Preparation and characterization of ultrafiltration membranes for toxic removal from wastewater

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ELSEVIER Desalination 168 (2004) 259-263 DESALINATION www.elsevier.corn/locate/desal Preparation and characterization of ultrafiltration membranes for toxic removal from wastewater .a* N. Saffaj , S. Alami Younssi a, A. Albizane a, A. Messouadi a, M. Bouhria a, M. Persin h, M. Cretin h, A. Larbot u aLaboratoire des Matdriaux Catalyse et Environnement, Facult~ des Sciences et Techniques, Mohammedia BP 146, Mohammedia 20650, Morocco Tel. +212 (63) 323683; Fax +212 (23) 315353; e-mail: ~lnstitut Europ~en des Membranes, UMR 5635 CNRS ENSCM UMII, 1919 Route de Mende 34293 Mon{pellier, Cedex 5, France Received 13 February 2004; accepted 23 February 2004 Abstract The use of ceramic membrane in liquid pollution treatment is actually limited due to the price of ceramic membrane, nevertheless these membranes are more resistant to solvent, pH and oxidation. In this work the pre- paration of low cost TiO:(50%)-ZnAI204(50%) membranes deposited on an artificial cordierite support is pre- sented. The pores diameter of the membrane is centered on 4nm, the water flux is 6.4 F l rn -2 h -l bar -t, thickness and molecular weight cut-off are respectively 1.2 lma and 3,000Da. High rejection of classical ionic solutes, heavy metal and dyes were obtained by a classical mechanism of Donnan exclusion. This kind of membrane gave promising results for possible wastewater treatment in emergent countries. Keywords: Membrane; Ceramic; Ultrafiltration; Electric interaction 1. In t roduct ion Membranes processes are now more and more used in a number of industrial processes, *Corresponding author which include different operating conditions and module designs. The use of membrane tech- nology to replace a separation or purification step in an existing industrial process may re- duce the overall consumption of energy and produce acceptable results. Compared to or- Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter 2004 Elsevier B.V. All rights reserved doi;10.1016/j.desal.2004.07.006 260 N. Saffaj et al./ Desalination 168 (2004) 259-263 ganic membranes, inorganic membranes offer several advantages, such as superior thermal and chemical resistance and better mechanical strength [1 ]. Unfortunately, they are nowadays too expensive to consider for environmental de- polluting applications, therefore a great deal of research has been devoted to developing new types of low cost inorganic membranes. In accordance with this idea, we will present in this work the preparation of a low ultrafiltration layer (50%TIO2, 50% ZnA1204) [2] deposited on a cheaper eordierite support [3]. The results of filtration tests performed by means of the prepared membranes with different salts, such as NaC1, Na2SO4 and heavy metal cations - - (Cr(I]l), Cd(II), Pb(II) - - and methylene blue dye will be presented, as well as a discussion on the concentration and pH effects. 2. Membrane preparation The particle size of the artificial cordierite powder used to prepare the paste is in the range of 0-1251am. The tubular support was obtained by extrusion of a mixture of eordierite and or- garlic additives in correct proportion to adjust the rehological properties of the paste. After drying at room temperature and sintering at 1275C, the support presents a porosity of 40% and an average pore size in the range of 7~xn. An intermediate layer made of zirconia was coated by slip casting on the prepared cordierite support using the suspended powder technique. After firing at a temperature of 1100C for 2 h, the ZrO2 microfiltration membrane obtained presented a pore diameter of 0.23 /am and an average thickness above 23 pan. The ultrafiltration top layer was then pre- pared by sol-gel route using TiO2 and ZnA1204 mixed sols. The mixture of TiO2 (50%) and ZnA1204(50%) sols was deposited on the ZrO2 mierofiltration layer by slip casting. Capillary forces suck the solvent through the support, leaving a layer of concentrated sol on the sur- face. The coated support was then dried for 24 h at room temperature and then fred at 400C for 2 h to finally obtained the composite membrane (Fig. 1). Tangential filtration tests were performed on a laboratory scale filtration pilot using a re- cycling configuration. It was equipped with an adjustable out-flow pump, a thermostatic feed tank and a vertical membrane module of 15 cm length. The transmembrane pressure was sup- plied by a N2 gas bottle. Fig. 2 shows that the water flux through the membrane depends on the Fig. 1. SEM micrograph of ZnAI204-TiO2 membrane. N. Saffaj et al. / Desalination 168 (2004) 259-263 261 60 ~- 40 . LL 20- 0 ' o ; 6 6 ;o Pressure (bar) 0,012 - 0.010 - 0,008- 0,006" 0,004- 0,002 2O pore diameter (A) Fig. 3. Pore size distribution for ZnA1204-TiO 2 UF top layer. Fig. 2. Water flux vs. working pressure. applied pressure. The average water permeabil- ity was about 6.4 1/ pore diameter of the final top layer measured by nitrogen ad- sorption-desorption is centred on 4 nm (Fig. 3). The molecular weight cut off (MWCO) of this membrane was determined by using dif- ferent solutions of calibrated polyethylene gly- col from 600-5000 Da at a 10 -3 mol.L -1 con- centration. The cut-off estimation of 3000 Da from Fig 4 data and the 4 nm pore size for the filtering layer are in good agreement with the retention of a low ultrafiltration membrane. 