Tolfenamic acid degradation by direct photolysis and the UV-ABC/H2O2 process: factorial design, kinetics, identification of intermediates, and toxicity evaluation

Abstract

This study employed direct UV-ABC photolysis and the UV-ABC/H₂O₂ process to investigate the degradation of tolfenamic acid (TA), a common anti-inflammatory drug used in both human and veterinary medicine. A 2³ factorial design with added center point was used to evaluate the effect of three independent variables-namely, H₂O₂ concentration ([H₂O₂]), TA concentration ([TA]), and experiment time (time)-on TA degradation and H₂O₂ photolysis during UV-ABC/H₂O₂ treatment using a high-pressure mercury vapor lamp (photon flux of 2.6307 × 10⁴ J s^-1) as the UV irradiation source. The responses yielded similar values, revealing a linear behavior, with correlation coefficients R = 0.9968 and R_adj = 0.9921 for TA degradation and R = 0.9828 and R_adj = 0.9570 for H₂O₂ photolysis. The most efficient combination of variables was [H₂O₂] = 255 mg L^-1 and [TA] = 25 mg L^-1, resulting in 100% TA degradation and 98.87% H₂O₂ photolysis by 90 min of treatment. Additionally, the second-order kinetic constant of the reaction between TA and HO^● was determined using a competitive kinetic model, employing 2,4-dichlorophenoxyacetic acid (2,4D) as the reference compound. The kinetic constant was 1.9 × 10¹⁰ M^-1 s^-1 in alkaline medium. TA degradation by direct photolysis generated quinone imines as by-products, responsible for the formation of a dark red "internal filter" that increased the value of acute toxicity to Artemia salina. The UV-ABC/H₂O₂ process did not promote formation of quinone imines by 90 min of treatment and therefore did not increase acute toxicity values. Several by-products generated during TA degradation were identified and possible degradation pathways for the UV-ABC and UV-ABC/H₂O₂ processes were proposed.

Keywords: UV-ABC/H(2)O(2) process; acute toxicity; degradation pathways; direct photolysis; second-order kinetic constant; tolfenamic acid.