Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
140 J. ION EXCHANGE
Article
The Inhibition of Fluoride Elution from Industrial Wastes with Portland Cement, Calcium and Magnesium Salts in Alkaline Region
Xiaoxu KUANG1*, Atsushi SASAKI2 and Masatoshi ENDO1
1Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering,
Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan 2 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University,
4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan
(Manuscript received March 31, 2018; accepted June 1, 2018)
Abstract
Recycling of fluoride industrial wastes is difficult to dispose, since the fluoride ions eluted again easily.
In this study, newly effective inhibition method in both neutral and alkaline region for fluoride elution
was investigated. Fluoride elution concentration from CaF2 was 9.8 mg/L at pH 8.1 and 288.5 mg/L at pH
12.2 with water/solid ratio of 10. Additions of Ca(OH)2, CaCl2 and NH4H2PO4 could restrain fluoride
elution concentration of CaF2 to 0.22 mg/L in the neutral region. This inhibition of fluoride elution was
due to a generation of hydroxyapatite (HAp) or chlorapatite (ClAp) which has high ion exchange capacity.
Additions of Portland cement, Ca(OH)2 and MgCl2 could restrain fluoride elution concentration of CaF2
to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution
concentration from CaF2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of
CaF2 with the high content of Ca2+ provided from Ca(OH)2 and cement hydrates. The carbonation of Ca2+
was prevented by the addition of Mg2+. Moreover, the forming of ettringite with a high ion-exchange
capacity and the solidification effect of Ca-bearing hydrates contributed to the inhibition of the elution of
fluoride. These additives for the practical industrial wastes, such as paper sludge and coal ash, with high
concentration of fluoride were conducted. The result indicates that the fluoride elution could be restrained
to meet the environmental standard (0.8 mg/L) in Japan in alkaline region. This inhibition method for
fluoride elution would be able to contribute to promotion of recycling of fluoride industrial wastes.
Keywords: Inhibition of fluoride elution, Recycling of fluoride wastes, Hydroxyapatite, Cement hydrates,
Alkaline region
1. Introduction
Fluorine, as an element of the halogen group, is widely
applied in semiconductors, medicines, glass and other
industries. To meet the growing demands for fluoride, the
productions of fluoride compounds have been also increasing.
It is known that excessive intake of fluoride could lead to the
disease of bone and teeth1). Recently, more and more reports
* Corresponding author E-mail: [email protected]
indicate that fluoride could affect the brain and spinal cord,
causing various diseases and even life-threatening ones2-6).
Treatment of fluoride as a significant industrial pollutant has
become an important issue. Fluoride industrial wastes, such as
fluoride-containing sludge, coal ash and so on, occur
mainlyduring the treatment of fluoride wastewater by co-
precipitating with calcium and aluminum salt and fluoride
( )100
Vol.29 No.3 (2018) 141
Article
The Inhibition of Fluoride Elution from Industrial Wastes with Portland Cement, Calcium and Magnesium Salts in Alkaline Region
Xiaoxu KUANG1*, Atsushi SASAKI2 and Masatoshi ENDO1
1Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering,
Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan 2 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University,
4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan
(Manuscript received March 31, 2018; accepted June 1, 2018)
Abstract
Recycling of fluoride industrial wastes is difficult to dispose, since the fluoride ions eluted again easily.
In this study, newly effective inhibition method in both neutral and alkaline region for fluoride elution
was investigated. Fluoride elution concentration from CaF2 was 9.8 mg/L at pH 8.1 and 288.5 mg/L at pH
12.2 with water/solid ratio of 10. Additions of Ca(OH)2, CaCl2 and NH4H2PO4 could restrain fluoride
elution concentration of CaF2 to 0.22 mg/L in the neutral region. This inhibition of fluoride elution was
due to a generation of hydroxyapatite (HAp) or chlorapatite (ClAp) which has high ion exchange capacity.
Additions of Portland cement, Ca(OH)2 and MgCl2 could restrain fluoride elution concentration of CaF2
to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution
concentration from CaF2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of
CaF2 with the high content of Ca2+ provided from Ca(OH)2 and cement hydrates. The carbonation of Ca2+
was prevented by the addition of Mg2+. Moreover, the forming of ettringite with a high ion-exchange
capacity and the solidification effect of Ca-bearing hydrates contributed to the inhibition of the elution of
fluoride. These additives for the practical industrial wastes, such as paper sludge and coal ash, with high
concentration of fluoride were conducted. The result indicates that the fluoride elution could be restrained
to meet the environmental standard (0.8 mg/L) in Japan in alkaline region. This inhibition method for
fluoride elution would be able to contribute to promotion of recycling of fluoride industrial wastes.
Keywords: Inhibition of fluoride elution, Recycling of fluoride wastes, Hydroxyapatite, Cement hydrates,
Alkaline region
1. Introduction
Fluorine, as an element of the halogen group, is widely
applied in semiconductors, medicines, glass and other
industries. To meet the growing demands for fluoride, the
productions of fluoride compounds have been also increasing.
It is known that excessive intake of fluoride could lead to the
disease of bone and teeth1). Recently, more and more reports
* Corresponding author E-mail: [email protected]
indicate that fluoride could affect the brain and spinal cord,
causing various diseases and even life-threatening ones2-6).
