Fluoride Removal from Water using Bio-Char, a Green Waste, Low-Cost Adsorbent: Equilibrium Uptake and Sorption Dynamics Modeling

Drinking water containing fluoride >1 mg/L is unsafe for human consumption. Higher intake of fluoride can cause potential health hazards. Low-cost pine wood and pine bark chars, obtained as a byproduct from fast pyrolysis in an auger reactor at 400 and 450 degrees C, were characterized and used as received for water defluoridation. Sorption studies were performed at different temperatures, pH values, and solid to liquid ratios in the batch mode. Maximum fluoride adsorption occurred at pH 2.0. A kinetic study yielded an optimum equilibrium time of 48 h with an adsorbent dose of 10 g/L. Sorption isotherm studies were conducted over a concentration range of 1-100 mg/L. Fluoride adsorption decreased with an increase in temperature. The char performances were evaluated using the Freundlich, Langmuir, Redlich-Peterson, Toth, Temkin, Sips, and Radke adsorption models. Based on average percent error, the best isotherm fits follow the orders for pine wood and pine bark: PWLangmuir approximate to PWRedlich-Peterson > PWToth > PWSips > PWRadke-Prausnitz approximate to PWFreundlich > PWTemkin and PBToth > PBRadke-Prausnitz approximate to PBFreundlich > PBRedlich-Peterson > PBLangmuir > PBSips > PBTemkin. The pine chars successfully treated fluoride-contaminated groundwater at pH 2.0. The chars swelled in water due to their high oxygen content (8-11%), opening new internal pore volume. Fluoride could also diffuse into portions of the chars' sub-surface solid volume promoting further adsorption. Ion exchange and metal fluoride precipitation are modes of adsorption. Remarkably, these chars (S-BET: 1-3 m(2)g(-1)) can remove similar amounts or more fluoride than activated carbon (S-BET: similar to 1000 m(2)g(-1)).