DESALINATION
Desalination 167 (2004) 273-279
ELSEVIER
www.elsevier.com/locate/desal
Silica removal using ion-exchange resins
"a*
M. Ben Sik Ali a, B. Hamrounl , S. Bouguecha b, M. Dhahbi b
°Laboratoire Eau et Technologies Membranaires, FacultO des Sciences de Tunis, Campus Universitaire,
1060 Tunis, Tunisia
Tel. +216 (71) 872600; Fax: +216 (71) 885008; email: bechir.hamrouni@fst.rnu.tn
bLaboratoire Eau et Technologies Membranaires, Institut National de Recherche Scientifique et Technique,
BP 95, 2050 Hammam-Lif Tunisia
Received 17 February 2004; accepted 27 February 2004
Abstract
In water treatment, silica is a problem because of its tendency to form deposits. It is often necessary to include
reducing silica concentration to permit increased cycles of concentration without scale. The behavior of aqueous silica
and the various water treatment processes used for its removal were reviewed. Silica that exists in water in equilibrium
with ion silicate can be removed by strong base anion-exchange resins operated in the hydroxide cycle. With the aim
of silica removal by ion exchange, some thermodynamic and kinetic aspects were studied at different temperatures.
Selectivity coefficients were determined and activity coefficients were calculated from an appropriate model. Silica
concentrations were obtained by the measure of the absorbance of yellow or blue-colored silicomolybdic acid. Anion
species concentrations were determined by ionic chromatography.
Keywords:
Silica removal; Ion-exchange; Selectivity coefficient; Thermodynamic equilibrium constant; Ion kinetic
exchange
1. I n t r o d u c t i o n
Scale formation in industrial and domestic
installations is still an important economic
problem. In water treatment, silica scaling is a
real and constant concern for plant operations. It
*Corresponding author.
is often necessary to include reducing silica
concentration to permit increased cycles of concentration without scale. Because the nature of
silica in solution has a significant influence on its
removal mechanism, the essentials of silica
chemistry are first reported.
The need to remove or to reduce the quantity
of silica and the various water treatment pro-
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation
between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European
Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved
doi;10.1016/j.desal.2004.06.136
274
M. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
cesses used for its removal were reviewed [1,2].
Recent papers on silica removal have been based
on chemical processes such as precipitation and
coagulation [3,4]. Ion-exchange technology encompasses the sciences of thermodynamics,
kinetics, ion chemistry, fluid mechanics, and
economics.
Understanding the finer points of ion
exchange helps to determine whether or not ion
exchange will be useful for a particular application. Thus, for a better understanding of silica
removal by ion exchange, the aim of this work
was to study some thermodynamic and kinetic
aspects at different temperatures. Selectivity coefficients were determined and activity coefficients were calculated by an appropriate model.
1.1. General chemistry o f silica
The silica in water exists in an amorphous
form whose solubility, at 25°C, lies in the range
of 100 to 140 mg/L as SiO 2 [5,6]. Dissolved silica
exists as ortho-silicic acid H4SiO4 or Si(OH)4.
Sillrn [7] has pointed out that it is a very weak
acid so that in marine water the pH range, only
about 5%, would be in the form H3SiO4. It can be
found in soluble, colloidal and suspended forms
[ 1,6]. However, in natural waters monositicic acid
may be stable for a long time if its concentration
is less than 100 mg/L at 25°C [6,7]. For higher
concentrations silica polymerization may occur
and leads to polymers, colloids and suspended
particles. Iler [6] found that, in the absence of
salts and at a 7-10 pH range, spherical colloids
grow without aggregation. When salts are present
and for pH less than 3 or in the range 7-10,
colloids associate to form aggregates and possibly
gels. Monosilicic and disilicic acids characterized
by their rapid reaction with molybdic acid are
called "reactive silica" [5]. More polymeric forms
and colloids are "non reactive silica". Thus, the
term "soluble silica" includes the reactive forms
and dissolved polymers [1,2].
Silica solubility depends on many factors such
as temperature, pressure, pH and ionic strength.
No significant variation is noted for pH values
below 9. At higher pH there is an apparent
increase in the solubility due to the formation of
silicates ions in addition to the monomer, which
is in equilibrium with the solid phase. Generally,
the presence of salts has significant influence on
silica solubility, which decreases when ionic
strength increases [2,8].
