vc/EPA
United States
Environmental Protection
Agency
Municipal Environmental Researc
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-082 July 1981
Project Summary
The Equilibrium Fluoride
Capacity of Activated Alumina
Gurmderjit Singh and Dennis A. Clifford
Because design limitations for a
defluoridation process were unavail-
able, laboratory studies were performed
to establish the maximum (equilibrium)
fluoride capacities for activated alumi-
na as a function of pH and competing
ions.
Fluoride adsorption tests were run
using mini-columns containing 1 gram
of acid-treated Alcoa F-1* activated
alumina equilibrated for 6 to 10 days
with a continuous flow of fluoride
solution at a constant pH. This proce-
dure is very different from the usual 1 -
hour batch equilibration test and
should give a more accurate estimate
of the maximum fluoride-adsorption
capacity of the alumina.
Maximum fluoride adsorption ca-
pacities in distilled water solutions of
sodium fluoride were found to be
8000 g FVm3 at 3.0 mg FYL and
9100 g FVm3 at 6.0 mg FVL. The
optimum pH range for fluoride adsorp-
tion was 5 to 6. Based on data now in
the literature and preliminary results
from pilot studies, less than 50 per-
cent of these capacities are estimated
to be attainable in actual municipal
defluoridation practice because of
poor kinetics and competing ion ef-
fects including silicate, which was not
tested here.
The alumina adsorbent preferred the
common groundwater anions in the
following order: OH~^>F~>SOr>Cr
> HCO3. Fluoride was highly preferred
*Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use by the U S Environmental Protection
Agency
over sulfate; however, at a typical
concentration of 250 mg SOijVL, the
fluoride adsorption capacity was re-
duced 28 percent at pH 6. Fluoride
capacities were not reduced signifi-
cantly by competition from chloride or
bicarbonate.
Although kinetics was not a major
objective of these studies, it was ob-
served that as pH increased from 6 to
8, the rate of fluoride adsorption also
increased. Adsorption rate should be
further studied.
This laboratory research on fluoride
removal was done in support of field
studies on the removal of fluoride with
the use of activated alumina columns
in a mobile pilot plant designed and
constructed at the University of Hous-
ton. The Mobile Drinking Water Treat-
ment Research Facility was completed
on April 9, 1980, and transported to
its first field location—Taylor, Texas, a
small community (population 13,000)
with a high-fluoride (3.0 mg/L)
groundwater supply. The mobile facil-
ity containing activated alumina and
ion exchange columns and reverse
osmosis and electrodialysis units may
be transported to any U.S. community
with an inorganic contaminant problem.
This Project Summary was devel-
oped by EPA's Municipal Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back). A second project
report by O. Clifford and M. Bilimoria
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covering the design, construction, and
operation of the Mobile Drinking
Water Treatment Research Facility
and a third report by D. Clifford, C. C.
Lin, and M. Bilimoria covering the
pilot-scale fluoride removal studies at
Taylor, Texas, will be published at a
later date.
Introduction
Packed beds of activated alumina
granules have been used for about 30
years to remove excess fluorides from
potable water, especially at levels > 5
mg/L current estimates are that there
are several thousand public water sup-
plies in the United States with excessive
fluorides, especially in the 1.4 to 5.0
mg/L range. Many of these communities
may eventually install activated alumina
treatment systems to remove the excess
fluoride pursuant to the requirements of
the 1974 Safe Drinking Water Act.
When this research began, reliable
process design criteria for defluorida-
tion of groundwater using activated
alumina were unavailable in the litera-
ture. This was especially true for fluoride
Plastic Tubing-^
Tee Connection_
For Influent
Sample
Mini-Column-
Glass Wool—
Pinch Clamp for—
Flow Adjustment
2 L Beaker
For Effluent
20 L Fluoride
Solution
Reservoir
Column
Stand
1 gm
- Pretreated
Alumina
Figure 1. Typical alumina mini-
column apparatus for
adsorption runs. Repre-
sents one often columns
used simultaneously.
10,000
3000
8000
II
o
i<
7000
6000
5000
2.0 4.0 6.0 8.0
Liquid Phase Fluoride Cone. mg/L
10.0
Figure 2.
Fluoride adsorption isotherm. pH 6.0; temperature 22°C; distilled water
solutions of NaF; pH adjusted with
concentrations less than 5 mg/L. There
was no systematic treatment available
of the effects of pH, competing ions, and
fluoride concentration on alumina's
capacity for fluoride at any concentra-
tion. Because fluoride "adsorption" on
acid-treated activated alumina is thought
to be an ion exchange process, all these
variables would be expected to influence
capacity. Furthermore, the available
literature data were conflicting and
obtained either from short contact time
(1 -hour) batch equilibration tests or
from actual municipal defluoridation
plant operating results. Both of these
sources have serious disadvantages.
Fluoride adsorption kinetics are known
to be very slow, and the effects of
variable feed water quality cannot
readily be studied in full-scale plants.
