comparison of adsorption and post-adsorption behavior of

1

Comparison of adsorption and post-adsorption behavior of oxyanions between ferrihydrite and schwertamnnite

Khandala Khamphila

Candidate for the Degree of doctoral Supervisor: Prof.Tsutomu Sato

Division of sustainable resources engineering

Introduction

Water contamination is a serious problem around the

world. Many toxic elements such as arsenic, chromium

and selenium are seriously problem in the surface and

groundwater, because most of them are oxyanions in the

natural water and they are highly mobiles over a wide

range of redox conditions.

The migration of dissolved trace species in surface

condition are initially retarded by the adsorption process

at mineral water interface. Especially, low crystalline

ferric oxides are known to be the most effective

scavengers for these species. The interaction of low

crystalline ferric oxide with cationic metal species have

been widely investigated by laboratory experiments and

field observations. However, these with anionic species

have been lacking information investigated due to their

complex behavior in the natural system. Ferrihydrite

and schwertmannite are iron oxide minerals and low

crystalline minerals had high specific surface area [1],

[2]. Ferrihydrite and schwertmannite occur in natural

with different pH conditions, ferrihydrite precipitated in

soil and sediment with pH in neutral conditions,

schwertmannite occurrences in pH acidic condition in

mining tailing [3], there are different with surface

properties, ferrihydrite surface are hydroxyl group with

had ligands exchange with oxyanions. Meanwhile,

schwertmannite has hydroxyl group and SO4 group and

there is some information adsorption arsenate [4], [5].

The low crystalline Fe(III) oxide are metastable phase

and eventually transform to more a stable phase with

time. Therefore, it is important to understand the

stability of the host mineral and sorbed species for the

prediction and assessments of long term behavior of

anionic species. Ferrihydrite and schwertmannite both

are metastable phase and transform to stable phase as

goethite by dissolution and reprecipitation. Ferrihydrite

is known as a ferric iron oxide mineral, which is highly

effective for waste water treatment and has application

to predict the adsorption capacity such as surface

complexation modeling, meanwhile, information is

lacking in which surface complexation modeling of

schwertmannite. The comparison of both ferrihydrite

and schwertmannite is important to better understand

the adsorption and post-adsorption properties.

Particularly, this study aims to

To understand the differences and similarities

in adsorption behavior of oxyanions by

ferrihydrite and schwertmannite

To understand the different and similarities in

post-adsorption behaviors of oxyanions

between ferrihydrite and schwertmannite

To apply surface complexation modeling for

different kinds of oxyanions on ferrihydrite and

schwertmannite to check applicability of

existing double layer model.

Method and material

Preparation of adsorbents

Ferrihydrite was prepare as adsorbent by using digital

titration machine TOADKK-AUT-701, 0.1M of

Fe(NO3)3.9H2O, 500 ml was prepared and set on the

stirrer, 0.1 of KOH solution was also prepared about

500 ml set into the machine, Auto titration was added

0.04ml/min (KOH) to the solution, until reach to pH7

[6], sample was centrifuge 3000rpm, 40 minutes’ wash

in 6 times, then filtered through 0.2µm cellulose

membrane. The resulting of solids was freeze dried and

identify the synthesized phases solids was examined by

Rigaku X-Ray diffractometer with CuK radiation

(40kV and 40 mA), the result of XRD pattern was

identical to that of the previously reported of

ferrihydrite [7]. Schwertmannite was prepared by the

method previously reported by Bigham et al [8]. Mixing

solution prepared by 0.04 M Na2SO4 solution and 0.04

M Fe(NO3)3.9H2O solution was held at 60C for 12

minutes, then cooled and dialyzed for 30 days,

deionized water used for the dialyzing was changed

every day. To remove the salt in the surface of mineral

the product was clean by using deionized water and was

filtered through a 0.2 µm cellulose membrane then

immediately freeze-dried to prevent transformation to

another phase. Further, X-ray diffraction (XRD)

analyses were conducted to identify the synthesized

phases by Rigaku X-Ray diffractometer with CuKα

radiation (40kV and 40mA). From the XRD analyses,

the synthesized products were identified as a

schwertmannite because the XRD pattern was identical

to that of the previously reported schwertmannite by

Bigham et al [2].

