             !"#$%&" ’()* +,,---"#$%&".’()*(/      Full Length Research Paper Water defluoridation by bauxite-gypsum-magnesite (B-G-Mc) based filters calcined at 350 – 500°C Thole B. 1 *, Masamba W. R. L. 2 and Mtalo F. W. 3 1 Ngurdoto Defluoridation Research Station, P. O. Box Usa River, Arusha, Tanzania. 2 Harry Oppenheimer Okavango Research Centre, University of Botswana, P/Bag 285, Maun, Botswana. 3 College of Engineering and Technology, University of Dar Es Salaam, P. O. Box Dar Es Salaam, Tanzania. Accepted 2 May, 2013 Research was carried out at Ngurdoto research station in Tanzania to ascertain the potential development of a water filter made of bauxite, gypsum and magnesite in an attempt to enhance the availability of low-fluoride water. The materials were sourced within Tanzania. The X-Ray fluorescence technique showed that the major components of the materials were: bauxite: Al 2 O 3 (30.33%), SiO 2 (15.0%) and Fe 2 O 3 (14.3%); gypsum: CaO (28.09%), SO 3 (34.96%), and SiO 2 (9.01%); and magnesite: MgO (34.57%) and SiO 2 (19.3%). The materials were calcined at 350, 400, 450 and 500°C. The activated materials were then mixed in mass ratios of 1:2:3, 1:3:2, 2:1:3, 2:3:1, 3:1:2 and 3:2:1 (bauxite: gypsum: magnesite). One gram of each composite was employed in the batch defluoridation of 1 L of water with fluoride concentration of 12.62 mg/L. The highest defluoridation capacity, 11.89 mg F - /g, was obtained with the 3:2:1 to 500°C composite. The quality of the treated water fell short of WHO standards on sulphates and iron but residual concentrations of Cl - , Al 3+ , Ca 2+ , Mg 2+ , Fe 2+ were within the prescribed limits. Sorption behavior followed strongly to Langmuir isotherm, except for the 450°C calcined samples for which the Temkin isotherm behavior was pronounced. Despite the limitations of high residual sulphates and iron, a composite filter of bauxite, gypsum and magnesite was shown to be workable. Key words: Defluoridation, bauxite, gypsum, magnesite, composite, calcine, isotherm. INTRODUCTION Fluoride and human health Fluoride has been identified as a cause of dental and skeletal fluorosis world over (Maliyekkal et al., 2010; Rango et al., 2010; Peter, 2009; Onyango et al., 2009). The World Health Organisation (WHO) set a guideline value of 1.5 mg/L as an acceptable upper limit (WHO, 2006). It has been established that drinking of water having fluoride concentrations between 1.5 to 3.0 mg/L results in dental fluorosis and the browning and mottling of teeth. Fluoride levels beyond 3 mg/L in potable water result in skeletal fluorosis and when extreme concentrations above 10 mg/L are obtained, crippling fluorosis ensues (Zhu et al., 2009; Sajidu et al., 2008). It is worth noting that low concentrations in drinking water, below 1.5 mg/L, are beneficial to dental health. Dissanayake (1991) summarized the fluoride linkage to human health as presented in Table 1. Proxy indicators for high fluoride in ground water are high levels of sodium, bicarbonate and pH above 7. High-fluoride ground waters typically have relatively low calcium and magnesium concentrations with sodium and bicarbonate as the dominant dissolved constituents (WHO, 2003). There are however, some exceptions. 01(($*)#&)/ ’,+( 2%& ,+$3$()(#4+,%&"%