Research PaperMicroaggregation of goethite and illite evaluated by mechanistic modeling
Graphical abstract
Introduction
Several biotic and abiotic processes can be involved in the formation of 20–250 μm sized soil microaggregates (Totsche et al., 2018), while the formation of their smaller building units (BUs) is largely determined by aggregation processes of fine-sized mineral and organic microaggregate forming materials (MFMs) (Chorover et al., 2004; Ilg et al., 2008). Their high specific surface area (SSA), diversity of shapes, presence of very small to large particles, and differences in surface charge (SC) have a decisive role for the mode of aggregation. A basic assumption evident from laboratory experiments (Tisdall and Oades, 1982; Barral et al., 1998; Duiker et al., 2003; Watts et al., 2005; Jozefaciuk and Czachor, 2014) is that the formation of microaggregate BUs in soils is mainly controlled by charge differences of the participating MFMs, whereby layer silicate clay minerals and oxyhydroxides of iron (Fe; summarized as ’Fe oxides') are considered the most important inorganic ones (Barral et al., 1998; Watts et al., 2005). In larger microaggregates >20 μm the role of charge differences for particle adhesion is decreasing and stabilization is mostly due to binding agents, which can be distinguished in gluing agents, e.g. root and microbial exudates, and in cementing mineral agents, such as carbonates, pedogenic hydroxides, oxyhydroxides and oxides (summarized as ‘oxides'), and silicates (Oades and Waters, 1991; Chorover et al., 2004; Ilg et al., 2008; Totsche et al., 2018).
Clay minerals have permanent negative charges on the basal planes and pH-dependent variable charges at their edge sites (Johnston and Tombácz, 2002), whereas pedogenic oxides have variable charges, with a point of zero charge (pzc) mostly between pH 7 and 9 (Kosmulski, 2020). Interactions of positive edge charges with negative basal SCs of 2:1 layer silicates produce T-type contacts and card-house type aggregation until the pzc of the edges at pH ≈ 6.5 is approached (Lagaly and Dékány, 2013), which is of high significance for the aggregation of these minerals (Keren and Sparks, 1995).
On the other hand, electrostatic attraction between negatively charged clay minerals and positively charged Fe oxides appears to be a prerequisite for aggregates with a high stability (Tisdall and Oades, 1982; Duiker et al., 2003; Jozefaciuk and Czachor, 2014). From the different shapes, especially varying in the aspect ratio (AR), particle sizes and SC properties of different clay minerals and pedogenic oxides it is likely that certain size and charge constraints exist for the formation of stable soil microaggregate BUs. Optimal neutralization of opposite SCs will occur at certain mixing ratios of negatively and positively charged particles only (Lagaly and Dékány, 2013). The size of the contact area between particles, which has a decisive role for aggregate stability, will depend on complex geometric preconditions. In a laboratory approach, Dultz et al. (2019) determined microaggregation kinetics of mono- and dual mineral mixtures at pH 6 using three size fractions of goethite and illite as model or prototype minerals for soils. The authors showed that aggregation occurred in all homoaggregation experiments, particularly for fine illite <0.2 μm with the highest share of edges on the external surface (lowest AR). Illite-goethite heteroaggregation experiments revealed a broad range of aggregation for mixtures with fine illite and a narrow range for combinations with medium and coarse illite (0.2–2 μm). Maximum aggregation typically occurred at the pzc of the respective mineral mixture (Dultz et al., 2019). This study highlighted the decisive role of the size of the contact area between particles and complex geometric preconditions in the aggregation and aggregate stability.
Particle arrangement, thus microstructure and pattern formation of microaggregate BUs could also be evaluated based on mechanistic models and related computer simulations. Yet most existing modeling approaches dealing with microaggregation considered only few different size classes as compartments without representing neither an explicit geometry of particles nor explicit attraction mechanisms (e.g. Segoli et al. (2013), see also the section on modeling in the review Totsche et al. (2018)). Recently Ritschel and Totsche (2019) presented an approach on the molecular scale using DLVO theory. Ray et al. (2017) and Rupp et al. (2018) introduced an operative tool based on a twophase cellular automaton method (CAM). Simulations allowed resolving single aggregation processes separately, but also their interplay. Moreover, in silico the variation of conditions is more flexible and can be realized in a much shorter time than in laboratory experiments. As in experimental studies, the microaggregation of different mono and dual mineral mixtures can be considered, but larger parameter ranges can be taken into account. Rupp et al. (2019) studied in silico structure formation of goethite, illite, and quartz particles at the scale of microaggregates with prototypes goethite, illite, and quartz with diameters of 3–41 μm. Basically, mirroring the results of laboratory experiments, Rupp et al. (2019) showed that the size of the interacting oppositely charged constituents controls the size, shape, and amount of aggregates formed and that the aggregation rate increases with particle concentration.
