Introduction Porous materials are of scientific and technological importance because of the presence of voids of controllable dimensions on the atomic, molecular, and nanometer scales, enabling them to discriminate and interact with molecules and chemical clusters. In 1992 the synthesis of mesoporous molecular sieves using a liquid-crystal templating mechanism (LCT) was reported [1, 2], immediately drawing attention to their textural and structural properties. In this process, the structure of the porous solid is defined by the organization of surfactant molecules into micelles which serve as templates for the formation of the pores. Silicates designated as MCM-41 have been synthesized with a regular hexagonal-array of uniform channels with pore sizes varying from 2 nm to about 10 nm. They seem to be very promising as sorbents for fundamental adsorption studies. The main research challenges include a fundamental understanding of structure-property relationships and tailor-design of structures for specific properties and applications. Research efforts in this field have been driven by rapidly growing emerging applications such as adsorption to remove specific contaminants in aqueous solution. This application offers exciting new opportunities for scientists to develop new strategies and techniques for the synthesis and applications of those materials. Chromium is quite a common substance in the environment and can be found in concentrations ranging from less than 0.1 µg/m3 in atmospheric air to 4 g/kg in soil. Chromium is present in the environment in two main oxidation states, trivalent and hexavalent. Naturally-occurring chromium is in the trivalent state in rocks, soil, plants, and volcanic emissions. Hexavalent chromium is mostly derived from industrial activities such as steel making, electroplating, and tanning [3]. The physiological effects of chromium on biological systems depend on its oxidation state. At low concentrations, Cr(III) is considered an essential element in mammals for the maintenance and control of glucose, lipids, and protein metabolism. However, at the same low concentrations Cr(VI) is toxic. In living systems it has been proven that Cr(VI) damages lungs, liver, nerves and kidney tissues in mammals, due to its tendency to oxidize other chemical species [4]. Most chromium wastes are in the form of Cr(III), that can be oxidized as a result of environmental conditions to its more hazardous form, Cr(VI). In fact, the International Agency for Research on Cancer (IARC) has determined that Cr(VI) is carcinogenic to humans; therefore, the removal of Cr(VI) from aqueous solutions and industrial effluents is becoming of increasingly public concern [5]. There are several methods of removing chromium from aqueous solutions, differing fundamentally in equipment utilization, process-efficiency and operational cost [6]. Among the known methods, adsorption is an alternate technology for removing chromium from aqueous solutions, based on the ability of some porous materials to bind chromium to their inner surfaces through several mechanisms such as physical adsorption, complexation, ion exchange, and surface micro-precipitation [7-9]. Due to the health hazards that chromium poses to persons and animals, numerous studies concerning its removal from aqueous solutions have been performed using different adsorbents [10-13]. An active search for newer and more selective adsorbents has led to the discovery of a wide variety of synthetic compounds [14]. Because of their higher chemical stability and greater affinity for some ions, Ti-modified materials are now regarded as promising for adsorbing chromium from industrial effluents, which might be utilized under extreme operating conditions, such as highly oxidizing effluents [15]. Chromium adsorbs onto the solid adsorbent surface from the aqueous solution and the quantity of the removed chromium is dependent on the adsorption capacity of the adsorbent. In this work, several Ti-Modified MCM-41 materials were prepared and characterized to study the effect of this modification on chromium-adsorption capacity. Two methods for post-synthetic incorporation of titania were combined: chemical grafting and incipient-wetness impregnation (Ti-MCM-G/I samples). Experimental Procedure Support Preparation Pure silica MCM-41 (Si-MCM-41) was synthesized according to the procedure reported elsewhere [16]. In short, 9.63 g of Ludox TM-40 (Aldrich 40 wt% colloidal silica in water) were poured into a 250 mL polypropylene beaker assisted by vigorous magnetic stirring. Then, 9.26 g of tetraethylammonium hydroxide solution (TEAOH, Aldrich, 20 wt% solution in water) was added followed by 8 g of hexadecyltrimethylammonium chloride solution (HDTMACl). Aldrich, (25 wt% in water, 1/3 of the total amount). Once the expected gel was formed, the remaining 2/3 of the surfactant (HDTMACl) was added. The gel was agitated for 1 min and then transferred to a 250 mL Teflon autoclave where it was left to react completely under gentle stirring (~120 rpm) for 42 h at 377 K. The resulting solid was recovered by filtration, washed with water, extracted with ethanol for 4 h in a