Processes influencing the emission of mercury from soils: A conceptual model

In contrast to recent rapid growth in the field data on mercury flux over soils, our knowledge and understanding of the emission processes and controlling mechanisms still remains limited. Many processes could control emission of mercury from soils. The emission processes may involve both Hg(0) and Hg(II) pools, both homogeneous and heterogeneous distributions and reactions, and both dark and light processes. The physical and chemical controlling processes mainly involve (1) distribution of different mercury species among different soil phases, largely associated with adsorption and desorption of Hg(0) and Hg(II) species, and (2) mercury redox reactions in soils, some of which could be induced by sunlight, especially in the UV. The distribution (sorption versus desorption) of Hg(0) in soils could be the rate-limiting process in the case where Hg(0) is the dominant mercury pool, and the redox reactions could be rate-limiting in the case where Hg(II) is the major pool. Various hypothetical scenarios of Hg(0) and Hg(II) transformation in soils are discussed here. However, the specific processes that actually occur in soils under natural conditions are far from clear and need to be identified and quantified in future research.