Transformation of metoprolol in UV/PDS process: Role and mechanisms of degradation and polymerization

Abstract

Advanced oxidation processes for the treatment of organic pollutants in wastewater suffer from difficulties in mineralization, potential risks of dissolved residues, and high oxidant consumption. In this study, radical-initiated polymerization is dominated in an UV/peroxydisulfate (PDS) process to eliminate organic pollutant of pharmaceutical metoprolol (MTP). Compared with an ideal degradation-based UV/PDS process, the present process can save four fifths of PDS consumption at the same dissolved organic carbon removal of 47.3%. Simultaneously, organic carbon can be recovered from aqueous solution by separating solid polymers at a ratio of 50% of the initial chemical oxygen demand. The chemical structure of products was analyzed to infer the transformation pathways of MTP. Unlike previous studies on simple organic pollutants that the polymerization can occur independently, the polymerization of MTP is dependent on the partial degradation of MTP, and the main monomer in polymerization is a dominant degradation product (4-(2-methoxyethyl)-phenol, denoted as DP151). The separated solid polymers are formed by repeated oxidation and coupling of DP151 or its derivatives through a series of intermediate oligomers. This proof-of-concept study demonstrates the advantage of polymerization-dominated mechanism on dealing with large organic molecules with complex structures, as well as the potential of UV/PDS process for simultaneous organic pollution reduction and organic carbon recovery from aqueous solution. Environmental implicationEmerging organic pollutants in aquatic environment pose potential hazards. Degradation processes that decompose organic pollutants into small molecules suffer from mineralization difficulties, potential risks by dissolved organic residues, and high oxidant consumption, especially for large molecules with complex structures. This study demonstrates (i) pharmaceuticals such as metoprolol can be simultaneously eliminated and recovered as organic carbon via radical-initiated polymerization with much lower oxidant consumption than degradation, (ii) UV/PDS process has the potential on simultaneous organic pollution reduction and organic carbon recovery. The detailed analysis reveals that specific degradation intermediates of large organic molecules can act as polymerization monomers.