Materials Science s a nd Applications, 2011, 2, 1654-1660
doi:10.4236/msa.2011.211220 Published Online November 2011 (http://www.SciRP.org/journal/msa)
Fluoride Adsorption onto Acid-Treated
Diatomaceous Mineral from Kenya
Enos W. Wambu1,2, Charles O. Onindo1, Willis J. Ambusso3, Gerald K. Muthakia4
1Department of Chemistry, Kenyatta University, Nairobi, Kenya; 2Current Address: Department of Chemistry, Bondo University Co-
llege, Bondo, Kenya; 3Department of Physics, Kenyatta University, Nairobi, Kenya; 4Department of Chemistry, Kimathi University
College of Technology, Nyeri, Kenya.
Received M ay 11 th, 2011 ; revised July 7th, 2011; accep ted September 23rd, 20 11.
ABSTRACT
Adsorption of Fluoride (F) from aqueous solutions using acid treated diatomaceous earth (ATDE) from a mining site in
Kenya was studied using batch experiments. The effect of F concentration, adsorbent dosage, contact time, temperature,
pH and competing anions was evaluated. The adsorption process was fast and an initial equilibrium could be attained
in just 10 min. Fluoride adsorption onto ATDE increased strongly from about 40% to over 92% when the solution
temperature was raised from 293 to 303 K. The process was however, less responsive to pH changes dropping by a
margin of less than 1% from 98.8% to 98% when the solution pH was raised from 1.59 to 6.89. It was clear that
increase in concentration of OH– ions did not affect F adsorption onto ATDE strongly. On the other hand, apart from
the Cl– ions which slightly reduced F adsorption onto ATDE, there was no obvious effect of the ,
2
4
SO 3 NO

and
3
4
P
O ions on F uptake by ATDE. Complete F removal (100% adsorption) could be achieved at 400 mg/L initial F
concentrations using 0.5 g/mL ATDE batch loading, 303 - 313 K and pH = 3.4 ± 0.2. The F adsorption data correlated
to the Freundlich and Langmuir models and could be classified as H-Type according to Giles classification of
isotherms. The maximum Langmuir F adsorption capacity of ATDE was 51.1 mg/g indicating that the mineral could be
used as an inexpensive adsorbent for the removal of F ions from aqueous streams.
Keywords: Acid Activated Diatomaceous Earth, Competing Anions, Fluoride Adsorption Isotherms
1. Introduction
At permissible levels, fluoride (F) is essential and desired
for development and maintenance of strong bones and
teet h [1, 2]. At hig her leve ls, ho weve r, p ro longed fl uori de
exposure leads to dental and skeletal fluorosis and in
severe cases to neurological and kidney disorders [3].
The primary cause of excessive human exposure to F is
drinking high F water when alternative low F domestic
water is not accessib le. Co untries alo ng the Ea stern Afr ica
Rift Va lley a re a mong re gions in the worl d wher e hi gh F
water and endemic fluorosis has been reported [4,5]. Ex-
cess fluoride in water sources in these regions results
from natural solubilization of F from high fluoride vol-
canic rocks associated with the Rift Valley [5].
Technologies for removal of fluoride from water in-
clude precipitation, distillation, ion exchange, membrane
technologies, reverse osmosis, and electro dialysis [6].
Application of these methods is however limited b y high
costs of operation, high energy and technical skill re-
quirements and inefficiencies of some of them parti-
cularly at low F contamination levels [7]. Adsorption uti-
lizing natural and inexpensive adsorbents and other mo-
dified materials which are adaptable to local needs would
be ideal alternative. Various low cost materials including
activated carbons, alumina, silica, some bio-sorbents and
a number of natural and synthetic resins have previously
been investigated for F adsorption [8-12]. Activated car-
bons, silica and alumina are however, expensive and re-
quire frequent regeneration to maintain their efficiency.
Many synthetic resins on the other hand, are non-bio-
degradable raising environmental concerns whereas bio-
sorbents are prone to chemical and biologica l attac k which
reduces their efficacy. Apparently, clays and similar soil
materials are most plausible alternative adsorbents be-
cause they are robust, readily available in stable and safe
forms and present attractive natural adsorptive properties
for various solutes [13] .

