Electric field assisted cross flow micellar enhanced ultrafiltration for removal of naphthenic acid

Separation and Purification Technology 98 (2012) 36–45

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Electric field assisted cross flow micellar enhanced ultrafiltration for removal of naphthenic acid B. Venkataganesh a, Abhijit Maiti b, Subir Bhattacharjee b, Sirshendu De a,⇑ a b

Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, Kharagpur 721 302, India Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G8

a r t i c l e

i n f o

Article history: Received 17 January 2012 Received in revised form 13 May 2012 Accepted 11 June 2012 Available online 18 June 2012 Keywords: Naphthenic acid Micellar enhanced ultrafiltration Electric field Permeate flux Retention

a b s t r a c t In this work, removal of naphthenic acid (NA) using micellar enhanced ultrafiltration has been reported using 10,000 cutoff polyethersulfone membrane. Sodium dodecyl sulfate (SDS) has been used as surfactant. Effects of various parameters like pH, surfactant concentration, transmembrane pressure drop, cross flow velocity and electrolyte concentration on the permeate flux and permeate quality have been investigated in detail. In order to increase the permeate flux, potential of external electric field has also been explored. The electric field has been applied in two modes. In mode 1, a step-wise electric field is applied, so that the operating field strength across the membrane increases stepwise. In mode 2, a fixed electric field is applied throughout the experiment. In second mode of operation, about 24% flux enhancement is achieved compared to 14% in mode 1. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Leaching of naphthenic acid (NA) from oil wells to ground water is a severe problem in oil rich provinces in Canada [1]. NA is a complex mixture of hundreds of relatively low molecular weight linear and cyclic saturated carboxylic acids. The general formula is CnH2n+zO2, where, n indicates the carbon atom numbers. The value of z varies in between 0 to 12, indicating the number of hydrogen ions removed to accommodate a cycloalkane ring structure [2]. Presence of NA in water is an important environmental concern as these compounds are toxic to aquatic and non-aquatic life and they are carcinogenic in nature [3]. NAs are corrosive in nature and damage the internals of equipment and pipelines in a plant [4]. Reports of use of equilibrium governed separation processes, like adsorption [5], chemical processes like decarboxylation [6,7], etc., are used for removal of NA. Adsorption is time consuming, expensive and slow process. Decarboxylation requires addition of chemicals and therefore, separation of products is a problem. On the other hand, rate governed processes, like membrane based systems can be an attractive alternative in this regard. Reverse osmosis has been used quite effectively for removal of lower molecular weight organics from aqueous solution [8,9]. The major limitation of reverse osmosis is that being a dense membrane, the operating pressure requirement is extremely high. To overcome

⇑ Corresponding author. Tel.: +91 3222 283926; fax: +91 3222 255303. E-mail address: [email protected] (S. De). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.06.017

this problem, micellar enhanced ultrafiltration (MEUF) can be another effective alternative. In MEUF, surfactants are used beyond their critical micellar concentration (CMC) to form the micelles. Micelles at lower surfactant concentration are spherical in shape [10]. The outer surface of the micelles is charged for ionic surfactants and inner core is hydrophobic in nature. Organics occurring in trace amount in aqueous solution get solubilized within micelle core readily and inorganic pollutants of opposite polarity (to that of the micelle outer surface) are attached to the outer surface by counter ion binding. Micelles loaded with pollutants are bigger in size and are removed using a relatively open membrane like ultrafiltration. Therefore, pressure requirement will be much less, resulting to higher productivity and efficient removal of pollutants. Concentration polarization, i.e., accumulation of solute particles over the membrane surface leading to flux decline is a major drawback in any membrane based operation. To minimize the flux decline, several techniques are explored. These include use of spacers, inserts [11–14], pulsatile flow [15,16], surface modification of membrane by chemical treatment [17,18], etc. Application of a suitable external electric field is another promising method to reduce concentration polarization, as micelles are charged in nature. It is used for flux enhancement in membrane filtration of charged solutes [19–26]. Use of electric field for flux enhancement of micellar system is also reported in case of removal of dye from aqueous stream [27]. Removal of various organic, inorganic, mixtures of organic and inorganic using MEUF are available in literature [28–40]. Use of

