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The impact of cerium (Ce) and neodymium (Nd) rare-earth metal doping of TiO2 prepared by hydrothermal method was investigated to tailor effective photocatalytic degradation of colored wastewater under UV or visible illumination. The hydrothermal treatment of TiO2 decreased the pHpzc from 6.3 to 3.1-3.8 favoring the affinity for cationic water contaminants. Doping with Ce and Nd modified the crystallinity and the morphology of the photocatalysts and significantly increased the BET surface area and the adsorption capacity of cationic dyes. The photocatalytic activity under UV light irradiation decreased due to shielding of the catalyst active area by excessive amount of dye adsorbed. Conversely, the photocatalytic activity of the Ce and Nd doped TiO2 increased under visible light irradiation by 1.2 times as a result of the dye photosensitization effect. It was demonstrated that two-steps dark adsorption and photocatalytic reaction or one-step simultaneous adsorption and reaction can produce significantly different results for the photocatalytic degradation of dyes in coloured waters, the rate being controlled by the competitive adsorption of the reacting organics and the H2O/OHspecies. The reaction is driven by the radical oxygen species (ROS) formed on the catalyst surface the nature of which, differs under UV or visible light irradiation. The Ce-doped TiO2 and Nd-doped TiO2 photocatalysts with 0.5% rare-earth content were found to be efficient in the degradation of MB in aqueous solution, removing the color and reducing the toxicity of wastewaters.

The catalytic activity of MnO, FeOOH and CeO nanoparticles was evaluated in the treatment of a synthetic petroleum refinery wastewater (SPRW) using O in a discontinuous reactor at 25°C and pH 5.5. The mineralization and partial chemical oxidation rates of SPRW using these metal oxides are in the same order of magnitude, and the catalytic activity in the mineralization of SPRW decreased in the order MnO>CeO> FeOOH. The mineralized fraction progressively increased with time in the catalytic process while in the non-catalytic process it remained constant. The effect of the operational conditions on the mineralization and partial chemical oxidation rates using MnO was investigated in detail. The mineralization rate was found to be lower than the partial oxidation rate due to the formation of partially oxidized by-products, and this is dependent on the solids dosage and pH. An investigation of the mechanism demonstrated that the enhancement effect could be attributed to the introduction of the manganese oxide nanoparticles, which could promote the utilization of O and/or enhance the formation of free radicals (•OH, •OH and •O) on the solid surface and further accelerate the degradation of the organic compounds present in the wastewater.

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