Effective separation of methylene blue dye from aqueous solutions by integration of micellar enhanced ultrafiltration with vacuum membrane distillation

Chemical Engineering Journal 375 (2019) 122015

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Effective separation of methylene blue dye from aqueous solutions by integration of micellar enhanced ultrafiltration with vacuum membrane distillation Sowmya Parakala, Siddhartha Moulik, S. Sridhar

T

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Membrane Separations Group, Process Engineering and Technology Transfer Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, Hyderabad 500007, India

H I GH L IG H T S

micellar enhanced ultrafiltration system for treatment of dye solutions. • Integrated of MEUF reject by VMD for water recovery and dye concentration. • Treatment to synthesize AC loaded polyethersulfone & TEOS crossed polystyrene membranes. • Easy resistance in series model for prediction of MEUF process flux. • Modified • A CFD model to estimate liquid entry pressure of hydrophobic membranes.

A R T I C LE I N FO

A B S T R A C T

Keywords: Micellar enhanced ultrafiltration Vacuum membrane distillation Modified resistance in series model Liquid entry pressure Computational fluid dynamics

Micellar enhanced ultrafiltration is widely used for separation of dyes and other dissolved organics from aqueous solutions. However, generation of reject water that contains highly concentrated dyes, surfactants and electrolytes is a major concern in this process. In this study, micellar enhanced ultrafiltration (MEUF) was integrated with vacuum membrane distillation (VMD) for effective removal of methylene blue dye from aqueous solutions with enhanced water recovery. Activated carbon loaded polyethersulfone (PES) ultrafiltration membrane was used in the process with sodium dodecyl sulphate as an anionic surfactant for micelle formation. The reject of MEUF was further processed via VMD for additional water recovery using a tetraethyl orthosilicate crosslinked polystyrene (PSt) membrane. Effect of feed dye concentration, surfactant concentration and feed pressure on MEUF process performance was evaluated. The effect of feed dye concentration, degree of vacuum and membrane thickness on VMD was also studied. A theoretical model based on modified resistance in series model was used to predict MEUF process flux. Also, an interesting model based on computational fluid dynamics was developed to predict the liquid entry pressure for dye solution which is an important parameter in VMD. The final concentrated dye in VMD retentate could directly be used as an emulsion in paint or textile industry, with simultaneous generation of utility water.

1. Introduction A variety of dyes that are widely used in textile, paper and leather industries are manufactured from coal tar-based hydrocarbons [1]. The processed water streams from these industries include synthetic dyes which are carcinogenic, mutagenic and teratogenic [2]. These industries are the leading consumers of water resources and thus cause severe damage to agriculture, aquatic and terrestrial life as well as humans. Since dyes are highly stable against environmental conditions

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such as temperature, light and pH, it is prudent to remove these from effluents to ensure safe discharge. Dyes are generally classified as anionic, cationic and non-ionic dyes. A deep blue coloured cationic thiazine dye named methylene blue, MB (3,7-bis (dimethylamino)-phenothiazin-5-ium chloride) is majorly used in textile, printing, dyeing, paper, rubber and plastic industries, etc. Various techniques have been employed for removal of such dyes from wastewater. Some of the published works include physical techniques such as adsorption [3] and coagulation/flocculation [4]; chemical techniques such as oxidation/

Corresponding author at: CSIR-Indian Institute of Chemical Technology, Tarnaka, 500007, India. E-mail address: [email protected] (S. Sridhar).

https://doi.org/10.1016/j.cej.2019.122015 Received 16 April 2019; Received in revised form 16 June 2019; Accepted 18 June 2019 Available online 19 June 2019 1385-8947/ © 2019 Published by Elsevier B.V.

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includes integration of two highly efficient processes. A TEOS crosslinked PSt hydrophobic membrane was used as a barrier for the liquid phase, allowing only water to pass through. Thus, pressure difference across the membrane acts as a driving force for the process. VMD is advantageous for dye recovery as it can utilize the industrial waste heat already present in the wastewater that is discharged from the processing industries [26,27]. Also, the dye can be recovered from the concentrate, which can be used by paint or textile industry [28]. The integrated process i.e., MEUF + VMD when compared to RO/ NF has several advantages such as lower energy requirement, reduced operating pressures, usage of high permeability membranes, higher recovery compared to RO for exploitation across a wide range of applications. VMD alone also offers low space and usage of low-grade waste heat. This study aims to develop a feasible solution to reject disposal problems faced in pressure driven processes by investigating VMD for water recovery from highly concentrated reject solutions coming from MEUF. VMD was tested under varying reject dye concentrations, permeate pressure and membrane thickness to prove its utility. A cross flow configuration with use of waste heat or solar energy could be an economical option for operating VMD integrated with low pressure MEUF to yield high water recovery for industrial reuse and concentrated dye as an input in paint production or textile manufacture [28]. In order to design a commercial unit of MEUF + VMD, reliable mathematical or theoretical models need to be developed to consider the effect of key operating parameters for proper prediction of the transport phenomena. Flux decline in ultrafiltration is a major setback. Numerical models have been developed in this study to analyse the process flux, and to estimate model parameters. Some of the widely used models for MEUF include resistance in series model [29,30] and gel polarization model [31]. Additionally, in VMD, membrane pore wetting and estimation of liquid entry pressure (LEP) are important parameters since the hydrophobic membrane used in the study, must be operated under LEP conditions that prevent wetting. Only a single study [32] was reported on calculation of liquid entry pressure (LEP) using computational fluid dynamics (CFD) models. Considering the above developments, the current study includes CFD for prediction of liquid entry pressure of the synthesized membrane in VMD process.

