International Journal of Physical Sciences

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: Al2O3 (30.33%), SiO2 (15.0%) and Fe2O3 (14.3%); gypsum: CaO (28.09%), SO3 (34.96%), and SiO2 (9.01%); and magnesite: MgO (34.57%) and SiO2(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-, Al3+, Ca2+, Mg2+, Fe2+ 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.


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.*Corresponding author.E-mail: tholebernard@hotmail.com.

Fluoride occurrence in Tanzania and Malawi
Fluoride occurs in a number of localized areas in Malawi, among them the well known areas are Nkhotakota boma and Nathenje in the central ragion, Mtubwi, Kangankhunde, Chiradzulu, and Bangula in the southern region (Sajidu et al., 2008).In Tanzania high fluoride levels in groundwater are found at Shingida, Shinyanga, Arumeru, Arusha and other places (Mjengera, 2002;Singano, 2000).In Malawi the fluoritic areas have fluoride levels ranging from 2 to 9 mg/L whereas in Tanzania the levels are as high as 3 to 30 mg/L.Dental fluorosis is observable in the fluoritic areas in Malawi where dental, skeletal and crippling fluorosis are evident in fluoride endemic areas.The research on fluoride contamination in water has been going in both countries to address this public health challenge.

Research on defluoridation
The research carried out on defluoridation in Malawi includes synthesis and use of hydroxyapatite and clay, the optimization of gypsum and bauxite for fluoride removal (Thole, 2005;Masamba et al., 2005), dental fluorosis mapping among school children (Msonda et al., 2007), fluoride mapping in southern Malawi (Sajidu et al., 2008) and specific short term investigations at a number of other sites.These studies have identified the fluoride spread in groundwater across the country and determined the defluoridation potential and characterized locally available materials for defluoridation (Sajidu et al., 2008;Msonda et al., 2007;Thole, 2005;Masamba et al., 2005).Research in Tanzania has been ongoing and includes the WHO's Best Available Demonstrated Technology (BADT) -use of the bone char (Mjengera, 2002;Mjengera and Mkongo, 2002;Mtalo, 1997), employment of magnesite (Singano, 2000) and use of bauxitic and kaolinitic clay (Peter, 2009).Fluorosis is greatly recognized in Tanzania, probably because of its severe extent in some regions such as Shingida, Arumeru and Shinyanga.World over, the search for fluoride removal options has revealed that there are number of synthetic and naturally occurring materials that have defluoridating properties.Some of these researched materials are zeolites (Onyango et al., 2009), Mg/Al-CO 3 hydrotalcite (Hongtao et al., 2007), magnesia amended SiO 2 (Zhu et al., 2009), Mg-Al-Zn alloys (Vasudevan et al., 2009), activated carbon (Melisa, 2001;Bablia, 1996), and alum and lime (Suneetha et al., 2008) SO in the water, whereas bauxite and clays raise the turbidity and colour of the water.
The current research investigated the defluoridation potential of a composite filter of calcined bauxite, gypsum and magnesite with the aim of identifying an optimum composite filter that would not alter the water quality beyond recommended WHO standard limits.The scope of the reported work was within calcine temperatures of 350 to 500°C with bauxite, gypsum and magnesite obtained within Tanzania.

EXPERIMENTAL
Bauxite, gypsum and magnesite were calcined in an open air furnace at temperatures of 350, 400, 450 and 500°C for 2 h.Finely divided powder (≤0.5 mm diameter) of each material were then mixed in 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 then placed in 1 L of water at an initial fluoride concentration of 12.62 mg/L.Fluoride concentrations, pH and the concentrations of were measured until equilibrium fluoride concentration was obtained.This was done for each calcine-temperature and for each composition at four different temperatures and six different compositions.Each experiment was replicated 3 times.Sorption capacity was determined through a mass balance as per Equation ( 1): where qe is the amount of adsorbed fluoride at equilibrium (mg g -1 ); V is the volume of the solution (L); Co and Ce are the initial fluoride Table 2. Alkalinity, pH and ion concentrations in water before defluoridation.

