1 COMPLEX CARBON-MINERAL SORBENTS FROM COMMON CLAY Authors: S. Cosin 1., C. A. Pinto 2, P. de Souza Santos 1, F.R. Valenzuela Díaz 1. 1) University of São Paulo, Polytechnic School, Materials and Metallurgical Engineering Department, Laboratório de Matérias-Primas Particuladas e Sólidos Não-Metálicos LMPSol, Brazil. 2) University of São Paulo, Polytechnic School, Chemical Engineering Department, Brazil s.cosin@zaz.com.br Keywords: mineral / carbonous material complex, red ceramics, adsorption, chemical attack, corrosion, clay, wastes. ABSTRACT Carbon/mineral complexes are materials with surfaces covered partially or totally by carbon materials contains. They have high industrial potential uses as material adsorbents and ceramic filters. The objective of this work is to evaluate the corrosion strength in acid and alkaline solutions, and the adsorption capacity of methylene blue of some materials prepared in the LMPSol. The compositions were prepared with an industrial common clay named taguá, an organic additive and a inorganic additive from industrial residues up to 50%. The corrosion test was performed in water at 60ºC and in aqueous solutions of HCl and NaOH at room temperature, with mass losses measured after 15 days. The samples were pressed manually and heated at different temperatures: 110ºC, 350ºC and 500ºC. The performed tests were transverse flexural strength, porosity and water absorption. INTRODUCTION Ab-absorbents mineral/carbon can be obtained from the pyrolysis of silicates covered by organic molecules or from chemical vapor deposition (CVD) techniques [1-3]. These materials can be used as catalyst supports or as absorbents in chromatography, trace analysis, and ad/absorption in general, as, for example, in the purification technology of water or in industrial wastes treatment. The main characteristics are porous texture and high specific surface area, combined to a non-polar surface (the part covered with carbon material) and polar parts (the not covered silicate surface), which can absorb many adsorbent types. According to manufacture process, the distribution of types of surfaces can be diversified, depending if the material is more or less polar: that means a greater or smaller percentual of silicate on the surface re-covered with carbon material. The adsorbents, mineral/carbon material, used in this work were obtained from an industrial clay named taguá and from organic and inorganic additives. The objective of this work is to evaluate the adsorption capacity of these materials in methylene blue, after heated at 350 C and at 500 C, as well as evaluating the resistance capacity to corrosion, in acid and alkaline solutions, with the samples prepared at the same temperatures, aiming to verify their potential for use in ceramic filters.
2 MATERIALS and METHODS Adsorbents Adsorbents were prepared in the laboratory, as a powder, passing throught mesh 80 (opening 0,18mm). The common clay taguá is described in reference [5]. The inorganic additive consists essentially of iron oxides and the organic contains a saccharide. Samples compositions A. Taguá with 10% of inorganic additive. B. Taguá with 20% of inorganic additive. C. Taguá with 30% of inorganic additive. D. Taguá with organic additive. E. Taguá with 10% of inorganic additive and with organic additive. F. Taguá with 20% of inorganic additive and with organic additive. G. Taguá with 30% of inorganic additive and with organic additive. H. Taguá with 50% of inorganic additive and with organic additive. The adsorption of methylene blue was performed in two different procedures: a) The samples already described were dried at 110 C and fired at 350 C; b) The samples were dried at 110 C and fired at 500 C. Samples weighting 0.19g were added to 5mL of water solutions with different concentrations of methylene blue (0.05g/L until 0,8g/L), or samples weighting 0.16g were added to 20ml of water solutions with 0.8g/L of methylene blue. After manual mixture, the system stood for 24 hours at 60 C. The system was maintained for more time until to obtain the maximum absorption of methylene blue from the samples. In the for the maximum absorption condition, the supernatan of the system was clear. Conformation and firing of samples The samples were prepared from the material in poedert form, passing throught mesh 80 (opening 0,18mm). They were humidified and conformed manually in prismatic metallic mold, with dimensions of 2cm x 2cm x 6cm. The samples were air dried at room temperature for 24 hours,. Then the samples staied at 60ºC by 1 week and at 110ºC also by another week. Six samples were prepared for each composition and condition (110ºC, 350ºC and 500ºC). The firing of the samples was performed in electric kiln at reducing atmosphere, with residence time of 30 minutes in the burning platform and natural cooling in the end of the process. Ceramic properties These tests were performed only with materials D, E and F. In the samples dried at 110 C the transverse strength was measured (the 3 points method), using universal machine Versat 500. The fired samples were measured by apparent porosity (AP), the apparent specific mass (ASM) and the water absorption (WA), using the described methods by Souza Santos [4]. Corrosion tests The corrosion tests were performed by measuring the mass losses for the samples D and F (fired at 500ºC) in distilled water and water solutions of HCl and NaOH, according to the conditions indicated below. Each sample with different conditions were tested one time.
