Defluoridation of drinking water using chitin, chitosan and lanthanum-modified chitosan

Chitin, a naturally occurring abundant biopolymer, and its deacetylated product chitosan has been widely used for metal adsorption from water and wastewaters, but the application of these materials as an adsorbent to remove fluoride from drinking water has seldom been explored. In this work the applicability of chitin, chitosan and 20%-lanthanum incorporated chitosan (20% La-chitosan) as adsorbents for the removal of excess fluoride from drinking water was studied. The effects of various physico-chemical parameters such as pH, adsorbent dose, initial fluoride concentration and the presence of interfering ions on adsorption of fluoride were studied. It was observed that the uptake of fluoride was maximum at original pH (pH 6.7). The equilibrium adsorption data were fitted reasonably well for Freundlich isotherm model. The presence of chloride, sulfate, carbonate and bicarbonate ions in drinking water greatly affect the uptake of fluoride indicating that these anions compete with sorption of fluoride on 20% La-chitosan. The rate of adsorption was rapid and maximum fluoride uptake was attained within 20 min. The mechanism of adsorption of fluoride on lanthanum-modified chitosan is also explained. The comparison of uptake of fluoride in distilled water and field water shows relatively higher uptake of fluoride in distilled water. This could be due to the competing effect of other anions present and higher pH of the field water.