Phototransformation of Lignin-related Compounds in Chromophoric Dissolved Organic Matter Solutions

Lignin is a major terrestrial source of chromophoric dissolved organic matter (CDOM), and studying the phototransformation of lignin monomers and their related compounds can enhance our understanding of CDOM intramolecular interactions. Coniferyl aldehyde (Coni) and sinapaldehyde (Sina) form ground-state complexes with CDOM, with equilibrium constants of 7,800 (± 1,800) and 20,000 (± 2,000) M^-1, respectively. In comparison, vanillin (Van) exhibits minimal affinity for CDOM complexation. The bimolecular reaction rate constants between singlet oxygen (¹O₂) and these phenolic carbonyl compounds ranged from 0.46 (± 0.02) to 1.8 (± 0.1) × 10⁷ M^-1s^-1, which is approximately one order of magnitude lower than their reaction rate constants (0.51 (± 0.02)-1.25 (± 0.02) × 10⁸ M^-1s^-1) with the triplet excited state of CDOM (³CDOM*). In acidic CDOM solutions (pH 5.0), ¹O₂, H₂O₂, and organic peroxyl radicals had negligible impact on the transformation. Comparing the initial transformation rate in the presence and in the absence of NaN₃ or furfuryl alcohol led to an overestimation of the contribution of ¹O₂ to the transformation of Van, Coni, or Sina. ³CDOM* scavengers could not fully inhibit the transformation of Coni or Sina. The remaining transformation is considered to arise from either the unquenched intra-CDOM phase ³CDOM* or a fraction of Coni⊂CDOM or Sina⊂CDOM complex, which underwent intramolecular photoinduced chemical reactions.