Treatment of fluoride as a significant industrial pollutant has
become an important issue. Fluoride industrial wastes, such as
fluoride-containing sludge, coal ash and so on, occur
mainlyduring the treatment of fluoride wastewater by co-
precipitating with calcium and aluminum salt and fluoride
( )100
waste products. Since fluoride easily eluted from the fluoride
industrial wastes, the disposal and/or recycle of those industrial
wastes also become difficult. Hydroxyapatite [HAp,
Ca10(PO4)6(OH)2] or chlorapatite [ClAp, Ca10(PO4)6Cl2] has
been reported to be able to remove fluoride ions in aqueous
solution because of its low solubility and excellent ion
exchange capability7-9). Moreover, hydrated cement containing
substantial amounts of Ca-bearing hydrates, such as portlandite
and ettringite, was also reported to be able to remove fluoride
ions by absorbing fluoride and co-precipitation10-13). However,
these studies focused on only removing of fluoride ions from
aqueous solutions and didn’t investigate an inhibition of
fluoride elution from the fluoride industrial wastes.
Additionally, reports about inhibition techniques of fluoride
eluted from the industrial wastes are also very few. Therefore,
the inhibition of fluoride eluted from the industrial wastes
needs further consideration. This study is aimed to develop an
effective inhibition method for fluoride elution. Effects of
forming of hydroxyapatite and cement hydrates and additions
of calcium and magnesium salts on an inhibition of fluoride
elution were studied. Additionally, tests of our fluoride
inhibition methods for paper sludge and coal ash as practical
industrial wastes, were also conducted from the viewpoints of a
practical application.
2. Experimental
2.1 Materials
These experiments described 2.2.1-2.2.3 with pure reagent
CaF2 (Wako Pure Chemical Industries Ltd., Japan) were
conducted for a simulation of fluoride elution from wastes.
Paper sludge (supplied by Nippon Paper Industries Co., Ltd.,
Japan) and coal ash (supplied by Fujico Co., Ltd.) were used as
source of fluoride wastes to test fluoride inhibitory effects.
The additives used to restrain the elution of fluoride were
Ca(OH)2 (99.9%), CaCl2·2H2O (99.0~103%), NH4H2PO4
(99.9%), Mg(OH)2 (96.0%), MgCl2·6H2O (99.0%) supplied by
Kanto Chemical Co., Inc, Japan and Portland cement supplied
by Sumitomo Osaka Cement Co., Ltd., Japan.
The equipment used in this study were Hitachi U-3000
Spectrophotometer, Rigaku Multipurpose X-ray diffraction
spectrometer (XRD Ultima IV) and Hitachi Field Emission
scanning electron microscope (SEM, SU8000).
2.2 The elution of fluoride ions from CaF2 pure reagent
CaF2 (10 g) was prepared by mixing with 3 mL of different
concentration of HCl or NaOH solution to adjusted initial pH
value of eluted solution to 4.6~12.2. They were placed at room
temperature for 6 h and then, dried with a muffle furnace
(Carbolite ESF3) at 60°C for 18 h. The dried samples were
conducted to fluoride elution test.
In this study, fluoride elution concentrations of the filtrates
were determined by measurements of fluoride concentration of
the filtrates with Lanthanum-alizarin complex
spectrophotometric method (ALC) using Hitachi U-3000
Spectrophotometer. These experiments were according to the
testing method for industrial wastewater of Japan (JIS K
0102)14). pH measurements of the filtrates were performed with
a Horiba pH Meter F-22. The crystal structure and phase
composition of dried mixtures were characterized by XRD. The
microstructures of dried mixtures were monitored by SEM.
2.3 Fluoride inhibition effect in neutral region
CaF2 (10 g) was mixed with 1.0 g of Ca(OH)2, 2.0 g of
CaCl2·2H2O, 0.78 g of NH4H2PO4. In this procedure, Ca(OH)2,
CaCl2·2H2O and NH4H2PO4 were added so that the molar ratio
of calcium/ phosphate was equal to 2. The pH values of eluted
solution of samples were adjusted to 3.6~12.7 by additions of
HCl or NaOH solution. The following experimental process
was the same as one in Section 2.2.
2.4 Fluoride inhibition effect in alkaline region
CaF2 (7 g) was mixed with 0.5 g of Ca(OH)2, 0.1~0.6 g of
MgCl2·6H2O and 3.0 g of ordinary Portland cement. Ultrapure
water was added to mixed samples, so that the water-to-solid
sample ratio was 0.3:1. The following experimental process
was the same as one in Section 2.2.
The experimental process of inhibition of fluoride elution
from CaF2 mixed with Ca(OH)2, Ca(OH)2 and Mg(OH)2, and
Portland cement respectively was the same as one descripted
above.
2.5 Application for practical wastes
Paper sludge and coal ash were mixed with CaF2 at weight
ratio of 9:1, respectively. Then Portland cement, Ca(OH)2 and
MgCl2·6H2O were added to these wastes to test fluoride
inhibitory effects. The following experimental process was the
same as one in Section 2.2.
