Methylmercury Photodegradation in Surface Water of the Florida Everglades: Importance of Dissolved Organic Matter-Methylmercury Complexation Chao Tai, †,§,∥,∇ Yanbin Li, ‡,§,∥,∇ Yongguang Yin, ⊥,§,∥ Leonard J. Scinto, #,∥ Guibin Jiang, ⊥ and Yong Cai §,∥, * † Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo, 454000, China ‡ Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao, 266100, China § Department of Chemistry & Biochemistry, Florida International University, Miami, Florida 33199, United States ∥ Southeast Environmental Research Center, Florida International University, Miami, Florida 33199, United States ⊥ State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China # Department of Earth and Environment, Florida International University, Miami, Florida 33199, United States * S Supporting Information ABSTRACT: Photodegradation is the major pathway of methyl- mercury (MeHg) degradation in many surface waters. However, the mechanism of MeHg photodegradation is still not completely understood. Dissolved organic matter (DOM) is expected to play a critical role in MeHg photodegradation. By using several techniques, including N 2 /O 2 purging and the addition of stable isotope (Me 201 Hg), scavengers, competing ligands, and a singlet oxygen ( 1 O 2 ) generator, the role played by MeHg−DOM complexation in MeHg photodegradation of Everglades surface water was investigated. DOM appeared to be involved in MeHg photodegradation via the formation MeHg−DOM complexes based on three findings: (1) MeHg was quickly photodegraded in solutions containing DOM extracts; (2) degradation of MeHg did not occur in deionized water; and (3) addition of competing complexation reagents (dithiothreitol-DTT) dramatically prohibited the photodegradation of MeHg in Everglades water. Further experiments indicated that free radicals/reactive oxygen species, including hydroxyl radical (·OH), 1 O 2 , triplet excited state of DOM ( 3 DOM*), and hydrated electron (e − aq ), played a minor role in MeHg photodegradation in Everglades water, based on the results of scavenger addition, 1 O 2 generator addition and N 2 /O 2 purging. A pathway, involving direct photodegradation of MeHg−DOM complexes via intramolecular electron transfer, is proposed as the dominant mechanism for MeHg photodegradation in Everglades water. ■ INTRODUCTION Methylmercury (MeHg) is the most notorious form of mercury in the environment, and has drawn public concern due to its toxicity, bioaccumulation, and biomagnification through food webs. 1−3 In aquatic environments, MeHg is primarily formed by sulfate reducing bacteria 4 or iron reducing bacteria 5 in anoxic sediment, and sometimes by photomethylation in surface water. 6 In addition to methylation, degradation of MeHg also plays an important role in the biogeochemical cycling of mercury in aquatic ecosystems. This process may occur through a number of abiotic or biotic pathways. 7−9 Although microbial demethylation of MeHg was observed both in sediment 10 and in the water column, 11 most previous studies demonstrated that photodegradation is the major pathway of MeHg degradation in surface water. 12−14 It was reported that photodegradation could remove a large proportion of the MeHg loading into lake water. 15,16 Since its importance was demonstrated, 14 several researchers have attempted to mechanistically determine the MeHg photodegradation process in aquatic environments. 17−20 Although wavelength-specific photodegradation of MeHg has been observed in aquatic environments, 13,21 much remains unclear about the chemical processes governing MeHg photodegradation. Previous reports suggest three potential pathways are responsible for MeHg photodegradation: (1) Received: January 21, 2014 Revised: April 24, 2014 Accepted: June 5, 2014 Published: June 5, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 7333 dx.doi.org/10.1021/es500316d | Environ. Sci. Technol. 2014, 48, 7333−7340