Concentrations of selected ions in raw water (mg/L)
Alkalinity (mg/L) pH (3) (dm 3 /g) and 1/n signify capacity and intensity, respectively.The intensity values <1 indicate favorable adsorption.A plot of lnqe vs lnce should yield straight line with slope 1/n and intercept lnKf, if the interaction between the adsorbents and fluoride is established by the Freundlich isotherm.The terms qe and ce denote equilibrium concentrations in the solid phase and liquid phase, respectively.The Temkin isotherm considers chemisorption of the solute on the adsorbent media.The mathematical expression for the Temkin isotherm is given as:

Capacity and water quality
Table 2 contains the initial alkalinity, pH and concentrations of selected ions in the water before defluoridation.The raw water quality was within the WHO recommended limits, except for pH that was 0.1 unit above the upper limit.The hardness of the water, 0.32 mg/L, also adhered well to WHO limits.The raw water was found to be very soft which confirmed the earlier findings (Mjengera, 2002;Singano, 2000).The maximum defluoridation capacity, 11.86 mg F/g, was obtained for the 3:2:1 composite calcined at 500°C.The recommended upper limit for sulphates in water being 400 mg/L, this calcine composite had a limitation of high residual sulphates, 2200 mg/L.Water quality from all composites did not adhere to the upper sulphate limit, but the 400°C calcined composite of the ratio 2:3:1 had the lowest sulphate concentration of 900 mg/L, giving some promise of the development of a potential composite with respect to the residual sulphate.The results portray that the best and optimum composites adhered to most of the water quality parameters considered in this research, except for the residual sulphates (all composites) and iron (2:3:1 to 350°C calcined sample).When the bauxite, gypsum and magnesite filters were compared it was observed that bauxite did not adhere to the Al upper limit, whereas gypsum failed with respect to sulphate.Magnesite increased residual hardness beyond the recommended upper limit.Bauxite obtained significantly greater capacities compared to gypsum and magnesite.Bauxite, gypsum and the composite filters decreased pH of the water, whereas magnesite resulted in the increase of pH.This was attributed to the possible fluoride sorption mechanisms of the bauxite, gypsum and magnesite.
The decrease in water pH following defluoridation using bauxite and gypsum may have resulted from two attributes; (1) the amphoteric nature of oxides of aluminium, and (2) the acidic behaviour of the oxides from anionic species in water, in particular the sulphite ions.Al 2 O 3 is amphoteric.Therefore, it reacts with both acids (Equation 5) and bases (Equation 6) as shown below: It has also been demonstrated by the XPS analysis that the sorption of fluoride at low pH (pH <pH pzc ) for nano-AlOOH can be explained by a two step protonation / ligand exchange reaction mechanism as described by Equations ( 7) and ( 8) (Shriver et al., 1994).
The overall reaction is represented by Equation 9O The sorption model proposed by Equations ( 8) and ( 9) should result in increase of pH, because there is a net removal of H + ions from solution.However, when the initial pH is greater than the point zero charge (pH pzc = 7.8), nano-AlOOH functions as a cation exchanger and adsorbs the sodium ions present in the solution, releasing protons resulting in the decrease of pH (Wang et al., 2009;Emamjomeh and Sivakumar, 2009).The probable mechanisms for the fluoride sorption at pH>7.8 by nano-AlOOH can be explained by Equation (10).
In the present experiments the initial water pH was 8.6, larger than the pH pzc of AlOOH.The X-ray fluorescence (XRF) technique showed that the bauxite employed in these experiments contained 30.33%Al 2 O 3 .The decrease in pH after defluoridation using the bauxite may entail the formation of AlOOH in water and subsequent protonation / ligand exchange reactions, as suggested earlier (Wang et al., 2009).The Al 2 O 3 being amphoteric may have reacted as acid in the basic water medium (pH = 8.60), as illustrated earlier via Equation ( 6).This lowers pH because it diminishes the OH - concentration in water and, then the following reaction may have occurred enhancing the decrease of water pH; The bauxite and gypsum contained 5.18 and 34.96% SO 3, respectively obtained through the XRF analysis.It is established that covalent oxides are largely acidic.Such oxides on dissolution bind water molecules and release protons to the surrounding medium (Shriver et al., 1994).
The behaviour of carbon dioxide in aqueous medium illustrates this point.
From the foregoing reactions (Equations 12 and 13) it is highly plausible that the sulphite from the bauxite and gypsum, upon entering the aqueous medium, reacted similarly as depicted via Equations ( 14) and (15).
The reactions proposed in Equations ( 14) and ( 15) would result in decrease of pH.This entails that the presence of both Al 2 O 3 and SO 3 in bauxite would function in synergy for decrease in pH during defluoridation using bauxite, in agreement with the experimental data, which shows that defluoridation using bauxite obtained lowest pH values amongst the three materials.A recap of the compositional quantities of SO 3 , Al 2 O, CaO, and MgO in bauxite, gypsum and magnesite depicted in Table 4 shows that bauxite and gypsum contained the sulphite in greater proportions compared to magnesite.Therefore, the effect of the sulphite reactions proposed in Equations ( 14) and ( 15) may have been negligible for magnesite.Ionic oxides being largely basic (Shriver et al., 1994) will lower pH of an aqueous medium as illustrated below.
The effect of the CaO reaction may have been overshadowed by the effect of the reactions of SO 3 as proposed in Equations ( 16) and ( 17) resulting in a net decrease of pH during defluoridation with gypsum, SO 3 being in greater proportion to CaO in gypsum.On the other hand, magnesite having a very low content of SO 3 , increased pH as effects of MgO reactions overshadowed the effect of SO 3 reactions because of the greater proportion of MgO compared to SO 3 .From the foregoing discussions, it is plausible that defluoridation with gypsum and magnesite followed the scheme proposed in Equations ( 18) and ( 19), respectively;

