New treatment method for dilute hydrochloric acid using magnesium-aluminum oxide

Article abstract of DOI:10.1246/bcsj.75.595

Magnesium-aluminum oxide (Mg-Al oxide) was shown to be superior to CO₃^2- intercalated hydrotalcite (CO₃-HT) for removing dilute hydrochloric acid. Cl^- in a 0.5 M HCl solution could be quantitatively removed by adding 1.75-times the stoichiometric quantity of Mg_0.70Al_0.20O at 60 °C for 1 h. The solution after the reaction was weakly alkaline. This reaction could be adequately described as being first order for the HCl concentration, and the apparent activation energy was 49.3 kJ mol^-1. The amount of Cl^- removal per 1 g of Mg-Al oxide in the first use was 3.08 meq at a Mg/Al molar ratio of 2.0 and 2.89 meq at a ratio of 3.5.

Full text of DOI:10.1246/bcsj.75.595

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 595–599 (2002)
595
e Chemical
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New Treatment Method for Dilute Hydrochloric Acid Using Magnesium-
Aluminum Oxide
Removal of Hydrochloric Acid by Mg-Al Oxide
T. Kameda et al.
Tomohito Kameda,*,# Toshiaki Yoshioka, Miho Uchida, Yoshinori Miyano, and Akitsugu Okuwaki
02
5
9
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
Aoba 07, Aoba-ku, Sendai 980-8579
SJA8
09-2673
230
(Received July 13, 2001)
Magnesium-aluminum oxide (Mg-Al oxide) was shown to be superior to CO32− intercalated hydrotalcite (CO3-HT)
for removing dilute hydrochloric acid. Cl− in a 0.5 M HCl solution could be quantitatively removed by adding 1.75-
times the stoichiometric quantity of Mg0.70Al0.20O at 60 °C for 1 h. The solution after the reaction was weakly alkaline.
This reaction could be adequately described as being first order for the HCl concentration, and the apparent activation en-
ergy was 49.3 kJ mol−1. The amount of Cl− removal per 1 g of Mg-Al oxide in the first use was 3.08 meq at a Mg/Al
molar ratio of 2.0 and 2.89 meq at a ratio of 3.5.
01
01
.2
HT ([Mg2+1−xAl3+x(OH)2](An−)x/n•mH2O: 0.20 ꢀ x ꢀ 0.33)
is a layered double hydroxide with anion-exchange properties.1
CO3-HT transforms into Mg-Al oxide when heated to 450–800
°C, as expressed by2
2. We have thus reported briefly on this matter recently.6 In
this study, Mg-Al oxide was applied for treating dilute hydro-
chloric acid. The effects of the time, Mg-Al oxide quantity,
HCl concentration, and temperature on Cl− removal were re-
vealed in detail. The difference on Cl− removal between CO3-
HT and Mg-Al oxide was also examined.
Mg1−xAlx(OH)2(CO3)x/2
→
(1 + x/2)Mg2(1−x)/(2+x)Al2x/(2+x)O + x/2CO2 + H2O. (1)
Experimental
CO3-HTs were prepared by a common precipitation method.7
Mixed Mg(NO3)2•6H2O and Al(NO3)3•9H2O solutions with Mg/
Al molar ratios of 3.5 and 2.0 were added to 1.0 M Na2CO3 solu-
tions. When the pH of the mixture was lower than 10, a solution
of 2.0 M NaOH was added to maintain the pH at 10.0. The mix-
ture was stirred and aged for a certain period of time, and the re-
sulting product was washed, dried and calcined at 500 °C for 1 h.
The Mg-Al oxide can intercalate anions in solution, and then
the original HT structure can be reconstructed, as expressed
by2–4
(1 + x/2)Mg2(1−x)/(2+x)Al2x/(2+x)O + x/n An−
+ (1 + x/2)H2O → [Mg1−xAlx(OH)2]Ax/n + xOH−. (2)
The feature of this reaction is the release of OH− along with
the intercalation of anions, and it is known that the selectivity
of anion intercalation into the Mg-Al oxide increases with in-
creasing charge density of the anion.2 In the case of anions
whose charge density is smaller than that of OH−, OH− > F−
0.01–0.5
M HCl solutions (10 mL) and [Mg0.70Al0.20-
(OH)1.80](CO3)0.10, Mg0.70Al0.20O or Mg0.58Al0.28O of 0.5–2.0 times
the stoichiometric quantity, according to Eqs. 3–5, were placed in
a 50 ml screw tube and shaken at 25–80 °C for 0.2–3 h.
[Mg0.70Al0.20(OH)1.80](CO3)0.10 + 0.20HCl →
−
> Cl− > Br− > NO3 > I− , the Mg-Al oxide preferentially
[Mg0.70Al0.20(OH)1.80]Cl0.20 + 0.10CO2 + 0.10H2O
(3)
(4)
(5)
intercalates the released OH− rather than those anions in a neu-
tral solution. Therefore, the Mg-Al oxide can not be used as a
remover of those anions in a neutral solution. In addition, be-
cause the solution after the reaction is strongly alkaline, it is
difficult to apply Mg-Al oxide for treating a waste neutral sa-
line solution. Accordingly, we propose that the Mg-Al oxide
can be utilized for waste acid treatment as both a neutralizer
and a fixative of anions.
Mg0.70Al0.20O + 0.20HCl + 0.80H2O →
[Mg0.70Al0.20(OH)1.80]Cl0.20
Mg0.58Al0.28O + 0.28HCl + 0.72H2O →
[Mg0.58Al0.28(OH)1.72]Cl0.28
The pH of the solution was measured after the reaction, and the
Cl− concentration was determined by ion chromatography. The
degrees of Mg2+ and Al3+ leaching were determined by ICP-AES.
The precipitates were identified by X-ray diffraction (XRD).
There have been many studies on the reaction for such neu-
tral saline solutions as NaCl, Na2SO4 and NaNO3.3–5 In con-
trast, no study had been made for an acidic solution below pH
Results and Discussion
1.
Difference between CO3-HT and Mg-Al Oxide.
# Present adress; Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome,
Aoba-ku, Sendai 980-8577
Figure 1 shows the effect of time on Cl− removal using
Mg0.70Al0.20O or [Mg0.70Al0.20(OH)1.80](CO3)0.10 of 1.75-times

