ECO-FRIENDLY HYDROXYAPATITE-BASED MATERIALS AS METAL ADSORBENTS: OPTIMIZATION OF SURFACE PROPERTIES FOR WATER AND WASTEWATER REMEDIATION

Water protection is a global priority due to its critical role in human health and environmental sustainability. One of the 17 United Nations Sustainable Development Goals (SDGs) focuses on clean water and sanitation, with a key target of improving water quality by reducing pollution and limiting the release of hazardous chemicals. This doctoral dissertation, conducted in collaboration with the national water management company A2A, addresses the challenge of water pollution by focusing on hexavalent chromium (Cr(VI)) contamination – an environmental hazard known for its high toxicity even at low concentrations. Among available remediation techniques, reductive adsorption is gaining attention as it combines both reduction and adsorption into a single process, necessitating the development of tailored adsorbent materials. The aim of this work is to design and develop tin-modified calcium phosphate-based adsorbents for the removal of Cr(VI). Two distinct synthetic approaches were employed, yielding two different materials: (i) Sn-modified hydroxyapatite (Sn/HAP) – prepared by surface deposition of tin species on preformed hydroxyapatite – and (ii) tin-enriched calcium phosphate (Sn-CaP) – prepared by a one-pot co-precipitation. A comprehensive characterization study using XRPD, N2 adsorption/desorption isotherms, XPS, 119Sn Mӧssbauer spectroscopy, TEM, and HAADF-STEM/EDS provided insights into tin speciation, structural composition, and morphological features, revealing structural differences between the materials. Sn/HAP was found to be crystalline, indicating that Sn surface deposition did not alter the crystalline structure of HAP. In contrast, the one-pot synthesis strategy resulted in a multiphasic material (Sn-CaP), which contained both amorphous and crystalline phases, specifically amorphous calcium phosphate, hydroromarchite (Sn6O4(OH)4), and cassiterite (SnO2). Despite their structural differences, both adsorbents proved to be efficient in the reductive adsorption of Cr(VI), underlining the crucial role of tin species. In particular, the oxidation state of Sn was found to be a key factor, with Sn2+ playing a pivotal role in the reductive adsorption process. Remarkably, the adsorbents maintained their Cr(VI) removal efficiency even in the presence of competing ions, highlighting their potential for real-world water treatment applications. In addition to Cr(VI) removal, this thesis explored strategies for the valorization of spent adsorbents, including recycling and upcycling approaches. Notably, spent Sn-CaP was successfully repurposed as a catalyst for environmental gas-phase reactions. Specifically, the upcycled catalyst exhibited moderate activity in the oxidation of NO to NO2, a key reaction in NOx abatement. This finding underscored the potential for circular economy applications by transforming spent adsorbents into functional materials for environmental use. Overall, this dissertation advances current water treatment strategies by demonstrating the feasibility of Sn-modified calcium phosphate adsorbents for treating Cr(VI)-contaminated water, even in complex matrices. Furthermore, the successful upcycling of spent adsorbents into catalysts aligns with sustainability principles, offering a promising route for waste valorization in environmental applications.