Magnetic biochar-supported layered double hydroxide for simultaneous remediation of As and Cd in soil: Effectiveness, retention durability, and insight into a new immobilization mechanism

Immobilization/stabilization is a common technique for remediating As and Cd-contaminated soil. However, the effectiveness of employed stabilizer plays a crucial role in determining the fate of these contaminants. This study investigated the effectiveness and durability of a magnetic biochar-supported Ca–Mg–Al layered double hydroxide composite (FLBC) and its components in simultaneous remediation of As and Cd in Phaeozem (BS) and Krasnozem soils (RS). Results demonstrated that 5 wt% FLBC amendment immobilized >85% of As and Cd and converted them into immobile fractions even in a reduction environment. Moreover, FLBC treatment effectively resisted leaching of As and Cd by > 70% under acidic rainwater filtration, thus displaying excellent retention durability. The new immobilization mechanisms were proved using transmission electron microscope technology, where layered double hydroxide (LDH) acted as a nano-bridge to bind FLBC and soil colloids ensuring FLBC stability and synergistic immobilization. During this process, X-ray photoelectron analysis demonstrated the re-oxidation of As(III) to As(V) driven by ferric reduction that resulted in the formation of multiple stable species on the soil surface via the confirmation of X-ray absorption near fine structure and extended X-ray absorption fine structure. This species included Ca/Fe/Cd–As(V) and γ-Cd(OH)2 co-precipitation, as well as As(V)–O–Fe(III)/Ca and As–Fe tridentate-hexanuclear corner-sharing complexes. Furthermore, FLBC amendment favored the immobilization of As and Cd in RS soil compared to BS soil due to a weaker reduction potential and an increased participation of metal-(hydr)oxides for As and Cd complexation. This study provides valuable references for predicting the impact of LDH-coupled magnetic biochar technology on the fate of As and Cd in actual soil environments.