3. Results and Discussion
3.1 Fluoride inhibition effect in neutral region
Figure 1 shows effect of pH for fluoride elution
concentration from CaF2 with and without additions of
Ca(OH)2/CaCl2 and NH4H2PO4. The fluoride elution
concentration from CaF2 without additives is 9.8 mg/L (pH=8.1)
in the neutral region. And then, it rapidly increases when the
pH value less than 8 or over 10. It could reach to 29.9 mg/L at
pH 4.6 and 288.5 mg/L at pH 12.2. The change in fluoride
elution concentration could be explained by the following
chemical equations:
CaF2 Ca2+ + 2F-
Ksp, CaF2 = [Ca2+][F-]2 = 3.95×10-11 (1)
H+ + F- HF
Ka, HF = [H+][F-]/[HF] = 3.5×10-4 (2)
Ca2+ + 2OH- Ca(OH)2
Ksp, Ca(OH)2 = [Ca2+][OH-]2 = 5.02×10-6 (3)
where Ka is acid dissociation constant, Ksp is solubility product.
( )101
142 J. ION EXCHANGE
[H+], [F-], [Ca2+] and [OH-] are activities of H+, F-, Ca2+ and
OH-, respectively. According to the Eqs. (1) and (2), CaF2 is
easy to be decomposed into calcium ions and fluoride ions in
the acid condition. With the increase of [H+], the ionization
equilibrium of HF moves to the right, F- has been consumed,
promoting the decomposition of CaF2. It ultimately leads to the
increase of concentration of fluoride ions in the solution.
According to the Eq. (3), with the increase in pH, the reaction
equilibrium of Ca(OH)2 moves to the right, Ca2+ has been
consumed. Consequently, this consuming of Ca2+ leads to
increase concentration of fluoride by the Eq. (3).
Fig. 1 Effect of pH for fluoride elution concentration from CaF2 with and without an addition of Ca(OH)2/CaCl2 and NH4H2PO4.
Generation of hydroxyapatite and chlorapatite reported to be
accelerated at Ca/P molar ratio of 1.67 or more15,16). Therefore,
the Ca/P molar ratio of Ca(OH)2/CaCl2 and NH4H2PO4 in this
study was set to 2. When Ca(OH)2/CaCl2 and NH4H2PO4 were
added, the lowest fluoride elution concentration was found at
pH 8.5 and was 0.22 mg/L which is below the concentration
specified in the environment standard of Japan. This fluoride
elution concentration: 0.22 mg/L, was only 2.3% of one
without the additions: 9.8 mg/L at pH 8.1. The elution of
fluoride was restrained by a large amount of Ca2+ dissolved
from Ca(OH)2/CaCl2 and the products ClAp and HAp. The
chemical reaction equation postulated among Ca(OH)2/CaCl2,
NH4H2PO4 and CaF2 are shown in Eqs. (4) -(7).
10Ca2+ + 6PO43- + 2Cl- Ca10(PO4)6Cl2 (4)
10Ca2+ + 6PO43- + 2OH- Ca10(PO4)6(OH)2 (5)
Ca10(PO4)6(OH)2 + 2F- Ca10(PO4)6F2 + 2OH- (6)
Ca10(PO4)6Cl2 + 2F- Ca10(PO4)6F2 + 2Cl- (7)
Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and/or chlorapatite
(ClAp, Ca10(PO4)6Cl2) would be formed by the reaction of Eqs.
(4) and (5). Additionally, its precursors, tricalcium phosphate
[Ca3(PO4)2] and/or dicalcium phosphate [CaHPO4], would be
also formed. It was reported that hydroxide ions (OH-) or
chloride ion (Cl-) of those apatites and its precursors restrained
the fluoride elution by exchanging with the free fluoride ions.
Moreover, those apatites or its precursors probably restrained
more the elution of fluoride ions with wrapping on the surface
of fluoride compound during its forming. These inhibition
behaviors of apatites and its precursors were shown in Fig. 2.
Fig. 2 Schematic illustration for the inhibition of fluoride with hydroxyapatite (HAp) or chlorapatite (ClAp), or its precursors (Ca3(PO4)2 and/or CaHPO4).
Fig. 3 XRD patterns of CaF2 which mixed with (a) CaCl2 and
NH4H2PO4 at pH 3.6, and (b) Ca(OH)2 and NH4H2PO4 at pH 8.5.
The reflections assigned to calcium hydrogen chloride
phosphate hydrate (CaClH2PO4·H2O, ICSD file No.00-044-
0746), chlorapatite (Ca10(PO4)6Cl2, ICSD file No.00-027-0074)
and hydroxyapatite (Ca10(PO4)6(OH)2, ICSD file No. 00-009-
0432) were observed in Fig. 3. The result suggests that HAp
and ClAp had been generating from the reactions of Ca(OH)2,
CaCl2 and NH4H2PO4. The dissolution of Ca(OH)2/CaCl2 and
the generation of ClAp and HAp decreased the fluoride elution
concentration from CaF2 below the environmental standards in
the neutral region. However, the fluoride elution concentration
was still reaches to 196.9 mg/L at pH 12.7 and far more than
environmental standards in the alkaline region. It was because
that, in alkaline region, Ca2+ was consumed not only by OH- to
generate Ca(OH)2 (Eq. (3)), but also by PO43- to generate HAp
(Eq. (5)). The consuming of large amounts of Ca2+ contributes
to the elution of fluoride. On the other hand, because of the
similarity in OH- and F- in charge, ionic radius and adsorption
sites in apatite, OH- will compete with F-, resulting into the
050100150200250300350
2 4 6 8 10 12 14
F co
nc. [
mg/
L]
pH
Without the additives With the additives
Calcium fluoride Portlandite ●Hydroxyapatite▼Chlorapatite
Calcium hydrogen chloride phosphate hydrate
( )102
Vol.29 No.3 (2018) 143
[H+], [F-], [Ca2+] and [OH-] are activities of H+, F-, Ca2+ and
OH-, respectively. According to the Eqs. (1) and (2), CaF2 is
easy to be decomposed into calcium ions and fluoride ions in
the acid condition. With the increase of [H+], the ionization
equilibrium of HF moves to the right, F- has been consumed,
promoting the decomposition of CaF2. It ultimately leads to the
increase of concentration of fluoride ions in the solution.