Many technologies have been developed for
the removal of silica. An increase of silica
solubility at high pH and/or temperature may
avoid its scaling [9,10]. Chemical processes were
used such as precipitation and coagulation [3,4].
Polymeric dispersant as inhibitors on silica
polymerization were also applied in desalination
plants [11,12]. In particular applications, strong
base anion exchange can remove virtually all reactive silica.
1.2. Thermodynamic aspects o f the ion-exchange
process
Synthetic ion-exchange resins are widely used
in water treatment to remove many undesirable
dissolved species, most commonly hardness, from
water. These resins are based on a cross-linked
polymer skeleton called the "matrix". Most commonly, this matrix is composed of polystyrene
cross-linked with divinylbenzene. Functional
groups are attached to the matrix through
covalent bonding. There are strong or weak acid
cation exchangers and strong or weak base anion
exchangers [13]. Silica removal is a specific case
of anion exchange and is typically accomplished
with a strong base resin in the hydroxide form.
Since silica acts as a very weak acid, the exchange reaction for silica removal is as follows:
H3SiO4 s + OH-R ~" H3SiO4 g + OH-s
(1)
HaSiO4 s + C1-R '~ H3SiOj R + Cl-s
(2)
M. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
Anion-exchange resins are commonly regenerated with caustic soda, in accordance with the
following reaction:
H3SiO4 R + OH-s '=" HsSiO4 s + OH-R
oHsSiO4- (HsSiO;) R (OH-)s
K OH- =
(H3SiO4-)s(OH -)R
(4)
H3SiO4-
.,S,O;
KOH-
=
[HsSiO;]s [OH _]R
.,s,o:
/~Cl-
=
(7)
(3)
In the above equations subscript "R" refers to the
ion exchanger (resin) and "S" to the mobile phase
(solution). Concentrations in the resin and in the
solution are, respectively, expressed in mmole/g
of resin and mmole/mL of solution.
The law of mass action to ion exchange is
applicable, and the ability of an ion exchanger to
select one of two ions present in the same
solution can be characterized quantitatively. For
ion-exchange equilibrium a thermodynamic equilibrium constant K ° (expressed in terms of ion
activities) or a selectivity coefficient K (expressed
in terms of ion concentrations) and a distribution
coefficient D are defined. They are calculated for
the ion exchange between silica and resin in the
hydroxide or chloride form, as follows:
(YH,S<)s (':O"-)R
275
Kon-
(5)
[n4siO4]R HsSiO4- [OH-]R
D- [H4SiO4]s- KOH- [OH_]s
(8)
2. Experimental
2.1. Materials and chemical products
Analytical-grade shipped chemicals and reagents were used. Silica solutions were prepared
using a soluble salt of metasilicate (Na2SiO3,
9HzO) and stored in polyethylene bottles. Distilled water and all solutions were microfiltred
before use. Experiments were made on the strong
anion-exchange resin (Dowex 1X8-100 mesh)
presented as a spherical particle of about 150 #m
diameter. De-ionized water with a resistance
greater than 18 M ~ was used for ion chromatography.
Laboratory ion-exchange processing was
accomplished by batch and column methods. In
the first method, the resin and solution are mixed
in a batch tank, the exchange is allowed to come
to equilibrium, then the resin is separated from
solution. The resin columns used were 170 ×
10 mm in size and made of glass. Each column
was slurry packed with about 10 g of the prepared
resin. The column was sealed with two plastic
caps at the top and bottom.
2.2. Analytical procedure
oHsSiO4- (H3Si04) R (C1-)s
K el=
(HsSiO4)s(C1-)R
(6)
Silica was determined by measuring the
absorbance of the yellow-colored silicomolybdic
acid at 380 nm or the blue-colored silicomolybdic
obtained by reduction with the 1 amino-2
naphtol-4 sulfonic acid at 810 nm. Addition of
tartaric acid removes interference from
276
M. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
Table 1
Parameters for ion chromatography analysis
Eluent, 10 -3 M
NaHCO3
2
1.3
Pressure, MPa
4.4
Analysis time, min
24
Loop, #L
20
Temp., °C
20
Suppressor: regenerating agents 20x 10-3 M H2904;
ultra pure water
auto step with fill
Na2CO 3
density and exchange capacity were determined.
