Defluoridation kinetics of 200 °C calcined bauxite, gypsum, and magnesite and breakthrough characteristics of their composite filter

Reaction kinetics and breakthrough characteristics in water defluoridation were studied through experiments with 200 °C-calcined bauxite, gypsum and magnesite and their composite filter. The aim was to determine defluoridation potential of a composite filter of the three locally sourced natural materials in contribution towards fluorosis mitigation. The materials were crushed and sieved to particle sizes of 1.2–1.4 mm diameter, and then heat-treated at 200 °C for 2 h. Their defluoridation capacities and reaction kinetics were determined in batch. A composite was then prepared in the ratio of the loading capacities. Breakthrough characteristics were experimented on in fixed bed through bed depth service time (BDST) design model, empty bed residence time (EBRT) optimisation model and the two parameter-logistics (2-PL) model. Mean loading capacities of 5.6, 3.4 and 1.7 mg F−/g were obtained for bauxite, gypsum and magnesite, respectively. Loading capacities decreased, while sorption percentages increased, with increase in dose level. Second order kinetics observed had rate constants 4.07 × 10−2, 1.87 × 10−2, 1.59 × 10−2 g mg−1 min−1 for bauxite, gypsum and magnesite, respectively. Composites, bauxite and gypsum decreased, while magnesite increased water pH. Time at 50% breakthrough (τ) obtained experimentally compared well with τ obtained through the two-parameter logistics model indicating good fitness of data to the model. Greater doses obtained higher breakthrough times that were, 120, 210, 255 and 360 min for 45, 75, 120 and 150 g, respectively. Critical bed depth (Zo), 7.71 cm and an operating line, ℏ = 4 × 10−4δ − 0.0757δ + 4.86 (ℏ = adsorbent exhaustion rate, δ = EBRT) were obtained. The water quality was within recommended quality limits for pH, apparent colour, hardness, and residual concentrations of SO42−, Cl−, Fe2+, and Al3+ in fixed bed. The research showed that a composite filter of the three materials, prepared in the ratio of their loading capacities and calcined at 200 °C, is a potential defluoridating filter in fixed bed configuration.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Bauxite–gypsum–magnesite composite filter was designed as per loading capacities. ► Composite filter had optimum defluoridation in fixed-bed and decreased water pH. ► Bauxite, gypsum decreased while magnesite increased water pH in defluoridation. ► Breakthrough times increased with increase in dose in fixed bed. ► In batch, capacity decreased while sorption increased with increased dose.