Groundwater Defluoridation With Raw Bauxite, Gypsum, Magnesite, and Their Composites

Breakthrough characteristics, kinetics, and dose-effect in defluoridation with bauxite, gypsum, magnesite, and their composites were determined. The aim was to identify optimum filter and configuration viable for groundwater defluoridation. Bed depth service time (BDST) design model and empty bed residence time (EBRT) optimization model were employed to characterize breakthrough. Higher doses obtained lower loading capacities but higher sorption percentages and breakthrough times. Breakthrough times obtained were 50 400, 32 400, 25 200, and 19 800 s for 150, 120, 75, and 45 g, respectively. The equation ħ = 1.0 × 10−4 δ2 −0.022 δ + 1.5053 defined the operating line with ħ, adsorbent exhaustion rate, in g L−1 and δ, EBRT, in seconds. A critical bed depth (Zo) of 6.56 cm was obtained. Second order kinetic rate constants were 0.73, 1.17, and 1.81 g mg−1 s−1 for magnesite, gypsum, and bauxite, respectively. The composite, gypsum and bauxite decreased water pH but magnesite increased pH in water defluoridation. Experimental data did not fit the two-parameter logistics model; model values were significantly different from experimental values. Optimum defluoridation characteristics were obtained in fixed bed. Despite high residual sulphates and apparent color, fixed-bed defluoridation with raw composites of these materials, treated in this manner, is viable.