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Removal of Nickel from Electroless Nickel Plating Rinse Water with Di(2-Ethylhexyl)phosphoric Acid-Impregnated Supports Hai Trung Huynh and Mikiya Tanaka * Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba, Ibaraki, Japan ABSTRACT Electroless nickel plating technology is playing an increasingly important and indispensable role in many fields such as the electronic and automobile industries. As a result, the treatment of the rinse water containing about 50 mg=dm 3 of nickel is becoming a serious environmental problem. Although this water is currently treated by the conventional precipitation method, a method without sludge generation is highly desired. This study explores the possibility of removing and recovering nickel from the rinse water with di(2-ethylhexyl)phosphoric acid-impregnated supports (D2EHPA-IS). Macroporous polymer and oil adsorbents made of synthetic and natural fibers as the supporting materials were tested for the nickel * Correspondence: Mikiya Tanaka, Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba, Ibaraki 305-8569, Japan; E-mail: [email protected]. SOLVENT EXTRACTION AND ION EXCHANGE Vol. 21, No. 2, pp. 291–305, 2003 DOI: 10.1081=SEI-120018951 0736-6299 (Print); 1532-2262 (Online) Copyright # 2003 by Marcel Dekker, Inc. www.dekker.com 291

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Removal of Nickel from ElectrolessNickel Plating Rinse Water with

Di(2-Ethylhexyl)phosphoric

Acid-Impregnated Supports

Hai Trung Huynh and Mikiya Tanaka*

Research Institute for Green Technology, National Institute of

Advanced Industrial Science and Technology (AIST),

Onogawa, Tsukuba, Ibaraki, Japan

ABSTRACT

Electroless nickel plating technology is playing an increasingly important

and indispensable role in many fields such as the electronic and automobile

industries. As a result, the treatment of the rinse water containing about

50 mg=dm3 of nickel is becoming a serious environmental problem.

Although this water is currently treated by the conventional precipitation

method, a method without sludge generation is highly desired. This

study explores the possibility of removing and recovering nickel from

the rinse water with di(2-ethylhexyl)phosphoric acid-impregnated supports

(D2EHPA-IS). Macroporous polymer and oil adsorbents made of synthetic

and natural fibers as the supporting materials were tested for the nickel

*Correspondence: Mikiya Tanaka, Research Institute for Green Technology, National

Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba,

Ibaraki 305-8569, Japan; E-mail: [email protected].

SOLVENT EXTRACTION AND ION EXCHANGE

Vol. 21, No. 2, pp. 291–305, 2003

DOI: 10.1081=SEI-120018951 0736-6299 (Print); 1532-2262 (Online)

Copyright # 2003 by Marcel Dekker, Inc. www.dekker.com

291

removal abilities from simulated rinse water. In the batch experiments,

more than 90% of the nickel can be adsorbed by these D2EHPA-IS without

pH adjustment. The adsorption of nickel reaches the equilibrium within

1.2 ks at 298K at a shaking rate of 140 rpm. The pH-dependency of the

nickel adsorption by the D2EHPA-IS shows that the nickel is adsorbed by a

cation exchange reaction. The adsorbed nickel can then be readily eluted

with mineral acids. Most of the IS can be used many times without losing

their adsorption abilities. In the column experiments, the breakthrough

curves of nickel for these supports indicate that the nickel–D2EHPA

complex formed at the high nickel loading region tends to dissolve into

the aqueous phase. These findings lead to the conclusion that most of the

studied D2EHPA-IS are effective for the removal and recovery of nickel

from an electroless nickel plating rinse water in batch mode.

Key Words: Nickel; Adsorption; Di(2-ethylhexyl)phosphoric acid;

Electroless nickel plating rinse water; Impregnated supports.

INTRODUCTION

Many researchers have made efforts to remove heavy metals from

industrial wastewater by using several methods such as chemical precipitation,

adsorption, solvent extraction, ion exchange, and reverse osmosis.[1–4] Some

of these methods are expensive and have limitations. Currently, the usual

treatment technology of metal-bearing wastewater is chemical precipitation.