Soxhlet apparatus, and finally calcined in air at 877 K for 22 h. The molar composition of the silica gel was 1SiO2:0.29CTMACl:0.19TEAOH:27H2O. Ti-modified MCM-41 Method Ti-modified MCM-41 supports were prepared using two different methods and a combined method of post-synthetic Ti incorporation: chemical grafting (Ti-MCM-G sample) and incipient-wetness impregnation (Ti-MCM-I samples). In both methods titanium (IV) butoxide (Ti(n-BuO)4, Aldrich 97% solution) was utilized as the titania source and n-propanol (n-PrOH) as the solvent. In the G procedure, a calcined Si-MCM-41 was turned into a slurry in dry n-PrOH containing Ti(n-BuO)4. The filtered material was then washed with dry n-PrOH three times. The solid was finally dried in air at (393 K, 3 h) and calcined (823 K, 5 h). For the preparation of Ti-MCM-G/I samples, calcined Ti-MCM-G was impregnated with an n-PrOH solution containing the required amount of Ti(n-BuO)4. Then, the samples were dried and calcined as described above. In this case no additional washing was performed after Ti incorporation. Support and Ti-Modified MCM-41 Material Characterization The support materials were characterized by N2 adsorption, X-ray diffraction (XRD) and measurement of the surface hydroxyl-group content. Nitrogen adsorption/desorption isotherms were measured with a Micromeritics ASAP 2010 automatic analyzer at liquid N2 temperature. Specific surface areas were calculated from the adsorption isotherms by the BET method, and pore size distributions from the desorption-isotherms by the BJH method. Prior to the BET measurements, all samples were out-gassed for 6 h at 453 K. In general, the errors found in repeated measurements of surface area were within 2-3% of the total surface area. X-ray diffraction patterns were recorded in the 3º≤2 Θ ≤80º range in a diffractometer model Equinox system EQUI22102003, (40 kV, 30 mA, λ=1.5458 Ǻ). Measurement of the surface hydroxyl-group content was measured by a modified Boehm’s method with a 420 A meter Orion 213154-A01 and electrode ion selective Orion 9157BN. Adsorption Isotherms of Cr(III) and Cr(VI) onto Ti-Modified MCM-41 Materials Ti-modified MCM-41-G and Ti-modified MCM-41-G/I-containing suspensions (5 mg/mL) and Cr(III) and Cr(VI) solutions (5, 10, 15, 20, 30 and 40 ppm) were put in contact for 1, 3, 5 and 7 days at 20°C until equilibrium was reached. After that, the adsorption was suspended by centrifugation and the amount of Cr(III) or Cr(VI) remaining under equilibrium conditions was determined. MCM-41 and TiO2 were used as control porous-adsorbent materials. The initial and final pH values of the solutions were measured, and those pH levels were controlled using HCl or NaOH over the whole adsorption period for all the experimental runs. Removal Of Cr(VI) From The Saturate Ti-Modified MCM-41 Materials To remove the chromium adsorbed into the porous materials, the saturated Ti-modified MCM-41 materials were washed with hot water for 10 min several times until no chromium was detected in the leachant. The recovery of chromium was estimated in each case. Chromium Determination An Atomic Absorption Spectrometer Perkin Elmer Analyst 100 was utilized for the chromium determination. The calibration curves were determined for every chromium-chemical species. A correlation coefficient of 0.95 or greater was successfully attained. The instrument response was periodically checked with chromium solutions of known concentrations. The difference between the initial chromium concentration and the chromium concentration in the supernatant after the reaction was assumed to be the chromium-sorbed species. The amount of chromium-sorbed species was estimated from the difference between the initial concentration and the concentration measured in the solution. All the experiments were carried in triplicate and control solutions were used as well. Results and Discussion Support and Ti-Modified MCM-41 Materials Characterization The physicochemical properties of Ti-modified MCM-41 materials are listed in Table 1. Support (silica) and TiO2 were used as sample controls. The table shows that textural properties decrease as TiO2 load increases. In the meantime, the amount of OH increases with TiO2 load. Table 1. Support and Ti-modified MCM-41 materials characterization. | | | | | | | | Si-MCM-41 | 0.0 | 860 | 0.70 | 3.0 | 1.8 | | Ti-MCM-G | 12.5 | 766 | 0.54 | 2.7 | 3.4 | | Ti-MCM-G/I | 32.5 | 560 | 0.40 | 2.8 | 9.5 | | TiO2 | 100.0 | 44 | 0.15 | 13.7 | 57.1 | Figure 1 shows the adsorption-desorption isotherms of Ti-MCM-G/I materials. This figure illustrates the typical shape of the MCM-41 isotherm usually present in Ti-MCM-G/I materials. The isotherm shape changes slightly with an increase in TiO2 load. The latter indicates a well-achieved distribution of Ti species in the pores of the MCM-41-type materials. Figure 2 shows the BJH pore-size distribution measured for the Ti-MCM-G/I materials. The pore-size-distribution intensity as observed in the curves of Figure 2 decreases as the TiO2 load increases. This effect is associated with thicker walls of the pores comprising the porous materials, loading to the narrower void conducts. 
