Microstructure and molecular mechanism of chromium retention to schwertmannite in acidic sulfate environments

Abstract

Schwertmannite affects the immobilization of chromium in acidic environment, yet the precise mechanism underlying chromium retention remains obscure and poorly understood. This study systematically examines incorporation behavior of Cr(VI) into schwertmannite which may be an important Cr host phase in acidic iron and sulfate rich environments. Through controlled synthesis of schwertmannite with varying Cr/Fe ratios (0.002–0.2), we employed a comprehensive analytical approach combining acidic ammonium oxalate (AAO) dissolution kinetics, SEM-EDS-Mapping, FTIR, XRD and the high-resolution XPS to provide detailed information regarding the Cr incorporation mechanisms. Long-term (31-day) incubation experiments were conducted to assess the impact of Cr incorporation on schwertmannite formation and the behavior of S and Cr during mineral phase transformation. It is found that moderate Cr incorporation (Cr/Fe ≤ 0.05) significantly enhances schwertmannite stability, following the order of SCH-Cr-0.05 > SCH-Cr-0.01 > SCH-Cr-0.002 > SCH, while effectively retarding Cr release. However, structural collapse and decreased stability occur when the Cr/Fe ratio exceeds 0.05. The outer-sphere bounded sulfate decreases from 52.2 % to 48.6 % and 41.4 % in SCH, SCH-Cr-0.01 and SCH-Cr-0.1, respectively. Two distinct incorporation pathways of Cr substitution for SO₄ and Cr substitution for Fe were deduced by extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculations. These insights implies that Cr-incorporated schwertmannite could serve as a viable, long-duration sequestration site for Cr in acid mine drainage (AMD) environments.