Solvent Extraction Research and Development, Japan, Vol. 21, No 2, 163 – 171 (2014)

Mutual Separation of Gallium, Indium and Zinc Using Silica Gel

Modified by a Surfactant Micelle Containing D2EHPA

Kazuo KONDO,* Ai MATSUOKA and Michiaki MATSUMOTO

Department of Chemical Engineering and Materials Science,

Doshisha University, Kyotanabe, Kyoto 610-0321, Japan

(Received December 25, 2013; Accepted January 22, 2014)

Solvent extraction is widely used as a universal method to extract and separate metals in industry. However

solvent extraction has some disadvantages, for example, use of a large amount of organic solvent which

could be an environmentally hazard. Therefore a solid-phase extraction method is proposed in this research.

First of all, a silica gel modified with a non ionic surfactant (TritonX-100) micelle including

di(2-ethylhexyl) phosphoric acid (D2EHPA) as a metal adsorbent has been prepared. The stability of the

modified silica gel was examined. Next, the adsorption behavior of gallium, indium and zinc onto the

modified silica gel were investigated and compared with the adsorption behavior when using silica gel

alone. The amount of D2EHPA impregnated in the modified silica gel was 0.138 mmol/g-silica gel. The

modified silica gel can be used as a stable adsorbent below pH 2.0 without leaking D2EHPA. The higher

the pH, the higher was the extent of metal adsorbed. Gallium was adsorbed on unmodified silica gel

preferentially at pH 3.0 from a mixed metal system. In contrast, indium was adsorbed onto the modified

silica gel preferentially at pH 1.1 in the same system. The modified silica gel could separate the metal ions

at lower pH much better than the unmodified silica gel.

1. Introduction

In recent years, the demand for rare metals has increased, because of their use as constructional,

electronic, magnetic and functional materials. In particular, gallium and indium are nowadays used for

semiconductor materials. Gallium and indium are very widely dispersed on the earth’s surface, but little of

these elements occur naturally in Japan. They are often found in industrial wastes because the amount of

consumption of these metals is still increasing in Japan. From this point of view, it is necessary to recover

and separate these metals from waste materials. Today solvent extraction is widely used as a universal

method in industry to extract and separate metals. However solvent extraction has some disadvantages such

as heavy use of organic solvents which could be an environmentally hazard.

In order to overcome these disadvantages, several methods with respect to solid-phase extraction

have been developed. Silica gel modified chemically [1-6] and nano-size alumina particles modified

chemically [7, 8] were used for extraction and separation of transition and heavy metals. For gallium and

indium extraction, Nishihama et al. [9] studied the extraction and separation of the metals using

organophosphorus extractants to determine the extraction mechanism. They also presented new ideas to

separate and recover gallium and indium [10]. We have studied the equilibrium and kinetics of gallium

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extraction [11], the separation of gallium and indium by a supported liquid membrane [12], the separation

and concentration of indium by an emulsion liquid membrane [13] and the extraction mechanism of

gallium and indium by microcapsules containing organophosphorus extractants [14]. Other than the studies

mentioned above, the extraction of gallium and indium has been studied using several methods [15-20].

In the light of the research described above, we report here a solid-phase extraction method for

gallium and indium. We used silica gel as a solid support and di(2-ethylhexyl) phosphoric acid (abbreviated

as D2EHPA) as the metal adsorbent. D2EHPA cannot be combined directly to silica gel due to its strong

hydrophobicity. So we proposed the use of a surfactant micelle. Through this method, D2EHPA can stay in

the micelle because the interiors of the micelles are hydrophobic.

First of all, silica gel modified with nonionic surfactant (TritonX-100) micelle including D2EHPA

was prepared and the stability of the modified silica gel was examined. Next, the adsorption behavior of

gallium, indium, and zinc onto the modified silica gel was investigated. The results were compared with

that of the adsorption behavior when using silica gel alone as an adsorbent to discuss the adsorption

mechanism.

