Evaluation of Bone Char Defluoridation of Water using Adsorption Isotherms and the Bed Depth Service Time (BDST) Model

Abstract

Sustained intake of elevated fluoride concentrations cause dental or skeletal fluorosis, a chronic disease manifested by mottling of teeth in mild cases and softening of bones and neurological damage in severe cases. Excessive and undesirable levels of fluoride in drinking water supplies are a major problem in the Rift Valley of Kenya. Fluoride concentrations range from 5 to 15 mg/L as compared to the WHO maximum permissible levels of 1.5 mg/L in drinking water. Current methods of fluoride removal from water include adsorption onto activated alumina, bone char and clay, precipitation with lime, dolomite and aluminum sulphate, ion exchange, and membrane processes such as reverse osmosis, electrodialysis and nano filtration. Most of these methods are expensive and technically non-feasible for rural communities in Kenya. This study evaluated the efficacy of locally available bone char in removal of fluoride from water. The bone char reduced fluoride concentration from 8.1 mg/L to within WASREB threshold of 3 mg/L for exceptionally high concentration of fluoride in 50 minutes and to within the WHO limit concentration of 1.5 mg/L in 2 hours. Fluoride removal was analyzed using adsorption isotherms; namely, Freundlich and Langmuir isotherms. Adsorption of fluoride on to bone char conforms to Freundlich Isotherm but not to Langmuir Isotherm indicating that the bone char media is not a homogenous media. Freundlich adsorbate capacity, Kf, increased with media size from 3.95 x 10-3 to 1.75 x 10-2 (L/g) for fine and medium size media, respectively, but adsorption intensity, 1/n, decreased from 1.657 to 1.043, which may explain the lack of significant difference between the fine and medium size media. To predict the fluoride breakthrough in a bone char fixedbed system, fluoride removal was modeled using the bed depth service time (BDST) model with bed capacity, No, of 79, 411 and 1269 mg/cm3 and adsorption rate constant, K, of0.139, 0.017 and 0.005 (cm3/mg-hr) for 80, 60 and 40 ml/L flowrates respectively. The study recommended further work on optimization of the continuous process with other adsorption configurations such as up flow fixed and moving bed.