Oxalic acid destruction at high concentrations by combined heterogeneous photocatalysis and photo-Fenton processes

Heterogeneous photocatalysis (HP) using UV/TiO2, photo-Fenton (PF) reaction using UV/Fe/H2O2 and the combination UV/TiO2/Fe/H2O2 (HP-PF) were tested as processes to degrade oxalic acid (Ox) at relatively high concentrations (0.032 M). PF reactions were generally more efficient than HP including the reaction in the absence of H2O2. Oppositely to previous results (e.g., with EDTA), HP-PF combinations did not result, in the case of oxalate, better techniques for degradation than systems in the absence of TiO2. The kinetic behavior was not unique and two parameters were taken to evaluate the efficiency of each system: initial rates (R0) and time to 95% of total mineralization (TOC95). Addition of hydrogen peroxide improves the initial HP reaction rate and reduces TOC95. Addition of Fe3+ also affects the reaction parameters but the effect of H2O2 seems to be higher, at least under the present conditions. When both H2O2 and iron were added simultaneously, the efficiency was higher. The optimal H2O2:Ox:Fe molar ratio was established and the results indicated that, at a fixed iron concentration, H2O2 increased R0 until a limit beyond which it did not cause any effect. No intermediates were formed in the reaction, oxalate being degraded directly to CO2. Analogies and differences with the EDTA system are presented.