HYDROXYAPATITE-BASED MATERIALS FOR ENVIRONMENTAL PROCESSES

In this dissertation, hydroxyapatite-based and hydroxyapatite with carbon composite materials have been rationally designed, synthesized, characterized and applied to processes of environmental concern, namely wastewater remediation (as sorbents) and electrochemical CO2 reduction reaction, CO2RR (as electrocatalysts). Pristine hydroxyapatite (HAP, Ca10(PO4)6(OH)2) and composite HAP/carbon materials have been synthesized according to a simple wet co-precipitation route. Composition, structure and morphology of samples have been investigated by a plethora of physical-chemical techniques (i.e. ICP/OES elemental analyses, N2 adsorption/desorption isotherms, XRD, TEM (and STEM) imaging, TEM/EDX mapping, transmittance FT-IR and Raman spectroscopies, and XPS). Since adsorption onto materials’ surface is crucial in both sorption and catalytic applications, a special attention has been devoted to surface properties, determined by means of zeta potential measurements, gas-solid calorimetric/volumetric titrations, and liquid-solid volumetric titrations, thus assessing the amphoteric nature of HAP-based surfaces. At first, pristine pristine HAP has been studied as sorbent for the remediation of inorganic pollution in wastewater (i.e. heavy metal cations such as Cu(II), Pb(II), Cr(III), Ni(II) and Co(II)). Stirred batch adsorption tests and collection of adsorption isotherms in solutions containing both individual and mixtures of heavy metal cations allowed to quantitatively assess HAP sorption ability. Ad-hoc experiments (i.e. microcalorimetric adsorption isotherms) and physical-chemical investigation of metal-loaded samples shed light on the adsorption mechanisms of several polluting species onto HAP surface. Eco-friendly HAP/carbon composites from renewable resources have been instead applied to the remediation of more complex effluents, where simultaneous organic and inorganic pollution may occur. Thanks to their dual nature, HAP/carbon composites exerted outstanding adsorption performances towards both class of polluting species. Both in the form of pristine HAP and HAP/carbon composites, the sorbents exhibited a strict retention of adsorbed pollutants when undergoing leach testing, therefore ensuring no secondary pollution issues. In an exploratory study, HAP/carbon composites have been also implemented as modifiers in self-standing and mechanically stable electrodes for the electrochemical detection of heavy metal cations traces in waterbodies. Benefitting from the pronounced HAP affinity towards such species, encouraging results have been obtained on the detection of some benchmark pollutants, although the registered limits of detection (ca. one order of magnitude improvement to match legislative requirements) and linearity ranges currently limit HAP/carbon modified electrodes applications. Finally, during a 6-months stay at University of California Irvine (UCI), hosted by Prof. Plamen Atanassov group, HAP has been applied as dopant in CO2RR electrocatalysts. The peculiar ability of HAP amphoteric surface to adsorb and destabilize CO2 (i.e. breakage of its linearity) has been exploited in composite electrocatalysts, composed by copper nanoparticles (active phase) and HAP (dopant), supported on N-doped 3D assembly of graphene nanosheets (3D-GNS, carbonaceous support). When tested in a lab scale CO2 electrolyzer, HAP-doped electrocatalysts displayed a faradic efficiency (FE) towards CO2RR ≥ 80% throughout the whole potential range under investigation, minimizing parasitic Hydrogen Evolution Reaction (HER). Interestingly, HAP doping actually altered CO2RR product distribution: in particular, small quantities of C2+ products were produced under low applied overpotential while at higher voltage formate production was boosted. Product distribution alteration has been rationalized taking into account the ability of HAP acid-basic surface groups to stabilize CO2 reduction intermediate species, directing the the selectivity of electrochemical CO2RR processes. Overall, the present study demonstrates that the unique features of HAP make it a versatile material which may be applied for the protection of the environment at 360 degrees, from water to air remediation, as pristine and/or composite material, as sorbent and/or catalysts’ dopant.