Photovoltaic Cells and Systems: 34-44 SQU Journal for Science, 16 (2011) © 2011 Sultan Qaboos University 34 Synthesis of Highly Ordered Crystalline Wurtzite (ZnO) Membrane for Selenium- Chromium Separation Sh. Labib and N.A. Tadros Nuclear Chemistry Department, Hot laboratories Center, Atomic Energy Authority, P.O.Box 13759, Cairo, Egypt, Email: dr_sh_labib@yahoo.com. تكوين غشاء رقيق من بلورات أكسيد الزنك عالية التنظيم لفصل السيلينيوم والكروميوم تادروس نادية ولبيب شيراز سييٌا احسييٌ ٌكر تثرييصة ٌريي أ - سييٌا احرص نييٌ أ سييٌا احرص نييٌ ك يي أهيي ا احث يي حيير ح مييٌ يي فييً :ملخصص ظ كف ح ميٌ ي ك حيف ح ني فً اح لك احهال ً ت ٌر سٌا اححٌحصنٌكر ثصسحراارأ تالة على اعص صة ننع ٌيكا ا اسيصة ثثحيةأ يص نيحظر احك احش (wurtzite)غشٌ اة احتك احثصثة أحر ح مٌ اح كر ك احسٌلٌنٌكر. عنن ي سيٌا أة نسيث جير جسيٌ صة اشغشيٌ اح مي ة ثاٌيصا فيًحيك ظ كجيكا اٌيصاة ك ا . أشع سٌنٌ كاح جه اإلح ح كنً اح صسح احثليك ياٌيصاة ح سيٌ احسيلكف فيً يص ٌكميح احياك اح عيص حهي ا اش سيٌا ) ٌ ك ي 3إحيى 1..5( ي احسٌ ٌكر اح مصف ي . كاح ي كر كاحسيٌلٌنٌكر عنني يحألغشٌ اح م ة. حر ع ا اس نل على رنصئص اح ن حألغشٌ اح رحص ة على اح عيص ح ي ي ك ي ثير نسيحنحل احياك اح ثٌي كثنسيث عصحٌي رص ني ثعنني اح ي كر. ه ه احا اس حر فن عنن احسيٌلٌنٌكر ي ك عنني فيًاحريكاص اح سيص ٌ حألغشيٌ اح مي ة فني عنني احسيٌلٌنٌكر ك فيًش سيٌا احرص نيٌ احرتثيًاحسلكف اح كر رالحهص. ABSTRACT: Pure ZnO and composite ZnO-CeO2 coated thin films on titania substrates were prepared using a chelating sol-gel method under controlled conditions to be used in the separation and/or rejection of Se(IV) and Cr(VI). XRD and SEM studies confirmed the formation of highly ordered arrays of crystalline wurtzite microstructure with mixed orientation. An increase in average particle size of the prepared membranes from 1.25 to 3 µm with increasing CeO2 percent was observed indicating the vital role of CeO2 as secondary phase in improving the crystallinity of the coated films formed. Detailed study of the separation performance of the selected membranes toward Se(IV)-Cr(VI) was studied. A high separation performance for the different membranes prepared was observed toward Se(IV) with respect to Cr(VI). The polar characteristic of ZnO thin films played an essential role in Se(IV) separation. However, a high permeation flow of Cr(VI) through the membranes was governed by the capillary pressure induced within the porous structure of the membranes. KEYWORDS: Rutile substrate; Pure and composite ZnO membranes; Polymeric sol-gel; SeIV separation; CrVI permeation. SYNTHESIS OF HIGHLY ORDERED CRYSTALLINE WURTZITE (ZnO) MEMBRANE 57 1. Introduction eavy metals discharged into the environment from various industries constitute one of the major causes of water and soil pollution (Oboh and Aluor, 2008) and in turn, their presence and accumulation have a toxic effect on human, animal and plant life. (Oboh and Aluyor, 2008). Selenium and chromium, as examples of heavy metals, contain undesired properties that affect humans and environments. Selenium, when it is present in the form of seleno-proteins, is an essential element to human nutrition. Moreover, as inorganic, Se(IV), Se(VI) and SeCn - are very toxic to wildlife (Cafferky et al. 2006). Chromium naturally occurring as trivalent Cr(III) is an essential nutrient, necessary to life, while hexavalent chromium Cr(VI) is known to be highly toxic; its compounds are highly soluble in water, and carcinogenic in mammals. Commercially, Cr(VI) is available in the form of potassium chromate and potassium dichromate (Jeyasingh and Philip, 2005). Membrane processes are one of the most efficient methods used to remove toxic elements due to the simplicity in treatment (Saffag et al. 2004), higher selectivity and permeability, excellent chemical, thermal and mechanical stability under operating conditions, low maintenance and defect free production (Chen, 2002). Owing to its specific chemical, surface and microstructural properties, ZnO has many industrial applications such as the production of adhesive tapes, automotive tires, ceramics, glasses, varistors, gas sensors, SAW devices, and catalyst