Effect of silica fouling on fluoride rejection in RO membrane system
(1) General introduction
Silica (SiO2)n is one of the foulants present in feed waters in membrane filtration processes.
Silica can be found in many different forms and is influenced by a variety of variables including
temperature, pH, metal ions, and ionic strength (S.D.N. Freeman and R.J. Majerle, 1995).
R.Y. Ning (2002) proposed that silica is the final and most stable form of silico-oxygen acid
polymerization at normal conditions. First, monosilicic acid (Si(OH)4), which is a weak
tetravalent acid with pKa values of 9.9, 11.8, and 12 will polymerize by dehydration to form SiO-Si anhydride bonds: n Si(OH)4 → (OH)3Si-O-Si(OH) dimer. Then, dimer (two similar
subunits linked) and oligomers (a finite number of subunits linked) are formed, respectively.
Finally, colloidal polymers and particulate silica will be formed. The silica (SiO2)n formula is
shown in Figure 1. Thus, the molecules of silica represented by the formula SiO 2 is polymeric
form, and is more accurately represented by the formula of (SiO2)n where n is infinited in
number, allowing for extensions in term of amorphous and crystalline forms of silica.
(Source: K.D. Demadis et al., 2005)Figure 1 Silica (SiO2)n formula
In crystalline silica the tetrahedral structure is continued over a large range, forming a wellordered lattice (crystal). Whereas, amorphous silica is the non-crystalline form of silica. In
amorphous silica, this long range order does not present due to the disordered nature of the
material, that some atoms are missing a neighbor to which it would be able to bind
(http://en.wikipedia.org).
P. Sahachaiyunta et al. (2002) reported that silica is generally found in natural waters in three
different forms (L.F. Comb, 1996), namely, reactive silica (dissolved silica), colloidal silica and
particulate silica. Reactive silica in water can be found in many forms i.e. monosilicic acid,
disilicic acid and polysilicic acid with molecules smaller than the colloidal range (G.B.
Alexander, 1953). Colloidal silica is widely thought to be both silica that has polymerized with
infinited units of silica dioxide and silica that have formed with organic compounds or with other
complex inorganic compounds. Particulate silica is larger in size and mostly comprised of sand
or suspended solids in water.
The dissolution of silica involves a chemical reaction can be expressed by:
SiO 2  2H 2 O  SiOH4
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The dissolution rate mainly depends on pH and temperature. The soluble form of silica is initial
monomer, as it contains only one silicon atom. In this monomeric form, or often called
monosilicic acid. Monosilicic acid is generally deionized at natural pH. At a pH of 8.5 only 10%
of the monosilicic acid is ionized and when the pH reaches at 10, 50% is ionized (R.
Sheikholeslami and S. Tan, 1999). Besides, high water temperature can accelerate hydrolysis of
SiO2.
In contrast, Silanols (Si-OH) can react with each other to form a siloxane bond (Si-O-Si), which
is the beginning of the polymerized silica as discussed previously (www.solgel.com):
~ Si  OH  HO  Si ~~ Si  O  Si ~ H 2 O
(2) Silica fouling and hyperfiltration (NF and RO filtration)
Silica fouling mechanisms can be separated into three categories. These phenomena will reduce
water flux in membrane filtration processes.
(1) Polymerization of dissolved silica when concentration exceeds its solubility. According
to R. Sheikholeslami and S. Tan (1999), the solubility of silica was observed of 6 ppm
and 100-104 ppm for crystalline silica and amorphous silica, respectively (K.B.
Krauskoph (1956); G.B. Alexander, W.M. Heston and R.K. Iler (1954). When silica is
dissolved in water, it forms monosilicic acid, which will remain in the monomeric state
as long as its concentration remains less than about 2 mM (R. Sheikholeslami and S.
Tan, 1999).
(2) Deposition of colloidal and particulate silica.
(3) A deposition of silica as copolymerization of silicic acid with hydroxideof metal
elements or reaction of silicic acid with organic compound to form anhydrides (R.Y.
Ning, 2002). These formations of silica scale in hyperfiltration systems leads to limited
recovery in many installations and unnecessary increase in costs.
When Fe(OH)3, Al(OH)3, Ca(OH)2, and Mg(OH), are presented in water at suitably high
pH, these hydroxides can react in the copolymerization resulting in the incorporation of
Fe, Al, Ca and Mg into the complex anhydrous silicate structures (R.Y. Ning, 2002).
