Indian Journal of Chemical Technology
Vol.17, March 2010, pp. 120-125
Fluoride removal from water using crushed limestone
Suresh K Nath & Robin K Dutta*
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, India
Email: [email protected], [email protected]
Received 15 April 2009; revised 22 January 2010
An examination of defluoridation by industrial grade limestone indicated that a combination of precipitation and
adsorption of fluoride can be more effective for defluoridation of water. Pre-acidified fluoride water using two edible acids,
viz., acetic acid (AA) and citric acid (CA) have been used for treatment by crushed limestone of diameter 3-4 mm to
precipitate fluoride as CaF2 in addition to adsorption of fluoride on limestone. The study has been carried out in batches by
varying the acid concentration and contact time. Addition of the acids to the water before treatment with the crushed
limestone in batch tests significantly improved the fluoride removal and this increased with the increase in the
concentrations of the acids. The concentration of CA and AA required bringing down the fluoride concentration from 10 to
1.5 mg/L, are 0.05 M and 0.033 M, respectively, when the crushed limestone chips sample was used for the first time with
contact time of 12 h. The acids are neutralized by limestone during the defluoridation process and the resulting final pH of
the treated water was found to be in the ranges of 6.2 and 7.0 for CA and 5.7 and 7.0 for AA.
Keywords: Defluoridaton, Limestone, Calcite, Acetic acid, Citric acid
Fluoride contamination of groundwater is a serious
problem in several countries spread throughout the
world as ingestion of excess fluoride, most
commonly,
through
drinking
contaminated
groundwater causes fluorosis. Mainly two factors are
responsible for contamination of groundwater with
fluoride – geological and anthropogenic1. Rock
geochemistry has a major control on geological
fluoride contamination. Physiological conditions of
rock, like decomposition, dissociation and subsequent
dissolution along with long residence time may be the
responsible factors for F- leaching2. Among
anthropogenic factors industrialization, urbanization
and improper utilization of water resources are of
prime importance, in case of the developing countries.
Fluoride can be released in higher concentrations to
the atmosphere than it occurs naturally, through a
number of industrial processes. Some of them are the
manufacture of cement and bricks, electronic
industries, the refining of aluminium and the
processing of slag from electric furnaces and from
steel manufacture, where a fluorite flux is used3,4.
Long term ingestion of fluoride in high doses can
lead to severe skeletal fluorosis5,6. In India the
groundwater of most of the states contains high
concentration of fluoride4-9. Multidimensional health
problems, mainly dental and skeletal fluorosis are
prevalent in these states6,7. The prevention of fluoride
contamination of water is critical; however, fluoride
removal from water is under practice for several
decades to prevent fluorosis.
The principles underlying the fluoride removal
are based on adsorption10-12, precipitation–
coagulation13,14,
ion-exchange15,16,
membrane
17-19
separation process , electrolytic defluoridation20,21,
electrodialysis22-24, etc. Some recently used adsorbent
for fluoride are laterite25, waste residue from alum
manufacturing plant26, activated carbon prepared from
Morringa indica bark27 and Neem (Azadirachta
indica) bark & Kikar leaves (Acacia arabica)28, alumimpregnated activated alumina29, etc. Beyond these
methods, there are miscellaneous methods for fluoride
removal, e.g., nanofiltration, reverse osmosis, use of
different adsorbent like bone char, sunflower
(Helianthus annuus) plant dry powder, etc. Although
some of these fluoride removal methods have the
potential for use in treating wastewater, either due to
low efficiency, high cost or production of huge
amount of sludge containing fluoride they may not be
preferable for application in fluoride removal from
groundwater.
Calcium containing compounds/materials have
been tried by several workers to remove fluoride from
water due to the potentiality of this metal for this
purpose30. Addition of lime to precipitate fluoride can
lower the fluoride concentration to about 1 mg/L but
it increases the pH of water. Nalgonda technique, a
NATH & DUTTA: FLUORIDE REMOVAL FROM WATER USING CRUSHED LIMESTONE
low cost method of fluoride removal which uses lime
and alum also suffers from some disadvantages, viz.,
it adds toxic aluminium to the water and adjustment
of the pH of water is necessary. Similarly, the method
developed at the IISc, India also uses alkaline
MgO and CaO followed by adjustment of pH with
NaHSO431,32. Calcium hydroxide [Ca(OH)2], calcium
chloride [CaCl2] and calcium sulphate [CaSO4]
have already been tried for fluoride removal33.
