No abstract

In this study, conventional TiO 2 powder was heated in hydrogen (H 2 ) gas at a high temperature as pretreatment. The photoactivity of the treated TiO 2 samples was evaluated in the photodegradation of sulfosalicylic acid (SSA) in aqueous suspension. The experimental results demonstrated that the photodegradation rates of SSA were significantly enhanced by using the H 2 -treated TiO 2 catalysts and an optimum temperature for the H 2 treatment was found to be of 500-600°C. The in situ electron paramagnetic resonance (EPR) signal intensity of oxygen vacancies (OV) and trivalent titanium (Ti 3þ ) associated with the photocatalytic activity was studied. The results proved the presence of OV and Ti 3þ in the lattice of the H 2 -treated TiO 2 and indicated that both were contributed to the enhancement of photocatalytic activity. Moreover, the experimental results presented that the EPR signal intensity of OV and Ti 3þ in the H 2 -treated TiO 2 samples after 10 months storage was still significant higher than that in the untreated TiO 2 catalyst. The experiment also demonstrated that the significant enhancement occurred in the photodegradation of phenol using the H 2 -treated TiO 2 .

Abstract:A series of cerium ion-doped titanium dioxide (Ce 3+ -TiO 2 ) catalysts with special 4f electron configuration was prepared by a sol-gel process and characterized by Brunauer-EmmettTeller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce 3+ -TiO 2 catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce 3+ -TiO 2 catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs.The experimental results verified that both the adsorption equilibrium constant (K a ) and the saturated adsorption amount ( max ) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti 3+ , Ce 3+ , and Ce 4+ ions reside in the Ce 3+ -TiO 2 catalysts. The results of DRS analysis indicated that the Ce 3+ -TiO 2 catalysts had significant optical absorption in the visible region between 400-500 nm because electrons could be excited from the valence band of TiO 2 or ground state of cerium oxides to Ce 4f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4f level) in Ce 3+ -TiO 2 might be a critical reason to eliminate the recombination of electron-hole pairs and to enhance the photocatalytic activity.

 the landfill leachate in Hong Kong usually contains quite high NH 4 + -N concentration, which is well known to inhibit nitrification in biological treatment processes. A common pre-treatment for reducing high strength of ammonium (NH 4 + -N) is by an air-stripping process. However, there are some operational problems such as carbonate scaling in the process of stripping. For this reason, some technical alternatives for NH 4 + -N removal from leachate need to be studied. In this study, a bench-scale experiment was initiated to investigate the feasibility of selectively precipitating NH 4 + -N in the leachate collected from a local landfill in Hong Kong as magnesium ammonium phosphate (MAP). In the experiment, three combinations of chemicals, MgCl 2 6H 2 O + Na 2 HPO 4 12H 2 O, MgO + 85% H 3 PO 4 , and Ca(H 2 PO 4 ) 2 . H 2 O + MgSO 4 . 7H 2 O, were used with the different stoichiometric ratios to generate the MAP precipitate effectively. The results indicated that NH 4 + -N contained in the leachate could be quickly reduced from 5,618 mg/l to 112 mg/l within 15 minutes, when MgCl 2 6H 2 O and Na 2 HPO 4 12H 2 O were applied with a Mg 2+ :NH 4 + :PO 4 3mole ratio of 1:1:1. The pH range of the minimum MAP solubility was discovered to be between 8.5 and 9.0. Attention should be given to the high salinity formed in the treated leachate by using MgCl 2 6H 2 O and Na 2 HPO 4 12H 2 O, which may affect microbial activity in the following biological treatment processes. The other two combinations of chemicals [MgO + 85% H 3 PO 4 and Ca(H 2 PO 4 ) 2 . H 2 O + MgSO 4 . 7H 2 O ] could minimise salinity after precipitation, but they were less efficient for NH 4 + -N removal, compared with MgCl 2 6H 2 Oand Na 2 HPO 4 12H 2 O. COD had no significant reduction during this precipitation. It was found that the sludge of MAP generated was easily settled within 10 minutes to reach its solids content up to 27%. The other characteristics including capillary suction time (CST) and dry density (DD) of the MAP sludge were also tested. The experimental results indicate that the settled sludge is quite solid and can be directly dumped at a landfill site even without any further dewatering treatment.

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