(3) Mechanisms of solutes through silica fouling layer
Solutes transport through the silica layer could be either convective or diffusive depending on
the structure of the silica layer. If the silica layer is made of a cake layer of colloidal or
particulate silica, then the solutes transport through the layer dominated by convection flow,
whereas if the silica layer is made of polimerized silica layer, then the solutes can move through
the layer only by diffusion.
(4) Effect of physicochemical on silica fouling
R. Sheikholeslami and S. Tan (1999) reported laboratory tests with simulated waters in the range
of those in a prospective desalination plant. The objective of this study was to determine the
water quality effects (i.e. cations) on silica precipitation both in batch and dynamic tests using
RO membranes. Operating pH was set at 6.5 for all experiments, which prevented
polymerization as well as precipitated of silicates. Results indicated that magnesium hardness
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was more effective than Ca hardness in silica polymerization rate. At a given hardness,
decreasing the ratio of calcium to magnesium was found to increase the polymerization rate. This
was apparent in batch tests and during the initial period of dynamic tests. Moreover, batch tests
showed that polymerization increases with the degree of supersaturation. However, in dynamic
tests the flux decline was higher at low feed silica concentrations and higher Ca: Mg ratio after a
period of operation. This inflection on the effect of feed silica concentration and Ca: Mg ratio in
dynamic tests is might be due to precipitation of monomeric silica and deposition at the surface
resulting in a lower feed silica concentration. While at high silica concentrations or when
polymerization is rapid (at high Ca: Mg), bulk polymerization results and deposits are of more
porous colloidal silica.
R. Sheikholeslami et al. (2001) reported that the pretreatment for reducing silica fouling potential
was operating pH range in absence of cations was above 9.5 and below 5.5; while in presence of
cations, it was for pH<5.5. It was also found that the polymerization of silica was increased with
incrasing of intial silica concentration. The increasing in polymerization rate was also observed
when calcium and magnesium increase, and magnesium hardness seemed to be more effective
than calcium hardness. Effect of increasing in pH of sodium aluminate, lime and soda ash
solutions on silica reduction was experimented and the relationships for them were carried out. It
was observed that the increasing in pH in lime softening was much more effective in silica
reduction than addition of sodium aluminate, that was up to 70% reduction in silica was obtained
at pH of 10.2 as opposed to 50% reduction with sodium aluminate (at about dosing rate of 22
ppm). However, it was not cleared for relationship between silica reduction and lime/soda-ash.
T. Koo, et al. (2001) studied silica fouling and membrane cleaning of reverse osmosis
membranes. In batch test experiments, it was found that increasing the concentration of calcium
and magnesium also enhanced the polymerization of silica. This also means that increasing the
total hardness of the solution enhances silica polymerization. The presence of calcium and
magnesium in low concentrations did not help catalyze silica polymerization. But the presence of
calcium and magnesium in higher concentrations did help catalyze silica polymerisation. At
silica concentrations greater than 300 ppm, polymerization will take place even in the absence of
calcium and magnesium. Moreover, carbonate was found to have greater effect on silica
polymerization than chloride. It was found that carbonate ions promote silica polymerisation
more compared to chloride ions. The cleaning of membranes was using distilled water. However,
distilled water did not restore the membrane fluxes properly.
P. Sahachaiyunta et al. (2002) investigated the effect of silica fouling in presence of minute
amounts of inorganic salts in reverse osmosis membranes. The various cations including iron,
manganese, nickel, and barium, which existed in industrial and mineral processing wastewaters,
on silica fouling was studied. Structure of fouled membrane is analysed using scanning electron
microscope (SEM). The concentration of soluble silica and total silica (soluble plus colloidal) in
the feed solution monitored during the runs using UV spectrophotometer and ICP/AES,
respectively. It was found that additions of manganese, nickel and barium in silica feed solution
accelerated flux decline of the feed silica solution. Since the structure of the deposit and absence
of these cations was the same, thus it might be concluded that manganese, nickel and barium do
not react with dissolved silica, and the reduction of flux was probably due to these cations
accelerating polymerization of silica. However, when iron of 3 ppm was added, the foulant
structure was changed drastically. It was observed monolayer structure with cracks developing.
This layer probably developed due to association of iron (III) with dissolved silica to form the
complex iron silicate, Fe(OH)3 · SiO2.
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(5) Chemical cleaning of silica fouling
Chemicals used for silica fouling control can be divided into three categories: (1) chemicals for
silica inhibition, (2) chemicals for silica dispersion (E. Neofotistou and K.D. Demadis, 2004),
and (3) chemicals for silica scale removal from membrane surfaces. Figure 2 shows silica
inhibition and silica dispersion processes.