2. Experimental

2.1 Reagents

Silica gel 60, with a particle size of 70-230 mesh, and the nonionic surfactant, Triton® X-100

(p-(1,1,3,3-tetramethylbutyl)phenoxypoly(oxyethylene)glycol, abbreviated as TX-100) were obtained from

Nacalai Tesque Inc., Japan. Di(2-ethylhexyl) phosphoric acid (D2EHPA) was supplied by Tokyo Chemical

Industry Co., Ltd., Japan and used without further purification. Aqueous solutions were prepared using 0.1

mol/dm3 HCl-CH3COONa buffer solution containing the desired amount of metal ions. The pH of the

aqueous solutions was adjusted by means of a pH meter (Horiba F-23). As the metal sources, special grade

metal chlorides (GaCl3, InCl3 and ZnCl2) were used.

2.2 Adsorption experiments of TX-100 to silica gel

Adsorption experiments for TX-100 on silica gel were carried out batch wise as follows. Silica gel,

(0.5 g), and 10cm3 of an aqueous solution containing TX-100 were contacted in a vial for 24 h to attain

equilibrium. The silica gel was sieved beforehand to prepare a particle size fraction of 125-210 µm. The

vials were shaken in a thermostat bath at 303K. The TX-100 concentrations in the aqueous solutions before

and after equilibrium were measured using a UV-VIS spectrophotometer (UV-2500, Shimadzu Co. Ltd.,

Japan) at 280 nm. The TX-100 concentration adsorbed onto the silica gel was calculated from the mass

balance before and after equilibrium.

2.3 Preparation of modified silica gel

The immobilization of D2EHPA into the micelles of TX-100 was carried out as follows. An ethanol

solution containing D2EHPA and a 0.08 mol/dm3 aqueous solution containing TX-100 prepared using the

buffer solution (pH 7.0) were mixed and the final concentration of D2EHPA was so controlled as to be 30

mmol/dm3 in the D2EHPA-TX-100 solution. This D2EHPA-TX-100 solution (0.02dm

3) and silica gel were

mixed in a vial, and then stirred in a thermostat bath at 303K for 24 h. The modified silica gel so obtained

was collected by filtration and then dried at room temperature. This is named DTS.

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2.4 Stability of DTS

DTS was added to the buffer solution and shaken for a fixed period of time while varying the pH

value of the aqueous solution. The extent of D2EHPA desorbed from the silica gel was determined by

measuring the phosphorus concentration using inductively coupled plasma spectrometry (ICP-AES,

ICPS-8000, Shimadzu Co. Ltd., Japan).

2.5 Metal extraction equilibria

Metal extraction equilibrium experiments were carried out batch wise as follows. DTS, (0.5 g), and

10cm3 of the aqueous solution containing each metal ion were contacted in a vial to attain equilibrium. The

vials were shaken in a thermostat bath at 303K. The pH and the metal concentration in the aqueous solution

before and after equilibrium were measured using the pH meter and ICP-AES, respectively. The metal

concentration extracted into DTS was calculated from the mass balance before and after equilibrium. The

initial pH of the aqueous solution ranged from 1.1 to 4.0. In order to compare the adsorption behavior of

the metals, similar experiments using silica gel alone were also performed.

3. Results and Discussion

3.1 Adsorption of TX-100 onto silica gel

Figure 1 shows the relationship between the amount of TX-100 adsorbed on silica gel and the

concentration of TX-100 in the aqueous solution at equilibrium. The higher the concentration of TX-100,

the higher the amount of TX-100 was adsorbed, when the concentration of TX-100 is low. However, the

amount of TX-100 adsorbed approaches a constant value when the concentration of TX-100 is higher than

0.08 mol/dm3. This means that the amount of TX-100 adsorbed onto silica gel reached saturation. The

maximum adsorption of TX-100 was consistently achieved by using solutions wherein the concentration of

TX-100 was higher than 0.08 mol/dm3. TX-100 and silica gel are considered to be combined through

hydrogen bonding. Since TX-100 has hydrophilic groups containing hydroxyl groups (-OH) and silica gel

has a silanol group (-SiOH), the hydroxyl groups can form hydrogen bonds with the silanol groups.

