Assessing changes in the physico-chemical properties and fluoride adsorption capacity of activated alumina under varied conditions

• Published in: Applied geochemistry 2017-01, Vol.76, p.112-123
• Main Authors: Craig, Laura, Stillings, Lisa L., Decker, David L.
• Format: Article
• Language: English
• Subjects: Activated alumina; Adsorption; Fluoride; Surface complexation
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2016.11.011

Adsorption using activated alumina is a simple method for removing fluoride from drinking water, but to be cost effective the adsorption capacity must be high and effective long-term. The intent of this study was to assess changes in its adsorption capacity under varied conditions. This was determined by evaluating the physico-chemical properties, surface charge, and fluoride (F−) adsorption capacity and rate of activated alumina under conditions such as hydration period, particle size, and slow vs. fast titrations. X-ray diffraction and scanning electron microscopy analyses show that the mineralogy of activated alumina transformed to boehmite, then bayerite with hydration period and a corresponding reduction in adsorption capacity was expected; while surface area analyses show no notable changes with hydration period or particle size. The pH dependent surface charge was three times higher using slow potentiometric titrations as compared to fast titrations (due largely to diffusion into pore space), with the surface acidity generally unaffected by hydration period. Results from batch adsorption experiments similarly show no change in fluoride adsorption capacity with hydration period. There was also no notable difference in fluoride adsorption capacity between the particle size ranges of 0.5–1.0 mm and 0.125–0.250 mm, or with hydration period. However, adsorption rate increased dramatically with the finer particle sizes: at an initial F− concentration of 0.53 mmol L−1 (10 mg L−1), 90% was adsorbed in the 0.125–0.250 mm range after 1 h, while the 0.5–1.0 mm range required 24 h to achieve 90% adsorption. Also, the pseudo-second-order adsorption rate constants for the finer vs. larger particle sizes were 3.7 and 0.5 g per mmol F− per min respectively (24 h); and the initial intraparticle diffusion rate of the former was 2.6 times faster than the latter. The results show that adsorption capacity of activated alumina remains consistent and high under the conditions evaluated in this study, but in order to increase adsorption rate, a relatively fine particle size is recommended. [Display omitted] •Activated alumina transforms to boehmite and bayerite with hydration period.•The fluoride adsorption capacity is not diminished with hydration period.•Fluoride adsorption rate increases dramatically with decreased particle size.