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

Xiaobo Wang; Yuanyuan Qin; Juanjuan Qin; Xinxin Long; Ting Qi; Rongzhi Chen; Kang Xiao; Jihua Tan

doi:10.1016/j.scitotenv.2021.145261

Spectroscopic insight into the pH-dependent interactions between atmospheric heavy metals (Cu and Zn) and water-soluble organic compounds in PM_2.5

Sci Total Environ. 2021 May 1:767:145261. doi: 10.1016/j.scitotenv.2021.145261. Epub 2021 Jan 20.

Authors

Xiaobo Wang¹, Yuanyuan Qin¹, Juanjuan Qin¹, Xinxin Long¹, Ting Qi², Rongzhi Chen¹, Kang Xiao¹, Jihua Tan³

Affiliations

¹ College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, 100049, PR China.
² School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, PR China.
³ College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, 100049, PR China. Electronic address: tanjh@ucas.ac.cn.

PMID: 33550065
DOI: 10.1016/j.scitotenv.2021.145261

Abstract

Taking Cu and Zn as examples, the pH-dependent interactions between atmospheric heavy metals (AHMs) and water-soluble organic compounds (WSOCs) in PM_2.5 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 μmol·L^-1), but had a relatively slight change with the stepwise addition of [Zn²⁺] (from 0 to 500 μmol·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 acid-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 (Q_ExL/H) 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 (HIX_em) declined more than 15% as adding [Cu²⁺] at pH 5.6 and 7.5, 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.

Keywords: Atmospheric particulate matter; Environment health risks; Toxic elements; Water-soluble organic matter; pH-dependent interaction characteristics.