Insight into photocatalyzed transformations of multiclass organic contaminants in water

Abstract

This study investigates three understudied emerging contaminants: pharmaceutical ramipril (RAM), artificial sweetener neotame (NEO), and herbicide cycloxydim (CYC). Laboratory simulations of natural photoinduced transformations were conducted exploring direct photolysis and TiO₂ photocatalysed process, followed by analysis with tandem high-resolution mass spectrometry (HRMS). The contaminants exhibited low-to-medium degree of photolysis, but rapid dissipation under photocatalysis. The assessment of transformation products (TPs) revealed several analytical challenges, particularly in peak detection and data management. The structural elucidation of the abundant TPs was achieved through multi-stage MS fragmentation studies and the transformation pathways were proposed based on the identified structures, evolution profiles, and polarities. The hydroxylation was the most common transformation for all contaminants, although each showed unique additional pathways. For instance, RAM underwent intermolecular cyclization, forming a diketopiperazine-like TP, as well as ketone formation and cleavage into low molecular mass TPs. NEO experienced ester hydrolysis, reduction, resulting in C=C bond formation, and oxidative aromatic ring-opening reactions. Lastly, CYC displayed (poly)hydroxylation or transformations of its oxime ether moiety, which underwent hydrolysis, detachment, or rearrangement, leading to oxazole TPs. The ECOSAR software was used for the prediction of toxicities of the identified TPs. In-silico toxicity prediction showed that the hydroxylated phenolic TPs of NEO and RAM, as well as certain oxazole TP isomers of CYC, have the potential to increase overall toxicity during the degradation. Overall, this paper addresses the analytical challenges and innovative methodologies for studying TPs, highlighting their growing relevance in environmental analytical chemistry.