Indi an Journ al or Chemi cal Technology
Vol. II , March 2004, pp. 185- 189
Study on the fluoride removal characteristics of mineral (fl uorapatite)
I B Singh* & M Pras ad
Reg ional Research Labo ratory (CS IR), Bhopal 462026, India
Rl'cl' ivl'd 25
.IWI/WIT
2003; revised received 8 Seple111i1er 2003; (lcu'l, led 5 Fehmary 200-1
The cfTect ive ness of low grade Iluorapatit e bearin g mineral on Iluoride remova l has not been exami ned earl ier.
For identification of such mineral. a preliminary assessme nt of performan ce of mineral is essenti al In orde r to kno w
which as pect is necessary to carry out further research for development of mineral based delluoridat iun tec hnology.
In thi s study batch adso rpti on st udy was performed to eva luate the Il uorid e re moval potentia l of low cost rock
phosphate (tluroapatite) mineral at pH 4, 5 and 6. The optimum dose was determined to be 1.5. 2.5 and 3.5 g mineral
at pH 4, 5 and 6, respectively. The first order adsorption rate constants derived by Lagergren equ atio n at different
pH , clearl y indi cates th e fast remo val kinetics at pH 4. The compara ti vely hig h so lu bi lit y of mineral and an increase
of calcium concentrati on at pH 4 appears to be the main reason for fas t remova l reacti on. There was no efrect of
minera l quantit y 0 11 flu oride remova l aft er so luti on pH beca me alkaline. The va rious as pects of th e study including
recyclabi lity of mi neral are discussed.
IPe Code: C02F I/28
Keywords: Fluorap;ll ite, Fluorid e remova l, Delluorid ati on technology, Adsorpti on, Lagergren equat ion
To control flu oros is du e to drinking of excess fluoride
bearing water, several deflu oridati on methods have been
developed during last si x decades!". But none of them
could solve th is problem at compl ete acceptab le level.
The co mpl ex ity of wate r quality and necess ity of
maintaining desirable level of fluoride in drinking water
are the main constraints for success of any treatment
method. Fluoride removal based on adsorption and ion
exchan ge by bone char, synthetic ion exc hange material
and activated alumina are so mewhat successful.
However, du e to high installation and maintenance cost,
these tec hnol ogies cannot be adapted in large scale in
country like India especially in its rural bases. During
ea rl y 1970, National Environmental Engineering
Research Institute developed alum, lime based treatment
method, known as " Nalgonda Technology"4 which was
preferred because of utili zation of lime. Unfortunately
thi s technology is not very successful in India due to
troubl esome operation and lack of awareness among
peopl e. Recently, generation of residual aluminum by
alum-lime lreatment is reported due to initiation of
co mpl exation reaction between alum and flu ori de'.
Uptake of solubl e aluminum may initiate secondary
toxicity problem. This seems to be another limitati on
of the use of alum in fluorid e remov al processes for
drinking purposes.
Now-a-days attempts are being made to repl ace
conventional adsorbent i.e. acti vated carbon by low
cost clays and min erals for th e removal of in orga ni c
pollutants including heavy meta ls' ·7. Mineral apa tites
(hydroxyapatite and fluorapatite) have been utili zed for
the removal of heavy metals like lead , cadmium , zi nc
etc using sorption technique x. 12 . Acco rdin g to av ailable
literature the fluoride remova l potential of th ese min erals
has not been examined earli er. Present in ves ti gati on
was aimed to see the possibilities of fluorid e remova l
in the presence of fluorapatit e (low g rade rock
phosphate) in diluted acidic sol uti on under different
ex perimental con ditions . The PlO, co ntent of thi s
mineral is low (-10 %) an d is not fo und su itable in
fertilizer manufacturing . It is av ailable as low cos t
mineral in Jhabua (MP) region .
*For
co rr es pond e nce
(E -m ai l:
ib s ingh @rrlbpl.org/
[email protected]; pax: 9 1 755 2587042)
Experimental Procedure
The rock phosphate (flu orapatite mineral) was
procured from the Jhabua region with th e help of M.P.
