Application of red mud acid activation treatment leaching solution for preparation of functionalized layered double hydroxides

Abstract

The conventional metal recovery process is challenged by the polymetallic, low-concentration acid leachate derived from the acid activation treatment of red mud (RM). Since the elemental requirements for layered double hydroxide (LDH) synthesis are aligned with the ionic profile of these leachates, a sustainable circular strategy is proposed in this study to transform RM-derived acid leachate into functional LDH for heavy metal remediation. Complete dissolution of alkaline minerals and selective extraction of Ca²⁺ and Al³⁺ were enabled through optimization of HCl leaching parameters, while the dissolution of Fe, Si, and Ti was effectively suppressed. Subsequent synthesis optimization enabled near-complete metal mineralization (99.9 % efficiency) into LDH structures. Advanced characterization (XRD, FTIR, XPS) revealed the synthesized material (designated RM-LDH) was Ca₂Al-LDH with Fe³⁺/Ti⁴⁺ dopants in cationic layers, and exhibited enriched active sites through variable valence metal incorporation. The RM-LDH demonstrated exceptional adsorption capacities of 665.99 mg/g (Cd²⁺) and 369.5 mg/g (Cu²⁺), achieving rapid removal through isomorphic substitution of Ca²⁺ by heavy metals, coupled with hydroxyl coordination and electrostatic attraction. In multi-ion systems, RM-LDH displayed selective adsorption in the order: Fe³⁺ > Cu²⁺ > Cr³⁺ > Zn²⁺ > Cd²⁺ > Co²⁺. This waste-to-resource strategy concurrently addresses RM valorization and heavy metal remediation.