Investigation of the light-enhanced emission of mercury from naturally enriched substrates

Incident radiation has been reported to facilitate mercury release from soils. In this study the influence of light on mercury emissions from substrates amended with pure synthetic mercury species, and from naturally and anthropogenically mercury-enriched substrates were investigated using laboratory experiments and in situ flux measurements. Light-enhanced emissions were found to occur from substrates amended with HgS, and from elemental mercury (Hg-0) and HgCl2 amended iron oxide and organic containing substrates. The magnitude of light-enhanced emissions for natural substrates ranged from 1.5 to 116 times that occurring in the dark at the same substrate temperature. Substrates containing corderoite, metacinnabar and "matrix bound mercury" (that bound to organic or inorganic phases) exhibited a higher degree of light-enhanced emissions relative to that containing predominantly cinnabar. Calculated activation energies for both laboratory and field data indicate that photo-reduction is a process associated with the light-enhanced emissions. Activation energies, derived using in situ mercury fluxes and soil temperatures, indicated that photo-reduction was a dominant process facilitating release of Hg from substrates with sunrise. Activation energies, calculated using daytime data, were less than those calculated for sunrise. This is hypothesized to be due to a pool of Hg being developed with photo-reduction at first light that is released as soil temperatures and convective heat transfer increase during the day. This study demonstrated that light energy is the more dominant process controlling mercury emissions from naturally enriched substrates than soil temperature. (C) 2002 Elsevier Science Ltd. All rights reserved.