Adsorption experiment

All adsorption experiments were conducted at a

constant ionic strength (I=0.01 M, NaNO3) by using a

50 ml centrifuge tube adding 40 ml solution of

Na2HPO4, Na2HAsO4, Na2CrO4, and Na2SeO4 with

concentrations from 0 to 2 mM and 40 mg of the

synthetic schwertmannite and ferrihydrite. The pH of

the adsorption media was adjusted to 7.00 ± 0.15 by

0.1M NaOH solution. The samples were placed in a

2

reciprocal shaker at 25C, 100 rpm for 24 hours. The

adsorption experiments for surface complexation

modeling were performed as function of pH from pH 3

to pH 12. The filtered solids were freeze-dried for

measurements of Zeta potential (Malvern Zetasizer

Nano series Nano-ZS90 instrument) and liquid samples

were used for the inductively coupled plasma atomic

emission spectroscopy, (ICPE-9000, ICP-AES) and ion-

chromatography (Metrohm 861 Advanced Compact IC

instrument) to determine the concentration of elements

after the adsorption. The released SO42-

in the

equilibrium solutions were also determined for

schwertmannite.

Alteration experiment

The dried powders of schwertmannite and ferrihydrite

were added to the solutions of the anions, for the

adsorption process, the pH was adjusted by 0.1 M

NaOH and 0.1 HNO3 to 7.00±0.12 and 30-35 mg/g of

the solid phase of anions of arsenate, phosphate,

chromate or selenate were adsorbed on the

schwertmannite, separately. The solids after the

adsorption were mounted and dried on silica glass. The

glasses with the mounted solid samples were kept in

boxes with wet cotton at 60C to accelerate alterations

in a moisture condition. At the different aging times, the

samples were analyzed by XRD to determine the extent

of the phase transformation.

Result and discussion

Adsorption behavior of oxyanions onto ferrihydrite and schwertmannite

The result of adsorption capacities between ferrihydrite

and schwertmannite, under these conditions showed that

the schwertmannite’s adsorption capacity is higher than

the ferrihydrite’s adsorption capacity. However, the

adsorption selectivity of oxyanion adsorption on both

schwertmannite and ferrihydrite decreases in the

following order: arsenate phosphate > chromate

>>selenate (Figure 1). The change of zeta potential for

ferrihydrite and schwertmannite before and after

adsorption shown the changing of zeta potential, when

decreasing of zeta potential consistent with previous

work which explain the mechanism of oxyanion

adsorption. Arsenate, phosphate might form inner-

sphere complexes with the surface of ferrihydrite and

schwertmannite. The different behavior is the selenate

and sulfate ions form outer-sphere complexes with the

surface of schwertmannite. Chromate in between

arsenate and selentate with surface of schwertmannite

assumed that chromate might make intermediate

complexes. For ferrihydrite chromate and selenate

might be made similar behavior as intermediate on

ferrihydrite. Strong base anions such as arsenate and

phosphate can form inner-sphere complexes, which

induces a strong adsorption with ferrihydrite and

schwertmannite as well as provides a high adsorption

capacity (Error! Reference source not found.)

________________________________________________________________________________________________

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0 0.5 1 1.5

Ad

sorp

tio

n (

mo

l/m

2)

Initial concentration (mmol)

Ferrihydrite

a

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0 0.5 1 1.5

Ad

sorp

tio

n (

mo

l/m

2)

Initial concentration (mmol)

Schwertmannite

b

0

20

40

60

80

100

0 0.5 1 1.5

Figure 1. Oxyanions adsorption on ferrihydrite and schwertmannite as function of the initial concentration of oxyanion

in solution, with pH adjustment (: Arsenate, : Phosphate, : Chromate and : Selenate)

