Vermelding onderdeel organisatie

April 18, 2023

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Modeling subsurface iron removalGeochemical modeling of subsurface aeration at Schuwacht Lekkerkerk

Harmen van der Laan

Faculty of Civil EngineeringMsc. Water ManagementSpecilization Drinking Water

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Content

• Subsurface iron removal• Objective research• Results• Conclusions• Next steps & Recommendations

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Content

• Subsurface iron removal• Objective research• Results• Conclusions• Next steps & Recommendations

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Subsurface iron removal

O2

front

0

1

inje

cted

wat

er

O2

fro

nt

injected

water

Fe2+

front 0

1

Fe2+

front

Fe2+

front

0

1

0

1

Fe2+

front

inje

cted

wat

er

Injection Extraction

Measured data

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Measured data

Development final iron concentration

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35

injection - extraction cycles

Fe (

mg/

L)

July 1998 – June 2000 March 2004 – June 2005

‘Original’ Model

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V/Vi

Expanded model

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main limitation original model

No explanation for the phenomena of the increasing spread of the (iron) front over the successive cycles

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Content

• Subsurface iron removal• Objective research• Results• Conclusions• Next steps & Recommendations

Objective

The objective of this research project is to• Find a theoretical foundation to explain the

development of the iron front over the successive cycles

• Enhance the existing model to obtain a proper description of the measured iron concentrations

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Content

• Subsurface iron removal• Objective research• Results• Conclusions• Next steps & Recommendations

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Results

I. Goethite / FerrihydriteII. Ion exchange vs. AdsorptionIII.Transport modeling

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Goethite vs. Ferrihydrite

FerrihydriteHfo, FeOH3, Fe5HO8·4H2O

Unstable, unstructured Amorphous / Aquaeous Spec. area ≈ 600 m2/g 2 sites / nm2 pHPZC 8.1 Solubility log K = 2 –

4.5

Observed by: KIWA Research

Goethiteα-FeOOH

Stable Crystalline Spec. area ≈ 60 m2/g 2 - 10 sites / nm2 pHPZC 8.7 Solubility log K = -1

Observed by: Mettler (2002)

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Surface complexation theory

OHOH

OHOH

OH + / - depends

on pH

Number of sites

Surface area

Fe2

+

Fe2

+

Fe2

+

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Site density

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Surface characterization

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Goethite vs. Ferrihydrite ?

• Variations in site density and surface area only result in small bandwidth

• Differences between Goethite and Ferrihydrite not the main limitation

α-FeOOH

Ferrihydrite

‘solubility’ decreases in time

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Results

I. Goethite / FerrihydriteII. Ion exchange vs. AdsorptionIII.Transport modeling

Exchange

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Exchange

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Exchange and adsorption hard to distinguish

Exchange capacity = lumped parameter

Empirical formula

does not give correct output

CEC (meq/kg) = 7 · (% clay) + 35 · (% C)

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Results

I. Goethite / FerrihydriteII. Ion exchange vs. AdsorptionIII.Transport modeling

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Retardation / Front spreading

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Microdispersion Macrodispersivity

Injection well

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Dispersion

Stagnant zones

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Stagnant zones

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Results

• Subsurface iron removal• Objective research• Results• Preliminary Conclusions• Next steps & Recommendations

Preliminary Conclusions

Objective is: Theoretical foundation and a better model

I. Many ‘flavors’, but I need too pick oneII. Model starts to describe the correct retardation

mainly because of dispersion / stagnant zones

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Possible explanation

Ferric

iron

I. Kinetics: decrease SI in time / combination Ferrihydrite – Goethite

II. Stagnant zones III. Or:

Biological activity?Complexes? Change groundwater composition?

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Next steps & Recommendations

Next steps Tracer in groundwater for dispersion? Implementation stagnant zones Influence conclusions on accumulation Finish model

Recommendations Column experiments:

separate transport model from geochemistry Research Ferrihydrite / Goethite

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Questions?