Defluoridation by highly efficient calcium hydroxide nanorods from synthetic and industrial wastewater

Calcium hydroxide nanorods (CHN) were synthesized through ultrasonic assisted chemical method to remove fluoride from the aqueous medium. The BET surface area, Barrett-Joyner-Halenda (BJH) surface area, pore volume and average pore diameter of CHN were found to be 367 ± 10 m²/g, 501 ± 8 m²/g, 0.54 ± 0.02 mL/g, and 4.4 ± 0.1 nm, respectively. Experimental data was applied to Freundlich, Langmuir, Temkin and Two-step adsorption isotherm models, but neither model is fitted well to the experimental data. The unsatisfactory isotherm model fit was found due to the predominant mechanism of fluoride removal through dissolution and co-precipitation of calcium ions (from CHN) with fluoride ions under various experimental conditions along with little adsorption. Batch study of fluoride removal using CHN reveals that with an initial fluoride concentration of 50–250 mg/L and fixed CHN dose of 0.12 g/L, fluoride removal was found to be 450 ± 10 mg/g of CHN within the 45 min of contact time at pH ∼6.5. The fluoride adsorption kinetic data was applied to pseudo-first order and pseudo-second order kinetic models. Fluoride removal follows pseudo-second order kinetics in this study. Pseudo-second order rate constants at initial acidic and basic pHs of 4.5 and 9.5 were found to be 0.00041 (g/mg-min) and 0.00116 (g/mg-min), respectively. Surface complexation model was applied at varying pHs of fluoride contaminated solutions, and mainly co-precipitation of calcium and fluoride ions was found here as a governing mechanism for removal of fluoride ions. Fluoride removal study was performed successfully using synthetic solutions and electroplating industrial wastewater in a wide range of pHs (2–12). The aqueous solution with an initial fluoride concentration of 550 mg/L was treated with 2 g/L of CHN and 99.27% fluoride removal was achieved.