around 280 and 430 mm, respectively. The effective length and membrane area of each module is about 22 cm and 120 cm2, respectively. The feed solution was circulated at the shell side of the hollow fiber module at

Table 3 Molecular structures of dyes and polydiallyldimethyl ammonium chloride.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

ð1Þ

where Q is the volume (L) of permeate collected during a specific period of sampling time t (h), A is the effective membrane area (m2), and ΔP is the transmembrane pressure (bar). The permeate flux (J, L m  2 h  1) can be determined by the following equation: J¼

Q At

ð2Þ

where Q is the volume (L) of permeate collected during a specific period of sampling time t (h), and A is the effective membrane area (m2). 2.5. Physicochemical analyses Two methods were chosen to determine the dye concentration; namely, total organic carbon (TOC) method and UV–vis integral method. The implementation of these two methods depends on specific situations as discussed later. The TOC method was based on the TOC concentration determined by a TOC analyzer (TOC, ASI 5000A, Shimazu). Appendix Fig. 1 shows the calibration curves of TOC values as a function of dye concentration for various dyes. The TOC value has an almost linear relationship with dye concentration in the range of 0 to 1000 ppm. The dye removal (Rd, %) was calculated by the following equation: Rd ¼

ðC b  C a Þ  100% Cb

ðI b  I a Þ  100% Ib

ð4Þ

where Ib and Ia are the UV–vis integral values of the dye solution before and after the CF treatment, respectively. The dye removal (Rd, %) by the NF membrane was calculated by the following equation: Rd ¼

ðI f  I p Þ  100% If

ð5Þ

where If and Ip are the UV–vis integral values of the feed and the permeate solutions, respectively. It is worthy to point out that the concentrated dye solutions had to be diluted to around 100 ppm when the UV–vis integral method was used in order to achieve reasonable accuracy. The normalized efficiency of coagulant/flocculant (Ec) is defined as Ec ¼

md mc

ð6Þ

where md is the weight of the dye removed by CF, while mc is the total weight of coagulant/flocculant added for the CF treatment. The particle size of the settlement after CF was analyzed by using a laser diffraction particle size analyzer (Beckman Coulter LS230, analysis range: 0.04 mm to 2000 mm). The solution pH was determined by using a pH meter (pH/ion S220, Mettler Toledo). The aluminum concentration was measured using Inductively

100

4.0 3.5

90 MDW=1000 ppm

ð3Þ

80 70

2.0

Ec

1.5 1.0 0.5

50 0

400

800

1200

1600

100

4.0 3.5

Rd (%)

Dye removala (%) INCA

RBBR

BB-R

RB-5

AB-8

3.0 MDW=100 ppm

80

Rd Ec

70

MDW

95.6 64.2 51.1 72.8 27.4 0 29.2

90.0 42.6 42.7 59.2 44.9 0 12.6

85.5 81.0 84.3 84.6 84.1 43.1 24.5

87.6 44.3 25.6 22.3 21.3 0 0

0 0 0 0 0 75.2 0

90.2 75.7 72.1 69.5 65.3 59.9 34.4

a Dye removal was based on the TOC method. The coagulant dosage was 1000 ppm, the original dye concentration was 1000 ppm.

2.5 2.0 1.5 1.0

60 PAC AS APS IC IS CO MC

0.0 2000

PAC dosage (ppm)

90

Coagulant

2.5

Rd

60

where Cb and Ca are the TOC concentrations of the dye solution before and after the CF treatment, respectively. The UV–vis integral method was based on the UV–vis integral (integrated range: 350–650 nm) obtained through scanning the sample by a UV–vis spectrophotometer (Pharo 300, Merck). Appendix Fig. 2 shows the calibration curves of UV–vis integral values as a function of dye concentration for various dyes. The UV– vis integral has a linear relationship with dye concentration when Table 4 Preliminary performance evaluation of coagulants on dye removal.

3.0 Ec

Q ΔPAt

Rd ¼

Ec

PWP ¼

the dye concentration is below 100 ppm. However, the relationship becomes non-linear when the dye concentration is above 100 ppm. Based on UV–vis integral, the dye removal (Rd, %) by CF was calculated by the following equation:

Rd (%)

a flow rate of 0.15 L/min and a pressure of 1.0 bar. The permeate solution was collected from the lumen side of the membrane. The retentate stream was circulated back to the feed solution. Each NF experiment included 3 steps: (1) the initial pure water permeability (PWP, L m  2 bar  1 h  1) was measured using DI water at 1 bar; (2) a 1.0 L dye solution was used as the feed solution and circulated at 1 bar. Then permeate samples were taken at each consecutive time interval of 1 h. Finally, DI water was used to flush the system and the module thoroughly until the circulating water was clean, then the final PWP was measured. Pure water permeability (PWP, L m  2 bar  1 h  1) was calculated according to the following equation:

309

0.5 50 0

40

80 120 160 PAC dosage (ppm)

0.0 200

Fig. 1. Coagulation performance of PAC for MDW treatment at different dye concentrations: (a) the dye concentration in MDW was 1000 ppm; (b) the dye concentration in MDW is 100 ppm. Dye removal is based on the UV–vis integral method.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

3. Results and discussions 3.1. Coagulation and flocculation Both single dye and multiple dye wastewaters (referred to as SDW and MDW, respectively, hereafter) were studied in order to evaluate the effectiveness of coagulants and flocculants in removing various types of dyes, and to select the most proper coagulant and flocculant. The following procedures were carried out: firstly to screen and select a proper coagulant; secondly to optimize the coagulant dosage; thirdly to identify the suitable flocculant (or coagulant aid); and finally optimize the flocculant dosage.

3.1.2. Optimization of the coagulant dosage The dye removal is also studied by the UV–vis integral method to identify the optimal coagulant dosage because of the following reasons: (1) the PAC solution induces no visible color change; (2) the organic flocculant may contribute to the TOC value. Fig. 1 shows examples of multiple dyes removal from MDW at different dye concentrations. For the 1000 ppm MDW treated by PAC, the dye removal increases sharply with an increase in PAC dosage initially, but finally approaches a plateau, which indicates that the maximum dye removal by coagulation is around 97%. Mathematically, the 2.5

90

2.0

80

1.5

70

1.0 Rd

50 0

100

200

300

0.0 400

Dosage of PDDA (ppm) Fig. 3. Effect of different dosages of PDDA combined with 400 ppm PAC to treat 1000 ppm MDW. Ec refers to the overall normalized efficiency of PAC and PDDA. The dye removal was based on the UV–vis integral method.

100

100

80

80

60

60

40

0.5 Ec

20

40 20

0

0 APAM

PAC

PAC -APAM

100

100

80

80

60

60

Rd (%)

Rd (%)

100

60

Rd (%)

Rd (%)

3.1.1. Selection of the proper coagulant Table 4 summarizes the dye removal for both SDW and MDW based on the TOC method. The reasons of choosing the TOC method are: (1) to exclude the color interference introduced by ironic ion (Fe3 þ ); (2) To avoid a large pH change that may change the intensity of UV–vis absorbance caused by the coagulants. This is particularly true for calcium oxide (CO) because 1000 ppm CO can dramatically change the pH value of dye solutions; (3) there is an almost linear relationship between TOC and dye concentration in the range of 0–1000 ppm, which makes it simple and reliable for analyses. It can be observed from Table 4 that an inorganic coagulant can be very effective in treating certain dyes, but totally ineffective for the others. In the case of MDW, every coagulant appears to be more or less effective, this may suggest that interactions between dyes take place and subsequently change the MDW's charge characteristics. As a result, when a coagulant is added into the dyes solution, charge neutralization initially occurs and promotes to formation of particles/flocs; then dyes may be also involved in the process of co-precipitation and sorption onto flocs; finally the

coagulation happens and dyes are removed [7,15,17,40]. The ability of removing dyes from the MDW sample by the 7 coagulants is in the order of PAC 4AS 4APS 4IC 4IS 4CO4MC. Therefore, PAC was identified as the most proper coagulant for further studies.

Ec

Coupled Plasma-Optical Emission Spectrometry (ICP-OES, iCAP 6000 Series, Thermal Scientific).

Rd (%)

310

40 20

CPAM

PAC

PAC -CPAM

PDDA

PAC

PAC -PDDA

40 20

0

0 CYGU

PAC

PAC -CYGU

Fig. 2. Performance of PAC and different organic flocculants to treat 1000 ppm MDW. The dosage of each PAC, APAM, CPAM, CYGU and PDDA was 400 ppm; For the PACflocculant combination, the dosage of each component was 400 ppm. Dye removal was based on the UV–vis integral method.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

8

6 Volume (%)

6

4

4 PDDA=0 ppm PDDA=100 ppm 2

PDDA=200 ppm PDDA=300 ppm PDDA=400 ppm

2

0 3

6

9 12 Particle size (µm)

15

0

0

20

40

60

80

100

Particle size (µm) Fig. 4. Particle size (or diameter) distributions of CF settlements when 1000 ppm MDW was treated by 400 ppm PAC without and with PDDA.

