Water Research 37 (2003) 2496–2504 Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters Jing Chen a , Simo O. Pehkonen a, *, Che-Jen Lin b a Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Department of Civil Engineering, Lamar University, Beaumont, TX 77710-0024, USA Received 27 August 2002; received in revised form 25 November 2002; accepted 15 January 2003 Abstract The degradation of methylmercury chloride by hydroxyl radicals ( d OH) has been investigated using nitrate photolysis from 285 to 800nm with a 450 W Xenon lamp as the d OH source. The identified products are Hg 2+ , Hg 0 , CHCl 3 and formaldehyde. The second-order rate constant at pH of 5 at room temperature was determined to be 9.83(+0.66)  10 9 M 1 s 1 using benzoic acid as the d OH scavenger. The effects of chloride concentration and methylmercury speciation have also been investigated. A mechanism of the CH 3 HgCl– d OH reaction has been proposed. The calculated methylmercury degradation rates in natural waters using the above rate constant were comparable to the in situ photodegradation rates reported previously, indicating that degradation by  OH may be one of the important pathways of methylmercury degradation in sunlit surface waters. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: OH radical; Natural waters; Methylmercury; Formaldehyde; Degradation; Mechanisms 1. Introduction Mercury is a global pollutant that can be released from a variety of anthropogenic and natural sources. In the past, mercury pollution was mainly caused by gold and silver mining. Presently, mercury is emitted into the environment from numerous industrial processes, in- cluding power generation, chemical production and waste combustion. Most of these point sources release mercury into the atmosphere. The dynamic redox chemistry of mercury in the atmosphere [1,2], followed by dry and wet deposition of atmospheric mercury leads to widespread contamination of water and soil. The chemical and biological activities in soils and water can convert the deposited mercury to gaseous forms and release the toxic metal into the atmosphere, acting as so- called ‘‘natural’’ mercury emissions from soils, lakes, wetlands and open oceans [3]. This global cycling of mercury causes the contamination of biosphere, even in remote areas such as the Arctic [4] and has drawn a great public health concern. The major health concern of mercury pollution is the presence of methylmercury in marine and freshwater fish. Methylmercury is a neurotoxin and can be formed through both biotic [5,6] and abiotic [7] methylation of inorganic mercury in natural waters. It is the most toxic form of mercury and highly biologically accumulated. Elevated mercury levels in fish, frequently exceeding World Health Organization (WHO) consumption guide- lines (0.5 mgg 1 ), have been found in lakes remote from direct mercury sources [8,9]. Consumption of fish contaminated by methylmercury poses a risk of direct mercury poisoning [10]. Several studies have suggested photodegradation of methylmercury as a potential sink in lake waters [11,12]. Inoko [13] reported that methylmercury can be degraded *Corresponding author. Tel.: +65-6874-4729; fax: 65-6779- 1936. E-mail address: chesop@nus.edu.sg (S.O. Pehkonen). 0043-1354/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00039-3