Ultrasonic/microwave-assisted synthesis of organosilicon quaternary ammonium-modified montmorillonite for selective removal of molybdenum from tungstate feed solution.

The selective separation of molybdenum (Mo) from tungsten (W) remains challenging due to their similar chemical properties. In this study, organosilicon quaternary ammonium salts with varying alkyl chain lengths, including methyl, tetradecyl (TD), and octadecyl, were employed to modify montmorillonite (Mt) via a combined ultrasonic/microwave-assisted protocol in different solvent systems for the separation of Mo from W in simulated W-Mo solutions. The results showed that TD-modified Mt prepared in water (W-TD) and cyclohexane (C-TD) exhibited superior adsorption selectivity and amount for MoS42- over WO42- in single-solute systems. In lab-prepared simulated mixed W-Mo feed solutions, pre-sulfurization significantly enhanced Mo removal. Response surface methodology identified initial Mo concentration as the most significant negative factor affecting separation coefficient, while pH and sulfide concentration showed interactive or quadratic effects depending on the type of modified Mt. A stepwise dosing strategy further improved performance. With a low sulfide dosage, three-step addition with C-TD raised Mo removal to 96.3% with a residual [Mo]/[WO3] ratio of 3.2 × 10-4, superior to many reported methods. Adsorption of MoOxS4-x2- induced a conformational transition of alkyl chains from gauche to trans. Ion exchange and desorption-adsorption accounted for the selective removal of MoOxS4-x2-. Density functional theory calculations supported the greater binding affinity of TD+ to MoOxS4-x2- than WO42-, suggesting the high selectivity after moderate sulfurization. Based on the high stability, low sulfide requirement, and superior separation performance, organosilicon quaternary ammonium-modified Mt samples, particularly W-TD and C-TD, are promising for efficient and selective Mo removal from W-rich solutions, offering a potential alternative to conventional precipitation and solvent extraction methods.