12

100

10

80 Rd: PAC

8

Rd: PAC-PDDA

60

Final pH: PAC 6

Final pH: PAC-PDDA 40

Final pH (-)

3.1.4. Optimization of the flocculant dosage The effects of different PDDA dosages on dye removal were studied by fixing the PAC dosage at 400 ppm. Fig. 3 displays that the dye removal increases with an increase in PDDA dosage. This might be attributed to: (1) PDDA possesses a high positive charge density (as shown in Table 3) and water solubility. It can function as an effective coagulant in the PAC-dye solution system; (2) PDDA serves as a flocculant that enhances the bridging mechanism for coagulation and improves the aggregating capacity [16], thus increasing the coagulation ability of PAC. However, the Ec value (i.e., the efficiency per gram of the total coagulants (400 ppm PACþ PDDA)) decreases when the PDDA dosage increases. Therefore, The CF settlement's particle size was investigated to optimize the PDDA dosage. Fig. 4 displays the particle size (or diameter) distributions of CF settlements when 1000 ppm MDW was treated by 400 ppm PAC

8

Volume (%)

3.1.3. Identification of the suitable flocculant A typical coagulation/flocculation process involves two stages of mixing process; namely, rapid stirring (75–700 rpm) for 0.5–3 min in order to achieve a good dispersion of the coagulant/flocculant, and slow stirring (30–150 rpm) for 5–30 min in order to propagate the growth of flocs and minimize the breakdown of formed aggregates [16,42]. In this work, the experiments associated with both coagulant and flocculant, the rapid and slow stirring processes were combined into one single stirring scheme; namely, stirring at 200 rpm for 1 min. Adopting the single stirring scheme is due to the following reasons: (1) to simplify the process; (2) to allow the coagulation and flocculation take place together. Some organic polyelectrolytes, such as CPAM and PDDA, exhibit the ability of functioning as both coagulant and flocculant, which is demonstrated in Fig. 2 with following discussions. In terms of dyes removal by coagulation/ flocculation, as depicted in Fig. 2(a) and (c), neither APAM nor CYGU is effective in dye removal. The combination of PAC-APAM does not make much improvement compared with PAC alone; while the combination of PAC-CYGU produces antagonistic effect, which reduces the efficacy of PAC. As shown in Fig. 2(b) and (d), both CPAM and PDDA alone are effective but with less efficiency than PAC. The combinations of PAC-CPAM and PAC-PDDA generate synergetic effects, which improve the total dye removal compared to that of PAC alone. Both CPAM and PDDA are positive charged, but PDDA is superior to CPAM according to its individual or combined performance. Additionally, the suspension produced by PAC-PDDA was easier to filter out than that by CPAM-PDDA. This is due to the fact that CPAM possesses a huge molecular weight (800–1000 million) which is prone to entangling with the filter paper and forming a gellike sticky film that blocks the filter pores. In summary, PDDA was identified as the most proper flocculant among the 4 examined flocculants.

without and with PDDA. The particle sizes were analyzed after CF experiments and 1 h settling. As observed at the bottom of Fig. 4, trace amounts of large-size particles in the range of 50–100 mm are formed with the addition of PDDA. In contrast, there are no particles with a size larger than 30 mm for the CF settlement without adding PDDA. This is probably owing to the fact that the fine precipitates formed by PAC are not able to grow bigger without the aid of flocculant (PDDA). As shown in the enlarged picture of Fig. 4, the particle size distribution curve shifts toward the right side when increasing the PDDA dosage. This suggests that the mean particle size becomes larger with an increase in PDDA dosage. Without PDDA, a mean particle size of 7.4 mm is produced by PAC, the mean particle size increases to about 7.8, 9.2, 9.3, or 9.7 mm when adding 100, 200, 300 or 400 ppm PDDA, respectively. The bigger the particle size, the easier the filtration is. It is found that the combination of 400 ppm PAC and 200 ppm PDDA produces a settlement with a moderate size of  9.3 mm and an acceptable dye removal of  90% with a relatively high normalized efficiency (Ec) of 1.5. Therefore, the (400/200 ppm) PAC/PDDA dosage was chosen to study the effects of pH.

Rd (%)

removal efficiency per gram of the coagulant (Ec) declines accordingly. For the 100 ppm MDW treated by PAC, Fig. 1(b) shows that the dye removal reaches a maximum when about 100–150 ppm PAC is added and then decreases with a further increment in PAC dosage. The exceeded amount of PAC might lead to the reverse of surface charge of the coagulated particles/flocs, thus resulting in particles re-stabilization and lowering the treatment efficiency [41]. Before the dye removal reaches the maximum, the Ec value of Fig. 1 (a) is always greater than that of Fig. 1(b), which implies coagulation taking place more favorably in a high dye concentration than a low dye concentration. In order to achieve a high dye removal without much compromising the coagulant efficiency, we chose a PAC dosage of 400 ppm because it can remove about 85% dyes with an Ec value of about 2.1 for further studies to identify a proper flocculant.