According to the Eq. (3), with the increase in pH, the reaction
equilibrium of Ca(OH)2 moves to the right, Ca2+ has been
consumed. Consequently, this consuming of Ca2+ leads to
increase concentration of fluoride by the Eq. (3).
Fig. 1 Effect of pH for fluoride elution concentration from CaF2 with and without an addition of Ca(OH)2/CaCl2 and NH4H2PO4.
Generation of hydroxyapatite and chlorapatite reported to be
accelerated at Ca/P molar ratio of 1.67 or more15,16). Therefore,
the Ca/P molar ratio of Ca(OH)2/CaCl2 and NH4H2PO4 in this
study was set to 2. When Ca(OH)2/CaCl2 and NH4H2PO4 were
added, the lowest fluoride elution concentration was found at
pH 8.5 and was 0.22 mg/L which is below the concentration
specified in the environment standard of Japan. This fluoride
elution concentration: 0.22 mg/L, was only 2.3% of one
without the additions: 9.8 mg/L at pH 8.1. The elution of
fluoride was restrained by a large amount of Ca2+ dissolved
from Ca(OH)2/CaCl2 and the products ClAp and HAp. The
chemical reaction equation postulated among Ca(OH)2/CaCl2,
NH4H2PO4 and CaF2 are shown in Eqs. (4) -(7).
10Ca2+ + 6PO43- + 2Cl- Ca10(PO4)6Cl2 (4)
10Ca2+ + 6PO43- + 2OH- Ca10(PO4)6(OH)2 (5)
Ca10(PO4)6(OH)2 + 2F- Ca10(PO4)6F2 + 2OH- (6)
Ca10(PO4)6Cl2 + 2F- Ca10(PO4)6F2 + 2Cl- (7)
Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and/or chlorapatite
(ClAp, Ca10(PO4)6Cl2) would be formed by the reaction of Eqs.
(4) and (5). Additionally, its precursors, tricalcium phosphate
[Ca3(PO4)2] and/or dicalcium phosphate [CaHPO4], would be
also formed. It was reported that hydroxide ions (OH-) or
chloride ion (Cl-) of those apatites and its precursors restrained
the fluoride elution by exchanging with the free fluoride ions.
Moreover, those apatites or its precursors probably restrained
more the elution of fluoride ions with wrapping on the surface
of fluoride compound during its forming. These inhibition
behaviors of apatites and its precursors were shown in Fig. 2.
Fig. 2 Schematic illustration for the inhibition of fluoride with hydroxyapatite (HAp) or chlorapatite (ClAp), or its precursors (Ca3(PO4)2 and/or CaHPO4).
Fig. 3 XRD patterns of CaF2 which mixed with (a) CaCl2 and
NH4H2PO4 at pH 3.6, and (b) Ca(OH)2 and NH4H2PO4 at pH 8.5.
The reflections assigned to calcium hydrogen chloride
phosphate hydrate (CaClH2PO4·H2O, ICSD file No.00-044-
0746), chlorapatite (Ca10(PO4)6Cl2, ICSD file No.00-027-0074)
and hydroxyapatite (Ca10(PO4)6(OH)2, ICSD file No. 00-009-
0432) were observed in Fig. 3. The result suggests that HAp
and ClAp had been generating from the reactions of Ca(OH)2,
CaCl2 and NH4H2PO4. The dissolution of Ca(OH)2/CaCl2 and
the generation of ClAp and HAp decreased the fluoride elution
concentration from CaF2 below the environmental standards in
the neutral region. However, the fluoride elution concentration
was still reaches to 196.9 mg/L at pH 12.7 and far more than
environmental standards in the alkaline region. It was because
that, in alkaline region, Ca2+ was consumed not only by OH- to
generate Ca(OH)2 (Eq. (3)), but also by PO43- to generate HAp
(Eq. (5)). The consuming of large amounts of Ca2+ contributes
to the elution of fluoride. On the other hand, because of the
similarity in OH- and F- in charge, ionic radius and adsorption
sites in apatite, OH- will compete with F-, resulting into the
050100150200250300350
2 4 6 8 10 12 14
F co
nc. [
mg/
L]
pH
Without the additives With the additives
Calcium fluoride Portlandite ●Hydroxyapatite▼Chlorapatite
Calcium hydrogen chloride phosphate hydrate
( )102
decrease in fluoride inhibitory effect in alkaline region7,8,17,18).
Consequently, more effective fluoride inhibition method in the
alkaline region is needed.
