microaggregation of goethite and illite: linking ...€¦ · enlarge the range of conditions and...

Microaggregation of goethite and illite: Linkingmechanistic modeling and laboratory experimentsAlexander Prechtel1, Simon Zech1, Stefan Dultz2, Georg Guggenberger2, Nadja Ray1

© The Authors1 Mathematics Department, Friedrich-Alexander Universität, Erlangen-Nürnberg, Germany2 Institute of Soil Science, Leibniz Universität, Hannover, GermanyEGU online, SSS5.6, 06.05.2020

Study microaggregate formation in silico

Why?

I study the interplay of particles in precisely defined settingsI in contrast to lab scenarios: easy variation of a manifold of

conditionsI isolation of mechanisms / effects possibleI direct access to all parameters of resulting structures - at

every time step (even not measurable ones)I mathematical upscaling of structural properties possible

A. Prechtel · · Microaggregation of goethite and illite: Linking mechanistic modeling and laboratory experiments EGU online, SSS5.6, 06.05.2020 2

ReferencesZech, S., Dultz, S., Guggenberger, G., Prechtel, A., Ray, N. (2020):

Microaggregation of goethite and illite evaluated by laboratory experiments andmechanistic modeling, Preprint Reihe Angewandte Mathematik, ISSN 2194-5127,Erlangen.

Relates to experimental studyDultz, S., Woche, S.K., Mikutta, R., Schrapel, M., Guggenberger, G. (2019):Size andcharge constraints in microaggregation: Model experiments with mineral particlesize fractions. Appl. Clay Sci. 170, 29-40. doi.org/10.1016/j.clay.2019.01.002Extension of the workI Rupp A., Guhra T., Meier A., Prechtel A., Ritschel T., Ray N., Totsche KU. (2019):

Application of a cellular automaton method to model the structure formation insoils under saturated conditions: A mechanistic approach. Front. Environ. Sci.7, doi.org/10.3389/fenvs.2019.00170

I Rupp A., Totsche KU., Prechtel A., Ray N. (2018): Discrete-Continuum MultiphaseModel for Structure Formation in Soils Including Electrostatic Effects. Front.Environ. Sci. 6, doi.org/10.3389/fenvs.2018.00096

I Ray N., Rupp A., Prechtel A. (2017): Discrete-continuum multiscale model fortransport, biomass development and solid restructuring in porous media. Adv.Water Resour. 107, 393-404, doi.org/10.1016/j.advwatres.2017.04.001


doi.org/10.1016/j.clay.2019.01.002

doi.org/10.3389/fenvs.2019.00170

doi.org/10.3389/fenvs.2018.00096

doi.org/10.1016/j.advwatres.2017.04.001

Enlarge the range of conditions and compositions of aggregate formation - use alsoparticles used in wet lab experiments

Investigation ofI Prototypic microaggregate forming materials (MFM): goethite, and illite; different

in size, shape, charge loading, concentration.I impact of shape (aspect ratio) and sizeI impact of edge charges of illite/point of zero chargeI stability vs. particle sizeI excess particles


Stable aggregation in terms of size and aspect ratioC

ont.

Are

a/Vo

l.[L−

1 ]

0 20 40 60 80 1000

0.2

0.4

0.6 Particle size: 3 12 27 48 75

5,000Time steps

0 20 40 60 80 1000

0.05

0.1

0.15

0.2

Time steps

Aspect ratio: 1.2 3.3 7.5 30

5,000Time steps

Figure: Influence of aspect ratio and size of illite building units on aggregate stability.

⇒ Small in size and aspect ratio MFMs are most stable.


Different phenotypes of structures while aggregation

Figure: Microstructures due to heteroaggregation of fine illite with negative edge charge andcoarse goethite. Left : 5% of goethite (stable disperse system), middle: 55% of goethite(compact structures), right : 95% of goethite (pillar structures with hidden places)


Optimal aggregation in terms of compositionMicroaggregation of goethite and illite: Comparison of results from laboratory experiments and mechanistic

modeling

Microaggregation of goethite and illite: Comparison of results from laboratory experiments and mechanistic

modeling


modeling

Meanhydr.

diam

[nm]


modeling

6

22

4757

20

0

0.2

0.4

0.6

0

500

1,000

1,500 (a) Fine goethite/�ne illite

8

45

83

59

32

0

0.2

0.4

0.6

0

500

1,000

1,500 (b) Coarse goethite/�ne illite

7

25

44

64

82

54

8

0

0.2

0.4

0.6

0

500

1,000

1,500 (c) Fine goethite/medium illite

21

5653

150

0.2

0.4

0.6

0 10 20 30 40 50 60 70 80 90 1000

500

1,000

1,500

goethite addition (%)

(d) Coarse goethite/medium illite

Figure 1: Mean hydrodynamic diameter (in nm, blue marks), contact area per aggregate volume (red marks)

and percentage of excess building units (illite in gray, goethite in brown) over composition of positively charged

goethite and negatively charged illite particles after 5000 steps of the Cellular Automaton Method. Point of

zero charge in green.a

Zech et al.: Preprint submitted to Elsevier Page 6 of 7

Contact

area/volume[L

−1 ]

0.4. Heteroaggregation scenario


goethite addition (%)



Goethite addition (%)



Figure: X-axis: % of portion of goethite(remaining portion is illite with negativeedge charge). Left Y-axis: Meandiameter for various compositions ofsolid. The blue dots show the medianof the mean diameter in 10 simulationruns, the black bars indicate the lowerand upper quartile of the 10 simulationruns. Right Y-axis: contact area peraggregated volume (red dots). Thegreen lines depict the point of zerocharge of the system leading to strongaggregation. Bars with % of excessbuilding units (illite in gray, goethite inbrown).


Different phenotypes of structures for varying edge charge of illite

Figure: Microstructures due to heteroaggregation of fine illite withpositive edge charge and coarse goethite. Left : 5% of goethite(cardhouse structures), middle: 55% of goethite (compactstructures), right : 95% of goethite (pillar structures with hiddenplaces)

Figure: Recall situation ofnegative edge charge-...


Comparison with experiment

20 40 60 800

2,000

4,0000

500

1,000

1,500

Fine goethite addition (%) Time

stepsM

eanhydr.diam.[nm]

0 20 40 60 80

20

400

1,000

3,000

5,000

Fine goethite addition (%) Time

(min)M

eanhydr.diam.[nm]

Figure: Mean (hydrodynamic) diameter (in nm) over time for different mixtures of fine goethite andfine illite with positive edge charge. Results from simulations (left) and experiments (right) show asimilar qualitative behaviour.

Forward Simulation! NO FITTING OF ANY PARAMETERS!


Simulations elucidate the impact ofI composition of solution/ charge and ratio between MFM

. Shielding

. Distinct aggregate phenotypes (coiled chain like, thin long chains,..)I relative size of MFM

. surface caoting inhibiting aggregation vs. bridging supporting aggregationI concentration of solution/porosity

. higher concentration leading to larger aggregates

. increasing complexity and size over timeon aggregation and quantification by means of characteristic values (average particlesize, specific surface, compactness, ...)

This research was kindly supported by the DFG RU 2179 “MAD Soil - Microaggregates:Formation and turnover of the structural building blocks of soils”.


microaggregation of goethite and illite: linking ...€¦ · enlarge the range of conditions and...

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