The Effect Of Environmental Factors On Flux And Isotopic Compositions Of Hg~0 Emission From Soil

Mercury(Hg~0)released from soil is an important source of atmospheric Hg in China and worldwide.Knowledge with respect to the contribution of Hg released from soil to the atmospheric Hg is an important step to establish global and regional Hg biogeochemical cycling model,and also contribute significantly to the political measures aimed at reducing and/or eliminating regional and global Hg pollutions.During the last decade,significant advances have been achieved in Hg stable isotope techniques and applications,which explore a new dimension of information on identifying the sources of Hg and tracking the transformation and transport pathways of Hg in the environments.Knowledge on isotopic compositions of Hg released from soil to the atmosphere is crucial for tracing sources of Hg in the atmosphere and establishment of global Hg isotope model.In this study,we investigated the effect of environmental factors on the flux and isotopic compositions of Hg~0 emission from soil.Two typical Hg-enriched soils,agricultural soil(218±19.20 ?g·g-1)and forest soil(15.4±0.40 ?g·g-1)in the Wanshan mercury minging areas were selected for the laboratory studies.In the laboratory,we measured Hg~0 emission fluxes and the isotopic compositions of emitted Hg~0 from soil and total Hg in reactant soil at rising soil temperature(40,100,and 130 °C),under sunlight(38217w·m-2)and sunlight+soil water(0%,5%,10%,20%,and 30%)conditions.The results showed that rising temperature,sunlight,and increasing soil water would dirve an increase in Hg~0 emissions.The Hg~0 emission fluxes at rising temperature(100? and 130°C)were relatively higher than that under sunlight.Under all the selected environmental conditions,both mass-dependent fractionation(MDF)and mass-independent fractionation of odd-mass isotopes of emitted Hg~0 were detected.?202Hg values of Hg~0 emitted from the agricultural and forest soils were in the ranges of-2.87 to-1.10‰ and-4.64 to-1.42‰,respectively,and ?199Hg values of Hg~0 emitted from the agricultural and forest soils were in the ranges of 0.08 to 1.34‰ and-0.06 to 0.34‰,respectively.No significant MIF of even-mass isotopes were observed during the experiments.These results indicate lighter isotopes and odd-mass isotopes are preferentially reduced and emitted to the atmosphere from soils.Our findings are similar to the studies on photo-reduction of Hg2+ coordinated to S-ligands and dark recution of Hg2+ in water,suggesting the portions of Hg in agricultural and forest soils that are readily recuded and evaporated from are bound to S-ligands.?199/201 Hg slopes of emitted Hg~0 from agricultural and forest soils under sunlight are 0.99 and 0.82,respectively.?199/201 Hg slopes of emitted Hg~0 from agricultural and forest soils at rising temperature are 1.34 and 1.41,respectively.This indicates magnetic isotope effect(MIE)played the dominant role in the MIF of Hg emitted from soil under sunlight,while nuclear volume effect(NVE)played the dominant role in the MIF of Hg emitted from soil at rising temperature.