In-depth study of the mechanism of heavy metal trapping on the surface of hydroxyapatite

n the present study, the efficiency and selectivity and the role of surface groups of hydroxyapatite in the Cr(III)trapping, present in solution as single species or in co-presence of Ni(II) and Pb(II), have been investigated.Transmission electron microscopy (TEM) analysis and synchrotron radiation X-ray powder diffraction (XRPD) mea-surements provided comprehensive descriptions of the HAP surface and structure. The surface chemical properties,densityandstrengthoftheacidandbasesitesofHAP,havebeendeterminedby2-phenylethylamineandbenzoicacidprobeadsorption,respectively.TheamphotericpropertiesofHAPturnedovertoonlyacidpropertyinwatersolution.ThesorptioncapacityofHAPhasbeenbatch-testedinsimulatedpollutedwaterscontainingCr(III)atdifferentinitialconcentrations(fromca.0.3toca.6mM)andpHvalues(from4to9).Competitiveadsorptionphenomenainmulti-metalsystems(Cr(III)withNi(II)andPb(II))havealsobeenstudied.Anappropriatesetofanalyticaltechniques,XRPD,TEM, STEM-EDX, N2adsorption-desorption analyses, FT-IR spectroscopy, have been used to characterize the metalloaded hydroxyapatite samples. Different mechanisms of metal uptake were active depending on the nature of themetalspeciesandpHvalueofsolutions:surfacecomplexation,precipitation,ion-exchange,ordissolution-precipitation.