Trichloroacetic acid reduction by an advanced reduction process based on carboxyl anion radical

• CO2- radical reacted with TCAA at a rate constant of 3.2 × 108 M−1 s−1.
• Good degradation efficiency was shown in presence of dissolved oxygen.
• High dechlorination rate was obtained throughout the whole process.
• Both CO2- radical and eaq- were involved in the degradation mechanisms.

UV254 irradiation combined with TiO2 in the presence of formate (FM) was used to develop a carboxyl anion radical (CO2-)-based process to degrade trichloroacetic acid (TCAA). The efficiency and mechanism of TCAA decontamination were examined, and its potential for practical use was then assessed by evaluating the effects of process variables (FM dosage, solution pH, temperature, HCO3-, dissolved organic matter, and NO3-), the treatability of residual FM, and the technical economy on TCAA. Results showed that even without O2 stripping, the UV/TiO2/FM process degraded TCAA more efficiently than the UV/TiO2/O2 aeration process. The kinetic and transformant investigation suggested that CO2- and hydrated electron (eaq-) were involved in TCAA degradation by attacking the C–Cl bond. The CO2- precursor (FM) had an optimum dosage. The redundancy analysis indicated that the effect of six variables on degradation efficiency followed the order of temperature > FM dosage > HCO3- > NO3- > pH. The remaining FM cannot be eliminated by the adsorption by activated carbon and magnetic ion exchange resin. The economic evaluation revealed that most of the expenses went to the production of primary radicals such as HO and eaq- (sources of secondary radical CO2-). Therefore, a simple, harmless, efficient, and low-energy-consuming process is desired to generate CO2- for halogenated pollutant removal.