and transparent electrodes (Roy and Basu, 2002). So, ZnO is considered to be a promising material to be studied as a ceramic membrane. This paper deals with the preparation of pure ZnO and composite ZnO-CeO2 ceramic membranes to remove or separate Se(IV) and Cr(VI) elements from their aqueous solutions using a chelating sol-gel process (Guizard et al.1999). In general the sol-gel process provides a simple method used for the preparation of inorganic membranes with high thermal and chemical stability, controlled microstructure and uniform pore size (Srinivasan and Kumar 2006; Xomeritakis et al.2003). Highly crystalline and crack free films of pure ZnO and composite ZnO-CeO2 ceramic membranes deposited over porous ceramic substrates were prepared under controlled parameters of processing. Separation performance of the optimized pure and composite ZnO membranes was determined by their higher flux, separation and or rejection toward the toxic elements Se(IV) and Cr(VI). 2. Experimental 2.1 Substrate preparation Disk rutile substrates of thickness (0.1cm), diameter (10 cm), pore size distribution (0.1-1μm) and porosity (55.8 Vol.%) were used for pure ZnO and composite (ZnO-CeO2) deposition. These substrates were home made from titania powder [Acros rutile powder, 99.5% ,USA]. The starting powder was thermally treated in an air oven at 500 °C for 30 min before any treatments, ground in an agate mortar for 30 min, pressed uniaxially at 12 K.N using P.V.A as binders and sintered at 1200 ° C/1h (Labib, 2006). 2.2 Membrane preparation Pure and composite ZnO sols were prepared using zinc nitrate hexahydrate [99% Merck, Germany], cerium (III) nitrate hexahydrate [99.5% Alpha Aesar, Germany], isopropyl alcohol [99.8% Scharlau, Spain], acetylacetone [≥ 99% Merck, Germany] and distilled water. All the reagents used were A.R. grade. The molar ratio of the precursors used and the composition of the samples prepared are shown in Table 1. Polymeric zinc oxide sol was prepared by drop-wise addition of an alcoholic mixture of isopropyl alcohol (5 mol) and distilled water (0.5 mol) to the stirred alcoholic solution of zinc nitrate and acetyl acetone having molar ratio (Isoprop.alc./zinc nitrate /acetyl acetone) 10:0.5:2 respectively. Otherwise, the different composite zinc oxide- cerium oxide sols were prepared by adding 10, 30 and 50% cerium nitrate to each of the alcoholic solutions of 90, 70 and 50% zinc nitrate, followed by adding acetyl acetone. (Isoprop.alc./zinc nitrate-cerium nitrate/acetyl H SH. LABIB and N.A.TADROS 58 acetone: 10:0.5:2). The mixed polymeric sols were prepared by drop-wise hydrolysis with the alcoholic mixture of isopropyl alcohol and distilled water. Table 1. Optimized precursor's molar ratios and the composition of the different samples. Metal Salt [M] Isopropyl Alcohol [M] Acetyl Acetone [M] Distilled Water [M] 0.5 15 2 0.5 Sample ZnO % CeO2 % 100z 100 0 90z 90 10 70z 70 30 50z 50 50 The different prepared polymeric sols were stirred vigorously for 20 min. The dip coat process was performed at room temperature by immersing rutile substrates into the different prepared polymeric sols for 24h (immersion and withdrawal rate: 0.5mm/s). After dip coating, the different ceramic membranes were left overnight to dry at room temperature. The prepared membranes as well as the different prepared polymeric sols were then dried at 200 °C/1h with a heating and cooling rate of 4°C/min. The process of dipping, withdrawing and drying was repeated 4 times to obtain a suitable coating thicknesses and to repair any defects in the first coating layer (Sekulic et al. 2006). Finally, both the supported and unsupported membranes were sintered at 500 ° C/1h with a heating and cooling rate of 4 ° C/min. 2.3 Membrane characterization The crystal structures of the different prepared membranes were identified using X-ray diffraction XRD (Philips X’pert multi-purpose diffractometer). A copper-tube x-ray tube operating at 40 KV and 30Ma was used, and the wavelength Kα1used was 1.54056 A°. The scan was performed over the range 2(5-80). The identification of the crystalline phases present was done using the JCPDS database cards. The microstructures and coating thicknesses of the prepared membranes were studied using a scanning electron microscope SEM (JEOL JEM-1200 EX II, made in Japan). Prior to SEM analysis the samples were sputtered with a thin layer of gold using JEOL Fine Coat model JFC-1100E Ion Sputter. 