REMOVAL OF EXCESS FLUORIDES FROM DRINKING WATER BY MULANJE
BAUXITE
B. Thole, 2W. Masamba, 2S. M. Sajidu and 2J.F. Mwatseteza
1
The Polytechnic, University of Malawi, Malawi2 Chancellor College, University of Malawi
ABSTRACT
Water defluoridation experiments were carried out at Chancellor College with bauxite phases
obtained through calcinations of Malawi’s bauxite at temperatures of 200 ºC, 300 ºC, 400 ºC and
500 ºC. 200ml of 8 ppm fluoride solutions and 2.5 g of bauxite were employed in experiments to
determine defluoridation capacities, effects of various ions, effects of pH and effects of
temperature on defluoridation. The highest defluoridation capacity of 95.3% was obtained with
the 200 ºC calcine and the second best capacity was 94.80% that was registered for the raw
bauxite. The lowest defluoridation capacity was obtained with the 500 ºC calcine and was
87.12% Presence of carbonate and chloride ions in solution reduced defluoridation whereas
calcium ions enhanced defluoridation. The ions sulphate, phosphate, nitrate, potassium and
sodium had no effect on defluoridation. Low pH of 2 and high pH of 10 registered low
defluoridation capacities of 14.63 and 15.38% respectively with best capacity of 96.2% being
obtained at pH 4. Increase in temperature decreased defluoridation capacity and decrease in
particle size increased the capacity. The 0.5 mm particles obtained defluoridation capacity of
96.1 whereas the 2.0 mm particles registered a capacity of 57.99%. Powder X-Ray Diffraction
revealed that the PXRD profile of the bauxite matched the Joint Committee on Powder
Diffraction Standard number 29-1488 and based on that match the bauxite was identified to be
Al2Si2O5(OH)4 aluminium silicate hydroxide. Defluoridation with the 200 ºC bauxite calcine had
first order kinetics with a rate constant of 3.5 10-3. Bauxite introduced aluminum and silicon in
the solutions with concentration increases of 0.13 ± 0.01 ppm for aluminum and 4.11 ± 0.07 ppm
for silicon. The pH increased with 0.5 ± 0.03 units after defluoridation with bauxite. Ground
water defluoridation was then carried out to compare the results obtained in fluoride solution
defluoridation and ground water defluoridation. It was therefore evidenced that raw bauxite from
Mulanje has a defluoridation and ground water defluoridation capacity of 94.80 % and its
defluoridation capacity increases slightly to 95.3% with calcinations at 200 ºC. It is conclusive
that Mulanje bauxite is a potential defluoridating material that can be employed in local
defluoridation technologies in Malawi.
Key words: bauxite, calcinations, defluoridation, fluoride, X-Ray Diffraction, calcium,
potassium, sodium, carbonate, nitrate, phosphate, chloride, sulphate, pH
INTRODUCTION
World over million of people particularly in developing countries are suffering from fluorosis
due to high fluoride concentrations in their drinking water PHE [1)] Fluorosis is characterized by
discoloured, blackened, mottled or chalky white teeth and, in its advanced stages, weakening of
the skeleton occurs. In Malawi this problem is common in places such as Nkhotakota, Karonga,
some parts of Nsanje, Chikwawa, Machinga, Mangochi Sajidu [2] and Lilongwe Msonda [3)]
Free fluoride levels in drinking water of up to 8.6 ppm have been reported at Ulongwe in
1
Machinga district Sibale et al [4], 7 ppm at Mazengera in Lilongwe, 9.6 ppm in Nkhotakota, 8.0
ppm in Karonga, 5.8 ppm in Nsanje and 3.4 ppm in Mwanza Carter and Bannet [5]. Work done
in determination of fluoride in Nathenje indicated a positive relation between prevalence of
fluorosis in children and levels of fluorides in potable water within the studied areas Msonda [6].
Although several defluoridation methods have been studied in detail and reported as appropriate
in other countries such as Kenya, Tanzania and India Susheela [7], NFI [8], PHE [9], REF [10]
Malawi has not attempted to undertake drinking water defluoridation seriously in the fluorosis
endemic areas. There is need therefore to institute defluoridation technologies in the affected
areas. The technologies of the existing methods, their running costs and the required level of skill
are crucial in selecting the appropriate technologies for Malawi. The present work explored the
viability of using local bauxite as a water defluoridation material in Malawi. Local bauxite was
selected as a good candidate for defluoridation in Malawi because of its availability and that
studies in India have shown that bauxite in that country has some defluoridation capacity NFI
[11]. Other materials well developed for defluoridation include alum, zeolites, gypsum and bone
char REF [12], NFI [13]. Some of these materials such as zeolites and alum were considered not
suitable for Malawi because they are costly and not readily available in rural areas of the
country. The defluoridation capacity, reaction kinetics and mechanisms of the local bauxite
material and effect of the material on water quality were investigated.