B. Venkataganesh et al. / Separation and Purification Technology 98 (2012) 36–45

MEUF for removal of NA by cationic surfactant, cetyl pyridinium chloride is also reported by Husein and co-workers [41,42]. In these studies, the use of MEUF for removal of NA was attempted using polymeric hollow fiber membranes [41] and ceramic membranes [42]. However, effects of cross flow velocity, solution environment, including solution pH and presence of electrolytes using anionic surfactant (like, sodium dodecyl sulfate) on the performance of MEUF were not reported in these works. A detailed investigation of effects of various operating parameters in a continuous cross flow mode is essential for an efficient scaling up of such systems. The present work is undertaken to fill this gap. Further, the effect of external electric field for reduction in concentration polarization is also attempted in this work. Two modes of application of electric field, namely, step-wise and single continuous mode are explored to observe their flux enhancement efficiency. 2. Experimental 2.1. Materials All chemicals used were of reagent grade. Anionic surfactant, sodium dodecyl sulfate, SDS (molecular weight 288.38) was obtained from Merck Ltd., Mumbai, India. Naphthenic acid (molecular weight: 180–350, density: 0.92 g/ml at 20 °C, boiling point: 132–243 °C at 760 mm Hg and acid value: 230) was purchased from Sigma Aldrich Chemical Company, USA. Methylene chloride of HPLC grade (molecular weight 84.93) was procured from Spectrochem Pvt. Ltd. Mumbai, India. Hyamine 1622 (0.004 M) (C27H42NO2Cl, molecular weight 448) and disulfine blue VN (C27H31N2NaO6S2, molecular weight 567) was obtained from Merck KGaA, Darmstadt, Germany and dimindium bromide (C20H18BrN3, molecular weight 380) was procured from Loba Chemie Pvt. Ltd., Mumbai, India. Critical micelle concentration (CMC) of SDS was 2320 mg/l (8.1 mM) [10] without any electrolyte. CMC value of SDS decreases with electrolyte concentration and it is as low as 470 mg/l (1.64 mM) at 0.1 M NaCl [43]. With pH, CMC of SDS decreases up to 7 mM at pH 2.0 and it remains constant at pH 5.0 and above [44]. Thus, in this manuscript, feed concentration of the surfactant is specified in terms of CMC value 8.1 mM (2320 mg/l) to maintain uniformity and clarity of information. The surfactant was used as supplied, without any further treatment. All the feed solutions were prepared using double distilled water.

37

was measured by a rotameter in the retentate line. Pressure inside the electro-ultrafiltration cell was maintained by operating the bypass valve and measured by a pressure gauge. Permeate was collected from the bottom of the cell and it was recycled back to the feed tank to ensure the constant feed concentration and to attain the steady state quickly. The effective filtration area was 0.0108 m2. The details of schematic diagram of the experimental set up are shown in Fig. 1a and b. 2.3. Experimental design Experiments were designed to observe the effects of operating conditions (pH, surfactant concentration, transmembrane pressure, cross flow velocity, electric field and electrolyte concentration) on the steady state permeate flux and retention. For all experiments, the concentration of NA was chosen as 500 mg/l. This concentration was selected as it is the maximum concentration found in the ground water of the province of Alberta, in Canada. pH of the feed solution was used at 3, 5, 7 and 9. The operating variables used in the experiments were different transmembrane pressure drop (276, 414 and 552 kPa), and cross flow rates (40, 60 and 80 l/h). Effect of electrolyte concentration was observed at four different concentrations (0, 0.01, 0.05 and 0.1 M of NaCl). During experiments, one parameter was varied while other parameters were kept unchanged to get the exact picture of dependence. Six distinct electric field strengths (0, 200, 400, 600, 800 and 1000 V/m) were studied. Electric field was applied in following two modes. 2.3.1. Mode 1: stepwise increase of electric filed In this mode, experiment was started at zero electric field at particular operating conditions. After permeate flux reaching a steady state, the field strength was set to 200 V/m without disturbing other operating conditions. Next field strength of 400 V/m was set after steady state was attained. This process was repeated in a step of 200 V/m up to 1000 V/m. 2.3.2. Mode 2 of operation: continuous electric field from starting In this mode, a fixed electric field was applied from the beginning of the experiment so that the field strength remains constant throughout the experiment. Once, that experiment was over, the membrane was cleaned and a new experiment was started with another value of electric field strength.

2.1.1. Membrane An ultrafiltration membrane (polyethersulfone) of molecular weight cut-off (MWCO) 10 kDa, obtained from M/s, Permionics Membranes Pvt. Ltd., Boroda, Gujarat, (India), was used for all the MEUF experiments. The membrane was hydrophilic, compatible in a pH range 2–12. Membrane permeability was measured using distilled water and was found to be (1.01 ± 0.05)  1011 m/Pa s.

2.4. Procedure

2.2. Electro-ultrafiltration cell

2.4.2. Conduction of experiments The membrane was compacted at a pressure of 690 kPa (higher than the highest operating pressure used in this study) for 3 h using distilled water. Pure water fluxes at different operating pressures were measured next and plotted against pressure difference. The membrane permeability was obtained from the slope of this curve as (1.01 ± 0.05)  1011 m/Pa s. During ultrafiltration experiments, permeate samples were collected at different times. The duration of each experiment was 45–60 min for mode 2 of operations. For mode 1, the duration was 30–40 min to reach steady state for each single electric field. The total duration for mode 1 of operation was 3–3.5 h. All experiments were conducted at

The experiments were conducted in continuous mode of operation. From the feed tank, feed solution was pumped and allowed to flow tangentially over the membrane surface through a thin channel of 37 cm in length, 3.6 cm in width and 6.5 mm in height. The anode was platinum coated titanium sheet (length 33.5 cm, width 3.4 cm, thickness 1.0 mm) obtained from Ti Anode Fabricators, Chennai (India), and mounted parallel to the flow position just above the ultrafiltration channel. External electric field from a regulated d.c. power supply was applied across the membrane surface. The retentate was recycled to the feed tank. The flow rate

2.4.1. Preparation of feed solution Feed solution was prepared by dissolving required amount of SDS and NA in 4 l of doubled distilled water. The pH of the solution was adjusted by using H2SO4 and KOH solution. The solution was kept under slow stirring for about 15 min and then kept static for around 2 h.