3.2 Fluoride inhibition effect in alkaline region
The fluoride elution concentration was decreased
significantly with the increase in the addition amount of
Ca(OH)2 (Table 1). When the addition amount of Ca(OH)2 was
over 0.7 g, the fluoride concentration could be reduced to less
than 4 mg/L, this concentration was only 1.38% of 288.5 mg/L
which was the fluoride elution concentration from CaF2 at pH
12.2. A large amount Ca2+ provided from Ca(OH)2 would
inhibit the elution of fluoride ions from CaF2 by the reactions
according to Eqs. (1) and (3). However, the fluoride
concentration had been hardly satisfied the environmental
standards only by the addition with Ca(OH)2.
Table 1 Effect of Ca(OH)2 for fluoride elution concentration
from 10.0 g of CaF2.
Table 2 Effect of Mg(OH)2 for fluoride elution concentration
from 10.0 g of CaF2 and 1.0 g of Ca(OH)2 with heating
at 60°C or vacuum drying at room temperature.
A decrease in concentration of fluoride ions was observed
with the increase in the addition amount of Mg(OH)2 with both
heat drying and vacuum drying methods (Table 2). It was
considered that the elution of fluoride ions would be restrained
by the addition of Ca(OH)2 and Mg(OH)2, since Ca2+ and Mg2+
provided from Ca(OH)2 and Mg(OH)2 could react with free F-
to generate CaF2 and MgF2. The fluoride elution concentration
of CaF2 with vacuum drying at room temperature was lower
than that with heating at 60°C. The reflections assigned to
portlandite (Ca(OH)2, ICSD file No.00-004-0733) and calcite
(CaCO3, ICSD file No.01-072-1652) were observed at the
condition with an addition of Ca(OH)2 and heating at 60°C (Fig.
4). Brucite (Mg(OH)2, ICSD file No.00-007-0239) was
appeared when Mg(OH)2 was added. On another hand, at the
conditions with vacuum drying at room temperature, the
reflection assigned to calcite was disappeared. Furthermore, the
reflection assigned to portlandite became stronger. The Ca2+
consumption from carbonation of Ca2+ by reacting with CO2 in
the air could be controlled in vacuum drying condition. The
decrease in the elution concentration of fluoride under vacuum
drying condition indicates that the inhibition of the carbonation
of Ca2+ could be a way to promote the inhibitory effect of
elution of fluoride ions.
Fig. 4 XRD patterns of CaF2 which mixed with (a) Ca(OH)2
(b) Ca(OH)2, and 0.5 g of Mg(OH)2 with heating at
60°C (c) Ca(OH)2 and (d) Ca(OH)2 and 0.5 g of Mg(OH)2
with vacuum drying at room temperature.
Table 3 Effect of ordinary Portland cement for fluoride
elution concentration from 7.0 g of CaF2.
Fig. 5 XRD pattern of CaF2 which mixed with ordinary
Portland cement at weight ratio of 50%.
A significant decrease in fluoride elution concentration was
observed with an increase in an addition amount of Portland
cement (Table 3). The lowest fluoride elution concentration:
0.68 mg/L was found when the weight ratio of Portland cement
was 50%, it was only 0.24% of 288.5 mg/L which was the
fluoride elution concentration from CaF2 at pH 12.2. The
reflections assigned to portlandite (Ca(OH)2, ICSD file No.01-
Addition amount of Ca(OH)2 (g)
0.2 0.4 0.5 0.7 1.0
F conc. [mg/L] 75.8 73.9 38.2 3.3 3.9
Drying method
Addition amount of Mg(OH)2 (g)
0.0 0.1 0.3 0.5
F conc. [mg/L] vacuum 12.4 11.3 9.2 8.7 F conc. [mg/L] oven 13.8 14.3 13.4 11.0
Addition of Portland cement (wt.%)
10 20 30 40 50
F conc. [mg/L] 3.9 1.7 1.2 0.86 0.68
ΔPortlandite ○Calcite Magnesium hydroxide
(d)
(c)
(b)
(a)
ΔPortlandiite ○Calcite ■Ettringite ▼Calcium silicate hydrate
( )103
144 J. ION EXCHANGE
076-0571), calcite (CaCO3, ICSD file No.00-005-0586),
calcium silicate hydrate (Ca3SiO5, ICSD file No.01-086-0402)
and ettringite (Ca6Al2(SO4)3(OH)12·26H2O, ICSD file No.00-
041-1451) were observed in Fig. 5. The elution of fluoride ions
was restrained by the coprecipitating of CaF2 with the high
content of Ca2+ provided from cement hydrates. In addition, the
forming of ettringite with high ion-exchange capacity and the
solidification effect of Ca-bearing hydrates contributes to the
inhibition of the elution of fluoride.
Table 4 Effect of MgCl2 for fluoride elution concentration
from 7.0 g of CaF2, Ca(OH)2 and Portland cement
under the drying conditions at 60°C or room
temperature.
A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2
All pH values of samples in Table 4 were above 12.2. The
fluoride elution concentration was restrained to less than 1.5
mg/L with an addition of Portland cement with both drying
conditions. Then, it slightly decreased with an increase in
addition amounts of Ca(OH)2 and MgCl2·6H2O. The highest
effect on an inhibition of a fluoride elution was found when the
samples were mixed with Portland cement, Ca(OH)2 and 0.4 g
of MgCl2·6H2O. Those fluoride elution concentrations were
restrained to 0.78 mg/L with drying at 60°C and 0.47 mg/L at
room temperature. In addition, the samples treated with drying
at the room temperature showed a higher effect in an inhibition
of fluoride elution compared with ones with drying at 60°C.
