Adsorption of fluoride from groundwater by Alumina Olivine Composite

The removal of fluoride from groundwater is a major challenge in environmental engineering especially while dealing with real-life groundwater from field. The presence of other ions in groundwater impacts significantly which renders the efficacy of many adsorbent which may show significant performance in synthetic water. The present study was conducted with groundwater of fluoride level 8.8 mg/L that mostly representative the moderate to high level fluoride concentration in the area of the western part of the state of West Bengal, India. The adsorbent chosen in the study as Alumina Olivine Composite (AOC) prepared by a wet impregnation method followed by calcination.

The adsorption system with real‒time groundwater was test with variation of dose, reaction time and agitation rate. The isotherm study, the variation of the adsorbent dose was chosen as 0.5 to 6 g/L. The best fitted model was found as Freundlich isotherm (R²=0.907 and K_f =0.527 (mg/g)(L/mg)^1/n, 1/n = 0.641) along with the three parameter isotherm models, such as Hill, Redlich–Peterson, Toth with comparable R². Thus, the adsorption model exhibited a favourable adsorption with significant capacity. The Kinetic study was conducted for 5‒480 min. The adsorption kinetic models are chosen as pseudo‒first-order (PFO), pseudo‒second‒order (PSO) and Elovich’s equation (EL) were used as the reaction‒based kinetic models, whereas, the liquid film diffusion model (LFD), intraparticle diffusion model (IDS and IDL) and double-exponential model (DEM). The Elovich’s equation was found to be the best‒fitted among the reaction based models with R² value of 0.970. The applicability of double-exponential equation significantly described the adsorption diffusion model with a R² value of 0.983, showed that the mechanism of adsorption govern by both intraparticle diffusion and film diffusion. This is further supported by the significant C value of 6.40 from intraparticle diffusion model. The present work established the wide applicability of the present adsorption system in field level water treatment units.

Keywords Isotherm, Kinetics, Batch Study, Water treatment, Fluoride