Kinetic modelling of aromaticity and water colour changes during diclofenac oxidation by UV/H2O2

Abstract Diclofenac (DCF) is degraded by UV/H₂O₂ following first-order kinetics to highly aromatic species, while generating an intense yellow tint in the water. During the oxidation, turbidity remains under 1 NTU. The formation of aromatic intermediates has been modelled considering a first-order series reaction system, where DCF degrades to aromatic species (anilines and phenols), according to a kinetic constant kDCF,₂₅₄=0.3 (1/min), until the maximum concentration of aromatic compounds \({[{A}₂₅₄]}ₘₐₓ=0.2116 {[{H}₂{O}₂]}₀0.6 – 0.9327 {[{H}₂{O}₂]}₀0.3+ 3.3602\). The aromaticity then decreases, following kArom (1/min), as carboxylic acids (acetic, formic) and inorganic ions (CO₂, NO₃⁻, Cl⁻) are created. The evolution of colour presents the kinetics of a reaction intermediate following first-order kinetics, where DCF degrades to coloured intermediates (quinone-like compounds), in agreement with kDCF,455=0.2 (1/min), until reaching a maximum intensity \({[Colour]}ₘₐₓ=0.0043 {[{H}₂{O}₂]}₀^{} - 0.0538 {[{H}₂{O}₂]}₀0.5+ 0.2758. \) After, tint diminishes following a kinetic constant kColour (1/min). Conducting with ratios over 1 mol DCF : 6 mol H₂O₂, waters with colourless appearance, low aromaticity and DCF degradation output >95% are attained. Both aromaticity and colour elimination rates increase with the oxidant concentration, reaching their maximum kArom=0.08 (1/min) and kColour=0.14 (1/min) when using 1 mol DCF : 150 mol H₂O₂. When performing with higher molar ratios, the kinetic constants slow down, due to the self-decomposition of H₂O₂ that acts as a HO• scavenger agent.