Removal of chromium from synthetic wastewater using MFI zeolite membrane supported on inexpensive tubular ceramic substrate

A mordenite framework inverted (MFI) type zeolite membrane was produced on inexpensive tubular ceramic substrate through hydrothermal synthesis and applied for the removal of chromium from synthetic wastewater. The fabricated ceramic substrate and membrane was characterized by diverse standard techniques such as X-ray diffraction, ﬁ eld emission scanning electron microscope, porosity, water permeability and pore size measurements. The porosity of the ceramic substrate (53%) was reduced by the deposition of MFI (51%) zeolite layer. The pore size and water permeability of the membrane was evaluated as 0.272 μ m and 4.43 × 10 – 7 m 3 /m 2 s.kPa, respectively, which are lower than that of the substrate pore size (0.309 μ m) and water permeability (5.93 × 10 – 7 m 3 /m 2 s.kPa) values. To identify the effectiveness of the prepared membrane, the applied pressure of the ﬁ ltration process and initial chromium concentration and cross ﬂ ow rate were varied to study their in ﬂ uence on the permeate ﬂ ux and percentage of removal. The maximum removal of chromium achieved was 78% under an applied pressure of 345 kPa and an initial feed concentration of 1,000 ppm. Finally, the ef ﬁ ciency of the membrane for chromium removal was assessed with other membranes reported in the literature.


INTRODUCTION
In recent years, environmental concerns have been increasing, particularly on the subject of heavy metals' existence in water. Heavy metals, such as chromium, lead, cadmium, mercury, nickel, copper and zinc, are, unlike organic contaminants, non-biodegradable and lead to accumulation in the human body, which causes health hazards due to their toxicity. Among these, chromium is a widespread pollutant in surface water and presents principally either in trivalent or hexavalent oxidation states. Hexavalent chromium is 10-100 times more toxic and difficult to remove from water than trivalent chromium. Moreover, it is carcinogenic to humans and causes considerable hazards to the environment (Shpiner et al. ; Kim et al. ). Therefore, the World Health Organization (WHO) set 0.05 mg L -1 as the highest acceptable limit for chromium in drinking water (Mandal et al. ). A variety of techniques have been established to treat chromium, including chemical precipitation (Gheju & Balcu ), liquid-liquid extraction (Sacmaci & Kartal ), chemical/electrochemical reduction (Dhaz et al. ), ion-exchange (Rengaraj et al. Membrane technology is the most prominent and alternative emerging process, particularly for the removal of heavy metals. This process is more economical than that of conventional/alternative techniques and requires much less land area than competing technology (Chougui et al. ). In addition, the level of efficiency is far greater than conventional treatment processes, and it allows for a high level of automation to potentially save on labor costs (Roberts et al. ). In membrane technology, ceramic membranes are extremely adaptable. They can be operated at higher temperature ranges, and many ceramic membranes are very stable at above 1,000 W C. In addition, ceramic membranes are highly stable to chemical attacks due to an extensive variety of materials utilized in preparation, which resist corrosive liquids and gases, even at higher temperatures. In the various harsh operational environments discussed above, polymeric membranes will not perform well, or will not survive at all.
To reduce high transmembrane pressures and achieve higher flux than ultrafiltration, the microfiltration technique is preferable to further lower the treatment cost. Also, the ceramic membrane is prepared with a composite arrangement using an active top layer that will determine its separation effi-