reduction and advanced oxidation processes [5]; biological techniques like aerobic and anaerobic digestion [6,7] and electrochemical techniques such as electrodialysis/electrocoagulation [8,9]. Due to low degradability of dyes, conventional biodegradation in wastewater treatment plants is not very efficient for the treatment of wastewater containing toxic dyes. Adsorption has been used extensively for the removal of pollutants from wastewater compared to coagulation due to its high efficiency and economic feasibility [10]. However, the process is slow and requires regeneration of adsorbents [11]. Recently, membrane separation processes have been widely explored for the treatment of dye solutions. Membrane processes such as reverse osmosis (RO) and nanofiltration (NF) are widely recognised as prominent technologies. However, higher operating pressure, membrane fouling, and generation of concentrated reject streams are some of the major drawbacks of these processes. Ultrafiltration (UF) is another membrane separation process which is not so widely explored for the treatment of dye solutions. It operates at lower pressures, but the desirable rejection of dye cannot be achieved due to relatively large pore size. Current developments dealing with complex dye separations for industrial growth include integrated techniques for process intensification with better product quality. Some integrated processes such as a combination of Electrodialysis and UF [12], electrochemical and biological treatment [13], adsorption and UF [14] etc., were studied for the removal of dye from aqueous solutions. This paper reports a new hybrid membrane process that combines micellar enhanced ultrafiltration (MEUF) with vacuum membrane distillation (VMD) for the first time, for the effective removal of methylene blue from aqueous solutions to generate greater quantity of utility water and a dye concentrate for paint and textile manufacture. MEUF is one promising technology that employs surfactant micelles to solubilise inorganic and organic contaminants present in the aqueous stream. An anionic surfactant such as sodium dodecyl sulphate (SDS) and an electrolyte such as sodium chloride (NaCl) was used to create micellar solutions in the study. An activated carbon loaded PES membrane was synthesized for the process. Addition of surfactant of concentration higher than its critical micelle concentration (CMC) into the wastewater enables the formation of large amphiphilic aggregate micelles. The lowpressure ultrafiltration membrane rejects these higher molecular weight micelles formed due to the hydrophobic and hydrophilic groups of the surfactant. Although MEUF can efficiently remove dyes from aqueous solutions, it also generates reject with highly concentrated solutes, surfactants and electrolytes, which is a major concern as its treatment is cumbersome and does not yield satisfactory results. Over the years, various review articles on MEUF were published [15,16] explaining nitty-gritty of the process including a range of investigations performed by different researchers for removal of different dyes [17,18] as well as phenolic derivatives [19,40], metal ions such as cadmium, nickel, zinc, copper, hexavalent chromium etc., from wastewater [20–23,41]. Also, MEUF has been integrated with coagulation for decolourization of reactive dyes [42]. The studies reported in literature mostly deal with treatment of low dye concentrations while a few were focussed on surfactant recovery from the MEUF retentate. Liquid-liquid extraction, stripping and precipitation are a few techniques proposed in literature out of which [24] showed precipitation as an efficient way to recover surfactants i.e., by addition of ions of a charge opposite to that of the surfactant. Also, orthoethoxycarboxylate was used as a surfactant for copper extraction and studied surfactant recycling from retentate using cloud point extraction (CPE) at a very low pH range of 1,2 [25]. The study showed that utilization of a surfactant which is ionic at higher pH and non-ionic at lower pH, makes CPE an effective approach for surfactant recycle. Reuse or recovery of valuable products from the reject stream would be an efficient route rather than resorting to incineration. To separate water from the reject stream and reuse the solute, a membrane process such as VMD which is efficient in concentrating non-volatile solutes (dye) by removing water further, is proposed. The novelty of this work

2. Material and methods 2.1. Materials Methylene blue (MB) (C6H18ClN3S), a monovalent cationic dye (molecular weight: 319.85 g/mol) was procured from S. D. Fine Chemicals, Mumbai, India. Polyethersulfone (PES) and Polystyrene (PSt) and tetraethyl orthosilicate (TEOS) were obtained from Sigma Aldrich Chemical Co., Milwaukee, WI, USA. Dimethylacetamide (DMAc) was supplied by S. D. Fine Chemicals Limited, Mumbai, India. Hydrochloric acid (HCl) of 35 wt% was procured from SD Fine Chemicals Ltd., Mumbai, India. Activated carbon with particle size 300 mesh and MB adsorption of 270 mg/g was obtained from S. D. Fine Chemicals Ltd., Mumbai. Demineralised water was generated in the laboratory itself by using a double pass reverse osmosis (RO) system. Sodium dodecyl sulphate (SDS), an anionic dye was used as a surfactant in the current study. The critical micelle concentration (CMC) of the surfactant, SDS in distilled water, considered in this study was 8.2 × 10−3 M at 25 °C as determined by linear plot of surface tension and average equivalent conductance [43,44]. 2.2. Membrane synthesis 2.2.1. Activated carbon loaded PES membrane for MEUF Polyethersulfone (PES) membrane loaded with activated carbon was prepared by phase inversion technique. 15 (w/v) % polymer solution was prepared in DMAc at an ambient temperature. To this homogenous 2