Hydrogen bonding is stronger than Van der Waals' forces, and so due to this chemical bonding between

TX-100 and silica gel, DTS is considered to be stable.

3.2 Preparation of modified silica gel

DTS obtained after filtration and drying at room temperature consisted of white particles. SEM

photographs of the surface of the unmodified silica gel and DTS were taken and are shown in Figure 2.

From Figure 2, no distinct difference was observed in the surface of the unmodified silica gel (a) and that

of DTS (b). The amount of D2EHPA modified into the silica gel was 0.138 mmol/g-silica gel.

3.3 Stability of DTS

Figure 3 shows the effect of pH on the extent of D2EHPA leaked from DTS. As seen in Figure 3, the

amount of D2EHPA leaked was dependent on the pH. The higher the pH, the higher was the amount of

D2EHPA leaked. The D2EHPA pKa value is 3.82. When the hydrogen ion concentration is low, the

hydroxyl group in D2EHPA dissociates in aqueous solution. Therefore, D2EHPA leaks out as a polar

molecule from the inside of the surfactant micelle to the aqueous solution. Below pH 2.0, the amount of

leaked D2EHPA is lower than 1%. Therefore, the modified silica gel is a stable adsorbent below pH 2.0 as,

under these conditions, no D2EHPA is leaked.

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3.4 Metal extraction equilibria

Figure 4 shows the effect of pH on the amount of metal adsorbed on unmodified silica gel in single

metal systems. Under the experimental conditions wherein each metal concentration is 1 mmol/dm3 and the

pH is adjusted using a 0.1 mol/dm3 HCl-CH3COONa buffer solution as described in Section 2.1. The extent

of metal adsorbed was calculated by the following equation:

Figure 1. Amount of TX-100 adsorbed on

silica gel.

Am

ou

nt

of

TX

-100 a

dso

rbed

[m

mol/

g-s

ilic

a g

el]

Concentration of TX-100 [mol/dm3]

(a) (b)

Figure 2. SEM photographs of the surface of

unmodified silica gel (a) and that of DTS (b).

Figure 3. Effect of pH on the amount of

D2EHPA leaked.

Figure 4. Amount of metal adsorbed on

silica gel in single metal systems.

C0,Ga=1mmol/dm3,C0,In=1mmol/dm

3,

C0,Zn=1mmol/dm3.

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0

0 adsorbed metal ofExtent C

CC － × 100 (1)

where C0 [mmol/dm3] is the initial concentration of metal and C is the concentration of metal at equilibrium.

Figure 4 shows that the amount of metal adsorbed on silica gel was dependent on the pH for the adsorption

of Ga and In. The higher the pH, the higher was the amount of metal adsorbed on silica gel. It is anticipated

that the metal adsorption on silica gel is caused by a cation exchange reaction with the silanol group.

Therefore, the metal cation and hydrogen ion compete with each other when the hydrogen ion

concentration is higher. The amounts of Ga and In adsorbed on silica gel was higher than that of Zn. The

ion exchange reaction tends to favor higher valence species. Hence as Zn is a divalent cation, and Ga and In

are trivalent cations, silica gel is extracted to adsorb Ga and In preferentially. This may be the reason for the

higher affinity of silica gel for Ga and In than that of Zn.

Figure 5 shows the effect of pH on the amount of metal adsorbed on silica gel in the mixed metal

system under the experimental condition in which the total concentration of the three metals is 1 mmol/dm3.

From a comparison of the adsorption behavior in the single metal systems and that in the mixed metal

system, the amount of Ga adsorbed on silica gel was found to be higher than that of In.

