An Assessment of Soil Materials in Kenya as Sorbent Media for Defluoridation of Water

Abstract

Three clay soil materials obtained from different parts of Kenya were studied for their feasibility as adsorption media for fluoride removal from water through batch tests. The soil materials were characterized by particle size distribution based on sieve analysis and oxides fraction percentage determined by X-ray fluorescence analytical technique. The soils clay content ranged from 68% to 75% and dominant oxides were SiO2(39-53%), Al2O3(35-39%) and Fe2O3(8-22%). Soils thermally treated at temperatures ranging from 500 to 850o C as well as pristine soil samples were compared for their fluoride adsorption efficiency. Fluoride removal efficiency of the soil materials increased with the clay content and the total percentage of the dominant oxides. The soils as collected in the field were of low permeability and; achieved very low fluoride removal of between 27-48%, whilst materials calcined at 650oC had the highest removal efficiency of between 52-80%, using raw water of 5mg/l fluoride concentration. Optimum conditions were achieved at pH 6, media dosage of 8g/200ml and contact time of 1hr. The soil sample MN obtained from Meru North achieved the highest removal of 80%. The Langmuir isotherm analysis indicated both the soil materials from Meru North and Nyeri to be feasible as an adsorption media for fluoride removal and further tests and actual performance evaluation will be undertaken on the Meru North soil.

Key words: fluoride, adsorption, calcined clay material, isotherm