Al3+-assisted adsorption mechanism of salicylhydroxamic acid on kaolinite

Based on DFT calculations and flotation experiments, this study systematically investigated the adsorption behavior of salicylhydroxamic acid (SHA) on kaolinite in three typical configurations (SHA-Kao, SHA_[Kao-Al], and [SHA-Al]_Kao). Results showed that SHA preferentially formed stable adsorption on the kaolinite (0 0 1) surface, and the presence of Al 3+ significantly enhanced adsorption. The SHA_[Kao-Al] configuration exhibited the most favorable adsorption energy (−550.87 kJ/mol) and Mulliken charge transfer (0.85 |e|), indicating that Al 3+ activates the kaolinite surface and facilitates electron transfer, thereby lowering adsorption energy. Flotation tests confirmed that SHA_[Kao-Al] achieved markedly higher recovery under acidic conditions (pH < 6), while the hydrolysis and precipitation of Al 3+ at higher pH suppressed surface activation, reducing recovery. FTIR and XPS results further confirm that SHA predominantly adsorbs on the Al-OH surface of kaolinite, and that the presence of Al ions markedly enhances both the coordination ability and the hydrogen-bonding strength between SHA and Al-OH sites on kaolinite, whereas only weak interactions occur on quartz. These results provide solid theoretical support for Al 3+ assisted selective flotation of kaolinite with anionic collectors.