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Fig. 1. (a) Schematic diagram of electro-ultra filtration system; (b) configuration of the electro-ultra filtration chamber.

30 ± 2 °C. After each experiment, the membrane was cleaned using the following procedure: After completion of an experiment, membrane was washed thoroughly for 45 min by recirculating distilled water at room temperature 30 ± 2 °C, followed by 45 min of acid (pH 3.0) washing using HCl, 30 min of washing using distilled water, 30 min of alkaline (pH 10) washing and finally membrane was washed with distilled water. After such thorough washing, water flux was measured with distilled water. This procedure was allowed for recovery of the pure initial water flux within 95%. All the steps were carried out at room temperature. Application of external d.c. electric field had a significant effect on membrane fouling. In presence of electric field, the deposition over the membrane surface was less and therefore less time was needed for its cleaning compared to without electric field. Following sequence of experiments were followed: (i) selection of solution pH; (ii) selection of suitable surfactant concentration;

(iii) effects of operating conditions, namely, transmembrane pressure drop, cross flow rate, modes of application of external electric field; (iv) effects of electrolyte concentration on the permeate flux and permeate quality. 2.5. Analysis 2.5.1. Analysis of NA concentration Concentration of NA in permeate was measured by Fouriertransform infrared (FTIR) spectroscopy method. The aqueous samples (150 ml) were adjusted to a pH of 2.0–2.5 using 9 (M) H2SO4. The acidified sample was extracted twice with 25 ml portions of dichloro methane (HPLC grade, Spectrochem Chemicals Pvt. Ltd., Mumbai, India). Aqueous samples contained small fraction of SDS forming the globules in organic phase and took nearly 10–12 h to settle. The solution was filtered through whatman filter paper to collect the SDS-globule free organic phase. Next, it was dried over-

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SDS concentration¼

x  molar concentration of hyamine  288:38 5 ml of sample ð2Þ

where x is the volume (ml) of hyamine 1622 required for titration [26]. Retention of SDS is defined as,

concentration of SDS in permeate  100% Retention of SDS ¼ 1  concentration of SDS in feed ð3Þ

2.5.3. Measurement of NaCl concentration NaCl concentration in the permeate was measured by measuring equivalent chloride ion concentration in the permeate. This was carried out by using a chloride ion selective electrode fitted to a benchtop pH/ISE meter (Model: Orion 4 star, supplied by M/ S Thermo Scientific Co., USA). 2.6. Statistical analysis Following statistical analyses were performed on the experimental data:

Surfactant concentration was selected by performing experiments at pressure drop 414 kPa, cross flow rate 60 l/h, 500 mg/l of feed NA and pH 3.0 using different concentration of surfactant varying from 2 CMC to 10 CMC. The results at the steady state are presented in Fig. 3a. It is observed from this figure that the retention of NA increases from 68% to 97% as the surfactant concentration increases from 2 CMC to 10 CMC. At 8 CMC, retention was about 96%. As more surfactants are present, micelle concentration also increases, thereby, solubilizing more NA leading to high retention of NA. However, at higher surfactant concentration, retention of NA becomes almost invariant due to saturation of solubilization of NA in micelles. Surfactant retention also increases from 78% to 94% with surfactant concentration in feed. Surfactant concentration in the permeate is slightly less than CMC. Thus, as the feed concentration increases, observed retention of surfactant also increases. On the other hand, the steady state flux decreases with surfactant concentration. For example, it decreases from

2.60

100

2.59

95 90

2.58

85

2.57

80 2.56 75 2.55 70 2.54 2.53 2.52 2.51

65

NA= 500 mg/l, SDS= 5 CMC, ΔP= 414 kPa, Flow rate = 60 L/h D Steady State Flux, Retention of NA Retention of SDS 3

4

5

6

7

8

9

pH Fig. 2. Effect of pH on steady state flux, retention of NA and SDS.

60 55 50

% Retention of NA and SDS

(i) In case of MEUF experiments, more than 70 experiments were conducted. Out of these, 30 experiments were repeated thrice under the same operating conditions. The uncertainties were calculated using standard error analysis and the permeate flux values were found to lie within ±3%. The average values of the permeate flux were reported in the figures and the text of the manuscript. (ii) For a particular experiment, three samples of permeate were analyzed for determination of concentration of NA and SDS. This was performed for all the experiments. The standard deviation values were calculated. The average values were reported. It was observed that for NA, the standard deviation of concentration beyond 100 mg/l, was 3 mg/l and that for less than 100 mg/l was 5 mg/l. Thus, it affects the observed retention value of NA by ±1% only. In case of SDS, the standard deviation of concentration of SDS was only 2 mg/ l. This leads to variation of observed retention of SDS by less than 1%.

