The “virtuous cycle” of hydroxyapatite : from remediation of heavy metal pollution to catalytic/electrocatalytic applications

Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) is a calcium phosphate which finds application from biomedical (as prothesis coating and filler) to heterogeneous catalysis (like Guerbet reactions, thanks to HAP amphoteric surface, and redox reactions on metal functionalized HAP). HAP presents good chemical/thermal stability, water insolubility (Kps ≈ 2.34∙10-59), and the ability to exchange Ca2+ ions with several metal ions (not only bivalent). HAP sorption ability can be exploited for de-metallation of polluted wastewaters; immobilization of heavy metal cations occurs with mechanisms and efficiencies dependent on operative conditions and metal species under study. Adhering to circular economy principles, metal-loaded HAP coming from de-metallation processes could have a “second life” in catalysis. For example, copper and iron loaded HAP display interesting activity in NOx abatement, while nickel containing HAP could be exploited as electrocatalyst, promoting small molecules oxidation.