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Slide 1

Eva Walpersdorf, Lisa Heiberg, Charlotte Kjærgaard, Christian B. Koch, David O'Connell, Henning S. Jensen, Hans Christian Bruun Hansen

University of Copenhagen, University of Aarhus and University of Southern Denmark

Does vivianite (FeII3(PO4)2 8H2O) control

phosphate solubility in anoxic soils?

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Slide 2

What is vivianite?

Vivianite as P trap

• Vivianite is an Fe(II)-phosphate (Fe3(PO4)2 8H2O)

• Commonly found in lake sediments, sewage sludges, river sediments

• Forms under non-acid conditions, and at "high" Fe2+ concentrations

• When FeII is oxidized to FeIII vivianite turns blue → metavivianite and santabarbarite

Vivianite

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Slide 3

Phosphate retention in anoxic soils

Vivianite as P trap

• Lowland soils potentially accumulates much P

• Retention due to sorption, precipitation, and plant uptake

• P retention is strongly linked to Fe biogeochemistry, and reduction of FeIII oxides may strongly affect retention

Oxic

Anoxic

P in solution (µM)

Sandy meadow soil

Heiberg et al. (2010)

• Distinct increase in Pi

solubility when soil is reduced

• Higher sorption in anoxic soil at high Psol concentrations!

→ Does vivianite precipitate?

EPC0

Pi sorption isotherm

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Slide 4

Theory: vivianite solubility and Pi retention

Vivianite as P trap

+ −+ + €2 34

3 2 2

3Fe 2PO 8H2O

Fe (PO4) 8H O

⇒ potential high-affinity Pi

sorption, low EPC0

⇒ points to vivianite as a very important "sorbent" in anoxic soils

Solubility product for vivianite: 10-35.8

(Al-Borno & Tomson, 1994)

EPC0 = 0.12 µM

Conditions: pH 7.0, PCO2 0.00036 atm, Fe2+ 200 µM, Ca2+500 µM, Mg2+ 100 µM, Cl- 1000 µM.

MINTEQ modelled Pi

“sorption isotherm”

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Slide 5

Experimental work

Vivianite as P trap

Main question: Does vivianite reach

solubility equilibrium?

A peat soil from Northern Jutland,

Denmark examined.

•Soil material → chemical and

mineralogical analyses

•Pore water analyses:

10 times, Nov. 2009 - June 2010

4 depths (30 - 135 cm)

• Material from the gyttje layer used for lab investigations:

- dissolution experiment: solubility of vivianite

- sorption isotherm experiment: precipitation of vivianite

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Slide 6

Field survey

Vivianite as P trap

Noerre river valey

The soil profile

Piezometer nest for water sampling

A

Box

Bred

Gyttje

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Slide 7

Field survey

Vivianite as P trap

Soil core with white speckles of vivianite

Soil aggregate with blue stains of oxidized vivianite (24 h after O2 exposure

White speckles of vivianite on soil aggregate

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Slide 8

Vejrumbro Profile- soil properties -

Vivianite as P trap

A Box Bred Gyttje Peat

Depth (cm) 0-15 15-35 35-60 60-90 > 90

pH 5.3 6.6 6.6 6.8

Clay (%) 10 20 10 26 -

Total C (%) 17 12 25 6.3 -

Total P (mmol kg-1) 106 164 183 198 220

FeCBD (mmol kg-1) 779 1700 1310 542 1550

Feox (mmol kg-1) 640 965 800 548 1470

Alox (mmol kg-1) 186 172 143 87 78

vivianite-containing horizon

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Slide 9

Minerals in the soil profile

Vivianite as P trap

A (0-15 cm)

B oxidized (15-35 cm)

B reduced (35 - 60 cm)

Gyttje (60 - 90 cm)

Peat (> 90 cm)

Q

V

Q

F

Q: QuartzF: FeldsparsV: VivianiteC: Clays

QQ Q Q

C

Vivianite observed only in the gyttje layer

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Slide 10

Minerals in the gyttje layer- close-up on white speckles -

Vivianite as P trap

V

V

V

S

S

S

S

S

SS S

Q

V V V VS

V: Vivianite, Fe3(PO4)2 8H2OS: Siderite, FeCO3

Q: Quartz, SiO2

Close-up on selected parts of gyttje layer enriched with white material

Very distinct peaks due to vivianite

There is also:- Siderite- Quartz- Goethite (Mössbauer spectroscopy)

