Selective extractions to assess the biogeochemically relevant fractionation of inorganic mercury in sediments and soils

Here we present a new method for sequential selective extractions (SSEs) for Hg in geological solids, validated with extensive quality assurance procedures. Mercury was separated into fractions which “make sense” biogeochemically, rather than being identified by specific compounds. Experiments elucidated the effects of extraction time, solids-to-liquid ratio, and alternate solvents in natural samples, reference materials, and pure compounds. Compounds tested included HgS (red and black), HgCl₂, Hg⁰, Hg₂Cl₂, HgSe, HgO, Hg(II) adsorbed on goethite, Hg-humate, and gold amalgamated Hg. Based on these findings, a five-step sequence of extractions was established to separate the compounds into biogeochemically distinct categories. The fractions and leaching media were as follows: F1 (deionized water), F2 (0.01 M HCl+0.1 M CH₃COOH), F3 (1 M KOH), F4 (12 M HNO₃), and F5 (aqua regia). Method blanks and method detection limits (MDLs) of 0.1-5 ng/g were obtained for the various analytical fractions, depending on the reagent concentrations used. Precision ranged from 2 to 8% for the major fractions in a sample, but increased to 2-40% for fractions making up <5% of the total. Recovery of total Hg by the sum of species in reference materials showed that the accuracy of the method ranges from 90 to 105%. Methylation potential, determined by anoxic incubation sample aliquots with biologically active sediments, showed that inorganic Hg extracted in the F3 fraction is most strongly correlated with methylation potential. In most natural and sediment incubation samples, the majority of Hg present was found either in the F3 or F5 fractions.