Performance assessment of an analcime-C zeolite–ceramic composite membrane by removal of Cr(VI) from aqueous solution

Abstract

This article describes the synthesis of an analcime-C zeolite membrane on a ceramic support by in situ hydrothermal crystallization. A circular shaped ceramic support was firstly fabricated using low cost raw materials by a uni-axial pressing method and sintering process. Subsequently, the zeolite composite membrane was prepared with the repeated coating of analcime-C on the ceramic support through an in situ crystallization technique. The synthesized zeolite composite membrane was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), porosity, average pore size and pure water permeability. The influence of the number of coatings on the characteristics of the zeolite membrane was also explored. The obtained results clearly demonstrate that the porosity, pore size and water permeability of the membrane decrease significantly with the multiple coating of zeolite over the ceramic support. The porosity, average pore size and pure water permeability of the zeolite membranes are estimated to be 38–24%, 285–155 nm and 2.18 × 10⁻⁷ to 4.53 × 10⁻⁸ m³ m⁻² s⁻¹, respectively for various coatings (1–3). Finally, the separation performance of three times coated zeolite membrane was evaluated by removal of chromium(VI) from aqueous solution by ultrafiltration (UF) at various operating conditions (applied pressure, concentration and pH). The maximum rejection of 84% is achieved at an applied pressure of 207 kPa. Moreover, the separation performance of membrane is better as compared to other membranes reported in the literature.

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1. Introduction

Chromium is one of the most useful and dangerous materials having a wide range of applications in the metal and chemical industries such as steel, chemical, leather, textile manufacturing, and in electroplating.¹ These industries produce waste matter with a huge quantity of chromium primarily in two forms, viz. Cr(III) and Cr(VI), and amongst these, Cr(VI) is extremely harmful.^1–3 Through the above industrial activities, chromium compounds easily get dissolved in water, raising its concentration in the environment. Cr(VI) distresses skin, the respiration system as well as kidneys.^4,5 The permissible discharge of Cr(VI) into the environment is <0.05 mg L⁻¹.⁶ From an environmental point of view, Cr(VI) is necessary to be treated before being discharged from the metal and chemical industries. The conventional methods employed in chromium recovery and removal are: liquid–liquid extraction,^7–11 chemical precipitation,^12,13 reverse osmosis¹⁴ and dialysis.¹⁵ Many of these techniques, including liquid–liquid extraction, chemical precipitation and ion exchange processes have severe limitations and are also believed to be contaminating processes themselves, owing to the utilization of expensive organic solvents and other chemicals. Moreover, these techniques are highly expensive. Therefore, the necessity of finding cheaper and non-polluting methods has led to the development of the membrane separation process. This process is very attractive, inexpensive and can be carried out at room temperature in addition to its other advantages, such as good stability at high temperature and extreme pH conditions. It also has the merits of low energy requirements, better selectivity and nearly total recovery without addition of any chemicals.

Recently, much research has been focused mainly on chromium removal using charged ultrafiltration and microfiltration membranes.^16–20 Bernat et al.¹⁶ showed that the chemical speciation of metals in aqueous solution was strongly correlated with the retention suggesting a possible interaction between the chromium species and the membrane materials. In another study, Dzyazko et al.¹⁷ developed ceramic membranes containing hydrated zirconium dioxide for Cr(VI) removal. The membranes were permeable to anions in an acidic medium, although they revealed cation-exchange characteristics in an alkaline medium. Tang et al.¹⁸ optimized the factors to make consistent magnetite crystalline membranes on a porous stainless steel support via a sol–gel technique for the removal of Cr(VI). The prepared membrane showed the highest removal at 92.5%. Badawy et al.¹⁹ demonstrated the significance of operating factors on the rejection. The investigation suggested that separation was mainly dependent on formation of a film of water-soluble positively-charged hydrolyzed species adsorbed onto the surface of the membrane. A recent study revealed that chromium removal can be achievable even using charged membranes with a reasonably larger range of pore sizes.²⁰ The materials generally used for the development of ceramic membranes are α-alumina, γ-alumina, titania, zirconia, zeolites, and microporous glasses. Amongst these, zeolites as a separating layer provide numerous benefits; for instance the pore size of the membrane can be regulated using an appropriate zeolite type.^21–23 Moreover, their hydrophilic/hydrophobic character can be changed by the introduction of various functional groups on the zeolite framework.²⁴ Zeolites contain distinct cavities and channels of molecular dimensions and these are more or less regular in size, providing a narrow pore size distribution within the inorganic separating layer. There has been specific interest in the applications of zeolite thin films, which are highly selective separation membranes.