However, this method often creates secondary problems with sludge

generation.[5]

Among the many methods mentioned above, solvent extraction and ion

exchange are known to be effective for metal removal. Although they offer

many advantages, there are several unsolved problems such as (i) the loss of

organics; and the contamination of the water with organics for solvent

extraction, and (ii) slow kinetics for ion-exchange.[6] Extractant-impregnated

resins have been shown to be effective adsorbents for the metal removal from

diluted aqueous solutions.[7] They combine the advantages of the solvent

extraction and ion-exchange processes.[8] Also, oil adsorbents made of

synthetic and natural fibers can adsorb an extractant used in metal solvent

extraction and are expected to be the supports of the impregnated metal

adsorbents. Thus, extractant-impregnated supports (extractant-IS) would be

widely applicable for the treatment of heavy metals in wastewater.

Electroless nickel plating is a typical surface finishing technology and

plays an important role in the high-tech industries. As a result, the rinse water

from the electroless nickel plating containing about 50 mg=dm3 of nickel is

becoming a serious environmental problem. The objective of this study is to

292 Huynh and Tanaka

explore the possibility of removing and recovering nickel from the electroless

nickel plating rinse water with di(2-ethylhexyl)phosphoric acid (D2EHPA)-IS.

Di(2-ethylhexyl)phosphoric acid was selected as the extractant, because this is

a typical organophosphorous acid and known to extract nickel at a pH greater

than 2.9.[9,10]

EXPERIMENTAL

Simulated Rinse Water and Reagent

The simulated rinse water was prepared by diluting the spent bath

discharged from an electroless nickel plating plant in Japan. Ion exchange-

distilled water was used throughout this study. Chemicals used in this study

were all reagent grade except for the extractant. Di(2-ethylhexyl)phosphoric

acid was the product of Daihachi Chemical Industry Co., and was used as

received. Various concentrations of sodium hydroxide and hydrochloric acid

were used as the pH adjusting reagents. As eluting reagents, 2 mol=dm3

hydrochloric and 1 mol=dm3 sulfuric acids were used.

Support

The thermally bonded fabrics, KFO Mat P-185 (KFO), made of poly-

ethylene and polypropylene, the non-woven fabrics, Static Resistant Oil

Sorbent HP-556 (OS), made of polypropylene, and the natural fiber, Oil

Catcher KT-65 (OC), made of kapok fiber, produced by Kyushu Filter Industry

Co., Ltd., 3M Co., Ltd., and Kakui Co., Ltd., respectively, were supplied in the

form of sheets. Before impregnation, all the fibers were cut into

0.5 cm� 0.5 cm pieces.

The macroporous resin Amberlite XAD7HP, supplied by Rohm and Haas

Co., is a polymeric adsorbent with an acrylic ester matrix. On a dry basis, it

has a specific surface area of more than 400 m2=g, a porosity more than 0.5, an

average pore size of 45–50 nm, and a pore volume of 0.5 cm3=cm3.

Impregnation Procedure

The D2EHPA-IS were prepared as follows:[8,11] The supports were

washed with methanol, dried, and contacted with 10 vol.% D2EHPA in ethanol

in the phase ratio of 50 cm3=g at 298K at a shaking rate of 140 rpm overnight.

The supports were then removed by filtration and washed with an excess

volume of water. Finally, the supports were dried in an oven overnight at

353K. The concentration of D2EHPA held in the supports was determined by

Electroless Nickel Plating Rinse Water 293

the difference in weight before and after the impregnation. In some cases, the

concentrations of D2EHPA in the IS were also determined by the digestion

with the mixture of sulfuric and nitric acids followed by the phosphorous

analysis by ICP-AES (Seiko SPS4000). The results agreed with each other

within �5%.