311

4 20

2 0

0 0

2

4

6

8

10

12

Initial pH (-) Fig. 5. Effects of the initial pH on the dye removal for 1000 ppm MDW when using PAC (400 ppm) and PAC-PDDA (400/200 ppm). Dye removal was based on the UV–vis integral method.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

3.2. Combination of CF and NF treatments Since the CF treatment is not able to achieve a complete dye removal and a strong color is still visible, NF was adopted to further process the wastewater after the CF treatment. 1.5 L of 1000 ppm MDW was first subjected to the CF treatment using the optimal CF formulation of (400/200 ppm) PAC/PDDA. After 90.4% dyes were removed and their settlements were filtered out, the left-over CF-treated solution was used as the feed solution for subsequent NF experiments. 3.2.1. Performances of the NF treatment NF experiments were performed for various dye solutions. As shown in Table 5, almost 100% dye removal was achieved for all dye solutions. In order to evaluate membrane fouling, pure water permeability was measured before and after NF experiments. The membrane modules have initial PWP values of around 3.0 to 3.2 L m  2 h  1 bar  1. The final PWP values vary from 0.9 to 3.0 L m  2 h  1 bar  1. Except for AB-8, the overall PWP recovery of all membrane modules is about 30%. The PWP reduction caused by fouling on such positively charged hollow fiber membranes can be effectively recovered through chemical cleaning, which has been reported in our recent paper [43] and is out of the scope in this study. It was observed that the membranes were colored after NF experiments, this happened because the hollow fiber membrane used in this study was made of polyamide–imide (PAI). Polyamide

based fibers can be dyed by most acid and reactive dyes through physiochemical bonding and impregnation of colloidal dye particles into the fiber [1]. Interestingly, for the case of AB-8, the membrane was not colored after NF experiments, and its final PWP was nearly equal to its initial PWP. This suggests that the membrane was not fouled by the AB-8 dye probably due to: (1) AB-8 has a relative large molecular weight (1299 g/mol) and therefore it is difficult to enter the pores of membranes; (2) Both AB-8 and the membrane are positively charged, they would repulse each other; and (3) A basic dye does not often adhere onto the polyamide-based fiber without properly regulating pH [1,20]. In summary, the NF process reveals excellent results in terms of dye removal or rejection regardless of their types and molecular weights. Except for the case of AB-8, the permeate flux is around 1.0 L m  2 h  1, which is about one third of the initial PWP. By comparing the case of 1000 ppm MDW with the one of 100 ppm, the initial dye removal and permeate flux are independent of dye concentration in these wastewater streams. The charge of acid dyes is negative while that of basic dyes is positive. As shown in Tables 2 and 5, the dye removals for both acid dyes (Indigo carmine (INCA), Brilliant Blue R (RB-5)) and basic dye (Alcian Blue 8GX (AB-8)) are nearly equally high. This might be ascribed to the double-repulsion effect coupling with the sterichindrance and the solute electro-neutrality effects [21]. In other words, since the selective layer (PEI) of the membrane is positively charged while the support (PAI) is negatively charged [19,21], the NF membrane possesses double-repulsion functions. The positively charged selective layer repulses positive dyes (basic dyes) while the negatively charged substrate repulses negative dyes (acid dyes). As

1.5

100.0

1.3 95.0

1.1 90.0

Rd (%)

The effects of the initial pH on dye removal when PAC (400 ppm) and PAC-PDDA (PAC/PDDA ¼400/200 ppm) were applied for the treatment of 1000 ppm MDW are shown in Fig. 5. Both PAC and PAC-PDDA exhibit a very stable and good dye removal capacity at the initial pH ranging from 3 to 10. Above pH 10, dye removal by PAC-PDDA increases with an increase in pH. On the contrary, dye removal efficiency by PAC alone decreases. This implies that an initial pH value higher than 10 is favorable for PAC-PDDA but unfavorable for PAC. However, at an initial pH value lower than 3, both PAC and PAC-PDDA show much lower performance. Moreover, the dye removal efficiency of PAC-PDDA is superior to PAC in the whole pH range. For a given dye solutions, in the CF process, the final pH is determined by the initial pH and the added chemicals (coagulant/flocculant). The original pH of 1000 ppm MDW is around 4.6, after CF treatment, the initial alkali pH ( 47) decreases dramatically and downs to below 5. When PAC is used with PDDA, the final pH is slightly higher than that of using PAC alone, this might suggest that the final pH is majorly affected by PAC. Based on the above results and discussions, and by considering the balance between the factors of dye removal efficiency, normalized coagulant/flocculant efficiency (Ec), easiness of filtration and workable pH values, a conclusion is therefore drawn that PAC/PDDA ¼400/200 ppm is chosen as the optimal CF formulation to treat 1000 ppm MDW for the further study in the following CF–NF process.

