Efficient absorption properties of surface grafted HEDP-HAP composites for Pb²⁺ and Cu²⁺: Experimental study and visualization study of interaction based on Becke surface analysis and independent gradient model

A simple hydrothermal method was employed to prepare the surface grafted 1-hydroxyethylidene-1,1-diphosphonic acid-hydroxyapatite (HEDP-HAP) composites, which were termed as 0.2HEDP-HAP, 0.5HEDP-HAP and 1HEDP-HAP respectively, according to the HEDP content. Adsorption experiments showed that the maximum adsorption capacity of 0.5HEDP-HAP for Cu²⁺ reached 168.9 mg/g, which was 4.19 times that of HAP, while the maximum adsorption capacity of 1HEDP-HAP for Pb²⁺ was as high as 1521 mg/g, which was approximately 10 times that of HAP. For the first time, the interaction mechanism of HEDP with HAP, or HEDP-HAP composite with the two heavy metal ions was illustrated by the combination of quantum chemistry (QC) calculation, quantitative analysis of molecular surface, Becke surface analysis and Independent Gradient Model (IGM) method. The active reaction sites, the types, relative strength and essence of the interactions were revealed based on the visual analysis of interactions. The results demonstrated that the phosphonic acid group (PO₃H⁻) of HEDP^2- can form hydrogen bonds with the phosphate group (PO₄^3-) and the hydroxyl group (OH) of HAP. The oxygen atom of PO₃H- provided lone pair electrons to form chelate with Ca²⁺ of HAP, which resulted in the strong affinity of HEDP with HAP. Moreover, the significant improvements of uptake of surface grafted HEDP-HAP for Cu²⁺ and Pb²⁺ were due to the formation of the ternary surface complexes.