3.2. Selection of surfactant concentration

2

2.5.2. Measurement of SDS concentration SDS concentration was determined by a two-phase titration. The titrant was benzothonium chloride (often called hyamine 1622), a cationic surfactant, the indicator was an acidic mixture of a cationic dye (dimindium bromide) and an anionic dye (disulfine blue VN). The titration was carried out in a water chloroform medium. SDS concentration was determined using the following equation:

Selection of operating pH was done by carrying out experiments at different pH values using 5 CMC SDS solution with 500 mg/l NA, at 414 kPa operating pressure and 60 l/h flow rate. The results are shown in Fig. 2. It is observed from Fig. 2 that the permeate flux and observed retention of SDS at the steady state are independent of pH. Since, retention of SDS does not change with pH values, the thickness of gel layer formed by SDS micelles also remains constant. As a result, the resistance against the solvent flux remains invariant and thereby the permeate flux does not change. On the other hand, pH has a negative effect on the retention of naphthenic acid. Retention of NA decreases from 82% to 68% as the solution pH increases from 3 to 9. At lower pH, carboxylic acid functional groups of NA become neutral and the molecules become non-polar. This leads to more solubilization of NA in the micelles. Moreover, at lower pH, CMC of SDS decreases [44], resulting to increase in micelle concentration and subsequently, solubilization capacity of the micelles increases. At higher pH, NA dissociates and its water solubility increases. Also, CMC values of SDS micelles remain constant at higher pH (pH > 5) [44]. Therefore, retention of NA is more at lower pH as is evident from Fig. 2. Thus, pH 3.0 was selected for the subsequent experiments.

3

ð1Þ

3.1. Selection of operating pH

6

concentration of NA in permeate concentration of NA in feed  100%

Retention of NA ¼ 1 

3. Results and discussion

Steady state flux ( 10 m /m .s)

night under a flow of compressed air to concentrate the sample to 1.5 ml prior to FTIR analysis. After that the sample was analyzed by FTIR spectroscopy method, by using KBr cell. FTIR response and calibration plot for different concentrations of NA are presented in Appendix A. The absorbance of the samples measured at wave number of 1701 cm1 [45]. This method to determine low concentration of NA in water is used by many researchers [45–49]. Retention of NA is defined as,

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100

2.5

3.8 3.6 ΔP = 552 kPa

2.1

80

NA= 500 mg/l, pH= 3.0, Δ P = 414 kPa, Flow rate = 60 l/h Steady State Flux Retention of NA Retention of SDS

2.0 1.9

4

6

8

2

2.8 ΔP = 414 kPa

2.6 2.4 2.2 2.0 1.8

ΔP = 276 kPa

1.6

70

1.4 1.2

1.8 2

3.0

3

2.2

NA= 500 mg/l, SDS= 8 CMC, pH=3.0 NaCl =0 (M), Flow rate in l/h 60 l/h, 80 l/h, 100 l/h

3.2

6

6

3

90

Flux (×10 m /m .s)

2

3.4 2.3

% Retention of NA and SDS

Steady state flux ( 10 m /m .s)

2.4

10

0

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1000

1500

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2500

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Time (s)

SDS concentration (CMC)

(a)

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6

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2.8

6

2.7

2

Flux ( 10 m /m .s)

2.8

Steady State Flux (×10 m /m .s)

NA= 500 mg/l, pH=3.0, Δ P= 414 kPa, Flow rate= 60 l/h SDS Concentration = 2 CMC SDS Concentration = 4 CMC SDS Concentration = 6 CMC SDS Concentration = 8 CMC SDS Concentration = 10 CMC

2.9

2.4 2.0

1.2 0.8

0

500

1000

1500

2000

NA= 500 mg/l, SDS = 8 CMC pH=3.0, NaCl = 0 (M) Flow rate = 60 l/h Flow rate = 80 l/h Flow rate = 100 l/h

1.6

276

2500

Time (s)

(b)

2

6

3

2

2.4  10 m /m s to 2.1  10 m /m s as surfactant concentration increases from 2 CMC to 8 CMC. At higher surfactant concentration, concentration of micelles becomes high. Thus, more micelles deposit on the membrane surface, forming a thicker layer of micelles, thereby increasing the resistances against the solvent flux, resulting to lower permeate flux. Therefore, at higher surfactant concentration, increase in retention of NA is associated with decrease in permeate flux. To attain a trade-off, 8 CMC concentration of surfactant was selected for subsequent experiments. Transient profiles of flux decline at different concentration of surfactant in feed are shown in Fig. 3b. Three general trends are observed from this figure. First, the permeate flux decreases with time of filtration progresses. More micelles deposit on the membrane surface with time of operation, leading to an increase of thickness of gel type of layer. This offers more resistance to the solvent flux, resulting to a decrease in flux. However, the cross flow of the retentate does not allow the growth of the gel layer unhindered and thickness of this layer gets limited by the shearing action of the cross flow. This results in attainment of a steady state. Second, the permeate flux is less at higher feed concentration of surfactant. At higher surfactant concentration, gel layer thickness of surfactant micelles is more from the beginning of the experiment, leading to higher resistance and thus permeate flux decreases. Third, the steady state is attained faster at higher surfactant concentration in feed. At higher surfactant concentration, gel layer thickness grows faster as discussed earlier and the steady state is reached

98

% Retention of NA and SDS

3

552

100

Fig. 3. Effect of surfactant concentration on steady state permeate flux, retention and flux decline profiles. (a) Steady state flux and retention of NA and SDS; (b) flux decline profiles.