2.4 Membrane performance studies The most suitable membranes were selected for the separation of the mixed and individual selected heavy metals (SeI(V) and Cr(VI) from their solutions. Synthetic solutions of 100, 50 and 10 ppm of both Se(IV) and Cr(VI) were prepared from selenium oxide (99.9% Sigma-Aldrich, Germany) and potassium dichromate (99.5% ADWIC, Egypt) respectively. The separation experiments were conducted at room temperature under atmospheric pressure, and the selected membranes of an area 0.0314 m 2 were placed inside a home-made cross flow tubular filtration apparatus made from glass. The cross flow tubular filtration apparatus was adjusted in the vertical position such that the ceramic membranes could be held inside its inner diameter as marked by the grey arrow in Figure 1. This position allowed a uni-directional flow of the feed solution from the ceramic support toward the external ceramic membrane surface. After each run, the membranes were washed several times with distilled water and isopropyl alcohol. The separation permeability of the ceramic membranes was determined in terms of rejection coefficient R or separation coefficient S that was calculated as a percentage according to Eq, (1) (Yang et al.1998): S = (1-Cp/Cf) x 100 (1) SYNTHESIS OF HIGHLY ORDERED CRYSTALLINE WURTZITE (ZnO) MEMBRANE 59 Where, Cp is the heavy metal concentration in the permeate solution and Cf is the heavy metal concentration in the feed solution (Yang et al.1998). The concentration of the heavy metal in the permeate solution was measured using an atomic absorption spectrophotometer (MS Spect. Soolar AA, England). Also, the fluxes J of the membranes were determined using Eq. (2) (Idris and Zain, 2006): J = V/A.Δt (2) Where, V is the permeate volume (L), A is the membrane area (m 2 ) and Δt is the permeation time (hour) (Idris and Zain, 2006). Figure 1. Cross flow tubular filtration apparatus used in the separation experiments. The selectivity coefficient S was defined as the ratio of initial fluxes for heavy metal 1 and heavy metal 2 respectively, as given in Eq. (3) (Kozlowski et al. 2002): S = JM1/JM2 (3) 3. Results and discussion 3.1 Structural properties Figure 2 shows the XRD patterns of pure ZnO and composite ZnO-CeO2 membranes thermally treated at 500 °C/1h. The intensities of the given wurtzite peaks were very low because the films obtained were very thin as shown in the SEM investigations given in Figure 5. Minor diffraction peaks of the wurtzite structure were identified at scattering angles 2(56.83, 62.75, 72.5, 76.79) corresponding to the basal planes (110, 103, 201, 004) respectively. X-ray patterns matched the patterns of polycrystalline wurtzite (JCPDS Card no. 03-0888) with unit cell parameters, a = b = 3.2489Å and c=5.2Å (Kenanakis et al. 2007). The crystalline rutile phase was identified (JCPDS Card no. 84-1283) with unit cell parameters, a = b = 4.593Å and c = 2.958Å respectively. The strongest diffraction patterns of the rutile phase existed at 2(27.5, 36.11, 41.27 and 54.5) corresponding to the basal planes (110, 101, 111, 211) respectively. The sharpness of the diffraction peaks was a reflection of the high ordered domains and this in turn was clarified by the uniform microstructure of the membranes obtained (Guizard et al.1999; Panda et al. 2008). SH. LABIB and N.A.TADROS 5: As shown in Figure 2, there was no peak difference between the different membranes prepared. This was due to the lower coating thickness values of the obtained films as outlined previously. No diffraction peaks of CeO2 phase were observed. CeO2 phase fluorite structure (JCPDS card no. 75-0076) was clearly observed in the diffraction pattern