Quantification of Metals and Semimetals in Carbon‐Rich Rocks: A New Sequential Protocol Including Extraction from Humic Substances

Abstract

We have developed a new sequential extraction technique that does not require complex procedures and is efficient in determining metal and semimetal contents of carbon‐rich rocks. Six geological reference materials (SBC‐1, SCHS‐1, SCo‐1, SDO‐1, SGR‐1b and SLg‐1) and an in‐house black shale (SH‐1) were selected to test the method, which consists of four main digestion steps involving: (a) dilute HCl acid; (b) NaOH; (c) aqua regia; and (d) hydrofluoric acid. Compared with traditional aqua regia + hydrofluoric acid attack, this new protocol recovers more of the moderately volatile elements during early extraction of humic substances. In addition when compared with reference values, those for most elements are in agreement within uncertainty. Furthermore, this new protocol reveals important information on the partitioning of elements; for instance, steps one and two indicate which elements are associated with carbonates and organic phases from fulvic and humic acid extractions, whereas step three provides results for which elements are associated with sulfide minerals and step four indicates which elements remained in the silicate and oxide phases.