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bottom chamber (which acts as a permeate chamber) were separated by the membrane which was supported by a stainless steel mesh to provide a uniform surface. Both glass couplers were secured tightly on to the membrane with Teflon gaskets using high vacuum silicone grease. The permeate line was connected to the cell by means of a B–24 glass cone which was fixed to high vacuum glass valve on one side followed by a glass condenser trap consisting of a small detachable collector on the other side. A rotary vacuum pump was used to apply vacuum on the permeate side, whichwas measured using an Edwards’s McLeod gauge. The feed side was maintained at atmospheric pressure and the downstream side pressure was controlled using a valve. The collected permeate was weighed using sartorius electronic balance (Model no: BSA224S-CW) within an accuracy of ± 0.0001 g, in order to calculate flux. On a laboratory scale, experiments pertaining to both MEUF and VMD processes had to be conducted separately since the membrane area is not large enough to permit continuous inflow of MEUF reject into the VMD system. However, the reject solution from MEUF was collected, analyzed for its composition before being subjected to VMD. Samples of feed, permeate and reject in all the experiments were analysed using a UV/VIS spectrophotometer supplied by LabIndia UV3092. Absorbance values were recorded at a wavelength of 661 nm for maximum absorbance (λmax) corresponding to each dye. A calibration curve was prepared for each dye using known concentrations and measuring the absorbance in each case.

polymer solution, varying wt% of activated carbon was added, stirred overnight and sonicated for 30 min before casting. The solution was cast to the desired thickness using a doctor's blade on to a polyester non-woven fabric support, which was affixed onto a glass plate. After the solution was cast onto the support, the glass plate was immersed into a non-solvent bath for 10 min to obtain a porous membrane. Different membranes were prepared by varying the amount of AC added with respect to the polymer wt% (i.e., 0, 5, 10, 15 and 20 of AC in polymer). 2.2.2. Synthesis of TEOS crosslinked polystyrene membrane for VMD TEOS crosslinked PSt membrane was synthesized by phase inversion technique, 30% (w/v) polymer solution was prepared in DMAc solvent at an ambient temperature. To this, TEOS was added as a crosslinker along with 1 mL of 35 wt% concentrated HCl as a catalyst. The mixture was stirred overnight at 60 °C to synthesize polystyrene-silicate hybrids. Addition of TEOS created covalent bonds with PSt, enhancing the membrane strength and creating a rather robust silica network within [33]. The de-aerated mixture was spread on a non-woven polyester fabric maintaining a fixed thickness followed by immersion within a non-solvent water bath for 10 min. The volume ratio of TEOS to PSt polymeric solution was varied as 0, 0.02, 0.03, 0.04, 0.05 and the corresponding membranes were named as M-1, M-2, M-3, M-4 and M-5, respectively. 2.3. Experimental setup

2.4. Membrane calculations

The schematic of the hybrid process i.e., MEUF + VMD proposed in this study is depicted in Fig. 1. MEUF experiments were carried out in an ultrafiltration cell using synthetic solutions with pre-defined amounts of dye (MB), surfactant (SDS) and electrolyte (NaCl) mixed properly in distilled water. Micellar solutions of varying dye concentrations were passed through an AC loaded PES membrane of area 0.009 m2. A low-pressure diaphragm pump (DP-125-100-1) forces the feed in crossflow mode across the membrane where the reject stream was recycled back to the feed tank. A Baumer pressure gauge was used to measure the pressure which was regulated by using a valve in the bypass line. The time taken to collect the permeate was noted down to calculate the process flux. Experiments were carried out with a recovery of 80% and the remaining solution i.e., reject wastewater was subsequently used as feed for VMD. A TEOS crosslinked PSt membrane of area 0.002 m2 was used for VMD. The cell consists of two bell-shaped B–24 size glass column couplers attached together with external padded flanges by means of tie rods to make the arrangement vacuum tight. The feed chamber and the

Permeate flux, J, in (m3/m2s) and % dye rejection were calculated using Eqs. (1) and (2) respectively.

VP ⎞ J=⎛ t AM ⎠ × ⎝ ⎜

⎟

(1)

where, Vp is the volume of the permeate (m3), Am is the effective membrane surface area (m2) i.e., t is the permeation time (s).

C % R = ⎛1 − P ⎞ × 100 CF ⎠ ⎝ ⎜

⎟

(2)

where R is the percentage of observed rejection of dye, CP is the concentration of dye in permeate (kg/m3), CF is the concentration in feed solution (kg/m3), respectively.

Fig. 1. Schematic of the proposed integrated process (MEUF + VMD). 3

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Fig. 2. (A): Mass balance over the concentration boundary layer in MEUF process; (B): Development of CFD model for the VMD study.

the more volatile component in the form of vapor from the feed to permeate side. When applied feed pressure is higher than the surface energy at the pore entrance, the liquid stream enters the pores of the membrane causing membrane wetting. This factor plays an important role in VMD since it jeopardizes the process performance despite the hydrophobicity and the controlled pore size. Liquid entry pressure (LEP) is the minimum pressure difference over the membrane at which the feed solution penetrates through the dried membrane pores. It depends on surface tension, contact angle of the liquid and pore size of the membrane. Thus, it is important to perceive the LEP of a membrane and operate under the LEP conditions to prevent membrane wetting. The membrane was placed in a static acrylic cell between the feed and permeate chamber. The feed chamber was filled with a feed solution containing dye, surfactant and electrolyte whereas the permeate side was connected to a capillary of 1 mm diameter. Initially, low pressure was applied on the feed side for 10 min, while the permeate side was maintained at atmospheric pressure. The feed side pressure was increased in small steps (10 kPa each). The capillary was filled with water, thus forming a stagnant meniscus when there was no applied pressure. With a step increase in hydrostatic pressure, stagnant meniscus moves up, which is measured using a cathetometer for determining the LEP value [34].