The aim of this work is to fabricate an analcime-C composite membrane by an in situ hydrothermal crystallization method and investigate its separation potential by the ultrafiltration of Cr(VI) from aqueous solution. The membrane was prepared with repeated cycles of the hydrothermal crystallization method on the ceramic support. Water flux, hydraulic permeability and structural characteristics were measured for the prepared zeolite composite membrane. To estimate the separation ability of the membrane, the influence of the significant process factors such as the applied pressure, concentration and pH of the solution on the rejection and permeate flux of Cr(VI) was examined.

2. Experimental

2.1. Materials

Clays (kaolin, ball clay, pyrophyllite, feldspar and quartz) used for the synthesis of the ceramic support were of mineral grade and procured in the vicinity (Kanpur, India). Calcium carbonate (Rankem Chemicals, New Delhi), sodium hydroxide (Merck (I) Ltd, Mumbai), polyvinyl alcohol (PVA) (Loba Chemie, Mumbai), Aerosil 200 (CDH Laboratory Reagent, New Delhi), triethanolamine (TEA) (Merck (I) Ltd, Mumbai), calcium chloride (Titan Biotech Limited, Bhiwadi), ammonium chloride (Rankem Chemicals, Faridabad), hydrochloric acid (Merck (I) Ltd, Mumbai), aluminium fine powder (Merck (I) Ltd, Mumbai), Eriochrome Black T (Merck (I) Ltd, Mumbai), ammonia (Merck (I) Ltd, Mumbai), ethylenediaminetetraacetic acid (S.D. Fine-Chem Limited, Vadodara), and chromium(VI) oxide (Merck (I) Ltd, Mumbai) were utilized as received without further purification.

2.2. Fabrication of the analcime-C zeolite–ceramic composite membrane

The protocol adopted for the synthesis of the ceramic support and its composition were elaborated in our previous work.²⁵ For fabrication of the ceramic support, the raw materials kaolin (14.45 g), ball clay (17.58 g), feldspar (5.60 g), quartz (26.59 g), calcium carbonate (17.14 g) and pyrophyllite (14.73 g) were mixed in a ball mill for 20 min at 40 rpm after adding 4 mL of aqueous PVA (2 wt%). The required quantity of the resulting powder was pressed at 50 MPa using a hydraulic pressing machine to make a circular ceramic support. To remove moisture, the obtained green disk was firstly placed in a hot air oven at 100 °C for 24 h and then dried at 200 °C for 24 h. Finally, the sintering process was carried out at 950 °C for 6 h in a muffle furnace. The obtained porous sintered support was burnished with abrasive paper (no. C-220) to obtain a smooth, polished and uniform surface. Subsequently, the support was placed under water in an ultrasonic bath (Elma, T460) to eliminate the unstitched powders formed during burnishing. After that, the surface of the porous ceramic support was moderately plugged with the analcime-C zeolite crystals developed by in situ hydrothermal crystallization.²⁶ The mixture for the hydrothermal reaction was prepared by dissolving 4.188 g of sodium hydroxide in 150 mL of Millipore water. Then aluminium metal powder (0.6607 g) was slowly added into the NaOH solution, after which an estimated amount of Aerosil 200 (fumed silica) and triethanolamine (TEA) were added and then the reaction mixture was stirred. The molar composition of the final gel was 4.2 Na₂O : Al₂O₃ : 21.3 SiO₂ : 340 H₂O : 2.2 TEA. A flat ceramic support was placed in a Teflon-coated stainless steel autoclave reactor and the above prepared reaction mixture was transferred to the reactor. The hydrothermal crystallization was then carried out at 200 °C for 12 h. During hydrothermal crystallization, the zeolite particles precipitated inside the pores of the ceramic support. After the synthesis, the zeolite membrane was carefully washed several times with Millipore water and dried at 100 °C for 24 h followed by calcination at 400 °C for 6 h in an air atmosphere during which triethanolamine (TEA) was removed.