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya1655
The diatomaceous earth used in this work, and similar
mineral adsorbents, for instance, are well known for their
adsorptive properties. The mineral is obtainable from a
mining site located in areas reported for high F waters [5]
which makes it an ideal starting material in search of a
water defluoridant. It was ground and treated with excess
HCl before it was assessed for F removal from aqueous
solutions. The effect of pH, F concentration, and time of
contact, adsorbent dosage and temperature was studied
and Freundlich and Langmuir isotherms used to validate
the adsorption equilibrium.
2. Experimental
2.1. Preparation of the Materials
The adsorbent material was collected from natural depo-
sits at Kariandusi in Gilgil District, Nakuru County,
Kenya. It was air dried and crushed to pass through <2
mm mesh. Ten-gram portions of ground samples were
soaked in 100 mL of 0.1 M HCl and the mixture mag-
netically agitated for varying length of time between 0
and 420 minutes. At the end of the shaking period, each
of the samples was suction filtered, washed with excess
deionized water and dried at 383 K overnight before be-
ing assessed for F uptake. The fluoride adsorption tests
were carried out using initial F concentration of 1000
mg/L, at 293 K, and adsorbent dosage of 0.1 g/mL. The
samples with highest F adsorption efficiency were de-
signated Acid Treated Diatomaceous Earth (ATDE) and
employed in the rest of the tests. The F adsorption effi-
ciency of ATDE was then compared with that of untreated
samples.
2.2. Characterization of the Adsorbent Materials
The pH of ATDE was measured using a Hanna Instru-
ments pH-211-microprocessor pH meter in 1 M KCl by
24-hour soaking of 5 g of ATDE in 50 mL aliquots of the
salt solution and the zero point of charge, pHznc, by fast
alkalimetric titration [14]. Chemical analysis was deter-
mined using a Varian Atomic Absorption Spectrophoto-
meter (AAS) model spectr AA and loss o n ignitio n (LOI)
determined by ashing known mass (g) of the sample at
1273 K. X-ray diffraction analysis which was employed
in mineralogical analysis of the adsorbent was carried
out on a P-Analytical X’ pert PRO PW-3040/60 diffrac-
tommeter with Cu Kα radiation at a scan speed of 1.2˚
min–1 over a range of 5˚ to 70˚. Analytical grade reagents
were used throughout the tests.
2.3. Adsorption Experiments
All adsorption tests unless otherwise specified, were car-
ried out on batch basis at room temperature (298 ± 0.5 K)
as follows: known amounts of ATDE were placed in a
100 mL stopperred Erlenmeyer flasks containing 50 mL
of F ions solution of a known concentration and shaken
for a given time length. The solutions were centrifuged
and F concentration in the supernatant measured using a
Tx EDT Model 3221 direct-ion electrode. The amount
of F adsorbed, qe (mg/g), was calculated from the ex-
pression:

eei
V
qCC
m
 (1)
whereas, the percentage F adsorption from the relation-
ship:


100
ei
i
CC
C
 (2)
where, Ci and Ce are initial and final equilibrium F con-
centrations (mg/L) respectively, V (mL) is the volume
and m (g) is the mass of ATDE used.
3. Results and Discussion
3.1. Effect of Acid Treatment
Effect of acid-treatment of Diatomaceous Earth (DE)
whose major characteristics are presented in Table 1 wa s
examined by batch adsorption tests using 1000 mg/L ini-
tial F concentration, 0.1 g/mL adsorbent dosage ratio at
293 K for different time intervals of mineral exposure to
0.1 M HCl. The results of these tests are presented in
Figure 1.
Table 1. Summary of mai n characteristics of ATDE.
Property Value (%)
SiO2 70.40%
Al2O3 9.29%
Fe2O3 3.76%
K2O 2.36%
Na2O 1.00%
Fe2O3 1.00%
CaO 0.77%
MgO 0.33%
TiO2 0.61%
MnO 0.08%
LOI 11.88%
pH 3.4 ± 0.2
PHznc 3.8 ± 0.2
Main minerals
Quartz, HP SiO2
Montmorillonite-22A Na0.3(Al, Mg)2Si4O10(OH)2.8H2O

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya 1656
050100 150 200 250 300 350 400 450
0
10
20
30
40
50
60
15 m i n ad sorp tion
30 m i n ad sorp tion
60 m i n ad sorp tion
120 m in adso r pti o n
Time of acid activation (min)
% F removal
Figure 1. Effect of time of acid activation on F adsorption
onto ATDE from 1000 mg/L solution at 293 K using 0.1
g/mL adsorbent dosage in a batch system.
