Photodegradation of methylmercury in Jialing River of Chongqing, China

Photodegradation（PD） of methylmercury（MMHg） is a key process of mercury（Hg） cycling i water systems, maintaining MMHg at a low level in water systems. However, we posses little knowledge of this important process in the Jialing River of Chongqing, China. In sit incubation experiments were thus performed to measure temporal patterns and influencin factors of MMHg PD in this river. The results showed that MMHg underwent a ne demethylation process under solar radiation in the water column, which predominantl occurred in surface waters. For surface water, the highest PD rate constants were observed i spring(12 × 10-3± 1.5 × 10-3m2/E), followed by summer(9.0 × 10-3± 1.2 × 10-3m2/E), autum(1.4 × 10-3± 0.12 × 10-3m2/E), and winter(0.78 × 10-3± 0.11 × 10-3m2/E). UV-A radiatio（320–400 nm）, UV-B radiation（280–320 nm）, and photosynthetically active radiation（PAR400–700 nm） accounted for 43%–64%, 14%–31%, and 16%–45% of MMHg PD, respectively. PD rat constants varied substantially with the treatments that filtered the river water and amended with chemicals（i.e., Cl-, NO3-, dissolved organic matter（DOM）, Fe（III））, which reveals tha suspended particulate matter and water components are important factors in affecting the PD process. For the entire water column, the PD rate constant determined for each wavelengt range decreased rapidly with water depth. UV-A, UV-B, and PAR contributed 27%–46%, 6.2%12%, and 42%–65% to the PD process, respectively. PD flux was estimated to be 4.7 μg/(m2·yea in the study site. Our results are very important to understand the cycling characteristics o MMHg in the Jialing River of Chongqing, China.