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.