The reflections assigned to portlandite (Ca(OH)2, ICSD file
No.00-004-0733), calcite (CaCO3, ICSD file No.00-005-0586)
and calcium silicate hydrate (Ca3SiO5, ICSD file No. 01-086-
0402) were observed at all the experimental conditions (Fig. 6).
The reflections assigned to portlandite as cement hydrates
became stronger when Ca(OH)2 was added. The reflections
assigned to portlandite, calcite and calcium silicate hydrate
weakened clearly when addition amounts of MgCl2 increased,
while gypsum appeared. Mg2+ was reported to have an
inhibitory effect on the growth of calcite in many researches. In
a saturated calcite solution, the presence of Mg2+ can enhance
the solubility of calcite by incorporating into the calcite lattice
even at low Mg concentration or aqueous Mg/Ca ratio19-22).
Actually, the peak of reflection assigned to calcite decreased
with an increase in an addition of MgCl2. Additionally, in the
coexistence of MgCl2 and Ca(OH)2 of Portland cement paste,
Mg2+ was reported to react with hydroxyl ions to generate the
precipitation of Mg(OH)2 because of its low solubility,
Fig. 6 XRD patterns of CaF2 which mixed with (a) Portland
cement; (b) Portland cement and Ca(OH)2; (c) Portland
cement, Ca(OH)2 and 0.4g of MgCl2; (d) Portland
cement, Ca(OH)2 and 0.4 g of MgCl2 with drying at 60°C
and (e) Portland cement, Ca(OH)2 and 0.4 g of MgCl2
with drying at room temperature.
(a)
(b)
Fig. 7 SEM imagines for (a) cement paste powder and (b)
ettringite from dried samples which mixed with
Portland cement, Ca(OH)2 and 0.4 g of MgCl2.
Additives
Drying condition
60°C Room Temp.
F conc.
[mg/L]
pH [-] F conc.
[mg/L]
pH [-]
cement 1.44 12.5 1.48 12.4
A 1.18 12.5 1.08 12.5
A+0.1 g of B 1.21 12.5 1.12 12.5
A+0.2 g of B 1.24 12.3 0.96 12.4
A+0.4 g of B 0.78 12.3 0.47 12.4
A+0.6 g of B 0.98 12.2 0.74 12.3
Ettringite
1.0μm
15.0μ
ΔPortlandiite ○Calcite □Gypsum
▼Calcium silicate hydrate
(e)
(d)
(c)
(b)
(a)
( )104
Vol.29 No.3 (2018) 145
076-0571), calcite (CaCO3, ICSD file No.00-005-0586),
calcium silicate hydrate (Ca3SiO5, ICSD file No.01-086-0402)
and ettringite (Ca6Al2(SO4)3(OH)12·26H2O, ICSD file No.00-
041-1451) were observed in Fig. 5. The elution of fluoride ions
was restrained by the coprecipitating of CaF2 with the high
content of Ca2+ provided from cement hydrates. In addition, the
forming of ettringite with high ion-exchange capacity and the
solidification effect of Ca-bearing hydrates contributes to the
inhibition of the elution of fluoride.
Table 4 Effect of MgCl2 for fluoride elution concentration
from 7.0 g of CaF2, Ca(OH)2 and Portland cement
under the drying conditions at 60°C or room
temperature.
A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2
All pH values of samples in Table 4 were above 12.2. The
fluoride elution concentration was restrained to less than 1.5
mg/L with an addition of Portland cement with both drying
conditions. Then, it slightly decreased with an increase in
addition amounts of Ca(OH)2 and MgCl2·6H2O. The highest
effect on an inhibition of a fluoride elution was found when the
samples were mixed with Portland cement, Ca(OH)2 and 0.4 g
of MgCl2·6H2O. Those fluoride elution concentrations were
restrained to 0.78 mg/L with drying at 60°C and 0.47 mg/L at
room temperature. In addition, the samples treated with drying
at the room temperature showed a higher effect in an inhibition
of fluoride elution compared with ones with drying at 60°C.
The reflections assigned to portlandite (Ca(OH)2, ICSD file
No.00-004-0733), calcite (CaCO3, ICSD file No.00-005-0586)
and calcium silicate hydrate (Ca3SiO5, ICSD file No. 01-086-
0402) were observed at all the experimental conditions (Fig. 6).
The reflections assigned to portlandite as cement hydrates
became stronger when Ca(OH)2 was added. The reflections
assigned to portlandite, calcite and calcium silicate hydrate
weakened clearly when addition amounts of MgCl2 increased,
while gypsum appeared. Mg2+ was reported to have an
inhibitory effect on the growth of calcite in many researches. In
a saturated calcite solution, the presence of Mg2+ can enhance
the solubility of calcite by incorporating into the calcite lattice
even at low Mg concentration or aqueous Mg/Ca ratio19-22).
