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Chemical Engineering Journal xxx (2012) xxx–xxx
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge
ash for soil amendment
Takaaki Wajima ⇑, Kenzo Munakata
Graduate School of Engineering and Resource Sciences, Akita University, 1-1 Tegata-gakuen-cho, Akita 010-8502, Japan
h i g h l i g h t s
" Zeolite K-F and hydroxysodalite were formed in KOH–NaOH solutions.
" Zeolite K-F and calcium hydrate minerals, such as katoite, were formed in KOH–LiOH solutions.
+
" Properties for K release depend on formation of an amorphous gel and zeolite K-F.
2+
" Properties for Ca
release depends on formation of an amorphous gel and calcium hydrate minerals.
" Zeolite K-F formation in KOH–LiOH solution with Li/(Li + K) = 0.25 is faster than that in KOH alone.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Paper sludge ash
Zeolite K-F
Alkali synthesis
Soil amendment
a b s t r a c t
Zeolitic material including zeolite K-F (KAlSiO41.5H2O) with soil amendment properties was synthesized
at 90 °C from paper sludge ash in KOH–NaOH and KOH–LiOH mixtures. The total alkali concentration in
each solution was maintained at 4 mol/L and the relative amounts of the cationic species (K+/Na+ and K+/
Li+) were varied. Zeolite K-F crystal could be obtained in KOH–NaOH solutions with Na/(Na + K) ratios
lower than 0.5 or KOH–LiOH solutions with Li/(Li + K) molar ratios lower than 0.25, while hydroxysodalite (Na6Al6Si6O248H2O) and katoite (Ca3Al2(SiO4)(OH)8) were formed at other cation molar ratios in each
mixture. The observed concentrations of Si and Al in the solution during the reaction explain the synthesis of reaction products with soil amendment properties. The properties of a product intended for K+
release depend on formation of an amorphous gel and zeolite K-F, while that for Ca2+ release depends
on formation of an amorphous gel and calcium hydrate minerals, such as katoite. The formation of zeolite
K-F in a KOH–LiOH solution with Li/(Li + K) ratio = 0.25 is faster than that in KOH alone and the product
with good properties, high cation exchange capacity and high released amount of Ca2+, for soil amendment could be obtained.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Paper sludge is generated as an industrial waste during the
manufacture of recycled paper products and the amounts generated are increasing annually. Over 3 million tons of sludge are discharged annually in Japan and approximately 8 and 2 million tons
are discharged in the United States and the United Kingdom,
respectively [1–3]. The sludge consists of organic fibers, inorganic
clay-sized materials, and about 60% water and can be incinerated
to produce paper sludge ash (PSA) by burning out the organic
materials, thereby reducing the volume of waste. Although a small
portion of the ash has been used as cement fillers, lightweight
aggregates in the construction industry and other minor applications [4,5], most is dumped in landfills. The large daily output of
⇑ Corresponding author. Tel./fax: +81 18 889 2748.
E-mail address: [email protected] (T. Wajima).
PSA and the limited landfill capacity causes social and environmental problems. It is therefore essential to develop new ash utilization techniques for improved recycling.
PSA contains SiO2, Al2O3, CaO and MgO and normally has extremely low potentially toxic components and has shown good capability for removal for phosphate ions from solution [6–8]. It has
been reported that PSA can be treated with NaOH solution at low
temperature (<100 °C) to improve the cation exchange capacity
(CEC) through the synthesis of zeolite crystals on its surface [9–
13]. The resulting product has the ability to simultaneously remove
ammonium and phosphate ions through ion exchange of the zeolite crystals with ammonium ions and chemical precipitation of
phosphate ions with calcium ions dissolved from PSA [8,14–18].
Thus, this method can potentially produce environmentally
friendly materials from waste PSA.
