

J. Hot Spring Sci. /2 1-ῌ2* ,**2ῌ

῎Adsorption of Zinc in Hot Spring Waters on Shirasu
Hiroo ISAGAI+
(Received Feb. +., ,**2, Accepted Aug. 2, ,**2)

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5 ,**- 6 Zn 7879: ;5<5= > ?7@. Zn, A
B + mg dm- CD)*+# EF @7 Zn 7GHI87JKL+*9MNOPQR= Zn 7GHI87JK. pH S# T< 7 pH  . U"#% Zn 7JKV,)= pH W+.GHI%XYZ#% pH [\Z#$%]^()=
5=R* @7 Zn, .GHIJK() @_%`a !"b+%3",)=
cdedf : JK gh  GHIAbstract
The drainage water of hot springs has recently been pointed out to a#ect the environment since some heavy metals are present in hot spring water. In Japan, the Ministryof the Environment included Zn among the items indicating the environmental quality
standard for aquatic organisms in ,**-. Zn, was present in hot spring waters that havea low pH and belong to the Kirishima volcanic group. The maximum concentration ofZn, in those hot spring waters was + mg dm-. In this study, fundamental research on
the adsorption of Zn, in hot spring water on Shirasu was carried out.
Adsorption of Zn, on Shirasu was not observed in hot spring water with a pH of .. However, the amount of Zn, adsorbed increased with increasing pH. Therefore, it wassuggested that Zn, in the drainage water of acidic hot springs would be adsorbed onShirasu and hardly inﬂuence on the environment if the pH of the acidic hot spring waterincreased by neutralizing the water with Shirasu.
Key words : adsorption, Zn, hot spring, Shirasu++23*ῌ**0/    +ῌ,+ῌ-/Faculty of Science, Kagoshima University, +ῌ,+ῌ-/, Korimoto, Kagoshima, 23*ῌ**0/, Japan73





+.

Introduction

The drainage water of hot springs has been pointed out to a#ect the environment sincesome heavy metals are present in hot spring water (Sakai, ,**-). The discharge region of thehot springs of the Kagoshima Prefecture, Japan, is covered with a pyroclastic ﬂow deposit,
locally known as Shirasu. Shirasu is volcanic ash that is sandy in texture ; it is generally
attributed to pumice fall. These secondary sediments produce the glowing clouds that areobserved when a caldera is formed (Iwamatsu et al., +323 ; Yokoyama, ,**-). Most of the volcanic deposit from an enormous eruption that occurred approximately ,/,*** years ago in the“Aira Caldera,” located in the Kagoshima Bay, comprised of pyroclastic ﬂow deposits (Yokoyama, ,**-).
The major chemical component of Shirasu is SiO,, which is approximately 1*ῌ in contentof the Shirasu (Isagai, ,**2). The other components are Al,O-, Na,O, K,O, Fe,O-, etc. Shirasu
is used for preparing zeolite because of its considerable porosity (VSI Kenkyuukai, +33/).
In Japan, Zn has recently become a subject of interest ; for example, the Ministry of theEnvironment included Zn among the items indicating the environmental quality standard foraquatic organisms in ,**-.
In previous studies (Isagai and Sakamoto, ,**/ ; Isagai, ,**2), the fundamental adsorption
experiments for determining the e#ect of the grain size of Shirasu were conducted by using
Zn, or Cu, solutions and samples that were grouped on the basis of grain size : groups withShirasu grains sized 1. mm, 1.ῌ+*/ mm, +*/ῌ+/* mm, +/*ῌ,/* mm, and ,/*ῌ/** mm. The amountof Zn, adsorbed on Shirasu increased with a decrease in the grain size of Shirasu. It wasrecognized that the amount of Zn, adsorbed was the largest for Shirasu grains sized 1. mm.
An adsorption experiment to determine the e#ect of pH was also conducted by using
various Zn, concentrations (*./**, +.**, +./*, ,.**, ,./*, and -./* mg dm-) with pH values of +.**,-.**, and /.** and a Shirasu sample with a grain size of 1. mm (Isagai and Sakamoto, ,**/ ;Isagai, ,**2). Adsorption of Zn, was not observed at a pH of +.** but was observed at pH values of -.** and /.** ; thus, the amount of Zn, adsorbed increased with an increase in pH.
Many studies reported that the higher the pH, the adsorption ability of the soil increases formetallic ions (Palherios et al., +323 ; Jain et al., ,**. ; Agrawal et al., ,**.). It was suggested
that the increase in the amount of Zn, adsorbed was due to an increase in the negative
surface charge of Shirasu.
In this study, the adsorption behavior of Zn (II) present in the drainage water of hot
springs on Shirasu was investigated via an adsorption experiment that was performed using
the batch method. The adsorption behavior of Zn, on Shirasu was examined by focusing on
the pH and the major chemical components of hot spring water.

