The current study was conducted to investigate the photocatalytic degradation kinetics of humic
acid at different light intensities using commercial TiO2 powders. The pseudo first order kinetic model and
Langmuir-Hinshelwood (L-H) rate equation in modified forms were used to compare the photocatalytic activities
of TiO2 materials as a function of light intensity. Under constant irradiation conditions, the pseudo first
order reaction rates as well as L-H rates were found to be decreasing in the following trend; Degussa P-25,
Millennium PC-500 and Millennium PC-100. The pseudo first order rate constants showed the same decreasing
trend as the pseudo first order reaction rates while L-H rate constants exhibited a light intensity related
change in the ordering of the photocatalysts. At the lowest light intensity, L-H rate constants decreased as
follows: Millennium PC-500 > Millennium PC-100 > Degussa P-25. However, increasing the light intensity
changed the order to; Millennium PC-100 > Millennium PC-500 > Degussa P-25 revealing the significance
of the L-H adsorption constant. Under constant irradiation conditions, ionic strength dependent changes in
the structure of humic acid did not alter degradation efficiency trend of the photocatalyst specimens and
they were ordered such as; Degussa P-25 > Millennium PC-500 > Hombikat UV-100 > Millennium PC-100 >
Merck. The results presented in this research also confirmed the effectiveness of Degussa P-25 as a photocatalyst
for the degradation of humic acid.
