Phosphate-modified attapulgite for efficient Pb(II) and Cd(II) removal: surface functionalization and adsorption mechanisms.

Heavy metal ions such as Pb(II) and Cd(II) in aquatic environments and contaminated soils pose severe risks to ecosystems and human health, thereby necessitating their effective removal and immobilization. In this study, a phosphate-functionalized attapulgite material (C-ATP) was synthesized through a calcination-assisted modification strategy, achieving efficient phosphate incorporation while preserving the intrinsic crystalline framework of attapulgite (ATP). The resulting C-ATP exhibited significantly enhanced maximum adsorption capacities for Pb(II) and Cd(II), with increases of 80.89 % and 21.44 %, respectively, compared to unmodified ATP. Moreover, C-ATP exhibited remarkable adsorption stability over wide variations in pH and temperature, while sustaining high adsorption performance even under elevated concentrations of competing Ca(II) ions. In soil remediation experiments, C-ATP achieved immobilization efficiencies of 77.9 % for Pb(II) and 35.59 % for Cd(II) within 7 days, which were 2.8 and 1.5 times higher than those of unmodified attapulgite, and it maintained stable solidification performance after 56 days. X-ray photoelectron spectroscopy (XPS) analysis verified that the phosphate functionalities were actively involved in the adsorption of heavy metal ions through the generation of inner-sphere coordination complexes at the material surface. Atomic force microscopy (AFM) further revealed that phosphate groups exhibited stronger binding affinity toward Pb(II) than the original hydroxyl groups on the attapulgite surface, providing a molecular-level explanation for the superior performance of C-ATP. This work provides a simple and cost-effective strategy for fabricating phosphate-modified attapulgite, elucidating the molecular mechanism by which phosphate groups enhance adsorption performance, and offering new insights into efficient heavy metal removal and contaminated soil remediation.