,.
,. +Experiment

Samples,. +. +Hot spring water

Hot spring water samples were collected from +- sites from the Amori River area in74 /2  ,**2 Fig. + Sampling sites of hot spring waters and Shirasu in Kagoshima andMiyazaki Prefectures, Japan.

Kagoshima and Miyazaki Prefectures, Japan (Fig. +). These sites belong to the Kirishima
volcanic group.
,. + . , .

Shirasu (pyroclastic ﬂow deposits)

Shirasu was collected from an outcrop in Makizono town in Kagoshima Prefecture. Thesampling site was near the discharge region of hot springs (Fig. +) ; sampling was performedat the same site as that in our previous study (Isagai and Sakamoto, ,**/). After excluding thesurface part of the outcrop, which was approximately +* cm thick, samples were collected ina polyethylene bag, brought back to the laboratory, and dried at ./ for - days by using anautomatic oven (EYELA NDO-.** ; Tokyo Rikakikai, Tokyo, Japan).
,. ,Adsorption experiment
In order to analyze the adsorption of Zn, contained in hot spring water on Shirasu, abatch adsorption experiment was conducted by using Shirasu samples with grains sized1.
mm and hot spring water. For the analysis, *.+ g of Shirasu was added to ,* ml of hot springwater in a +** ml Erlenmeyer ﬂask with a ground-glass stopper and stirred using a multishaker(EYELA MMS--*+* ; Tokyo Rikakikai, Tokyo, Japan) in a constant-temperature chamber(EYELA FMC-+*** ; Tokyo Rikakikai, Tokyo, Japan) for + h. All operations were conducted
at ,/. After stirring, the contents of the ﬂask were ﬁltered by using a *../-mm cellulose
acetate syringe ﬁlter (Advantec), and the equilibrium concentration of Zn, in the ﬁltrate wasdetermined.
Ultrapure water prepared by water puriﬁcation systems (Elix / UV and Mill-QSP ; Nihon
Millipore, Tokyo, Japan) was used throughout the study.

75,. -Analytical method
Na, K, Zn,, and Cu, contents were determined by using an atomic absorption spec-trometer (AAS, Shimadzu AA-0,** ; Kyoto, Japan ; C,H,/airframe). Ca, and Mg, were quantiﬁed by performing chelate titration with an ethylenediaminetetraacetic acid (EDTA) solution (*.*+ M). SO., and Cl contents were determined by turbidimetry (The Japan Society forAnalytical Chemistry, Hokkaido Branch, +33.) using a Hach DR/.*** spectrophotometer.
BCG-MR alkalinity (pH ..2) was determined by titrating the hot spring water samples with*.*, M H,SO. (The Japan Society for Analytical Chemistry, Hokkaido Branch, +33.).

-.
-. +Result and Discussion

Grain size composition of Shirasu
The collected Shirasu was divided into six groups on the basis of grain size by using asieve. The grain size compositions of Shirasu were as follows :1. mm, ,/.*ῌ ; 1.ῌ+*/ mm,2.,-ῌ ; +*/ῌ+/* mm, +-..ῌ ; +/*ῌ,/* mm, +..+ῌ ; ,/*ῌ/** mm, +-.1ῌ ; and /** mm, ,/./ῌ.
-. ,.