In our previous study, zeolite K-F (KAlSiO41.5H2O) was synthesized from PSA in KOH solution [19]. Zeolite K-F is a potassium-type
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.136
Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
2
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
zeolite in the edingtonite group and has a high affinity towards NHþ
4,
making it potentially useful for soil amendment [20]. Because potassium, ammonium and phosphate ions are important ions in agricultural fertilizers, there is good potential for producing a useful soil
amendment material that can simultaneously adsorb both ammonium and phosphate ions by treatment of PSA with KOH. However,
the reaction rate for zeolite synthesis from PSA using KOH is slower
than with other alkali species, such as NaOH and LiOH [19]. The reaction rate depends on many factors, such as temperature, pH and alkali species. It would be possible to promote the zeolite K-F
synthesis reaction rate using two-component alkali solutions,
NaOH/KOH and LiOH/KOH solution. The obtained product including
zeolite K-F crystal, which has high cation exchange capacity, could
be used for soil amendment by KCl washing to remove other cations,
Na+ and Li+. Also, little information is available on the conversion of
PSA into a product for removal of ammonium and phosphate using
KOH solution. To our knowledge, no previous efforts have been
made to determine the effect of alkali species during product synthesis on the latter’s soil amendment characteristics.
In the present study, the effects of alkali species on the formation of K-F zeolite from PSA were examined. The aim of this study
was to improve the formation rate of zeolite K-F in the products
and to obtain the product with high soil amendment abilities, high
cation exchange capacity (CEC) and high released Ca2+ to adsorb
both ammonium and phosphate ions.
component alkali solutions, NaOH/KOH and LiOH/KOH, were used
as alkali sources. The total alkali concentration in the solution was
maintained at 4 mol/L to clarify the difference of reaction behavior
by the reference of our previous study [19]. The amounts of Na+
and K+ or of Li+ and K+ were varied under a constant OH concentration. Each reaction, using these alkali solutions, was carried out
as follows. 100 g of ash were added to 1 L of alkali solution in a 1 L
Erlenmeyer flask (made of poly methyl pentene) with a dimroth
condenser and the mixture (slurry) was continuously stirred at
90 °C. Five mL aliquots of each slurry were removed at various time
intervals to monitor the reaction process over a period of 24 h. The
aliquots were filtered, the solid residue was washed with purified
water (using a Millipore Milli-Q Labo system, USA) and dried for
12 h at 60 °C in a drying oven. The solid residue was then analyzed
by XRD to determine the minerals present. The intensity of the major XRD peaks for mineralogical phases: zeolite K-F (2 2 2),
hydroxysodalite (2 1 1), and katoite (4 2 0), were used to determine
changes in the mineralogical phases. The chemical composition of
the product was analyzed by the same method as used for the ash.
The filtrates were analyzed by ICP-AES to determine the concentration of Si and Al in the alkali solution during the reaction.
2.3. Properties of the product for soil amendment
The amounts of K+, Ca2+, Na+ and Li+ released from each product
in an ammonium solution and CEC of the product were examined
to indicate the soil amendment material properties by modified
Schöerrenberg’s method [21]. 0.1 g of the solid residue was treated
with 10 mL of 1 M ammonium acetate solution, separated from the
solution by centrifugation, and added again to fresh 1 M ammonium acetate. This process was repeated three times for 20 min
per exchange. The total amounts of K+, Ca2+, Na+ and Li+ ions in
the ammonium acetate solution were analyzed by ICP.
After three times ammonium acetate treatments, the sample
washed with 80% EtOH solution two times in preparation of the
+
next step replacement. The NHþ
4 replaced by K in 10% KCl solution,
in a procedure repeated three times. Finally, total amount of NHþ
4
in the solutions was analyzed by the method of Koyama et al.
[22] to determine the CEC of the sample.
The amounts of K+ and Ca2+ released from the product and CEC
of the product were calculated as follows.
2. Materials and methods
2.1. Paper sludge ash
Raw PSA was obtained from a major paper manufacturer in Japan. The chemical composition of the ash, determined by scanning
electron microscopy (SEM) (Hitachi, S-2600H, Japan) equipped
with energy dispersive spectrometry (EDS) (Horiba, EX-200, Japan)
[19], is shown in Table 1. It is noted that Li content is analyzed by
an inductively coupled plasma method (ICP-AES) (SPS4000, Seiko,
Japan) after dissolving the sample in aqua regia, because EDS cannot detect Li content. The ash consisted mainly of SiO2 (43.0%),
Al2O3 (23.9%) and CaO (22.9%) in the form of amorphous matter
and the minerals gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8),
determined by X-ray diffraction (XRD) (Rigaku, Rint-2200U/PC-LH,
Japan), as shown in Fig. 1. The remaining components were essentially low-concentration impurities, such as Na2O, K2O, MgO, Fe2O3
and TiO2.