3

-60

-40

-20

0

20

40

3 4 5 6 7 8 9 10 11

Zet

a p

ote

nti

al

(mV

)

pH

-60

-40

-20

0

20

40

3 4 5 6 7 8 9 10 11

Zet

a P

ote

nti

al

(mV

)

pH

(a) (b)

Figure 2. The zeta potential with function of pH, (a) display Zeta potential for ferrihydrite, (b) display zeta potential

for schwertmannite (: Original, : Arsenate, : Phosphate, : Chromate and : Selenate)

________________________________________________________________________________________________

Post-adsorption behavior of oxyanions onto ferrihydrite and schwertmannite

The post-adsorption behavior of oxyanion onto

ferrihydrite and schwertmannite was investigated. To

better understand the stabilization of minerals. The

solubility of schwertmannite with different oxyanions

was calculated by the solid solution theory. A

comparison of post-adsorption behavior between

schwertmannite and ferrihydrite showed that solubility

of ferrihydrite is lower than schwertmannite’s solubility

that is why ferrihydrite is more stable than

schwertmannite shown in XRD pattern (Error!

Reference source not found. and Error! Reference

source not found.). In case of comparison of other

oxyanions adsorption on schwertmannite showed that

the degree of retardation in transformation to goethite

decreased as

arsenate=phosphate>chromate>selentate>sulfate

(Error! Reference source not found.). The solubility

of mineral after adsorption increase in the following

order: arsenate<phosphate<chromate

<selenatesulfate.

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Fh-Chromate

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Fh-Selenate

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Ferrihydrite

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Fh-Phosphate

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Fh-Arsenate

º2CuK º2CuKº2CuKº2CuKº2CuK

Figure 3. X-ray diffractogram of synthetic ferrihydrite and ferrihydrite after adsorption of each oxyanion with different

aging times

4

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Chromate

G

G G

G=Goethite

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Selenate

G

GGG

G=Goethite

G

10 20 30 40 50 60 70

7 days

14 days

21 days

30 days

GG G

GG

G

Sulfate

G

G

0 day

G=Goethite

G

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Phosphate

10 20 30 40 50 60 70

0 day

7 days

14 days

21 days

30 days

Arsenate

º2CuK º2CuKº2CuKº2CuKº2CuK

Figure 4. X-ray diffractogram of the synthetic schwertmannite and schwertmannite after adsorption of each oxyanion

with different aging times.

________________________________________________________________________________________________

Therefore, oxyanions with a high selectivity can

stabilize schwertmannite by lowering the solubility of

schwertmannite after adsorption of oxyanions. The

similar characteristic of post-adsorption is the oxyanions

effected to stabilization of both minerals. The different

shown ferrihydrite’s solubility is lower than

schwertmannite’s solubility. But in pH acidic condition

to pH neutral condition schwertmannite’s solubility

decreased to similar with ferrihydrite’s solubility.

-10

-9

-8

-7

-6

-5

-4

-3

-2

3 4 5 6 7 8 9 10 11

log

tota

l F

e(II

I) a

ctiv

ity

pH

Solubility of Fe

Ferrihydrite Anion free, I=0.01 (Fukushi et al, 2005)

Ferrihydrite containing SO4 (Fukushi et al, 2005)

Ferrihydrite containing PO4 (Fukushi et al, 2005)

Ferrihydrite containing As(V) (Fukushi et al, 2005)

Ferrihydrite containing Cr(VI) (This study)

SCH anion free, I=0.01 (This study)

SCH containing Cr(VI) (This study)

SCH containing PO4 (This study)

SCH containing As(V) (This study)

Figure 5. Solubility of diagram of Fe(III) for

ferrihydrite and schwertmannite

Surface complexation modeling for many kinds of oxyanions onto ferrihydrite and schwertmannite

The surface complexation modeling which is known as

a theoretical method and a tool for prediction of

adsorption in the natural system was applied. However,

ferric oxide has already been established in many of the

adsorption conditions, such as ferrihydrite adsorbing

arsenate and phosphate as inner-sphere complexes [9],

they was modeled by extended triple layer modeling

(ETLM).

