Mechanisms Governing the Oxidation of Leachate Organic Matter from Tire Wear Particles Coupled with Reductive Dissolution of Manganese (Hydr)oxides

Transformation pathways of leachate organic matter from tire wear particles (TWPs) in the environment remain elusive. Herein, we studied the oxidation of a TWP leachate (85 mg C/L) by three common manganese (Mn) (hydr)oxides (β-MnO₂, δ-MnO₂, and γ-MnO₂) (500 mg/L) in aqueous suspensions at varying pH (3, 4, and 5). The released amount of Mn²⁺ positively correlated with the TOC loss of leachate, indicating the coupled oxidation of leachate with reductive dissolution of Mn (hydr)oxides. The strongest reduction shown for β-MnO₂ at pH 3 was accounted for by the intensified leachate adsorption assisted by acidic dissolution. This was further verified by electrochemical quartz crystal microbalance (EQCM) analysis to in situ monitor changes of mass and redox potential at the surface of electrodeposited Mn (hydr)oxide by comparison with inert polystyrene nanoparticles. Fluorescence analysis showed that new quinone compounds were formed after reacting with β-MnO₂, whereas some quinone compounds were mineralized or structurally altered after reacting with δ-MnO₂. Compared with pristine TWP leachate, only the leachate reacted with δ-MnO₂ at pH 3 exhibited enhanced luminescent bacterial toxicity (by 96.2%) attributable to oxidized quinone compounds and lower-molecular-weight compounds produced from oxidation desulfurization, as revealed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis.