14 Research Report
Fluoride Vol. 32 No. 1 14-19 1999
FLUORIDE UPTAKE CHARACTERISTICS OF FLY ASH
R Piekos and S Paslawska
Gdansk, Poland
SUMMARY: Retention of fluoride ion in dynamic experiments on columns
packed with fly ash was studied at 20ºC with a series of aqueous solutions
containing 1, 5, 10, 20, 50, and 100 mg F-/L. The flow rate through a 450-g
bed was ≤ 2 mL/hr. At the lowest F - concentration (1 mg/L), the F - level in
the effluent initially increased and then gradually decreased down to 0 mg/L
after 120 hours. With higher F - concentrations in the feed solutions, the F concentration in the effluent steadily decreased reaching 0 mg/L after 120 168 hours. We conclude that coal fly ash is an effective sorbent for F - ions,
especially at high concentrations in water.
Key words: Coal burning, Fluoride removal, Fly ash, Water.
INTRODUCTION
Wastewaters from phosphate fertilizer plants may contain up to 2 per cent
of fluoride.1 Increased levels of fluoride can also be present in effluents from
the fluorine industry,1 glass etching,1 and in ground water around aluminum
smelters.2,3 The problem of high fluoride concentration in groundwater resources has become an important health-related geo-environmental issue in
some areas. Examples include the state of Rajasthan, India, where nearly 3
million people are reported to consume excess fluoride-containing water,4,5
and the upper regions of Ghana, where 23 per cent of wells have fluoride concentrations above the WHO recommended maximum guideline limit of 1.5
mg/L.6 In the Gdansk region, high fluoride levels (1.90 - 3.00 mg/L) were
detected in Malbork drinking water supplies.7 Since excessive amounts of
fluoride may cause adverse health effects to humans and animals, there is a
need for defluoridation of industrial wastewaters. Classical procedures of
defluoridation involve precipitation, adsorption, ion exchange, and membrane
techniques.8 Of these, the most popular and cost-effective is precipitation of
fluoride with lime. However, the resulting calcium fluoride, though sparingly
soluble, still poses environmental concern and must be safely disposed of.
Fly ash is the major solid waste by-product from coal-fired power plants. It
is produced as a fine residue carried off in the flue gases with relatively uniform particle size distribution in the 1 to 100 μm range. The main components
of fly ash are silica, alumina, iron oxides, calcium oxide, and residual carbon.
The fineness of the fly ash particles and the inherent large surface area (1 to 6
m2 g-1),9 together with the content of unburnt carbon, make it a good candidate
for utilization as an inexpensive sorbent.
A literature survey revealed that fly ash has been used for removing heavy
metals10-17 and radionuclides18 from aqueous solutions, for treatment of
wastewaters to remove organic compounds 19-22 and color,23 as a coal desulphurization agent,24 and - together with hydrated lime - for SO2 removal from
flue gases.25
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*Medical University of Gdansk, Faculty of Pharmacy, Al. Gen. Hallera 107, PL - 80-416
Gdansk, Poland
Fluoride uptake by fly ash
15
Dynamic experiments on leaching of trace elements from fly ash, carried
out by Dybczynski and co-workers26 have shown that distilled water (1800
mL) leached out ca 23% of fluoride
and simulated acid rain (H2SO4, pH
2.5, 1800 mL) ca 38% (both figures
taken from a bar graph) of fluoride
from 10 g of the material placed in a
thin layer between two filter papers,
during 6 hours.
There is also evidence27 that the
capital and operating costs of
wastewater treatment by fly ash to
remove turbidity, fluoride, and to reduce COD, are lower than those by
conventional lime precipitation.
In view of the advantages of fly ash
as a low-cost sorbent, it seemed worthwhile to study its efficiency in removing fluoride ion from high-fluoride
waters.
MATERIALS AND METHODS
Fly ash was collected directly at the
electrostatic precipitator of the Gdansk
Thermoelectric Power Plant. The grain
size of the ash ranged between 1-90
μm with a mean diameter of 20-30 μm.
The results of its chemical analysis are
as follows: SiO2 52.7%, Al2O3 21.9%,
Fe2O3 8.4%, CaO 7.2%, loss on ignition 9.1% (a class F fly ash according
to ASTM standards). A slurry prepared
from 10 g of the fly ash and 100 mL of
water had a pH of 10.1.
All chemicals were of analytical reagent grade, and distilled water was
used throughout.
Sorption experiments were conducted at 20oC in a column (see Figure) packed with 450 g of fly ash.
The concentrations of fluoride in
water fed on the column were 1, 5, 10,
20, 50 and 100 mg/L.
The procedure was as follows: 250
mL of a fluoride solution was first
Figure. Schematic of the
experimental setup.
Fluoride 32 (1) 1999
16
Piekos and Paslawska
poured onto the column to moisten the fly ash. Then a 250-mL portion of the
solution was placed in the top reservoir and the draining rate was adjusted to
ca 2.0 mL/hr. The fluoride level in the effluent was determined every 24 or 48
hrs. potentiometrically by using a fluoride ion-selective electrode. Particulars
concerning the analytical procedure are reported elsewhere.28
Each experiment with solution of a given concentration was run in duplicate with a fresh portion of fly ash.