Experimental results from dried resin at l l0°C
indicate that the humidity percentage and the
density shipping were, respectively, 37.12% and
0.7 g/L. Ion-exchange capacity calculated was:
Ce = 2.74:t:0.09 mmol/g.
3.2. Selectivity coefficients determination
The selectivity coefficients between silica and
the strong anion resin in hydroxide and in
.....
phosphorous by destroying phosphomotybdate
within few minutes [5,6].
Anions were determined by ion chromatography using a Metrohm 761 Compact IC with
conductivity detection. The anion chromatography measurements with chemical suppression
were made with a Metrosep anion dual 2 column
(4.6×75 mm) with a particle diameter of 6 #m.
The operating parameters for ion chromatography analysis are described in Table 1. The
operating conditions for selectivity coefficient
determination by batch method were as follows:
• To make resin in hydroxide form, 100 mL of
2 M NaOH solution recently prepared with
boiled water were passed through the column
containing 10 g of resin. Then the resin was
rinsed with boiled distilled water till neutrality
of effluent solution was reached.
• A 0.1 M HCI solution was passed through the
column: the first 2 mL corresponding to the
interstitial volume were removed and the
following 100 mL were collected.
• The excess HC1 was neutralized by 0.4 M
NaOH and the ion-capacity exchange was
deduced.
3. Results and discussion
3.1. Preliminary characteristics
Characteristics of the resin Dowex 1×8-100
mesh such as humidity percentage, shipping
H3SiO4-
. .~.H3SiO4-
chloride forms, respectlvelY,hon_ ana,,~cl_
were determined.
The ability of the ion-exchange resin to select
between the two ions, C1- and H3SiO4, given by
Eqs. (2) and (7) was determined by batch method.
A mass of resin mR was stirred with a volume Vs
of a solution of silica. After equilibrium, [Cl-]s is
determined by ion chromatography and [CI-]R is
deduced from the mass balance in the resin:
[C1-]R = Ce - Vs "[C1-]s
mR
(9)
Table 2 summarizes the results obtained for
the selectivity coefficient where
KCl3_si°4- = 0.76 + 0.04
The selectivity coefficient KoHHS_
i°4- , according to
Eqs. (1) and (5), was determined by a graphical
method (Fig. 1). Fig. 1 gives the silica concentration in resin (CR) VS. silica concentration in solution (Cs) for a given hydroxide concentration
(c0).
Because the quantities of silica fixed in the
resin were quite small, [OH-]R and [OH-]s were
assumed, respectively, to be CE and Co. Eq. (8),
where D tends towards D °, becomes:
D°
[H4SiOa]R
H3SiO4-CE
- [H4SiO4]s - KOH-
Co
(10)
277
34. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
Table 2
Selectivity coefficient determined by batch method
Sample
[C1-]s, retool-L- ~
[Cl-]g,mmot.g I
[SiO2]R,mmol.g-1
[SiO2]s,mmoI.L-~
Kc~3-si°;
1
2
30.23
30.41
2.52
2.51
0.11
0.11
1.94
1.75
0.71
0.80
3
30.84
2.51
0.11
1.86
0.76
IT} ]
Table 3
Distribution and selectivity coefficient determination
y=5x
/ y+ 2.
80
/
,,""
y=1.1186x
ft Y ' / ' . . . .
~.6o-
f /
II
,;
,,"
t. i
,./
W' <'.;/
"20 ~/,,,;i./¢..
0
Co, rnlTlol.L -1
D°
KO-H-O'l-l'l''lill
Co= 2.5 mmol.L-1
1.25
2.50
5.25
5.00
2.22
1.12
2.28
2.03
2.14
)7'- . . . .
~.. 40 : //;":" /~i/"
"
Co= 1,25 mmoI.L-t
y_o- ,,2_5 __mmol.L-I
......... . _-.e- . . . . . . . . . . .
2~0
4])
6'1)
80
ll~)
17.0 14(I
.,sio: _ (re,)s
Fig. 1. Graphical representation of ion equilibrium
exchange, CR =f ( Cs).
In Fig. 1 D ° was given by the tangent to the
curve at axial intercepts. Table 3 gives
D°
..d Ko" S
values The values
/q.f!o;
H,sio,
(12)
Cs ( m g per L of solution)
Activity coefficients in solution, at low ionic
strength /, were calculated using the DebyeHiJckel model [2] according to Eq. (13) with subsequent parameters A, B and a/° [14].