MATERIAL AND METHODS
The bauxite sample studied in this work was obtained in dust form the Geological Survey of
Malawi in Zomba but was originally acquired from Mulanje Mountain in Mulanje District.
Analytical reagent grade chemicals for analyses were purchased from Technilab Company (Ltd)
Malawi who sourced the chemicals from Association Chemical Enterprises cc (RSA) and
Saarchem-Holpro Analytical (PTY) LTD (RSA). Deionised tap water was used in all solutions
preparations and analyses. The bauxite dust was ground into powder form. It was then calcined
in air using a muffle furnace at temperatures of 200, 300, 400, and 500°C to acquire different
phases of bauxite for experimentation so as to determine the phase with the highest
defluoridation capacity. The samples were held at each reaction temperature for 2 hours and
quench-cooled to room temperature. Defluoridation capacity determinations were carried out by
mixing 200 ml of 8 ppm fluoride solution with 2.5 g of defluolidating material (bauxite or a
calcined phase of bauxite) and shaking for 30 minutes. Fluoride concentration in the solution was
monitored hourly for twelve hours. Twelve hours was chosen as being long enough for
comparison purposes. Fluoride levels in water samples were determined by using fluoride ion
selective electrodes Orion number 9409 on a Sargent Welch pH/activity meter model PAX 900
as described in the Association of Official Analytical Chemists AOAC [14]. To determine the
effect of bauxite on water quality pH and, aluminum, silicon, sulphate, carbonate and phosphate
concentrations in water were determined. A pH meter model 5000, s-30002 digital Sargent
Welch was utilized to determine pH. Aluminium and silicon were analysed using Atomic
Absorption spectrophotometry. Sulphate were determined by a turbidimetric method on Jenway
6405
uv-visible
spectrophotometer,
phosphates
were
determined
through
a
vanadomolybdophosphoric acid colorimetric method and carbonates were determined through
titrimetrically as described in APHA [15]. The % defluoridation capacity was calculated by
expressing the decrease in fluoride concentration as a % of the starting concentration. The
starting incalcined gypsum and the phases obtained upon calcinations were characterized by
2
Powder X-Ray Diffraction (PXRD) using Shimadzu 600 X-Ray Diffractometer and phase
identification was made using a search-match computer program supported by the Joint
Committee on Powder Diffraction Standards database JCPDS [16]. In order to determine the
dehydration temperature and also the amount of water in the bauxite occurring both as surface
physisorbed water and water coordinated to the bauxite, thermogravimetric analysis (TGA) was
crudely done by heating the bauxite sample in air in a muffle furnace at increasing specified
temperatures and measuring the weight loss of the material using an analytical balance.
RESULTS AND DISCUSSION
Water defluoridation Capacity Defluoridation Mechanims
Table 1: Fluoride concentrations and defluoridation capacities of bauxite phases
Bauxite phase
Raw
200 ºC
300 ºC
400 ºC
500 ºC
Initial Fluoride
Concentration
(ppm)
8.00
8.00
8.00
8.00
8.00
Equilibrium
Fluoride conc.
(ppm)
0.42
0.38
0.77
0.90
1.03
Fluorides
removed from
solution mg/1
7.58
7.62
7.25
7.10
6.97
%
Defluoridation
capacity
94.8
95.3
90.6
88.8
87.1
The highest defluoridation capacity of 95.3% was obtained in defluoridation with the 200 ºC
calcine and this was followed closely by that obtained with the raw bauxite at 94.8%. These two
results did not differ significantly at 5% level of significance; as such the calcinations is not
necessary. The lowest defluoridation capacity of 87.1% was obtained in defluoridation with the
500 °C bauxite calcine.
Figure 1 is the PXRD profile of raw bauxite that matched the Joint Committee on Powder
Diffraction Standard number 29-1488 JCPDS [17]. This match let to the identification of the
Mulanje bauxite as Al2Si2O5 (OH)4. The generally high defluoridation capacities were attributed
to presence of exchangeable anions (the hydroxide ions) that potentially exchanged with fluoride
ions PHE [18]
3
Fig 1. PXRD profile of raw Mulanje bauxite
The increase in defluoridation from raw bauxite to the 200 ºC calcine was attributes to absence
of physisorbed water that was lost during heating which increased available surface for
adsorption (Coulson and Richardson) [19]. The Thermogravimetric plot in Figure 2 shows that
water was lost around 200 ºC.
Fig 2: Thermogravimetric plot of Mulanje bauxite
4
The defluoridation capacities decreased steadily from 300 ºC through 400 ºC to 500 ºC calcine.