The F adsorption capacity of the mineral rapidly in-
creased with increasing time o f contact with the acid from
just about 24% for untreated samples (time zero) to about
40% for 60 minutes acid treated materials. Thereafter, the
increase in F uptake was marginal; barely reaching 50%
for 420-min acid-treated samples. The 60-min contact of
the mineral with the acid was sufficient to activate t he mi-
neral for F adsorption.
The pH and the pHznc of the 60-min acid treated ma-
terial which was then designated as acid treated diato-
maceous earth, ATDE, were determined and the results
included in Table 1. As expected, the pH of ATDE was
lower than the pHznc indicating that as a result of acid
treatment, the mineral surface had developed a net posi-
tive surface charge requisite for anionic adsorption. This
indicates that on contacting the adsorbent with the acid,
H+ ions rapidly adsorbed on to the mineral surface in-
creasing the surface potential for adsorption of F ions.
The ATDE surface was strongly enhanced for F uptake
by treatment in dilute HCl. Alternative activation proto-
col could however be exploited to further improve the
performance of the adsorbent.
3.2. Effect of Adsorbent Dosage
The amount of contact surface between an adsorbent and
adsorbate solution plays an important role in adsorption
process. The effect of varying adsorbent mass while main-
taining the adsorbate volume and concentration constant
was studied as follows: 15, 20, 25, 30 and 35-gram por-
tions of the adsorbent were placed in 50 mL aliquots of
the adsorbate solution containing 1000 mg/L of F and
agitated on a DKZ model 1 shaking water-bath at 293 K
for 20 min. The percentage F removal was determined and
plotted against the mass of the adsorbent and the results
presented in Figure 2.
The overall percentage F removal from solution ra-
pidly increased with increase in ATDE dosage. More
than 90% F adsorption could be achieved at 30 g per 50
mL ATDE batch dosage. Adsorbent doses greater than
35 g per 50 mL of solution were however, not possible
because the slurry became too thick to agitate effectively.
This rise in F removal efficiency with increasing adsor-
bent dosage was ascribed to increased availability of ad-
sorptive surface in the solution [15]. The adsorbent do-
sage ratio of 0.5 g/mL was adequate for 80% F adsorp-
tion. These ratios were therefore adopted in the rest of
the tests.
3.3. Effect of Temperature
Batch experiments were carried out using 1000 mg/L F
solutions at selected temperatures between 293 and 323
K by agitating the adsorption mixture for 60 min and the
results presented in Figure 3.
The percentage F uptake increased sharply with in-
creasing temperature from just about 40% at 293 K to
92% at 303 K before the system gradually approached
the equilibrium with 100% F removal at 313 K. It can be
assumed that increase in temperature strongly increased
the energy of the ions. More F ions could therefore in-
teract effectively with the surface groups in ATDE for
adsorption to occur resulting in higher percentage F re-
moval from solution. The most efficient temperatures
were therefore within the ambient daytime tropical tem-
0
20
40
60
80
100
120
10 15 2025 30 35 40
Adsorbent dosage g/50 ml
% F Removal
Series1
Figure 2. Effect of adsorbent dosage using 50 mL of the
adsorbate solution containing 1000 mg/L of F being shaken
for 20 min at 293 K.
0
20
40
60
80
100
120
290 300 310 320 330
Temperature (K)
% F Removal
Series1
Figure 3. Effect of temperature studied at pH 3.4 using 0.5
g/mL adsorbent dosage and 1000 mg/L initial fluoride con-
centration.

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya1657
peratures (298 - 313 K) showing that F ions could, effect-
tively, be removed from solutions using these materials
without prior temperature adjustments. The process was
however reverseble at lower temperatures indicating that
the adsorption of fluoride on ATDE was endothermic.