þ
qM ¼ ðC M Þ=ðM wÞ½M : Kþ ; Ca2þ ; Naþ ; Li ; NHþ4 where CM is the total amount of each ions from samples (mg), M is
the molar mass of K+, Ca2+ and NHþ
4 (g/mol), and w is the mass of
PSA added (g). The units of qM were converted from mmol/g to
cmol/kg, which is the general unit for soil properties, by multiplied
by 10. This procedure was done three times, and the average of
these data was used.
2.2. Zeolite synthesis
PSA was partially converted to zeolites and other minerals by
reaction with alkaline solutions. To investigate the effect of the
cationic species in alkali solutions on zeolite synthesis, two-
Table 1
Chemical compositions of PSA and the products.
Reaction solution
PSA
The product
4M
1M
2M
3M
4M
3M
2M
1M
4M
KOH
NaOH + 3 M KOH
NaOH + 2 M KOH
NaOH + 1 M KOH
NaOH
KOH + 1 M LiOH
KOH + 2 M LiOH
KOH + 3 M LiOH
LiOH
Chemical composiiton (wt.%)
SiO2
Al2O3
CaO
Na2O
K2O
MgO
Fe2O3
TiO2
Li
43.0
23.9
22.9
0.3
0.0
7.3
0.8
1.8
0.0
41.6
37.9
37.9
39.2
38.2
38.7
37.2
39.4
31.9
21.0
19.4
18.1
19.7
19.5
20.1
20.0
22.0
27.4
20.9
23.5
25.9
25.0
26.3
24.1
29.0
25.6
26.5
0.1
1.5
3.4
6.0
6.4
0.0
0.0
0.0
0.1
8.1
9.9
6.2
2.2
0.0
8.1
2.6
0.4
0.0
6.3
5.6
6.3
6.0
7.1
6.2
6.5
6.7
7.5
0.7
0.7
0.9
0.6
0.9
0.6
0.8
0.5
0.9
1.3
1.5
1.2
1.2
1.7
1.6
1.7
2.1
1.6
0.0
0.0
0.0
0.0
0.0
0.6
2.2
3.3
4.2
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amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
3
: Anorthite [CaAl2Si2O8]
200
100
0
0
10
20
2
30
[CuK
40
50
60
(degree)]
Fig. 1. Powder X-ray diffraction patterns of PSA.
500
300
400
200
300
200
100
100
0
0
0
0.25
0.5
0.75
Ratio of Na+/total cation
Zeolite K-F
K+
Na+
3. Results
3.1. Reaction in KOH–NaOH solution
PSA was reacted in NaOH–KOH mixed solutions at 90 °C for
24 h. XRD patterns of the five reaction products are shown in
Fig. 2. In the original ash (Fig. 1), two mineral phases, gehlenite
and anorthite, existed. The intensity of the anorthite diffraction
peaks in all products diminished, indicating that anorthite dissolves in NaOH–KOH mixed solutions, while the intensity of the
gehlenite diffraction peaks in all products did not change over
24 h. The zeolite K-F (KAlSiO41.5H2O) formed when using Na/
(Na + K) molar ratios below 0.5, while hydroxysodalite (Na6Al6Si6O248H2O) formed at Na/(Na + K) molar ratios of 0.75 and 1.0.
The chemical compositions of products synthesized using KOH–
NaOH solution are shown in Table 1. The K content in the product
was higher than that in the ash and increased with increasing K
content in the mixed solution because of the formation of a crystalline zeolite K-F phase, while the Na content in the product increased with increasing Na content in the mixed solution, due to
the formation of a crystalline hydroxysodalite zeolite phase.
Intensities of the major mineralogical phases in the products, the
amounts of K+, Ca2+ and Na+ released from the products, and CEC of
the product after 24 h reaction are shown in Fig. 3. It is noted that
the released amounts of K+, Na+ from raw ash and CEC of the ash
are zero and the released amounts of Ca2+ is 30 cmol/kg. The intensity
of zeolite K-F was almost constant at Na/(Na + K) = 0 and 0.25 but
gradually decreased above Na/(K + Na) = 0.25, while that of hydroxysodalite was almost constant at Na/(Na + K) = 0 and 0.25 and gradually increased above Na/(K + Na) = 0.5. The amount of K+ released
from the product was almost constant at Na/(Na + K) = 0 and 0.25
and gradually decreased above Na/(K + Na) = 0.5, which correlated
with the amount of zeolite K-F in the product. On the other hand,
θ
α
1
and Na+ , and CEC (cmol/kg)
Intensity (cps)
: Gehlenite [Ca2Al2SiO7]
Released amounts of K+, Ca2+,
300
Intensities of zeolite K-F
and hydroxysodalite (cps)
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
Hydroxysodalite
2+
Ca
CEC
Fig. 3. Intensity of zeolite K-F and hydroxysodalite in the product and amounts of
K+, Ca2+, and Na+ released from the product, and CECs of the product after 24 h
reaction in each mixed solution.