(Source: E. Neofotistou and K.D. Demadis, 2004)
Figure 2 Silica inhibition and silica dispersion
Silica inhibitors can retard the polymerization of silica, which exceeded its solubility i.e
phosphonate-based chemicals. Dispersants can work by placing a surface charge on the
polymerized silica and cause repulsion and dispersion of these polymerized structures into water
phase (L. Y. Dudley and J. S. Baker, 1999).
Polymerization of dissolved silica (i.e. supersaturation and catalyzed by hardness) can be very
difficult to remove. It should be noted that this type of silica fouling is different from colloidal
and particulate silica, which may be associated with either metal hydroxides or organic matter
(Toray, 2003). Thus, it might difficult to remove polymerized silica by typical chemical cleaning
process. However, polymerized silica scales removed with a high pH cleaning solution (pH of
10-11) were reported. Caustic (sodium hydroxide) at the maximum pH allowed by the membrane
manufacturer will remove polymerized silica scales but it will take many hours to remove a silica
scale (David H. Paul, 1996). While colloidal silica and particulated silica coating, which not
associated with either metal hydroxides or organic matter can be removed from membrane
surface by physical flushing (Nitto Denko Corporation, 2000).
Sayed Siavash Madaeni et al. (2001) studied the fouling problem in the application of
membranes for water treatment. Chemical cleaning processes were carried out for membrane
regeneration. The chemical cleaning of fouled membrane using acid, alkaline, chelating agent,
and surfactant and detergent solutions has been investigated. It was found that cleaning
efficiency depended on the type of cleaning agent and its concentration. The results showed that
the efficiency increases with increasing of chemical cleaning concentration while the mix
cleaning solution of EDTA, SDS, and NaOH (0.05%ww) was the most effective solution for
foulants removal. Operating conditions such as cross flow velocity, turbulence in the vicinity of
membrane surface, temperature, pH, and cleaning time were also affected the cleaning process.
The increasing of these conditions to their optimum values resulted in the highest efficiency of
cleaning processes.
E. Neofotistou and K.D. Demadis (2004) investigated the use of antiscalants for mitigation of
silica fouling and deposition in desalination systems. Chemical for silica inhibition was
examined. The inhibition performance of Starburst® polyaminoamide (PAMAM) dendrimers of
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generations 0.5, 1, 1.5, 2, and 2.5 were investigated. The results showed that the NH2-terminated
dendrimers, PAMAM 1 and 2 were much more effective for SiO2 inhibitors with the optimum
dosage of 40 ppm. However, PAMAM 1 and 2 dendrimers also formed SiO2-PAMAM
composite precipitates after long period. Thus, formation of such materials should be considered.
Moreover, polyethyloxazolines were also effective for SiO2 inhibitors, but their performance is
less dependent on structural features.
L. Y. Dudley and J. S. Baker (1999) proposed that phosphonate-based chemicals were widely
used in water treatment as scale and corrosion inhibitors. In membrane systems, they act as scale
inhibitors. These products have the ability to hold highly supersaturated solutions in a stable
condition during the finite time it takes the water to exit the membrane system. They claimed
that the phosphonate-based chemicals offered the better bicarbonate rejection and less corrosive
to membranes.
(6) Preliminary experiment
(a) Introduction
The preliminary experiment was conducted on December 2006. The objectives of this
experiment were to investigate the optimal contact time of cleaning chemical solutions on silica
fouling membrane and to evaluate the performance of silica fouling membrane before/after
chemical cleaning process on fluoride and chloride removal.
(b) Methodology
Batch test was experimented. The operating pressure of 0.8 MPa and temperature of 25 °C were
controlled. Permeate flux was monitored by using balance and connected to PC. Surface
structure of membrane was observed by using SEM while fluoride and chloride were analyzed
by using IC. Cleaning efficiency was evaluated by comparing permeate flux recovery and
fluoride and chloride removal (feed concentration of 10 mg/L) both before and after chemical
cleaning processes.
The examined cleaning chemical was mixed of NaOH, SDS, and EDTA. The concentrations of
all cleaning chemicals were 0.05 % ww. The cleaning period for each batch experiment will be
varied of 30, 60, and 120 min, respectively. The procedure for cleaning experiments was set up
as followed:
(1) The membranes were previously fouled with silica solution at pilot plant of Tamakawa
water treatment plant.
(2) The silica-fouled membrane was washed for 20 minutes to remove weak-bonded particles
from membrane surface and cut into small pieces and preserved in milli-Q water.
(3) The chemical cleaning solutions were prepared and followed by washing the fouled
membrane from (2) with prepared solutions without applying pressure. The cleaning
period for each batch experiment will be varied of 30, 60, and 120 min, respectively.
