Photooxidative Degradation of Aromatic Carboxylic Acids in Water: Influence of Hydroxyl Substituents

Abstract

The UV/H2O2 process was applied for the degradation of aromatic carboxylic acids (ACAs). Benzoic acid, salicylic acid, gentisic acid, and gallic acid are chosen as typical water pollutants representing ACAs with the increased number of hydroxyl groups. The effect of the structural characteristics of ACAs and UV/H2O2 process parameters on their degradation kinetics was investigated by a statistical/empirical approach employing the design of experiments and response surface methodology. Different optimal conditions for maximal degradation rates were established, while degradation rates followed the decreasing order benzoic acid > salicylic acid > gentisic acid > gallic acid with the increasing number of hydroxyl groups. The inversed order was established in the case of mineralization kinetics. Structurally influenced degradation pathways of studied ACAs, with the shared sequence related to the preferable hydroxylation position at the benzene ring, are reflected in the observed changes in the biodegradability and toxicity toward Vibrio fischeri during UV/H2O2 treatment.