Removal of fluoride from spiked water in the batch or static mode and also in the column or dynamic mode

Abstract

Mature leaves of the neem tree (Azadirachta indica) were converted into fine powder, the neem leaf powder (NLP), which was used for adsorption of fluoride from water. Adsorption was 82.6% at pH 2.1, 97.5% at pH 7.0 and 99.4% at pH 10.0 at a fluoride concentration of 15 mg/L. Adsorption kinetics conformed to pseudo-second-order model with a rate coefficient of 2.04 × 10−2–11.83 × 10−2 min/mg/g for a NLP amount of 1.0–6.0 g/L. The adsorption enthalpy, ∆H, decreased slightly from 26.06 to 24.85 kJ/mol as the fluoride concentration increased (2.50–20 mg/L). In the same concentration range, the adsorption entropy, ∆S, varied from −66.92 to 86.47 J/mol/K. Spontaneity was ensured by Gibbs energy decrease from −5.75 to −0.99 kJ/mol (fluoride 15.0 mg/L) and from −1.79 to −2.65 kJ/mol (fluoride 20.0 mg/L) in the temperature range of 308–318 K. However, at lower concentrations of fluoride, ∆G had values >0 which still decreased with increasing temperature. The influence of competing ions on fluoride adsorption by NLP showed a significant influence of Cl−, HCO3-, SO42-, NO3- and HPO4- ions. Dynamic study on a NLP column showed a breakthrough volume of 2,800 mL for a fluoride loading of 20 mg/L, a flow rate of 0.4 mL/min and a column bed depth of 1.5 cm.