2.3. Characterization

Zeolite powder was obtained from the bottom of the autoclave reactor, which had been produced during the hydrothermal crystallization reaction. The zeolite powder was then washed several times with Millipore water, then dried and calcined under the same conditions as used for the composite membrane.

2.3.1. XRD analysis. X-ray diffraction of the zeolite powder under air at room temperature was performed using a machine (Bruker AXS instrument) with a Cu Kα radiation source. The patterns were obtained in the 2θ range of 2° to 80° with a scan speed of 0.05° s⁻¹.

2.3.2. FT-IR analysis. The FT-IR analysis was performed using a Shimadzu IRAffinity-1 model spectrometer in the wavenumber range of 4000–450 cm⁻¹. Samples of zeolite powder were prepared in the form of KBr tablets and the spectra were taken at room temperature and atmospheric pressure.

2.3.3. TGA analysis. Thermogravimetric analysis (TGA) of the zeolite powder (before calcination) was carried out using a Mettler Toledo TGA/SDTA 851® under a flowing nitrogen atmosphere with a heating rate of 10 °C min⁻¹ from 25 to 800 °C in a 150 μL platinum crucible.

2.3.4. N₂ adsorption/desorption isotherm. N₂ adsorption/desorption isotherms were measured at −196 °C on a Beckman Coulter surface area analyzer (SA™ 3100). Prior to the adsorption/desorption analysis, the zeolite powder was outgassed at 150 °C for 3 h. The specific surface area was evaluated using a multipoint BET method. The pore size distribution was calculated using a BJH method from the adsorption branch of the nitrogen isotherm. The pore volume was estimated at a relative pressure, P/P₀ of 0.99, assuming full surface saturation with nitrogen.

2.3.5. Point of zero charge (PZC) measurement. The solid addition method was employed to determine the point of zero charge of the composite membrane.²⁷ 50 mg of the solid sample was treated in 50 mL of 0.001 M NaCl solutions at various pH (2–12) for 24 h in a shaking bath. The initial pH (pH_i) of the solution was adjusted using 0.1 M HCl or NaOH. Then the suspensions were shaken and allowed to equilibrate for 24 h. After that, the pH value of the supernatant for each flask was measured. The difference between the final and initial pH (ΔpH = pH_f − pH_i) was calculated and the graph was plotted between ΔpH and pH_i. In the plot, the point of intersection of the resulting curve at which ΔpH = 0 is the pH_PZC value of the composite membrane.

2.3.6. Determination of cation exchange capacity (CEC). The cation exchange capacity (CEC) of the analcime-C zeolite particles was evaluated using the following procedure: 1 g of hydrated calcium chloride was dissolved in 1100 mL of Millipore water and the pH of the solution was adjusted to 9 with NaOH solution. From the solution, 100 mL was taken out to evaluate the Ca²⁺ content. After that, the dried zeolite (1.0 g) was dispersed in the remaining Ca²⁺ solution and stirred for 1 h at room temperature. Subsequently, the solution was filtered and the filtrate was recovered. The concentration difference between 100 mL of the original Ca²⁺ solution and the filtrate was then calculated, which gave the cation exchange capacity in milli equivalent per gram of the dried zeolite.²⁸

The concentration of Ca²⁺ in the solution was calculated by taking 50 mL of the sample and the pH of the sample was adjusted to 10.0 by adding buffer solution (a mixture of NH₄Cl and ammonia water). The solution was titrated against the 0.01 M ethylenediaminetetraacetic acid (EDTA) solution using an indicator, Eriochrome black-T. The end point was observed by the change of color from wine-red to blue. The cation exchange capacity of the zeolite was estimated using the following expression:

where A and B are the volume of 0.01 M EDTA solution consumed for the original and exchanged calcium ion solutions, respectively. C is the volume of the sample taken for titration and D is the weight of the dry zeolite.

2.3.7. FESEM analysis. A Field Emission Scanning Electron Microscope (FESEM, JEOL JSM-5600LV) was used to examine the surface properties of the prepared composite membrane. A small amount of the sample was fixed onto the stub and coated with gold using a JEOL JFC-1300 auto fine coater prior to the morphology assessment.