2.5. Membrane characterization 2.5.1. Tensile strength The mechanical properties of the synthesized membranes were evaluated via Shimadzu tensile testing machine (Model AGS-10KNG) with a cross head speed of 10 mm/min with the sample length being 50 mm. The measurement was performed at ambient temperature and 27% relative humidity. 2.5.2. SEM, FTIR and contact angle The prepared membranes were characterized by using scanning electron microscopy (SEM) (JEOL JSM 5410, Japan) to study the surface and cross-sectional morphologies of the membranes. Fourier transform infrared (FTIR) spectroscopy was performed using a Shimadzu FTIR instrument to analyse the chemistry of the membrane surface after the addition of the crosslinking agent i.e., TEOS. Contact angles of indigenously synthesized hydrophobic membranes were measured by introducing a water droplet of 5–8 µL volume using a micro-syringe over the dry membrane surface in a Goniometer (Model no.: 190-F2, rame-hart instrument co., United States) equipped with high-resolution image capture system with Surfaceware 7 analysis software. 2.5.3. Measurement of liquid entry pressure (LEP) In VMD, a hydrophobic membrane acts as a barrier and transports 4

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3. Theory of mathematical and CFD models

The resistance to concentration polarization, Rp can be given by the following equation:

3.1. Calculation of mass transfer coefficient for MEUF

−

Rp = αδC Over the years, a few simple models were developed for explaining the limiting flux, identifying the flux decline in ultrafiltration, and determining the mass transfer coefficients. Gel polarization model, which is one of the most widely used models to analogize UF experimental data, was used to predict the mass transfer coefficient. Thus, the limiting flux ‘Jplim’ can be predicted from thin-film theory and is represented as:

Jplim = k ln

where α is the specific resistance of the layer which is deposited on the membrane surface and can be given as α =α 0 ΔP , and, α 0 is the specific resistance coefficient. Thus, Eq. (10) can be further modified into: −

Rp = α 0 δC ΔP

(11)

Substituting the mean concentration term as in Eq. (7), in the above equation, we get:

Cg C0

(10)

(3)

Rp =

where Cg and C0 are solute concentrations at the membrane surface and in the bulk fluid, respectively, and k is the average mass-transfer coefficient which can be obtained from the slope of the graph Jplim v/s lnC0. It was observed that, experimentally, the flux remained constant at a certain pressure. This is called the limiting flux. The gel polarization model assumes that the concentration of the gel layer is constant and the thickness of the gel layer increases with an increase in applied pressure [35].

α 0 C0 δ (exp Pe − 1)ΔP Pe

(12)

Therefore, the permeate flux using modified resistance in series model can be represented as:

ΔP

Jv = μRm +

(

α 0 C0 δμ Pe

) (exp Pe − 1)ΔP

(13)

To determine the model parameters, the above equation can be rewritten in a linear form ΔP as: Jv

α C δμ ΔP = μRm + 0 0 (exp Pe − 1)ΔP Pe Jv

3.2. Modified resistance in series model The assumptions in the gel polarization model are not practically viable since the gel layer concentration is not constant but a function of both feed velocity and bulk concentration. Classical resistance in series model only relates permeate flux to TMP and other resistances due to membrane fouling. Thus, the classical resistance in series model was further modified [36]. Among the various models developed for ultrafiltration, modified resistance-in-series model predicts the influence of mean solute concentration and also relates permeate flux to transmembrane pressure (TMP) and other constant resistances arising from concentration polarization Rp. Fig. 2(A) shows the mass balance over the concentration boundary layer and can be represented by:

CJv − D

∂C =0 ∂x

3.3. Computational fluid dynamics model for LEP Fig. 2(B) represents CFD model developed by considering (a) crosssection of the membrane having cylindrical pores and (b) the VMD modular configuration. The computational domain (c) is shown with feed side containing methylene blue solution and a single pore geometry of the membrane with circular cross-section. The blue colour area represents the feed chamber containing MB solution while the golden area exists in the air phase. A 2D solution of single pore geometry was chosen to eliminate intricacy and reduce the total number of mesh cells. The meshed (triangular) computational domain of the process built for CFD simulation is also shown in (d). The direction of the flow is from feed to permeate side. One major challenge of this study is capturing the movement of the fluid/fluid interface at a sub-micrometre pore scale to scrutinize LEP. Thus, a multiphase flow simulation method such as the laminar two-phase flow level-set method (LS) was used for the modelling. The assumptions used for developing the model are as follows [32]:

(4)

Which results in:

J C (x ) = C0 exp ⎛ v ⎞ x ⎝D⎠

(5)

where D is the diffusion coefficient and Jv is the permeate flux. Mean − concentration of C in the concentration boundary layer whose thickness is δ can be written as:

• The methylene blue solution and air are assumed to be immiscible and incompressible. • The physical properties of both methylene blue solution and air were assumed constant at 30 °C. • The membrane pore wall was considered as an adiabatic rigid wall. • The hydrophobicity of the pore wall was assumed uniform with constant contact angle (113°). • Contact angle hysteresis was neglected. • No-slip boundary condition at the pore wall was assumed.