3 a) The samples were dried for 24 hours at 110ºC, weighted and put in a glass beaker. Distilled water was added at the beaker until complete covering of the sample. The beaker was covered with aluminum foil. The system was maintained at 60ºC. After 15 days they were removed from the beaker, the water excess was eliminated by a cloth, the samples were dried at 110ºC and weighted. b) Same procedure as described in 1, however maintaining the sample at room temperature in different solutions concentrations: 1.8%, 3.6%, and 7.2% of HCl (% in weight of HCl); 5% of sodium hydroxide (% in weight of NaOH). The samples D were analysed in solutions of 10%, 15% and 20% of NaOH. Before the final drying, the samples were washed with distilled water. RESULTS and DISCUSSIONS Table 1 presents results for adsorption in methylene blue tests. Table 1 - Methylene Blue concentration adsorbed (mg of methylene blue /g adsorbent) Test 350 C 500 C A 21,0 5,3 B 21,0 5,3 C 21,0 5,3 D 5,3 5,3 E 1,3 5,3 F 21,0 100,0 G 21,0 100,0 H 5,3 5,3 It can be seen at Table 1 that the samples in which 20 and 30% of inorganic residue was incorporated, and also organic residue (F and G), presented the highest adsorption capacity of methylene blue (100mg/g). The samples without organic residue (A, B and C) had lower values for adsorption after fired at 350ºC and at 500ºC. In the other hand, samples with the specific organic residue tend to improve the adsorption power of methylene blue with the increase of firing temperature. The samples D, E and F, dried at 110ºC and fired at 350ºC and at 500ºC, were submitted to transverse strength test, the results are presented in Table 2.
4 Table 2 - Values for the transversal strength of the samples after drying 110ºC and firing at 350ºC and at 500ºC: Tests Transverse strength (MPa) 110ºC 350ºC 500ºC D 19,8 5,0 3,7 E 21,6 11,3 2,9 F 13,9 6,2 2,6 After drying at 110ºC, the materials that presented highest values were D (19,0MPa) and E (21,6MPa), containing respectively 0% and 10% of inorganic residue. The material F with 20% of inorganic residue presented the lower value (13,9MPa), indicating that the residue must be functioning as an inert material. The samples that were fired at 350ºC presented significantly lower values of flexion strength comparing to the samples dried at 110ºC. This indicates that the organic component must be acting as a low temperature binding material and after firing this bond action is reduced, but remains due to the carbon formed in the reducing atmosphere. After 350ºC firing, the material with 10% of inorganic residue (E) presented higher values to the transverse strength (11,3MPa) compared to the material without residue (D) that was 5,0MPa. Comparing E with the material containing 20% of residue (F) the strength was higher. This indicates the possibility of an optimum amount of inorganic residue that supplies the highest value of strength and that this value may be placed between 0% and 20% of residue. After it is fired at 500ºC the values of transverse strength decreases to values between 2,6 and 3,7MPa, indicating a reduction of the bonding power with organic additive. The lowest value required, usually for products of structural ceramics [2], is 5MPa and if it is considered as a minimum value also for the use as ceramic filters, in this case it would be necessary to use a material with 10% of inorganic residue fired at temperature between 350ºC and 500ºC. The results for water absorption, apparent porosity and apparent specific mass of the samples D, E and F, fired at 350ºC and at 500ºC are shown in Table 3. Table 3 - Values of the water absorption (WA, %), Apparent Porosity (AP, %) and Apparent Specify Mass (ASM, g/cm 3 ): Tests WA AP ASM D-350ºC 14,1 25,0 1,7 D-500ºC 14,8 27,2 1,8 E-350ºC 16,2 27,8 1,7 E-500ºC 18,3 31,8 1,7 F-350ºC 18,4 30,6 1,6 F-500ºC 22,4 36,5 1,6
