Coupled Transformation Processes of Cr-Adsorbed Schwertmannite and Chromium Redistribution Controlled by Ca(II) Speciation

Schwertmannite (Sch) is a widespread iron oxyhydroxysulfate mineral in acid mine drainage (AMD) systems, and its transformation strongly influences the environmental fate of chromium (Cr). However, the role of Ca(II), which is commonly introduced during alkaline neutralization of AMD, in regulating the transformation of Cr(VI)-adsorbed schwertmannite (Cr-Sch) and subsequent Cr redistribution remains insufficiently understood. In this study, transformation experiments were conducted under various pH conditions (3.0, 7.0, and 10.0) to investigate the effects of Ca(II) speciation on mineral transformation and Cr behavior. The results demonstrated that the transformation of Cr-Sch was predominantly pH-dependent. Under acidic conditions, Cr-Sch transformed into goethite via dissolution–recrystallization, resulting in transient Cr release followed by partial refixation. The presence of Ca(II) exerted only a minor influence due to weak interactions between Ca2+ and positively charged mineral surfaces. Under alkaline conditions, Cr-Sch preferentially transformed into hematite through dehydroxylation and cation rearrangement, leading to the sustained release of adsorbed Cr(VI). In contrast, Ca(II) predominantly precipitated as CaCO3 precipitate (calcite, aragonite, and vaterite) under alkaline conditions, which coated mineral surfaces and inhibited phase transformation and Cr release. These findings reveal that Ca(II) regulates Cr redistribution primarily through pH-dependent speciation and mineral–surface interactions, highlighting coupled geochemical processes governing iron mineral transformation and contaminant mobility in AMD environments. This study provides mechanistic insights for predicting Cr behavior and optimizing alkaline remediation strategies in mining-impacted systems.