Ultrasonic Synthesis of Magnesium-Iron Layered Double Hydroxides and Their Sorption Properties Toward Chromate Anions

Layered double hydroxides (LDHs) are promising anion sorbents, but conventional Mg-Fe LDH synthesis requires prolonged aging. The effects of ultrasound application stage on Mg-Fe LDH microstructure and chromate uptake remain insufficiently clarified. This study compared ultrasonic treatment during and after coprecipitation and related XRD-derived microstructural descriptors to Cr(VI) sorption. Mg-Fe LDHs were synthesized using 28 or 40 kHz ultrasound during or after coprecipitation and 1.7 MHz ultrasound after coprecipitation; 24 h thermal aging was used as a reference. The products were characterized by ICP-MS, FTIR, TGA/DSC, SEM, and XRD and tested for chromate adsorption, kinetics, recyclability, multicomponent-solution performance, and soil Cr(VI) immobilization. Fifteen minutes of ultrasonication yielded Mg/Fe ≈ 2 LDHs and shortened synthesis compared with 24 h aging. Ultrasound during coprecipitation at 28 kHz gave the best sorbent, increasing experimental adsorption capacity to 80.35 mg/g versus 53.70 mg/g for the reference LDH. Sorption followed pseudo-second-order kinetics and was best described by the Freundlich model. In a multicomponent solution, this sample removed 68% Cr(VI) at 1.0 g/L and reduced water-soluble Cr(VI) in soil from 14.31 to 0.26 mg. Ultrasound application during coprecipitation improves Mg-Fe LDH structure-related characteristics and chromate sorption.