Water Research 38 (2004) 2859–2864

ARTICLE IN PRESS

*Correspond

E-mail addr

(A. Nakajima).

0043-1354/$ - se

doi:10.1016/j.w

Mechanism of hexavalent chromium adsorptionby persimmon tannin gel

Akira Nakajimaa,*, Yoshinari Babab

aDepartment of Chemistry, Faculty of Medicine, Miyazaki Medical College, University of Miyazaki, Kiyotake,

Miyazaki 889-1692, JapanbDepartment of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192, Japan

Received 28 October 2003; received in revised form 8 March 2004; accepted 6 April 2004

Abstract

Mechanism of chromium adsorption by the persimmon tannin (PT) gel was examined. The PT gel can adsorb Cr

highly effectively from aqueous solutions containing Cr(VI), while it adsorbed far smaller amounts of Cr from the

solution containing Cr(III). The maximum Cr adsorption from the Cr(VI) solution occurred at pH 3. The Cr

adsorption from the Cr(VI) solution by the PT gel was rapid, was faster than VO2+ and Fe3+ adsorptions, and was

obeyed the Langmuir adsorption isotherm (Qe=5.27mmol g�1 and K=16.2mM). The gel adsorbed Cr from the Cr(VI)

solution (pH 1 and 3) showed no ESR signal of Cr(III), while the ESR signal of Cr(III) was observed in the residual

solution at pH 1. Hexavalent chromium was, therefore, adsorbed on the PT gel through the esterification of chromate

with catechol group. In other words, Cr(VI) should combine with catechol as a hard acid, CrO22+ cation. Through the

treatment of a Cr(VI) solution with the PT gel, chromium should be recovered as a Cr(IV)�tannin complex at pH 3 or a

Cr(III) solution at pH 1 or lower pH region.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Chromium adsorption; Persimmon tannin; Hexavalent chromium ion; ESR

1. Introduction

Chromium is used widely in anodizing, electroplating,

corrosion control, oxidation, wood treatment, leather

tanning, and several other industrial applications.

Normally, industrial wastes contain both hexavalent

and trivalent forms of chromium. However hexavalent

chromium, Cr(VI), is more hazardous, carcinogenic and

mutagenic to living organisms [1]. It is, therefore,

worthwhile subject to study on remove of chromium

from industrial effluent, environmental aqueous system

and soil. Some researchers tried to remove chromium

from aqueous systems by using various biosorbents [2–

ing author. Tel.:/fax: +81-985-85-1185.

ess: akanaka@med.miyazaki-u.ac.jp

e front matter r 2004 Elsevier Ltd. All rights reserve

atres.2004.04.005

10] and activated carbon [11,12]. One of the present

authors has shown that persimmon tannin (PT) gel,

newly developed, could be useful for the recovery and

removal of heavy metals such as uranium [13], iron [14]

and vanadium from aqueous solution [15]. In these

cases, metal ions were combined as hard acids, UO22+,

Fe3+, and VO2+, with hard bases, catechol and

pyrogallol groups in the gel [16], according to the hard

and soft acids and bases (HSAB) principle first proposed

by Pearson [17–19]. Recently, Nakano et al. [20]

examined the adsorption mechanism of hexavalent

chromium, Cr(VI), by mimosa tannin gel, and explained

with the four reaction steps, the esterification of

chromate with tannin molecules, the reduction of Cr(VI)

to trivalent chromium, Cr(III), the formation of

carboxyl group by the oxidation of tannin molecule,

and the ion exchange of the reduced Cr(III) with the

d.

ARTICLE IN PRESSA. Nakajima, Y. Baba / Water Research 38 (2004) 2859–28642860

carboxyl and hydroxyl groups. However, their explana-

tion was restricted to the lower pH region (mainly at pH

1) and the higher Cr concentration (1000mgL�1).

Under these conditions, the oxidation ability of Cr(VI)

is dominant, which should complicate the reaction

mechanism. In the present study, therefore, the mechan-

istic study of Cr(VI) adsorption by another condensed-

tannin, PT gel, at higher pH region (mainly at pH 3) and

lower Cr concentration (0.2mM, about 10mgL�1) will

be conducted.

Fig. 1. Effect of pH on the chromium adsorption by the PT gel.