J (Lm-2h-1)

312

0.9 J:100 ppm J:1000 ppm J: After CF Rd:100 ppm Rd:1000 ppm Rd: After CF

0.7

0.5 1

2

3

85.0

80.0 4

5

6

7

8

Running time (hour) Fig. 6. Performance of the NF process for MDW without and with the CF treatment. Operating pressure is 1 bar. Dyes removal was based on the UV–vis integral method.

Table 5 Performance of the NF treatment on various dye solutionsa. Dye concentration:

1000 ppm

Dye solution 2

1

1

Initial PWP (L m h bar ) Permeate flux (L m  2 h  1) Dye removalb (%) Final PWP (L m  2 h  1 bar  1) a b c

After CF

c

100 ppm

INCA

BRBR

RBBR

RB-5

AB-8

MDW

MDW

MDW

3.0 1.0 99.9 0.9

3.0 0.7 99.9 0.9

3.0 0.6 99.9 1.1

3.2 0.9 100.0 1.1

3.1 3.0 100.0 3.0

3.0 1.1 99.9 1.3

3.2 0.9 99.3 1.0

3.2 1.1 99.7 1.2

Operating pressure was 1 bar. The values were the average of two results measured at the first two hours of each experiment. Dyes removal was based on the UV–vis integral method. 1000 ppm MDW was treated by CF, 90.4% dyes were removed and about 100 ppm dyes were remained.

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

60.0

100.0 90.0

70.0

40.0

60.0 30.0

NF permeate

40.0 CF treated MDW

10.0

CF settlement stream

20.0

50.0

0.0

30.0

Distribution of Al (%)

80.0

Total added Al for CF

Concentration of Al (ppm)

50.0

20.0 10.0 0.0

Total

Solid

Feed

Effluent

Fig. 7. The fate and distribution of aluminum during the CF–NF treatment process.

Original After CF After CF-NF

50

40 Absorbance

a result, the NF membrane shows excellent rejections for both positively and negatively charged dye molecules. Fig. 6 displays the permeate flux and dye removal as a function of time for MDW. Two MDW concentrations were employed; namely, 100 and 1000 ppm. The 1000 ppm MDW was treated with and without the CF treatment. For all cases, the permeate flux declines initially but becomes stable after 7 h, while the dye removal remains above 99%. For the 100 ppm and 1000 ppm MDW, they have almost the same initial permeate flux and decline in the same speed until the fourth hour. After the fourth hour, the permeate flux of the 100 ppm sample tends to drop slightly, whereas that of the 1000 ppm sample drops dramatically. After 7 h, both permeate fluxes become stable, and the permeate flux of the 100 ppm sample is approximately 20% higher than that of the 1000 ppm sample. These phenomena may be explainable as follows: (1) the dyes adhered to the membrane surface and partially blocked the surface pores in the beginning; (2) membrane fouling due to the concentration polarization of the dyes became serious for the 1000 ppm sample after 4 h. Since the higher the dye concentration, the heavier the concentration polarization is [44], the flux decline of the 100 ppm sample is mainly controlled by membrane fouling, while that of the 1000 ppm sample is affected by both membrane fouling and concentration polarization fouling. Since about 90% dyes were removed after the CF treatment, the dye concentration in the CF treated sample is around 100 ppm. As a result, the relationship between permeate flux vs. time of the CF treated sample is nearly parallel with that of the 100 ppm MDW sample, but the former has a lower flux (i.e., about 10%) than the latter. The similar relationship between permeate flux vs. time indicate that membrane fouling for both cases are similar due to their similar dye concentrations. The lower flux of the CF treated sample may arise from the osmotic pressure increase in the CF treated sample. As shown in Fig. 7, about 24 ppm aluminum ion is found in the CF treated solution. As a consequence, the effective driving force across the NF membrane is reduced even though they have the same transmembrane pressure [45,46]. Since the CF treated sample has less fouling propensity, it has a higher flux than that of the 1000 ppm sample after the 7-h test (i.e., about 10% higher). Clearly, applying the CF process as a pretreatment, not only can it lower membrane fouling but also sustain the permeate flux. In terms of rejections, nearly 100% dyes were removed by the NF for all the 3 cases over 8-h tests. The fate of added aluminum was also studied for the CF–NF process. As shown in Fig. 7, about 58% of the total added aluminum (i.e., 57 ppm) reacted and

313

30 20

a

10

b 0 200

c 300

400

500

600

700

800

Wave length (nm) Fig. 8. The UV–vis absorbance spectra of (a) the original MDW (1000 ppm), (b) solution after the CF treatment and (c) solution after the NF–NF treatment. The UV–vis spectrum of the original MDW was obtained via diluting the original MDW by 20 times, then the diluted sample was measured, the resultant absorbance was multiplied by a factor of 20.