of the non-supported membrane (50z) as shown in Figure 3. The broadness of the ceria peak is indicative of the presence of smaller size ceria crystallites in the ZnO matrix, (i.e. nano crystallite ceria particles in the ZnO matrix) (Mishra et al. 2003). Figure 2. XRD patterns of pure ZnO and composite ZnO-CeO2 membranes thermally treated at 500 0 C/1h. (a) 100z, (b) 90 z, (c)70z and (d) 50 z. Figure 3. XRD patterns of non supported-membrane 50z thermally treated at 500 0 C/1h. The JCPDS card number of the prepared powder 50z was 36-1451, corresponding to pure zincite, which indicated no impurities in the unsupported membranes. SYNTHESIS OF HIGHLY ORDERED CRYSTALLINE WURTZITE (ZnO) MEMBRANE 5; 3.2 Microstructural Studies Figures 4 (a-d) show the SEM at a magnification power of 1500x of pure ZnO and composite ZnO-CeO2 thin films coated over rutile substrates. No variation in particle microstructure was observed in these figures. A continuous hexagonal array of wurtzite particles was shown in the different figures separated by uniform pores. It was observed that no aggregated particles were present. The particle size increased from 1.25μm to 1.56μm with the addition of 10% CeO2, see Figures 4 (a-b). A further increase in CeO2% was accompanied by an increase in particle size to 2.3 μm and 3 μm, as shown in Figures 4 (c-d) respectively. The increase of grain size can be attributed to the increased ability of CeO2 atoms to move toward stable sites in the lattice of ZnO leading to the crystallinity improvement (Öztaş, 2008). An inhomogeneous grain size distribution was observed in sample 70z, Figure 4(c), making it non homogeneous thin film. The average pore diameter values for 100z, 90z and 50z were 0.94, 0.63 and 1.25μm respectively as shown in Figures 4 (a, b, d). Figures 5 (a-d) show the cross SEM of ZnO membranes thermally treated at 500 0C/1h. The fractured coating thicknesses of the different films were very thin and uniform confirming that the bonding strength between the film and the substrate was strong (Hwang et al.2005). The coating thicknesses of the different ZnO films deposited were found to be 375 nm for 100z, 1071 nm for 90z, 1428 nm for 70z and 1785 nm for 50z. The increase of the different films coating thickness with increasing CeO2% means that a fundamental factor impacting the grain size is the increase of coating thickness leading to the decrease of micro-strain and energy band gap in the coated films (Öztaş, 2008). Figure 4. SEM of ZnO membranes thermally treated at 500 0 C/1h (a) 100z, (b) 90 z, (c) 70 z and (d) 50 z. 3.3 SeIV-CrVI separation characteristics The separation studies of Se(IV) and Cr(VI) were performed on the three different membranes 100z, 90z and 50z. High separation rate of Se(IV) values of 80-90% were obtained through the different membranes as SH. LABIB and N.A.TADROS 64 outlined in Table 2. However very low percentages of Cr(VI) separation were observed through the different membranes. Slight differences in the separation percentage from one membrane to another were due to the characteristic atomic configuration of wurtzite crystal structure, which possesses crystal polarity (Jang at al, 2005; Ban et al. 2007). This polarity played an important role in selenium separation, leading to strong interaction between Se particles and ZnO surface charge (Saffag et al. 2004). As concluded, the membrane surface charge was significant to the membrane performance, because it affected the electrostatic repulsion between the ions or charged molecules and the membrane surface (Childress and Elimelech, 2000). On the other hand, the high rejection rate of Cr(VI) through the different membranes was governed by the capillary pressure induced within the porous structure of the membranes (Guizard et al. 2002). Figure 5. Cross SEM of ZnO membranes thermally treated at 500 0 C/1h, magnification 2500 x, (a) 100z, (b) 90 z, (c) 70 z and (d) 50 z. With a mixture of 100 SeIV-50 Cr(VI), a 100% separation rate of Se(IV) and a 100% flow rate of Cr(VI) were observed through the different membranes, as