3. Salts rdtrat ion 3.1. Filtration of NaCl and Na2S04 The performances of a low ultrafiltration membrane in terms of rejection towards elec- trolyte solutions, depends on the steric and elec- tric interaction between the surface of the mem- brane and the ions. In our case, due to the com- pared size o f the ions and the pore size of the membrane, the main parameters that must be considered are the electric interactions. In the goal to control this, classical electrolyte solu- tions (NaC1, Na2SO4) were filtered at different pH value (between 3 and 11) fixed by acid or base adding. The rejection rate of NaC1 (Fig. 5) remains constant between pH3 and 8 and then it decreases at pH9.8. For a higher pH value, an increase o f the rejection is also observed. For the 100 - 60 - i 40. 20. 0 i , i - 1000 2000 Moleou lar we ight (Da) . J . 3o~o - - - -~ Fig. 4. Rejection rates of PEG at different molecular weight. 80- 60. i 40, 20- ! pH ml * 10 Fig. 5. Evolution of NaCI rejection vs. pH for the ultra- filtration membrane, c = 10-3M, AP = 10 bars. 262 N. Saffaj et al. / Desalination 168 (2004) 259-263 100 - 80, 60- 40 , 20 . pH t \ ; 1'o Fig. 6. Evolution of Na2SO4 rejection vs. pH for the ultrafiltration membrane, c = 10-3mol.1 -~, AP = 10 bars. experiment performed with the NaaSO4 electro- lyte (Fig. 6), the rejections obtained are in the range of 40% from pH3 - 8 and pass by a mini- mum at pH9.7; for higher pH values, the rejec- tion increased again. This behaviour is well known and observed in a number of studies [4,5]. In a former study [2,5] devoted to TiO2- ZnA1204 layer properties, we showed that the isoelectric point of the filtering material is at pi l l0; at this pH the surface charge of the material is very low, this is why a decrease of the rejection rate is observed when the pH values are around 9.7. For a higher pH value than 10.5, the increase of the rejection of NaC1 or Na2SO4 can be explained by the repelling of the anion, which is more important for the dianion SO42- because the membrane surface becomes more and more negatively charged. Hence, we con- firm that the behaviour of the membrane can be explained taking into account the evolution of the material membrane charge with the pH of the filtered solution. 3.2. Filtration of heavy metals Due to their toxicity for human health, heavy metals concentration in waste-water are limited by strict standards. The use of ceramic membrane in de-polluting membrane processes is actually limited because the cost of this kind of membranes is too high. The behaviour of the low cost prepared membrane suggests the pos- sibility of using it to filtrate solutions which con- Table 1 Rejection of different heavy mol.l -t, AP = 10bars metals solutions, 10 -3 Pb (NO3) 2 pH R, % 3.9 94 5.0 93 5.8 93 Cd (NO3h 4.5 87 5.3 87 6.2 87 Cr (NO3)3 2.6 93 3.5 94 3.7 95 tain heavy metallic cations as Cr(lJI), Cd(IO and Pb(IO. The ZnAI204/TiO2 ultrafiltration membrane deposited on the cordierite support was then tested for the filtration of Cr(NO3)3, CdfNO3)2 and Pb(NO3)2 solutions at different pH in the goal to evaluate the efficiency of membranes in rejecting the toxic metals. The rejections are grouped in Table I. As for the NaCI and Na2SO4salts, the positive membrane surface charge is responsible for the high rejec- tion (90%) of the heavy metal ions by a clas- sical Donnan exclusion mechanism [6]. 3.3. Filtration o f methylene blue Methylene blue is a dye used in the textile industry; it can lead to various harmful effects by inhalation. Decolorizing waste-water con- taining methyllene blue, using a ZnA1204-TiO2 membrane should be interesting to consider. With this in this mind, filtrations of methylene blue dye (50ppm) at different pH values from 2-9 were performed. The evolution of the rejection rate is presented in Fig. 7. The rejec- tion rates decrease from 81% at pH3 - 54% at pH9. Here also the decrease of the rejection rates is due to the decrease of the positive sur- face charge, which leads to the decrease of the electric interaction between the methylene blue cation positively charged and the membrane. N. Saffaj et al. / Desalination 168 (2004) 259-263 263 100- 80- A t,-- 40 . 20. a ~ ~ 10 pH Fig. 7. Evolution of methylene blue rejection vs. pH for the ultrafiltration membrane, c = 50 ppm, AP = 10 bars. 4. Conclusion In this work we have shown that a low cost composite ultrafiltration ceramic membrane can be successfully developed using low cost cera- mic support in the place of a classical o~ alumi- na support. The mechanism responsible for the rejection of salts is the electric interaction be- tween the charged surface groups of the cera- mic material and the ions present in the filtered solution. This membrane gives promising re- suits for the retention heavy metals and dyes. Acknowlegment This joint program was made possible thanks to the comit6 Mixte inter Universitaire Franco- Marocain (A.I. 213/SM/00). We gratefully ac- knowledge funding through Institut Europren des Membranes for technical support and ana- lytical screening of samples. References [1] A.J. Burggraf and L. Cot, Fundamentals of Inorganic Membranes, Science and Technology, Elsevier Science and Technology Series 4, Elsevier, Am- sterdam, 1996. [2] Y. Elmarraki, M. Cretin, M. Persin, J. Sarrazin and A. Larbot, Mater. Res. Bull., 36 (2001) 227-237. [3] L. Broussous, Elaboration de nouvelles gromrtrie tubulaires de membrane crramiques: application la rrduction du eolmatage, Thesis, Montpellier, France, 1999. [4] T. Van Gestel, C Vandecasteele, A. Buekenhoudt, C. Dotremont, J. Luyten, R. Leysen, B. Van der Bruggen and G. Maes, J. Membr. Sci., 209 (2002) 379-389. [5] Y. Elmarraki, M. Cretin, M. Persin, J. Sarrazin and A. Larbot, Sep. Purif. Technol., 25 (2001) 493-499. [6] J. Palmed, P. Blanc, A. Larbot and P. David, J. Membr. Sci., 160 (1999) 141-170.


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