Sorption and desorption of mercury(II) in saline and alkaline soils of Bahia Blanca, Argentina

The objective of this work was to study sorption-desorption and/or precipitation-dissolution processes of Hg(II) compounds considering an eventual contact of soils with Hg-bearing wastes. In addition, this study contributes new data about Hg(II) chemistry in alkaline systems. Saline and alkaline soils with low organic matter (< 1 %) and high clay content (60-70 %) were obtained near a chlor-alkali plant. Batch techniques were used to perform the experiments using 0.1 M NaNO3 solutions. Total Hg(II) concentrations ranged from 6.2 x 10(-8) to 6.3 x 10(-3) M. Sorption of Hg(II) was evaluated at two concentration ranges: (a) 6.2 x 10(-8) to 1.1 x 10(-4) M, and (b) 6.4 x 10(-4) to 6.3 x 10(-3) M. At low Hg(II) concentrations, adsorption occurred with a maximum sorption capacity ranging from 4 to 5 mmol/kg. At high Hg(II) concentrations, sorption-precipitation reactions occurred and maximum sorption capacity ranged from 17 to 31 mmol/kg. The distribution of Hg(II) hydrolysis products showed that Hg(OH)(2) was the predominant species under soil conditions. According to sorption experiments, X-ray diffraction and chemical speciation modelling, the presence of Hg(OH)(2) in the interlayer of the interstratified clay minerals can be proposed. Hg(OH)(2) was partially desorbed by repeated equilibrations in 0.1 M NaNO3 solution. Desorption ranged from 0.1 to 0.9 mmol/kg for soils treated with 5.8 x 10(-5) M Hg(II), whereas 2.1-3.8 mmol/kg was desorbed from soils treated with 6.3 x 10(-3) M Hg(II). Formation of soluble Hg(II) complexes was limited by low organic matter content, whereas neutral Hg(OH)(2) was retained by adsorption on clay mineral surfaces.