This study aimed to assess the performance of three zeolite membranes in the removal of trivalent metal ions from aqueous solution using a cross-flow mode of operation. Three types of zeolite membrane, MCM-41, MCM-48 and FAU, were prepared on a low-cost, circular ceramic support by hydrothermal treatment. The three zeolite membranes were characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and contact angle measurements. The XRD results confirmed the formation of zeolites. The deposition of zeolite on the ceramic support and hydrophilicity of zeolite membranes were monitored by FESEM and contact angle measurement, respectively. The pore size of the MCM-41, MCM-48 and FAU membrane was found to be 0.173 μm, 0.142 μm, and 0.153 μm, respectively, which was lower than that of the support (1.0 μm). The fabricated zeolite membranes were used to investigate the separation behavior of trivalent metal ions (Al3+ and Fe3+) from aqueous solution at various applied pressures. It was observed that an increase of applied pressure leads to a slight decrease in the removal efficiency. Among the various zeolite membranes, the FAU membrane showed the maximum rejection of 88% and 83% for Fe3+ and Al3+ separation, respectively.
Metal ions in wastewater are a serious environmental concern owing to their high toxicity and tendency to accumulate in living organisms (Mohammad et al. 2004). These metals are non-biodegradable in the environment; therefore, environmental regulations are designed to reduce the level of concentration in wastewater to the safe limit specified by legislation. Sources of contamination in wastewater include printing, tanning, electroplating, dying and textile industries, steel working and finishing industries, battery manufacturing units, chlorinating agents in metallurgical and organic synthesis, and catalysts (Gherasim et al. 2013). When wastewater from various industrial activities is discharged into the environment, the standards of environmental regulations should be implemented. Therefore, the treatment of wastewater for the removal of contaminants becomes an important and challenging task.
Conventional techniques for the removal of effluent coming from various sources of streams are liquid–liquid extraction, precipitation, adsorption and ion exchange (Kumar et al. 2015a, 2015b). These methods are time consuming, laborious, expensive and cannot reduce the pollutants to the limit framed by legislation. In addition, reverse osmosis, electrodialysis, and nanofiltration have been used for wastewater treatment (Gherasim et al. 2013). However, these processes are expensive and have limitations due to high pressure requirements. Therefore, it is necessary to explore an alternative, cheaper, efficient and non-polluting separation technique. Charged ultrafiltration membranes are gaining popularity in wastewater treatment due to their capability for electrostatic interactions between a charged membrane and metal ions, even when wide pore membranes are used (Arunkumar & Etzel 2015).
Membrane technologies are promising methods for the separation of heavy metals from aqueous solutions (Frares et al. 2005). Moreover, membrane-based separations are more effective in terms of energy saving, higher removal efficiency and stability in operation. Conversely, organic polymeric membranes showed instability at high temperature and in harsh environments. Over the past two decades, supported inorganic zeolite membranes have been utilized in various applications such as separators, sensors, reactors and electrical insulators because of their uniform pore structure framework and high thermal stability (Yu et al. 2007; Ahmad & Hagg 2013). Sandstrom et al. (2011) prepared an MFI zeolite membrane on an alumina support to separate CO2 gas from synthesis gas and natural gas. Wirawan et al. (2011) fabricated a silicate-1 composite membrane on an α-alumina support and studied H2/CO2 permeation through the membrane. Maghsoudi & Soltanieh (2014) developed a CHA zeolite membrane on α-alumina disks by in situ crystallization to separate CO2 and H2S from CH4. Iglesia et al. (2006) synthesized an MCM-48 membrane on an α-alumina tube and used it for the separation of gas mixtures. In addition, it is well known that the separation of metal ions by ultrafiltration and microfiltration is not only based on the pore size, but also depends on other factors such as the surface charge of the membrane and electrostatic interactions between the membrane and charged ions (Monash et al. 2010). This means that the interaction between membrane and metal ions can significantly affect the performance of the ultrafiltration/microfiltration membranes (Monash et al. 2010). Zeolites have potential for use in removing diverse materials because of their properties, including high surface area, excellent thermal/hydrothermal stability, high shape-selectivity and superior ion-exchange ability, which form the basis for their traditional applications in catalysis and separation of small molecules (Ozin et al. 1989; Kumar et al. 2015a, 2015b).
Many researchers have used α-alumina as a support material to fabricate zeolite membranes. From an industrial point of view, alumina is a very expensive material. Moreover, the majority of researchers have utilized these fabricated zeolite membranes for gas phase separation applications. The present work focuses on the preparation of MCM-41, MCM-48 and FAU (faujasite) zeolite membranes on low-cost support material and their performance in the separation of Al3+ and Fe3+ from aqueous solution.
