Accumulation and oxidation of elemental mercury in tropical soils

•Retention and oxidation of Hg0 in tropical soils depends on soil properties.•Those processes vary greatly both quantitatively and qualitatively with soil type.•Organic matter is the main soil characteristic in Hg adsorption and oxidation.•pH and CEC also appear to be important soil characteristics in Hg adsorption.•Hg0 oxidation occurred at the air/soil interface rather than only in the air. The role of chemical and mineralogical soil properties in the retention and oxidation of atmospheric mercury in tropical soils is discussed based on thermal desorption analysis. The retention of gaseous mercury by tropical soils varied greatly both quantitatively and qualitatively with soil type. The average natural mercury content of soils was 0.08±0.06μgg−1 with a maximum of 0.215±0.009μgg−1. After gaseous Hg0 incubation experiments, mercury content of investigated soils ranged from 0.6±0.2 to 735±23μgg−1, with a mean value of 44±146μgg−1. Comparatively, A horizon of almost all soil types adsorbed more mercury than B horizon from the same soil, which demonstrates the key role of organic matter in mercury adsorption. In addition to organic matter, pH and CEC also appear to be important soil characteristics for the adsorption of mercury. All thermograms showed Hg2+ peaks, which were predominant in most of them, indicating that elemental mercury oxidized in tropical soils. After four months of incubation, the thermograms showed oxidation levels from 70% to 100%. As none of the samples presented only the Hg0 peak, and the soils retained varying amounts of mercury despite exposure under the same incubation conditions, it became clear that oxidation occurred on soil surface. Organic matter seemed to play a key role in mercury oxidation through complexation/stabilization of the oxidized forms. The lower percentages of available mercury (extracted with KNO3) in A horizons when compared to B horizons support this idea.