Adsorption and Elution Procedures

In the batch equilibrium experiments, the desired amount of each

D2EHPA-IS and 10 cm3 of the simulated rinse water with the liquid–solid

phase ratio of 50 cm3=g dried non-impregnated support (DNIS) except the

experiments for nickel adsorption isotherm, and the very small amount of the

pH adjusting reagent (less than 2 cm3 of the 1 mol=dm3 HCl solution per 1 dm3

of the rinse water), when necessary, were placed in a stoppered 50 cm3-conical

flask and shaken at a rate of 140 rpm in a water bath maintained at 298K for

more than 12 hours to ensure equilibrium. A small amount of the aqueous

phase was then removed and appropriately diluted in order to determine the

nickel concentration. The nickel concentration in the IS was calculated on the

basis of mass balance. The adsorbed nickel was eluted separately by

2 mol=dm3 HCl and 1 mol=dm3 H2SO4 with the liquid solid phase ratio of

50 cm3=g DNIS by vigorous vertical shaking for one hour. When the durability

for repeated use of the IS was investigated, the D2EHPA-IS after the

adsorption–elution cycle was washed with water until chloride ion was not

detected in the filtrate, dried at 353K, and submitted to another adsorption–

elution cycle.

In the kinetic runs, the contact of the two phases began under the same

conditions as those for the batch equilibrium experiments without adding the

pH adjusting reagent to the rinse water. During the experiment, the pH of

the aqueous phase was not kept at constant. At preset time intervals, the two

phases were separated, and the nickel concentration in the aqueous phase was

determined after appropriate dilution.

In the column experiments, 1 g of the D2EHPA-IS was packed in a glass

column with an inner diameter of 11 mm. The rinse water or the eluting

reagent was fed to the bottom of the column maintained at 298K, and the

effluent from the head was collected by a fraction collector.

The nickel concentrations in the aqueous solutions before and after

adsorption, and after elution were determined by ICP-AES. The pH values

in the aqueous phases were measured by a pH meter (Toa HM-60G). Sulfate,

phosphinate, and phosphonate ions, as well as lactic and propionic acids in

the simulated rinse water were analyzed using a capillary electrophoresis

apparatus (Otsuka CAPI-3200).

294 Huynh and Tanaka

RESULTS AND DISCUSSION

Composition of the Rinse Water

The composition of the simulated rinse water was measured, and the result

is shown in Table 1. The simulated rinse water is weakly acidic with high

concentrations of sodium, phosphonate, sulfate ions, and organic acids. Since

the nickel concentration in the spent bath was gradually changed due to

the slow precipitation of nickel phosphonate, the nickel concentration in the

simulated rinse water was varied from 20 to 50 mg=dm3; thus, the actual nickel

concentration will be described in the legend of each figure or table.

Batch Experiment

The relationship between the nickel adsorption percentage and equili-

brium pH is shown in Fig. 1. At the equilibrium pH of 3.9 for the D2EHPA-

impregnated OC and XAD7HP, and 4.2 for the impregnated KFO and OS

(these pH values are attained if the adsorption of nickel from the simulated

rinse water is done without pH adjustment), more than 90% of the nickel is

adsorbed. The nickel removal efficiency is abruptly reduced with decreasing

pH, suggesting that nickel is adsorbed by a cation exchange reaction. For

solvent extraction of nickel using D2EHPA,[10] nickel is not extracted at a pH

lower than 2.9. In addition, nickel is extracted from the simulated rinse water

using 48.5 g=dm3 D2EHPA dissolved in Shellsol D70 (Shell Chemicals) with

the phase ratio of 1 : 1; however, at the equilibrium pH of 3.9, the extraction

efficiency is only 50% in spite of the fact that the amount of D2EHPA per unit

Table 1. The composition and pH value ofthe simulated rinse water.

Component Concentration (mg=dm3)

Sodium 1020

Nickel 20–50

Iron <1

Zinc <0.1

SO42� 630

H2PO2� 230

HPO32� 1200

Lactic acid 550

Propionic acid 90

pH 5.3

Electroless Nickel Plating Rinse Water 295

volume of the aqueous phase was more than three times higher in the solvent

extraction (SX) than those in the IS. According to the nickel removal

percentages by adsorption and extraction, D2EHPA-IS are more effective

than SX using the same extractant.

The blank experiments show that OC itself adsorbs nickel to a slight extent;

that is, without pH adjustment, about 40% of the nickel is adsorbed at the

equilibrium pH of 5.0. This is probably because natural fibers (OC) have some

functional groups like ��OH. For other supports, nickel is not adsorbed at all.

The difference among the pH dependency of the nickel adsorption by

each D2EHPA-impregnated fiber (OC, OS, and KFO) is not found; however,

over the entire investigated pH range, nickel adsorption is reduced in the order

of the impregnated supports: OC>OS>KFO in accord with the decreased

order of the D2EHPA concentrations in the supports: OC>OS>KFO. On

Figure 1. The effect of equilibrium pH on the nickel adsorption. The nickel

concentration in the rinse water was 48.4 mg=dm3. The D2EHPA concentrations in

the IS (g=g IS) were 0.27 (KFO), 0.28 (OS), 0.39 (OC), and 0.37 (XAD7HP).

296 Huynh and Tanaka

the other hand, the pH dependency of the nickel adsorption by the D2EHPA-

impregnated XAD7HP is slightly less than those by the D2EHPA-impregnated

fibers. The reason for this is not clear, but would be related to the difference in

the structures of the fibers and XAD7HP.

Without adding the pH adjusting reagent to the rinse water, 100% of the

iron and zinc are adsorbed by all the IS, and due to their very low concentrations

we did not pay special attention to the adsorption of those metals hereafter.

The time courses of the nickel removal for the different D2EHPA-IS are

shown in Fig. 2. Under the present experimental conditions, the adsorption

equilibria are reached within 1.2 ks (20 min) for all the D2EHPA-IS, which

would be fast enough to remove nickel in the rinse water by batch mode. The

fibers seem to exhibit faster removal rates than the resin. This is probably due

to the diffusion resistance in the XAD7HP pores.

Figure 2. Time courses of nickel removal with D2EHPA-IS. The nickel concentration

in the rinse water was 46.3 mg=dm3. The D2EHPA concentrations in the IS (g=g IS)

were 0.23 (KFO), 0.48 (OS), 0.55 (OC), and 0.26 (XAD7HP).

Electroless Nickel Plating Rinse Water 297

Figures 3 and 4 show the adsorption isotherm of nickel with D2EHPA-IS.

The isotherm was obtained by varying the solid–liquid ratios. The pH of the

aqueous phase was not adjusted. As shown in Figs. 3 and 4, the amount of

adsorbed nickel increases with the rise in the aqueous nickel concentration,

particularly in the low nickel loading region. In the high nickel loading region,

the amount of adsorbed nickel tends to decrease with the rise in the aqueous

nickel concentration for the D2EHPA-impregnated fibers. This phenomenon,

though not clear for the D2EHPA-impregnated resin, is probably due to the

dissolution of the nickel–D2EHPA complex into the aqueous phase and will

be discussed more later.

The adsorbed nickel is readily and completely eluted with 2 mol=dm3

hydrochloric or 1 mol=dm3 sulfuric acids. The elution efficiency does not

Figure 3. Adsorption isotherm of nickel with the D2EHPA-impregnated fibers. The

nickel concentration in the rinse water was 47.6 mg=dm3. The D2EHPA concentrations

in the IS (g=g IS) were 0.43 (KFO), 0.29 (OS), and 0.60 (OC).

298 Huynh and Tanaka

depend on the kinds of supports and acids, and achieves more than 95%

(unbalance within 5% is due to the experimental error) for all the D2EHPA-IS

(Table 2).

The durability for repeated use of the investigated D2EHPA-IS was tested

at a low nickel loading (phase ratio was 50 cm3=g DNIS). For the D2EHPA-

impregnated OS, the nickel removal efficiency remarkably decreases, and the

IS became an inert adsorbent for nickel after the 5th cycle (Fig. 5). However, for

other IS, the change in the nickel adsorption percentages is not found, and they

can be used many times without losing their adsorption abilities, indicating that

the loss of D2EHPA is negligible at the low nickel loading region.

