Fe and associated As(V) reactivity

in wetland soil : Kinetic modelling

approach

Mélanie Davranche, Aline Dia, Mohamad Fakih,

Bernd Nowack, Guillaume Morin, and Gérard Gruau

EMEC 2010

Why focusing effort on Fe (III)-oxyhydroxides behaviour understanding?

Key factor on (bio)geochemical process controlling trace metal distribution in soils and waters

Ubiquitous in soils, sediments, geological materials Major trace elements carrier Fe Redox behaviour controls trace element mobility

Study focus : Reductive dissolution in wetland soil system

Fe oxide reactivity upon dissolution depend on:

•crystal structure•crystal size distribution.

All modifications changes of the reaction kinetic and subsequent reactivity

Kinetic : Alternative to the classical chemical Kinetic : Alternative to the classical chemical extractions or thermodynamic methods. extractions or thermodynamic methods.

Why studying Fe reactivity with a kinetics approaches?

Methodological procedure

Fe-oxides fixed on a slide system

Quantitative dissolution analyses (XRF)Quantitative dissolution analyses (XRF) Mineralogical analysis (DRX, EXAFS)Mineralogical analysis (DRX, EXAFS)

Fe-oxides : Fe-oxides : ferrihydrite and lepidocrocite Different crystallinity Contrasted surface areas

Associated element : Arsenic (V) Potentially toxic Redox sensitive

Iron oxide

2mm

2cm

0.1mm

Iron oxide

2mm

2cm

0.1mm

2mm

2cm

2mm

2cm

0.1mm

2 cm

2 cm

Methodological procedure

Experimental insight :Anoxic Incubation in equilibrium soil column

-Soil sample : organo-minral horizon of the Naizin Kervidy wetland soil

- Soil Solution analysis : pH, Eh, As(V)*, As(III)*, Fe(II), NO3, SO4, acetate

Peristaltic pump

Synthetic solution

Soil

0.6 mol.L-1

Slide+Fe-oxides

Stirrer

References experiments

• As-ferrihydrite: As-Fh (Bacteria) (Autochtonous bacteria), (Burnol at al., 2007)

• As-Ferrihydrite : As-Fh(Ascorbate) (Ascorbic acid)

• Ferrihydrite : Fh (Ascorbic acid and S. Putrefaciens) (Roden, 2006)

• As-lepidocrocite: As-Lep (Ascorbic acid)

• Lepidocrocite : Lep (S. Putrefaciens)

Kinetic framework

J

m0k'(

m

m0)?

dt

dmJ

Non linear least-square regression

γ and k’

(Postma, 1993)

Generalized rate law

As-Fh

As-Lp

Applied to mineral dissolution

As-Lp:

J

m00.047(

m

m0)1.13As-Fh:

J

m00.015(

m

m0)0.18

γ

Undissolved mineral fraction

Rate of dissolution

Depends on morphology, size distribution and reactive site density of the oxide during dissolutionRate constant

m

m0

γ =

k’=

J

m0

NO3- , Fe(II) , SO4

2- , and Acetate

Typical redox evolution of waterlogged soils

-1

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200 1400 1600

Temps (heure)

Fe

(II)

, Fe

(tot

), N

O3-

et S

O42

- (m

g.L

-1)

0

50

100

150

200

250

Acé

tate

(m

g.L

-1)

N03

S04

Fe (II)

Fe (tot)

Acétate

Time (hours)

Ace

tate

(m

g L

-1)

Fe(

II),

Fe(

tot)

, NO

3- and

SO

42- (

mg

L-1)

Reductive dissolution

‘Slide’ system macroscopic observations

Progressive dissolution of Fe-oxides stuck onto slides

Reduction

1 week

- +

2 monthsTime

SEM observations

Bacterial colonization

Thick biofilms

Surface alteration

Diversity of newly formed minerals

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80Time (d)

mt/

m0

As-Fh(ascorbate)Fh-(ascorbate)As-Fh(bacteria)Fh-(bacteria)As-Fh(soil)

-11

-9

-7

-5

-3

-1

1

0 0.2 0.4 0.6 0.8 1-log (mt/m0)

log

(J/m

0)

