Adsorption of fluoride onto mixed rare earth oxides

Fluoride pollution of water is widespread in several parts of India. Fluoride although beneficial to humans in small quantities, causes dental fluorosis when consumed in larger quantities over a period of time. In the present study a novel adsorbent was used to remove fluoride from synthetic solutions. The adsorbent, which is a mixture of rare earth oxides, was found to adsorb fluoride rapidly and effectively. The effect of various parameters such as contact time. initial concentration, pH. and adsorbent dose on adsorption efficiency was investigated. More than 90% of the adsorption occurred within the first 5 - 10 min. Adsorption efficiency was found to be dependent on the initial fluoride concentration and adsorption behavior followed Langmuir adsorption model. The optimum pH was found to be about 6.5. The presence of other ions such as nitrate and sulphate did not affect the adsorption of fluoride significantly (adsorption efficiency reduced from 85 to 79%) indicating the selective nature of the adsorbent. The adsorbed fluoride could be easily desorbed by washing the adsorbent with a pH 12 solution. This study clearly shows the applicability of naturally occurring rare earth oxides as selective adsorbent for fluoride from solutions. (C) 2001 Elsevier Science BN. All rights reserved.