the amount of Na+ released from the product gradually increased with
increasing Na+ content in the solution, which correlated with the
amount of hydroxysodalite in the product. Regardless of Na+ content,
the CEC of the product is almost constant, which are almost same as
the sum of released amounts of K+ and Na+. The amount of Ca2+ released gradually increased to 500 cmol/kg and decreased to
300 cmol/kg at Na/(Na + K) = 1, with increasing Na/(Na + K) ratios in
the mixed solution, which mean that the product synthesized in
mixed solution contains higher releasable Ca than those in single
solutions.
The reaction process was monitored by measuring the concentrations of Al and Si in the solutions and analyzing the properties of the
solid product for release of K+ and Ca2+ during each 24 h experiment.
Although Ca is also a major elemental constituent of the ash, its concentration in solution is not a reliable indicator of the bulk system
chemistry, because Ca is incorporated into insoluble solid phases
in alkaline solutions [23]. Fig. 4 shows the concentrations of Al and
Si in the solutions, together with the properties of the product solids,
as a function of reaction time using the mixed solutions at each Na/
(Na + K) ratio. The concentration of Al in solution always exceeded
that of Si, even though the Si concentration exceeded that of Al in
the starting ash. In the case of 4 M KOH solution (Fig. 4a), the concentrations of Si and Al initially increased after introduction of PSA, then
became almost constant after 4 h, and thereafter the Si concentration rapidly decreased to approximately 30 mM, while Al remained
in the solution after 12 h. The K+ release from the solid product gradually increased to 100 cmol/kg, while that for Ca2+ release rapidly increased to 300 cmol/kg and became constant after 3 h of reaction.
With increasing Na content in the mixed solution, the initial concentration of dissolved Si was lower and the Si concentration decreased
more rapidly after its initial increase. On the contrary, the initial concentration of dissolved Al was higher and the Al concentration decreased more rapidly after its initial increase. The K+ release
property of the solid product gradually increased during the reaction
in all solutions but the amount of K+ released decreased with
increasing Na content in the solution because of the decrease in zeolite K-F content in the solid product. In contrast, the amounts of Ca2+
released rapidly increased during initial stages in all solutions,
which was correlated with the initial increases of the Si and Al concentrations, and then remained almost constant. With increasing Na
content in the solution, the amount of Ca2+ released from the solid
product increased, reaching a maximum amount of approximately
500 cmol/kg when Na/(K + Na) = 0.5 and 0.75, decreasing to
300 cmol/kg in 4 M NaOH solution.
3.2. Reaction in KOH–LiOH solution
Fig. 2. Powder X-ray diffraction patterns of the product derived from PSA with
KOH–NaOH mixtures; (a) 4 M KOH, (b) 3 M KOH + 1 M NaOH, (c) 2 M KOH + 2 M
NaOH, (d) 1 M KOH + 3 M NaOH, and (e) 4 M NaOH.
The experimental procedure described above was also applied
to KOH–LiOH solutions. XRD patterns of the reaction products in
Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
4
8
12
16
Reaction time (h)
0
24
20
500
150
400
100
300
Si
Al
50
K+
Ca2+
200
100
0
0
4
8
12
16
0
24
20
Fig. 5. Powder X-ray diffraction patterns of the product derived from PSA using
KOH–LiOH mixtures; (a) 4 M KOH, (b) 3 M KOH + 1 M LiOH, (c) 2 M KOH + 2 M
LiOH, (d) 1 M KOH + 3 M LiOH, and (e) 4 M LiOH.
