Probing structure-property relationships of calcium hydroxyapatite defluoridation to enhance performance

Fluoride (F-) is one of the most significant inorganic contaminants endemic to groundwaters worldwide. An estimated 200 million people, mostly in rural low-income regions, have or risk incurring fluorosis because they consume water with F- levels above the World Health Organization’s (WHO) recommended level of 1.5 mg/L. Calcium hydroxyapatite (HAP, Ca5(PO4)3X, where X=OH) nanoparticles (NPs) formed into pellets and used in fixed-bed column reactors are among a handful of technologies recommended by the WHO for low income contexts. In the environmental engineering discipline, HAP has historically been considered a F- adsorbent, understood as surface-limited uptake by replacement of OH- at surface-terminated X lattice sites. However, this work demonstrates that HAP NPs not only adsorb but also internalize F- into the bulk of its structure under environmentally relevant conditions (i.e. pH=5-9 and [F-]=1.5-30 mg/L) by the migration of F- to subsurface X and defect lattice sites, yielding fluoro-hydroxyapatite solid solutions (FHAP, Ca5(PO4)3X, where X=OH/F). The practical implication is that there is a potential four to ten-fold increase in F- removal capacity with the utilization of bulk sites. To accomplish this (Chapter 2), an array of experimental techniques were employed to develop a robust particle model to quantify the adsorption and total (i.e. adsorption and bulk) F- capacity specific to the B-type carbonated HAP sample under investigation. Comparison with batch test F- removal revealed uptake far exceeding the adsorption capacity, thereby indirectly validating the occurrence of F- internalization. Rietveld refinement of X-ray diffraction patterns yielded apatite unit cell parameter values a and c. Consistent with the fact that the a parameter of fluorapatite (FAP, Ca5(PO4)3X, where X=F) is significantly smaller than that of HAP, the a parameter of fluoridated HAP samples decreased with increased F- uptake, providing strong physical evidence of F- internalization. Time-resolved quantitative 19F and 1H solid-state Nuclear Magnetic Resonance spectroscopy (NMR) showed the speciation and quantities of removed F- as well as the corresponding losses of HAP OH-, suggesting that a significant portion of F- inserted into conventional and alternative lattice sites occurs without direct replacement of OH-. One dimensional and two dimensional 1H and 1H-19F NMR techniques demonstrated that the removed F- was heterogeneously distributed within the HAP NPs, indicating strong F- concentration gradients are formed at the NP surface with an inwardly migrating boundary. An attempt was made to directly observe the F- distribution in single fluoridated HAP NPs by utilizing a nascent and powerful microscopy technique, atom probe tomography (APT), at the Australian Centre for Microscopy and Analysis (Chapter 3). The particle model framework was used to investigate F- uptake for a number of HAP variants, revealing that F- does not internalize equally or sometimes at all in different types of HAP (Chapter 4). An investigation of their physicochemical properties helped to isolate factors that significantly affect both F- adsorption and internalization. This led to better insights about the potential mechanism of F- internalization as well as ways in which HAP NPs can be synthesized for enhanced performance.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Environ Engr in Civil Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Mosiman, Daniel S

Contributors dc:contributor

• Marinas, Benito J
• Bellon, Pascal
• Espinosa-Marzal, Rosa
• Cusick, Roland

• hydroxyapatite
• fluoride
• water treatment

Statement dc:rights

• Copyright 2021 Daniel Mosiman

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/113019

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/113019