6

414

(b)

96 94 92 90 88 86

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0, NaCl= 0 (M) Electric field = 0 V/m, Error bar: ± 2, Flow rate in l/h 60 l/h, 80 l/h, 100 l/h Retention of NA : 60 l/h, 80 l/h, 100 l/h Retention of SDS : 276

414

552

ΔP (kPa)

(c) Fig. 4. Variation of flux decline profiles, steady state permeate flux and retention of NA and SDS for different transmembrane pressure drop and cross flow rates. (a) Flux decline profiles; (b) steady state permeate flux; (c) retention of NA and SDS.

earlier. For example, at 4 CMC, steady state was attained at 1000 s, whereas, that at 10 CMC was about 750 s.

3.3. Effects of transmembrane pressure drop and cross flow rate Effects of transmembrane pressure drop and cross flow rate on the permeate flux and retention of NA and SDS are presented in Fig. 4. The transient flux profiles are shown in Fig. 4a. Three clear trends are observed from this figure. First, large flux decline occurs at higher transmembrane pressure drop. For example, at 276 kPa,

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3.4. Effects of external electric field 3.4.1. Mode 1 of operation: stepwise increase of electric filed As discussed earlier, the experiment was started without electric field. Once the steady state was reached, 200 V/m field strength was applied. Having attained the steady state, 400 V/m was applied and similarly, higher electric field was applied stepwise up to 1000 V/m with a step of 200 V/m. The transient flux decline profiles for three pairs of transmembrane pressure drop and cross flow rates are presented in Fig. 5. It is observed from this figure

that the permeate flux is more as the electric field strength increases. As the strength of external electric field increases, the negatively charged surfactant micelles are attracted by the positive electrode (i.e., the top plate of the channel) and the micelles are lifted up by electrophoresis from the gel layer on the membrane surface, thereby, reducing the thickness of gel layer and leading to flux improvement. This effect is more dominant at higher electric field strength. As discussed earlier, the permeate flux is more at higher transmembrane pressure drop and cross flow rate. The steady state values of permeate flux as a function of electric field strength for different pressure drop and cross flow rates are presented in Fig. 6a. It is observed from this figure that permeate

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0, NaCl= 0 (M) Flow rate: 60 l/h, 80 l/h, 100 l/h

3.2

6

3

2

Steady state flux (×10 m /m .s)

3.6

ΔP= 552 kPa

2.8 2.4

ΔP= 414 kPa

2.0 ΔP= 276 kPa

1.6 1.2

0

200

400

600

800

1000

Electric field strength (V/m)

(a) 16

NA= 500 ppm, SDS= 8 CMC, pH = 3.0, NaCl = 0 (M), Flow rate in l/h ΔP= 276 kPa, 60 l/h, 80 l/h, 100 l/h ΔP= 414 kPa, 60 l/h, 80 l/h, 100 l/h ΔP= 552 kPa, 60 l/h, 80 l/h, 100 l/h

14

% Enhancement in flux

80 l/h flow rate, permeate flux declines from 1.6  106 (initial flux) to 1.4  106 m3/m2 s (steady state flux) (about 8.8%). At 552 kPa and the same flow rate, permeate flux decreases from 3.6  106 (initial flux) to 3.0  106 m3/m2 s (steady state) (about 20%). At higher pressure drop, the gel layer thickness grows faster and its thickness becomes more than that at lower pressure. Thus, the concentration polarization is more, leading to large decline in permeate flux. Second trend is that permeate flux is higher at higher operating pressure. For example, at 80 l/h cross flow rate, permeate flux at 276 kPa is 1.4  106 m3/m2 s and that at 552 kPa is 2.8  106 m3/m2 s (about 100% increase). At higher operating pressure, two opposing phenomena occur. The driving force is higher at higher pressure drop leading to an increase in flux. Also at higher pressure, the gel layer thickness is more leading to reduction in flux. However, the first phenomenon becomes dominant, resulting to increase in flux with transmembrane pressure drop. Third trend is that at a fixed transmembrane pressure drop, the permeate flux is more at higher cross flow rate. For example, at 552 kPa pressure, the permeate flux increases from 2.8 to 3.2  106 m3/m2 s (14% increase) as the cross flow rate increases from 60 to 100 l/h. This is due to more shear induced by higher cross flow rate. The steady state values of permeate flux at different transmembrane pressure drop and cross flow rates are shown in Fig. 4b. This figure shows the expected trends as discussed earlier. Variation of observed retention of NA and SDS at steady state is presented in Fig. 4c. It is observed from this figure that retention of NA is in the range of 95–98% for all the operating conditions and that for SDS is in the range of 91–92%. The effects of transmembrane pressure drop and cross flow rate on the retention of NA and SDS are marginal. Therefore, the operating conditions like cross flow rate and transmembrane pressure drop influence strongly the permeate flux (productivity) values. Retention of NA and SDS is independent of these operating conditions and they are solely governed by the solubilization mechanism within the micelles.

