Molecular Mechanism of Selective Depression of Specularite by Cassia Seed Gum: Nonflocculating Adsorption via Fe–Galactose Coordination

The efficient flotation separation of Fe-bearing silicate minerals remains a major challenge due to their similar surface chemistry. In this work, the molecular mechanism underlying the selective depression of specularite by cassia seed gum (CG), a natural galactomannan polymer, was systematically investigated. Comparative flotation and turbidity analyses revealed that CG acts as a nonflocculating selective depressant, in contrast to the high-flocculating behavior of guar gum and xanthan gum. Surface characterizations including Zeta potential, adsorption density, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) measurements demonstrated that CG selectively adsorbs on specularite surfaces through Fe-O-H coordination, enhancing hydrophilicity and preventing collector adsorption, while exhibiting negligible interaction with chlorite. Density functional theory (DFT) calculations further confirmed that the C₆-hydroxyl group of the galactose unit serves as the most stable coordination site for Fe ions, with a binding energy of -24.76 kcal mol^-1, exceeding that of the C₂- and C₃-hydroxyl sites. The moderate adsorption strength and linear molecular conformation of CG effectively suppress multipoint Fe-O-C bonding and interparticle bridging, accounting for its nonflocculating selective adsorption behavior. This study establishes the structure-function relationship of cassia seed gum at mineral interfaces and provides a mechanistic framework for designing environmentally benign, nonflocculating natural depressants with controllable adsorption selectivity toward Fe-bearing mineral systems.