Mercury adsorption to elemental carbon (soot) particles and atmospheric particulate matter

An assessment of the adsorption/desorption of mercury onto/from particulate matter is presented. This assessment addresses both elemental carbon (soot) particles using published data and atmospheric particulate matter using new experimental data. Available experimental data on the adsorption of mercury onto elemental carbon particles have been reexamined in terms of their adsorption isotherms. The experimental data sets analyzed include experiments concerning mercury adsorption on activated carbon in water as well as in the gas phase. Our analysis using partition coefficients derived from published experimental data and typical atmospheric concentrations of elemental carbon particles shows that the amounts of Hg adsorbed to soot in the air or in atmospheric droplets is insignificant. However, if the particle surface/mass ratio is scaled from the experimental data to typical atmospheric conditions, an upper limit of about half the amount of Hg (primarily Hg(II)) could adsorb to elemental carbon particles, for the conditions considered here. Adsorption of Hg(0) appears to be negligible. Experiments were also conducted to investigate the partitioning of mercury between the dissolved (solution) and suspended particulate phases in rain water. The rain-water data are compared with laboratory experimental studies of Hg desorption and adsorption onto atmospheric particulate matter (APM). The results of these experiments suggest that a significant fraction of Hg(II) can be present in atmospheric particulate matter. Our analysis suggests that between 2 and 35% of the dissolved Hg species (e.g. HgCl2 and Hg(OH)2) could be adsorbed to APM, for the conditions considered here. The remainder of particulate Hg(II) consists of solid Hg species such as HgO and HgS. An analysis of the adsorption experiments as a function of time suggests that the kinetics of adsorption may need to be taken into account in atmospheric applications.