Study of extraction conditions for the quantitative determination of Hg bound to sulfide in soils from Almaden (Spain)

Almaden mine (Spain) is the largest source of cinnabar (HgS) in the world. The purpose of this study is to evaluate, compare and optimize the analytical conditions for the quantitative determination of Hg bound to sulfide in soil samples from mining area of Almaden. A sequential extraction procedure was performed in two stages. The first one was based on a nitric acid leaching. An optimization of certain extraction conditions in this stage was carried out. In order to, assess the suitability of the nitric acid leaching, an additional study of possible interfering compounds that might promote the solubility of HgS in nitric acid was developed. The quantitative determination of Hg bound to sulfide was considered in a second stage. A comparative study was carried out among three different procedures: i) extraction with a saturated Na2S solution from the residue remaining in the first stage, ii) microwave assisted dissolution of the remaining Hg in this residue with aqua regia, and iii) quantification by difference between total Hg content and Hg extracted in the first stage. The recoveries of Hg bound to sulfide were found to be comparable for the three proposed procedures. Soil samples coming from Almaden mining area were analysed by this method. The distribution of Hg chemical forms was found to be similar for the two parcels tested, and the recoveries of bound to sulfide Hg ranged from 83% to 96% of total Hg content.