Removal of monochloroacetic acid in water by advanced oxidation based on ozonation in the presence of TiO2 irradiated at λ > 340 nm

TiO2 photocatalysis is based on the photoexcitation of electrons from the valence band to the conduction band. Increasing the rate of photocatalytic reactions requires decreasing the rate r at which the electrons are de-excited back to the valence band. Ozone was expected to be a good candidate to lower r on the basis that it is a much better electron scavenger than oxygen. In addition, it does not leave residues. To illustrate that, monochloroacetic acid (MCAA) was chosen as a model water pollutant since MCAA itself or its intermediate oxidation products do not directly react with O-3 and since it is nonvolatile. At a dose rate of 290 mg h(-1) of nonphotoexcited O-3, the MCAA removal rate at pH 3.4 and 7 was multiplied by a factor of ca. 7 in both distilled and tap water. TOC associated with MCAA was removed at about the same rate as MCAA. Cl- ions were gradually oxidized into ClO3- ions. The chemistry underlying these O-3 effects is discussed.