In this work, HCl is used as a component in the aqueous solution. It is well known that In tends to

form chloride complexes in HCl solution. Ga and Zn also slightly form chloride complexes. The

distribution ratio of each metal chloride complex can be calculated using its formation constants. Figures 6,

7 and 8 show the distribution ratio of the chloride complexes of Ga, In and Zn, respectively. Since the HCl

concentration is lower than 0.1 mol/dm3 under our experimental conditions, Ga

3+ is considered to be the

dominant species in this HCl concentration range. On the other hand, InCl2+ is the dominant species in this

HCl concentration range. Because the valence of Ga3+

is larger than that of InCl2+, silica gel will adsorb Ga

preferentially.

Figure 5. Amount of metal adsorbed on silica

gel in the mixed metal system.

C0,Ga=0.33 mmol/dm3, C0,In=0.33 mmol/dm

3,

C0,Zn=0.33 mmol/dm3.

Figure 6. Distribution ratio of Ga chloride

complexes (CGa=1 mmol/dm3).

Ga3+

GaCl2-

GaCl2+

GaCl3

GaCl4-

Dis

trib

uti

on

rati

o o

f G

a c

hlo

rid

e co

mp

lex [

%]

Initial concentration of Cl- [mol/dm

3]

[mol/dm3]

ddm[mol/dm3] [mol/dm3]

[mol/dm3] [mol/dm

3]

Exte

nt

of

met

al

ad

sorb

ed o

n s

ilic

a g

el [

%]

Equilibrium pH

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As shown in Figure 5, at pH 3 Ga was completely adsorbed on silica gel, and In and Zn were poorly

adsorbed in the mixed metal system. Therefore, silica gel can separate Ga from In and Zn. In addition, Ga

was not adsorbed on silica gel below pH 1.5. Therefore, Ga adsorbed on silica gel can be desorbed using a

strong acid.

Figure 9 shows the effect of pH on the amount of metal adsorbed on DTS in the single metal systems.

DTS has a high adsorption ability for the metals at metal concentrations of 5 mmol/dm3. Figure 9 also

shows that the amount of Ga, In and Zn adsorbed on DTS was dependent on the pH. The higher the pH, the

higher was the amount of metal adsorbed on DTS, just as for silica gel. It is believed that the metal

adsorption on DTS is caused by a metal-chelating complex formation reaction with D2EHPA in addition to

adsorption onto silica gel.

The extraction of Ga with D2EHPA in a solvent extraction system was reported to be described by

the following extraction equilibrium equation [9, 11].

3HRHGaRRH2Ga 32

3 (2)

The following equation was reported for In extraction [9].

3HRHInRRH2In 32

3 (3)

Further, the following equations were presented for the extraction using microcapsules

containing D2EHPA [14].

3HRHGaRRH2Ga 32

3 (4)

3H2RHInRRH5/2In 32

3 (5)

Figure 7. Distribution ratio of In chloride complexes

(CIn=1 mmol/dm3).

Figure 8. Distribution ratio of Zn chloride

complexes (CZn=1 mmol/dm3).

Dis

trib

uti

on

rati

o o

f In

ch

lori

de

com

ple

x [

%]

Dis

trib

uti

on

rati

o o

f Z

n c

hlo

rid

e co

mp

lex [

%]

Initial concentration of Cl-

[mol/dm3]

InCl2+

In3+

InCl2+

InCl3

Zn2+

ZnCl+

ZnCl2

ZnCl3-

Initial concentration of Cl-

[mol/dm3]

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In the above equations, RH denotes D2EHPA. D2EHPA forms a neutral complex with the metal

cation accompanied by a release of three hydrogen ions. When the hydrogen ion concentration is high, the

equilibrium shifts to the left in the above equation. Thus, the extraction of metal ion is unfavorable at low

pH. In this work, it is believed that these metal ions form neutral complexes with D2EHPA in the nonpolar

inside of the surfactant micelle on DTS.

