Groundwater defluoridation by natural minerals : Understanding the process of fluoride removal in Tanzania

Abstract: Fluoride (F-) concentration in drinking water is one of the water quality parameters in countries with high concentrations in groundwater sources. Natural minerals have shown considerable effectiveness in F- removal. In this study, the performance of the locally available natural minerals, such as bauxite, magnesites, and gypsum, have been investigated at the laboratory scales and through modelling for F- removal from drinking groundwater sources in Tanzania. Batch experiments were carried out to examine the optimum conditions for F- removal by the calcined bauxite, magnesite, and gypsum. X-ray fluorescence spectroscopy (XRF) characterization showed that Al2O3, MgO, and SO3 were the major oxides in bauxite, magnesite, and gypsum, respectively. The experimental data for the three treated adsorbents fitted well with the Freundlich adsorption isotherm and the pseudo-second-order kinetics. The values of ∆G° and ∆H° indicate that the F- adsorption on bauxite and magnesite surfaces was spontaneous and endothermic. The randomness described by ∆S° at the solid–liquid interface was increased during the adsorption processes. While for gypsum, the reaction was spontaneous and exothermic, where the randomness at the solid–liquid interface decreased during the adsorption processes. At optimum conditions, calcined bauxite (400 °C), magnesite (650 °C), and gypsum (350 °C) lowered the F- concentration from 8.27 mg/L to 1.02, 0.233 and 1.99 mg/L, respectively. Bauxite and gypsum lowered the pH of water from 9.38 to 6.74 and 7.41, respectively. Magnesite raised the pH from 9.38 to 10.12, which is above the World Health Organization (WHO) (6.5 - 8.5) and Tanzania Bureau of Standards (TBS) (6.5 – 9.2) drinking water standard; therefore, pH adjustments are needed before water can be used for drinking. The Bed Depth Service Time (BDST) plot showed that the service times for F- adsorption on the calcined bauxite and magnesite surface increased with bed depth. The critical bed depths (Zo) for bauxite and magnesite obtained were 7.21 and 8.28 cm, respectively. The lower value of the kinetic rate parameter (Kα) for bauxite (1.43E-5 L/mg s) and magnesite (1.50E-5 L/mg s) highlighted that the breakthrough occurs in short beds; therefore, deeper beds are required to avoid breakthroughs. The experimental results and model predictions have helped to compare the adsorption processes as well as contrast their performance and sustainability for F- removal using the bauxite, magnesite, and gypsum in the drinking water resources in Tanzania. The adsorption results and the overall cost analysis show that the cost of calcined bauxite and magnesite is low compared to other available adsorbents; therefore, they can be used in F- removal from water.