Degradation of pharmaceutical memantine by photo-based advanced oxidation processes: Kinetics, pathways and environmental aspects

Memantine (MEM) is an important and widely used pharmaceutical for the treatment of Alzheimer's disease. However, due to MEM structural features, it is considered fairly persistent and bioaccumulative chemical and is therefore potentially harmful to the environment. Here, the degradation of MEM by UV-C/H2O2 and UV-A/TiO2 processes under optimized conditions was investigated. Complete degradation of MEM (0.1 mM) was achieved in < 3.5 min by UV-C/H2O2 process (pH 5.1, [H2O2]=15.21 mM) and in ∼90 min by UV-A/TiO2 process (pH 4, γ(TiO2)= 0.955 gL-1). Under the same conditions, 90% mineralization was achieved in ∼90 and ∼330 min, respectively. A second-order rate constant for the reaction of MEM with hydroxyl radical (HO·) (kHO•/MEM) was determined based on the competitive kinetic experiments, amounting 6.16 × 109 M-1s-1. The relative contribution of reactive oxygen species in the photocatalytic treatment was determined by selective scavenging, assigning a major contribution to HO·, while superoxide radicals played a minor role in the degradation of MEM. A detailed survey of the intermediates of MEM degradation was performed using accurate mass spectrometry, focusing on the correlation of 25 identified intermediates with changes in biodegradability and toxicity to Daphnia magna. It was found that MEM and its degradation intermediates are non-biodegradable and relatively toxic, requiring extensive mineralization, even above 50%, to improve these ecotoxicity parameters.