Calcium chloride and hydroxide34, cement paste35 and
combined use of calcium salts as precipitant and
polymeric aluminium hydroxide as coagulant have
been studied for the fluoride removal by precipitation
method. Another fact is that, when initial
concentration of fluoride is large it is easier to
precipitate fluoride as CaF2 since it attains
supersaturation level easily. However, when fluoride
concentration is low, it is difficult to precipitate36. So,
it is a challenge to decrease the fluoride from lower
strength, i.e., from 10-20 mg/L to drinking water level
by precipitation method alone.
Limestone filtration as such has been found to
reduce F- concentration to 4 mg/L. Reardon and
Wang14 achieved F- removal to below 2 mg/L by
passing CO2 through crushed limestone columns
during filtration. Precipitation of CaF2 by an increased
Ca2+ activity in the limestone column due to
dissolution of calcite by CO2 was reported to be the
main mechanism for the observed F- removal.
However, it may not be practical to have CO2
canisters in remote rural areas. Thus, although there
have been several attempts with different techniques
on defluoridation of groundwater, the area still
remains quite topical in view of the world-wide
fluoride contamination in groundwater. This is mainly
because a method which can be easily used by a
layman and at the same time is cheap and effective in
long term has yet to be definitely proven. It appears
that the above mentioned precipitation or adsorption
processes alone, without increasing the pH or adding
other impurity to the water, are not capable of
bringing fluoride level in water to below 2 mg/L,
which is safe for drinking according to WHO and
BIS37,38. Therefore, it was thought worthwhile to carry
out a systematic investigation of defluoridation of
water by combination of precipitation and adsorption
method with limestone. The studies on defluoridation
by crushed limestone using acidified synthetic
fluoride solutions in batches have been carried out.
Two edible acids, viz., acetic acid (AA) and citric acid
121
(CA) have been used in this study for pre-acidifying
the fluoride containing water to generate high
concentration of Ca2+ in the crushed limestone pack in
situ due to dissolution of CaCO3 by the acids. These
edible acids have been chosen because they do not
add any harmful contaminants or unpleasant taste to
the treated water39. The organic anions of the acids
remaining, after neutralization by limestone, in the
water also should not affect the acidity of gastric juice
which is due to dissociation of HCl. Acetic acid can
eliminate fatigue, because it is changed to citric acid
by combining with oxaloacetic acid with the help of
Coenzyme A and ATP (adensosine triphosphate)39.
Acetic acid has 1/60 power of hydrochloric acid, while
citric acid has 1/180 power of hydrochloric acid. So, it
is classified scientifically as one of the weaker acids,
having no power to cause a heart burn39.
Experimental Procedure
Materials
Acetic acid, citric acid and sodium fluoride
(AR Grade, Merck Ltd.) were used. Distilled water
was used for the preparation of F- solutions (10 mg/L)
by dissolving anhydrous sodium fluoride. Crude
limestone and powdered limestone of particles
size <170 µm were obtained from Bokajan Cement
Factory, Bokajan, Assam, India. The crude limestone
was crushed and the 3-4mm size segment was used
for the experiments. The composition of limestone
based on chemical analysis is given in Table 1.
Analysis of fluoride, calcium and pH
Concentrations of fluoride were measured on an
Orion Multiparameter Kit ion meter using an Orion
ion selective electrode for fluoride. TISAB-III was
used to control ionic strength and de-complex
fluoride. The calibration was done at 10, 1.0 and
0.1 mg/L standard sodium fluoride solutions. Calcium
was determined by using an atomic absorption
spectrophotometer - Perkin Elmer AA200. The pH
was measured on a Systronics µ-pH System.
Table 1—Chemical composition of limestone
Sl. No.
Composition
1
2
3
4
5
6
7
8
Calcium carbonate CaCO3
Magnesium carbonate MgCO3
Calcium oxide CaO
Magnesium oxide MgO
Silica SiO2
Aluminium oxide Al2O3
Iron oxide Fe2O3
Loss on ignition
% Weight of dried
material
85.0-90.0
0.8-1.6
40.0-45.0
0.8-1.2
10.0-14.0
4.0-5.0
2.5-3.5
33.5-37.5
122
INDIAN J. CHEM. TECHNOL., MARCH 2010
Defluoridation by limestone using acid treated water
Batch tests were carried out with crushed limestone
using synthetic fluoride solutions pre-acidified with
varying amounts of AA and CA. In the first batch
tests with crushed limestone, a set of half liter size
bottles were filled with 3-4 mm size limestone
particles and 10 mg/L F- solutions having varying
concentrations of 0.016 to 0.20 M of AA were added
to fill up to the top level of limestone. They were
allowed to stand without shaking. Parts of the
solutions were withdrawn after 3, 6 and 12 h by
decantation; filtered using Whatman 42 filter paper
and the concentration of F- and final pH in the
solutions were measured. The remaining solutions of
the bottles were drained out. Fresh F- solutions were
put into the respective bottles for the next day
containing the limestone packs. This experiment was
repeated once a day for five consecutive days. The
experiments were repeated with CA of varying
concentrations from 0.01 to 0.10 M and crushed
limestone chips of 3-4 mm.