Reaction time (h)
100
0
0
4
8
12
16
0
24
20
(cmol/kg)
K and Ca
200
2+
50
300
+
K
2+
Ca
(Fig. 5), which suggests that these two phases dissolve into the
alkaline solution. Also, the minerals formed at high Li content were
calcium hydrate minerals, such as hydrocalumite [Ca2Al(OH)7
3H2O], katoite (Ca3Al2(SiO4)(OH)8) and portlandite (Ca(OH)2). Zeolite K-F could be synthesized in mixed solutions with low Li/(Li + K)
ratios of 0 or 0.25.
The chemical compositions of the products synthesized in
KOH–LiOH solutions are shown in Table 1. The K content in the
product was higher than that in the ash and increased with
increasing K content in the mixed solution, through the formation
of a crystalline zeolite K-F phase. The Li content in the product increased with increasing Li content in the solution.
The intensities of the major mineralogical phases in the product,
the amounts of K+, Ca2+, and Li+ released from the product, and CEC
of the product after 24 h reaction are shown in Fig. 6. The intensities
of zeolite K-F in the products at Li/(Li + K) ratios = 0 and 0.25 were
higher than that at other ratios, while those of katoite in the products at Li/(Li + K) ratios = 0 and 0.25 were lower than at other ratios.
The amount of K+ released from the product was almost constant at
100 cmol/kg at Li/(Li + K) = 0 and 0.25 and gradually decreased to
zero above Li/(Li + K) = 0.25, which correlated with intensity of zeolite K-F in the product. The amount of Li+ released from the product
lineally increased with increasing Li content in the solution. The
CEC of the product gradually decreased with increasing Li content,
which are almost same as the amount of K+ released. This means
that CEC of the product mainly depends on zeolite K-F, and released
+
Si
Al
Released amounts of
400
100
Released amounts of
500
150
Reaction time (h)
500
150
400
100
300
Si
Al
50
+
K
2+
Ca
200
100
0
0
4
8
12
16
20
0
24
100
+
K 2+
Ca
400
300
200
50
100
0
0
4
8
12
16
Reaction time (h)
20
0
24
(cmol/kg)
500
Si
Al
K and Ca
150
+
Si and Al concentrations (mM)
Reaction time (h)
(e)
α
θ
+
Fig. 4. Concentrations of Si and Al in the solutions during reaction, and the K and
Ca2+ release properties of the solid products for soil amendment after synthesis, for
each mixed solution; (a) 4 M KOH, (b) 3 M KOH + 1 M NaOH, (c) 2 M KOH + 2 M
NaOH, (d) 1 M KOH + 3 M NaOH, and (e) 4 M NaOH.
each KOH–LiOH mixed solution after 24 h reaction are shown in
Fig. 5. For NaOH–KOH solutions, only anorthite dissolved into the
alkaline solution, while gehlenite remained in the solids throughout all reactions. In the case of KOH–LiOH solutions, however,
the reactions differed from the previous cases. First, both anorthite
and gehlenite decreased with increasing Li content in the solution
300
800
600
200
400
100
200
0
0
0
0.25
0.5
0.75
Ratio of Li+/total cation
Zeolite K-F
Katoite
K
Ca
Li
CEC
1
and Li+ , and CEC (cmol/kg)
0
Released amounts of K+, Ca2+,
100
0
Intensities of zeolite K-F
and katoite (cps)
200
50
Released amounts of
300
Released amoutns of
400
K+ and Ca2+ (cmol/kg)
100
K
2+
Ca
K+ and Ca2+ (cmol/kg)
Si and Al concentrations (mM)
500
+
Si
Al
2+
(d)
150
Released amounts of
(c)
Si and Al concentrations (mM)
(b)
Si and Al concentraitons (mM)
(a)
K+ and Ca2+ (cmol/kg)
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
Si and Al concentrations (mM)
4
Fig. 6. Intensity of zeolite K-F and katoite in the product and amounts of K+, Ca2+,
and Li+ released from the product and CECs of the product after 24 h reaction for
each mixed solution.
Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
200
40
100
20
0
0
4
0
24
8
12
16
20
Reaction time (h)
400
K+
2+
Ca 300
100
Si
Al
80
60
200
40
100
20
0
0
4
8
12
16
Reaction time (h)
20
100
0
24
400
+
Si
Al
80
K
Ca2+
300
60
200
40
100
20
0
0
4
8
12
16
20
Reaction time (h)
100
400
80
300
60
Si
Al
40
K+
2+
Ca
200
100
20
0
0
4
8
12
16
20
Reaction time (h)
0
24
K+ and Ca2+ (cmol/kg)
Released amounts of
0
24
K and Ca (cmol/kg)
300
(cmol/kg)
K+
2+
Ca
Si
Al
60
K and Ca
80
K and Ca (cmol/kg)
400
100
K and Ca (cmol/kg)
20
2+
8
12
16
Reaction time (h)
+
4
Released amounts of
0
the solid product for K+ and Ca2+ release during each 24 h
experiment (Fig. 7). As observed in KOH–NaOH experiments, the
concentration of Al was always higher than that of Si in solution.
The trends in the concentration curves in Fig. 7a and b are almost
same as those for KOH–NaOH solutions, except those in Fig. 7c–e
exhibit different shapes from the other reactions. In the case of
Li/(K + Li) = 0.25 (Fig. 7b), the concentrations of Si and Al increased
initially after introduction of PSA and then the Al content became
almost constant after 2 h, while the Si concentration rapidly decreased to approximately 30 mM. The changes shown in Fig. 7b occurred faster than those shown in Fig. 7a. The K+ release from the
solid product increased to 100 cmol/kg and became almost constant after 4 h, meaning that K+ release from the product at Li/
(K + Li) = 0.25 reached a maximum faster than when using KOH
alone. In contrast, the Ca2+ release rapidly increased to 300 cmol/
kg and became constant after 3 h of reaction, which was similar
to using KOH alone. In the case of Li/(K + Li) ratios higher than
0.5 (Fig. 7c–e), the concentrations of Si and Al initially increased
after introduction of PSA and the Si and Al concentrations decreased. The rates of decrease were faster with increasing Li content in the solution and the amounts of Si and Al dissolved
decreased. The amount of K+ released from the solid product decreased because of the decrease in zeolite K-F crystal content.
The Ca2+ release rapidly increased to 300 cmol/kg and then decreased, which was correlated with the Al content in the solution.
The rate of decrease in Ca2+ release was also higher with increasing
Li content in the solution.
2+
0
24
0
5
4. Discussion
+
100
20
Released amoutns of
200
2+
K
2+
Ca
Si
Al
40
+
+
Released amounts of
300
60
2+
Si and Al concentraitons (mM)
80
+
(e)
400
100
Released amounts of
(d)
Si and Al concentraitons (mM)
(c)
Si and Al concentrations (mM)
(b)
Si and Al concentrations (mM)
(a)
Si and Al concentrations (mM)
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
Fig. 7. Concentrations of Si and Al in the solution during reaction, and the K+ and
Ca2+ release properties of the solid product for soil amendment after synthesis in
each mixed solution; (a) 4 M KOH, (b) 3 M KOH + 1 M LiOH, (c) 2 M KOH + 2 M
LiOH, (d) 1 M KOH + 3 M LiOH, and (e) 4 M LiOH.
Li+ has no relationship to cation exchange reaction. The amount of
Ca2+ released gradually decreased to zero with increasing
Li/(Li + K) ratio in the mixed solution, which correlated with the
intensity of katoite.
The reaction process was monitored by measuring the concentrations of Al and Si in the solutions and analyzing the properties of
A number of studies have dealt with the alkaline reaction of
incinerated ash, including low Ca content (<10 wt.%) [9,24–34]. In
these studies, the concentration of Si in solution was invariably
greater than that of Al, under all reaction conditions. However, in
the case of PSA with high Ca content, the opposite concentration
relation was found in our previous study [8,19] and in the current
study, as shown in Figs. 4 and 7, in which the concentration of Al
was greater than that of Si under all reaction conditions. Catalfamo
et al. [35] reported that the high affinity of Ca2+ ions for the silicate
species inhibits the dissolution of Si into the alkali solution. The
reason for the difference in the relative concentrations of Si and
Al may be related in part to the much higher Ca content in the
starting ash used in the current study compared to that in the previous studies.
