Binding characters of biomass burning smoke-derived dissolved organic matter with Cu(II) in aqueous environment: Roles of functional groups and organic components.

The environmental effect of biomass burning smoke-derived dissolved organic matter (BBS-DOM) has attracted growing attention due to the increasing wildfire globally. BBS-DOM eventually deposits on the water and soil environments, thus altering the environmental behaviors of pollutants (e.g., heavy metals) in the surface environments of the wildfire region. However, presently, the binding characters between heavy metals and BBS-DOM remains unknown. In this study, alfalfa, pinewood, and corn straw were burned at 300 °C and 600 °C to produce BBS-DOMs and their binding characters with Cu(II) were investigated using fluorescence excitation-emission matrix spectra coupled with parallel factor (EEM-PARAFAC), synchronous fluorescence spectra combined with two-dimensional correlation spectroscopy (2D-SFS-COS) and FTIR combined with two-dimensional correlation spectroscopy (2D-FTIR-COS). The fluorescence quenching/enhancing results after Cu(II) addition suggested that the binding capacities with Cu(II) of various organic components in BBS-DOMs followed an order of polyphenols-like matters (Ex/Em: 220 nm/310 nm) > aromatic protein-like matters (Ex/Em: 275 nm/310 nm) ≈ small humic-like matters (Ex/Em: 300 nm/380 nm) > large humic-like matters (Ex/Em: 330 nm/410 nm). Interestingly, the quenching effect of Cu(II) addition on the fluorescence intensities of polyphenols-like matters and humic-like matters decreased with their increasing abundances, which possibly depended on the proportion of organic ligands of these components. Furthermore, 2D-FTIR-COS demonstrated that the binding sequence of different functional groups followed deprotonated -COOH→deprotonated phenol-OH→-C]O of aldehydes, ketones, and lactones/aromatic rings/-NH→C-O-C/C-OH of ethers and alcohols. Another novelty was that Cu(II) binding could increase the molecular size and humification of BBS-DOMs, due to the bridge effect of Cu(II). This work provides an importantly theoretical basis for deeply understanding the mechanism of BBS-DOM binding with Cu(II) at the molecular level, which is a key for reasonably predicting the multimedia-crossing effects of BBS-DOM and the environmental behavior of heavy metals in the wildfire region.