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

ctio

n (

%)

pH

Ferrihydrite, 1 g L-1

1 mM As(V), 0.1 M NaNO3

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11A

dso

rpti

on f

ract

ion

(%

)pH

Ferrihydrite, 1g L-1

Se(VI) 1mM, 0.1 M NaNO3

Figure 6. DLM of arsenate adsorption onto ferrihydrite

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

ctio

n (

%)

pH



0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

ctio

n (

%)

pH



Figure 7. DLM of selenate adsorption onto ferrihydrite

5

Meanwhile, adsorption information is lacking for

schwertmannite. The double layer modeling (DLM) was

performed following from previously study [10], by

using REACT in the Geochemist’s workbench (GWB)

[11], the result of arsenate adsorption onto ferrihydrite

with DLM, shown in Figure 6 the point is experimental

data and solid line is model, the experiment data was

fitting well with the model. The DLM was also applied

for selenate base on the data base of previously study

[10] by using REACT in GWB program. As shown in

the Error! Reference source not found., the

experiment data was not fitted well with this model

which just fit some of the data base, because selenate

was involved both surface species equation which are

inner-sphere and outer-sphere complexes. To better

fitted model, in this present study, extended triple layer

modeling (ETLM) was applied following previously

study [12], [13] for arsenate, chromate and selenate

adsorption onto ferrihydrite (Error! Reference source

not found., Error! Reference source not found. and

Error! Reference source not found.). The speciation

reaction equation for arsenate are following here:

O2HHAsOFeO)(AsOHFeOH2 222

0

43 (1)

O2HAsOOHFeO)(AsOHFeOH2 22

0

43 (2)

OHH2FeOAsOAsOHFeOH 2

2

3

0

43 (3)

The speciation reaction equation for selenate are

following here:

OHFeOSeOSeOHFeOH 23

-2

4 (4)

2

422

-2

4 _SeO)FeOH(SeOH2FeOH2 (5)

2

42

-2

4 _HSeOFeOHSeOH2FeOH (6)

The speciation reaction equation for chromate are

following here:

OHOOHFeOCrHCrOHFeOH 224 (7)

4224 _HCrO)FeOH(HCrO2HFeOH2 (8)

424 _HCrOFeOHHCrOHFeOH (9)

In case of applied ETLM onto schwertmannite for

oxyanions, in this present study surface protonation and

electrolytes adsorption equilibrium constants and

capacitances was calculated following previous study of

ferrihydrite. Because from the comparison adsorption

capacities, oxyanions selectivity and surface speciation

of ferrihydrite and schwertmannite. As known that

schwertmannite and ferrihydrite are precipitate in

mining site whereas rich iron; therefore, the different is

schwertmannite had SO4 sorbs in the tunnel structure. Although, the between schwertmannite and ferrihydrite

had similar, but additional to apply schwertmannite

ETLM, some reaction should be included

OHFeOSOSOHFeOH 23

2

4 (10)

42

2

4 _HSOFeOHSOH2FeOH (11)

As shown in Error! Reference source not found.

arsenate adsorption onto schwertmannite, the

experiment data was fitted to the model. In Error!

Reference source not found., dash line is the

estimating modeling, in this model involve only two

species surface reaction of chromate inner-sphere

complexes and bidentate outer-sphere complexes. From

the result of estimated modeling was fitted to the

experiment data.