Previous studies of alkali reactions with incinerated ash tested
the effects of pH, alkali concentration, temperature, or alkali species on reactions but, to date, none has studied the dependence
of the alkali species on the properties of the product derived from
high-Ca PSA, especially the synthesis of zeolite K-F. In the case of
KOH–NaOH solutions, only anorthite dissolved into the alkali,
which resulted in a solid reaction product containing hydroxysodalite and zeolite K-F, which crystallized from solution, and undissolved gehlenite. However, in the case of KOH–LiOH solution,
both anorthite and gehlenite dissolved into the solution, and the
calcium aluminosilicate mineral, katoite, was formed. Furthermore, the XRD patterns suggest that, for higher Li content in the
solution, the amorphous component of the products (indicated
by a broad hump between 20° and 40°) was less abundant than
with lower Li content solutions. It is considered that amorphous
phases are dissolved and subsequently converted into calcium aluminosilicate crystals in higher Li content solutions.
Murayama et al. [36] proposed the following mechanism for
zeolite formation from coal fly ash: release of Si and Al from the
ash into the alkaline solution, followed by the formation of an aluminosilicate gel as a precursor to crystalline solids, and finally
Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
6
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
crystallization of zeolites from the gel. In our experiments, the Al
concentration in solution was always higher than that of Si. In
the case of KOH–NaOH solutions and KOH–LiOH solutions with
Li content lower than Li/(Li + K) = 0.25, assuming that gel formation preceded crystallization in our experiments, the higher Al concentration resulted in an aluminosilicate gel with a low Si:Al ratio,
from which Al-rich zeolite phases formed (i.e., hydroxysodalite and
zeolite K-F). These results agree with the prediction of Barrer [37]
that elevated Al dissolution will lead to unique reaction products,
such as zeolites with low Si:Al ratios. However, in the case of
KOH–LiOH solutions with Li contents higher than Li/(Li + K) = 0.5,
a high Ca content was supplied into the solution by dissolution
of gehlenite and the production of an amorphous calcium aluminosilicate gel, using Si and Al in the solution, increased, to crystallize
katoite. Barrer [37] also concluded that the order for gelation rates
was Li+ > Na+ > K+, which corresponds to the order of hydrated ionic radius. The relative reaction rates observed in our experiments,
as shown in Figs. 4 and 7, are consistent with these results. Also, as
the rate of gelation reaction is faster, the dissolution rate promote
by keeping chemical equilibrium.
In our experiments, the products mainly contained zeolite K-F,
hydroxysodalite and katoite. Zeolite K-F is a member of the edingtonite group, hydroxysodalite is a member of the sodalite group,
and katoite is a member of the garnet group. Zeolite K-F and
hydroxysodalite have cation exchange ability, wherein K+ releases
from zeolite K-F and Na+ from hydroxysodalite by ion exchange
with NHþ
4 . Therefore, the product synthesized in KOH–NaOH solution has high CEC. On the other hands, katoite, hydrocalumite and
portlandite has little CEC, and the product synthesized in KOH–
LiOH solution with high Li content, including these calcium minerals, indicates low CEC. Okada et al. [38] reported that poorly crystalline calcium aluminum silicate hydrate (CASH) gels were
hydrothermally synthesized from mixtures of PSA, calcia and silica
in NaOH solution, and showed excellent simultaneous sorption
abilities for ammonium by ion exchange and for phosphate ions
by the reaction with dissolved Ca2+ ions. It can be considered that
release of Ca2+ mainly depends on the amorphous CASH gel in the
product and that part of the K+ release is from CASH gel. Furthermore, the released Ca2+ suggests that, for higher Li content solutions, the amorphous CASH in the products is less abundant in
katoite formation and the amount of released Ca2+ decreases. In
the case of KOH–NaOH solution, it is unclear why the amount of
Ca2+ released decreased to 300 cmol/kg in NaOH solution. The only
difference is that the solution is mixture of K+ and Na+ ions, which
may influence to gelation reaction by interaction of two cations.
These results are in good accordance with the change in Si and
Al concentrations in the solutions and the properties of the solids
during the reaction. As shown in Figs. 4 and 7a and b, part of the
soluble Si and Al content in raw ash was dissolved into alkali solution. Here, the Si content decreased due to gelation with Ca and Si,
and Si and Al, leaving Al in solution after 24 h of alkali reaction. As
shown in Fig. 7c–e, not only a major part of the Si and Al content
but the Ca content in the ash was dissolved into alkali solution,
where the Si and Al content decreased due to gelation with the
high Ca content. Because the Ca dissolution was higher, a larger
amount of CASH gel was formed, crystallizing katoite.
