Kinetic and isotherm studies on adsorption of fluoride by limonite with batch technique

Fluoride in drinking water has an intense effect on bones and teeth. The concentration of fluoride >1.5 mg l-1 in drinking water is harmful for human health. In the present study the naturally occurring geomaterial limonite (Iron Ore) has been used as the adsorbent for the investigation of defluoridation capacity by Batch adsorption technique in aqueous condition. Various parameters such as contact time, adsorbent dose, adsorbate concentration, adsorbent particle size and agitation speed at room temperature (298K) and pH (7?0.3), has been optimization for the adsorption of F- onto limonite. The initial rate of the adsorption was very fast and after 120 minutes it attained equilibrium. Three known Isotherm Models (Langmuir, Freundlich and Temkin) used to study the mechanism and nature of adsorption onto the surface of limonite. The experimental results are best fitted into the Langmuir and Temkin isotherm models (R2> 0.99). The average monolayer adsorption capacity (Qm) obtained for limonite is 0.269 mg g-1. The experimental data highlights that surface sites of adsorbent (limonite) are heterogeneous in nature and fits into a heterogeneous site binding model. The kinetic study revealed that the adsorption process followed Pseudo 2nd order model. The removal efficiency of limonite for the naturally high F- water sample (4.12 mg l-1) under optimized condition obtained 66.9%. The maximum adsorption capacity Qe (F- adsorbed) at equilibrium is calculated 2.208 mg g-1. The isotherm and kinetic studies reveal that limonite can be used as an adsorbent for fluoride removal. The OH-group of limonite acts as ion exchange with F- ion confirm by Fourier Transmission-IR spectroscopy analysis (FT-IR). XRD (X-ray Refraction Dispersive) peaks also give good evidence of surface adsorption onto limonite and surface morphology by Scanning electron microscopy (SEM) image. In the future we can use this technology for fluoride removal in large scale by using limonite which is cost-effective, eco-friendly, minimal processing and easily available in the study area.