Karstic bauxite as low cost fluoride absorbent

Fluoride is one of the hazardous geogenic contaminant. In fact, despite F concentrations less than 1 mg/L are beneficial for calcification of dental enamel and bone formation, excess intake would lead to various diseases. Fluoride-related health disease are a common issue in the Eastern Rift Valley where, the high concentration of F in soils and drinking waters is related to the alkaline volcanic activity of the Rift Valley. Taking into account this important issue, many fluoride removal methods have been developed. Probably, the most popular remedy consists in treating waters with bone char. Today many processes like ion exchange, precipitation, coagulation, membrane processes, distillation and electrolysis are used as absorption. Among these, absorption is one of the most used method and involves passage of water through an absorbent material where F is removed by ion exchange or surface chemical reaction with the solid. Most of the available defluoridation materials are expensive or technically non-feasible for rural areas. Hence, F removal studies using i) low-cost, ii) locally available, iii) safe and iv) easy to use materials is of basic importance. Among the natural locally available materials, this study focuses on bauxite. Bauxite is a rock of large availability and exploited (e.g. Lushoto district) in central Africa. Aim of the research is to examine the potential of bauxite F removal from waters, analyzing the effect of various factors that control the removal (flow rate, contact time, particle size, coexisting ions, adsorbent dose, pH, initial fluoride concentration…). Dynamic tests have been carried out using ppl columns with glass wool at the bottom (in order to prevent obstructions and avoid turbidity in the outgoing solution). An adjustable valve was placed in the bottom to control the flow rate. Working synthetic solution was prepared with NaF in order to obtain solutions at different concentration. The effluent was sampled each 25 ml. F dosage eluate was performed after pre-dilution with TISAB to reduce the ionic strength variation in the solutions. Preliminary data highlighted that the uptake capacity increase with the contact time. As for the grain size, 0.2-0.5 mm class provided better results: finer classes restricted the solution flow whereas larger classes provided less contact surface. The adsorption capacity was evaluated as mg of F removed per g of bauxite (Q value). In this way the maximum value obtained was higher than 0.10 mg/g. A second test using bauxite previously activated with H2SO4 0.4 N was able to increase Q value up to >1 mg/g. Concluding, these preliminary data suggest that bauxite may be used as a low-cost adsorbent for F removal especially in districts close to the ore deposits.