Mechanistic insights into heavy metal adsorption on hydroxyapatite: New perspectives from dynamic adsorption behavior

In the present study, the dynamic adsorption behavior and mechanisms of hydroxyapatite (HAP) toward Pb(II), Cd(II), and Cr(III) was systematically investigated, with a focus on the transitions of interfacial mechanisms. Kinetic analysis revealed a concentration-dependent shift from pseudo-second-order to Elovich models, indicating a transition from chemisorption to complex heterogeneous interfacial processes. The adsorption of Pb²⁺ was primarily governed by a dissolution-precipitation mechanism and lattice substitution, with ion exchange dominating at later stages. Cd²⁺ adsorption was governed by competitive dissolution-precipitation and surface reactions, exhibiting a temperature-sensitive transition from physical to chemical adsorption. Cr³ ⁺ adsorption was mainly controlled by ion exchange with limited structural disruption. Thermodynamic simulations and time-resolved solution chemistry analysis confirmed these stage-specific transitions and their dependence on metal type, concentration, and reaction time. These findings provide novel insights into the dynamic mechanism shift during heavy metal-HAP interactions, highlighting the necessity of multi-stage process understanding for optimizing heavy metal removal strategies.