Design and operation of fixed bed laterite column for the removal of fluoride from water

Laterite, the geomaterial, is used as an adsorbent for the removal of fluoride from drinking water. The equilibrium data obtained through batch study is fitted to the Langmuir and Freundlich isotherm. The values of thermodynamic constants and the separation factor indicate that fluoride removal on laterite is feasible. The solution movement through the laterite column bed was predicted from the mass balance concept. The nature of breakthrough curve was found to be influenced by several operational parameters, adsorption capacity and adsorption rate. The governing influence of primary adsorption zone formation was characterized by different secondary parameters. The laterite bed capacity under dynamic condition was determined from the breakthrough curve and is higher compared to that evaluated from static mode. Sharp breakthrough curve was obtained at higher feed fluoride concentration and lesser bed height. Column capacity does not change up to a flow rate of 6.5 cm3 min−1 even for the highest concentration studied. It is found that a maximum of 80.4% of retained fluoride was eluted at pH ≥ 9.8. In a typical set with initial fluoride concentration of 20 mg dm−3 and bed height 20 cm the retention–elution process was repeated for a number of times. It is found that the capacity though decrease with the cycle number, it remains within 68% after five cycles.