remained in the settlement as the solid stream, while around 41% dissolved and left in the treated solution as the feed stream for the NF process, and about 0.5% finally went to the permeate flux as the effluent stream. Ideally, there should be no aluminum in the NF feed stream after the CF process since the aluminum ion would lead to an increase in osmotic pressure and ultimately a decrease in effective driving force across the membrane. Additionally, the less aluminum in the effluent stream, the better it is since the effluent would eventually be discharged or reused. The UV–vis was also used to confirm the above conclusion. Fig. 8 displays the UV–vis spectra in a wavelength range between 200 and 800 nm for the original 1000 ppm MDW, the CF treated MDW, and the NF permeate of the CF treated sample. The absorbance decreases significantly after the CF treatment because of about 90% dyes removal at this stage. The UV absorbance drops to almost zero after the NF process. This is consistent with the aforementioned results of almost 100% dye removal. The pictures at the up right side of Fig. 8 show the color changes during the processes. The original 1000 ppm MDW is deeply dark blue. After the CF treatment, it becomes light blue, and then the color completely disappears after the CF-NF treatment.

4. Conclusions The treatments of synthetic dye wastewaters were studied by applying coagulation/flocculation (CF), nanofiltration (NF) and the combination of CF–NF. It is found that the removal of dye through CF from single dye wastewater (SDW) depends on both dye and coagulant. Some coagulants are very effective in treating certain SDW, but totally ineffective for the others. The efficiency of removing dyes from the highly concentrated multiple dyes wastewater (MDW, 1000 ppm) by the 7 coagulants follows the order of PAC 4AS 4APS4IC 4 IS4CO4MC. For the treatment of MDW, polyaluminum chloride (PAC) is the best coagulant and polydiallyldimethyl ammonium chloride (PDDA) is the best flocculant. A dosage of 400/200 ppm PAC/PDDA is the optimal CF formulation, which is able to remove about 90% of dyes from 1000 ppm MDW. With the optimal dosage, the CF process is more favorable for dye solutions with a higher concentration and pH. The positively charged hollow fiber NF membrane shows dye removal of almost 100% for both anionic and cationic dyes. The NF performance is also nearly independent of dye concentration with a permeate flux of about 1.0 L m  2 h  1 under an operating

314

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

pressure of 1 bar. Additionally, this NF membrane shows an excellent rejection of trivalent ions (e.g., Al3 þ ). The generation of concentrated stream and membrane fouling is inevitable in the NF process, while the production of sludge and the remaining strongly visible color are encountered in the CF process. The combination of CF and NF can complement each other's strengths and overcome their individual limitations. The NF treatment can completely remove the color left in the CF treated MDW, meanwhile the efficiency of coagulant/flocculant is significantly enhanced by treating the concentrated streams of the NF process. As a result, much less sludge is generated. In turn, by applying CF process as a pretreatment method, the membrane fouling is reduced and the NF permeate flux is increased. Those benefits and strengths lead to better performance for the treatment of synthetic dye wastewater. Future works will aim at developing a proper CF formulation and a higher flux positively charged NF membrane for the treatment of real dye wastewater.

Acknowledgments The authors would like to acknowledge the financial supports provided by National Research Foundation (NRF) of Singapore under its NRF Proof-of-Concept 8th Grant Call (NRF2012NRFPOC001-059) for the project entitled ‘Development of advanced nanofiltration membranes for high removing rate of dyes in textile wastewater’ (NUS grant number: R-279-000-389-281) as well as the Singapore-MITAlliance for Research and Technology (SMART) Centre under its Innovation Grant (ING12045-ENG) for the project entitled ‘Development of robust high-performance nanofiltration membranes for textile wastewater treatment’ (NUS grant number: R-279-000-377-592). Special thanks are given to Mr. Bai-Wang Zhao for his supports during the preparation of this work.