=
2.15+0.12 and Kcli3_
~i°2'-'° = 0.764-0.04 show that
log Yi -
Az?
o
(13)
l + ai B v/I
resin in hydroxide form has more affinity for
silica than in its chloride form.
The thermodynamic equilibrium constants
H3SiO4H3SiO4/COOn_
and K°cl_
calculated according, respectively, to Eqs. (4) and (6), require the knowledge of activity coefficients. Because activity
coefficients in the resin are inaccessible, they
may be assumed to be equal, and the above
equations are simplified as follows:
I = 0.005 and -.(YOH-ts -= x,(Y~si°4-t = 0.96. Thus, in
the two studied cases, thermSdynamic equilibrium constants and correspondent selectivity
coefficients have the same value.
KoHaSiO4- -
3.3. Kinetic aspects
(~'OH-)s
HsSiO4
F o r gcl~
io4-H.s calculation at 25°C, ionic strength
w a s I = 0 . 0 6 and (ycl-)s --- (YH SiO-/ = 0.8. For the
HsSiO4~ 3 4 ts
.
Kon_
calculation at 25°C, ionic strength was
The kinetics of exchange of silica and resin in
hydroxide form were carried out at different
M. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
278
temperatures. Fig. 2 shows that ion-exchange
equilibrium was established rapidly at less than
10 mn and no significant effect of temperature.
Fig. 3 represents the variations, respectively,
of fixed and removed silica concentrations vs.
time, according to Eq. (1), for the forward and
reverse ion-exchange reactions. We note the
reversibility of the ion-exchange reaction and
confirm the rapidity for reaching the equilibrium
state.
The rate of the ion-exchange reaction
according to Eq. (1) is proportional to the concentration of the reactants:
V =-k. [H38104 ]S [OH -JR2
"
-
~1
•
(14)
where k is the specific rate constant; cq and ct2 are
the reaction order with respect to silica and
hydroxide reagents. Assuming that hydroxide
ions in resin are in excess in respect to silica, the
following expressions are obtained:
1,6 I
"{,l
0
The third-order reaction and the rate constant
were determined at different temperatures: 20°C,
30°C and 40°C. The reaction rate integrated form
is:
1
1
[H3SiO4-]~ [H3SiO4-]2o
- 2.k,.t
2
2
4
6
8
I0
20 °C
•
30 °C
•
40 °C
12
14
16
T i m e (ran)
Fig. 2. Silica concentration vs. time at different
temperatures.
25O.
22q-~
6
20{) 1 \
i ~
'751
~ 15ol \
O
" -]a,S where, k' =k.[OH-]~, 2 (15)
v =-k'. [H 3SLO4
--~-
2 1,4t
[
0
'11
Forward rcaotion,
• - ,t- .. Reverse reaction
.-................. !--~......................... ~................
.
10
.
.
20
.
,
'~
30
40
50
N)
Time(ran)
Fig. 3. Reversibilityof ion-exchangereaction at 25°C.
6T
Y
R2
5 ~20 oC : 1.1679x 0,9814
'."
| 3 0 °C: 1.0266x 0,9996
.-"
(16)
In Fig. 4 experimental data were fitted for
each temperature to Eq. (16). The implication of
a given straight line with a very high correlation
coefficient is that the studied ion exchange is a
third-order reaction. This is especially true for
temperatures of 20 and 30°C, for which correlation coefficients were, respectively, 0.9996 and
0.9814. At higher temperatures the correlation
coefficients diverge from unity.
11
~-
30oc:.___
~ "
oc -
40°C:
Time
•
(ran)
Fig. 4. E x p e r i m e n t a l data fitted for t h i r d - o r d e r reaction at
different temperatures.
M. Ben Sik Ali et al. / Desalination 167 (2004) 273-279
4. Conclusions
A strong anion-exchange resin is often useful
in water treatment, especially for silica removal.
Some thermodynamic and kinetic aspects were
developed in this work. Thermodynamic equilibrium constants and selectivity selections determined by a graphical method have the same
value. They show a better selectivity for silica
when the resin is in the hydroxide form. With
silica, anion-exchange reactions were rapid and
reversible. They were in a third-order reaction at
20°C and 30°C. Rate constants were determined
at these temperatures. Applied to geothermal
water, silica removal by a strong ion-exchange
resin suffers from the competition of other anions
present.
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