Measurement Report: Effects of transition metal ions on the optical properties of humic-like substances revealing structural preference.

Humic-like substances (HULIS) are complex macromolecules in water-soluble organic compounds containing multiple functional groups, and transition metal ions (TMs) are ubiquitous in atmospheric particles. In this study, potential physical and chemical interactions between HULIS and four TM species including Cu²⁺, Mn²⁺, Ni²⁺, and Zn²⁺ were analyzed by optical method under acidic, weakly acidic and neutral conditions. The results showed that Cu²⁺, Mn²⁺, and<superscript />Zn²⁺ only slightly enhanced mass absorption efficiency (MAE₃₆₅) of HULIS in winter, and had indiscernible effects on absorption Ångström exponent (AAE) of HULIS in both seasons under all acidity conditions. All four TMs had fluorescence quenching effects on winter HULIS, and only Cu²⁺ had similar effects on summer HULIS, with the highest quenching coefficients found under weakly acidic condition in both seasons. The ¹H-nuclear magnetic resonance (¹H-NMR) and Fourier-transform infrared (FT-IR) spectra revealed that Cu²⁺ mainly bound with aromatic species and tightened the molecule structures of HULIS. The parallel factor analysis (PARAFAC) results extracted four components of HULIS, including low-oxidized humic-like substances (C1), N-containing compounds (C2), highly-oxidized humic-like substances (C3), and the mixing residentials (C4), from the fluorescence spectra in both winter and summer. The divergent variations of HULIS spectral components with Cu²⁺ additions under three acidity conditions indicated that electron-donating groups of HULIS mainly corresponded to C1 and C3, with Cu²⁺ binding with HULIS by replacing proton, while electron-withdrawing groups of HULIS could correspond to C2, with its connection with Cu²⁺ through electrostatic adsorption or colliding induced energy transfer. [ABSTRACT FROM AUTHOR]