δ

∫ C (x ) dx

−

C=

0

δ

(6)

Substituting Eq. (5) in Eq. (6), the mean concentration becomes: −

C=

C0 (exp Pe − 1) Pe

(7)

In the above equation, Pe represents Pectlet number and is given as:

Pe =

Jv δ J = v D k

The movement of the fluid/fluid interface in this model is governed by the momentum transport Eq. (15), a continuity Eq. (16) and a level set Eq. (17):

(8)

where k is the mass transfer coefficient. Classical resistance in series model can be represented as:

1 ΔP Jv = μ (Rm + Rp)

(14)

ρ (9)

where transmembrane pressure (TMP) is denoted by ΔP , while Rm and Rp are the membrane resistance and the resistance to concentration polarization, respectively. 5

∂u + ρ (u·∇) u = −∇P + ∇ ·(μ∇u) + ρg + Fst ∂t

(15)

∇ ·(u) = 0

(16)

∇φ δφ + ∊ls ∇φ⎞⎟ + u·∇φ = γ ∇ ·⎜⎛−φ (1 − φ) | ∇φ | δt ⎝ ⎠

(17)

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4.2. Micellar enhanced ultrafiltration of methylene blue dye

Table 1 Performance of membranes used in this study. S. No.

Membrane

Tensile strength (MPa)

% Elongation at break

Permeate flux (kg/ m2 h)

% Dye rejection

1 2 3 4 5

M-1 M-2 M-3 M-4 M-5

4.7 26.2 39.8 46.2 53.1

23.54 27.31 30.21 33.08 35.61

72.6 70.72 66.27 45.22 30.6

96.3 98.1 99.6 99.7 99.8

(0 g) (0.75 g) (1.5 g) (2.25 g) (3 g)

4.2.1. Effect of membrane thickness on MEUF performance The effect of varying membrane thickness on process performance has been studied with dye concentration kept constant at 0.01 kg/m3, surfactant concentration as 4 CMC, and pressure of 250 kPa. Three membranes have been prepared and tested in this study to understand the process performance. The thickness of the three membranes was found to be 123.9, 141.9 and 160.4 µm, respectively. As the thickness increased, the flux decreased from 1.6 × 10−5 m3/m2 s to 1.91 × 10−5 m3/m2 s and the % dye rejection slightly increased from 99.4 to 99.7%. Out of the three, the second membrane showed moderate flux of 1.84 × 10−5 m3/m2 s and a dye rejection of 99.6%, which has been used for further studies.

where ρ is the density (kg/m3), u is velocity (m/s), t is time (s), μ is dynamic viscosity (Pa.s), p is pressure (Pa), Fst is surface tension force (N/m3), φ is level set function, γ and ∊ls determine the amount of reinitialization and thickness of the interface, respectively. The density and viscosity are calculated from Eqs. (18) and (19)

ρ = ρ1 + (ρ2 − ρ1) φ

(18)

μ = μ1 + (μ 2 − μ1) φ

(19)

4.2.2. Effect of AC loading on MEUF performance The effect of AC loading on tensile strength and elongation (%) was studied for five different membranes, as presented in Table 1. It is observed that as AC loading increased, both tensile strength and % elongation enhanced. This could be attributed to the compatibility between the additive and the polymer, which tends to improve the mechanical properties [45]. It can also be observed that, at higher percentages i.e., beyond 15%, the AC blocks the pores, resulting in a stronger and denser membrane with reduced void volume, thus increasing mechanical strength. Mass transfer mechanism such as water flux and % dye rejection with varying AC loading was studied with 0.01 kg/m3 of dye concentration. With an increase in AC loading, flux of the membrane decreased, and the % dye rejection increased. M-1 which is pristine PES membrane showed higher flux and lower rejection of dye compared to M-2. M-4 and M-5 showed higher dye rejection but a decline in flux. Based on these observations, M-3 was selected as the most favourable membrane that can be used for MEUF experiments as it resulted in reasonable flux and % dye rejection with good mechanical strength.

where ρ1, ρ2 , μ1 and μ 2 are densities and viscosities of both fluids, respectively. To estimate the LEP, initially, both fluids were kept stationary. The hydrophobic wall of the membrane was in direct contact with methylene blue reject stream coming from MEUF process. The primary location of the interface was maintained at the pore entrance by defining the initial volume fraction distribution. The simulations were carried out first by applying an inlet pressure (Pin) at the feed side to observe the LEP at the pore entrance. The outlet of the pore was modelled as open to atmospheric pressure (Patm). The inlet pressure was increased in small steps of 10 kPa each to monitor the effect of increasing pressure on LEP whereas the outlet was kept constant at atmospheric pressure. 4. Results and discussion

4.2.3. Effect of surfactant concentration on % dye rejection and variation of flux The feed surfactant concentration is one of the most important factors in MEUF. The feed MB concentration was fixed at 0.01 kg/m3 and experiments were carried out at a pressure of 250 kPa. The influence of feed SDS concentration on % dye rejection could be observed in Fig. 3(A). As the feed SDS concentration increased from 0 to 6 CMC (0 to 14.184 kg/m3), the dye rejection increased from 12 to 99.4% and the permeate flux gradually decreased from 2.43 × 10−5 to 1.19 × 10−5 m3/m2 s. This is because, with an increment in CMC, more micelles were aggregated with greater solubilisation of MB molecules in the micelles. Without SDS, it was observed that the rejection of dye was low since no micelles were formed and free dye molecules passed through the membrane easily. 4 CMC of SDS showed high flux and dye rejection i.e., 1.8 × 10−5 m3/m2 s and 99.6%, respectively. Further increment in CMC did not show any significant effect on the process performance. Higher concentration of SDS induced deformation of micelles near the membrane surface, thus, blocking the membrane and increasing the operating costs. Also, the permeate concentration of SDS was below the CMC up to a feed concentration of 4 CMC. When the feed SDS concentration increased beyond 4 CMC, the permeate SDS concentration increased due to convective transport of SDS molecules through the membrane promoted by concentration polarization. Thus, 4 CMC of SDS was selected as the optimum surfactant concentration for 0.01 kg/m3 dye concentration.

4.1. Membrane characterization 4.1.1. SEM SEM micrographs of both AC loaded PES membrane and TEOS crosslinked PSt membrane have been depicted in Fig. S1(a)–(d). The membranes were synthesized via phase inversion and analysed by Field Emission Scanning Electron (FE-SEM) microscope. Fig. S1(a) shows some macro-void substructures visible due to the presence of powdered activated carbon, with an average pore size of 0.02 µm. From Fig. S1(c), it can be observed that pores of average diameter of 0.01 µm are available all over the surface. Fig. S1(b) and (d) depict the crossectional morphologies of both the membranes with an active layer thickness of 62.93 and 62.86 µm, respectively. It is evident that the solvent used in preparation of both the membranes produces a dual layer structure. The inner finger-like-pores are bordered by a porous outer surface implying that both the membranes have their own selective skin, making them integrally skinned asymmetric membranes. 4.1.2. FTIR Spectroscopic analysis plays a crucial role in characterization of modified membranes to study the presence of an incorporated constituent. The infrared spectrum was measured in the range 4000–400 cm−1. Fig. S2 shows FTIR spectra of (a) pristine and (b) TEOS crosslinked PSt membranes. The peaks obtained between 1300 cm−1 represent SieOeSi valent vibrations. Peaks obtained at 1068 cm−1 signify the typical absorption bands of SieOH groups which confirm crosslinking of the PST membrane by TEOS. In case of pristine PSt, the peaks between 3000 and 3200 cm−1 correspond to eCH stretching.

4.2.4. Effect of feed dye concentration on % dye rejection and variation of flux The feed MB concentration was varied from 0.005 to 0.04 kg/m3 and the SDS concentration was fixed at 4 CMC. The experiments were carried out at 250 kPa and 30 °C to study the effect of dye concentration 6

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Fig. 4. (A) Effect of varying pressure and (B) Effect of NaCl concentration on permeate flux and % rejection of methylene blue.

Fig 3. (A): Effect of SDS concentration on permeate flux and % dye rejection; (B): Effect of feed dye concentration on permeate flux and % dye rejection.

concentration on MEUF process [37–39]. Fig. 4(B) shows the effect of electrolyte on permeate flux and dye rejection while the concentration of feed dye and surfactant were fixed at 0.01 kg/m3 and 4 CMC, respectively. An optimum pressure of 250 kPa was used to run the experiments while the concentration of NaCl in the feed was varied from 0 to 20 kg/m3. It can be seen that % dye rejection was low when there was no electrolyte in the feed. As the electrolyte concentration was increased, the dye rejection enhanced from 71.8% to 99.6% while the permeate flux slightly decreased from 2.43 × 10−5 to 1.84 × 10−5 m3/ m2 s. Reported studies attribute this result to a decrease in CMC of the surfactant and an increase in aggregation number of the SDS micelles caused by the oppositely charged sodium ion of NaCl. It was further explained that the sodium ion of the electrolyte gets attached to the head groups of the surfactant thus, neutralizing the charge of micelles. This weakens the electrostatic repulsion between the head groups, thereby increasing the aggregation of micelles. The same phenomena could be observed in the present study resulting in higher dye rejection.

on membrane performance. The concentration of electrolyte was also fixed at 20 kg/m3. As shown in Fig. 3(B), with an increase in feed dye concentration from 0.005 to 0.04 kg/m3, a decrease in the permeate flux from 1.94 × 10−5 to 1.47 × 10−5 m3/m2 s and % dye rejection from 99.8 to 95.4 was observed. The slight decrement in permeate flux can be due to the additional resistance to flux created by accumulation of micelles on the membrane surface. Since the SDS concentration in the feed was kept constant, the concentration of micelles formed was also constant. Thus, the concentration of unsolubilized dye kept increasing which helped them to directly pass through the pores, decreasing the % dye rejection. 4.2.5. Effect of pressure on % dye rejection and variation of flux Variation of % dye rejection and permeate flux with applied pressure is shown in Fig. 4(A). The feed dye concentration, surfactant concentration and electrolyte concentration were fixed at 0.01 kg/m3, 4 CMC and at 20 kg/m3 respectively. An increase in pressure from 150 kPa to 500 kPa caused the % dye rejection to decrease from 99.7% to 94.81% while permeate flux increased from 1.18 × 10−5 to 2.21 × 10−5 m3/m2 s. The decrease in % dye rejection can be attributed to the decrease in solubilisation capability of micelles, thus, compacting and ramming the micelles down the permeate side. On the other hand, the increment in permeate flux could be due to increased driving force for mass transport by overcoming membrane resistance. Thus, a pressure of 250 kPa was considered optimum since it resulted in high rejection of dye along with a moderate water flux.

