Mechanisms and Patterns of Heavy Metal Release from Black Shale Gravel During Weathering as Characterized by Gradient Fragmentation

Aiming at the problem of heavy metal release from ultra-low-grade waste rock caused by the coupling of natural weathering and acid-rain leaching, black shale gravel of the Cambrian Series in western Hunan was taken as the research object. Gradient mechanical crushing was used to simulate physical weathering, and sulfuric–nitric acid-type simulated acid rain was prepared for continuous leaching experiments. Combined with ICP-MS monitoring and SEM-EDS characterization, the effects of crushing intensity on the physicochemical properties of leaching system and heavy metal release kinetics were systematically analyzed. The results showed that the pH of the leaching system presented three evolutionary stages: acid-dominated, alkaline transition and buffer stabilization. Heavy metal release could be divided into three types according to their occurrence forms: the sulfide-phase-sensitive type (Cd, Zn), secondary stable type (Pb), and silicate lattice bound type (Cu, Ni, Cr). The promotion effect of crushing on interface reaction activity showed diminishing marginal effect, and the particle fractal dimension increased from 2.15 to 2.67. It was concluded that the core controlling factor of heavy metal release risk is the selective exposure degree of occurrence mineral phases by physical disturbance. A coupling framework of “physical weathering–mineral exposure–release response” was established, providing a scientific basis for the differentiated management and control of heavy metals in filling sites.