12 10 8 6 4 2 0

0

200

400

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800

1000

Electric field strength (V/m)

(b) 100 96

3.5

1.65

2

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600 V/m

800 V/m 1000 V/m

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2.9 2.8

0 V/m 0

2000

6000

88 84 80

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0, NaCl= 0 (M) Open symbols: Retention of SDS for different operating conditions Closed symbols: Retention of NA for different operating conditions

76 72 0

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200 V/m

4000

92

2

600 V/m 400 V/m

3.0

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1.55

6

Flux (×10 m /m .s)

3.4

1.70

Flux (×10 m /m .s)

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0, NaCl Concentration = 0 (M) ΔP= 552 kPa, Flow rate = 80 l/h ΔP= 276 kPa, Flow rate = 80 l/h ΔP= 276 kPa, Flow rate = 60 l/h

% Retention of NA and SDS

3.6

1.30 8000

10000

(c)

12000

Time (Sec) Fig. 5. Flux decline profiles in mode 1 of operation at external electric field.

Fig. 6. Variation of steady state permeate flux, flux enhancement and retention of NA and SDS with electric field strength. (a) Steady state permeate flux; (b) flux enhancement; (c) retention of NA and SDS.

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dominant and the gel layer thickness becomes almost constant, leading to invariant flux values. However, maximum 12% flux enhancement is observed from this figure. At 276 kPa pressure, the gel layer is loosely packed and the effect of electric field becomes prominent and significant flux enhancement is observed. Variation of retention of NA and SDS for all the operating conditions are shown in Fig. 6c. It is observed from the figure that retention of NA is in the range of 94–98% and that for SDS is 90–92% for all the experiments. This figure confirms that retention of NA and SDS is independent of cross flow rate, operating pressure and electric field strength.

2.00 1.95

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0, ΔP = 276 kPa, Flow rate = 60 l/h 0 V/m, 200 V/m 400 V/m, 600 V/m 800 V/m, 1000 V/m

1.90 1.85 1.80 2

1.70

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Flux (×10 m /m .s)

1.75

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NA= 500 mg/l, SDS = 8 CMC, pH = 3.0 ΔP = 276 kPa, Flow rate = 80 l/h 0 V/m, 200 V/m, 400 V/m 600 V/m, 800 V/m, 1000 V/m

1.85 1.80

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NA= 500 mg/l, SDS = 8 CMC, pH = 3.0 ΔP = 552 kPa, Flow rate = 80 l/h 0 V/m, 200 V/m, 400 V/m 600 V/m, 800 V/m, 1000 V/m

4.2 4.1

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3

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500

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3500

Time (s)

(c) Fig. 7. Flux decline profiles for different electric field strength for mode 2 of operation. (a) DP = 276 kPa, cross flow rate 60 l/h; (b) DP = 276 kPa, cross flow rate 80 l/h; (c) DP = 552 kPa, cross flow rate 80 l/h.

flux is increased with electric field strength, for all the experiments. The percentage enhancement of permeate flux compared to the case without electric field for various operating condition is shown in Fig. 6b. From this figure, two trends are observed. First, the flux enhancement is saturated at 800 V/m at higher transmembrane pressure drop and it is monotonically increasing for 276 kPa. At higher operating pressure, gel layer becomes more compact and as a result, beyond 800 V/m, short range interactive forces becomes