4.3. Model parameters for modified resistance in series model 4.3.1. Validation of the mathematical model Model parameters such as membrane resistance (Rm), mass transfer coefficient (k), specific resistance coefficient (αo) were estimated by using the procedures described under “Section 3: Theory of Mathematical and CFD Models”. In the present study, the variation of permeate flux and other model parameters were studied by varying the feed concentration. As stated in gel polarization model, the mass transfer coefficient (k) was predicted from the slope of the graph Jvlim v/s lnC0 and which is shown in Fig. 5(A). It is also possible to estimate the gel layer concentration by extrapolating the Jvlim v/s lnC0 curve so that flux is zero and C0 = Cg. As feed concentration increases, a minor decline in

4.2.6. Effect of electrolyte on solubilisation of dyes Various researchers have studied the effect of electrolyte (NaCl) 7

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Table 2 Estimated parameters from resistance-in-series model. Dye concentration (kg/m3)

Specific resistance coefficient, α0 (m mol−1 pa−1)

Membrane Resistance, Rm (m−1)

Jvlim (m3/m2 s)

0.005 0.01 0.02 0.04 0.05

1.027 × 1013 8.38 × 1013 7.87 × 1013 9.92 × 1013 8.74 × 1013

1.05 × 1013 1.05 × 1013 1.05 × 1013 1.05 × 1013 2.11 × 1013

1.94 × 10−5 1.84 × 10−5 1.72 × 10−5 1.47 × 10−5 1.17 × 10−5

depicted in Table 2. The Rm value was greater for high dye concentration. Usually, membrane resistance (Rm) is the hydraulic resistance of the membrane, an intrinsic property which contemplates to pore structure and morphology. The increment in this resistance could be due to membrane tortuosity and pore structure. Also, as explained by S. Ghadge et al., Rm being a combination of membrane resistance and adsorption resistance, the marginal increase could be because of slender adsorption of dye molecules over the membrane surface and the pores [30]. Permeate flux was calculated using Eq. (1) at a constant pressure of 250 kPa and different dye concentrations. The second term in the denominator of the same equation corresponds to resistance due to concentration polarization. It was observed that the membrane resistance (Rm) and resistance due to concentration polarization (Rp) were high at higher dye concentrations. The results obtained for varying dye concentrations are shown in Table 3. The model fits best for all concentrations with minimum percentage error. 4.4. Vacuum membrane distillation of methylene blue 4.4.1. Effect of feed dye concentration on VMD flux and % dye rejection Fig. 6(A) shows the variation in the water flux and % dye rejection for different feed dye concentrations processed with TEOS crosslinked PSt membrane. The permeate pressure, membrane thickness was set at 20 mmHg, 140 μm. Permeate flux and % dye rejection were found decreasing from 1.28 × 10−6 to 0.31 × 10−6 m3/m2 s and 99.83 to 96.55, respectively, upon increasing the feed dye concentration from 0.028 to 0.22 kg/m3. This decrease in flux and rejection can be attributed to concentration polarization on the membrane surface. As the dye concentration increased and surfactant concentration kept constant, the formation of micelles was limited. The dye solubilisation capacity decreases, resulting in permeation of unsolubilized dye into the permeate side and hence reduces the observed retention of dyes. Also, since the dye molecules pass through the membrane pores, a resistance to solvent flux takes place which explains the flux decline. This study shows the ability of the synthesized membrane in the treatment of highly concentrated dye for the first time. The flux and % dye rejection values confirm that VMD could minimize energy costs for the treatment of reject from MEUF compared to multiple effect evaporators.

Fig 5. (A): Plot of JvLim v/s lnC0 representing variation in permeate flux; (B): Representation of ΔP/Jv v/s C0(exp Pe − 1)ΔP/Pe (a) for 0.005 kg/m3 dye concentration; (C): for 0.05 kg/m3 dye concentration.

flux was observed which indicates that the gel layer formation is negligible at lower dye concentrations. From the figure, it is found that the mass transfer coefficient obtained from the slope of the plotted graph is 5 × 10−6 m/s and the gel concentration is about 403 kg/m3, which is in corroboration with the literature where a 10 kDa hydrophilic membrane was used to study ultrafiltration of surfactant solutions. The specific resistance coefficient and membrane resistance were calculated from the graphs plotted for ΔP/Jv versus C0(exp Pe − 1)/Pe, for increasing dye concentration (0.005–0.05 kg/m3). Fig. 5(B) and (C) shows the plots for 0.005 and 0.05 kg/m3 dye concentration. The graph shows a straight line with a slope ‘α 0 μδ’ where α 0 is the specific coefficient resistance coefficient while Rm was calculated from the intercept obtained. The obtained values α 0 , Rm and Jvlim are

4.4.2. Effect of permeate pressure on VMD performance Fig. 6(B) shows the separation performance of TEOS crosslinked polystyrene membrane under varying permeate pressure of 2–30 mmHg at a constant membrane thickness of 140 μm and feed dye Table 3 Comparison of experimental flux with theoretical values. Rp (m−1)

Jvmod (m3/m2 s) 12

1.92 × 10 2.57 × 1012 3.88 × 1012 5.96 × 1012 3.84 × 1012

8

−5

2.11 × 10 2.01 × 10−5 1.83 × 10−5 1.59 × 10−5 1.06 × 10−5

Jexp (m3/m2 s) −5

1.94 × 10 1.84 × 10−5 1.73 × 10−5 1.47 × 10−5 1.17 × 10−5

% Error 8.88 9.08 5.64 8.42 9.71

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Fig. 6. (A): Effect of feed dye concentration on process flux and % dye rejection in VMD; (B): Effect of permeate pressure on membrane flux and % dye rejection in VMD; (C): Effect of membrane thickness on permeate flux and % dye rejection in VMD.

cannot be easily penetrated. Therefore, water flux falls, and the rejection of dye increases further.

concentration of 0.05 kg/m3. The water flux decreased from 2.15 × 10−6 to 0.57 × 10−6 m3/m2 s and the % dye rejection decreased from 99.7 to 97.2. The decrease in dye rejection was due to higher vacuum which floods the membrane pores with the feed liquid instead of, leading to a change in mass transfer mechanism inside the pores. The main contribution to the driving force is the vapour pressure gradient between the feed and permeate sides of the membrane. A decrease in flux is caused by an increase in permeate pressure, since maximum diving force is generally obtained at zero permeate pressure, a fact confirmed mathematically and experimentally [46]. Mericq et al., 2010 studied the effect of permeate pressure on VMD for treatment of seawater reverse osmosis brines and explained that a low permeate pressure could yield a high permeate flux. Also, even when the permeate pressure is gradually decreased, the energy requirement of VMD would be quite invariable as the energy necessary to maintain vacuum would only be a small fraction of the total energy requirement (less than 2%) [47].

