RESUMEN
Investigations of the redox process and chemical speciation of Hg(II) lead to a better understanding of biogeochemical processes controlling the transformation of Hg(II) into toxic and bioaccumulative monomethyl mercury, mainly in areas contaminated with Hg(0). This study investigates the speciation and redox processes of Hg in soil samples from a small area contaminated with Hg(0) as a result of gold mining activities in the rural municipality of Descoberto (Minas Gerais, Brazil). Soil samples were prepared by adding Hg(0) and HgCl2 separately to dry soil, and the Hg redox process was monitored using thermodesorption coupled to atomic absorption spectrometry. A portion of the Hg(0) added was volatilized (up to 37.4±2.0%) or oxidized (from 36±7% to 88±16%). A correlation with Mn suggests that this oxidation is favored, but many other factors must be evaluated, such as the presence of microorganisms and the types of organic matter present. The interaction of Hg with the matrix is suggested to involve Hg(II)-complexes formed with inorganic and organic sulfur ligands and/or nonspecific adsorption onto oxides of Fe, Al and/or Mn. The kinetics of the oxidation reaction was approximated for two first-order reactions; the faster reaction was attributed to the oxidation of Hg(0)/Hg(I), and the slower reaction corresponded to Hg(I)/Hg(II). The second stage was 43-139 times slower than the first. The samples spiked with Hg(II) showed low volatilization and a shifting of the signal of Hg(II) to lower temperatures. These results show that the extent, rate and type of redox process can be adverse in soils. Descoberto can serve as an example for areas contaminated with Hg(0).
Asunto(s)
Compuestos de Mercurio/metabolismo , Mercurio/metabolismo , Contaminantes del Suelo/metabolismo , Adsorción , Brasil , Monitoreo del Ambiente , Minería , Oxidación-Reducción , Espectrofotometría Atómica , VolatilizaciónRESUMEN
This paper reports on the speciation study and the Hg redox behavior in Amazon soils not influenced by gold mining and collected near Manaus, AM, Brazil. The samples were incubated by adding Hg(0) and HgCl(2) to dry soil. Solid phase Hg speciation analysis was carried out using a Hg thermodesorption technique with the aim of distinguishing elemental Hg(0) from Hg(II) binding forms. In the first case, we observed the conversion of Hg(0) to Hg(II) binding forms in the range of 28-68% and a correlation between the percent of oxidation and OM content. Samples incubated with Hg(II) showed the formation of Hg(I) and/or Hg(0) in the range of 19-69%. The lowest values corresponded to the samples with the lowest clay contents. The kinetics of conversion of Hg(0) as well as HgCl(2) were roughly fitted to the two first order reactions, a fast one and a slow one. It was not possible to evaluate differences between sampling sites and types of soils, but the mean half-life of the first order reaction obtained by the addition of Hg(II) was slower (t(1/2)=365d) than the one obtained by the addition of Hg(0) (t(1/2)=148d). Previous studies have shown the predominance of organically bound Hg in these samples. Thus, the kinetic difference between Hg oxidation and reduction in combination with the efficient retention processes by OM may explain the high background values found in Amazon soils.
Asunto(s)
Mercurio/química , Contaminantes del Suelo , BrasilRESUMEN
In order to investigate the content and distribution of Hg in soils from Amazon areas not impacted by gold mining activities, the concentration and granulometric distribution of Hg were determined in three different types of soils. Hg speciation data were obtained by a thermodesorption atomic spectrometry system and the role of physico-chemical and mineralogical characteristics on Hg content were also studied. Hg concentrations found in the soil fraction <2000 microm were similar to values found in other Amazon areas without anthropogenic sources. The fine fraction (<53 microm) of podzolized soils had a higher Hg content than clayey soils. Multivariate analysis shows that there are marking differences between Spodosols of two distinct areas (fine fraction Hg contents up to 10-fold as high) indicating the importance of pedogenic processes. All statistical data analysis indicated that organic matter plays an important role in Hg accumulation in the types of soils studied. Thermodesorption analysis revealed the presence of only Hg2+, and the predominance of organically bound Hg in the majority of the studied samples.