2.3.8. Porosity. To determine the porosity, the dry weight of the membrane was measured after drying at 120 °C for 3 h. The membrane was then placed in water for 24 h. After that, the wet weight was measured after removing all visible water from the surface of the membrane with tissue paper. Using the dry weight (W_dry), wet weight (W_wet) and total volume (V_membrane), the porosity of the membrane was calculated according to the following equation:²⁹

2.4. Water flux and Cr(VI) removal

The water permeability and separation potential of the composite membrane were measured using an in-house made dead end UF setup as shown in Fig. 1. The water flux was calculated at different applied pressures. At each applied pressure, the first 50 mL of water collected was disposed of and the time taken for the collection of the following 50 mL of water was used for the determination of flux using the following equation:where J_W is the water flux (m s⁻¹), Q is the volume of water permeated (m³), A is the effective membrane area (m²) and ΔT is the sampling time (s). The performance of the composite membrane in the removal of Cr(VI) was tested using the UF setup filled with 100 mL of the feed solution. The Cr(VI) solution was prepared with Millipore water and the concentration was measured using a conductivity cell (EUTECH instruments with model no. CON2700). In order to determine the permeate flux at a fixed applied pressure, the first 10 mL of the permeate collection was discarded and the time taken for collection of the second 10 mL was noted. The observed rejection was calculated using the following expression:where C_f is the concentration of the feed solution, C_p is the concentration of the permeate solution and R is the observed rejection (%). After every experimental run, the membrane was thoroughly cleaned by flushing with Millipore water at a higher pressure to regain the original pure water flux.

Fig. 1 Schematic of the in-house made dead end UF setup used for the permeation experiments.

3. Results and discussion

3.1. Characterization of the analcime-C zeolite powder and composite membrane

Analcime-C zeolite powder (before and after calcination) was subjected to analysis for its structure and purity verification by means of XRD patterns as shown in Fig. 2. The obtained XRD patterns closely match with ICDD data (COD 9008207 Al₂ H₄ Na₂ O1₄ Si₄ analcime) and the standard pattern of analcime.³⁰ The main crystalline phase identified corresponds to the ANA-type structure, although a few extra lines reveal the occurrence of traces of anatase and ilmenite. Moreover, there are no significant changes in the peak positions and intensities for the zeolite before and after calcination. Fig. 3 shows the FT-IR spectra of the analcime-C zeolite powders (as-synthesized and calcined). The bands (antisymmetric and symmetric) for the stretching vibration of hydroxyl groups appear in the wavenumber range of 3610–3550 cm⁻¹ and exhibit high intensity.³¹ The H–O–H bending vibration appears at 1630 cm⁻¹. The bands of the antisymmetric stretching vibration with the variable intensity of the tetrahedral T–O (T = Si, Al) appear in the wavenumber range between 1050 and 810 cm⁻¹.^31,32 The bands observed at 780–660 cm⁻¹ are attributed to the symmetric stretching vibration of T–O bonds. The bending vibration of the tetrahedral bonds are exhibited in the region of 615–450 cm⁻¹.^31,32

Fig. 2 XRD patterns of the analcime-C zeolite powder formed during the hydrothermal crystallization reaction.

Fig. 3 FT-IR spectra of the synthesized analcime-C zeolite powder.

Fig. 4 depicts the TGA curve of the synthesized analcime-C zeolite particles. The total weight loss is estimated to be approximately 8%. The weight loss at temperatures below 150 °C is owing to removal of the unbound moisture that was absorbed by the sample. The weight loss at higher temperature is due to the removal of structural hydroxyl groups. Moreover, the weight loss of the particles after 400 °C is found to be insignificant. Therefore, it can be inferred from the graph that the minimum calcination temperature after coating should be above 400 °C. During calcination at 400 °C, TEA is burnt to leave an open pore zeolite framework. The N₂ adsorption/desorption isotherm of the analcime-C zeolite powder is presented in Fig. 5. The obtained isotherm demonstrates that the adsorbed volume increases with increasing relative pressure and it gives an isotherm of type III. The pore size distribution of the zeolite is calculated from the adsorption isotherm using the BJH model as depicted in Fig. 6. This plot proves that the pore size of the zeolite is in the mesoporous range. It is observed that most pores are below 20 nm (around 80%). The BET surface area and pore volume of the analcime-C zeolite are determined to be 0.329 m² g⁻¹ and 0.0420 mL g⁻¹, respectively. The CEC of the calcined zeolite powder is estimated to be 1.84 meq g⁻¹.

