Molecular dynamics simulations of the binging affinity of 1-hydroxyethane-1, 1-diphosphonic acid (HEDP) with nano-hydroxyapatite and the uptake of Cu²⁺ by HEDP-HAP hybrid systems

The adsorption capacities of different ratios of 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP) and nano-hydroxyapatite (HAP) hybrid systems on Cu²⁺ were probed. The FTIR, XRD, SEM and EDS analyses showed that HEDP with Cu²⁺ adsorbed on the surface of HAP and a new crystal phase appeared. The content of adsorbed Cu²⁺ were 4.4% and 21.8% on the surface of single HAP and HEDP-HAP-0.5 hybrid system, respectively, and later was 4.94 times that of the former. Conversely, the Ca/P (mol) ratio decreased from 1.40 to 0.61, indicating more Ca²⁺ were replaced by Cu²⁺. Meanwhile, molecular dynamics (MD) simulations results showed that HEDP and water molecules both formed ordered adsorption layer with similar concentration profiles, but the former preferred to gather on the HAP surface than the latter. The electrovalence bonds between the phosphonic acid functional groups of HEDP and Ca^2＋ of HAP surface played the dominant role in their adsorption. The adsorption results showed that the maximum adsorption capacity of single hydroxyapatite for Cu²⁺ was 40.32 mg/g, while the maximum adsorption capacities reached 99.11, 171.8 and 147.27 mg/g for HEDP-HAP-0.2, HEDP-HAP-0.5 and HEDP-HAP-1.0 hybrid systems, respectively. The study illustrated that the adsorption process accorded with the pseudo-second-order kinetic and Langmuir isotherm model.