Abstract
Photocatalytic mineralisation of humic acids using TiO2 modified by tungsten dioxide/ hydrogen peroxide TiO2 of anatase structure was modified by tungsten dioxide and H2O2 in order to obtain WO3-TiO2 photocatalyst with enhanced photocatalytic activity under both, UV and artificial solar light irradiations. WO2 was dissolved in 30% H2O2 and mixed with TiO2 in a vacuum evaporator at 70°C. Such modified TiO2 was dried and then calcinated at 400 and 600°C. The prepared samples and unmodified TiO2 were used for the photocatalytic decomposition of humic acids (Leonardite standard IHSS) in the aqueous solution under irradiations of both, UV and artificial solar light. Modification of TiO2 with tungsten dioxide and H2O2 improved separation of free carriers in TiO2 which resulted in the increase of OH radicals formation. Calcination caused an increase of anatase crystals and higher yield in OH radicals. The uncalcined samples showed high abilities for the adsorption of HA. Combination of adsorption abilities and photocatalytic activity of photocatalyst caused that the uncalcined TiO2 modified with WO2/H2O2 showed the shortest time of HA mineralisation. The mineralisation of HA under the artificial solar light was much lower than under the UV. It was proved that, although OH radicals are powerful in the decomposition of HA, adsorption can facilitate the contact of the adsorbed molecules with the photocatalyst surface and accelerate their photocatalytic decomposition.
Highlights
Humic acids naturally present in the surface water state a problem because of their abilities to form the trihalomethanes (THMs) during the chlorination of water
It was reported that artificial sunlight irradiation was not sufficient to cause a significant decomposition of humic acids (HA), whereas TiO2 appeared to be very effective under the UV irradiation[8]
In the case of WO3/TiO2 photocatalysts the highest photocatalytic activity was achieved on the uncalcined sample, which showed high adsorption of HA, that could be caused by a better separation of free carriers in TiO2 and enhanced the yield of surface oxidative reactions
Summary
Humic acids naturally present in the surface water state a problem because of their abilities to form the trihalomethanes (THMs) during the chlorination of water. The efficiency of TiO2 photocatalyst can be increased by doping some metals as silver, gold or platinum, which are electron acceptors and can retard the inconvenient recombination reaction, which occurs in TiO2 after its excitation, giving better separation of the free carriers and increasing the OH radicals formation[9,10,11] Another solution of better photocatalytic performance is the preparation of a new generation of photocatalysts, which demonstrates activity under visible light. Because we would like to use for the preparation a raw industrially produced TiO2 material, which has very high BET surface area, around 300 m2/g The disadvantage of this method consists in the fact that it does not allow the narrowing of the band gap by introducing the tungsten oxide phase inside the forbidden band of TiO2, as it is possible using the sol-gel, excitation under visible light is only possible in the WO3 semiconductor. According to the elemental analyses obtained from the list of IHSS products Leonardite HA standard (IS104H) contains in % (w/w): 63.81 C, 3.70 H, 31.27 O, 1.23 N, 0.76 S, < 0.01 P, and 2.58 ash
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