Eight milligrams of the gel were suspended in 40mL of

solutions containing 0.2mM of K2Cr2O7 (closed circle) and

CrCl3 (open circle), respectively for 1 h at 30�C. Each points

represents mean7standard deviation of triplicate. Solid lines

indicate the calculated ratios of chemical species of hexavalent

chromium in a K2Cr2O7 solution (0.2mM Cr) using the acid

dissociation constants listed in Table 1.

2. Experimental

2.1. Materials

PT solution used throughout this study and its

gelation procedure were described previously [15]. The

resulting gel was crushed into small pieces (0.177–

0.250mm diameter), washed thoroughly with deionized

water. Potassium dichromate (K2Cr2O7), chromium(III)

trichloride hexahydrate (CrCl3 � 6H2O), and other che-

micals used for this study were obtained from Nacarai

Tesque, Inc. (Kyoto, Japan).

2.2. Adsorption experiments

The chromium adsorption experiments by using the

persimmon tannin gel were conducted as follows: 8mg

of the gel (dry weight basis) was suspended in 40mL of

the solution containing 0.2mM of chromium and the

suspension was stirred for 1 h at 30�C using magnetic

stirrer (250 rpm). Chromium was supplied as K2Cr2O7

and CrCl3 � 6H2O. The pH of the solution was adjusted

to desired values with 0.1M HCl and 0.1M NaOH.

Adsorption experiments for the time course of the

chromium adsorption were conducted as follows: 8mg

(dry weight basis) of the gel was suspended in 40mL of a

solution containing 0.2mM of chromium and the

suspension was stirred for 10min–1 h at 30�C. Adsorp-

tion experiments for the effect of chromium concentra-

tion were conducted as follows: 8mg of the gel was

suspended in 40mL of solutions containing 0.2–2mM of

chromium. The adsorption experiments were conducted

three times and averaged.

2.3. Electron spin resonance measurements

Five milligrams of the freeze-dried powder samples

were put into a quartz sample tube of 5mm diameter,

and then used for the electron spin resonance (ESR)

analysis. ESR spectra were measured using X-band ESR

spectrometer (JEOL JES TE-100) under the conditions

of the microwave power 5mW, the microwave frequency

9.44GHz, the external magnetic field 345mT, the field

amplitude 100mT, the field modulation 100 kHz, the

modulation width 0.32mT, the time constant 0.1 s, and

the sweep time 8min.

3. Results and discussion

3.1. Effect of pH on chromium adsorption by persimmon

tannin gel

The chromium adsorption by the persimmon tannin

gel was examined from aqueous solutions containing

hexavalent chromium ion, Cr(VI), as K2Cr2O7, and

trivalent chromium, Cr(III), as CrCl3. As shown in

Fig. 1, the maximum Cr adsorption from the Cr(VI)

solution occurred at pH 3. Above and below pH 3, rapid

and gradual decreases of the Cr adsorption were

observed. The pH value for the maximum Cr adsorption

from the Cr(VI) solution was different from those for

VO2+ (pHmax=6) [15] and Fe3+ (pHmax=5) [14]. The

difference between pH profiles of Cr and other two

metal ions suggests their different adsorption mechan-

isms. The chemical species of Cr(VI) in the Cr(VI)

solution (0.2mM) were calculated using formation

constants listed in Table 1 [21] (Fig. 1). As shown in

Fig. 1, the Cr adsorption from the Cr(VI) solution occur

in the pH range where HCrO4� species mainly exists.

Matsuo and Itoh [16] reported that PT consists of

catechin, catechin-3-gallate, gallocatechin and galloca-

techin gallate at the ratio of 1:1:2:2 (chemical formula

weight 2248). It contains two 1,2-dihydroxyphenyl

ARTICLE IN PRESS

Table 1

Acid dissociation constants of hexavalent chromium in aqueous

solution

Species pKML

[HCrO4�][H3O

+]/[H2CrO4] 0.7

[CrO42�][H3O

+]/[HCrO4�] �5.74

[Cr2O72�]/[HCrO4

�]2 1.97

Fig. 2. Schematic structures of hexavalent chromium–catechol

coupling in the PT gel: (a) the esterification between catechol

and HCrO4�, (b) the coupling between catechol anion and

CrO22+.

Fig. 3. Time course of hexavalent chromium adsorption by the

PT gel. Eight milligrams of the gel were suspended in 40mL of

0.2mM of K2Cr2O7 solution at 30�C. Each points represents

mean7standard deviation of triplicate. Solid line indicates the

time-dependent Langmuir equation, Q=Qe (1�exp(�k t)),

where Qe=1.86mmol g�1 adsorbent and k=0.155min�1.

