The complexation between transition metals and water-soluble organic compounds (WSOC) and its effect on reactive oxygen species (ROS) formation

Both Transition Metals (TMs) and Water–Soluble Organic Compounds (WSOC) in atmospheric particles can make a certain contribution to Reactive Oxygen Species (ROS), but the complexation between them and its subsequent effects on ROS formation are still unclear. Three typical TMs (Fe(II), Cu(II), Mn(II)) and two types of WSOCs (SRHA (Suwannee River Humic Acid) and SRFA (Suwannee River Fulvic Acid)) were chosen to study their complexation process and its effects on generation of ROS. The excitation-emission matrix (EEM) was used to obtain the three-dimensional fluorescence spectra of WSOC during complexation process, and the fluorescence regional integration method (FRI) and parallel factor analysis (PARAFAC) were used to quantitatively characterize the changes of different compositions of WSOC during complexation process. The results showed that the complexation between WSOC and Cu or Mn exhibit synergistic effects on ROS generation, while WSOC and Fe has a negative effect even though ROS produced by Fe is the highest among the three TMs. The hydrophobic fraction of WSOC and ROS generation rate decreased during the complexation process, despite ROS concentration increase. PARAFAC analysis results showed that the aromaticity of WSOC is greatly reduced by complexing with Cu, followed by Fe, and the least with Mn. Cu–WSOC exhibited the most significantly synergistic effect on ROS generation, suggesting aromatic and polar substituents (such as phenol and quinone groups) in WSOC dominated the complexation process. All of three TMs tend to form stable complexes with the polar groups in WSOC. Cu might be more capable of combining with the benzene ring by π-π electron transfer than Fe and Mn. The aromatic groups (π-π electron transfer) in WSOC could significantly enhance ROS generation, but polar groups not.