Mineral-dependent release, migration and enrichment of toxic elements during black shale weathering: An integrated study from profile scale to mineral scale

Chemical weathering of lithologies with high geochemical backgrounds such as black shale has been proposed to be a critical source for toxic elements in soil and water systems. However, mechanisms controlling the release, migration and enrichment of toxic elements during black shale weathering are poorly understood. This study utilized a suite of micro analytical techniques such as TESCAN integrated mineral analyzer (TIMA), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS), X-ray photoelectron spectroscopy (XPS) and electron micro-probe analysis (EMPA) to elucidate the intimate relationship between mineralogical transformations and elemental behaviors from profile scale to mineral scale. Mineralogical and elemental compositions for a black shale weathering profile (and surface strongly weathered materials) suggest a dominant sequence of mineral reactions as oxidation of sulfides, dissolution of carbonates, alteration of aluminosilicates, and transformation of clay minerals. Most of the toxic elements were largely released from the weathering profile and significantly enriched in the strongly weathered materials. Black shale weathering was initiated by oxidation of pyrite, sphalerite and molybdenite, and these chemical reactions dominated the release of toxic elements (e.g., As, Cd, Mo, Mn, Ni and Zn). During oxidation of pyrite, Fe (hydr)oxides pseudomorphically replaced pyrite grains, along with the release of As and Mn and their subsequent retention in Fe (hydr)oxides. Sulfuric acid generated by oxidation of sulfides firstly dissolved surrounding calcite and dolomite to significantly improve the pore-fracture networks in the weathered shale, allowing more water fluxes and transportation of Fe (hydr)oxides and concomitant migration of associated toxic elements. Then, albite and minor orthoclase were altered to illite that was responsible for the secondary enrichment of Tl throughout the weathering profile. In intense weathering stage, Fe/Mn (hydr)oxides were substantially delivered to re-precipitate in fractures and contributed to considerable enrichment of As, Ni, Co, Zn and Cd. Meanwhile, transformation of illite to kaolinite may also influence the enrichment of toxic elements. This work highlights the importance of understanding the control of mineralogical transformation on release, migration and enrichment of toxic elements during black shale weathering, such that this mineral-dependent mechanism can be implemented to risk prediction and assessment of toxic elements in black shale regions.