Appendix. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2014.06.057. References [1] A. Akbari, J.C. Remigy, P. Aptel, Treatment of textile dye effluent using a polyamide-based nanofiltration membrane, Chem. Eng. Process. 41 (2002) 601–609. [2] T. Vickerstaff, The Physical Chemistry of Dyeing, second ed., Oliver & Boyd, London, 1954. [3] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effuent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [4] H. Zollinger, Colour Chemistry-Synthesis, Properties of Organic, Dyes and Pigments, VCH Publishers, New York, 1987. [5] P. Nigam, G. Armour, I.M. Banat, D. Singh, R. Marchant, Physical removal of textile dyes from effluents and solid-state fermentation of dye-adsorbed agricultural residues, Bioresour. Technol. 72 (2000) 219–226. [6] E. Forgacs, T. Cserhati, G. Oros, Removal of synthetic dyes from wastewaters: a review, Environ. Int. 30 (2004) 953–971. [7] A.K. Verma, R.R. Dash, P. Bhunia, A review on chemical coagulation/flocculation technologies for removal of colour from textile wastewaters, J. Environ. Manage. 93 (2012) 154–168. [8] C. Hessel, C. Allegre, M. Maisseu, F. Charbit, P. Moulin, Guidelines and legislation for dye house effluents, J. Environ. Manage. 83 (2007) 171–180. [9] A.Y. Zahrim, N. Hilal, Treatment of highly concentrated dye solution by coagulation/flocculation–sand filtration and nanofiltration, Water Resour. Ind. 3 (2013) 23–34. [10] O.J. Hao, H. Kim, P.C. Chiang, Decolorization of wastewater, Crit. Rev. Env. Sci. Technol. 30 (2000) 449–505. [11] O. Tünay, I. Kabdasli, G. Erememktar, D. Orhon, Color removal from textile wastewaters, Water Sci. Technol. 34 (1996) 9–16. [12] M. Riera-Torres, C. Gutiérrez-Bouzán, M. Crespi, Combination of coagulation– flocculation and nanofiltration techniques for dye removal and water reuse in textile effluents, Desalination 252 (2010) 53–59.

[13] A. Bes-Piá, M.I. Iborra-Clar, A. Iborra-Clar, J.A. Mendoza-Roca, B. Cuartas-Uribe, M.I. Alcaina-Miranda, Nanofiltration of textile industry wastewater using a physicochemical process as a pre-treatment, Desalination 178 (2005) 343–349. [14] T. Chen, B. Gao, Q. Yue, Effect of dosing method and pH on color removal performance and floc aggregation of polyferric chloride–polyamine dualcoagulant in synthetic dyeing wastewater treatment, Colloids Surf. A 355 (2010) 121–129. [15] A.Y. Zahrim, C. Tizaoui, N. Hilal, Coagulation with polymers for nanofiltration pre-treatment of highly concentrated dyes: a review, Desalination 266 (2011) 1–16. [16] K.E. Lee, N. Morad, T.T. Teng, B.T. Poh, Development, characterization and the application of hybrid materials in coagulation/flocculation of wastewater: a review, Chem. Eng. J. 203 (2012) 370–386. [17] B. Shi, G. Li, D. Wang, C. Feng, H. Tang, Removal of direct dyes by coagulation: the performance of preformed polymeric aluminum species, J. Hazard. Mater. 143 (2007) 567–574. [18] K. Majewska-Nowak, Synthesis and properties of polysulfone membranes, Desalination 71 (1989) 83–95. [19] S.P. Sun, T.A. Hatton, T.S. Chung, Hyperbranched polyethyleneimine induced cross-linking of polyamide-imide nanofiltration hollow fiber membranes for effective removal of ciprofloxacin, Environ. Sci. Technol. 45 (2011) 4003–4009. [20] S. Cheng, D.L. Oatley, P.M. Williams, C.J. Wright, Positively charged nanofiltration membranes: review of current fabrication methods and introduction of a novel approach, Adv. Colloid Interface Sci. 164 (2011) 12–20. [21] S.P. Sun, T.A. Hatton, S.Y. Chan, T.S. Chung, Novel thin-film composite nanofiltration hollow fiber membranes with double repulsion for effective removal of emerging organic matters from water, J. Membr. Sci. 401–402 (2012) 152–162. [22] J. Gao, S.P. Sun, W.P. Zhu, T.S. Chung, Polyethyleneimine (PEI) cross-linked P84 nanofiltration (NF) hollow fiber membranes for Pb2 þ removal, J. Membr. Sci. 452 (2014) 300–310. [23] W.P. Zhu, S.P. Sun, J. Gao, F.J. Fu, T.S. Chung, Dual-layer polybenzimidazole/ polyethersulfone (PBI/PES) nanofiltration (NF) hollow fiber membranes for heavy metals removal from wastewater, J. Membr. Sci. 456 (2014) 117–127. [24] P.S. Zhong, N. Widjojo, T.S. Chung, M. Weber, C. Maletzko, Positively charged nanofiltration (NF) membranes via UV grafting on sulfonated polyphenylenesulfone (sPPSU) for effective removal of textile dyes from wastewater, J. Membr. Sci. 417–418 (2012) 52–60. [25] L. Shao, X.Q. Cheng, Y. Liu, S. Quan, J. Ma, S.Z. Zhao, K.Y. Wang, Newly developed nanofiltration (NF) composite membranes by interfacial polymerization for Safranin O and aniline blue removal, J. Membr. Sci. 430 (2013) 96–105. [26] X. Wei, X. Kong, C. Sun, J. Chen, Characterization and application of a thin-film composite nanofiltration hollow fiber membrane for dye desalination and concentration, Chem. Eng. J. 223 (2013) 172–182. [27] Y. Zheng, G. Yao, Q. Cheng, S. Yu, M. Liu, C. Gao, Positively charged thin-film composite hollow fiber nanofiltration membrane for the removal of cationic dyes through submerged filtration, Desalination 328 (2013) 42–50. [28] M. Li, J. Xu, C.Y. Chang, C. Feng, L. Zhang, Y. Tang, C. Gao, Bioinspired fabrication of composite nanofiltration membrane based on the formation of DA/PEI layer followed by cross-linking, J. Membr. Sci. 459 (2014) 62–71. [29] W. Fang, L. Shi, R. Wang, Interfacially polymerized composite nanofiltration hollow fiber membranes for low-pressure water softening, J. Membr. Sci. 430 (2013) 129–139. [30] L. Wang, S. Ji, N. Wang, R. Zhang, G. Zhang, J.R. Li, One-step self-assembly fabrication of amphiphilic hyperbranched polymer composite membrane from aqueous emulsion for dye desalination, J. Membr. Sci. 452 (2014) 143–151. [31] S.P. Nunes, K.V. Peinemann, Membrane Technology in the Chemical Industry, Wiley-VCH Verlag GmbH, Weinheim, 2001. [32] B. Van der Bruggen, L. Lejon, C. Vandecasteele, Reuse, treatment, and discharge of the concentrate of pressure-driven membrane processes, Environ. Sci. Technol. 37 (2003) 3733–3738. [33] M. Unlu, H. Yukseler, U. Yetis, Indigo dyeing wastewater reclamation by membrane-based filtration and coagulation processes, Desalination 240 (2009) 178–185. [34] B. Van der Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: a review, Sep. Purif. Technol. 63 (2008) 251–263. [35] L. Lei, X. Hu, P.L. Yue, Improved wet oxidation for the treatment of dyeing wastewater concentrate from membrane separation process, Water Res. 32 (1998) 2753–2759. [36] I.C. Escobar, B. Van der Bruggen, Modern Applications in Membrane Science and Technology, American Chemical Society(ACS), Washington, DC, 2011. [37] A. Mittal, J. Mittal, L. Kurup, Batch and bulk removal of hazardous dye, indigo carmine from wastewater through adsorption, J. Hazard. Mater. 137 (2006) 591–602. [38] S. Cheng, D.L. Oatley, P.M. Williams, C.J. Wright, Characterisation and application of a novel positively charged nanofiltration membrane for the treatment of textile industry wastewaters, Water Res. 46 (2012) 33–42. [39] S.P. Sun, K.Y. Wang, D. Rajarathnam, T.A. Hatton, T.S. Chung, Polyamide-imide nanofiltration hollow fiber membranes with elongation-induced nano-pore evolution, AlChE J. 56 (2010) 1481–1494. [40] B.Y. Gao, Y. Wang, Q.Y. Yue, J.C. Wei, Q. Li, Color removal from simulated dye water and actual textile wastewater using a composite coagulant prepared by