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ra

cti

on

%

pH


0.2 mM As(V), 0.01 M NaNO3

0.6 mM As(V), 0.01M NaNO3


0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ra

cti

on

%

pH


0.2 mM As(V), 0.1 NaNO3


1 mM As(V), 0.1M NaNO3

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ra

cti

on

%

pH




0

20

40

60

80

100

3 4 5 6 7 8 9 10

% A

rse

na

te s

pecie

s

pH



(>FeO)2AsO2-

HAsO4--

(>FeO)2AsOOH

>FeOAsO32-

H2AsO4-

a

dc

b

Figure 8. ETLM of arsenate adsorption onto

ferrihydrite

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

ctio

n %

pH


0.2 mM Se(VI), 0.01M NaNO3

0.6 mM Se (VI), 0.01M NaNO3

1 mM Se(VI), 0.01M NaNO3

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ract

ion

%pH

Ferrihydrite,1g L-1




0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ract

ion

%

pH




0

20

40

60

80

100

4 5 6 7 8 9 10

% S

elen

ate

sp

ecie

s

pH



(>FeOH2+)_SeO4

-

NaSeO4-

SeO4--

>FeOH2+_HSeO4

-

>FeOSeO3-

a

dc

b

Figure 9. ETLM of selenate adsorption onto ferrihydrite

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

cti

on

%

pH


0.2 mM Cr(VI), 0.01 M NaNO3

0.6 mM Cr(VI), 0.01M NaNO3

1 mM Cr(VI), 0.01M NaNO3

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fracti

on

%

pH


0.2 mM Cr(VI), 0.1 M NaNO3



0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fracti

on

%

pH




0

20

40

60

80

100

4 5 6 7 8 9 10

% C

hro

ma

te s

pecie

s

pH

>FeOHCrO4-

>FeOH2+_HCrO4

-

HCrO4-

CrO4--


1 mM Cr (VI), 0.1 M NaNO3

a

dc

b

Figure 10. ETLM of chromate adsorption onto

ferrihydrite

6

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ract

ion

%

pH

Schwertmannite, 1 g L -1



0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tion

fra

ctio

n %

pH




0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ract

ion

%

pH




0

20

40

60

80

100

3 4 5 6 7 8 9 10 11 12

% A

rsen

ate

sp

ecie

s

pH

Schwertmannite, 1 g L-1


1.46 mM SO4-- sorbing on

H2AsO4-

HAsO4--

(>FeO)2AsO2-

>FeOAsO3-

>FeOSeO3-

SO4--

(>FeOH2+)2_SO4

-

a

dc

b

Figure 11. ETLM of arsenate adsorption onto

schwertmannite

0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ra

cti

on

%

pH




0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ra

cti

on

%

pH




0

20

40

60

80

100

3 4 5 6 7 8 9 10 11

Ad

sorp

tio

n f

ract

ion

%

pH




0

20

40

60

80

100

4 5 6 7 8 9 10 11

% C

hro

ma

te s

pecie

s

pH


>FeOCr(OH)O2

>FeOSO3-

>FeOH2+_SO4

2-

>FeOH2+_HCrO4

-

CrO42-

SO42-

>(FeOH2+)2_HCrO4

-

1 mM Cr(VI), 0.1 M NaNO3

a

d

c

b

Figure 12. ETLM of chromate adsorption onto

schwertmannite

Conclusion

Oxyanions were classified that arsenate and phosphate

are inner-sphere complexes, chromate, selenate and

sulfate are intermediate complexes. The ferrihydrite’s

solubility is lower than schwertmannite’s solubility

indicate that ferrihydrite is more stable than

schwertmannite. Oxyanions were effected to

stabilization of both minerals. In the natural is complex

system, surface complexation model as ETLM is useful

to predicted the adsorption capacities for oxyanions

adsorption on ferrihydrite and schwertmannite. In the

natural water treatment systems for both acid mine

drainage and ground water system, schwertmannite is

the appropriate material use for water treatment system,

because schwertmannite had high adsorption capacities.

To choose the materials are depending on exciting of

the materials near in the water treatment system site and

the concentration of the toxic elements. In case of

disposal site for materials, if used schwertmannite

adsorbs arsenate and phosphate, the disposal of

adsorbent will be safe. Base on the elementary

properties of cation and base on this studies result, other

oxyanions which had similar properties such as arsenate

and vanadate may have made inner-sphere complexes;

selenate, manganate and chromate may have made

intermediate complexes, but to better understand we

need experiments to support this prediction. For

oxyanions, which made intermediate and outer-sphere

complex we can consider to choose ferrihydrite and use

surface complexation modeling to predict the

concentration of adsorbents.

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Organic Anions on the Crystallization of

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comparison of adsorption and post-adsorption behavior of

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