596 Bull. Chem. Soc. Jpn., 75, No. 3 (2002)
Removal of Hydrochloric Acid by Mg-Al Oxide
Fig. 1. Effect of time on Cl− removal using (a) Mg0.70Al0.20
O
or (b) [Mg0.70Al0.20(OH)1.80](CO3)0.10 of 1.75-times the sto-
ichiometric quantity. HCl concd: 0.5 M, Temp.:60 °C.
Fig. 3. Effect of the Mg0.70Al0.20O quantity on Cl− removal.
HCl concd: 0.5 M, Temp: 60 °C, Time: 3 h.
was a mixture of HT and a slight amount of Mg(OH)2. The re-
action until about 0.2 h was interpreted by both the intercala-
tion of Cl− and the dissolution of Mg0.70Al0.20O, as expressed
in Eq. 4 and
Mg0.70Al0.20O + H2O →
0.70Mg2+ + 0.20Al3+ + 2OH−.
(6)
It is conjectured that Cl− was removed as follows at 0.2–1 h:
Mg0.70Al0.20O + 0.10Mg2+ + 0.20Cl− + H2O →
[Mg0.70Al0.20(OH)1.80]Cl0.20 + 0.10Mg(OH)2.
(7)
Since the dissolution of Al was not recognized at 0.2 h, it is
supposed that Al was promptly hydrolyzed.
In sum, because an increase in the OH− concentration was
prevented by the precipitation of Mg(OH)2, as expressed in
Eq. 7, it may be concluded that Cl− was easily intercalated into
Mg0.70Al0.20O for a small amount of OH−.
Fig. 2. Effect of time on the pH or Mg2+ dissolved using
Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
HCl concd: 0.5 M, Temp: 60 °C.
2. Dilute Hydrochloric Acid Removal Using Mg-Al Ox-
ide. Figure 3 shows the effect of the Mg0.70Al0.20O quantity
on Cl− removal. Cl− removal increased with increasing
Mg0.70Al0.20O quantity, and Cl− was removed above 99% add-
ing Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
However, Cl− was not quantitatively removed at the stoichio-
metric quantity. This was due to the formation of OH-HT, as
expressed by
the stoichiometric quantity. The CO3-HT removed hydrochlo-
ric acid slightly. When a 0.1 M HCl solution (20 mL) and
CO3-HT of the stoichiometric quantity were shaken at 37 °C,
Cl− was removed 55% by an exchange reaction of HCl and
CO32−, as shown in Eq. 3.8 Because the HCl concentration
was high in this study, it is conjectured that the CO3-HT was
mainly dissolved in addition to the slight exchange reaction.
From the fact that the solution after the reaction was neutral, it
may be deduced that Cl− could hardly exchange with the
Mg0.70Al0.20O + H2O →
2−
CO3
.
[Mg0.70Al0.20(OH)1.80](OH)0.20
(8)
In contrast, Mg0.70Al0.20O exhibited a much higher degree of
Cl− removal, and Cl− was quantitatively removed above 1 h.
Figure 2 shows the effect of time on the pH or Mg2+ dissolved
using Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
The pH was rapidly increased and kept constant at about 10.
Although Mg from Mg0.70Al0.20O was dissolved at first, the dis-
solution was not recognized after 1 h. The precipitate after 1 h
after the release of OH−, as expressed by
Mg0.70Al0.20O + 0.20Cl− + H2O →
[Mg0.70Al0.20(OH)1.80]Cl0.20 + 0.20OH−.
(9)
Additionally, this is also attributable to the dissolution of

