Ozonolysis of phenolic aldehydes from industrial-based lignin wastewaters

Phenolic aldehydes are widespread pollutants in water and soil, originating from lignin-based agro-industries. With increasing wastewater pollution, improved treatment systems are necessary to degrade phenolic aldehydes into less hazardous compounds. Over the past two decades, ozonolysis wastewater treatment has been implemented in the United States, Japan, and South Korea. However, the mechanistic understanding of phenolic aldehyde ozonolysis in water remains incomplete. This study investigates the ozonolysis of three model phenolic aldehydes (syringaldehyde, vanillin, 4-hydroxybenzaldehyde) in representative concentrations for wastewater of 0.5-1.5 mM and pH 4-8. Each compound solution was sparged for 30 min at a fixed O₃(g) flow (0.20 to 1.00 L min^-1), providing steady-state dissolved concentrations of 5.4 to 16.2 μM. Reactant loss and product generation were monitored using UV-visible (UV-vis) spectroscopy, ultra-high pressure liquid chromatography (UHPLC) with UV-vis and mass spectrometry (MS) detection, and ion chromatography with conductivity and MS detection of anions. Identified products based on their mass-to-charge ratio (m/z^-) included oxalic acid (89), glycolic acid (75), formic acid (45), and maleic acid (115). Additional intermediate products were identified under various reaction conditions, revealing competing mechanisms in the degradative oxidation of aqueous phenolic aldehydes exposed to O₃(g). A unifying mechanism is proposed to explain the production of smaller, less toxic molecules during phenolic aldehyde ozonolysis, enhancing water quality. This mechanism serves as a basis for evaluating the implementation of ozonolysis in scaled-up water treatment processes.