A methodological framework to study the behavior and kinetic influence of V, Mn, Co, Ni, Cu, Zn, As, Se and Mo during pyrite formation via the polysulfide pathway at ambient temperature

Abstract

Pyritization links the biogeochemical cycles of Fe, S and C to those of trace elements (TE) such as As, Ni, Cu, Co, Mn, Zn, Se and Mo in reducing environments. The scavenging modes of such impurities in pyrite are parameters of importance for evaluating to which extent pyrite can immobilize TE under diverse (sub)surface conditions. Furthermore, determining the fate of TE during pyritization is a prerequisite to evaluate the ability of pyrite to record the chemical signature of surrounding waters at the time of precipitation. Here, we provide a methodological framework to study the incorporation of TE during a reaction sequence leading to pyrite formation via the polysulfide pathway. Laboratory syntheses were conducted by reacting a FeS precursor with elemental sulfur – both readily obtained by reducing FeCl3 with Na2S in solution under strict anoxia – at ambient temperature and in the presence of aqueous V, Mn, Co, Ni, Cu, Zn, As, Se and Mo in a TE/Fe molar ratio of 0.5 mol%. Within this simplified framework, our experiments aimed at reproducing the early formation of pyrite in water column aggregates/bottom sediments before the late diagenetic aging phases. Solids and liquids were sampled over time during the reaction sequence leading to pyrite formation, up to 3100 h, and further analyzed by ICP-OES, XRD, XRF and STEM-EDXS to obtain information on (1) the kinetics of pyrite formation and (2) the extent of TE incorporation in the solid phase, during and after the complete conversion of FeS to FeS2. Pyrite formation kinetic is observed to be strongly influenced by the identity of the added TE, with the delay before nucleation decreasing in the order Ni >> Mn, Co > Cu, Zn, Se > Control, V >> As, Mo. During the full reaction sequence, i.e. FeS plus S(0) conversion to FeS2, Mn remains in the liquid phase, Ni, Co, V, Cu, and Se quickly precipitate and remain sequestered overtime, Zn and Mo precipitate and are further released into solution, and As is partially incorporated in the solid. Elemental mapping at the microscale using STEM-EDXS show that, except for Mn, the TE are associated with pyrite. Moreover, while TE are homogeneously distributed at the particle scale for Co, Cu, Zn, As, and Se, Ni-pyrite exhibited an enriched core. The solid-liquid partition coefficients of each TE during our pyritization experiments in our simplified model system are calculated and ranks as follows: Cu ≥ Ni ≥ Co > Se > V > Mo >> As > Zn >> Mn. These partitions are discussed in the light of values reported in the literature from natural settings. The important differences observed exemplify the complexity to predict TE pyrite-water partition in sedimentary environments, in particular when considering additional multifactorial processes such as the local biogeochemistry of micro-environments, as well as the partial sequestration of TE by non-sulfide minerals such as authigenic clays and carbonates.