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The photocatalytic oxidation of humic substances in aqueous solutions and natural waters with TiO 2 attached to buoyant, hollow glass micro-spheres was studied. A maximum oxidation efficiency of 3.6 mg W 1 h 1 was achieved in neutral or alkaline media at a plane surface concentration of the catalyst attached to the micro-spheres of 25 g m 2 . Proceeding by different mechanisms in acidic and alkaline media, the photocatalytic oxidation efficiency did not benefit from an excessive presence of hydroxyl radical promoters, hydrogen peroxide and alkali.

The photocatalytic oxidation (PCO) of UV-irradiated aqueous solutions containing lignin onTiO2was studied for the influence of ferrous ions. The addition ofFe2+, up to 2.8 mgL−1, to the acidic lignin solution leads to the drastic, for about 25%, increase in PCO efficiency. A further increase in ferrous ion concentration results in a decrease in PCO efficiency of lignin. The maximum PCO efficiency, up to 9.2 mgW−1h−1, was observed in neutral and slightly basic media: the oxidation mechanism with OH-radicals seems to prevail. Also, the difference in the PCO performance with a different attachment mode of titanium dioxide on the catalyst support was observed. Sprayed catalyst exhibited 1.5 times higher efficiency than the one attached by submersion, although sprayed one was easily resuspended in acidic lignin solutions. The efficiency of the N-doped photocatalyst active in visible light was observed to be negligible with lignin.

The photocatalytic oxidation (PCO) of UV-irradiated aqueous solutions containing humic acids and lignin was studied. The photocatalystTiO2was attached to buoyant hollow glass microspheres and glass plates. A maximum oxidation efficiency as low as1.1and2.54mgW−1h−1for humic acids and lignin, respectively, was achieved in neutral and alkaline media with 25gm−2of the buoyant catalyst. In acidic media, efficiency was even lower. The photocatalytic efficiency with the photocatalyst attached to glass plates was about 3 to 4 times higher than that for the buoyant catalyst. Ferrous ions added to acidic solutions did not increase the rate of PCO of humic acids. However the addition ofFe2+ions, up to 0.05 mM, to the lignin solution leads to a dramatic increase, about 25%, in PCO efficiency. A further increase in ferrous ion concentration results in a decrease in the PCO efficiency of lignin. Proceeding most likely by a radical mechanism, the efficiency of PCO of humic acids did not benefit from an excessive presence of hydroxyl radical promoters, such as hydrogen peroxide, although the reaction rate increased. However, PCO of lignin in the acidic medium, where OH.-radical formation is suppressed, benefited from the introduction of hydrogen peroxide due to promoted radical formation.

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