Chemical composition of hot spring water-. ,. +

Major chemicals

The chemical composition of the hot spring water sampled in the study area was closelyrelated to the geology of the sampling site. It was reported that the hot springs around thesolfatara fumaroles in the Kirishima volcanic zone were rich in SO., and were strongly
acidic. Further, the hot springs in the peripheral area of the piedmont of the Kirishimavolcanic zone were rich in HCO-, and their temperature was also lower, i.e., 0* (Tuyuki,+33,).
The chemical compositions of hot springs are shown in Table +. Hot springs were classiﬁed as Group A (No. +ῌ2) and Group B (No. 3ῌ+-). The hot springs of Group A discharged ata height of about 2**ῌ+*** m a.s.l. ; the water was strongly acidic (pH : +.22ῌ-..0) and had ahigh concentration of SO.,. In general, acidic hot springs have extremely high SO., concentrations. It has been reported that SO., in hot spring is produced by self-oxidation of H,SO(Iwasaki and Ozawa., +30*). Thus, it can be concluded that the SO., in acidic hot springs
originate from the SO, in volcanic gas. The hot springs of Group B discharged at a low heightof +**ῌ,** m a.s.l. The pH values of Group B samples were higher than those of Group A
samples and ranged from weakly acidic to weakly alkaline (pH : /.32ῌ2.++). The SO., concentration in this water was low, while the CO-, and HCO- concentrations calculated from thealkalinity were high. Hence, it was suggested that the major chemical components of thiswater were HCO-, NaHCO-, and Ca (HCO-),.
-. ,. ,Zn,ῌ in hot spring water

The Zn, concentration in hot spring water with a low pH was higher than that in waterwith a high pH. It has been reported that the average Zn, concentration in the hot springwater in Japan is *.3 mg dm- (Iwasaki, +31*). The Zn, concentrations of No., and No.2 samples were higher than those of other samples. The pH values of these samples were +.22 and,.*., respectively. The relationship between the pH value and the Zn, concentration is76 /2  ,**2

Table +
Samples
Group A
No. +No. ,No. No. .
No. /
No. 0
No. 1
No. 2Group B
No. 3
No. +*
No. ++
No. +,
No. +-

Samples
Group ANo. +No. ,No. No. .
No. /
No. 0
No. 1
No. 2
Group B
No. 3
No. +*
No. ++
No. +,
No. +- 

Physicochemical properties and chemical components in hot spring waters

Air
Temperature
()

Water
Temperature()

pHElectricconductivity(mS m+)Alkalinity(pH : ..2)
(meq dm-)Na(mg dm-),140,.4*-*4+,/4.
,*4*,*4*,*4*,+4/--40-.40/40.4,/24*0,4*0,4*.-4*-4.0+422,401,402,4,+,4-+,4,.
,4*.

+,/
+*01
1140
3*43,4,3*4/
3/4,+/3





/243.,40,/42,/4-/4,.*4*.+4.
+/4*--4*-040,,4*,,4*-042

0241
134*
/.4*.34*.,4204+,
24++
/432
04*.
04*,

1.4.
+33+20+12+.2

14**,422+04/+04.
+/4*K(mg dm-)Ca(mg dm-)Mg(mg dm-)Cl(mg dm-)SO.,
(mg dm-),243,/42--4*,14*,041-/4*
-/4*
2431

++++-+++4,+34+,*4,+/4+243,14*,14*
,/42
24,*
242*
2432
342*
++41
+,4,

++1
-4.*
,04*
,*41
-,4*
-+4.
-+42+,4,.*+++10,-/
,//
.,.+.
./1
.2*+243-+40-241-042,,42/,42041*+-3+-2+-*+,4,,4**
034*
1243
0-4*

-+40/.42++.
++/
204,++*+20+.
/*
0*+-2+00+/++..
+..

Cu(mg dm-)Zn(mg dm-)ND*4**,*4*3+,*4*,-3*4*+,+*4*+.3*4*,/0*4*-,,*4+++

*4,-1
ND
ND
*4**1
*4**,0*4**2,*4**3+
ND
ND*4**./*4**/.
ND

ND*4**0.**4**,+**4*++,ND

ND : Not detectedshown in Fig. ,. Figure , shows that Zn, concentration is high at a low pH. The relationshipbetween SO., and Zn, is given in Fig. -. Figure - shows a positive correlation between theSO., and the Zn, concentrations. These results showed that the Zn, concentration increased with an increase in the SO., concentration at a low pH. Hence, it was suggested thatthe Zn, concentrations increased by the reaction of a dissolved Zn compound, for example,
ZnS, with the sulfuric acid in the hot spring water.

