Spectroscopic insight into the pH-dependent interactions between atmospheric heavy metals (Cu and Zn) and water-soluble organic compounds in PM2.5

Taking Cu and Zn as examples, the pH-dependent interactions between atmospheric heavy metals (AHMs) and water-soluble organic compounds (WSOCs) in PM2.5 were analyzed by a combination of UV-vis absorption, Fourier transform infrared (FTIR) spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). We found metal-H ion exchange, complexation and electrostatic adsorption might occur between AHMs and WSOCs, and were generally enhanced with the increase of pH. Furthermore, these interactions were strengthened with the stepwise addition of [Cu2+] (from 0 to 500 mu mol.L-1), but had a relatively slight change with the stepwise addition of [Zn2+] (from 0 to 500 mu mol.L-1) generally. This indicated that the above interactions depended on the types and the concentrations of AHMs. Carboxyl, hydroxyl, carbonyl and aromatic structures of WSOCs were the major binding sites with AHMs. Humic add-like substances were the dominant components of WSOCs binding with AHMs. The ratios of the apparent fluorescence quantum yields of the low and the high conjugation fractions of WSOCs (QE,) declined by more than 28% as adding [Cu2+], indicating the formers had more strong complexing capacity with AHMs. AHMs might significantly impact the light absorption capacity and the wavelength dependence of WSOCs. The humification index (HIXem) declined more than 15% as adding [Cu2+] at pH 5.6 and 75, indicating AHMs might weaken the oxidation capacity of WSOCs. These results implied the interactions between AHMs and WSOCs might play a profound role in atmospheric environment, human health, and global climate change. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The interactions between Cu2+ and Zn2+ with WSOCs in PM2.5 were found to be pH-dependent, with metal-H ion exchange, complexation, and electrostatic adsorption. The major binding sites of WSOCs with AHMs were identified as carboxyl, hydroxyl, carbonyl, and aromatic structures, with humic acid-like substances dominating the components. Additionally, Cu2+ significantly impacted the fluorescence quantum yields and humification index of WSOCs, indicating a potential impact on atmospheric environment, human health, and global climate change.