Integrating column leaching experiments and geochemical modelling to predict the long-term alkaline stability during erosion process for gypsum amended bauxite residue.

Gypsum amendment is widely used to resolve alkalinity issues and implement sustainable management for bauxite residue disposal areas (BRDAs). Amended BRDAs under natural conditions suffer from long-term erosion processes. Nevertheless, the effect of erosion on amendment efficacy is rarely assessed. In this study, by integrating the geochemical modelling of PHREEQC and column leaching experiments, the dissolution of alkaline solids in bauxite residue (BR) and gypsum amendment, as well as their environmental behaviors, were determined through a 1-year simulated rainfall leaching experiment. The PHREEQC simulation results demonstrated that Na ion strength, CO partial pressure and rainfall, all affected the saturation index (SI) of calcite significantly and accelerated its corrosion, leading to the dissolution of gypsum and calcite in a relatively stable state. However, Na ion strength and rainfall significantly acted on the SI of gypsum, which lead to loss of Ca and reduction of alkaline stability. In addition to the effects of Na and Ca on the saturation concentration of gypsum and calcite solution, Na and Ca also exhibited significant effects on the equilibrium of chemical species reactions. The column results confirmed that stability of gypsum and calcite was consistent with the simulation results of PHREEQC in the BRDAs environment. Furthermore, multiple linear regressions revealed differences in combined contributions of rainwater and atmospheric CO on the stability of calcite and gypsum. The PHREEQC simulation provides a new approach to predict long-term alkaline stability of BR as well as to establish sustainable remediation on BRDAs during erosion process.