186
INDIAN 1. CHEM . TECHNOL., MARCH 2004
Table I-Chemical analysis of rock phosphate (fluorapatite)
mineral
Chemical compositions
pps
SiOl
CaO (as calacite)
FePl
Alp]
MgO
TiO)
Loss of igni tion
Weight percent
13 .00
26.50
32.50
1.59
6.05
0.06
0.47
19 .11
Table 2-Effect of pH on mineral solubility, hardness and calcium
Water solubility
pH 4
pH 5
pH 6
Weight percentage
29.7
17.3
14.1
Total hardness as CaCO]
pH 4
pH 5
pH 6
mg/L ( after shaking I g mineral)
25.4
14.7
10.3
Soluble calcium
pH 4
pH 5
pH 6
mg/L ( after shaking Ig mineral)
10.1
6.9
4.1
State Mining Corporation. The molecular concentration
of different constituents of the mineral is given in Table
I . The surface area of the mineral determined by BET
was found relatively large (- 0 .152 m 2 gol). After
grinding the raw mineral samples into different size
fractions, particles with size less than 150 mm was
se parated and used for fluoride removal. For
determination of solubility in water having pH 4, 5 and
6, I g mineral was shaked for 60 min . The insoluble
mineral left in the solution was filtered, dried and
reweighed. The measured weight loss was considered
as solubility of mineral which is given in Table 2. The
hardness and calcium content of dissolved mineral in
water were determined by EDTA titrimetric methd 13.
The stock solution of fluoride (1000 mg/L) was
prepared by dissolving required quantity of analytical
':!':lcle ( S. D. Fine Chemicals) dried sodium fluoride in
triple distilled water. The working solutions of fluoride
10 mg/L) were made by successive dilution of the
lock solution. pH of the fluoride solution was adjusted
ldding dil..HCI and NaOH . Therafter, fluoride
,oval exercises, utilizing batch shaking process were
performed. In each experiment, 100 mL of 10 mg/L
fluoride solution of pH 4, 5 and 6 was taken in a
conical flask containing known quantity of mineral.
pH of the treated solution was measured at the end of
batch experiment. Residual fluoride left after removal
in the treated solutions was analyzed by SPANDS
method l3 of chemical analysis. Analytical error during
analysis was kept <2% at I mg/L range in calibration.
GB make Citra 40 UV visible spectrophotometer and
Systronic make pH meter were employed for residual
fluoride analysis and pH measurement, respectively.
All the batch experiments and analysis works were
carried out at room temp (-25"C).
Results and Discussion
Since no report is available on fluoride removal
by low grade rock phosphate mineral, it is difficult to
explain this phenomenon without carrying out detailed
studies. Investigations related to removal of heavy
metals H. 11.I 2. 14 have shown that dissolution of rock
phosphate in acidic environment followed by their
precipitation is likely to be the reason for sorption of
metals. In case of fluoride ion, mechanism of its removal
may be quite different as compared to positively charged
metal ions. A strong affinity of fluoride for calcium
that is available in rock phosphate mineral as calcite,
can play an effective role in fluoride removal process .
Free calcium after dissolution of calcite from mineral,
is likely to be involved with fluoride ion as per the
following reaction,
.. . ( I )
This could be the most probable process which may be
responsible for removal of fluoride by calcium bearing
rock phosphate mineral. Based on this hypothesis
fluoride removal exercises were carried out at pH 4,5
and 6.
Effect of mineral quantity and shaking time
Different quantities of mineral were taken
separately in 100 mL of 10mg/L fluoride solution of
pH 4, 5 and 6 and kept for batch shaking for one hour.
Thereafter, analysis of residual fluoride in the treated
solutions was performed and a relationship between
fluoride removal percentages and mineral quantities was
established as shown in Fig.l. From the trend it can be
seen, that, a maximum level of 64% fluoride removal
occurs in the presence of 3.5 g of mineral at pH 6.
