Comparative adsorption mechanism and transport behaviors of 2,4-D and 4-CPA in soil column with addition of Mg-Al layered double hydroxides

• 2,4-D and 4-CPA retention in soil-LDHs columns increased as pH and IS decreased. • Carbonate and decreasing IS would release 2,4-D and 4-CPA adsorbed by LDHs. • 2,4-D and 4-CPA caused each other to release in soil-LDHs columns. • LDHs exhibited better loss control on simultaneous application of herbicides. 2,4-Dichlorophenoxyacetic acid (2,4-D) and 4-chlorophenoxyacetic acid (4-CPA) exhibit high water solubility and mobility, leading to low utilization efficiency and environmental contamination. Research into controlling the migration and leaching of multiple pesticides in complex soil environments remains relatively limited. This study employed Mg-Al layered double hydroxides (LDHs) to control the loss of 2,4-D and 4-CPA simultaneously. Here, adsorption kinetics experiments, density functional theory (DFT) calculations, column experiments and numerical models were conducted to investigate the adsorption mechanisms of the two herbicides onto LDHs, the co-/transport and co-/release behaviors of the herbicides in soil with the addition of LDHs. Outer-sphere complexation was the predominant adsorption mechanism for 4-CPA with LDHs, while outer-sphere and inner-sphere complexation occurred between 2,4-D and LDHs. DFT calculations indicated lower adsorption energy and greater adsorption strength between LDHs and 2,4-D compared to that between LDHs and 4-CPA. The retention of two herbicides in the soil increased by 8.99–22.46 % with 0.5 wt% LDHs and increased with the decrease of pH values and ionic strength (IS) of soil solution. The release amounts of 4-CPA and 2,4-D from LDHs in soil columns increased by 9.06 % and 12.05 % when the IS of K + decreased from 100 to 0 mM, and increased by 11.12 % and 15.49 % in the presence of 25 mM carbonate. LDHs exerted a greater loss control effect on the simultaneous application of both herbicides. Either 2,4-D or 4-CPA would lead to the release of each other with the addition of LDHs. Pot experiment results further verified that the addition of LDHs in soil could reduce the loss of the herbicides by up to 77.29 %, which brought greater germination control of Abutilon theophrasti . This study provides novel mechanistic insights into the differential adsorption and transport of coexisting herbicides controlled by LDH S under complex environmental conditions, offering promising strategies for enhancing herbicide efficacy and mitigating environmental risks in sustainable agriculture.