Mineral phase transformation drives molecular fractionation of dissolved organic matter during organic acid-driven weathering of bauxite residue

Dissolved organic matter (DOM) sequestration is critical to soil formation of bauxite residue (BR) in which BR undergoes mineral weathering and neutralization / acidification driven by microbial and/or plant root-derived organic acids (OA). The present study aimed to understand the molecular fractionation and adsorption of DOM in BR minerals during key stages of mineral weathering and neutralization, using mixed organic acids to simulate the mineral weathering process in the ecological engineering context. Selective adsorption and molecular fractionation of DOM derived from microbial decomposition of plant mulch were characterized using the high-resolution electrospray ionization Orbitrap mass spectrometry (Orbitrap-MS). The OA-treatments progressively weathered sodalite-like alkaline minerals to form amorphous Al/Si/Fe phases and lowered pH, increasing surface reactive sites in mineral phases. The slightly weathered (pH 9–7) and highly weathered and acidified BR minerals (pH 5–4), preferentially adsorbed high-molecular-weight organics rich in aromatic and/or polyphenolic domains. Synchrotron-based Al, Si, and Fe K-edge X ray absorption spectroscopic analyses (XAS) and Fourier transform infrared (FTIR) mapping, revealed strong associations of secondary Al/Si/Fe phases with protein-, carboxylic-, aromatic-, and oxygenated lipid-like compounds. These findings highlight the critical role of mineral weathering in BR in controlling the stabilization of DOM during eco-engineered soil formation processes.