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