Photocatalytic mineralization of codeine by UV-A/TiO₂—Kinetics, intermediates, and pathways

Highlights: • Codeine was completely removed in 30 min under irradiated UV-A/TiO₂. • Codeine was mineralized in 90 min with near complete N conversion to NH₄⁺ and NO₃⁻. • Morphine was authenticated and four others were identified as intermediates. • Degradation proceeded via ipso-substitution and hydroxylation of the aromatic ring. - Abstract: This study investigated the photocatalytic degradation of codeine by UV-irradiated TiO₂. The degradation kinetics was determined under varied conditions including the TiO₂ loading, codeine concentration, and pH. Codeine and several reaction intermediates including morphine were identified and tracked during degradation using HPLC/MS–MS technique, along with TOC and IC measurements. Specifically, removal of 100 μg/L of spike codeine was complete in 3 min by contact with a 0.1 g/L suspension of TiO₂ under UV irradiation at pH 7. The degradation kinetics of codeine was first-order with respect to both the catalyst TiO₂ and the reactant codeine, with enhanced reaction rates with increasing pH up to pH 9. Mineralization of codeine was possible upon prolonged contact; near complete mineralization of 10 mg/L of codeine was achieved in 90 min with 0.1 g/L TiO₂ under irradiation at pH 5, during which the organic nitrogen was converted to NH₃-N (74%) and NO₃-N (22%). Based on the identified intermediates, two degradation pathways were proposed of which one involved ipso-substitution followed by cleavage of the aromatic ring and another involved repeated hydroxylation of the codeine molecule followed by its fragmentation.