EXPERIMENTAL MFI zeolite membrane preparation
The comprehensive procedure adopted for the fabrication of the MFI zeolite membrane is presented in Figure 1. The methodology followed for the fabrication of the MFI zeolite membrane on inexpensive tubular ceramic substrate and its characteristics are extensively presented in our earlier published work (Kumar et al. b). Firstly, the tubular substrate was fabricated using cheaper natural clays with the composition of ball clay (18 wt%), feldspar (6 wt%), kaolin (15 wt%), pyrophyllite (15 wt%), quartz (28 wt%) and calcium carbonate (18 wt%). The clay mixtures were well mixed with Millipore water obtained from Milli-Q system to create a paste for the extruding tube. The paste was fed into the extrusion cylinder and extruded with the dimensions of 100 mm length, and an exterior and interior diameter of 11.5 mm and 5.5 mm, respectively. The obtained tube was air-dried for 12 h under atmospheric conditions.
where W wet , W dry are the wet and dry weights of the membrane (dried at 120 W C for 3 h), respectively. V membrane is the total volume of the membrane and ρ water is the density of the water. In order to estimate the wet weight of the membrane, the membrane was soaked in water for 24 h. Then, the wet weight was measured after wiping all the water from the membrane surface with tissue paper. Five measurements were conducted for each sample and the average value was reported. The water permeability and pore size of the membrane was calculated by various methods, which are extensively presented in our earlier published work (Kumar et al. b).
Water flux and chromium removal tests The water flux of the membrane was obtained at diverse applied pressures (69-345 kPa) using the laboratory-scale cross-flow MF test unit (see Figure 2).
The water flux was calculated using the following equation: The chromium removal experiments were conducted using the same MF set up illustrated in Figure 2. The percentage of chromium removal was calculated using the following equation: The synthesized MFI zeolite was characterized to substantiate its pureness by the XRD profile as presented in  The average porosity is measured as 53% for the ceramic substrate and 51% for the zeolite membrane according to methods reported elsewhere (Kumar et al. b). The water flux obtained for the ceramic substrate and zeolite membrane is presented in Figure 6. As expected, water flux increases linearly with an increase in the applied pressure and follows Darcy's law. Figure 6(b) depicts the pure water flux of the ceramic substrate along with the zeolite membrane as a function of applied pressure. It is evident from the figure that the water flux of the zeolite membrane is lower than the ceramic substrate. It is accredited by the reduction in pore radius on hydrothermal treatment. The water permeability is calculated as 5.93 × 10 -7 m 3 /m 2 s kPa for the ceramic substrate and 4.43 × 10 -7 m 3 /m 2 s kPa for the zeolite membrane from the slope of the water flux versus pressure across the membrane (from Figure 6(b)).
The average hydraulic pore size is determined using the Hagen Poiseuille expression by assuming pores are cylindrical in shape (Kumar et al. b): where ε is the porosity of the membrane, r is the pore radius of the membrane, l is the pore length, τ is the tortuosity factor, μ is the viscosity of water, L h is water permeability and ΔP is the applied pressure. The average hydraulic pore diameter of the ceramic substrate is estimated to be 0.309 μm, whereas for the zeolite membrane it is found to be 0.272 μm, which represents the pore size of the top separating layer. It is worth mentioning that the hydrothermal coating of zeolite on the ceramic substrate reduced the porosity, permeability and pore size values. and it can be either positive or negative. In order to find out the surface charge of the MFI membrane, the zeta potential of the MFI particles (calcined) was measured using an electrophoretic light scattering method in Delsa Nano C (Beckman coulter) by keeping the powders in water suspension at different pH. The surface charge of the MFI powder was altered by the addition of HCl to make a lower pH, and for a higher pH, NaOH solution was added. At a lower pH, the surface charge of the MFI powder becomes positive due to a larger quantity of H þ ions, whereas at higher pH, negative hydroxyl (OH À ) ions increase owing to the addition of NaOH (see Figure 8). The zeta potential value of MFI is around þ11.5 mV at an acidic pH (pH ¼ 2) and at pH 12, it is approximately À63 mV (Figure 8). It is clear from these data that the electrostatic interactions play a major role in the surface activity of MFI. In this study, the iso-electric point (IEP) of the MFI powder is found to be 4, which is in good agreement with the IEP value reported by Kosmulski () for MFI zeolite.
Since the IEP of the MFI is 4, the membrane is positively charged at a pH less than 4, and the membrane is negatively   probably due to the chromium ions fouling (Habibi et al. ). It is apparent that the percentage removal of chromium augments with an increment in the initial feed concentration of chromium, as displayed in Figure 10(b). This is primarily attributed to various factors such as concentration polarization, osmotic pressure and membrane fouling across the membrane surface. In the studied concentration range (500-3,000 ppm) the highest percentage removal obtained was 76% for the initial chromium concentration of 3,000 ppm.
Effect of cross flow rate  From the assessment survey (Table 1), the highest removal of 78% with a permeate flux of 1.42 × 10 -4 m 3 /m 2 s achieved in this work is better than that of other membranes. Even though some of the membranes showed a higher percentage removal of chromium in comparison with this work, the flux (1.42 × 10 -4 m 3 /m 2 s) of the present study is 1-6 orders higher when compared with other membranes (see Table 1), and also the membrane displays a reasonable removal percentage (78%). Thus, the prepared MFI zeolite

CONCLUSIONS
An MFI-type zeolite membrane was successfully produced on inexpensive tubular ceramic substrate through a hydrothermal synthesis method. The ceramic substrate was layered with homogeneous dispersion of MFI zeolite crystals to form a uniform MFI zeolite membrane. The efficiency of the prepared zeolite membrane was tested by the removal of chromium present in the aqueous medium.
The highest rejection of 78% was achieved with a permeate flux of 1.42 × 10 -4 m 3 /m 2 s. The performance comparison analysis clearly indicated that the prepared membrane has better potential in the removal of chromium while offering better flux.