3.4. Effect of Initial Adsorbate pH and Time of
Contact
The time-dependent F adsorption onto ATDE was studied
by varying equilibration time from 0 to 80 min using
initial F concentration of 1000 mg/L. The adsorption stu-
dies were carried out under constant agitation on a DKZ
model 1 shaking water bath at 303 K and initial pH va-
lues of 1.56, 3.32, 5.24 and 6.89 respectively. The pH
adjustments were achieved b y addition of small amounts
of 1 M NaOH or 1 M HCl using 50 µL burettes. The per-
centage F adsorption was plotted against contact time and
the results presented in Figure 4.
As it can be seen from Figure 4, 98.8% F adsorption
could be attained in just 10 min at initial pH o f 3.32. T he
initial rapid F adsorption then gave way to a very slow
process and gradual increase in percentage F uptake
could still be observed at 80 min. This also meant that,
the equilibrium removal of fluoride increased with in-
creasing pH to a pick of 98.8% at 3.32 and decreased
there on as it has also been reported for similar adsorb-
ents in the literature [9]. The observed lower F adsor-
ption at very low pH values could be ascribed to com-
petition for sorptive sites by Cl– ions from HCl in
solution and to the collapse of the crystalline chemical
structure of the adsorbent under strong acidic conditions.
However, the F adsorption changed only insignificantly
from about 98.8% to just below 98.0% when the solution
pH was raised from 1.56 to 6.89. It was evident that, al-
though F adsorption on ATDE was somehow favored by
strongly acidic pH values of about 3.32, change in the
solution pH did not affect the adsorp tion of F onto ATDE
strongly. This shows that the F ions has got such strong
0 102030405060708090
94
95
96
97
98
99
100
pH=1.59
PH=3.32
PH=5.24
PH=6.89
Time
% Fl uoride rem o va l
Time (min)
Figure 4. Effect of initial adsorbate pH and time of contact
using 1000 mg/L F solution at 303 K and 0.5 g/mL ATDE
batch dosag e.
affinity for ATDE surface that it could approach and ad-
sorb into the adsorbent material against strong coulombic
repulsions from adsorbed surface ions at high pH
values. This means that ATDE could be used to defluo-
ridate aqueous systems at neutral pH values of most wa-
ter solutions without need for prior pH adjustments.
OH
3.5. Effect of Dissolved Competing Ions
Because of the apparent indifference of F adsorption to
pH changes and hence to the concentration of OH– ions
discussed in sections 3.4, the influence of potential com-
peting anions was studied using 1000 mg/L F adsorbate
solutions in 0.1 M background solutions containing res-
pective competing anions as potassium salts. The results
for these tests are presented in Figure 5.
As expected, apart from the chloride, the oxo-anions
appeared to have no obvious influence on F adsorption.
The F ions were able to adsorb preferentially at ATDE
surfaces in presence of the other anions. It can be as-
sumed that the chloride having a smaller spatial radius
and less steric hindrance than the oxo-anions, had higher
mobility and penetration in solution and therefore com-
peted more effectively with F for adsorptive sites in
ATDE thereby reducing F adsorption more strongly. How-
ever, the preferential uptake of the F ion shows that F
ions were able to penetrate and adsorb to inner sites in
ATDE which were inaccessible to the other ions. This
shows that the adso rption of F o nto ATDE is not a f fected
by these ions. The F ions can therefore selectively be re-
moved from solution in presence of the nitrates, sulphates
and phosphates using this defluoridat ion prot ocol.
3.6. Equilibrium Analysis
Equilibrium experiments are useful in elucidating ad-
sorption mechanism, characterization of adsorbates, as-
sessment of adsorbent surfaces and they can provide
useful insight in the dynamics of the adsorption process
[16]. For equilibri um test s of F upta ke by ATDE, 25-gra m
0
20
40
60
80
100
0.1 M
Chloride 0.1 M SulphateBLANK0.1 M Nitrate0.1 M phosphat e
Com peting i o n
% Fluoride Adsorption
10 min20 min40 min80 m in
Figure 5. Effect of dissolved competing ions using 1000
mg/L F adsorbate solutions prepared in 0.1 M background
solutions containing respective competing anions as potas-
sium salts at 0.5 g/mL adsorbent dosage, 303 K and pH 3.4.