Figure 1. Adsorption-desorption isotherms of Ti-MCM-G/I materials. 
Figure 2. BJH pore size distribution of Ti-MCM-G/I materials. Figure 3 shows the X-ray diffraction patterns for Ti-MCM-G/I materials. The characteristic TiO2 reflections (anatase) increase with increase in TiO2 load. 
Figure 3. X-ray patterns diffraction of Ti-MCM-G/I materials. Figures 4 and 5 show the adsorption isotherms of Cr(III) and Cr(IV) respectively. Figure 4 shows that the silicates-based material (SiO2) has a better absorption capacity for Cr(III), 1.2 mg Cr(III)/ gads. On the other hand Figure 5 shows that anatase (TiO2) has a better adsorption capacity for Cr(VI), 2 mg Cr(VI)/ gads. 
Figure 4. Adsorption isotherms of Cr(III). 
Figure 5. Adsorption isotherms of Cr(VI). Conclusions The present study introduces a good alternative method for Cr(VI) removal from aqueous and industrial wastewater solutions, allowing the development of newer, lower operational cost, and more efficient technology than other processes already in use. The grafting method for Ti loading to porous materials shows an improved dispersion of TiO2 in the pores of MCM-41-type materials. The adsorptive process for Cr(III) and Cr(VI) chemical species from aqueous solutions by Ti-MCM-G/I or Ti-MCM-G was investigated in a batch-wise mode, observing clear differences in the characteristics of adsorbed Cr species. In both porous materials, the Cr(VI) chemical species was adsorbed approximately twice as efficiently as the Cr(III) chemical species. References 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck. “Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism”, Nature, Vol. 359, 710-712, 1992. 2. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, “A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates”, J. Am. Chem. Soc., Vol. 114, 10834-10843, 1992. 3. C. Pellerin and S. M. Booker, “Health Hazards of an Industrial Heavyweigh”, Environ. Health Perspect, Vol. 108, 12-23, 2000. 4. K. R. Reddy, C.Y. Xu and S. Chinthamreddy, “Assessment of electrokinetic removal of heavy metals from soils by sequential extraction analysis”, J. Hazard. Mater., Vol. 84, 279-286, 2001. 5. World Health Organization 6. J. Kotas and Z. Stasicka, “Chromium occurrence in the environment and methods of its speciation”, Environ. Pollut., Vol. 107, 263-283, 2000. 7. C. P. Jordão, J. L. Pereira and G. N. Jham, “Chromium contamination in sediment, vegetation and fish caused by tanneries in the State of Minas Gerais, Brazil”, Sci. Total Environ., Vol. 207, 1-11, 1997. 8. United States Environmental Protection Agency, (US-EPA), National Primary Drinking Water Regulations, Sec 141.2 (1992) 93. 9. California Public Health Goal (C-PHG), Public Health Goal for Chromium in Drinking Water, California (1999). 10. World Health Organization (WHO), Guidelines for Drinking-Water Quality, Geneva (1993). 11. S. Kocaoba and G. Akcin, “Removal and recovery of chromium and chromium speciation with MINTEQA2”, Talanta, Vol. 57, 23-30, 2002. 12. I. Gaballah and G. Kilbertus, “Recovery of the heavy metal ions through decontamination of sintethic solutions and industrial effluents using modified barks”, J. Geochem. Explor., Vol. 62, 241-286, 1998. 13. N. Daneshvar, D. Salari and S. Aber, “Chromium adsorption and Cr(VI) reduction to trivalent chromium in aqueous solutions by soya cake”, J. Hazard. Mater., Vol. 94, 49-61, 2002. 14. Z. Aksu, Ü. Açikel, E. Kabasakal and S. Tezer, “Equilibrium Modelling of Individual and Simultaneous Biosorption of Chromium(VI) and Nickel(II) onto Dried Activated Sludge”, Water Research, Vol. 36, 3063-3073, 2002. 15. O. Franke, J. Rathouský, G. Schulz-Ekloff and A. Zukal, “Synthesis of MCM-41 mesoporous molecular sieves”, Preparation of catalysts VI. In: G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds), Scientific Bases for the Preparation of Heterogeneous Catalysts. Studies in Surface Science and Catalysis, Elsevier, Amsterdam, Vol. 91, 309-318, 1995. 16. Martínez, J. M., “Diseño de materials mesoporosos tipo MCM-42 modificados con titanio para reacciones de hidroprocesamiento”, Ph. D. Thesis, UNAM. México (2002). Contact Details |