Figure 4. Adsorption isotherm of nickel with the D2EHPA-impregnated XAD7HP.

The nickel concentration in the rinse water was 43.3 mg=dm3. The D2EHPA concen-

trations in the IS (g=g IS) were 0.18 [XAD7HP(1)], 0.26 [XAD7HP(2)], and 0.37

[XAD7HP(3)].

Electroless Nickel Plating Rinse Water 299

Column Experiment

Due to the remarkable loss of D2EHPA for the impregnated OS (Fig. 5),

the column experiments were conducted only for the impregnated KFO, OC,

and XAD7HP. Breakthrough curves of nickel for these IS are shown in Fig. 6,

where the relative nickel concentration is the nickel concentration in the

effluent divided by the nickel concentration in the feed solution. The relative

nickel concentrations for the D2EHPA-impregnated KFO and OC exceeded

unity. For the D2EHPA-impregnated XAD7HP, at the lower D2EHPA content

(0.18 g D2EHPA=g IS), the relative nickel concentration does not exceed unity

and the shift of the breakthrough curve during the second cycle from that

during the first cycle is small; however, at the higher D2EHPA content

(0.37 g D2EHPA=g IS), the relative nickel concentration exceeds unity and

the shift of the breakthrough curve during the second cycle from that during

the first cycle is much larger.

One possible explanation for the fact that the relative nickel concentration

exceeds unity might be that the adsorption of other cation in the rinse water by

D2EHPA-IS caused the nickel elution. If that happened, however, the break-

through curves of nickel for the D2EHPA-impregnated XAD7HP in the

second cycle would be identical with those in the first cycle. Therefore, the

following two hypotheses can be formulated:

The first one considers the adsorption of sodium by the D2EHPA-IS and

the dissolution of the Na–D2EHPA complex thus formed into the aqueous

phase. The second one considers the dissolution of the Ni–D2EHPA complex

formed at the high nickel loading region into the aqueous phase.

Table 2. Nickel elution efficiency from the adsorbed impregnated supports. Thenickel concentration in the rinse water was 19.4 mg=dm3.

Elution efficiency, %

Extractant- D2EHPA Solid Ni

impregnated

support

concentration

in the IS, g=g IS

concentration,

mg=g IS

2 mol=dm3

HCl

1 mol=dm3

H2SO4

D2EHPA-KFO 0.26 0.68 95 97

D2EHPA-OS 0.32 0.64 97 96

D2EHPA-OC 0.46 0.49 99 96

D2EHPA-XAD7HP 0.18 0.63 99 96

Note: D2EHPA is di(2-ethylhexyl)phosphoric acid; IS is an impregnated support; and

KFO, OS, and OC are KFO Mat P-185, Static Resistant Oil Sorbent HP-556, and Oil

Catcher KT-65, respectively.

300 Huynh and Tanaka

Sodium can be extracted by D2EHPA dissolved in a nonpolar organic

solvent;[12,13] thus, the extraction of nickel and sodium from the simulated

rinse water by D2EHPA dissolved in Shellsol D70, and Shellsol D70 alone

was done at the various phase ratios, and the nickel and sodium concentrations

in the obtained organic phases were determined by the analysis of the aqueous

phase formed after stripping by 2 mol=dm3 hydrochloric acid with the phase

ratio of 1 : 1. The result shows that the sodium concentrations in the organic

phases at various phase ratios are almost constant, indicating that sodium is

not extracted or extracted to a very small extent under the conditions of the

present study (Table 3). This result rejects the first hypothesis.

For the nickel extraction using D2EHPA dissolved in a nonpolar organic

solvent, the formation of Ni–D2EHPA polymer complexes at a high nickel

Figure 5. Repeated usage of D2EHPA-IS. The nickel concentration in the rinse water

was 48.2 mg=dm3. The D2EHPA concentrations in the IS (g=g IS) were 0.57 (KFO),

0.26 (OS), 0.51 (OC), and 0.37 (XAD7HP).