shown in Figure 6. This was due to the increase in the membrane separation performance with the decrease in the effluent concentrations. Figure 7 shows a decrease in Se(IV) and Cr(VI) flux through the different membranes as a function of Se(IV) and Cr(VI) concentrations. This decrease is explained as follow: the higher the concentration, the higher the osmotic pressure and the lower the permeation flux (Saffag et al. 2004). Permeation flux must be proportional to the reciprocal of the membrane thickness, but for a very thin layer, the flux doesn't vary with the thickness of the layer (Byun, 2009). In this case, the flux is controlled by the kinetics of transport through the porous structure and / or the surface exchange reaction (Byun, 2009). Thus the lowest values of Se(IV) flux through the different membranes with respect to Cr(VI) flux were related to the higher separation coefficient of Se(IV) in comparison to Cr(VI) as given previously. Cr(VI) flux increased with increasing membrane pore diameter as shown in Figure 7b, where the flux order was 50z > 90z > 100z, meaning that wider pore diameter had lesser resistance to Cr transport and so higher flux SYNTHESIS OF HIGHLY ORDERED CRYSTALLINE WURTZITE (ZnO) MEMBRANE 64 value. This result confirmed that the transport of Cr(VI) through the different membranes was by the capillary pressure induced within the porous structure of the membranes as outlined before (Guizard et al. 2002). The selectivity coefficients for the competitive transport of Cr(VI) and Se(IV) from the prepared aqueous solutions through the membranes used are given in Table 3. The decrease of the selectivity coefficient values for the competitive ions with increasing their concentrations is due to the lower permeation flux at higher concentration (Saffag et al. 2004). Table 2. Se(IV) and Cr(VI) separation coefficient results obtained with the selected membranes. Membrane type Average pore diameter (µm) Average particle size (µm) Heavy metal concentration Separation Coefficient for Se(IV) Separation Coefficient for Cr(VI) 100z 90z 50z 0.94 0.63 1.25 1.25 1.56 3.0 100 ppm 90-92% 2-10 % 100z 90z 50z 50 ppm ≥ 86% 0.1- 2 % 100z 90z 50z 10 ppm 82-86% 0.1-2 % Figure 6. Performance of zinc oxide membranes toward 100Se(IV)-50Cr(VI). The highest selectivity was obtained through the synthesized membrane 90z. This result is mainly due to its having the narrowest pore size of 90z with respect to the other membranes as shown in Table 2. SH. LABIB and N.A.TADROS 64 Figure 7. Variation of flux of a - Se(IV) and b - Cr(VI) through different membranes as a function of Se(IV) and Cr(VI) concentrations respectively. 4. Conclusion The objective of this study was to prepare pure ZnO and composite ZnO-CeO2 membranes with high permeability to be used in the separation and or rejection of Se(IV) and Cr(VI). Highly ordered hexagonal arrays and crack free thin films of ZnO and ZnO-CeO2 deposited over titania substrates were prepared using a chelating sol-gel process. The addition of CeO2 to a ZnO matrix had a great effect on grain size and pore size as well as to the coating thickness of the different films. This result affected the separation performance as well as the selectivity and the flux. A high separation performance for the different membranes prepared was observed toward Se(IV) with respect to Cr(VI). The higher separation rate of Se(IV) was mainly due to the polar characteristic of wurtzite structure. On the other hand, the higher rejection rate of Cr(VI) was governed by the capillary pressure induced by the porous structure of the different membranes selected; the membrane 90z, having the narrowest pore diameter, was characterized by the highest selectivity in comparison to 100z and 50z membranes. So, it is concluded that the ZnO membranes described in this paper show high separation SYNTHESIS OF HIGHLY ORDERED CRYSTALLINE WURTZITE (ZnO) MEMBRANE 65 performance and can be used with high accuracy in the separation of Se(IV) from its aqueous solutions. 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