A. Nakajima, Y. Baba / Water Research 38 (2004) 2859–2864 2861

(catechol) groups and seven 1,2,3-trihydroxyphenyl

(pyrogallol) ones, acting as hard bases, in a chemical

formula unit. In the Cr(VI) solution, therefore, Cr(VI)

should combine with catechol as a hard acid, CrO22+

cation (Fig. 2b), in similar manner as Mo(VI)-catechol

complex [22]. This consideration should be equivalent to

the esterification of chromate with catechol group (Fig.

2a) proposed by Nakano et al. [20].

As shown in Fig. 1, the amounts of Cr adsorbed by

the gel from the Cr(III) solution increased gradually

from pH 2 to 6. However, the amounts of Cr adsorbed

from the Cr(III) solution at pH 6 were much smaller

than those from the Cr(VI) solution at pH 3. As the

precipitation of Cr(OH)3 was observed in the Cr(III)

solution, the adsorption experiments was not conducted

in the higher pH range above 7.

3.2. Kinetics of hexavalent chromium adsorption by the

persimmon tannin gel

Kinetics of Cr(VI) adsorption by the persimmon

tannin gel was examined at pH 3 (Fig. 3). The

adsorption of Cr from the Cr(VI) solution by the PT

gel was rapid, reaching a plateau within 40min. The

results were fitted by a time-dependent adsorption

equation according to the Langmuir model [23]

Q ¼ Qe½1� expð�j tÞ�; j ¼ ka þ kd; ð1Þ

where Qe is the equilibrium adsorption amounts, ka, the

adsorption rate constant, and kd, the desorption rate

constant. The values, Qe and j, were estimated to be

1.86mmol g�1 and 0.155min�1, respectively, and simu-

lated results using these values were also shown in Fig. 4.

The estimated j value for Cr adsorption, being near to

that for VO2+ adsorption from VOCl2 solution

(0.132min�1) [15], is larger than those for V adsorption

from NH4VO3 solution (0.0918–0.0978min�1) [15] and

Fe3+ adsorption (0.0633min�1) [14]. While the former

case for VO2+ adsorption is the direct adsorption of

metal cation, the latter cases for VO3� and Fe3+

adsorptions contain red-ox reaction of metal ion in the

adsorption process. These results suggested that Cr

should be adsorbed directly by the PT gel without any

reduction process.

The Cr(VI) adsorption by the PT gel was saturable as

the concentration of Cr in the solution was increased

(Fig. 4). The results were analyzed using the Langmuir

adsorption isotherm [23]

Qe ¼ K QmCe=½1þ K Ce� ð2Þ

where Qe is the equilibrium adsorption amounts, Qm, the

maximum adsorption amount, Ce, the residual vana-

dium concentration, and K, the constant. The values,

Qm and K, were estimated to be 5.27mmol g�1

ARTICLE IN PRESS

Fig. 5. ESR spectrum of PT gel adsorbed chromium. Eight

milligrams of the gel were suspended in 40mL of solutions

containing 0.2mM CrCl3 (pH 6) (a) and 0.2mM K2Cr2O7 (pH

3) (b), respectively for 1 h at 30�C. The amounts of Cr adsorbed

by the gel were 0.038mmol g�1 for Cr(III) and 0.096mmol g�1

for Cr(VI), respectively.

Fig. 4. Isotherm of chromium adsorption by PT gel. Eight

milligrams of the gel were suspended in 40mL of solutions

containing 0.2–2mM of K2Cr2O7 solution for 1 h at 30�C. Each

points represents mean7standard deviation of triplicate. Solid

line indicates the time-dependent Langmuir equation, Q=k Qm

Ce/(1+k Ce), where Qm=5.27mmol g�1 adsorbent and

k=16.2mM�1.

A. Nakajima, Y. Baba / Water Research 38 (2004) 2859–28642862

(274mg g�1) and 16.2mM, respectively, and simulated

results using these values were also shown in Fig. 5. The

Qm value obtained for Cr(VI) is larger than those for

VOCl2 (3.41mmol g�1) [15] and Fe3+ (2.15mmol g�1)

[14]. As described in previous section, PT contains two

catechol groups and seven pyrogallol ones, acting as

hard bases, in a chemical formula unit. When each

ligand combines with one metal ion, the assumed

capacity of the gel should be estimated to be 4mmol g�1.

