Mercury emissions in equilibrium: a novel approach for the quantification of mercury emissions from contaminated soils

Mercury emissions from soil samples with different mercury contents have been estimated using a closed circuit array. The samples were collected from the Almaden mercury mining district. The emissions confirmed that temperature and light radiation favour mercury desorption due to the increase in the mercury vapour pressure. An additional positive factor could be the photocatalytic reduction of soluble Hg2+ to volatile Hg-0 at the soil surface. A physicochemical model based on mass transfer and equilibrium was developed and was used to reproduce the mercury emissions at the laboratory scale. The use of this model allowed us to obtain the unknown mass transfer coefficient (K-L) and adsorption parameters required to quantify the possible gaseous mercury fluxes from these contaminated soils. Experimental results indicate that an equilibrium between the solid and gas phases was established. The proposed kinetic model reproduced perfectly the experimental data, with KL found to be proportional to the inverse of temperature and independent of the radiation. The concentration of mercury in the gas phase was mainly dependent on the soluble mercury content (Hg-S). Equilibrium data were fitted by Langmuir and Freundlich models and the best fit was obtained using the multi-layer model attending to the convex shape of the curves, which is characteristic of non-porous or possibly macroporous materials having a low adsorption energy. The Freundlich constant (K-F) was also fitted as a polynomial function with temperature and this gave a straight line for the light radiation and a second grade equation for dark conditions. Once the parameters had been obtained, the Hg emission fluxes from contaminated soils were estimated and the values were between two and three orders of magnitude higher than those published in the literature for non-contaminated soils.