汞的稳定同位素分馏机理

The Mechanism of Mercury Stable Isotope Fractionation:A Review

汞是唯一能够以气态单质形式进行长距离传输的有毒重金属元素,其环境行为和健康危害受到广泛关注。近十多年来发展起来的汞稳定同位素技术为研究环境中汞的来源、迁移转化过程以及相应的生态环境效应提供了新的视野。汞同位素是自然界中唯一表现出多种显著非质量分馏(MIF)的独特金属同位素体系,对汞同位素的研究一直偏重应用,而对其分馏机理的认识十分有限。本文从汞稳定同位素分馏理论、分馏实验研究和实际环境过程的汞同位素分馏三方面系统阐述了近十多年来关于汞同位素分馏机理的研究成果、最新进展和未来发展方向。尽管目前的研究普遍认为无机汞的光化学氧化还原和甲基汞的光化学降解是环境中汞同位素MIF的主要产生机制,然而MIF程度和方向的影响因素还不完全清楚,其量化理论还未完全建立,实际环境过程中的分馏机理研究还相对缺乏。未来需要结合理论和实验研究进一步明确大气、陆地、海洋、极地、古环境等实际环境体系中的汞同位素分馏机理,进而拓展汞同位素的应用。

Mercury(Hg)is the only metal contaminant that can transport globally as a gas,thus its environmental behavior and impact have received wide attentions.The stable isotopes of Hg provide a novel tracer for the sources and transformations of Hg,as well as the related environmental processes.The mercury isotope system is unique as it exhibits multiple types of mass independent fractionation(MIF)in nature.But the current research on the Hg isotope system mostly focus on its application,whereas many unknowns still exist for the mechanism of Hg isotope fractionation.In this paper,we systematically review the recent progress of theoretical and experimental studies on the mechanism of Hg isotope fractionation and its application in tracing environmental processes.While it is well established that the photochemical redox reactions of inorganic Hg and photodegradation of methylmercury are the main processes that give rise to Hg MIF in the environment,the factors that determine the extent and direction of MIF are not completely understood.Moreover,a quantitative theoretical framework for Hg MIF is still lacking,and the research on Hg isotope fractionation mechanism under realistic environmental settings is still scarce.We recommend that the future research should emphasize a combination of theoretical and experimental approaches to better understand the Hg isotope fractionation mechanism in atmospheric,terrestrial,oceanic,polar and paleo-environments,and thus expand the application of Hg isotopes.