Preparation Of Activated Carbon Supported TiO₂ And Its Application In Humic Acid Photocatalytic Degradation

In recent years, photocatalyst TiO₂ has become a research hot spot in environmental and material field, because of its high photocatalytic activity, chemical stability, low cost, and nontoxicity in water treatment, air purification, etc. Most reported photocatalytic studies deal with dispersions, where the main disadvantage is that such suspensions are not suited for practical applications. The principal advantage of supported photocatalytic system is that the catalyst can be reused and there is no need for any post-treatment catalyst filtration step. A catalyst regeneration step, when needed, is also possible for immobilized catalysts. But the activity of TiO₂ may be reduced due to the decrease of its surface area contacting the reactant when it is immobilized. Supporters such as adsorbent materials with porous structures which have large surface area are always used to remedy this kind of loss. And because of their high adsorption capacity, by increasing the concentration of the reactant around the TiO₂ nanos, the adsorption action of the reactant on the surface of TiO₂ can become much easier, which is all-important to the photocatalytic reaction, thus, enhanced photocatalytic activity of the catalyst will appear.In this study, granular activated carbon is selected as the TiO₂ supporter. The immobilized catalyst activated carbon supported TiO₂ (TiO₂/AC) is prepared by a sol-gel method, and humic acid (HA) is chosen to characterize its photocatalytic behavior. The photocatalytic experiments are performed in a quartz glass reactor. The aqueous suspensions are magnetically stirred under the irradiation of a high pressure mercury lamp with a full emission intensity at wavelength of 365nm. Ultraviolet absorbency at wavelength of 254nm (UV₂₅₄) of the solution is analyzed, and as well as total organic carbon (TOC) where needed. The following sub-topics are studied: 1) preparation of TiO₂/AC: studies on TiO₂ load, surface area, and TiO₂ properties, the effect of granule size of activated carbon, concentration of TiO₂ colloid, dipping time, calcination temperature, coating times, and multi-coating process; X-ray diffraction (XRD) tests for crystalline forms and particle size of TiO₂, N₂ adsorption-desorption analysis for BET surface areas and aperture distributions; 2) studies on the degradation of HA under the photocatalytic action of TiO₂/AC: the direct photolysis of HA and the adsorption of HA on TiO₂/AC in dark; the effect of concentrations of TiO₂/AC and HA, pH, temperature, light intensity and air flow; the activity of reused TiO₂/AC; the comparison of suspended and supported systems; TOC and SUVA (UV₂₅₄/DOC) changes during the photocatalytic degradation of HA; and 3) kinetic studies: the direct photolysis of HA, the adsorption of HA on TiO₂/AC in dark, photocatalytic degradation of HA by TiO₂/AC, respectively.This study indicates that the supported catalyst TiO₂/AC owning the highest photocatalytic properties can be prepared in the following manners: TiO₂ sol is formed by hydrolyzing tetrabutyl titanate under a proposed formulation of 1tetrabutyl titanate: 10ethanol: 0.2hydrochloric acid: 0.1 H₂O: 0.3acetyl acetone (in molar ratio). Keep clean activated carbon granules (70-100 mesh) being dipped in the sol for 2h, subsequently get the coated granules out of the sol, dried at 120℃and calcined at 300℃for 1h. Repeat dipping, drying, and calcining process above, and at the third coating process calcine at 500℃for 1h. Finally, get them clean with distilled water and dried. The supported TiO₂ nanoparticle size calculated using Scherrer formula is 11.7nm, which indicates high photocatalytic activity.The results show that, The supported TiO₂/AC as an efficient catalyst has high photocatalytic ability for degradation of HA. During the photocatalytic degradation of HA, synergistic reaction caused by the positive cooperativity between TiO₂ and its supporter activated carbon happens, which effectively enhances the degradation rate of HA. The supported TiO₂/AC can give 96% removal of UV₂₆₄, and large decrease of TOC and SUVA in 3h irradiation time under the 2g/L concentration of TiO₂/AC and 7.7mg/L concentration of HA. It shows that the HA molecules are decomposed into small and simple molecules and then be mineralized during the photocatalytic reaction. The degradation rate of HA is shown to depend on the catalyst and initial HA concentrations, the pH, the temperature, and the light intensity. Activity of the supported catalyst is slightly reduced when be reused.A Langmuir-type relationship between the initial degradation rate and the initial concentration of HA, indicates that adsorption plays a key role in the photocatalytic reaction. The kinetic equation of the degradation of HA under the photocatalytic actions of TiO₂/AC can be expressed as the pseudo-first-order Langmuir-Hinshelwood (L-H) equation. The pseudo-equilibrium constant of HA adsorption on TiO₂ determined from the photocatalytic results, was about 1.7 times higher than the equilibrium constant measured in dark. The difference of the calculated adsorption constant may be related to changes occurring on the adsorption sites of TiO₂ surface under irradiation.