3.4.2. Mode 2 of operation: continuous electric field from starting In mode 2 of operation, a uniform electric field is applied from the beginning of the experiment. The transient flux decline profiles for various electric field strength are shown in Fig. 7a–c for different operating conditions. Two general trends are observed from these figures. First, effect of electric field is more dominant at lower transmembrane pressure drop. For example, at 276 kPa and 60 l/h flow rate, permeate flux increases from 1.3 to 1.6  106 m3/m2 s (about 24% increase), as the electric field strength increases from 0 to 1000 V/m (refer Fig. 7a). It is observed from Fig. 7b that flux increases from 1.35 to 1.6  106 m3/m2 s as electric field strength increases from 0 to 1000 V/m for 276 kPa and 80 l/h cross flow rate. This is equivalent to 19% enhancement in flux. Corresponding flux enhancement is only 8% at 552 kPa and 80 l/h flow rate (Fig. 7c). The reason of these has already been discussed earlier. Second trend that can be observed from these figures is that the steady state is attained earlier at higher electric field strength. For example, at 276 kPa and 60 l/h flow rate, steady state is attained at about 2200 s without any electric field. At 1000 V/m, steady state is attained in about 1500 s. Corresponding data for operating conditions in Fig. 7b are almost same. At higher operating pressure (at 552 kPa) in Fig. 7c, these values are 1850 and 1600 s, respectively. As discussed earlier, steady state is attained by the forced convection imposed by the cross flow of retentate on the gel layer that restricts the thickness of gel layer. The mechanisms exist to attain the steady state is a balance between convective solute flux towards the membrane (due to pressure gradient) and diffusive solute flux away from the membrane (due to concentration gradient between gel layer and bulk), in case of without electric field. In presence of external electric field, another solute flux comes to play, i.e., the flux due to electrophoresis of charged micelles away from the membrane surface. At higher field strength, electrophoretic flux is more and a steady state is attained quickly. Fig. 8a shows the variation of flux enhancement with electric field strength using both the modes at different operating conditions. It is observed that mode 2 results in higher enhancement of permeate flux compared to mode 1. In mode 2 of operation, a constant but enhanced electric field is applied from the starting of the experiment. This would result in enhanced electrophoretic mobility of micelles leading to lowering in gel layer thickness and higher permeate flux. In this mode, maximum about 24% flux enhancement is achieved at 276 kPa pressure and 60 l/h flow rate. At same pressure and 80 l/h flow rate, flux enhancement is about 18%. At higher transmembrane pressure drop, at 552 kPa and 1000 V/m field strength, the flux enhancement decreases as discussed in preceding section, to only 7%. But, this value is higher than the value (5%) at same operating conditions under mode 1 of operation. Thus, mode 2 of operation is more advantageous than mode 1 for flux enhancement by electric field. Flux enhancement is more at lower transmembrane pressure drop. The retention of NA and SDS both are not affected by the mode of operation and electric field strength. The retention of NA is

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B. Venkataganesh et al. / Separation and Purification Technology 98 (2012) 36–45 100

30

2

2.20 2.18

6

3

20

15

10

5

0

200

400

600

800

1000

2.12

94

2.10

93

2.08

92

2.06

91

2.04

90 89 0.02

0.04

2.65 2.60

96

2.55

2

2.45 2.40

3

2.35

6

92 90 88 86

NA= 500 mg/l, SDS = 8 CMC, pH = 3.0, NaCl = 0 (M) ΔP= 276 kPa, Flow rate = 60 l/h: NA, SDS ΔP= 276 kPa, Flow rate = 80 l/h: NA, SDS ΔP= 552 kPa, Flow rate = 80 l/h: NA, SDS

80 400

0.10

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0 ΔP = 414 kPa, Flow rate =60 l/h NaCl Concentration = 0 M NaCl Concentration = 0.01 M NaCl Concentration = 0.05 M NaCl Concentration = 0.1 M

2.50

94

2.30

Flux (×10 m /m .s)

% Retention of NA and SDS

2.70

98

200

0.08

(a)

100

0

0.06

NaCl concentration (M)

(a)

82

97

95

2.14

Electric field strength (V/m)

84

98

96

2.16

2.02 0.00

0

99

% Retention of NA and SDS

NA= 500 mg/l, SDS= 8 CMC, pH= 3.0 ΔP = 414 kPa, Flow rate = 60 l/h Steady state Flux Retention of NA Retention of SDS

2.22

Steady state flux (×10 m /m .s)

25

% Enhancement in Flux

2.24

NA= 500 mg/l, SDS= 8 CMC, pH = 3.0, NaCl = 0 (M) ΔP= 276 kPa, Flow rate = 60 l/h: Mode 2, Mode 1 ΔP= 276 kPa, Flow rate = 80 l/h: Mode 2, Mode 1 ΔP= 552 kPa, Flow rate = 80 l/h: Mode 2, Mode 1

600

800

1000

Electric field strength (V/m)

(b)

Fig. 8. Enhancement of steady state permeate flux and observed retention for mode 2 of NA and SDS at different electric field strength. (a) Comparison of enhancement of steady state permeate flux between both modes; (b) retention of NA and SDS in mode 2.

about 94–98% and SDS is 90–92% which is same for both modes shown in Fig. 8b. 3.5. Effect of electrolyte concentration Variation of steady state values of permeate flux and observed retention of NA and SDS with electrolyte concentration at zero electric field is shown in Fig. 9a. It is observed from the figure that permeate flux at steady state decreases with electrolyte concentration. The flux decreases from 2.2  106 to 2.0  106 m3/m2 s (9% decrease) as the electrolyte concentration increases up to 0.1 (M). In presence of electrolyte, more counter ions (Na+) are attached on the outer surface of the negatively charged micelles due to electrostatic attraction. This makes the gel layer more compact because inter-micellar repulsion is minimized due to screening effects of electrolytes. This phenomenon leads to an increase in resistance against the solvent flow and the permeate flux declines. This effect becomes more pronounced at higher electrolyte concentration. As observed from this figure, retention of SDS is not affected by electrolyte concentration. An interesting observation is noted regarding retention of NA. When no electrolyte is added, the retention of NA was about 98%. On addition of 0.01 (M) electrolyte, NA retention is reduced to about 94.5% and it remains unaltered at higher electrolyte concentration. Addition of electrolyte leads to