Fig. 4 Thermogravimetric analysis of the as-synthesized analcime-C zeolite powder.

Fig. 5 N₂ adsorption/desorption isotherm of the analcime-C zeolite powder.

Fig. 6 BJH pore size distribution of the analcime-C zeolite powder.

The PZC measurement of the composite membrane was performed at various pH (2–12) and the obtained result is illustrated in Fig. 7. The surface of the membrane changes its polarization according to the pH of the solution and the pH_PZC of the composite membrane. In the present work, the PZC of the composite membrane is found to be pH 5.6. At pH values below the point of zero charge (pH_PZC), the membrane surface is positively charged and at pH values higher than the (pH_PZC), the membrane surface is negatively charged.

Fig. 7 Point of zero charge measurement for the composite membrane.

Fig. 8(a) shows the FESEM image of the fabricated membrane support, and it is apparent that the membrane support is highly porous and crack-free. Fig. 8(b) demonstrates that the zeolite crystals are spherical in shape with sizes ranging between 15 and 20 μm. Fig. 8(c–e) depict the surface images of the analcime-C zeolite ceramic composite membranes with multiple cycle of coatings (1–3, respectively). It is evident that the zeolite particles adhered more with repeated coatings. Fig. 8(f) displays the cross-sectional view of the zeolite membrane. It indicates that the pores of the ceramic support are roofed with a dense film of zeolite and the typical structure of the zeolite is observable at higher magnification. Hence it can be concluded that the analcime-C is formed on the surface of the membrane support.

Fig. 8 FESEM images of (a) the ceramic support, (b) analcime-C zeolite particles, (c–e) the prepared composite membrane after different coating cycles (1–3, respectively) and (f) the cross sectional view of the analcime-C zeolite composite membrane.

The hydraulic permeability is estimated from the fundamental theory of porous membranes. Further, the average pore radius of the membrane is determined using the following expression:³³

where μ is the viscosity of water, l is the pore length, L_h is the permeability of the membrane, and ε is the porosity of the membrane. Table 1 presents the characterization results of analcime-C zeolite ceramic composite membranes with different coatings. As expected, the membrane porosity decreases with increasing number of coatings. The sequential reduction in pore size of the membrane is because of deposition of zeolite on the membrane surface. The deposition gradually decreases with an increase in the number of coating cycles (Table 1). After the third coating, there was no significant amount of weight increment. Therefore, coating was stopped and the prepared membrane was applied for the removal of Cr(VI).

Table 1 Characterization results of the analcime-C ceramic composite membranes

Membranes	Porosity	Average pore size (μm)	Water permeability (m^3 m^−2 s^−1 kPa^−1)	Weight increment (g)
Ceramic support	44	0.969	3.63 × 10^−6	—
Coating 1	38	0.285	2.18 × 10^−7	1.058
Coating 2	26	0.170	5.88 × 10^−8	1.668
Coating 3	24	0.155	4.53 × 10^−8	1.883

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Fig. 9 depicts the variation of water flux with applied pressure for the support and zeolite membranes. As expected, the water flux increases linearly with an increase of applied pressure for all the membranes. It is also observed that the water flux of the composite membrane decreases with increasing number of coatings. This illustrates the reduction in pore size and porosity (Table 1), which results in decreased hydraulic permeability of the analcime-C zeolite composite membrane. The pure water permeability of the support, the once-coated, twice-coated and thrice-coated composite membrane is found to be 3.63 × 10⁻⁶, 2.18 × 10⁻⁷, 5.88 × 10⁻⁸, and 4.53 × 10⁻⁸ m³ m⁻² s⁻¹ kPa⁻¹, respectively.

Fig. 9 Variation of water flux for the ceramic support and zeolite ceramic composite membranes with respective coatings.

3.2. Chromium removal from aqueous solution

Studies on Cr(VI) removal using the synthesized, thrice-coated membrane demonstrate that the applied pressure, Cr(VI) concentration, solution pH and membrane charge have significant influence on the separation efficiency.

