Fixed-Bed Adsorption Comparisons of Bone Char and Activated Alumina for the Removal of Fluoride from Drinking Water

Abstract

Fluoride is commonly found at elevated concentrations in groundwaters worldwide and is difficult to remove even with activated alumina (AA) adsorption, the best available technology. Consequently, alternative treatment technologies for fluoride removal continue to be researched, including the use of bone char (BC) adsorption. However, BC studies are limited mostly to batch or equilibrium studies, which can be difficult to extend to full-scale applications. Therefore, the goal of this study was to compare a BC to a commercial AA for treating groundwater with a naturally occurring fluoride concentration of 8.5 mg/L using pilot- and bench-scale fixed-bed adsorption tests. At the pilot scale using an empty bed contact time (EBCT) of 10 min, fluoride breakthrough reached 1.5 mg/L within 450 bed volumes (3.1 days) and 650 bed volumes (4.5 days) for BC and AA, respectively. Two designs of the rapid small-scale column test (RSSCT) were applied to simulate the pilot columns, where the proportional diffusivity RSSCT (PD-RSSCT) design provided a more adequate prediction of fluoride breakthrough to 1.5 mg/L. The PD-RSSCT was also used to evaluate BC and AA EBCTs of 10 and 20 min, with the longer EBCT providing no significant increase in adsorbent use efficiency. Complete BC regeneration using countercurrent flow to the PD-RSSCT was limited by exposure to the sodium hydroxide regenerant solution and, potentially, the presence of arsenic.