After this, removal rate became almost constant with
the increase of mineral quantity. At pH 4 and 5, fluoride
removal rates were found unexpectedly less as compared
187
SINGH & PRASAD el at. : THE FLUORIDE REMOVAL CHARACTERISTICS OF FLUORAPATITE
to pH 6. As seen in Fig.l, only 45% and 50% fluoride
removal occur at pH 4 and 5, respectively. Afterwards,
fluoride removal rate became independent with the
increase of mineral quantity. Mineral quantities 1.5,
2.5 and 3.5 g at which maximum fluoride removal
70r---------------------------~
60
50
~
ii
~40
E
I!
•
t,. 30
iL
20
10
o+----.----,----.----,---~----~
o
2
3
4
5
6
Mineral, g
Fig.I-Auoride removal percentages obtained in the presence of
different doses of mineral at pH 4 ( . ), 5 ( • ) and 6 (A.).
10
9
-Initial pH
.. Final pH
8
i
occurred at pH 4, 5 and 6, respectively, can therefore
be considered to be as optimum doses. The initial and
final pH of pretreated and finally treated solutions are
presented in bar chart ( Fig. 2). Shift of pH from 4 to
8.8, 5 to 8.5 and 6 to 8.3 of finally treated solution as
demonstrated in Fig. 2 suggests the development of
alkalinity in the presence of mineral. Solubility of calcite
of mineral could be the main reason for shift of pH in
the alkaline region. Solubility of mineral in water was
determined at pH 4, 5 and 6 (Table 2). Higher solubility
of mineral - 30% at pH 4 as compared to pH 5
(- 17%) and 6 (-14%) (Table 2) could be the main
reason for shift of maximum pH. Shifting of pH towards
alkaline region at pH 4 was confirmed by calcium
hardness determination of mineral in water ( Table 2).
It is interesting to mention, that, more than two times
higher calcium hardness and solubility of calcium were
found in mineral soluble water of pH 4 as compared
to pH 6.
The effect of contact time on fluoride removal
rate of mineral at pH 4, 5 and 6 was determined by
using its optimum doses i.e. 1.5, 2.5 and 3.5 g,
respectively. The batch experiments for 10 mgIL fluoride
were conducted at different intervals of time in 100 mL
solutions of pH 4, 5 and 6. After every 10 min of shaking,
residual fluoride in the treated solution was analyzed.
The fluoride removal gradually increased with time and
attained maximum value after 30, 50 and 70 min of
shaking at pH 4, 5 and 6 respectively ( Fig. 3). Afterwards,
fluoride removal became constant with time. The above
durations could be considered as optimum time for
maximum level of fluoride removal at respective pH.
7
70r-----------------------------~
6
60
5
50
4
3
2
20
o
10
2
3
Removal ExerdMs
o+-----,-----.-----~----_.----~
o
20
40
60
60
100
Time, min
Fig.2-Initial and final pH of treated solution of pH 4, 5 and 6
Fig .3-F1uoride removal rate with time in the presence of
optimum doses of mineral at pH 4( . ), 5 (. ) and 6 (A. ).
188
INDIAN J. CHEM. TECHNOL. , MARCH 2004
Reaction rate
The kinetics of fluoride removal was desc ribed
by following Lagerg ren first order equ at ion:
... (2)
log (qc - q ) = log qe - (Kat! / 2.303) I
where qc is the amount of f luoride adsorbed by per unit
weight of mineral (mg g'l) at eq uilibriu m time, q is the
amoun t of fl uoride adsorbed (mg g'l ) at a particular
time ( min ) and Kad is the first order rate constant. The
straight linear plot of log (qc - q ) vers us t as shown
in Fig. 4 , suggests the applicabi li ty of Lagergren
equation . T he rate consta nts ( Ka) at diffe rent pH were
calc ulated to be O. 106, 0.0785 and 0.0596 min' l for
pH 4, 5 and 6 respectively. The regress ion coeffic ients
(R2) were al so hi g h (0.985 , 0.9802 and 0.949) to
validate the a ppl ication of Lage rg re n equation .
