Binding of heavy metals by oxidised kerogen in (palaeo)weathered black shales

Abstract

Sub-aerial weathering of black shales drives the gradual leaching of sulphur- and organic-bound heavy metal elements, which are usually abundant in these rocks due to depositional conditions. The formation of oxygen functional groups in kerogen, however, can lead to an opposing mechanism - metal adsorption and binding, similar to a process common in soils. An increase in cation exchange capacity (CEC) measured previously using metal complexes on black shales oxidised under laboratory conditions implies that the same phenomenon may occur in a naturally oxidised black shale. This idea was tested on a unique, well-developed and -preserved Permian palaeoweathering profile containing two neighbouring but diverse black shales from the Devonian/Carboniferous boundary in the Holy Cross Mountains (Poland). In the studied black shale beds, the oxygen groups formed in kerogen in the partially-weathered zone were found to be responsible for significant changes in adsorption properties measured using hexamminecobalt(III) and Cu(II)‑triethylenetetramine cations, which are common probes for CEC. Compared to a pristine part of black shales, the partially weathered zone was depleted of total organic carbon (TOC), sulphur, and sulphur- and organic-bound metals, and highly enriched in Cu, which is generally present in low levels in the nascent shales. In the partially weathered zone, where TOC content is reduced, apparent CEC values surpass the CECs predicted from the contents and structures of clay minerals, and correlate linearly with the content of oxygen groups developed during weathering. The adsorption properties of carboxyl groups in the oxidised kerogen are suggested as being responsible for the syn- or post-weathering enrichment in Cu caused by the remobilisation of older Cu-sulphide ores present in the area. As opposed to natural weathering, aggressive oxidation, e.g. under laboratory conditions produces a high proportion of cross-linked oxygen groups that do not participate in metal cation adsorption. The CEC values of artificially oxidised samples reached a limit corresponding to those of naturally oxidised shales.