Abstract: Acid mine drainage (AMD) resulting from mining activities is a global environmental issue. Investigating the acid generation mechanisms of AMD and the biogeochemical processes affecting the release, migration, and transformation of heavy metals and sulfates provides a scientific basis for quantitatively predicting AMD formation and developing remediation strategies.
In this paper, a multiphase reactive transport model for water-gas two-phase flow is constructed based on the TOUGHREACT/EOS3 code to simulate the acid generation process of sulfide minerals (such as pyrite) in tailings impoundment under the influence of atmospheric precipitation and oxygen. It further considers the physical and geochemical reaction processes of associated species in the migration and transformation within the aqueous medium. The reaction network includes the release of H⁺ from the oxidation and dissolution of sulfide minerals, the migration and transformation of iron ions and sulfate, and the pH buffering processes caused by the dissolution and precipitation of major minerals. Finally, by incorporating sulfur stable isotopes into the reactive transport model, the study identifies the bacterial sulfate reduction processes and corresponding sulfur isotope fractionation occurring during the formation of AMD. The model quantitatively assesses the acid production contributions under different oxidation pathways of pyrite, revealing that simultaneously considering the parallel oxidation processes of O₂(aq) and Fe³⁺ increases the concentrations of acid, iron ions and sulfate. The pH is controlled by the release of H⁺ from pyrite oxidation and the neutralization reactions corresponding to mineral dissolution and precipitation. Stable isotope reactive transport model indicates significant differences in δ³⁴S values under different fractionation kinetic enrichment factor, and using the Rayleigh fractionation model in an open system can greatly overestimate the enrichment factor. This research aids in understanding and predicting the formation and migration processes of acidic water in mining environments, which is crucial for addressing environmental pollution issues caused by mining activities. 

Key words: Acid mine drainage, Water-air two-phase flow, reactive transport modeling, sulfur stable isotope