Study of the suitability of HNO3 and HCl as extracting agents of mercury species in soils from cinnabar mines

Abstract

The aim of this study was to compare the influence and feasibility of two common extracting agents (50% v/v HCl and 50% v/v HNO₃) on the leaching of Hg from soils. The solubility of a number of Hg species in each acid solution was evaluated under selected conditions. Most species were quantitatively dissolved in both acids with the exception of HgS. The application of both acid solutions to a soil sample from the Almaden mining area provided different recoveries of Hg: about 5% in 50% v/v HNO₃ and 50% in 50% v/v HCl. The following experiments were designed and developed in order to evaluate the matrix influence on HgS solubility and leaching: (1) study of the solubility of HgS in the presence of different potential interfering compounds such as FeCl₃, KCl, KI, Fe₂O₃, CuSO₄, FeSO₄, MnO₂ and NaNO₃; (2) study of the recovery of HgS spiked in soil samples; (3) study of the extraction process in soil samples spiked with the critical interfering compounds. Results showed the existence of a greater matrix influence with the HCl solution, since much higher Hg recoveries were obtained with this reagent. In addition, the presence of nitrates and Mn oxides drastically promotes the solubility of HgS in an HCl solution. On the other hand, halide compounds drastically enhanced the extractability of Hg in the HNO₃ and they must be considered as potential interfering compounds when this acid solution is used as extracting agent. In summary, neither acid is totally free of matrix effects from common soil constituents; conclusions about mercury mobility resulting from the general application of these extraction procedures must therefore be made with caution.