The higher value of rate constant at pH 4 supports
the ex istence of a fast reacti on kinetic s as compared to
pH 5.
Recycling probability
The probability of recyclab ility of adsorbed
mineral has been carried o ut at pH 6. In this st udy
optimum dose (3.5 g) of minera l wa s kept for batch
shaking for one hour in 10 mg/L fluorid e containin g
100 mL soluti o n. In next two treatments, fres h mi nera l
of same quantity was used for the re moval of residual
fluoride of first time treated water. After eac h treatment
an increase of pH towards alkaline reg ion was observed.
The pH of the finally treated (3 rd time) so luti on was
aro und 9.
The res idu al concentrations of fluoride , as shown
in Bar chart (Fig, 5) after r, II and ill cycles of treatment
were 3 .5, 3 , 1 a nd 3 m g/L, respect iv e ly . The
ineffectiveness of mineral in reduc ing the fluoride
concentration in II and III cycle may be attributed to
the shift of pH towards a lk aline regi on. Development
of a lkalinity due to calcium as calcium carbonate ( pH
> 8.3), after fiJ'st treatment seems to reduce the flu oride
remova l reaction . The free calcium ion released by
di ssolution of calcite of minera l can preferabl y be
involved with CO)' of soluti on. T hi s cou ld be the reason
for neg lig ible re moval of fluoride after first treatme nt.
The recyclability of adsorbed mineral was al so tested.
Sim il ar to preceding ex periment, optimum dose of
mineral (3.5 g) was used for fluoride removal at pH 6.
After shaking for one hour, used min eral was separated
by filtration, dried and re used for fluorid e removal in
the fresh fluorid e solution (10 mg/L). T his process was
repeated for the 3 rd time under identical conditi ons.
T he residual concentrations of fl uoride after I, II and
III cycles of treatment were 4.4, 4.2 and 4. 1 mg/L
respectively, as shown in Fig. 6. Thi s study suggests
th at adsorbed mine ral can be recyc led for fluoride
remova l of fresh so luti on.
Un like activa ted a lum ina, low g rad e rock
phosphate can be preferred for fluoride removal because
of its low cost and easy avai lability. This ca n be used
-1.8 , - - - - - - - - - - - - - - - - - - ,
1 2r---------------------------------~
10
-1.3
•
a
Q)
~
Ini tial
Final
-.J
C. 6
E
- 0.8
'"
'0
"§
Ol
~
,g
6
c;;
"
'0
- 0. 3
'iii
Q)
a:
0.2
.(
2
o
10
20
30
40
50
Time It), min
Fig.4---Lagergrcn I order kineti cs plots for pH 4( . ),5 ( • ) and
6 ( ~).
0
1st
2nd
3rd
Removal steps
Fig.5-Fluoride removal in three consecuti ve steps of once treated
solution of pH 6
SINGH & PRASAD el al. : THE FLUORIDE REMOVAL CHARACTERIST ICS OF FLUORAPATITE
12
Acknowledgement
_Treatment
8
Authors are grateful to Dr. N. Ramakri shnan ,
Director, Regional Research Laboratory, Bhopal for
his encouragement and provid ing laboratory facilities
for carrying of this work.
6
References
After Treatment
10
..!
189
Cl
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ai
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0
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:;::
'iii
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4
I
2
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4
o
1st
2nd
3m
5
Removal steps
Fig.6-Fluoriue removal by once used mineral in fresh tluoride
bearing solutions of pH 6
as such without any purification or specific treatment.
However, control of alkalinity in the presence of mineral
seems to be an important aspects for further research.
Secondly, improvement in reduction of contact time for
fluoride removal in less acidic solution (pH 5 and 6)
can enhance the fluoride removal efficiency of minerals.
To develop a successful technology by using low grade
rock phosphate, it is therefore necessary to conduct
removal process in such a way, where a minimum
quantity of mineral is required. Simultaneously,
maintaining of pH in the neutral range is equally
important.
6
7
8
9
10
28 ( 1994) 1472.
II
12
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