3.2.1. Effect of applied pressure. The flux and retention behavior of Cr(VI) with applied pressure are shown in Fig. 10. The concentration and pH of the Cr(VI) solution used in this study are 1000 ppm and 2.3, respectively. The effect of the applied pressure on the permeate flux and Cr(VI) removal was studied in the range of 207–483 kPa. It can be seen that the permeate flux increases with increasing applied pressure. However, the permeate flux is lower than that of the pure water flux. This indicates that the existence of chromate ions causes an extra restriction (concentration polarization) to the flow of solvent. The observed rejection decreases with increasing applied pressure. This shows that the concentration polarization effect is a large and substantial loss of efficiency.³⁴ The work described here is preliminary in nature, executed with an unstirred in-house made dead-end setup; further experiments will be carried out to decrease the concentration polarization using a continuous cross-flow setup. Besides the applied pressure, the charge density, solute concentration and interaction of membrane surface charges with ionic solutes play an important role in determining the rejection. However, concentration polarization effects cannot be ignored and can lead to a considerable loss.

Fig. 10 Effect of applied pressure on the permeate flux and removal of Cr(VI). (Cr(VI) concentration = 1000 ppm, pH = 2.3.)

3.2.2. Effect of feed concentration. The effect of feed concentration of Cr(VI) on the permeate flux and rejection was measured at a constant pressure of 207 kPa and pH of 2.3. It is evident from Fig. 11 that the flux declines to some extent with an increase of feed concentration. This is due to concentration polarization and partial plugging of the membrane at higher concentration. The rejection values obtained at these concentrations also demonstrate that the observed rejection decreases with increasing feed concentration. This is a typical characteristic of charged membranes, for which Donnan exclusion plays a vital role.³⁵ With increasing Cr(VI) concentration, the effect of Donnan exclusion declines and also the surface concentration increases, which leads to the harsh concentration polarization. Consequently, the solute permeation by diffusion increases and hence the permeate concentration also raises.

Fig. 11 Effect of concentration on the permeate flux and removal of Cr(VI). (Pressure = 207 kPa; pH = 2.3.)

3.2.3. Effect of pH. Generally, chromium exists in two oxidation forms, Cr(VI) and Cr(III). In aqueous solution, Cr(VI) species are dissociated in the following forms:³⁶

H₂CrO₄ + H₂O ⇄ HCrO₄⁻ + H₃O⁺, pK₁ = −0.5

HCrO₄⁻ + H₂O ⇄ CrO₄²⁻ + H₃O⁺, pK₂ = 5.8

2HCrO₄⁻ ⇄ Cr₂O₇²⁻ + H₂O, pK₃ = 2.2

HCrO₄⁻ dominates in the pH range of 0 to 5.8 except that above a certain Cr(VI) concentration (∼10⁻³ M) it coexists with Cr₂O₇²⁻; at pH > 8, Cr(VI) exists only as CrO₄²⁻.

In general, the surface charge of the membrane depends on the pH of the solution.³⁷ It is especially a key factor to recognize the efficiency of a membrane separation process during the removal of ionic species. In this context, the rejection was measured over a range of pH (2.3–11) for a fixed concentration of Cr(VI) (1000 ppm), and at an applied pressure of 207 kPa. Fig. 12 depicts the observed rejection as a function of pH. The result shows that the removal is strongly dependent on the working pH and the highest rejection is obtained at pH = 2.3. Since the point of zero charge (PZC) of the composite membrane is 5.6 (see Fig. 7), the membrane is positively charged at pH < 5.6 and negatively charged at pH > 5.6. While the charged membrane is in contact with the chromium solution, the concentration of ions with the same charge as the membrane will be lower near the surface of the membrane than that in the solution, and the other ions, which have the opposite charge, have a higher concentration in the membrane than in the solution. On account of this concentration difference, a potential difference is generated at the interface between the membrane and the solution to maintain electrochemical equilibrium between the solution and the membrane. With this potential, the membrane repels ions with the same charge as the membrane.³⁷ When the pH of the solution increases from 2 to 5.6, the magnitude of the membrane charge (positive) declines and this is evidenced from the decrease in ΔpH value as shown in Fig. 7. As a result, the repulsion between the positively charged membrane and positive species (H₃O⁺) decreases and the rejection reduces from 80%. Because the cation and anion cannot act independently, HCrO₄⁻ is also rejected to maintain electroneutrality. Upon increasing the pH of the solution from 5.6 to 11 by adding NaOH, OH⁻ ions are accumulated on the membrane surface. Accordingly, the membrane acquires a greater negative charge while in contact with the feed. It is noteworthy to mention that when the pH increases, the CrO₄²⁻ concentration also increases compared to HCrO₄⁻. As the pH increases from 5.6 to 11, the magnitude of the membrane charge (negative) also increases as evidenced from Fig. 7 and hence the higher surface charge (negative) leads to an increase in the intensity of the electrostatic repulsion between the ions present in the solution (CrO₄²⁻ and Cr₂O₇²⁻) and the membrane surface. This explains an increase of rejection with increasing pH from 5.6 to 11.