XRD

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Slide 11

FeIII

Vivianite in the soil- Mössbauer spectrum -

Vivianite as P trap

Species

εεεε(mm s-1)

δδδδ(mm s-1)

f(%)

εεεε(mm s-1)

δδδδ(mm s-1)

f(%)

Vejrumbro Reference*

FeIIA 1.14 2.42 28 1.15 2.48 25

FeIIB 1.22 2.97 25 1.21 3.00 40

FeIIIA 0.45 0.64 47 0.42 0.69 37

Room temperature

* Dormann & Poulen (1980), Bull. Mineral. 103, 633 - 639

Spectrum of white (bluish) material:

•Confirms the presence of vivianite

•vivianite is partly oxidized

FeII

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Slide 12

Minerals in the soil- SEM -

Vivianite as P trap

Iron phosphate "sponge" structure + underlying iron oxide

Vivianite "sponge" coating (similar morphology seen by Peretyazhko et al., 2010)

Iron oxide

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Slide 13

Pore water data- Nov. 2009 - June 2010 -

Vivianite as P trap

Param. Unit Value Comment

pH 6.5 - 6.8

Na+ 100 - 400 increasing with depth

K+ ∼ 50

Mg2+ 25 - 150 incresing with depth

Ca2+ 1200 - 2300 highest in gyttje layer

NH4+ 50 - 350 highest in gyttje layer

Fe2+ µM 500 - 2700 highest in gyttje layer

Mn2+ 30 - 100 constant at 85 µM in gyttje layer

Cl- 400 - 550 very uniform

SO42- 20 - 800 low at top and bottom

HCO3- 2000 - 9000 highest in gyttje layer

PO43- 0.2 - 30 highest in peat layer

Pore water composition from the same depth rather constant for most elements

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Slide 14

Pore water composition- pH, FeII and Pi -

Vivianite as P trap

• Constant pH of about 6.8

• Highest FeII

concentrations

• Pi concentrations low

(0.2 - 6 µM), and lower

than for other horizons

FeII (µM)

PO4 (µM)

pH

Gytt

je (

viv

) layer

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Slide 15

Pore water: equilibrium with vivianitein the gyttje layer?

Vivianite as P trap

Ksp (viv)

• Near to equilibrium with vivianite late winter; oversaturated during spring

• Not oversaturated with respect to calcium phosphates

oversaturation

undersaturation

IAP =[Fe2+]3[PO43-]2

⇒ pore water data points to equilibrium with vivianite or slight

oversaturation

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Slide 16Lab

exp

erim

: R

ele

ase o

f Fe a

nd

Pi

(V

ivia

nit

e d

isso

luti

on

)

Vivianite as P trap

slopesFeII: 251 µM d-1

Pi: 20.6 µM d-1

•FeII concentrations as in the field

•Pi concentrations much higher than in the field

• Highly oversaturated with respect to vivianite; no equili-brium with vivianite

FeII

Pi

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Slide 17

Pi sorption isotherms for the gyttje layer- Vivianite precipitation -

Vivianite as P trap

Non-typical isotherm shapes with no sorption maxima as Psol increases→ precipitation

Very high desorption and very high EPC0

Highly oversaturated with respect to vivianite; no equilibrium with vivianite

Vivianite probably precipitates, but no equilibrium with vivianite in lab experiments → slow dissolution/precipitation kinetics

[Fe2+] sol≅ 200 µM

[Fe2+]sol ≅ 500 µM

EPC0

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Slide 18

So.... - does vivianite control Pi in reduced soil?

Vivianite as P trap

Field•Vivianite control of Pi solubility during winter times•Oversaturation with respect to vivianite when biological activity increases, more flow, oxidation...

Laboratory•Solution Pi concentrations very high over extended periods•Although vivianite dissolves or precipitates, oversaturation remains high: no fast equilibriation

Does vivianite control Pi solubility?•Systems without perturbation: Yes•Systems with perturbations: NoIn perturbed systems equilibrium takes time to establish - and vivianite-containing soils/sediments rather act as Pi sources

Uncertainty in stoichiometry of the soil vivianite (content of FeIII) ⇒What is the true solubility of the soil vivianite.