Abstract
Batch kinetic tests and continuous-flow column experiments were conducted to study fluoride adsorption on porous hydroxyapatite ceramics. Batch tests showed that fluoride uptake occurred faster for smaller adsorbent particles, which is expected for ceramics with internal sorption sites. Columns with different flow rates had different breakthrough curves. Fluoride loading on the adsorbent (q) at point of exhaustion (Ceff = 0.85 C0) increased with decrease in flow rate, while the mass loading at breakthrough (Ceff = 0.15 C0) stayed nearly constant. Flow interruption at different points led to a temporary decrease in the effluent fluoride concentration demonstrating that the columns experienced nonequilibrium conditions, and intraparticle diffusion played a significant role in fluoride uptake. The Rapid Small-Scale Column Test (RSSCT) concept was tested as a scale-up approach. Small-scale columns were designed using the constant diffusivity (CD) and proportional diffusivity RSSCT approaches, with CD found to be the more suitable approach to predict the breakthrough of a large-scale column. This serves to further validate the use of porous hydroxyapatite ceramics for fluoride uptake and provides insights into the design of full-scale systems.
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