Fig. 12 Effect of pH on the permeate flux and removal of Cr(VI). (Pressure = 207 kPa; concentration = 1000 ppm.)

3.2.4. Performance comparison of the prepared membrane with other membranes. It is apparent from Table 2 that the rejection (80%) reported in this work at a lower applied pressure of 207 kPa with Cr(VI) concentration of 1000 ppm is comparable or even better than those described in the literature. Moreover, a maximum rejection of 84% is obtained with a Cr(VI) concentration of 250 ppm at 207 kPa. In the work reported by Pugazhenthi et al.,³⁸ the carbon composite membrane demonstrated 96% rejection of Cr(VI) with a permeate flux of 1.464 × 10⁻⁸ (m³ m⁻² s⁻¹) for a chromium concentration of 1000 ppm (Table 2). In another work by Sachdeva et al.,³⁹ around 90% rejection with 1.464 × 10⁻⁸ (m³ m⁻² s⁻¹) permeate flux was obtained for a styrene acrylonitrile composite membrane. In comparison with results described in the literature, the permeate flux of the membrane reported here (5.96 × 10⁻⁶ m³ m⁻² s⁻¹) is higher even at a lower operating pressure of 207 kPa. From the comparison study, it can be concluded that the synthesized membrane is better than the other membranes reported in the literature.

Table 2 Rejection comparison of other membranes with the prepared membrane

Membrane material	Pore size	Feed concentration (ppm)	Solute permeability (m s^−1)	Rejection (%)	Reference no.
PMMA–EGDM	8.5 kDa	1000	5.96 × 10^−10	68	2
Zeolite-clay membrane	30 nm	1000	2.73 × 10^−6	66	34
Styrene acrylonitrile	55 nm	1000	2.38 × 10^−6	90	39
Clay-carbon	2 nm	1000	1.46 × 10^−8	96	38
Analcime-C ceramic	155 nm	1000	5.96 × 10^−6	80	Present work

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4. Conclusions

An analcime-C zeolite composite membrane has been successfully fabricated through an in situ hydrothermal crystallization technique over a ceramic support. FESEM images show that the zeolite particles adhere more on the surface of the membrane with repeated coatings. The porosity, average pore size and water permeability of the composite membrane (3 times coated) are found to be 24%, 155 nm and 4.53 × 10⁻⁸ m³ m⁻² s⁻¹ kPa⁻¹, respectively. Overall results suggest that the membrane morphology, porosity, average pore size, and pure water permeability of the prepared zeolite membrane vary significantly by the repeated coating on the ceramic support. The thrice-coated membrane has been used for chromium removal with an in-house made dead end UF setup at room temperature and the performance of the membrane has been investigated for the effect of several parameters: applied pressure, feed concentration and pH. The highest separation (84%) is achieved at pH 2.3 with a permeate flux of 6.85 × 10⁻⁶ m³ m⁻² s⁻¹ at 207 kPa and a feed concentration of 250 ppm. The separation test displays that the membrane has a high removal efficiency for Cr(VI) from an aqueous solution and is comparable to those reported in literature.

Acknowledgements

The part of the work reported in this article was financially supported by a research grant under the Fast Track Scheme (SR/FTP/ETA-44/2010) from the Department of Science and Technology (DST), Government of India. We would like to thank the Central Instrument Facility, IIT Guwahati for helping to perform FESEM analysis. The XRD was financially supported by a FIST grant (SR/FST/ETII-028/2010) from the Department of Science and Technology (DST), Government of India.

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