Removal of trivalent metal ions from aqueous solution via cross- ﬂ ow ultra ﬁ ltration system using zeolite membranes

Thisstudyaimedtoassesstheperformanceofthreezeolitemembranesintheremovaloftrivalentmetal ions from aqueous solution using a cross- ﬂ ow 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), ﬁ eld emission scanning electron microscopy (FESEM) and contact angle measurements. The XRD results con ﬁ rmed 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 μ m, 0.142 μ m, and 0.153 μ m, respectively, which was lower than that of the support (1.0 μ m). The fabricated zeolite membraneswereusedtoinvestigatetheseparationbehavioroftrivalentmetalions(Al 3 þ andFe 3 þ )from aqueoussolutionatvariousappliedpressures.Itwasobservedthatanincreaseofappliedpressureleads toaslightdecreaseintheremovalef ﬁ ciency.Amongthevariouszeolitemembranes,theFAUmembrane showed the maximum rejection of 88% and 83% for Fe 3 þ and Al 3 þ separation, respectively.


INTRODUCTION
Metal ions in wastewater are a serious environmental concern owing to their high toxicity and tendency to accumulate in living organisms (Mohammad et al. ). 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. ). 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. a, b). 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. ). 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 ).
Membrane technologies are promising methods for the separation of heavy metals from aqueous solutions (Frares et al. ). 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  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. ). This means that the interaction between membrane and metal ions can significantly affect the performance of the ultrafiltration/microfiltration membranes (Monash et al. ).
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. ; Kumar et al. a, b). 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 ). 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 W 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 treat- where V is the volume of permeate ( where J is the permeate flux (μm/s), L h is water permeability (μm/s kPa), ΔP is the applied pressure across the membrane (kPa), μ is the viscosity of water (kPa s), l is pore length  with time at different applied pressures for a fixed cross-flow rate of 1.11 × 10 À7 m 3 s À1 is presented in Figure 7(a)-7(c).
For all the zeolite membranes, the rejection of trivalent metal ions slightly increases with the duration of the process. This is possibly due to a build up of the concentration polarization until a steady state is reached at the membrane surface (Danis & Keskinler ). The rejection also increases with increasing applied pressure for all the zeolite membranes. The convective transport becomes more important than the diffusive transport at elevated applied pressure, and hence the retention will increase.
Thus, the rejection of trivalent metal ions increases with 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 counterions (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. ). We found the isoelectric point of the