With increase of Cr(VI) concentration, more Cr2O7
2−
species are formed, which have a weaker adsorption ability
than HCrO4
− and are able to repel the adsorption of
HCrO4
− species on TiO2. The result is that the photocatalytic
oxidation ability of TiO2 decreases as the initial
Cr(VI) concentration increases.