High fluoride (F-), &gt;2 mg/L, in drinking water causes chronic fluorosis. Toxic F- levels are endemic to groundwater aquifers around the world and disproportionately affect poor, rural populations who lack access to technically involved or financially prohibitive treatment technologies. As of 2008, it is estimated that 200 million people worldwide have or risk incurring fluorosis because they consume water above the World Health Organization’s recommended level of 1.5 mg/L. Hydroxyapatite (HAP, Ca5(PO4)3OH) is an appropriate F- removal technology that has been used for the past 70+ years. Throughout this time environmental engineers have considered HAP an adsorbent, where F- exchanges with surface OH-. Yet, our group recently proved that F- also migrates to the HAP interior to exchange with OH- at internal lattice sites – a process we call internalization – by demonstrating quantitatively that long term experimental F- uptake significantly exceeds the adsorption capacity of carbonated HAP NCs. Adsorption sites were quantified by combining a particle size distribution analysis (using transmission electron microscopy, TEM) coupled with a high resolution TEM assessment of particle morphology and crystal habit to generate an evidence-based particle model. The reality of internalization prompts new questions regarding how to best utilize the material for optimal performance. Specifically, what are the structure-function relationships that affect F- uptake? Using our adsorption site quantitation approach, we assessed the internalization capability of HAP variants produced by heat treating the virgin material to 160 °C, 300 °C, 500 °C, and 900 °C. Internalization decreased with increasing heat treatment. We then investigated the relationship between physiochemical changes with heat treatment and its impact on uptake. Notably, while increased particle size and crystallinity play a role in decreased adsorption and perhaps internalization, the most substantial decrease in internalization occurred between the virgin material and HAP@300 °C where particle size and crystallinity are not significantly different. The only distinction we found between these variants was a correlation between structural water loss in the material (due to heating) and a contraction of the a/b unit cell parameter. This finding suggests that c-axis diffusion along the NC’s OH- channels is the primary internalization mechanism and that contraction of the channel diameter decreases diffusivity.

Structure-function relationships that dictate fluoride uptake in hydroxyapatite nanoparticles

ACS Fall 2020 Virtual Meeting

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