T. Kameda et al.
Bull. Chem. Soc. Jpn., 75, No. 3 (2002) 597
Fig. 6. Effect of the temperature on Cl− removal using
Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
HCl concnd: 0.5 M, Time: 1 h.
Fig. 4. Effect of the Mg0.70Al0.20O quantity on the pH or
Mg2+ dissolved. HCl concd: 0.5 M, Temp: 60 °C, Time: 3
h.
mainly intercalated due to an increase in the OH− concentra-
tion due to the formation of NaOH, as expressed by
Mg0.70Al0.20O + 0.20NaCl + H2O →
[Mg0.70Al0.20(OH)1.80]Cl0.20 + 0.20NaOH.
(10)
It must be emphasized that the capacity of Mg-Al oxide for
Cl− removal could be attained maximally in a strong hydro-
chloric acid below 0.1 M.
Figure 6 shows the effect of the temperature on Cl− removal
using Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
Cl− removal increased with increasing temperature, and Cl−
was quantitatively removed above 60 °C. The thermostable
temperature of the anion-exchange resin is below 60 °C. It
should be noted that a hydrochloric acid treatment at over 60
°C is possible using Mg-Al oxide instead of the resin.
From the fact that the removal rate of Cl− was fast with in-
creasing temperature, it is implied that this reaction proceeded
under chemical reaction control. The Cl− removal rate at 25–
60 °C was arranged according to the following equation using
the results shown in Fig. 1 and 6:
Fig. 5. Effect of the HCl concentration on Cl− removal using
Mg0.70Al0.20O of 1.75-times the stoichiometric quantity.
Temp: 60 °C, Time: 1 h.
−ln(1−x) = kt,
(11)
Mg0.70Al0.20O. About 5% of Mg was dissolved at the stoichio-
metric quantity, as shown in Fig. 4. Considering the formation
of OH-HT and the dissolution of Mg, Mg-Al oxide is required
above the stoichiometric quantity for a quantitative removal of
Cl−.
where x is the degree of Cl− removal, and k (min−1) is the ap-
parent rate constant. Figure 7 shows a first-order plot of Cl−
removal using Mg0.70Al0.20O of 1.75-times the stoichiometric
quantity. This reaction is described as being first order for the
HCl concentration because of the good linearity at each tem-
perature. An Arrhenius plot of k, determined from the slope of
the straight lines in Fig. 7, is shown in Fig. 8. The apparent ac-
tivation energy was 49.3 kJ mol−1. This value confirms that
Cl− removal by Mg-Al oxide proceeded under chemical reac-
tion control.
Figure 5 shows the effect of the HCl concentration on Cl−
removal using Mg0.70Al0.20O of 1.75-times the stoichiometric
quantity. Cl− removal increased with increasing HCl concen-
tration. With increasing HCl concentration, the concentration
of Cl− should be higher than that of OH− at the pH at which
the once dissolved Mg2+ catch the released OH− to precipitate
as Mg(OH)2. Consequently, it can be stated that Cl− was easi-
ly intercalated into Mg0.70Al0.20O. The intercalation capacity
of Mg-Al oxide for Cl− has been investigated in a NaCl solu-
tion,3,5 and it is known that Cl− removal is low, since OH− is
3. An Application of Mg-Al Oxide to Dilute Waste Hy-
drochloric Acid Treatment. Table 1 gives the amounts of
Cl− removed per 1 g of Mg-Al oxide in the first use. The Mg-
Al oxide has a large capacity for removing Cl− relatively, be-

