Effect of activated carbon loading on the photocatalytic decolorization of methyl orange using activated carbon combined with nanotitania

• Main Author: Cabral, Kerry P.
• Format: text
• Language: English
• Published: Animo Repository 2010
• Subjects: Sewage—Purification; Textile waste; Carbon, Activated; Nanotechnology; Environmental Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/6140; https://animorepository.dlsu.edu.ph/context/etd_masteral/article/13156/viewcontent/CDTG004799_P.pdf
• Institution: De La Salle University

Colored wastewater is the major problem of textile industries. There are typical wastewater treatment processes available but they do not degrade dye molecules completely. Most of them just transfer the pollutant from one phase to another creating secondary pollution. In this study, photocatalytic decolorization of methyl orange (MO) using nanotitania supported by activated carbon (nanoTiO2/AC) was investigated. A support material was needed in order to aid in the adsorption of large dye molecules that eventually would lead to the photocatalytic degradation of the pollutant by activated nanoTiO2 using a UV light source. Sol-gel was the method used to synthesize nano-sized TiO2 as well as to its attachment to the support material which was AC. Improved surface area was achieved by producing nanoTiO2 and by adding AC. Moreover, the hydrolyzing agent used during the synthesis of the catalyst was glacial acetic acid (i.e. a weak acid). Its reaction with the titanium (IV) isopropoxide (i.e. TiO2 precursor) was so rapid that formation of linear polymers was promoted instead of forming bulk polymers. Bulk polymers form larger crystallite with smaller surface areas. The catalysts used in the study were 1:10, 2:10, and 3:10 AC-nanoTiO2. The ratios referred to the proportion of the amount of AC added in grams to the volume of sol in milliliters. In this case, the AC proportion was varied at a fixed volume of sol. The ratios were used in the optimization process, thus, the supported catalysts in the study were conveniently expressed as AC-nanoTiO2 instead of the conventional nanoTiO2/AC. In terms of percentage nanoTiO2, 1:10, 2:10, and 3:10 AC-nanoTiO2 corresponded to 3.6%, 5.4%, and 10.2% amount of theoretical nanoTiO2, respectively. The catalysts were subjected under several characterization techniques. The surface area of the catalysts was determined by Brunauer-Emmett-Teller (BET) analysis. The amount of AC loaded to the catalysts was determined by thernogravimetric analysis (TGA). Morphology and elemental composition was determined by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The TiO2 crystallite size was estimated using high-resolution images generated using transmission electron microscopy (TEM). TiO2 crystallite phase and size were determined by X- ray powder diffraction (XRD). Lastly, organic functional groups present at the surface of the catalysts were determined by Fourier Transform Infrared (FT-IR). The increase in surface area of the catalysts was mainly due to the addition of AC. Bare nanoTiO2 only has 109.7 m2 /g as compared to 711.8, 837.2, and 851.1 m2 /g of 1:10, 2:10, and 3:10 AC-nanoTiO2, respectively. As the amount of AC loaded to the catalysts increased, the surface area approached to that of pure AC (i.e. 944.1 m2 /g). Moreover, comparing the surface area of the bare nanoTiO2 with that of commercial Degussa P-25 (i.e. 50±15 m2 /g), it can be said that an improvement has been done in terms of the surface area. The amount of AC present in the catalysts as revealed by thermograms were 77.03%, 84.03%, and 91.66% for 1:10, 2:10, and 3:10 AC-nanoTiO2, respectively. This showed that there were a lot of available areas for the attachment of nanoTiO2. Comparing the results from the thermograms with the theoretical amount of AC loaded, a discrepancy can be observed. For 1:10, 2:10, and 3:10 AC-nanoTiO2, theoretical percentages of AC were computed as 89.84%, 94.65%, and 96.37%, respectively. AC lost during the preparation procedure was the possible cause of the difference. Images taken at 100x magnification of the catalyst using SEM showed that the catalysts were made up of a variety of small and large particles. On the other hand,the images taken at 1000x magnification revealed that 1:10, 2:10, and 3:10 AC- nanoTiO2 had more porous structure than bare nanoTiO2. Elemental maps producedshowed that nanoTiO2 was well-dispersed on the surface of AC. Elemental composition results confirmed the presence of %C, %Ti, and % O. The %C was observed to increase as the amount of AC added to the catalyst was increased. This finding was in good agreement with the thermograms and diffractograms. TiO2 crystallite sizes of the catalysts were estimated using the high-resolution images (i.e. 10 nm scale) obtained from TEM. Measured TiO2 crystallite sizes were 11.58, 11.41, 9.38, and 8.58 nm for bare nanoTiO2, 1:10, 2:10, and 3:10 nanoTiO2-AC, respectively. These results were comparable with the XRD machine-generated TiO2 crystallite sizes which were computed based on Scherrer’s formula. The catalysts were all found to have anatase TiO2 crystallite phase. The crystallite size, on the other hand, was found to have a decreasing trend as the AC added to the catalyst was increased. This was due to the increased dispersion of nanoTiO2 on the surface of AC as the amount of AC increased. Bare nanoTiO2, 1:10, 2:10, and 3:10 AC-nanoTiO2 had 12.9, 10.3, 9.3, and 8.8 nm crystallite sizes, respectively. The organic functional groups found on the surface of the catalysts were: O-H in stretching mode which corresponded to the O-H vibration of the Ti-OH groups and water molecules; O-H in bending mode which indicated the basic characteristics of the catalysts; and C=O in stretching mode which corresponded to the presence of carbon in the samples. It was also observed that as the amount of AC was increased, the O-H group, indicating the basic characteristics of the catalysts, decreased. From this finding, it was expected that 3:10 AC-nanoTiO2 would perform the best among the catalysts prepared in terms of activity towards MO. Since MO is an anion when dissolved in water, its attraction to a surface that has less hydroxide ion is favorable than a surface that has more. Photocatalytic activity test was performed in accordance with the Box-Behnken design of experiment. The parameters considered were the AC proportion of the catalysts, initial dye concentration, and pH of the solution represented by X1, X2, and X3, respectively. Levels corresponding to low (-1), center (0), high (+1) for each parameter were the following: for AC proportion, 1:10, 2:10, and 3:10 AC-nanoTiO2; for initial dye concentration, 10, 15 and 20 ppm; and for pH of the solution, 4.0, 7.0, and 10.0. The response, rate of MO removal, was represented by Y (mol/kg cat-s) and was computed using the formula, Y = (N0-N)/Wcat (t-t0), where N0 is the initial number of moles of MO and N is the number of moles of MO after 100 min of photoreaction. Taking into consideration the 20 min allotted for dark adsorption, time t therefore was 120 min (i.e. 7,200 s). Design Expert 8.0.3 (Trial Version) was used to carry out the statistical analyses. The reactor used was supplied by Riko-Kagaku Sangyo Co., Ltd. The UV lamp used has a capacity to emit 254-nm wavelength of light.