ADSORPTIVE REMOVAL OF FLUORIDE AND NITRATE FROM SYNTHETIC WATER USING SURFACE MODIFIED LATERITE SOIL

Abstract

Drinking water containing high amount of fluoride and nitrate can cause serious health hazards to the people across the globe. Fluoride concentrations between 0.5-1.5 mg/l are beneficial, for the prevention of dental caries or tooth decay, but concentrations above 1.5 mg/l can cause fluorosis (dental or skeletal) and neurological disorders in severe cases. For, nitrate concentrations above 45 mg/l can cause methemoglobinemia, also known as “blue baby syndrome” especially to infants. Hence it is required to keep the concentrations of fluoride and nitrate well in the limits prescribed by WHO. Techniques available for fluoride and nitrate removal include adsorption, chemical treatment, electrochemical methods, dialysis and ion exchange process. Among them, adsorption is found to be effective, environmental friendly and economical. Under the above backdrop, the present study has been undertaken to prepare efficient fluoride and nitrate adsorbent from raw laterite by acid followed by alkali treatment. Batch experiments were performed to study the adsorption of fluoride and nitrate ions on Treated Laterite (TL) adsorbent with fixed size. Characterization of TL was carried out using various techniques such as BET Surface Area Analysis, FTIR, XRD, FE-SEM and EDAX. Effects of process parameters like pH, Adsorbent dose, contact time and initial ion concentration on the removal of fluoride and nitrate from synthetic water have been investigated using TL as adsorbent to determine optimum process conditions for maximum removal. The fluoride concentration of 10 mg/l is removed upto 86% by TL effectively. Optimum adsorbent dose is found to be 12.5 g/l. The optimum time is determined to be 150 min at optimum pH of 6. Adsorption of fluoride ions on TL has been found to pseudo-second model among pseudo-first order, pseudo-second order and Webber-Morris model. Among the conventional isotherms, such as Langmuir, Freundlich and Temkin isotherms, the Temkin isotherm gave better prediction of specific uptake for fluoride ions at equilibrium. For nitrate adsorption on TL, optimum adsorbent dose has been found to be 20 g/l for initial nitrate concentration of 100 mg/l at optimum pH of 5. TL could remove 80% nitrate ions effectively and the equilibrium was set in 120 min. Langmuir isotherm fitted well to the data than Freundlich and Temkin isotherm models and adsorption followed pseudo-second order kinetics.