(4) Fouled membrane after chemical cleaning process was washed again with milli-Q water.
(5) The permeate flux after chemical cleaning process was measured using milli-Q water.
(6) The permeate flux recovery is determined.
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(4) Results
Figure 3 shows the SEM images of membrane surfaces of each condition: (a) silica fouled
membrane, (b) silica fouled membrane with 30 min chemical cleaning, (c) silica fouled
membrane with 60 min chemical cleaning, and (d) silica fouled membrane with 120 min
chemical cleaning. It was found that increase in chemical cleaning time, increasing of clearance
on membrane surface was observed. And it leaded to increasing in permeate flux of membrane
significantly.
(a)
(b)
(c)
(d)
Figure 3 SEM images of membrane surfaces (Yasuhiro Matsui, D3)
Figure 4 shows concentrations of fluoride and chloride in permeate water of each experimental
condition. It was found that increasing of contact time of 30 and 60 min, fluoride and chloride
concentration in permeate water was reduced. It means that chemical cleaning with these two
contact time values were successfully to clean the silica fouled membranes and efficiently to
recover the membrane performance. However, fluoride and chloride concentration in permeate
water were increased in the case of contact time of 120 min. This phenomenon might be due to
the rupture of membrane surface when contacted with cleaning chemical for prolonged periods.
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concentration (mg/L)
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
30
60
contact time (min)
90
120
fluoride in permeate
chloride in permeate
Figure 4 Fluoride and chloride concentration in permeate versus contact time
(7) Preliminary research plan
(7.1) Chemical cleaning experiment
(a) Objectives
(1) To study the effects of physicochemical conditions on chemical cleaning of silica fouled
membranes including chemical compositions, contact time, and pH.
(2) To study the efficiency of a virgin membrane, a fouled membrane, and a membrane after
chemical cleaning on permeate flux recovery and rejection of fluoride.
(3) To observe the effect of chemical cleaning on zeta potential and surface charge density of
membrane surface before and after cleaning process.
(b) Methedology
Batch test will be conducted. Silica fouled membranes at pilot plant of Tamakawa water
treatment plant, which tested in preliminary experiment will be experimented. Experiments will
be conducted with 64 (4 x 4 x 4) combinations. Chemical solutions of NaOH, NH4OH,
NaOH+EDTA+SDS, and NH4OH+EDTA+SDS, will be examined at controlled temperature of
25 °C. Contact time of 30, 60, 90, and 120 min, will be investigated. And pH values of chemical
solutions in range of 8 – 11 will be evaluated. Flow diagram of this preliminary research plan is
illustrated in Figure 5.
In the literatures, the procedure for cleaning experiments was set up as followed (M.J. MunozAguado et al., 1996).
-
-
The permeate flux of virgin membrane was measured by using DI water (jo).
The virgin membrane was fouled by observed foulant solution; the monitoring of water
flux during this process was also done. After fouling was finished, the permeate flux of
fouled membrane was measured by using DI water (ji).
The fouled membrane was washed for 20 minutes to remove weak-bonded particles from
membrane surface and the permeate flux was measured again by using DI water (jk).
The chemical cleaning solutions were prepared and followed by washing the fouled
membrane from (3) with prepared solutions without applying pressure.
The permeate flux after chemical cleaning was measured using DI water (jc).
The permeate flux recovery is determined as jc/jo.
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Figure 5 Flow diagram of chemical cleaning experiment
(c) Materials and methods
(1)
(2)
(3)
(4)
IC: fluoride
ATR-FT-IR, SEM: surface of membrane
Nanotrack Particle Size Analyzer: silica species size
Zeta potential measurement: an automated electrophoresis method (ELS-8000)
(7.2) Effect of silica fouling on fluoride removal & Transportation mechanism of fluoride
through silica fouled layer investigations
(a) Objectives
(1) To study the effect of silica fouling (both of Batch test and cross-flow mode) on flux
decline and fluoride removal by the variation of colloidal silica and dissolved silica,
operating pressure, and temperature.
(2) To observe the effect of silica fouling on flux decline and fluoride removal.To study the
transportation mechanism of fluoride through silica fouled layer.
(b) Materials and methods
(1)
(2)
(3)
(4)
(5)
(6)
ICP-AES: total silica
IC: fluoride ion
ATR-FT-IR, SEM: surface of membrane
Nanotrack Particle Size Analyzer: silica species size
Molybdosilicate Method, UV spectrophotometer: dissolved silica
Zeta potential measurement: an automated electrophoresis method (ELS-8000)
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