These results suggested that extra hydroxyl group in

pyrogallol unit and hydroxyl group in catechin unit in

PT gel should be also concerned with the Cr adsorption.

Similar consideration was presented previously in the

uranium adsorption by PT gel [13]. The value K for

Cr(VI) is much larger than those for VOCl2 (5.82mM)

[15] and Fe3+ (6.20mM) [14]. These results indicate that

catechol group in PT should couple with Cr(IV) far

more strongly than VO2+ and Fe3+.

3.3. Electron spin resonance study on chromium

adsorption by PT gel

To examine the chemical state of Cr adsorbed on the

PT gel, electron spin resonance spectra of the gel were

measured. Trivalent chromium, Cr(III), containing

unpaired electron, is ESR positive, while Cr(VI) with

no unpaired electron is ESR negative. Pentavalent

chromium, Cr(V), being also ESR positive, is unstable

and exists only in restricted biosystems [24]. Thus,

Cr(III) is only an ESR-positive state in the present

study. Fig. 5 shows the ESR spectra of PT gels adsorbed

Cr from CrCl3 (pH 6) and K2Cr2O7 (pH 3) solutions.

The electronic levels for Cr3+ ion with its three unpaired

electrons (total spin S=2/3) in an octahedral crystal

field can be described by a spin Hamiltonian of the

form [25]

H ¼ gmBHS þ D½S2Z � ð1=3ÞSðS þ 1Þ� þ EðS2

x � S2yÞ;

ð3Þ

where the terms involving D and E are the axial and

orthorohmbic crystal-field components, respectively,

and mB indicates the Bohr magneton, H; outer magneticfield, S, the spin operator, Sx, Sy, and Sz, x, y and z

components of the spin operator, g, the gyromagnetic

tensor. The g-value for the Cr(III) ion is almost isotropic

and in the range of 1.96–1.99 [26]. In the present case,

parameters D and E should be so small that the ESR

spectrum becomes almost a symmetric single line. As

shown in Fig. 5, the typical ESR signal with g=1.971 for

Cr(III) was observed with a small signal of a free radical

(g=2.004) in the PT gel adsorbed Cr from CrCl3solution. On the other hand, no ESR signal was

observed in the gel adsorbed Cr from K2Cr2O7 solution

(pH 3) except for that of the free radical. These results

indicate that most of Cr in the PT gel adsorbed from the

K2Cr2O7 solution exist as Cr(VI), which suggests that

the adsorption process does not contain reduction of

Cr(VI) to Cr(III).

As described above, no reduction of Cr(VI) to Cr(III)

was observed in the adsorption process from the

K2Cr2O7 solution (pH 3, 0.2mM Cr). However, Nakano

ARTICLE IN PRESS

Table 2

Adsorption or complexation of hexavalent chromium by

various materials

Adsorbent Cr adsorbed

(%)

Cr in residual

solution (mM)

Cr(III) in

residual solution

(mM)a

PT gel 78.0 4.4 0.81

Cellulose 4.7 19.1 1.34

Albumin —b 20.0b 13.3

One hundred milligram of each material were suspended in 100

mL of a solution (pH 1) cotaining 20mM of Cr(VI) as chromate

for 1 h.aAmounts of Cr(III) in the residual solution was determined

from the signal intensity of ESR spectra.bAs albumin and its Cr complex were completely dissolved in

the solution under the present conditions, all Cr should be in

the residual solution after the treatment.

A. Nakajima, Y. Baba / Water Research 38 (2004) 2859–2864 2863

et al. [20] claimed the four reaction steps for the Cr(VI)

adsorption by mimosa tannin gel from the solution (pH

1) containing Cr(VI) (1000mgL�1, about 20mM),

namely, the esterification of chromate with tannin

molecules, the reduction of Cr(VI) to Cr(III), the

formation of carboxyl group by the oxidation of tannin

molecule by chromate, and the ion exchange of the

reduced Cr(III) with the carboxyl and hydroxyl groups.

To clarify the distinction, the ESR measurements were

conducted for the adsorbents and residual solutions

(20mM Cr as K2Cr2O7, pH 1) after treatments with

various substances, such as albumin, cellulose and PT

gel. The ESR spectra of the residual solution were

depicted in Fig. 6, and the adsorption information was

listed in Table 2. As shown in Fig. 6, typical Cr(III)

signals were observed in each residual solution after

treatments by these substances. Albumin was dissolved

in the solution so homogeneously that Cr(VI) in the

solution oxidized it effectively. About 26.8% of Cr(VI)

in the solution was used for the cellulose oxidation, and

16.1% for the PT gel oxidation. These results indicate

that the PT gel is more stable than cellulose against the

Cr(VI) oxidation, which is caused by the strong coupling

between the PT gel and Cr(VI) through esterification.