As-Fh(ascorbate)Fh-(ascorbate)Fh-(bacteria)As-Fh(soil)As-Fh(bacteria)

not total Fe dissolution with bacteria

• same initial dissolution rate with bacteria

•Intermediary Reductive dissolution in soil limited Fe decreasing reactivity

Ferrihydrite kinetical modeling

Lepidocrocite kinetical modeling

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80Time (d)

mt/

m0

As-Lep-(Ascorbate)Lep-(bacteria)As-Lep(soil)

-2.2

-1.8

-1.4

-1

-0.6

-0.2

0.2

0 0.2 0.4 0.6 0.8 1-log(mt/m0)

log

(J/m

0)

As-Lep-(Ascorbate)Lep-(bacteria)As-Lep (soil)

Total As dissolution whatever the reducing agent

• lower initial dissolution rate in soil

•Intermediary Reductive dissolution in soil limited As decreasing reactivity

As kinetical modeling

-4

-3

-2

-1

0

1

0 0.2 0.4 0.6 0.8 1

-log(mt/m0)

log

(J/m

0)

As-Fh(ascorbate) As-Fh(soil) As-Lep (soil)As-Lep(ascorbate) As-Fh(bacteria)

Discussion

Ferrihydrite:

•Fe decreasing dissolution rate with bacteria:

- Fe(II) readsorption

- Secondary mineral precipitationIn soil : Fe(II) complexed by dissolved organic matter

and soil mineral Limited Fe(II) readsorption and subsequent newly Limited Fe(II) readsorption and subsequent newly

mineral formationmineral formation

0

100

200

300

0,2 µm 10 kDa 5 kDa 2 kDacutting size

DO

C (

mg

L-1

)

0

20

40

60

Fe(

II) a

nd

As

T (

mg

L-1

)

DOCAsTFe(II)

Fe(II) bound to dissolved organic matterFe(II) bound to dissolved organic matter

Discussion

Lepidocrocite :

•Lower Fe decreasing dissolution rate than Fe from ferrihydrite:

- stronger solubility

- lower Fe(II) readsorption (lower surface aera) and subsequent secondary mineral precipitation

•In soil : lower initial rate dissolution : soil bacteria consortium

Discussion

As:

•As and Fe have closed dissolution rate : Fe reactivity control in part As reactivityFe reactivity control in part As reactivity

•As stronger solubilized from lepidocrocite than Ferrihydrite

•Ferrihydrite : As lesser solubilized than Fe

• As readsorbed on newly formed mineral

Conclusion

•Kinetic modeling relaible to predict coprecipitated As reactivity from Fe-oxide dissolution

•In wetland soil : organic matter controlled

• Fe reactivity from ferrihydrite dissolution

•As reactivity from ferrihydrite dissolution

•In wetland soil : As readsorption on secondary phases (hygher for ferrihydrite than lepidocrocite)

Are Wetland soils source of As for hydrosystems ?Are Wetland soils source of As for hydrosystems ?

Newly formed phases

New Fe- and S-rich minerals evidenced: iron sulphides?

None other minerals formed in simple experimental system.

- Ex. green rusts, magnetite, vivianite or siderite

• Blocking Fe-oxide surface sites by adsorption• Complexing Fe(II) preventing re-adsorption and re-precipitation onto Fe-oxides

Key control of organic oxy-anions (such as acetate >240 mg L-1)

Conclusions Important bacterial colonization and biofilms occurrence

b

i

o

l

o

g

i

c

a

l

l

y

-

m

e

d

i

a

t

e

d

p

r

o

c

e

s

s

e

s

Two dissolution ways : Fh -2D & Lp-3D

Dissolution rates remained fairly constant through time :

F

e

(

I

I

)

-

M

O

c

o

m

p

l

e

x

e

s

Prevention of Fe(II) adsorption and hygh Fe phases precipitation

Secondary minerals (Iron sulphides ?) ,

Arsenic behaviour:

As(V) As(III) => Bacterial reduction Fh Re-adsorption

Lp Destruction of adsorption sites Release or adsorption onto other soil minerals