Electroless Nickel Plating Rinse Water 301

loading region in the organic phase were reported by some authors.[14,15]

Especially, the Ni–D2EHPA complex proposed by Sato and Nakamura[15]

exists as a trimer, has the octahedral structure, and contains coordinated water

molecules. This complex is likely to have high solubility in an aqueous

solution. In the present study, in order to detect the Ni–D2EHPA complex in

the aqueous phase, several effluent fractions during the column experiment

with D2EHPA-impregnated OC at the flow rate of 2.1 cm3=ks were mixed with

Shellsol D70 with the aqueous to organic phase ratio of 5 : 1 by vigorous

vertical shaking for one hour. The organic phases thus obtained were stripped

Figure 6. Breakthrough curves of nickel at the flow rate of 1.67 cm3=ks. The nickel

concentration in the rinse water was 35.1 mg=dm3. The D2EHPA concentrations in the

IS (g=g IS) were 0.26 (KFO), 0.46 (OC), 0.18 [XAD7HP(1)], and 0.37 [XAD7HP(2)].

302 Huynh and Tanaka

by 2 mol=dm3 HCl with the phase ratio of 1 : 1. For the effluent fractions with

the nickel relative concentrations of 0.11, 0.55, 1.22, and 2.09, the nickel

concentrations in the aqueous solutions after stripping were 0.25, 1.18, 4.14,

and 14.7 mg=dm3, respectively. This fact qualitatively indicates that the Ni–

D2EHPA complex is present in the effluent, and its concentration increases

with the rise in the effluent volume. This result supports the second hypothesis

that the Ni–D2EHPA complex formed at the high nickel loading region

dissolves into the aqueous phase.

CONCLUSIONS

The adsorption properties of nickel from the electroless nickel plating

rinse water by the D2EHPA-IS have been studied at 298 K using synthetic

fibers (KFO and OS), natural fiber (OC), and resin (XAD7HP) as the

supporting materials. The results are summarized as follows:

(i) Under the batch contact, more than 90% of the nickel can be

removed within 1.2 ks by the D2EHPA-IS without pH adjustment

via a cation exchange reaction.

(ii) The adsorbed nickel can be readily eluted by mineral acids.

Table 3. The extraction of nickel and sodium from the simulated rinse water withD2EHPA dissolved in Shellsol D70. The nickel concentration in the rinse water was32.5 mg=dm3.

D2EHPA

concentration Aqueous to Equilibrium

Equilibrium

organic phase,

mg=dm3

in the Shellsol organic aqueous

No. D70, g=dm3 phase ratio pH Na Ni

1 10.5 1 : 1 4.7 23.7 11.7

2 10.5 5 : 1 4.9 19.8 23.4

3 10.5 10 : 1 5.0 26.7 23.4

4 10.5 15 : 1 5.0 23.6 17.1

5 0 1 : 1 5.3 23.6 0

6 0 5 : 1 5.3 21.4 0

7 0 10 : 1 5.3 22.8 0

8 0 15 : 1 5.3 23.8 0

Electroless Nickel Plating Rinse Water 303

(iii) As long as the nickel loading is low, the D2EHPA-IS can be used

many times without losing its adsorption ability except that using

OS as a supporting material.

(iv) When the nickel loading becomes high, the nickel–D2EHPA

complex tends to dissolve into the aqueous phase; thus, the

D2EHPA-IS lose their adsorption abilities.

Therefore, D2EHPA-IS using KFO, OC, and XAD7HP as supporting

materials can be used for the batch treatment of the rinse water in the low

loading region.

ACKNOWLEDGMENT

One of the authors, Hai Trung Huynh, gratefully acknowledges the

financial support provided by the Japan Science and Technology Cooperation

under the STA fellowship.

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Received May 31, 2002.

Electroless Nickel Plating Rinse Water 305