Geochemical Behavior and Environmental Effects of Arsenic and Molybdenum During Black Shales Weathering in Central Hunan, China

Arsenic (As) is a toxic element that has garnered significant attention in environmental studies, whereas molybdenum (Mo) is an essential element for both plants and animals; however, excessive Mo is considered detrimental to human health. Black shale serves as a crucial natural reservoir for As and Mo within the Earth′s crust and is highly susceptible to weathering when exposed to the surface. Understanding the geochemical behavior of As and Mo during the weathering process is of great scientific significance for assessing the ecological and environmental risks associated with As and Mo in areas where black shale is distributed. This study focuses on the Muzi (MZ) black shale weathering profile in the central Hunan region as the research subject and systematically analyzes the content, distribution, migration characteristics, and release flux of major elements as well as As and Mo across different weathering layers. The results indicate that from fresh black shale (FBS) to weathered black shale (WBS) and subsequently to black shale-derived soil (BSS), the Chemical Index of Alteration (CIA) gradually increases alongside heightened weathering intensity. The contents of As and Mo in MZ black shale range from 12.60 to 266.00 mg/kg and 3.77 to 301.20 mg/kg, respectively. Compared to the upper crust, fresh black shale layers exhibit extreme enrichment of As and Mo (average enrichment coefficient >40). During the weathering process, significant leaching of As and Mo occurs; the mass transfer coefficients (τ) of As and Mo for WBS and BSS are all less than 0 when compared to FBS. The initial weathering stage (transition from fresh black shale layer to initial weathering layer) represents a significant release period, closely associated with the strongly acidic environment (pH<3) of the initial weathering layer. The average mass transfer fluxes (Mi) of As and Mo reached –864.96 g/m² and –510.18 g/m², respectively, indicating substantial release amounts; this situation may pose a potential threat to local soil, surface water, and groundwater, necessitating serious consideration.