5 It is observed in Table 3 that the values of absorption in water have a range from 14% to 22%. The apparent porosity varies from 25% to 36%, those high values are beneficial from the point of view of the use of these materials as ceramic filters. The densities of the materials varied between 1,6 and 1,8g/cm 3, indicating that the inorganic residue did not have extreme influence in the density. The results from the corrosion tests are in Table 4. Table 4 - Results of the corrosion tests. Test Mass loss (%) H 2 O 1,8%HCl 3,6%HCl 7,2%HCl 5%NaOH 10%NaOH 15%NaOH 20%NaOH D 500ºC 0,7 1,3 1,2 n.d. crumble n.d. n.d. n.d. E 500ºC 1,7 1,8 1,3 1,4 44,5 n.d. n.d. n.d. F 500ºC 0,2 1,2 1,3 1,5 0,3-1,1* 6,3 crumble n.d. = not determinated; *- Lightly crumbled All the samples presented good stability in water at 60ºC, giving mass losses lower than 1,8%. The best result was obtained from the sample with 20% of inorganic residue (F, 20%). All the samples presented a good resistance in HCl (water solutions containing up to 7,2% in mass of HCl) at room temperature, getting losses of mass lower than 1,6%. The material containing 20% of inorganic residue (F), supported the attack in 5% NaOH solution. The other materials either crumbled or had a high mass loss. In higher concentrations of NaOH, the sample F presented an increase in its weight (with 10% NaOH solution); the same sample presented mass loss higher (6,3% in 15% NaOH solution). Therefore, the inorganic residue added 20%, is the only sample which gave a small resistance to the 5% NaOH solution at room temperature. CONCLUSIONS The materials analysed presented, after fired at 350ºC or at 500ºC, capacity of adsorption of methylene blue up to 100mg/g adsorbent. The samples prepared with the materials containing between 0% and 20% of inorganic residue had presented higher values of the transverse strength after drying at 110ºC. Strength values after firing at 350ºC and lower values than 5MPa after firing at 500ºC, indicates that the organic additive acts as a temperature-dependent binding and this action decreases with the increase of the firing temperature. The fired samples presented high porosity and absorption of water, which is benefitial from the point of view as potential use as ceramic filters. A good corrosion resistance was noticed in samples after permanence in water at 60ºC and in HCl solutions at room temperature. For the fired materials at 500ºC, the sample containing 20% of inorganic residue was the only one that presented a small resistance to corrosion in 5% NaOH. REFERENCES 1) Leboda, R. Mineral carbon adsorbents- New type of sorbents. 2. Surface properties and methods of their modification. Materials Chemistry and Physics, 34(2), 123-141 (1993).
2) Leboda, R.; Charmas, B. Area evaluation of surface of carbon component of model carbon-silica adsorbents from adsorption dates of p-nitrophenol from aqueous solutions, Colloid and Surfaces. Physicochemical and Engineering Aspects, 135, 267-275 (1998). 3) Leboda, R.; Turov, V.V.; Charmas, B.; Skubiszewska-Zieba, J.; Gunko, V.M. Surface properties of mesoporous carbon-silica gel adsorbents, Journal of Colloid and Science Interface.223, 112-125 (2000). 4) Santos, P.S. - Science and Technology of Clays, 2 nd ed., Edgard Blücher, São Paulo, 1989 (vol1) and 1992 (vol. 2 and 3). 5) Cosin, S. Santos H. S, Santos, P. S, Toledo, S. Characterization of the component mineral argilous of taguás of the region of Jarinu - Jundiaí, SP. Part I - Ceramic Information, Ro Claro, SP. Brazil, volume 23 pg. 43 to 47. 6