inorganic materials lab. skku

Inorganic Materials Lab. SKKUInorganic Materials Lab. SKKU

Sorption Reaction of Aquatic TcO4- or CrO4

2-

on Calcined Mg/Al Layered Double Hydroxide:

Reaction Equilibria and Characterization

Seog Woo Rhee1, Mun Ja Kang2 and Duk-Young Jung*, 1

1 Department of Chemistry, SungKyunKwan University

2 Radioactive Waste Disposal Team, Korea Atomic Energy Research Institute

e-mail: [email protected]@[email protected]

Inorganic Materials Lab. SKKU

Abstract

A layered double hydroxide (LDH) is referred to as anionic clay and easily synthesized in the laboratory. The reconstruction reaction of calcine

d LDH may prove it to be useful for sorbing anionic species from wastes. TcO4- and CrO4

2- are one of hazard elements in the nuclear and industria

l wastes, respectively. The sorption reaction of aquatic TcO4- or CrO4

2- on calcined Mg/Al LDH was investigated. The calcined LDH was prepared

by heating a synthesized Mg/Al LDH to 560 ºC. The batch sorption experiments were carried out in an inert atmosphere and at the constant tempe

rature of 25 ºC. The liquid-solid reaction of TcO4- or CrO4

2- on calcined LDH was proposed to be stepwise ion-exchange reaction: generation of L

DH hydroxide, Mg6Al2(OH)18, as an intermediate and then replacement of the OH- by TcO4- or CrO4

2- ions. The equilibrium constants (K) for ion-

exchange reactions Mg6Al2(OH)18(s) + 2TcO4- = Mg6Al2(OH)16(TcO4)2(s) + 2OH- or Mg6Al2(OH)18(s) + CrO4

2- = Mg6Al2(OH)16(CrO4)(s) + 2OH-

were evaluated by a non-linear least squares fit procedure. The ReO4- was used as a TcO4

- surrogate. Calcined Mg/Al LDH, before and after the so

rption reaction with ReO4- or CrO4

2- was characterized to study reaction mechanism. The analyses by powder X-ray diffraction, 27Al MAS NMR a

nd FT-IR spectroscopy were carried out. The XRD pattern of LDH perrhenate or chromate shows the typical reflections for the layer-structured m

aterials. The NMR and FT-IR spectra reveal that the calcined LDH was reconstructed after intercalation of ReO4- or CrO4

2- in the prepared aquatic

solution. The detailed XRD analysis for the CrO42- intercalated material showed that the hydroxyl group on LDH surface is produced along with L

DH chromate in the reaction solid. This result implies that the liquid-solid reaction of oxometallate on calcined LDH involves the ion-exchange pr

ocess.


Hydrotalcite is rare but naturally occurring mineral. It has a layered structure and anion-exchange capacity. It is

also called layered double hydroxide (LDH).

The layered structure of LDH is destroyed when LDH is calcined. However, this structure is reconstructed with

anionic species such as Cl-, CrO42-, and PO4

3- in aqueous solution. This property of calcined LDH may prove to be u

seful for sorbing anionic species from industrial and nuclear waste.

Technetium-99 is a hazardous element because it exists as appreciable amounts in nuclear waste and has very

long half-life. TcO4- anion is highly soluble and mobile.

Chromate anion is one of a toxic element of the industrial wastewater. It is used by industries such as metal

platting, leather tanning and textile dyeing.

The purpose of this study is to investigate the solid-liquid reaction of TcO4- or CrO4

2- with calcined LDH as an

inorganic sorbent. Emphasis of the work is placed upon understanding the reaction equilibrium and mechanism.

Introduction


Schematic Representation and General Properties

OH

OH

OH

OH

CO32-, nH2O

M(II), M(III)

M(II), M(III)