2.25 2.20 2.15 2.10 2.05 2.00

1.95

0

500

1000

1500

2000

2500

3000

Time (s)

(b) Fig. 9. Variation of steady state observations and flux decline profiles with electrolyte concentration without electric field. (a) Steady state permeate flux, retention of NA and SDS; (b) flux decline profiles.

competitive solubilization between counter ion (Na+) and NA on the micellar surface. More adsorption of Na+ on the micellar surface hinders the solubilization of NA in the micelles and due to this, the solubilization of NA is slightly lowered, thereby reducing its observed retention. However, competitive solubilization is present at higher electrolyte concentration as well but this effect is marginal. At higher electrolyte solution 0.1 (M), CMC value of SDS micelles also decreases to 1.64 (mM) [43]. This increases the micelle concentration and hence its solubilation capacity. Because of the presence of two opposing effects, i.e., less retention due to competitive solubilization and more solubilization of NA in micelle phase due to lowering of CMC, the retention of NA is not significantly different between 0.01 (M) and 0.1 (M) of NaCl. Flux decline profiles without electric field at various electrolytic concentrations are shown in Fig. 9b. This figure shows the expected transient profiles with steady state values lower for at electrolyte concentration. Variation of steady state permeate flux with electric field strength and the electrolyte concentration is presented in Fig. 10. This figure shows that the steady state flux decreases with electrolyte concentration at the same electric field strength. The explanation is same as in Fig. 9b without electric field. In presence of electrolyte, the gel layer becomes compact due to screening effects of electrolytes and also, the effective charge on SDS micelles are reduced due to same effect. Thus, the electropho-

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B. Venkataganesh et al. / Separation and Purification Technology 98 (2012) 36–45

4. Conclusions

2.4

6

3

2

Steady state flux (×10 m /m .s)

2.5

2.3

2.2

NA= 500 mg/l, SDS= 8 CMC, pH = 3.0, ΔP= 414 kPa, Flow rate = 60 l/h NaCl Concentration = 0 (M) NaCl Concentration = 0.01 (M) NaCl Concentration = 0.05 (M) NaCl Concentration = 0.1 (M)

2.1

2.0 0

200

400

600

800

1000

Electric field strength (V/m) Fig. 10. Variation of steady state flux with electric field for different electrolyte concentrations.

0.7

NA concentration = 50 mg/l NA concentration = 100 mg/l NA concentration = 200 mg/l NA concentration = 300 mg/l NA concentration = 500 mg/l

0.6

Absorbance (A)

0.5 0.4

Applicability of MEUF in context of removal of NA from aqueous stream has been explored in this study. Effects of various process parameters (transmembrane pressure drop, cross flow rate, external electric field) and solution chemistry (pH and electrolytic concentration) have been investigated in detail. It was observed that at pH value of 3.0 and surfactant concentration of 8 CMC, about 98% removal of NA (at 500 mg/l in feed) was possible. Operating conditions like transmembrane pressure drop and cross flow rate have no influence on the retention of NA and surfactant as well. However, they significantly influence the permeate flux. Application of external d.c. electric field was also investigated for flux enhancement. Two modes of application of electric field were used. Maximum 12% flux enhancement was achieved in mode 1 of operation and about 24% was attained in mode 2. However, electric field has no effect on retention values of NA as well as surfactant. Presence of electrolyte significantly decreases the permeate flux up to 9% at 0.01 (M) electrolyte. Retention of NA was marginally reduced by electrolyte concentration and that of SDS remained unaffected. Therefore, retention of NA by MEUF is entirely governed by the solubilization of NA in surfactant micelles and solution chemistry (pH and electrolyte concentration).

Acknowledgements

0.3

Financial support from Shastri Indo-Canadian Foundation is gratefully acknowledged. Support from the NSERC Industrial Research Chair in Water Quality Management for Oil Sands Extraction is gratefully acknowledged.

0.2 0.1 0.0 1900

1850

1800

1750

1700

1650

1600

-1

Appendix A

Wavenumber (cm )

(a)

Fourier transform infra red response (FTIR) for various concentration of NA is presented in Fig. A1a. Based on the maximum absorbance values at 1701 cm1, the calibration curve of NA is prepared and presented in Fig. A1b.

0.7 0.6

Absorbance (A)

0.5

References

0.4 0.3 0.2 0.1 0.0

0

100

200

300

400

500

600

Concentration of NA (mg/l)

)b(

Fig. A1. FTIR response and calibration curve to analyze NA. (a) FTIR response for different NA concentrations; (b) calibration plot.

retic mobility in presence of electric field decreases. This effect is stronger at higher electrolyte concentration and flux is reduced. At the same electrolyte concentration, the flux increases as the field strength increases due to enhanced electrophoretic mobility. For example, at electrolyte concentration of 0.1 (M), permeate flux increases from 2.0 to 2.3  106 m3/m2 s (12% increase). Therefore, presence of electrolyte is detrimental to MEUF of NA as it reduces the permeate flux significantly and also decreases the retention of NA marginally.

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