MATERIALS AND METHODS
The raw materials used for the preparation of ceramic supports (quartz, feldspar, ball clay, pyrophyllite and kaolin) were acquired from Kanpur, India. Calcium carbonate, polyvinyl alcohol (PVA), tetraethyl orthosilicate (TEOS), aluminum fine powder and hydrochloric acid were purchased from Merck (I), Mumbai, India. Fumed silica (Aerosil 200) was procured from CDH, Laboratory Reagents, Mumbai. Ferric chloride, aluminum chloride (hexahydrate), and NaOH were bought from Loba Chemie (Laboratory Reagents & Fine Chemicals), Mumbai. All chemicals and reagents were used directly without further purification. Water was taken from the Millipore water system (ELIX-3).
Preparation of zeolite membranes
Ceramic supports were fabricated using inexpensive clay materials available in India. Details of the composition of raw materials and preparation procedure were reported in our earlier publication (Monash & Pugazhenthi 2011). Ball clay (17.58 g), pyrophyllite (14.73 g), quartz (26.59 g), kaolin (14.45 g), feldspar (5.60 g) and calcium carbonate (17.14 g) were mixed with 4 ml of 2 wt% of aqueous PVA in a ball mill. An estimated quantity of powder mixture was pressed at 50 MPa in a hydraulic press and the raw ceramic supports were sintered at 950°C in a muffle furnace. The sintered ceramic supports were polished with abrasive paper (no. C-220) and the loose particles produced while sizing were removed in an ultrasonic bath with Millipore water for 15 min. Finally, the dried ceramic supports were subjected to hydrothermal treatment to deposit the MCM-41, MCM-48 and FAU zeolites individually on the surfaces. The cost of the ceramic support was evaluated to be Rs. 480/m2 ($10/m2) based on the cost of raw materials (Monash & Pugazhenthi 2011).
The MCM-41 zeolite membrane was synthesized with the molar composition of 1TEOS:0.1CTAB:0.3NaOH:60H2O according to the procedure described in our previous publication (Basumatary et al. 2015). The synthesized gel was subjected to hydrothermal reaction by placing a ceramic support in the bottom of a Teflon container autoclave reactor at 110°C for 96 h. The MCM-48 zeolite membrane was fabricated with a gel composition of 1TEOS:0.25Na2O:0.65CTAB:0.62H2O and subjected to hydrothermal treatment at 110°C for 72 h. After treatment, the membranes and zeolite powders (MCM-41 and MCM-48) were washed with Millipore water and dried at 100°C for 24 h. The membranes and zeolite powders were calcined at 550°C for 5 h for MCM-41 and 6 h for MCM-48 at a heating rate of 1°C/min. MCM-41 and MCM-48 deposition on the ceramic support were done with three cycles of coating in the same manner. After the third cycle, no significant weight increment was observed for MCM-41 and MCM-48 membranes.
The FAU zeolite membrane was fabricated with a gel mixture of 70Na2O:Al2O3:20SiO2:2000H2O. The prepared solution mixture and ceramic support were kept in the bottom of the autoclave reactor and treated hydrothermally at 75°C for 24 h. After completion of the reaction, the membrane and FAU powder were washed with Millipore water and dried at 110°C for 24 h.
The phase purity and crystallinity of the MCM-41, MCM-48 and FAU zeolite powders were confirmed by X-ray diffraction (XRD) analysis. The XRD profile was recorded on a Bruker D8 Advance using CuKα (λ = 1.5406 A°) radiation operating at 40 kV and 40 mA. The XRD patterns were taken in the 2θ range of 1–10 ° for the MCM-41 and MCM-48 powder and 1–50 ° for the FAU powder at a scanning speed of 0.02 °s−1. The structural morphology of the ceramic support and zeolite membranes were investigated by a field emission scanning electron microscope (FESEM) using a Zeiss Sigma instrument at 3-Kev acceleration voltage. Prior to FESEM analysis, the surface of the samples was coated with gold by means of a JEOL JFC-1300 auto fine coater. The wettability surface nature of the ceramic support and zeolite membranes (MCM-41, MCM-48 and FAU) were measured using a contact angle Drop Shape Analyzer-DSA 25 (Kruss), maintaining a fixed water droplet size of 4 μL and a falling rate of 0.16 mL/min. Five measurements were taken at different positions of membrane surfaces and the average value was reported.
Pure water flux
Cross-flow ultrafiltration of trivalent metal ions
Cross-flow filtration experiments were performed using the same setup depicted in Figure 1. Trivalent metal salts (Al3+ and Fe3+) solutions were prepared individually with a concentration of 250 ppm using Millipore water and pH 2. A typical cross-flow ultrafiltration run involves the measurement of permeate flux at time intervals of 5 min for the separation of trivalent metal ions from aqueous solution for the time period of 1 h. With the fixed cross-flow rate (1.11 × 10−7 m3s−1), all experiments were conducted at different applied pressures (69–345 kPa) to evaluate the effect of applied pressure on the separation characteristics of all zeolite membranes. The feed solution was pumped from the feed tank container into the membrane module by means of a high pressure peristaltic pump. To examine the influence of applied pressure, the cross-flow rate was made constant by controlling the retentate valve (V) and the applied pressure was varied by adjusting the speed of the pump. In order to maintain constant concentration in the feed tank throughout the experiments, the retentate and permeate streams were returned back to the feed tank.