Actually, the peak of reflection assigned to calcite decreased
with an increase in an addition of MgCl2. Additionally, in the
coexistence of MgCl2 and Ca(OH)2 of Portland cement paste,
Mg2+ was reported to react with hydroxyl ions to generate the
precipitation of Mg(OH)2 because of its low solubility,
Fig. 6 XRD patterns of CaF2 which mixed with (a) Portland
cement; (b) Portland cement and Ca(OH)2; (c) Portland
cement, Ca(OH)2 and 0.4g of MgCl2; (d) Portland
cement, Ca(OH)2 and 0.4 g of MgCl2 with drying at 60°C
and (e) Portland cement, Ca(OH)2 and 0.4 g of MgCl2
with drying at room temperature.
(a)
(b)
Fig. 7 SEM imagines for (a) cement paste powder and (b)
ettringite from dried samples which mixed with
Portland cement, Ca(OH)2 and 0.4 g of MgCl2.
Additives
Drying condition
60°C Room Temp.
F conc.
[mg/L]
pH [-] F conc.
[mg/L]
pH [-]
cement 1.44 12.5 1.48 12.4
A 1.18 12.5 1.08 12.5
A+0.1 g of B 1.21 12.5 1.12 12.5
A+0.2 g of B 1.24 12.3 0.96 12.4
A+0.4 g of B 0.78 12.3 0.47 12.4
A+0.6 g of B 0.98 12.2 0.74 12.3
Ettringite
1.0μm
15.0μ
ΔPortlandiite ○Calcite □Gypsum
▼Calcium silicate hydrate
(e)
(d)
(c)
(b)
(a)
( )104
resulting in the decrease in pH value and an increase in the
solubility of Ca23,24). The reflection assigned to ettringite was
not observed. The addition weight ratio of Portland cement in
Fig. 6(a) was less than that of Fig. 5. Little amount of
production of ettringite caused this weak diffraction.
Coagulation of cement paste and the generation of ettringite
were confirmed in Fig. 7. Ettringite exhibit a rob or needle-like
particles with 1-2µm length. The elution of fluoride ions was
restrained by the coprecipitating of CaF2 with the high content
of Ca2+ provided from Ca(OH)2 and cement hydrates. The
carbonation of Ca2+ was prevented by the addition of Mg2+.
Moreover, the forming of ettringite with high ion-exchange
capacity and the solidification effect of Ca-bearing hydrates
contributed to the inhibition of the elution of fluoride. Water in
cement paste is more easily lost with drying conditions at 60°C.
The loss of water delayed hydration of cement paste, resulting
into a low solidification effect of cement paste. Additionally,
Ca(OH)2, added initially and generated in hydration process, is
easily to dissolve and react with CO2 in the air in the fluoride
elution experiment. Therefore, the fluoride inhibitory effect is
better with the drying at room temperature.
3.3 Application for practical wastes
The practical fluoride inhibition test for the addition of
Ca(OH)2, MgCl2 and Portland cement was conducted with
using of the paper sludge and the coal ash. The composition of
the two industrial wastes showed distinct differences (Table 5).
Paper sludge showed high content of SiO2 (32.44 wt.%), Al2O3
(7.28 wt.%), Fe2O3 (5.95wt.%), CaO (5.47 wt.%) and S (16.50
wt.%). As for coal ash, it consisted mainly of CaO (16.59
wt.%), MgO (7.50 wt.%), SiO2 (4.96 wt.%), Na2O (1.66wt.%),
S (20.50 wt.%) and Cl (4.66 wt.%). The initial fluoride
concentration eluted from paper sludge and coal ash without
CaF2 was 0.43 mg/L and 1.45 mg/L, respectively.
Table 5 Chemical compositions of paper sludge and coal
ash (wt.%).
ND: no detection
The paper sludge samples treated by CaF2 indicate paper
sludges mixed with CaF2 by the method described in Section
2.5. The fluoride elution concentration of this treated paper
sludge and coal ash samples without any additives were 3.51
mg/L (pH=8.3) and 1.2 mg/L (pH=12.0), respectively (Table
6). The fluoride elution concentrations from treated paper
sludge and coal ash samples were restrained to below
environmental standard 0.8 mg/L with an addition of only
Portland cement. The highest fluoride inhibitory effect was
found when treated paper sludge and coal ash mixed with 3.0 g
of cement, 0.5 g of Ca(OH)2 and 0.4 g of MgCl2. The fluoride
elution concentrations from paper sludge and coal ash samples
were 0.35 mg/L and below 0.1 mg/L.
Table 6 Fluoride concentration of treated paper sludge and
coal ash samples prepared with Portland cement,
Ca(OH)2 and MgCl2 with drying at 60°C. Paper sludge
and coal ash were mixed with CaF2 at weight ratio of
9:1. The addition amount of these fluoride wastes was
7.0 g.
Additives F conc. [mg/L]
paper sludge coal ash
none 3.51 1.2 cement 0.75 0.55 A 0.65 0.68 A+0.1 g of B 0.52 0.84 A+0.2 g of B 0.43 0.56 A+0.4 g of B 0.35 0.85
A+0.6 g of B 0.35 <0.10 A+0.8 g of B 0.37 – A+1.0 g of B 0.39 – A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2; – : no determination.
These results indicate that the additives, Portland cement,
Ca(OH)2 and MgCl2, could be used for inhibition of fluoride
elution from industrial waste in alkaline region to meet the
environmental standard in Japan. The industrial wastes, paper
sludge and coal ash, could be reused for civil resource, such as
road base or ground consolidation after treatment.