77





Fig. , Relationship between pH and Zn, concentrations.

Fig. - Relationship between the SO., and the Zn, concentrations.

-. -E#ect of major ions on the adsorption of Zn,ῌ on Shirasu
The adsorption experiment was conducted by using Shirasu (except for the Shirasu sam-ple with grains sized /** mm) and Zn, solutions (+.** mg dm-) prepared at almost equalionic strength with hot spring water. The molality of Zn, solutions of the required ionicstrength (*.*,M) was achieved using NaCl, Na,SO., KCl, K,SO., CaCl,, and MgCl,. Table ,shows that Zn, adsorption was not observed for any sample ; the exceptions were the Zn,solutions prepared using NaCl and Na,SO.. This ﬁnding suggested that Na did not strongly
compete with Zn, for adsorption on Shirasu. The Ca, and Mg, concentrations were lower
than the Na concentration in the hot spring samples. Therefore, it was expected that theadsorption of Zn, could be observed without any inhibition from major ions in this sample.
-. .

Adsorption of Zn,ῌ present in hot spring waters on Shirasu
The adsorption experiment was conducted by using Shirasu with grains sized

781. mm /2  ,**2Table ,

E#ect of major ions on the Zn, adsorption on Shirasu

Solutes

Decreasing concentration of adsorbed Zn, (ῌ)

NaClNa, SO.
KClK, SO.
CaCl,MgCl,--43,.4+****

Fig. . E#ect of pH on the adsorption of Zn, present in the hot spring water.

and hot spring water sample (No. 2). No adsorption of Zn, on Shirasu was observed in thisexperiment. This was considered to be due to the low pH of the hot spring water. Therefore,
after the pH of the hot spring water was adjusted by adding ,.*ῌ+*- M NaOH solution, theadsorption experiment was repeated. The relationship between the pH of the hot springwater and the amount of Zn, adsorbed is shown in Fig. ., which shows that Zn, was notadsorbed when the pH of the hot spring water was lower than .. The amount of Zn, adsorbed increased with increasing pH, and the adsorption was maximum at the pH of 0.1-. At
the pH of 0.3*, Zn, concentration in the hot spring water was below the detection limit ofAAS. This suggested that the Zn, present in the hot spring water formed Zn (OH),. It wasreported that H was released from the oxide surface of an adsorbent for high values of thepH, which caused a change in the electric charge of the oxide surface (Salomons and Förstner,+32.). This suggested that the Zn, present in the hot spring water was adsorbed on Shirasu
when the drained hot spring water was neutralized by an alkali oxide.

..

Conclusion

Hot spring water samples collected from Amori River area in Kagoshima and Miyazaki
Prefectures were classiﬁed into two groups on the basis of chemical components. Samples ofthe ﬁrst group had a low pH ; these contained a large amount of SO.,. Samples of the other79῕ΐῑῒ῎῔῏ῐgroup had near neutral pH ; these contained a large amount of HCO-῍. Zn,ῌ was present inalmost all the samples. However, the Zn,ῌ concentration in the hot spring water with a low
pH was higher than that in the hot spring water with a high pH. The amount of Zn,ῌ adsorbed on Shirasu increased with an increase in the pH of the hot spring water, while no Zn,ῌadsorption was observed at a low pH. This result was consistent with that obtained in amodel adsorption experiment in which a standard Zn,ῌ solution was used. Although it isconsidered that the Zn,ῌ present in the drainage water of hot springs is scarcely adsorbed ata low pH, if the pH of the hot spring water is neutralized by the addition of Shirasu, the Zn,ῌpresent in the drainage water of the hot springs may be adsorbed on Shirasu.
(A part of this study was presented at the /2th Annual Meeting of the BalneologicalSociety of Japan on September 2, ,**/.)

Acknowledgements
The author would like to thank to Dr. H. Sakamoto for helpful discussions and suggestions.
References
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80