REMOVAL OF FLUORIDE FROM DRINKING WATER BY ADSORPTION TECHNIQUE

Abstract

Fluoride content in groundwater, greater than the WHO limit of 1.5 mg/L, causes dental and, skeletal fluorosis. Concentrations in the range of 1.5-4 mg/L result in dental fluorosis whereas with prolonged exposure at still higher fluoride concentrations (4-10 mg/L) dental fluorosis progresses to skeletal fluorosis. High fluoride concentrations in groundwater, up to more than 30 mg/L, occur widely, in many parts of the world. In India, several states are affected with excess fluoride in groundwater. Adsorption of fluoride was studied using three adsorbents namely, bagasse fly ash (BFA), Native rice straw (NRS) and Modified rice straw (MRS). BFA is a waste material obtained from the flue gas of the bagasse-fired boilers of sugar mills. Rice straw was thermo chemically modified with citric acid (CA) as esterifying agent. The adsorbent has been characterized with. BET, FTIR, XRD, TGA, SEM and EDAX studies. Batch adsorption studies were conducted to evaluate the effect of various parameters such as pH, adsorbent dose, contact time, temperature and initial fluoride concentration on the removal of fluoride. The effective pH was 6 (BFA) and 4 (NRS, MRS) for adsorption of fluoride by the three adsorbents. Equilibrium reached in about 7 h (BFA), 6 h (NRS) and 5 h (MRS) contact time. Optimum BFA, NRS and MRS dosages were found to be 10, 10 and 7.5 g/L, respectively. For 15 mg/L of fluoride solution, the percentage fluoride removed is up to 80.5%, 86.1% and 91.7% by BFA, NRS and MRS adsorbents, respectively. The adsorption followed pseudo-first-order and pseudo-second-order kinetics. Equilibrium adsorption data on BFA, NRS and MRS were analyzed by Freundlich and Langmuir isotherm equations using regression analysis.