Schematic representation

General formula of synthetic LDH

M(II)1-xM(III)x(OH)2(Am-)x/mnH2O

M(II) = Mg, Ni, Zn M(III) = Al, Cr, Fe Am- = exchangeable anion 0.2 ≤ x ≤ 0.4

Anion-exchange property : high anion-exchange capacity: 2 5 meq g-1

Mg6Al2(OH)16(Cl)2 + CO32- Mg6Al2(OH)16CO3+ 2Cl-

Memory effect

450600 C Mg6Al2(OH)16CO3 Mg6Al2O9 + CO2 + H2O

An- Mg6Al2O9 + H2O Mg6Al2(OH)16An- + OH-

Use in removing anionic species from wastewater

An- : Cr2O72-, CrO4

2-, HPO42-, TcO4

-, SO42-, MnO4

-


Experimental Sections

Materials & Methods

TcO4- : 0.3M NH4TcO4 / 0.1M NH4OH / 0.01M HTcO4 stock solution

ReO4- : 0.3M NH4ReO4 / 0.01M HReO4 stock solution

CrO42- : 0.1 M Na2CrO4

.4H2O / 0.1M NaOH stock solution

Calcined LDH : Mg/Al system, x = 0.25, calcined at 560 C for 3hours

Batch experiments with inert atmosphere at 25 C

Concentration Determination

TcO4- : Liquid Scintillation Analysis (290keV -radiation of Tc-99)

ReO4-, CrO4

2- : UV-Visible Spectroscopy and ICP-AES

Sorption reactor with inert atmosphere and constant temperature


Sorption Experiments with TcO4- and ReO4

-

MO4- mLDH Vol. [LDH]0

* [MO4

-]0[MO4-]eq [MO4

-]sorbed pH0 pHeq

(mg) (ml) (mol/L) (mol/L) (mol/L) (mol/L)

TcO4- 20 247 2.36E-4 1.44E-5 9.60E-6 4.80E-6 5.80 10.59

51 247 5.99E-4 1.33E-5 6.24E-6 7.06E-6 5.75 10.87102 247 1.20E-3 1.29E-4 4.46E-5 8.44E-5 5.20 10.85100 246 1.18E-3 1.12E-3 2.50E-4 8.67E-4 5.46 10.72151 247 1.78E-3 1.53E-5 4.16E-6 1.11E-5 5.52 10.92148 253 1.70E-3 1.34E-5 6.43E-6 6.97E-6 11.03 11.29144 250 1.67E-3 1.40E-5 1.02E-5 3.81E-6 11.70 11.72147 250 1.71E-3 1.39E-5 1.20E-5 1.87E-6 12.06 12.10

ReO4- 204 490 1.20E-3 1.16E-4 3.29E-5 8.27E-5 5.29 10.78

204 495 1.20E-3 5.14E-4 1.33E-4 3.81E-4 4.93 10.81205 495 1.20E-3 1.14E-3 2.96E-4 8.44E-4 4.82 10.58203 495 1.19E-3 2.00E-3 6.75E-4 1.32E-3 5.23 10.43204 495 1.20E-3 4.62E-3 2.81E-3 1.81E-3 5.46 9.76206 495 1.21E-3 9.55E-3 7.70E-3 1.85E-3 5.39 9.34

Mg6Al2O9 : M = 343.79 g/mol


Sorption Experiments with CrO42-

mLDH Vol. [LDH]0* [CrO4

2-]0 [CrO42-]eq [CrO4

2-]sorbed [OH-]0 [OH-]eq(mg) (ml) (mol/L) (mol/L) (mol/L) (mol/L) (mol/L) (mol/L)