Also, some of our preliminary experi-
ments indicated that it would take
several days of continuous exposure to
continuously flowing fluoride solution
to achieve 90 percent of equilibrium
adsorption capacity. Thus itwasdecided
to perform long-duration column tests
using feed solutions of varying fluoride
concentration, pH, and anion composi-
tion to reliably establish the effects of
these variables on the equilibrium ad-
sorption capacity of activated alumina.
The ultimate objective was to develop
design limitations for municipal defluo-
ridation processes. The laboratory data
would also be used to plan pilot-scale
defluoridation experiments in various
U.S. communities.
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Methods and Materials
Alcoa, 28 x 48 mesh, F-1 grade
activated alumina was conditioned by
decanting to remove fines and washing
exhaustively in a column with hUSOA-
acidified, pH 5, fluoride-free tap water
before drying at 110°Cfor 48 hours. An
alumina exhaustion run consisted of
passing 10,000 to 30,000 bed volumes
of fluoride solution through a mini-
column containing 1 gram of conditioned
sulfate-form alumina over a period of 6
to 10 days. A sketch of the apparatus
showing one of the 11-mm ID mini-
columns is shown in Figure 1. Fifty runs
were performed in all. The early runs
were duplicated to verify that, indeed,
the procedure was reproducible. The
fluoride capacity of the exhausted alu-
mina was determined by eluting the
fluoride from the alumina over 4 hours
using a 50-fold stoichiometric excess of
caustic (100 ml of 1 percent NaOH solu-
tion). This procedure recovered at least
95 percent of the adsorbed fluoride.
Regenerant solutions were analyzed for
fluoride with the use of a fluoride-
selective ion electrode and a special
buffer to prevent interference from the
dissolved aluminum. Sulfate was deter-
mined by a BaCI2 turbidimetric method,
bicarbonate by the inorganic channel of
a TOC analyzer, and chloride by poten-
tiometric titration using a Ag/AsS
electrode.
Fluoride concentrations in distilled
water in the range of 0 to 10 mg/L and
pH in the range of 5 to 8 were evaluated.
Minimal concentrations (< 0.006 meq/L)
of H2S04, NaaCOs, and NaOH were used
to adjust pH. Individual competition
from sulfate, chloride, or bicarbonate
was evaluated in concentrations in the
range of 0.5 to 1 5 milliequivalents/L.
The effects of high ionic strength (high
TDS) were evaluated using solutions
containing the same number of equiva-
lents of sulfate, chloride, and bicarbon-
ate at total solution concentrations in
the range of 3.8 to 56 millimoles/L.
Reagent grade sodium salts were used
throughout the experiments.
Fluoride analyses of column effluents
were made periodically throughout
each run to determine when a given
column was exhausted. Although pre-
cise flow control was not maintained,
the total volume fed to a given column at
a particular time was precisely mea-
sured. These data were used to deter-
mine average flow rates, which varied
from 1 /2 to 4 ml/min.
Results
The effects of fluoride concentration
on fluoride adsorption capacity at pH 6
are shown in Figure 2; the effects of pH
at constant fluoride concentration are
illustrated in Figure 3. Table 1 sum-
marizes both these effects on fluoride
adsorption capacity.
Figure 4 depicts the effects of varying
concentrations of sulfate or chloride on
the equilibrium fluoride capacity of the
alumina at pH 6 and constant fluoride
concentrations (5.7 mg/L). Finally, the
time to 90 percent of equilibrium at
various pH values and fluoride concen-
trations is depicted in Figure 5; for all
the data points, throughput (T = fluoride
fed to column/fluoride capacity of
column) was approximately 2.5.
Table 2 is a very useful compilation of
relevant conversion factors for design
purposes and literature comparisons. It
is included because the defluoridation
literature contains so many different
units expressing fluoride adsorption
capacity.
The full report was submitted in ful-
fillment of Grant No. R806073010 by
the University of Houston under the
sponsorship of the U.S. Environmental
Protection Agency.
10,000
8000
u
<0
5 5000
4000
2000
Fluoride
Concentration
j_
Figure 3.
4.0 5.0 6.0 7.0 8.0 9.0
pH
Effects of pH on fluoride adsorption capabity. Distilled water solutions
of NaF; pH adjusted with Na2C03 and HzSO*
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Table 1. Effects of pH and Aqueous-Phase Fluoride Concentration on Fluoride
Adsorption Capacity*
Fluoride Adsorption Capacity in gm F /m3
of Alumina
Fluoride Concentration
mg/L
5.0
Adsorbate pH
6.0 7.3
250
8.0
2.0
4.0
6.0
8.0
10.0
7390
8680
9360
9200
9395
7445
8605
9075
8965
9340
3030
4580
5510
6195
6645
2160
3280
3835
4250
4650
^Capacities are based on an alumina packed bed density of 55 Ib/ft3.
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G. Singh and D. A. Clifford are with the University of Houston, Houston, TX
77004.
Thomas J. Sorg is the EPA Project Officer (see below).
The complete report, entitled "Equilibrium Fluoride Capacity of Activated
Alumina," (Order No. PB 81-204 075; Cost: $8.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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