Microaggregation of goethite and illite: Linking mechanistic modeling and laboratory experiments

Microaggregates are the fundamental building blocks of soils and thus important for their structure, properties, and functions. Hence, experimental aggregate formation studies (Dultz et al. 2019) were conducted to reveal the mechanisms leading to the establishment of soil microaggregates from mixtures of the mineral building units goethite and illite. Mathematically based modeling can further illuminate the mechanisms and factors behind structure formation as well as facilitate this understanding, even if the experimental capability is limited.

To this end, we present and extend a mechanistic modeling approach (Rupp et al. 2018, Rupp et al. 2019) which is based on a cellular automaton method that resolves explicitely particles at the micrometer scale. Thus it is capable to represent structural changes originating from (electrostatic) interaction of building units (aggregate forming materials). As prototypic building units goethite and illite with needle like and platy shapes of different size and charge are implemented. The operational, comprehensive model allows studying structure formation as a function of composition and charge of such mineral mixtures. Along this line, homoaggregation as well as heteroaggregation scenarios are investigated. The resulting microaggregates are investigated with respect to size, structure, and stability. Moreover, the role of the aspect ratio for stability, the point of zero charge for aggregation, and the amount of excess particles with respect to time is illustrated. Finally, the results are evaluated and compared to experimental data given in Dultz et al. (2019), and extend the scenarios studied there

S. Dultz, S.K. Woche, R. Mikutta, M. Schrapel, G. Guggenberger (2019): Size and charge constraints in microaggregation: Model experiments with mineral particle size fractions. Applied Clay Science 170, 29-40.

A. Rupp and K. Totsche and A. Prechtel and N. Ray (2018): Discrete-continuum multiphase model for structure formation in soils including electrostatic effects. Frontiers in Environmental Science, 6, 96.

A. Rupp, T. Guhra, A. Meier, A. Prechtel, T. Ritschel, N. Ray, K.U. Totsche (2019): Application of a cellular automaton method to model the structure formation in soils under saturated conditions: A mechanistic approach. Frontiers in Environmental Science 7, 170.