time with residence time (t) of 12 h. CA was added
only up to 0.1 M because above that concentration,
excessive precipitation of calcium citrate was
observed as confirmed from IR and XRD data.
Although this precipitate makes the water turbid,
those were settled down on the bottom of the
container on standing for 2-3 h. Thus the turbidity of
the solution can be removed by filtration.
Batch test with added AA
The results of the batch experiments with AA
and crushed limestone are shown in Fig. 2. Lowering
of the F- concentration from 10 to 1.5 mg/L was
achieved with an initial concentration of 0.033 M of
AA, which is not only better than limestone filtration
alone but also better than limestone filtration with
CO2 bubbling14. These batch test results also indicated
a slightly better efficiency of AA in comparison to
CA in F- removal at equal concentration of the acids.
Results and Discussion
Defluoridation by limestone using acid treated water
Batch test with added CA
The results of batch experiments of F- removal with
crushed limestone chips of 3-4 mm size, as a function
of initial concentrations of CA up to 0.1 M are shown
in Fig. 1. It can be seen from the figure that addition
of the acid to water before treatment with crushed
limestone lowers the F- concentration from its initial
value of 10 mg/L to below 2 mg/L, which is much
better compared to the fluoride removal by limestone
filtration alone14. The ability of the crushed limestone
chips to remove F- increased with increasing the acid
concentration. However, the removal became poorer
with the number of repeated use (η) of the same lot of
limestone chips. This happens due to the adsorption of
fluoride onto the surface of the limestone. Turner
et al.41 have shown that limestone can precipitate as
well as adsorb fluoride. As the acid reacts with
limestone generated Ca2+ and it precipitates as CaF2.
At the same time new surfaces of limestone are
created and fluoride also gets adsorbed onto the
surface. With increase in number of repeated use (η),
adsorption of fluoride and calcium citrate on the
limestone surface decreases the ability of the
limestone for fluoride adsorption. An initial CA
concentration of 0.05 M is required to bring the
F- concentrations from 10 to 1.98 mg/L when the
crushed limestone chips sample was used for the first
Fig. 1—Concentration of [F-] and pH of water after treatment with
crushed limestone versus concentration of citric acid. Size of
crushed limestone used, 3-4 mm; time, 12 h; [F]o=10 mg/L.
η = no of repeated use of same limestone.
Fig. 2—Concentration of [F-] and pH of water after treatment with
crushed limestone versus concentration of acetic acid. Size of
crushed limestone used, 3-4 mm; time, 12 h; [F]o=10 mg/L.
η = no of repeated use of same limestone.
NATH & DUTTA: FLUORIDE REMOVAL FROM WATER USING CRUSHED LIMESTONE
Moreover, one can see from Fig. 2 that the remaining
F- concentration can be further reduced to 1 mg/L on
increasing [AA]o to 0.2 M. The F- removal ability of
the crushed limestone chips gradually decreased
with η in the case of AA also. An important point to
be noted here is that the in situ generation of high
concentration Ca2+ ions by dissolving limestone with
CA and AA can lower the F- concentration much
more than the crushed limestone chips alone as is
evident from the results in Figs 1 and 2. Another
interesting observations from the batch tests with
crushed limestone and added acid is that the final pH
of the water which come to the near neutral range and
is acceptable for drinking. The final pH of the treated
water was found to be between 7.0 to 6.2 and 7.0
to 5.7 for water pre-acidified with CA and AA,
respectively, as shown in Figs 1 and 2 for η=1, 3 and
5. The slightly lower values of pH can be brought to
the acceptable range for drinking by passing the water
through a limestone column after this treatment14,41.
The final pH may also be maintained within the
acceptable limit by optimization of the process
parameters. Thus, the present method also has an
advantage over use of MgO or CaO for defluoridation
needing subsequent pH correction by adding more
chemicals31,32. It can be mentioned here that no odor
of vinegar was observed in the treated water which
can be attributed to the neutralization of the acetic
acid by the limestone. Occurrence of precipitation of
CaF2 in the experiment can be proved by calculation
of saturation index from the concentration of Ca2+ and
F- in the treated water which has been elaborated in
the next section.