RESULTS AND DISCUSSION
Separation of trivalent metal ions using cross-flow ultrafiltration
The surface charge of the membrane plays a significant role in determining the rejection efficiency of the membrane with ionic solution, and it differs with pH of the solution. When a charged membrane is in contact with the salt solution, the concentration of co-ions (ions with the same charge as the membrane) close to the surface of the membrane will be lower than that in solution, and the counter-ions (having 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. By this potential (known as the Donnan potential), co-ions are repelled by the membrane (Majhi et al. 2009). We found the isoelectric point of the MCM-41, MCM-48 and FAU zeolite membranes to be 3.9, 3.2 and 3.8, which was determined using zeta potential measurement (Delsa Nano) (Wu et al. 2008). The zeta potential value of MCM-41, MCM-48 and FAU zeolite membrane is +18.36 mV, +3.95 mV and +4.22 mV, respectively, at pH 2. Since the membranes are positively charged, Al3+ and Fe3+ ions are repelled by the membrane. As cations and anions cannot act independently, Cl− ions are also rejected to maintain electroneutrality. Additionally, the diffusion coefficient of Al3+ and Fe3+ also affects the retention behavior of metal ions (Schaep et al. 1998). Al3+ (5.5 × 10−9 m2s−1) has a higher diffusion coefficient than Fe3+ (1.4 × 10−9 m2s−1), which results in a lower rejection of Al3+ for all zeolite membranes (Jafarian et al. 2006; Haarberg & Keppert 2008). Peeters et al. (1998) pointed out that salts having a lower diffusion coefficient display higher retention.
In addition to charge density, the retention of trivalent metal ions also depends on the quantity of zeolite materials deposited on the ceramic support. The amount of FAU zeolite deposition (1.26 g) on the ceramic support is more than that of MCM-41 (0.89 g), whereas the zeta potential value of MCM-41 is higher than the FAU zeolite, and both membranes have very similar isoelectric point values However, the rejection of the FAU zeolite composite membrane is found to be higher when compared to the MCM-41 membrane, and this may be due to the more hydrophilic nature of the FAU zeolite membrane. Among the three zeolite membranes, the rejection of the FAU membrane is marginally higher as it has almost an equal amount of deposition to MCM-48 and its hydrophilicity is also greater than that of the other two composite membranes. The highest rejection values of 88, 86 and 85% were accomplished with the FAU, MCM-48 and MCM-41 zeolite composite membranes, respectively, at an applied pressure of 345 kPa for the removal of Fe3+. In the case of Al3+ retention, 84, 83 and 82% were achieved using FAU, MCM-48 and MCM-41 zeolite membranes at an applied pressure of 345 kPa.
The permeate flux of trivalent metal ions showed no declining trend (i.e. no fouling of the membrane) for the MCM-41, MCM-48 and FAU zeolite membranes. In a batch filtration, there is an occurrence of concentration polarization, implying the cross-flow mode of filtration is better than the batch mode. Among the membranes in this study, the FAU zeolite membrane was found to be better in terms of flux and removal efficiency. Besides, its preparation needs less synthesis time (24 h), a lower hydrothermal temperature (75 °C), a single stage of coating and no calcination step, which leads to a lower manufacturing cost.
Al3+ and Fe3+ can be effectively removed using zeolite membranes (MCM-41, MCM-48 and FAU) by a cross-flow ultrafiltration mode. MCM-41, MCM-48 and FAU zeolite composite membranes do not have a tendency for fouling and cake layer formation on the surface of the membrane in the removal of trivalent metal ions. The FAU zeolite membrane displays a slightly higher rejection when compared to the other membranes and is better than MCM-41 and MCM-48 zeolite membranes due to the shorter synthesis time, single cycle deposition and having no calcination step required in its preparation. The maximum rejection is observed with Fe3+ separation rather than Al3+ for all the zeolite membranes. In the experiment, a permeate flux decline trend was not observed during the whole period of the filtration study; hence the prepared zeolite membranes can be used for a longer duration without frequent regeneration of the membrane. Our results suggest that the zeolite membranes (MCM-41, MCM-48 and FAU) are promising candidates for the separation of trivalent metal ions from aqueous solution based on interaction phenomena between the charged membrane and trivalent metal ions.
We would like to express our sincere gratitude to the Central Instruments Facility of IIT Guwahati for helping us to perform FESEM analysis. The contact angle instrument used in this work was financially supported by a grant for the Center of Excellence for Sustainable Polymers at IIT Guwahati from the Department of Chemicals & Petrochemicals, Ministry of Chemicals and Fertilizers, Government of India.