Mechanisms of fixation and slow-release of 2,4-dichlorophenoxyacetic acid enhanced by biochar and MgAl-layered double hydroxide composites

The high mobility of 2,4-dichlorophenoxyacetic acid (2,4-D) reduces herbicidal efficacy and increases environmental risks. This study synthesized phosphorus-modified biochar and MgAl-layered double hydroxide composites (BC-LDHs) as a potential slow-release carrier for 2,4-D. BC-LDHs showed higher adsorption (156.35 mg g⁻ 1 ) and lower desorption (22.74%) than BC alone (128.41 mg g⁻ 1 , 43.55%). Adsorption mechanisms included pore filling, electrostatic attraction, π-π interactions, hydrogen bonding, and inner-sphere complexation, while release followed Fickian diffusion driven by different solvent concentrations. The adsorption binding energy of 2,4-D on BC-LDHs calculated by DFT was −0.90 eV. This indicated that 2,4-D has a strong reversible adsorption capacity, making it theoretically better suited to achieving slow release. Meanwhile, BC-LDHs exhibited greater stability with lower Mg 2+ release than LDHs alone. Transport experiments indicated that BC-LDHs retained 67.8% of 2,4-D, comparable to LDHs (69.4%) and significantly higher than BC (20.6%). Retention of 2,4-D increased under lower pH, reduced ionic strength, and Ca 2+ presence due to enhanced irreversible interactions. The weed control experiments confirmed that 2,4- D -loaded BC-LDHs matched commercial 2,4-D in short-term efficacy but provided superior long-term (>28 days) weed control. These findings provide valuable insights for developing novel slow-release herbicides based on BC-LDHs and reducing potential environmental risk.