Saturation index of fluorite (SIf)
Saturation index of fluorite (CaF2) in the water
after the batch tests with the crushed limestone chips
can be used to know whether precipitation of CaF2 is
a major mechanism of fluoride removal or not41. A
positive saturation index of fluorite (CaF2), SIf in the
water after treatment of the acidified water with
crushed limestone indicates precipitation to control
the F- removal. The SIf of the solutions have been
calculated using Eq. (1),
SIf = log10 (Q/Ksp)
…(1)
where, Q = (activity of Ca2+)(activity of F-)2 and Ksp=
fluorite solubility product41.
The SI values for acetic acid have been found
within the range of 0.07 to 0.2 and for citric acid
123
the range was 0.01 to 0.7. All the values are positive,
which indicates precipitation to control the Fremoval. However, since adsorption of fluoride from
water over calcite has been already known36, the same
may also take place along with precipitation in the
present case.
The excess free Ca2+ ions generated by the
dissolution of limestone by the acids combine with
the F- present in water are precipitated40. The Ca2+
concentration in the treated water was found to be
within the range of 8 mg/L initially which increased
to 109 mg/L on repeated use of the same limestone
sample for both the acids. Thus the values of
Ca2+ concentration remain within soft or moderately
soft range of total hardness42. The moderate hardness
can be removed by simple methods by using water
softeners. However, the slight increase in [Ca2+] above
the desirable limit can be removed or may be
acceptable as the increased Ca2+ concentration may
also work as a potential calcium supplement for
patients of fluorosis and osteosporosis43,44. An
analysis of the treated water showed that no harmful
chemicals such as Pb or Hg were leached from the
limestone during the process.
Effect of contact time
The dependence of the removal of F- on contact
time, t with crushed limestone in the batch was
examined by measuring remaining F- concentration in
water samples withdrawn from the containers after
t = 3, 6 and 12 h and the results are presented in
Fig. 3. The removal of F- was found to increase with
increasing the contact time. In the case of CA, the
slope rapidly decreased to almost zero at 0.10 M. This
means, the fluoride removal attains equilibrium fast
on the first use of the crushed limestone and the
equilibration time increases with η in the case of CA.
In the case of AA the removal continued to improve
even beyond 12 h up to the acid concentration of
0.10 M. The remaining F- concentration decreased
almost linearly with time within the experimental
range of 12 h. The slope slightly decreased with
increasing the concentration of the acid. This
indicates that at higher concentration of the acid the
F- removal is not only more but also quicker. It can be
seen from Figs 3 and 4 that the equilibration times for
F- removal in the presence of 0.1 M CA and AA are
about 6 and 12 h, respectively and the equilibration
time increases with decrease in the concentration of
the acids in both cases.
124
INDIAN J. CHEM. TECHNOL., MARCH 2010
the fluoride concentration from 10 to 1.74 mg/L
and 0.977 mg/L, respectively, without affecting the
taste and colour. Although, adsorption of fluoride
on limestone surface takes place, precipitation is
the major mechanism of fluoride removal in the
batch tests with acidified fluoride containing water
and crushed limestone. Therefore, limestone
defluoridation of fluoride containing water pretreated
with CA and AA can be potential ways of removing
fluoride from water.
Fig. 3—Concentration of [F-] in water after treatment with crushed
limestone equilibrated for varying contact time in presence of citric
acid for η = 1 of the batch test. Size of crushed limestone used,
3-4 mm; η = no of repeated use of same limestone.
Acknowledgements
The authors are thankful to the Department
of Science and Technology, New Delhi, India
for financial support under the projects
DST/TDT/WTI/2k7/24 and DST/TSG/WP/2007/14.
The authors also thank The Cement Corporation of
India, Bokajan, Assam for providing limestone
samples.
References
1
2
3
4
5
6
Fig. 4—Concentration of [F-] in water after treatment with crushed
limestone equlibrated for varying contact time in the presence of
acetic acid for η = 1 of the batch test. Size of crushed limestone
used, 3-4 mm; η = no of repeated use of same limestone.
7
8
Cost effectiveness and suitability
Limestone is a very low-cost material. Therefore,
the cost of the present fluoride removal method is
practically due to the cost of the acid only.
Considering the present market price of AA and CA
as Rs. 100/- and 125/- per kg, respectively, the cost of
fluoride removal by the present method should be
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Conclusions
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