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya 1658
portions of ATDE were agitated for 20 min in 50 mL
aliquots of adsorbate solution containing between 5 and
1000 mg/L F ions at a pH of 3.4 ± 0.2 and 303 K. The
tests were repeated at 313 K for comparison and the
results presented in Figure 6.
Complete removal of Fluoride io ns was recorded up to
200 mg/L initial F concentration at 303 K and up to 400
mg/L for 313 K respectively. The percentage F removal
thereafter declined sharply at both temperatures due to
rapid saturation of adsorptive sites in the adsorbent. Wi-
thin the concentration range used in this work, the per-
centage F adsorption at 303 K, leveled off at 400 mg/L.
However, even at 1000 mg/L initial F concentration, the
F removal efficiency of ATDE at 313 K was still re-
ducing. It was evident, therefore, that ATDE is a highly
porous material with high anion exchange capacity and
the adsorption capacity of the material is highly tempe-
rature dependent. The results showed that ATDE could
effectively be utilized in defluoridation of aqueous solu-
tions in the F contamination range of natural waters at
ambient tropical temperatures which approximates those
used in this study (303 K).
An adsorption isotherm was constructed by plotting
the amount of adsorbed F, qe (mg/g), versus the equili-
brium concentration, Ce (mg/L), and the results presented
in Figure 7.
Fluoride adsorption increased with increasing initial
concentration. It can be assumed that higher initial fluo-
ride concentration enhanced the anion exchange potential
of the adsorbate and resulted in higher adsorption capac-
ity [17]. Based on the initial part of the curve being
vertical to the concentration axis, the Fluoride adsorp tion
isotherm could be classified as an H type isotherm accor-
ding to Giles classification of adsorption isotherms. This
showed tha t ATDE has got such high affi nity for F that at
low concentrations the F ion were completely adsorbed.
This implies that ion-exchange mechanisms involving so-
lutes of lower affin ities for AT DE sur faces co uld be th e
Figure 6. Effect of change in the initial fluoride concen-
tration on its uptake by ATDE using 25 gram per 50 mL
ATDE batch dosage agitated for 20 min at pH 3.4 and 303
K and at 313 K temperature respectively.
main p ro cess in F ad so rp tio n o nto AT DE. It sho ws t ha t F
adsorption onto ATDE could therefore be chemical in
nature involving stoichiometric inner-sphere complexa-
tion with ATDE surface groups. This is consistent with
earlier postulates in Section 3.4 and 3.5 that electro nic and
geometrical attributes of the F appear to favor its prefe-
rential adsorption onto ATDE solutes.
The linear Langmuir and Freundlich models were plo-
tted for the F adsorption data and their respective iso-
thermal constants calculated. The Langmuir isotherm was
used in the fo rm:
max max
111
eq eq
qbqCq
 (3)
where Ce is the equilibrium concentration (mg/L), qe the
amount of metal ion adsorbed (mg/g), qmax is the qe for a
complete monolayer coverage of the material by the ad-
sorbate (mg/g), and b sorption equilibrium constant (L/
mg). The equation assumes that adsorption takes place at
specific energetically homogeneous sites within the ad-
sorbent. It is then assumed that once an adsorbate ion
occupies a site, no further adsorption can take place at
that site. The rate of adsorption to the surface should then
be proportio nal to the adsorbate concentration in sol ution
and the available area of free sites. The Langmuir iso-
thermal plot and the corresponding constants are given in
Figure 8 and Table 2 respectively.
Figure 7. A general equilibrium isotherm for fluoride ad-
sorption onto ATDE.
0. 0
0. 5
1. 0
1. 5
2. 0
2. 5
0.0 0.5 1.01.5 2.0 2.5
1/Ce (L/mg)
1/qe (g/m g)
1/C
e
(L/mg)
1/q
e
(g/mg)
Figure 8. Langmuir adsorption isotherm for fluoride ad-
sorption on to ATDE.

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya 1659
Table 2. Langmuir and Freundlich adsorption constants compared with those of selected low-cost fluoride adsorbents in
literature.
