Assessing the effect of inorganic anions on TiO2-photocatalysis and ozone oxidation treatment efficiencies

Abstract

Considering the application of AOPs might be limited for the treatment of industrial wastewater with high inorganic load, and that partial results reported to date regarding this particular are inconclusive, even opposite in some cases, the effect of inorganic anions on the oxidation efficiency of photocatalysis and ozonation has been further assessed with statistical significance. While the presence of sulphate and chloride did not appreciably affect the photocatalytic oxidation of phenol, nitrate significantly enhanced the removal of COD (≈8-15%). The addition of carbonate simply increased the pH, which strongly inhibited the photocatalytic process; whereas if pH=5 was kept constant, the reduction of the COD was not affected by the presence of carbonate. On the other hand, sulphate, chloride and nitrate did not significantly affect the degradation of phenol by ozonation; whereas the presence of carbonate apparently enhanced the reduction of COD. It is actually proved that this improvement in the efficiency of the treatment was produced by the pH buffering effect of these ions, rather than to its presence itself, which actually significantly reduced the removal of COD (5-10%) by radical scavenging action in comparison to when the treatment was performed in the absence of anions in the solution adjusting the pH to similar basic values (≈9.5-13.5). When ozonation was performed at a pH close to neutral (6.5 ± 0.2) or basic (12 ± 0.2), at which the indirect oxidation of hydroxyl radical is surely widely active, the results were significantly enhanced in any case (COD removal ≈70-75%), whether in the absence or the presence of these anions; despite the significant slight radical scavenging effect (COD removal ≈65-70%) that was attributed to the addition of carbonate.