In summary, we propose the following mechanism for the chemical conversion of PSA into the zeolite K-F. PSA is mainly composed
of gehlenite, anorthite, and other amorphous phases, and the order
of solubility with respect to alkali reaction is: amorphous
phases > anorthite > gehlenite. During alkali reaction in KOH–
NaOH solution, most of the amorphous phase and the anorthite dissolves into the solution, and precipitates as amorphous CASH and
aluminosilicate gel to form zeolite crystals, zeolite K-F in solutions
with Na/(Na + K) ratios lower than 0.5, and hydroxysodalite in solutions with other ratios, leaving Al3+ in solution. Under the condi-
tions for obtaining products containing zeolite K-F, with
increasing Na content in the solution, the amount of K+ released
gradually decreases and that of Ca2+ increases, along with the time
required to reach the maximum amount of Ca2+ released. In the case
of KOH–LiOH solutions, in solutions where the Li/(Li + K) ratio is
lower than 2.5, most of the amorphous phase and the anorthite dissolves into the solution and precipitates as amorphous CASH and
aluminosilicate gel to form zeolite K-F. At higher Li solution content, the amount of K+ released is similar and the time required to
reach the maximum amount of released K+ is shorter, while that
of Ca2+ is slightly lower. However, at Li content in the solution higher than Li/(Li + K) = 0.5, not only amorphous phase and anorthite
but also gehlenite dissolves in the solution and the precipitation
of amorphous CASH to katoite is promoted. The product displays
low release of K+ and Ca2+, which is not good for soil amendment.
From these results, products containing zeolite K-F can be synthesized in KOH–NaOH solutions with Na/(Na + K) = 0, 0.25 and
0.5, or in KOH–LiOH solutions with Li/(Li + K) = 0 and 0.25. Under
zeolite K-F synthesis conditions, for all five products, the amount
of Ca2+ released from the products for phosphate fixation are enough, and the time to the Ca2+ release capability are shorter than
4 h. The CEC of the product synthesized in all three solutions after
24-h reaction are almost same, 100 cmol/kg, but the time to reach
100 cmol/kg for the product synthesized in KOH–LiOH solution
with Li/(Li + K) = 0.25 is faster than those in other solutions. From
these results, the optimal conditions for the synthesis of soil
amendment materials from PSA would involve KOH–LiOH solutions with Li/(Li + K) = 0.25, because a product with high CEC and
high capability for releasing Ca2+ can be synthesized faster than
when using other synthesis solutions.
5. Conclusions
Soil amendment materials containing zeolite K-F were synthesized from PSA in KOH–NaOH and KOH–LiOH solutions at 90 °C.
Zeolite K-F, hydroxysodalite, katoite, hydrocalumite and portlandite were synthesized in the products. Using KOH–NaOH mixtures,
zeolite K-F was synthesized in solutions with Na/(Na + K) ratios
lower than 0.5, while hydroxysodalite was synthesized at other
Na/(Na + K) ratios. Increasing the Na/(Na + K) ratio to 0.75 gradually
decreased the K+ released from the product, increased the Na+ released from the product, and increased Ca2+ release, while the
CEC of the product is almost constant. In KOH–LiOH mixtures, zeolite K-F was synthesized at Li/(Li + K) ratios lower than 0.25, while
calcium hydrate minerals, such as katoite, hydrocalumite and portlandite, were synthesized using other ratios. Increasing the Li content to Li/(Li + K) = 0.75 gradually decreased K+ and Ca2+ release
from the product, increasing Li+ release from the product, while
the CEC of the product decreases. The concentrations of Si and Al
in the solution observed during the reaction explain the synthesis
of products with differing soil amendment properties. The release
of K+ from the product depends on formation of precursor gel and
zeolite K-F crystals, while Ca2+ release depends on formation of
CASH gel. The formation of zeolite K-F in the KOH–LiOH solutions
with Li/(Li + K) ratio = 0.25 is faster than using KOH solution alone,
and the product would have good properties for soil amendment
after KCl washing. This was identified as the best conditions for synthesis of soil amendment materials from PSA in this work.
Acknowledgments
This work was supported by a Research Promotion Grant from
the Iron and Steel Institute of Japan, and MEXT/JSPS Grant-in-Aid
for Young Scientists (B): 24710086.
Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136
T. Wajima, K. Munakata / Chemical Engineering Journal xxx (2012) xxx–xxx
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Please cite this article in press as: T. Wajima, K. Munakata, Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil
amendment, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.136