4.5. Computational fluid dynamics for VMD 4.5.1. Effect of contact angle and membrane thickness on LEP The contact angles of the synthesized membranes were found out using goniometer sessile drop technique by spreading a drop of the dye solution over the top surface of the dry membrane. Out of the five membranes, M-5 exhibited the highest contact angle i.e., 113°, whereas lowest contact angle was observed for M-1 i.e., 86° which indicated that TEOS improved hydrophobicity of the synthesized membrane. LEP was calculated experimentally for all five membranes. Fig. 7(A) shows the effect of contact angle on LEP for the five membranes. The LEP value was found to be 56 kPa for pristine polystyrene, whereas, the value significantly increased to 96 kPa with increase in contact angle. The simulated result of the CFD model developed for estimation of LEP is shown in Fig. 7(B). The volume fraction of water (VOF) plot for M-5 membrane is shown where initially an air-water interface at the pore wall was observed. With a positive pressure on the feed side increasing in steps, the movement of the interface through single pore geometry could be observed. The TEOS crosslinked PSt membrane with the highest contact angle of 113° (M-5) showed complete wetting at a pressure of 115 kPa. Thus, 115 kPa was considered the LEP for M-5 membrane when the feed solution contained 0.028 kg/m3 of dye. Both experimental and CFD results reveal that as contact angle of the membrane increased, LEP values increased as greater hydrophobic nature decreased the risk of wetting.

4.4.3. Effect of membrane thickness on VMD performance Fig. 6(C) represents the influence of membrane thickness that was varied from 100 to 180 µm, over process flux and % dye rejection. All other parameters such as permeate pressure, feed temperature and feed dye concentration were maintained constant at 20 mmHg, 30 °C and 0.05 kg/m3, correspondingly. With increasing thickness, the flux expectedly decreased from 0.99 × 10−6 to 0.14 × 10−6 m3/m2 s due to enhanced mass transfer resistance. Even, the dye rejection increased from 98.6 to 99.8%, as higher membrane thickness poses greater mass transfer resistance to the entry of larger dye molecules into the membrane pores. The hydrophobic membrane in VMD allows no interaction with feed components, but only vaporization of volatile components (in this case, water). Methylene blue is a non-volatile molecule. The membrane does not get wetted by water, which thus cannot drag dye along with it. The enhanced thickness enables a tortuous path which

5. Conclusions Micellar enhanced ultrafiltration was integrated with vacuum membrane distillation for treatment of aqueous solutions of methylene 9

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Fig. 7. (A) Correlation between contact angle and LEP obtained through experiments and CFD simulation, and (B) Volume fraction of water (VFW) contour plot at the initial stage, movement of the interface and under complete pore wetting conditions (Feed: Aqueous methylene blue solution, Outlet Pressure: 101.325 kPa).

velocity and enhance downstream vacuum to achieve commercially viable flux. The product from VMD would be pure water since the dye is a non-volatile solute. Moreover, the integrated process can be operated in continuous mode using greater membrane area in VMD to match MEUF process flux with permeate water being recycled to cooling towers, and the concentrated dye made use of as a useful ingredient in paint manufacture. The promise shown by the hybrid process could extend its applications in treatment of effluents coming from distillery, starch, pharmaceutical and tannery industries to achieve maximum water recovery with pollution abatement.

blue dye. A hydrophilic PES membrane loaded with activated carbon was used for micellar enhanced ultrafiltration, where sodium dodecyl sulphate and sodium chloride were used as surfactant and electrolyte, respectively to create micelles. A numerical model based on modified resistance in series model was developed to estimate the flux decline in ultrafiltration. The developed model fits well and was successful in predicting the operating parameters and generated a minimum error when experimental flux was compared with simulated flux. Further, water recovery from the reject stream of micellar enhanced ultrafiltration process was successful via vacuum membrane distillation. The synthesized tetraethyl orthosilicate crosslinked polystyrene membrane resulted in enhanced hydrophobicity. Both the membranes showed excellent stability and their potential in processing dye solutions. A computational study was carried out to detect the liquid entry pressure of the membrane. The developed model gave an interesting insight into the concept of liquid entry pressure and movement of fluid-fluid interface inside the membrane pore. It was observed that the CFD simulation provided liquid entry pressure results that were quite close to reality. Process integration of this type could lead to benefits in industrial economy especially if VMD is operated on solar power or waste heat is made available. The study has helped to devise a complete process for treating the reject coming from MEUF by VMD for simultaneous recovery of pure water and dye. During process scale up, VMD operating parameters can be optimized to reduce membrane thickness, increase cross flow

Acknowledgements The authors thank the Knowledge & Information Management (KIM) Department, CSIR-Indian Institute of Chemical Technology for plagiarism check of this manuscript. Manuscript Communication Number: IICT/Pubs./2018/374. Disclosure There are no financial conflicts of interest to disclose. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 10

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doi.org/10.1016/j.cej.2019.122015. [25]

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