Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters

The degradation of methylmercury chloride by hydroxyl radicals (OH) has been investigated using nitrate photolysis from 285 to 800 nm with a 450 W Xenon lamp as the OH source. The identified products are Hg2+, Hg0, CHCl3 and formaldehyde. The second-order rate constant at pH of 5 at room temperature was determined to be 9.83(+−0.66)×109 M−1 s−1using benzoic acid as the OH scavenger. The effects of chloride concentration and methylmercury speciation have also been investigated. A mechanism of the CH3HgCl–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.