598 Bull. Chem. Soc. Jpn., 75, No. 3 (2002)
Removal of Hydrochloric Acid by Mg-Al Oxide
incinerators is generally treated by blowing slaked lime pow-
der, and the produced calcium chloride etc. are discharged into
landfill.9 However, the calcium chloride generates highly con-
centrated saline leachate, and the leachate becomes a cause of
damage from salt to the surrounding water environment.10,11
We thus propose that Mg-Al oxide can be applied for treating
hydrochloric acid produced by washing waste hydrogen chlo-
ride with venturis scrubbers. After this treatment and a filtra-
tion process, the produced Cl-HT can be used as an ion ex-
changer for removing phosphorus to prevent eutrophication of
surface water by an algae bloom.12,13 It is known that the use
of Cl-HT in various anion intercalated HTs is suitable for treat-
ing phosphate ion.12 In the case of calcining Cl-HT at 450–
800 °C, Mg-Al oxide can be reproduced with hydrochloric ac-
id.14,15 This reproduction method doesn’t produce any new
drainage, contrary to the reproduction of an ion-exchange res-
in. The reproduced Mg-Al oxide can be used for another treat-
ment.
Fig. 7. First-order plot of Cl− removal using Mg0.70Al0.20O of
1.75-times the stoichiometric quantity. HCl concd: 0.5 M.
Conclusion
From the present results concerning the removal of dilute
hydrochloric acid using Mg-Al oxide, the following conclu-
sions may be drawn:
1. Mg-Al oxide was superior to CO3-HT as a remover of di-
lute hydrochloric acid.
2. 0.5 M hydrochloric acid could be quantitatively removed
by adding 1.75-2 times the stoichiometric quantity of
Mg0.70Al0.20O at 60–80 °C over a period of 1 h.
3. The reaction of Cl− removal by Mg-Al oxide could be ade-
quately described by first order for the HCl concentration. The
apparent activation energy was 49.3 kJ mol−1.
4. The amount of Cl− removal per 1 g of Mg-Al oxide in the
first use was 3.08 meq at a Mg/Al molar ratio of 2.0, and 2.89
meq at a ratio of 3.5.
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1
2
3
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Table 1. Amounts of Cl− Removed per 1 g of Mg-Al Oxide
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larger capacity for removing Cl− than that at 3.5. However, it
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