C.-Z. Liang et al. / Journal of Membrane Science 469 (2014) 306–315

ployferric chloride and polydimethyldiallylammonium chloride, Sep. Purif. Technol. 54 (2007) 157–163. [41] W.P. Cheng, F.H. Chi, C.C. Li, R.F. Yu, A study on the removal of organic substances from low-turbidity and low-alkalinity water with metalpolysilicate coagulants, Colloids Surf. A 312 (2008) 238–244. [42] J. Zou, H. Zhu, F. Wang, H. Sui, J. Fan, Preparation of a new inorganic–organic composite flocculant used in solid–liquid separation for waste drilling fluid, Chem. Eng. J. 171 (2011) 350–356. [43] Y.K. Ong, F.Y. Li, S.P. Sun, B.W. Zhao, C.Z. Liang, T.S. Chung, Nanofiltration hollow fiber membranes for textile wastewater treatment: lab-scale and pilotscale studies, Chem. Eng. Sci. 114 (2014) 51–57.

315

[44] L. Xu, L.S. Du, C. Wang, W. Xu, Nanofiltration coupled with electrolytic oxidation in treating simulated dye wastewater, J. Membr. Sci. 409-410 (2012) 329–334. [45] C.N. Lopes, J.C.C. Petrus, H.G. Riella, Color and COD retention by nanofiltration membranes, Desalination 172 (2005) 77–83. [46] C. Tang, V. Chen, Nanofiltration of textile wastewater for water reuse, Desalination 143 (2002) 11–20.