RESULTS
Results of the measurements are summarized in the Table.
Table Results of the measurements
F- conc.
Aliquot of ef- Sampling
in incoming fluent taken for
time
soln. – mg/L analysis - mL
hrs
0
4.0
1
1.0
5
1.0
10
1.0
20
0.1
50
0.1
100
0.1
Fluoride 32 (1) 1999
F- concentration in effluent
mg/L
First expt.
Second expt.
Mean
24
48
72
1.0
0.8
0.5
0.8
0.6
0.5
0.9
0.7
0.5
24
48
72
96
1.8
0.8
0.5
0
2.0
1.5
0.9
0
1.9
1.2
0.7
0
24
48
72
96
120
3.8
1.8
1.5
0.5
0
3.9
1.9
1.5
0.3
0
3.8
1.8
1.5
0.4
0
24
48
72
96
120
144
7.5
5.3
3.6
1.9
0.5
0
6.8
5.2
3.9
2.5
0.9
0
7.1
5.2
3.7
2.2
0.7
0
24
72
120
168
12.5
5.5
2.5
0
13
7.0
3.5
0
12.6
6.2
3
0
24
72
120
168
17
12.5
5.5
0
18
13.5
4.2
0
17.5
13
4.8
0
24
72
120
168
25
15
4.8
0
22.5
17.5
5.8
0
24
16
5.3
0
Fluoride uptake by fly ash
17
DISCUSSION
Inspection of the results presented in the Table shows that the sorption of
fluoride increases with increasing concentration of incoming solution. Pure
water leaches fluoride from fly ash in which it is likely to occur in the form of
fluorite and metal fluoride complexes. When a solution with low fluoride concentration (1 mg/L) is passed through the column, the fluoride level in the
effluent is initially higher than in the incoming solution (1.9 mg/L average
after 24 hrs) owing to concurrent leaching of the fluoride contained in fly ash.
However, after 120 hrs the effluent becomes completely free of fluoride.
It is remarkable that the retention capacity of fly ash increases with increasing fluoride concentration in incoming solution. This finding is best illustrated by the decrease in F- levels of the solutions after 24 hrs. For the 5, 10,
20, 50 and 100 mg/L F- concentrations the respective drops in F - concentration are 24, 28, 36.5, 65 and 76 per cent. Complete retention of the fluoride by
fly ash occurs after 120-144 hrs for the lower F - concentrations (1, 5, and 10
mg/L) and after 168 hrs for the higher F - concentrations.
The explanation for the high sorption capacity of fly ash with respect to the
fluoride ion seems quite straightforward if one considers its high CaO content
(7.2%) on the one hand, and a slow flow rate of the solution through the column (ca 40 mL/24 hrs) on the other. The hydration of the oxide is responsible
for the high pH of the water slurry of fly ash (pH exceeding 10). The resulting
calcium hydroxide reacts with the fluoride to afford a sparingly soluble calcium fluoride.
A simple calculation shows that in the total quantity of 450 g of fly ash
packed in the column, there is 32.4 g (0.58 mole) of CaO, whereas 500 mL of
the most concentrated F- solution (100 mg/L) contains only 0.0025 mole of F-.
Strong sorptive capacity of residual carbon is also likely to contribute to
this process. Recent investigations into the morphology and bulk physicochemical properties of this fly ash component, carried out by Hurt and associates,29 led to the following conclusions (numbering by the present authors):
i. “The residual carbon samples show signs of significant oxidation that
has led to highly porous, fragmented particle structures, and visible
fused surface ash.
ii. “Residual carbon does not consist of fuel particles that have largely
avoided high-temperature oxidizing trajectories in the boiler; rather it
has undergone complete devolatilization…, high-temperature treatment
and significant, though incomplete, oxidation;
iii. “The residual carbon particles are generally quite macroporous, providing good access to oxygen to penetrate to the particle interior;
iv. “There is no evidence that a significant fraction of the carbon is encapsulated by inorganic matter at the whole-particle scale; [and]
v. “The residual carbon samples have microporous surface area of 100 200 m2g-1 carbonaceous material.”
Fluoride 32 (1) 1999
18
Piekos and Paslawska
Especially supportive to our belief are conclusions (iii) and (v) above.
Firstly, large pores of the carbon which are capable of accommodating oxygen
molecules with the diameter of ca 300 pm, can also provide good access to
much smaller fluoride ion with the diameter of 172 pm. Secondly, the surface
area of the carbon (100 to 200 m2g-1) is much larger than that of bulk fly ash
(1 to 6 m2g-1).9
We conclude that a double mechanism is likely to operate in the retention
of fluoride by fly ash: chemical binding by calcium hydroxide and physical
sorption by residual carbon particles.
This paper was presented and discussed at the XXIInd Conference of the
International Society for Fluoride Research in Bellingham, Washington USA
(24-27 August, 1998).
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Published by the International Society for Fluoride Research
Editorial Office: 17 Pioneer Crescent, Dunedin 9001, New Zealand
Fluoride 32 (1) 1999