Adsorption performance and mechanism of MgAl-LDHs@FRBCs/Fe3O4 composite for multiple heavy metal ions in the aqueous extract of Ligusticum chuanxiong Hort.

Heavy metal contamination in traditional Chinese medicines (TCMs) and their preparations is an urgent quality and safety issue to be addressed in the industrialization of TCMs. Herein, a novel magnetic composite of MgAl-LDHs@FRBCs/Fe3O4 was prepared as the adsorbent, which was fabricated by using the leaves (Isatidis Folium) and roots (Isatidis Radix) of Isatis indigotica as the carbon source for biochar (FRBCs), followed by in situ growth of MgAl-layered double hydroxides (MgAl-LDHs). In the aqueous extract of Ligusticum chuanxiong Hort. (CX), the composite exhibited high simultaneous removal efficiencies for multiple heavy metal ions: 96.53% for Pb2+, 78.49% for Cu2+, 83.82% for Cd2+, and 35.48% for Ni2+. The adsorption process fitted better with the pseudo-second-order (PSO) kinetic model and Langmuir isotherm, indicating that the adsorption was dominated by monolayer chemisorption. Meanwhile, a certain correlation with the pseudo-first-order (PFO) kinetic model was also observed, reflecting the participation of the physisorption. Thermodynamic parameters (ΔH = 25.84-105.42 kJ mol-1, ΔS = 122.15-386.15 J mol-1 K-1, ΔG = -17.37 to -9.65 kJ mol-1) confirmed a spontaneous and endothermic process with increased disorder. Further mechanism investigation by pH-dependent experiments, zeta potential measurements, and XPS revealed that the adsorption occurred mainly through electrostatic interactions and chemical complexation. Importantly, HPLC analysis revealed no significant alteration of the major bioactive components in the CX extract after adsorption. The composite also exhibited fast adsorption kinetics, high removal efficiency under competitive conditions, good stability and reusability, and easy magnetic separation. The current work provides a high-performance adsorbent for heavy metal removal from TCM aqueous extracts, which also offers a sustainable strategy for valorizing TCM residues.