This could be a result of phase changes that were occurring as a result of high temperature. The
possible ion exchange was explained as per equation 1 below that shows a 1: 1anion exchange
between fluoride and hydroxide ions:
Al2 Si2 O5 (OH) 4 + 4F- Al2 Si2 F4 + 4OHThe amounts of hydroxide ions introduced in solution was therefore taken to approximate the
amounts of fluoride ions removed from solution through ion exchange. The positive difference
between the amounts of fluoride removed from solution and the amounts hydroxide ions
introduced in solution represented fluoride removed from solution through adsorption. Table 2
presents amounts of hydroxide ions introduced in solution bauxite calcined at 200 °C was
61.37% ion exchange and 38.63 % adsorption.
Table 2: Fluoride sorbed on, and hydroxide desorbed from bauxite
Phase of bauxite
Raw
200 ºC calcine
300 ºC Calcine
400 ºC Calcine
500 ºC calcine
Fluoride removed Hydroxide
ions
from solution (mg/l) introduced in solution
(mg/l)
7.14
3.09
7.64
4.00
7.92
3.94
7.94
3.98
7.94
4.57
Difference (mg/l)
4.08
3.64
3.98
3.96
3.37
Fig 3 presents kinetic plot for defluoridation with 200 ºC bauxite calcine.
Fig 3: Kinetic plot for bauxite
5
The plots of [F] against time, 1/[F] against time and In[F] against time had a best linear curve of
slope 0.0005 for the 1/[F] versus time plot. This shows that defluoridation with bauxite followed
second order kinetics with rate constant of 3.5 x 10-3ppm-1min-1 . The rate law therefore was of
the form rate = d[F] = [(F]2. The rate law and the rate constant show that with a starting
dt
fluoride concentration of 8 ppm and a water volume to bauxite ratio of 200 ml to 2.5 g; 3 hours
20 minutes of contact time would be required to defluoridate the water to 1.4 ppm fluoride
concentration.
Figure 4 presents a plot of equilibrium concentrations against mass of bauxite used in
defluoridation of 200 ml of 8ppm fluoride solutions.
Fig. 4 Adsorption isotherm for fluoride on 200 °C calcined bauxite
The plot shows that equilibrium concentrations decreased quickly with increase in mass of
bauxite used in defluoridation up to about 10 g, after this point the equilibrium concentration
were within 0.05 to 0.2 ppm. This implied that increasing the mass of bauxite per volume of
water would only increase defluoridation efficiency up to a water to bauxite ration of 200 to 10 g
(20 ml to 1 g).
Water quality and defluoridation
Effects of various ions on defluoridation
Table 3 presents correlation coefficients between initial concentration of various ions in solutions
and concentrations of fluoride that were attained in solution after defluoridation. The positive
correlation shows that more fluorides remained in solution where the concentration of the
6
particular ion was high. Negative correlation depicted that less fluorides remained in solution
where concentration of a particular ion was low.
Table 4: Correlation coefficients between increase in ion concentrations and decrease in fluoride
concentrations in defluoridation with bauxite
Ion
PO43Correlation -0.23
coefficient
SO42-0.59
CO32+0.85
NO3
-0.07
CI+0.71
Ca2+
-0.94
K+
-0.17
Na+
-0.14
Carbonates and chlorides showed high direct positive correlation indicating that higher initial
concentrations of carbonates and chlorides and chlorides resulted in more fluorides remaining in
solution (high concentration of residual fluoride in water). Carbonates and chloride hindered
defluoridation with bauxite. The negative effect of carbonates on fluoride sorption could be
explained on the basis of comparative solubility of carbonates and fluorides, Carbonates being
generally less soluble in water than fluorides interacted more strongly with A1+3 and Si4+ in the
Al2Si2O5(OH)4 than the fluorides. The ion that interacts more with active sites in the adsorbent is
selectively adsorbed compared to the other NAS [12].
Chloride hindrance to defluoridation was a result of similarity of chemistry between fluorides
and chlorides both ions being halides. General electivity trends for sorption also place chloride
before fluoride ion; sulphate>iodide>nitrate>bromide>Chloride>Fluoride NAS [21]. This
indicates that chloride is more prone to sorption than fluoride.
