Probing Methylmercury Photodegradation by Different Fractions of Natural Organic Matter in Water: Degradation Kinetics and Mercury Isotope Fractionation Characteristics

Lian Zhang; Qingliang Dai; Huaqing Liu; Yanbin Li; Yongguang Yin; Guangliang Liu; Peng Dai; Xiaoqiang Cao; Jian Zhang; Yong Cai

doi:10.1016/j.envpol.2024.125563

Probing Methylmercury Photodegradation by Different Fractions of Natural Organic Matter in Water: Degradation Kinetics and Mercury Isotope Fractionation Characteristics

Environ Pollut. 2024 Dec 19:125563. doi: 10.1016/j.envpol.2024.125563. Online ahead of print.

Authors

Lian Zhang¹, Qingliang Dai², Huaqing Liu², Yanbin Li³, Yongguang Yin⁴, Guangliang Liu⁵, Peng Dai⁶, Xiaoqiang Cao², Jian Zhang², Yong Cai⁷

Affiliations

¹ College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China; Shandong Key Laboratory of Environmental Processes and Health, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China.
² College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China.
³ Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China.
⁴ Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
⁵ Department of Chemistry & Biochemistry, Florida International University, Miami, Florida, 33199, United States.
⁶ Department of Environmental Engineering and Earth Sciences, Clemson University, South Carolina, 29631, United States.
⁷ Shandong Key Laboratory of Environmental Processes and Health, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China; Department of Chemistry & Biochemistry, Florida International University, Miami, Florida, 33199, United States.

PMID: 39709054
DOI: 10.1016/j.envpol.2024.125563

Abstract

Recent advancements in mercury (Hg) isotopic fractionation research have evolved from conceptual demonstrations to practical applications. However, few studies have focused on revealing fractionation fingerprinting for aqueous methylmercury (MeHg) photodegradation due to its sensitivity to natural organic matter (NOM). Here, the impact of NOM fractions with varying chemical properties on MeHg photodegradation kinetics and Hg isotope fractionation characteristics was investigated. Findings reveal that reduced NOM, containing alcohol/phenol groups, slows the degradation rate compared to the oxidized. Low-molecular-weight NOM, rich in thiol groups, enhances the degradation rate more effectively than high-molecular-weight counterparts. Hydrophilic/hydrophobic-acidic/basic NOM also significantly influence the rate constant, with the highest for hydrophilic-acidic NOM. Isotopic analysis showed that NOM's redox properties affect the extent and direction of Hg isotope fractionation. NOM with various molecular weights controls mass-dependent and mass-independent fractionation by regulating MeHg-NOM triplet radical pairs reactions, likely due to differences in functional groups. Similar effects were observed for different hydrophilic/hydrophobic-acidic/basic fractions. Further experiments with scavenger addition indicated that direct photodegradation of MeHg-NOM is a possible degradation mechanism, with free radicals/reactive oxygen species playing a minor role. These findings underscore the sensitivity of both the degradation rates and Hg isotope fingerprinting to different NOM fractions.

Keywords: Methylmercury; mercury isotope fractionation; natural organic matter; photodegradation.