Langmuir isotherm Freundlich isotherm
Adsorbent qmax (mg/g) b (L/mg) R2 Kf n R2
ATDE (in this work) 51.81 0.0223 0.9692 1.185 0.6524 0.9345
Alumimium sulphate [18] 1.7142 0.3744 - 1107.67 –1.8447 -
Montmorillonite clay [19] 1.485 4.202 0.995 0.279 2.551 0.829
Attapulgite [17] 40.09 0.00804 0.997 0.776 1.4728 0.994
An Activated carbon [11] 4.617 1.58 0.9990 3.024 5.0251 0.994
The linearized Freundlich isotherm was adopted as: ess is not entirely monolayer in coverage. This means
that the mineral surface does not have completely homo-
geneous surfaces as can be seen from mineralogical ana-
lysis in Section 3.1.
log loglog
eq feq
q=K+n C (4)
where, n is a constant characteristic of the intensity of
surface loading of the adsorbate on the adsorbent and Kf
is a constant indicative of the affinity of the adsorbent for
the adsorbate particles. Adherence of adsorption data to
the Freundlich isotherm postulates that adsorption pro-
ceeds by completely reversible multilayer physisorption
based on weak van der Waals type of interactions between
adsorbent and adsorbate particles. The isotherm does not
predict surface saturation by the adsorba te. T here fore, the
surface covering is mathematically unlimited. The Freun-
dlich plot and calculated isothermal constants values are
give n in Figure 9 and in Tab le 2 respectively.
The comparison of Langmuir and Freundlich isother-
mal constant with those of other low-cost adsorbents gi-
ven in Table 2 show that, ATDE has superior adsorp-
tion characteristic compared to those of the other low-
cost adsorbents as it had higher adsorption capacity than
any of the other low adsorbents, the corresponding Freun-
dlich affinity constant Kf was quite high and the Freun-
dlich intensity constant was between 0 and 1 signifying
effective sorption of F by the adsorbent. In general it can
be concluded that acid treated diatomaceous mineral,
ATDE , fr o m a mini ng site i n Ken ya ha s go t r ob ust a d sor -
ption properties and could efficiently be utilized as inex-
pensive and safe adsorbent for the removal of F from
aqueous systems.
As can be seen from the respective figures, the Lang-
muir and Freundlich isotherms fitted the adsorption data
with R2 values of 0.97 and 0.93 respectively. This shows
that the adsorption process could be described by these
models although the data fit for the Langmuir isotherm
was so me ho w be tt er . Simila r f ind in gs ha ve b e en r ep o rt ed
for heat treated diatomite [20]. The affinity coefficient Kf
and intensity parameter n values of the Freundlich iso-
therm indicate effective binding of adsorbate particles by
the material. More so, as can be seen, the value of n was
less than unity, which indicates that the adsorption proc-
4. Conclusions
From the results in this work, it was found that a diato-
maceous mineral from Kariandusi mining site in Kenya
could greatly be enhanced for F adsorption by simple
pretreatment in dilute HCl. Because of high affinity of
the acid pretreated mineral for F ions, the pH and the
presence of other competing ions could not affect the F
adsorption onto its surface. The F ions could adsorb
equally well on to the mineral surfaces even in high con-
centrations of other negatively charged ions. The tem-
perature was the main variable affecting F uptake onto
ATDE. The adsorption process was favored by elevated
temperatures range of 303 - 313 K. It was therefore sug-
gested that F adsorption onto ATDE was spontaneous
and endothermic. The equilibrium isotherm for the proc-
ess could be classified as H Type following Giles classi-
fication of isotherms; which attests to the high affinity
between F and the adsorbent surface. With a high Lang-
muir adsorption capacity of 51.1 mg/g, this study has
demonstrated that ATDE, a naturally occurring mineral
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.00.2 0.40.60.81.0 1.21.4 1.6
log Ce
Log qe
log C
e
log q
e
Figure 9. Freundlich isotherm for the fluoride adsorption
on to ATDE.

Fluoride Adsorption onto Acid-Treated Diatomaceous Mineral from Kenya 1660
abundantly and cheaply obtainable from a mining site in
Kenya could be used as a plausible, cheap and safe F ad-
sorbent for defluoridating fluoride polluted aqueous str-
eams.
5. Acknowledgements
This study was funded jointly by the International Foun-
dation for Science (IFS) and t he Kenya National Council
for Science and Technology (NCST).
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