The PT gel adsorbed Cr(VI) was ESR silent, which

indicates almost no Cr(III) in the gel. It is well known

that ordinarily catechol is oxidized to o-benzoquinone

by chromate [27]. Only under peculiar conditions

bicarbonate (muconic acid) can be obtained through

the oxidative intra-diol cleavage [28]. In the present

experiments, the PT gel is so stable against the oxidation

in lower pH region that the amounts of muconic acid

Fig. 6. ESR spectrum of residual solutions (pH 1) containing

20mM of K2Cr2O7 after the treatment with albumin (a),

cellulose (b), PT gel (c), or no treatment (d).

produced in the gel should be very small. A part of

Cr(VI) in the solution was reduced to Cr(III), which

could not be adsorbed by the gel and remained in the

residual solution. Thus, the most acceptable explanation

for the mechanism of Cr adsorption from 20mM

K2Cr2O7 solution (pH 1) by the PT gel is as follows:

Cr(VI) should be adsorbed on the PT gel as a hard acid,

CrO22+ cation, through esterification. Nakano et al. [20]

conducted their experiments in the solution containing

high concentration of chromate (100mgL�1), which

should oxidize phenols to carboxylic acids through

ketones. They explained these results with the four

reaction steps, the esterification of chromate with tannin

molecules, the reduction of Cr(VI) to trivalent chro-

mium, Cr(III), the formation of carboxyl group by the

oxidation of tannin molecule, and the ion exchange of

the reduced Cr(III) with the carboxyl and hydroxyl

groups. On the other hand, in the present experiments,

0.2mM Cr(VI), being about 10mgL�1, oxidized phenol

groups to ketones, as pointed out by Kehrmann and

Hoehn [27]. As remained phenols and resulted ketones

in the gel could couple with only Cr(VI) ion, the present

considerations are a little bit different from the

Nakano’s third and fourth steps.

As mentioned above, the PT gel can adsorb

5.27mmol g�1 of hexavalent chromium, Cr(IV), at pH

3. However, it is fairly difficult to desorb the hexavalent

chromium adsorbed on the gel by using acidic and basic

solutions. Its difficulty was also pointed out by Nakano

et al. [20]. Thus, hexavalent chromium could be

recovered as Cr(IV)–tannin complex. At the lower pH

range, such as pH 1, the PT gel reduced hexavalent

chromium, Cr(VI), to trivalent chromium, Cr(III),

which could not be adsorbed by the gel. It is, therefore,

possible to recover chromium as Cr(III) ion from a

Cr(VI) solution by the treatment with the PT gel under

suitable conditions.

ARTICLE IN PRESSA. Nakajima, Y. Baba / Water Research 38 (2004) 2859–28642864

4. Conclusion

The PT gel can adsorb hexavalent chromium highly

effectively, while it shows a lower effectiveness for

trivalent chromium. The adsorption of hexavalent

chromium, Cr(VI), by the PT gel was occurred through

the esterification of chromate with catechol group in the

gel. In other words, Cr(VI) should combine with

catechol group (hard base) in the gel as a hard acid,

CrO22+ cation. The trivalent chromium, Cr(III), resulted

through the reduction of Cr(VI) by the PT gel is also a

hard acid. However, the cation can be adsorbed by the

gel only in the higher pH region above 6. The proposed

adsorption mechanism for Cr(VI) adsorption by the PT

gel can well explain, its adsorption characteristics, such

as the pH dependency, the kinetics, and the ESR

profiles. As a whole, the adsorption of heavy metal

ions, such as uranium, iron, gold, vanadium and

chromium by the PT gel can be explained using the

mechanisms based on the two characteristics of the gel,

the strong coupling of the gel with hard acids and the

reduction ability of the gel. Through the treatment of a

Cr(VI) solution with the PT gel, chromium should be

recovered as a Cr(IV)–tannin complex at pH 3 or Cr(III)

solution at pH 1 or lower pH region.

Acknowledgements

This work has been supported by a grant-in-aid for

scientific research from the Ministry of Education,

Culture, Sports, Science and Technology of Japan.

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