From a comparison of the metal adsorption behavior onto DTS with that on silica gel in the single

metal systems, the amount of metal adsorbed on DTS was higher than that on unmodified silica gel for the

adsorption of Ga, In and Zn. In particular, the amount of In adsorbed is higher. However, silica gel cannot

adsorb In at pH 3, but DTS can. This result is due to the modification of silica gel with D2EHPA. D2EHPA

is well known to have a high affinity for In.

Figure 10 shows the effect of pH on the amount of metal adsorbed on DTS in the mixed metal system.

From a comparison of the adsorption behavior in the single metal systems with that in the mixed metal

system, the amount of In adsorbed on DTS was higher than that of Ga and Zn. In the mixed metal system,

the metal cations compete with each other, so DTS is considered to adsorb In preferentially since DTS has

a high affinity for In.

As shown in Figure 10, at pH 1.1 In was almost completely adsorbed on DTS, while Ga and Zn were

poorly adsorbed in the mixed metal system. From this result, DTS can separate In from Ga and Zn. From a

comparison of the metal adsorption behavior onto DTS with that for silica gel in the mixed metal system,

DTS can separate metals at a lower pH better than the unmodified silica gel. The modification of silica gel

with D2EHPA can reduce the need for pH adjustment and also the amount of the reagent used in pH

reduction practice where a low operating pH is preferable.

Furthermore, the adsorption behavior on DTS in the mixed metal system wherein the Zn content is

Figure 9. Amount of metal adsorbed on

DTS in the single metal systems.

C0,Ga=5 mmol/dm3, C0,In=5 mmol/dm

3,

C0,Zn=5 mmol/dm3.

Figure 10. Amount of metal adsorbed on DTS

in the mixed metal system.

C0,Ga=1.66 mmol/dm3, C0,In=1.66 mmol/dm

3,

C0,Zn=1.66 mmol/dm3.

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high was also investigated for the purpose of separating Ga and In from zinc minerals. Figure 11 shows the

effect of pH on the amount of metal adsorbed on DTS in the mixed metal system when [Zn]0:[Ga]0:[In]0 =

10: 1: 1, where [M]0 denotes the initial metal concentration. Figure 12 shows the result when

[Zn]0:[Ga]0:[In]0 = 100: 1: 1. Figures 11 and 12 suggest that DTS adsorbed In preferentially when the

aqueous solution contained very high Zn levels. Therefore, DTS is considered to have excellent properties

for separation of metals in any zinc residue treatment process.

Shortly, we will clarify the saturation amount of the metal adsorbed on DTS and the selective

desorption method for the metals from DTS.

4. Conclusion

In this study, a solid-phase extraction method was proposed. First of all, silica gel modified with the

non ionic surfactant TritonX-100 micelle including D2EHPA as the metal adsorbent was prepared. The

stability of the modified silica gel was examined to confirm its usability. Next, the adsorption behavior of

gallium, indium and zinc onto the modified silica gel was investigated and compared with the adsorption

behavior when using silica gel alone. The amount of D2EHPA impregnated in the modified silica gel was

0.138 mmol/g-silica gel. The modified silica gel could be used as a stable adsorbent below pH 2.0 without

leaking D2EHPA. The higher the pH, the higher was the amount of adsorbed metal. Gallium was adsorbed

on unmodified silica gel preferentially at pH 3.0 in the mixing metal system. In contrast, indium was

adsorbed onto the modified silica gel preferentially at pH 1.1 in the same system. It was confirmed that the

modified silica gel prepared in this study can separate the metal ions at lower pH better than the unmodified

silica gel.

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Figure 11. Amount of metal adsorbed on DTS.

C0,Ga=0.5 mmol/dm3, C0,In=0.5 mmol/dm

3,

C0,Zn= 5 mmol/dm3.

Figure 12. Amount of metal adsorbed on DTS.

C0,Ga=0.05 mmol/dm3, C0,In=0.05 mmol/dm

3,

C0,Zn= 5 mmol/dm3.

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