Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya

Abstract

Adsorption of Fluoride (F) from aqueous solutions using acid treated diatomaceous earth (ATDE) from a mining site in Kenya was studied using batch experiments. The effect of F concentration, adsorbent dosage, contact time, temperature,pH and competing anions was evaluated. The adsorption process was fast and an initial equilibrium could be attained in just 10 min. Fluoride adsorption onto ATDE increased strongly from about 40% to over 92% when the solution temperature was raised from 293 to 303 K. The process was however, less responsive to pH changes dropping by a margin of less than 1% from 98.8% to 98% when the solution pH was raised from 1.59 to 6.89. It was clear that increase in concentration of OH – ions did not affect F adsorption onto ATDE strongly. On the other hand, apart from the Cl – ions which slightly reduced F adsorption onto ATDE, there was no obvious effect of the  , 2 4 SO  3 NO and  3 4 PO ions on F uptake by ATDE. Complete F removal (100% adsorption) could be achieved at 400 mg/L initial F concentrations using 0.5 g/mL ATDE batch loading, 303 - 313 K and pH = 3.4 ± 0.2. The F adsorption data correlated to the Freundlich and Langmuir models and could be classified as H-Type according to Giles classification of isotherms. The maximum Langmuir F adsorption capacity of ATDE was 51.1 mg/g indicating that the mineral could be used as an inexpensive adsorbent for the removal of F ions from aqueous streams.