Sorption isotherm
Figures 1, 2 and 3 exemplify sorption isotherm plots obtained from the experimental data.Most of the data sets adhered more strongly to Langmuir sorption except for the data from the 450°C calcined samples that followed the Temkin isothermal sorption.Mixed sorption were, however, deciphered, the correlation coefficients of all three sorption isotherms being close to unity.The isotherm parameters obtained from the best fit linear equations are depicted in Table 5.

Conclusions
The composite-filter prepared by calcination obtained higher capacities compared to the stand-alone filters of bauxite, gypsum and magnesite, depicting a synergetic relationship among the three materials.The residual sulphate is the most important limitation of defluoridation using these composites.This appears to stem from the gypsum component.The composite materials have a net effect of slightly decreasing pH that is attributable to the behaviour of Al 2 O 3 and sulphite in aqueous media.Bauxite generally attained greater capacities compared to gypsum and magnesite at four temperatures.The composites with relatively larger bauxite quantities obtained the larger sorption capacities.The water quality adhered to the WHO upper limits of pH, hardness, Al indicating a promising scenario for developing a composite filter made of these three materials.

Further work
Defluoridation in fixed bed will be carried out to obtain the breakthrough characteristics using the composites of the three materials used in this study.
fluoride concentration at equilibrium (mgL -1 ), respectively, and, m is the mass of adsorbent (g).Sorption isotherm was studied by fitting the data with the Langmuir, Freundlich and Temkin isotherm expressions as per Equations 2 to 4. The Langmuir isotherm assumes that the adsorption sites are energetically the same and there is a monolayer formation with no movement of particles over the surface from one site to another.ce is the equilibrium fluoride concentration (mg/L), qe is the equilibrium amount on adsorbent (mg/g) Q (mg/g) and b (dm 3 /mg) are the Langmuir isotherm constants related to capacity and energy, respectively.A plot of isotherm is associated with multi-layer formation on heterogeneous sites and follows the expression; a and b obtainable from the plot of qe vs Log ce are the Temkin constants.qe and ce have the same meaning as earlier described.

Table 1 .
Summary of linkage of fluoride ingestion through water to human health.
Table 3 presents the highest capacity filter (C 1 ), the optimum filter (C 2 ) (best possible filter with respect to WHO potable water standards) and the average capacity (C µ ) for all the composite filters at each temperature.pH, residual hardness and residual concentrations for the Al are included in the table.

Table 3 .
Summary of highest and optimum capacities, residual ion concentrations, hardness and pH.

Table 4 .
Percentages of sulphites and metal oxides present in the three raw materials.