4. Conclusion
The fluoride concentration eluted from CaF2 was inhibited
successfully in both neutral and alkaline region. In neutral
region, the additions of Ca(OH)2, CaCl2 and NH4H2PO4 was
able to restrained the fluoride elution concentration to 0.22
mg/L by a large amount of Ca2+ dissolved from Ca(OH)2/CaCl2
and the products HAp and ClAp. In alkaline region, the
additions of Portland cement, Ca(OH)2 and MgCl2 was able to
restrained the fluoride elution concentration to 0.78mg/L with
drying at 60°C and 0.47 mg/L at room temperature. The elution
Chemical compositions (wt.%)
Paper sludge Coal ash
SiO2 32.44 4.96Al2O3 7.28 0.73 FeO 0.12 0.05
Fe2O3 5.95 0.53 CaO 5.47 16.59K2O 1.25 0.62 MgO 0.98 7.50 Na2O 0.95 1.66
Cu 0.10 0.11Zn 0.33 0.31S 16.50 20.50Cl ND 4.66
Other 28.58 66.89
( )105
146 J. ION EXCHANGE
of fluoride ions was restrained by the coprecipitating of CaF2
with the high content of Ca2+ provided from Ca(OH)2 and
cement hydrates. The carbonation of Ca2+ was prevented by the
addition of Mg2+. Moreover, the forming of ettringite with high
ion-exchange capacity and the solidification effect of Ca-
bearing hydrates contributed to the inhibition of the elution of
fluoride.
Practical tests for the industrial wastes, paper sludge and coal
ash with high concentration of fluoride were conducted. The
result indicates that the fluoride elution concentration of these
industrial wastes could be restrained to meet the environmental
standard in Japan in alkaline region.
References
1) C. Death, G. Coulson, U. Kierdorf, H. Kierdorf, W.K.
Morris and J. Hufschmid, Sci Total. Environ., 533, 528
(2015).
2) O.L. Adebayo, P.D. Shallie, B.A. Salau, E.O. Ajani and
G.A. Adenuga, J. Trace Elem. Med. Biol., 27, 370 (2013).
3) M. Li, J. Cao, J. Song, B. Zhou, C. Feng and J. Wang,
Chemosphere, 145, 365 (2016).
4) F. Liu, J. Ma, H. Zhang, P. Liu, Y.P. Liu, B. Xing and
Y.H. Dang, Physiol. Behav., 124, 1 (2014).
5) Z. Sun, W. Zhang, X. Xue, Y. Zhang, R. Niu, X. Li, B. Li,
X. Wang and J. Wang, Chemosphere, 144, 1012 (2016).
6) M. Bartos, F. Gumilar, C. Bras, C.E. Gallegos, L.
Giannuzzi, L.M. Cancela and A. Minetti, Physiol. Behav.,
147, 205 (2015).
7) S. Gao, J. Cui and Z. Wei, J. Fluorine Chem., 130, 1035
(2009).
8) M. Jiménez-Reyes and M. Solache-Ríos, J. Hazard.
Mater., 180, 297 (2010).
9) D. Zhang, H. Luo, L. Zheng, K. Wang, H. Li, Y. Wang
and H. Feng, J. Hazard. Mater., 241-242, 418 (2012).
10) Y. Tsunashima, A. Iizuka, J. Akimoto, T. Hongo and A.
Yamasaki, Chem. Eng. J., 200-202, 338 (2012).
11) S. Kagne, S. Jagtap, P. Dhawade, S.P. Kamble, S. Devotta
and S.S. Rayalu, J. Hazard. Mater.,154, 88 (2008).
12) W.H. Kang, E.I. Kim and J.Y. Park, Desalination, 202, 38
(2007).
13) S. Bibi, A. Farooqi, K. Hussain and N. Haider, J. Clean.
Prod., 87, 882 (2015).
14) JIS K0102, “Testing Methods for Industrial Wastewater”,
Japanese Industrial Standard, Japanese Standards
Association (2016).
15) S. Kim, H.S. Ryu and H.S. Hong, Metals Mater. Int., 10,
171 (2004).
16) M.C. Wang, W.J. Shih, I.M. Hung, H.T. Chen, M.H. Hon
and H.H. Huang, Ceram. Int., 41, 1223 (2015).
17) J.M. Astilleros, L. Fernández-Díaz and A. Putnis, Chem.
Geol., 271, 52 (2010).
18) M. Hong, J. Xu, H.H. Teng, Geochim. Cosmochim. Acta,
172, 55 (2016).
19) Y. Nie, C. Hu and C. Kong, J. Hazard. Mater., 233-234,
194 (2012).
20) C.S. Sundaram, N. Viswanathan and S. Meenakshi, J.
Hazard. Mater., 155, 206 (2008).
21) L.C. Nielsen, J.J. De Yoreo and D.J. DePaolo, Geochim.
Cosmochim. Acta, 115, 100 (2013).
22) Y.P. Lin and P.C. Singer, J. Cryst. Growth, 312, 136
(2009).
23) K. De Weerdt, A. Colombo, L. Coppolab, H. Justnes and
M.R. Geiker, Cem. Concr. Res., 68, 196 (2015).
24) M. Maes and N. De Belie, Constr. Build. Mater., 155, 630
(2017).
( )106