100 100.0 2.909E-3 1.02E-3 2.92E-6 1.02E-3 8.26E-3 10.2E-399.5 100.0 2.909E-3 1.28E-3 4.38E-6 1.28E-3 7.75E-3 10.1E-399.7 100.0 2.909E-3 1.28E-3 5.12E-6 1.27E-3 7.75E-3 10.1E-399.7 100.0 2.909E-3 1.53E-3 1.68E-5 1.51E-3 7.23E-3 10.3E-3100 100.0 2.909E-3 1.79E-3 2.63E-5 1.76E-3 6.71E-3 9.91E-399.8 100.0 2.909E-3 1.79E-3 3.28E-5 1.76E-3 6.71E-3 9.93E-399.5 100.0 2.909E-3 2.04E-3 1.72E-4 1.87E-3 6.20E-3 9.82E-399.8 100.0 2.909E-3 2.04E-3 1.33E-4 1.91E-3 6.20E-3 10.3E-399.6 100.0 2.909E-3 2.04E-3 1.33E-4 1.91E-3 6.20E-3 9.84E-3100 100.0 2.909E-3 2.30E-3 2.42E-4 2.06E-3 5.68E-3 9.64E-399.9 100.0 2.909E-3 2.30E-3 2.82E-4 2.02E-3 5.68E-3 9.44E-3100 100.0 2.909E-3 2.55E-3 5.25E-4 2.03E-3 5.16E-3 9.01E-399.8 100.0 2.909E-3 2.55E-3 4.50E-4 2.10E-3 5.16E-3 9.18E-3100 100.0 2.909E-3 2.81E-3 6.51E-4 2.16E-3 4.65E-3 8.77E-399.9 100.0 2.909E-3 2.81E-3 7.22E-4 2.09E-3 4.65E-3 8.63E-399.5 100.0 2.909E-3 3.06E-3 1.01E-3 2.05E-3 4.13E-3 7.96E-3100 100.0 2.909E-3 3.32E-3 1.14E-3 2.18E-3 3.61E-3 7.75E-3100 100.0 2.909E-3 3.32E-3 1.18E-3 2.14E-3 3.61E-3 7.54E-399.6 100.0 2.909E-3 3.57E-3 1.36E-3 2.21E-3 3.10E-3 7.15E-3100 100.0 2.909E-3 3.57E-3 1.36E-3 2.21E-3 3.10E-3 7.13E-3100 100.0 2.909E-3 3.83E-3 1.56E-3 2.27E-3 2.58E-3 6.70E-3100 100.0 2.909E-3 3.83E-3 1.68E-3 2.15E-3 2.58E-3 6.64E-399.7 100.0 2.909E-3 4.09E-3 1.91E-3 2.18E-3 2.07E-3 6.15E-3100 100.0 2.909E-3 4.34E-3 2.03E-3 2.31E-3 1.55E-3 5.71E-399.8 100.0 2.909E-3 4.34E-3 2.23E-3 2.11E-3 1.55E-3 5.75E-399.6 100.0 2.909E-3 4.60E-3 2.29E-3 2.31E-3 1.03E-3 5.25E-399.7 100.0 2.909E-3 4.85E-3 2.56E-3 2.29E-3 5.16E-4 4.76E-399.7 100.0 2.909E-3 5.11E-3 2.75E-3 2.36E-3 6.31E-6 4.33E-399.4 100.0 2.909E-3 5.11E-3 2.77E-3 2.34E-3 6.31E-6 4.30E-399.4 100.0 2.909E-3 5.11E-3 2.77E-3 2.34E-3 6.31E-6 4.24E-399.7 100.0 2.909E-3 5.11E-3 2.75E-3 2.36E-3 6.31E-6 4.27E-3

Mg6Al2O9 : M = 343.79 g/mol


Evaluation of Equilibrium Constants for MO4- (M: Tc or Re)

10-3

10-2

10-1

100

101

10-3 10-2 10-1 100 101 102 103

[MO4-]eq / [OH-]eq

ReO4-[M

O4- ]

sorb

ed /

[LD

H] o

TcO4-

Equilibrium reaction

K1

Mg6Al2(OH)18(s) + MO4- Mg6Al2(OH)17(MO4)(s) + OH-

K2*

Mg6Al2(OH)17(MO4)(s) + MO4- Mg6Al2(OH)16(MO4)2(s) + OH-

Equilibrium constants K1, K2

[LDH(MO4)(s)]eq[OH-]eq

K1 = [LDH(s)]eq[MO4

-]eq

[LDH(MO4)2(s)]eq[OH-]eq2

K 2 = = K1K2

[LDH(s)]eq[MO4-]eq

2

Evaluation of Kn

[MO4-]eq [MO4

-]eq2

K1 + 2K2 [MO4

-]sorbed [OH-]eq [OH-]eq

= [LDH]0

[MO4-]eq [MO4

-]eq2

1 + K1 + K2 [OH-]eq [OH-]eq

K1 = 1.40 0.11 K2 = 0.47 0.20

Plot of [MO4-]sorbed/[LDH]0 vs [MO4

-]eq/[OH-]eq


Evaluation of Equilibrium Constants for CrO42-

Plot of [CrO42-]sorbed/[LDH]0 vs [CrO4

2-]eq/[OH-]eq2

10-3

10-2

10-1

100

101

10-3 10-2 10-1 100 101 102 103

[CrO

42- ]

sorb

ed /

[LD

H] o

[CrO42-]eq / [OH-]eq

2

Equilibrium reaction of ion-exchange

K Mg6Al2(OH)18(s) + 2 CrO4

2- Mg6Al2(OH)17(CrO4)2(s) + 2OH-

Equilibrium constants K

[LDH(CrO4)(s)]eq[OH-]eq2

K = [LDH(s)]eq[CrO4

2-]eq

Evaluation of K

[MO4-]eq

K [CrO4

2]sorbed [OH-]eq2

= [LDH]0

[MO4-]eq

1 + K [OH-]eq

2

- Evaluation of K by non-linear least squares fit:

K = 25.3 3.5


Relative Fraction of Sorbed MO4-

0.0

0.2

0.4

0.6

0.8

1.0

0.01 0.1 1 10 100

[MO4-]eq / [OH-]eq

n = 1 n = 2

n [L

DH

(MO

4)n(

s)] e

q / [

MO

4- ]so

rbed

Relative fraction of sorbed MO4-

[LDH(MO4)(s)]eq K1xn = 1 : = [MO4

-]sorbed K1x + 2K2x2

2[LDH(MO4)2(s)]eq 2K2x2

n = 2 : = [MO4

-]sorbed K1x + 2K2x2

where x = [MO4-]eq/[OH-] eq


Relative Fraction of LDHs

Relative fraction of LDHs

[LDH(s)]eq 1n = 0 : = [LDH]0 1 + K1x + K2x2

[LDH(MO4)(s)]eq K1xn = 1 : = [LDH]0 1 + K1x + K2x2

[LDH(MO4)2(s)]eq K2x2

n = 2 : = [LDH]0 1 + K1x + K2x2

where x = [MO4-]eq/[OH-] eq

0.0

0.2

0.4

0.6

0.8

1.0

0.01 0.1 1 10 100

[MO4-]eq / [OH-]eq

n = 1

n = 2

[LD

H(M

O4)

n(s)

] eq

/ [L

DH

] o n = 0


Spectroscopic Results of LDHs

FT-IR spectra 27Al MAS NMR

LDH(CO3)

calcined LDH

LDH(CrO4)

LDH(CO3)

calcined LDH

LDH(CrO4)

TdOh


Characteristics of Solid Phases

Powder XRD patterns of LDHs

0 10 20 30 40 50 60 70

2 ( o )

7.8

2

3.8

9

2.6

0

2.3

4

1.8

9

1.5

31

.50

2.1

1

1.4

9

4.7

8

1.5

3

(a)

(b)

(c)

0 10 20 30 40 50 60 70

LDH(ReO4)2

LDH(OH)2

(a)

(b)

(c)

(d)

(e)

2 ( o )

LDH(CO3)

calcined LDH

LDH(ReO4)2

LDH(OH)2

Mixture of LDH(OH)2 & LDH(ReO4)2

LDH(ReO4)2


Schematic Model for Anion-Exchange

3 + + 6

parallel

serial

hydroxide ion chromate ion

Schematic model for anion-exchange Powder XRD patterns of LDHs

10 20 30 40 50 60

LDHCrO4

LDH(OH)2

2 , (o)

(a)

(b)

(c)

(d)

(e)

Mixture of LDH(OH)2 & LDH(CrO4)

LDH(CrO4)

LDH(OH)2


Conclusions

The sorption of TcO4-, ReO4

- or CrO42- on calcined LDH is found to be a stepwise ion-exchange reaction: ge

neration of LDH hydroxide, Mg6Al2(OH)18, as an intermediate and then replacement of the OH- by TcO4-,

ReO4- or CrO4

2- ions.

The equilibrium constants for ion-exchange reaction are obtained by a non-linear least squares fit procedure.

MO4- (M=Tc or Re): K1 = 1.40 0.11 K2 = 0.47 0.20

CrO42- : K = 25.3 3.5

The results of powder XRD, FT-IR and 27Al MAS NMR spectroscopy reveal that the layered structure de

stroyed by calcination is reconstructed after intercalating ReO4- or CrO4

2- in the aqueous salt solution.

The XRD patterns of mixture of LDH(CrO4) and LDH(OH)2 demonstrate that an intermediate of the sor

ption process is LDH hydroxide. It is proposed that the CrO42- ion incorporates in the interlayer through

a parallel route.

inorganic materials lab. skku

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