Calcium ions showed a high negative correlation indicating a decrease in residual fluoride
concentration with increase a calcium ion concentration; Calcium thus enhanced defluoridation a
result attributed to possible adsorption of calcium onto the bauxite that in turn increased active
sites for chemisorptions of fluorides. This only happened to a small extent with potassium and
sodium, which is explainable by the preference of higher positive charges to lower positive
charges in adsorption Zumdhal [22]. Sulphate ions had a moderate enhancement on fluoride
sorption a result that contradicts general selectivity trends that place sulphate way before fluoride
ion:
SO42- > NO3- > Br- > CT > FFrom such a trend sulphate would be expected to interfere with defluoridation yet sulphate
enhances defluoridation with bauxite. This is part explainable by specific resin-ion interaction
implying that fluoride interact more with bauxite compared to sulphate ion. There exist resin/ion
combinations that will not adhere to general selectivity trends Coulson Richardson [23]
7
Effects of pH on defluoridation
Table 5: shows defluoridation capacities at different pH levels
pH
2
% defluoridation 14.63
capacity
4
96.2
6
44.63
8
25.38
10
15.37
The defluoridation was much lower at pH 2 and pH 10. The low fluoride sorption at pH 2 was
explained by the greater tendency of Fluorides to form complexes with H- ions at low pH
Meenakshi et al [24]. Fluoride sorption at high pH of 10 could have been interfered by presence
of hydroxide ions.
Effects of temperature on defluoridation
Figure 5 is a plot of defluoridation capacities against temperature
Fig. 5: Plot of % defluoridation capacity against temperature
The plot shows that there was a decrease in defluoridation capacity with increase in temperature.
This was attributed to concentration of fluorides in water with increase in temperature.
Increasing water–temperature has an effect of concentrating fluorides in the water ICOH [25].
Surface area of bauxite and defluoridation
Figure 6 is a plot of % defluoridation capacity against surface area. The plot shows increase in
defluoridation capacity with increase in surface area. This was attributed to the increase in
8
adsorption with increase in the surface available for contact with fluoride ions in solution. It thus
supported the use of fine powder bauxite for defluoridation. However it was noted that the finer
was the bauxite powder the greater was the turbidity in the water after defluoridation.
Fig 6: Plot of % defluoridation capacity against surface area
Effects of bauxite on water quality
Table 6 shows effects of bauxite on water quality.
Table 6: Effects of bauxite on water quality
Parameter
Al+3
SO42PO43Si4+
CO32-
Initial
concentration
ppm
0.00
0.00
0.00
0.00
0.00
Concentration after Change
defluoridation ppm
concentration
ppm
0.13± 0.01
0.13 ± 0.01
bdl
bdl
4.11 ± 0.26
4.11 ± 0.26
bdl
WHO limit ppm
0.200
400.00
None
None
500
bdl: below detectable limits.
9
The pH changed from 6.9 ± 0.01 to 7.4 ± 0.07 representing a pH increase by 0.5 unit.
Bauxite introduced aluminum and silicon in the water. It also increased the pH of the water by
0.5 of a unit. The water quality changes however were to earth within WHO recommended limits
WHO [26]. The raw bauxite and its 200 ºC calcine coloured the water to earth brown making the
water unaesthetic for drinking. Clarification with Molinga oleifera could be explored after
defluoridating with these phases of bauxite. M. oleifera has demonstrated to clarify turbid river
water from 500 NTU to 5 NTU Warhurst et al [27], Folkard et al [28].
CONCLUSION AND RECOMMENDATIONS
Experiments on water defluoridation with Mulanje bauxite showed that the 200 °C calcined
bauxite has a defluoridation capacity of 95.2% whereas the raw bauxite has a capacity of 93.8%.
The difference between the two defluoridation capacities was found to be insignificant at 5%
level of significance. This implies that use of the raw bauxite in defluoridation is even better than
use of 200 ºC bauxite calcine considering that in rural settings attainment of 200 ºC temperature
may not be feasible. The raw bauxite introduced an earth brown colour in the water during
defluoridation, which requires exploring clarification methods. Clarification with Moringa
Oleifera is a potential method for trial since it was shown to clarify river waters Warhust et al
[29]. Use of small particle bauxite can enhance defluoridation. It has however the effect if
increasing levels of turbidity. This requires exploration along with water clarification methods.
Water: bauxite ratio of 200 ml to 2.5 g (80 ml to 1g) is recommended but other lower ratios may
be employed where fluoride concentrations are higher than 8 ppm or lower contact time required.
With ratio of 80 ml water to 1 g bauxite the recommended contact time is 3 hours 20 minutes
hence ways of reducing the contact time may be explored that may include use of more bauxite
per unit volume. The lowest ratio however worth exploring further is 20 ml water to 1 g bauxite.
Low and high pH lowered defluoridation capacity but these being pH of 2 and 10 the ordinary
potable water pH ranges of 6.5 to 8.5 would not interfere with water defluoridation. High
temperatures were noted to reduce defluoridation capacity thus it is advisable that where water
requires defluoridation with bauxite and is boiled for microbial disinfection it should be
defluoridated first before boiling.
A pilot project in a high fluoride area need to be carried out to test the applicability and
efficiency of the method in real life settings.
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12