This cellular automaton method opens new opportunities in the elucidation of processes in aggregation. Despite detailed insights from dynamic light scattering measurements on size and charge constraints in microaggregation, open questions exist which could be answered by analysis on the single particle scale, e.g. charge, size, and shape effects of MFMs for aggregation, quantification of discrete particles not included in microaggregate BU formation and contact area between MFMs as a measure of stability. This information is hardly available from even sophisticated high resolution techniques in the laboratory but easily obtainable in simulations, such as the two-phase modeling of Rupp et al. (2018) to determine structure formation on the single particle scale.
Here, the formation of microaggregate BUs was simulated with the CAM for the MFMs goethite and illite considering three different basic scenarios arising from SC, particle morphology, and mixing ratio, which affect particle interactions, aggregation kinetics, and BU microstructure and stability. With this approach, the following research questions were addressed:
- (i)
What is the role of the particles' SC properties with van der Waals attractive forces versus electrostatic interactions by charged sites?
- (ii)
Which constraints arising from particle morphology exist during aggregation for MFMs being characterized by their size and AR? What is the explanation of the primary role of fine illite <0.2 μm for microaggregation?
- (iii)
What is the role of SC, particularly positive or negative edge charges of illite and total charge in aggregation for microaggregate BU diameter and stability?
- (iv)
Are there principal differences in aggregation properties between mono-mineral systems and dual-mineral systems of goethite and illite?
For the determination of structure formation during aggregation on the single particle scale, several different items were investigated. Basically, MFMs and their combinations were characterized by the temporal development of microaggregate structures and diameters. For illite, additionally the role of AR for microaggregation was determined. The interplay of forces between MFMs in suspension was determined, on the one hand, for charged sites by electrostatic interactions and, on the other hand, for uniform short-range attractions by van der Waals forces. For combinations of goethite and illite, the role of balancing the positively and negatively charged sites given by the pzc for microaggregation was determined. Further, the role of discrete MFMs not involved in microaggregate BU formation and shielding effects due to attachment of illite on the goethite surfaces was investigated, which is not derivable from Zetasizer experiments. Finally, the stability of BUs was derived from their contact area in dependence of AR and SC.
In any of the simulations, physico-chemical or model-related parameters were not fitted but derived a priori. The model set up was strictly oriented on laboratory aggregation experiments with goethite and illite of Dultz et al. (2019), so that the findings from modeling could be validated with experimental data. Hence, for the first time independent simulation results were compared to microaggregation experiments that have been conducted under precisely defined conditions in a Zetasizer.
Section snippets
Laboratory method and mathematical model
In the laboratory experiments microaggregation of particles in suspension was determined by dynamic light scattering, a method which is well established in microaggregation studies (Kretzschmar et al., 1998). A measuring requirement of the laser based method is that the particle concentration is relatively low (≈5–500 mg l−1). In the experiments of Dultz et al. (2019) it was constant with 100 mg l−1. Hence, in contrast to size and charge constraints in microaggregation effects of particle
Surface charge, aggregation kinetics and aggregate stability
The homoaggregation of fine and medium illite with negative edge charges was simulated first under the sole influence of attracting van der Waals forces, i.e. equally favoring face-to-face and edge-to-face contacts, which resulted in compact structures (Fig. 2a,c). As simulation outcome, composites of illite MFMs that were connected by face-to-face contacts were predominantly observed as the contact area between two faces is larger than that between an edge and a face (Lagaly and Dékány, 2013).
Conclusion
In this research homo- and heteroaggregation of various illite and goethite mixtures was investigated with the help of a novel modeling approach based on mechanistic principles using a cellular automaton method. It could be shown that 2D simulations give temporally resolved insights into the structure formation during aggregation starting at the single particle scale and hereby increase the understanding of laboratory aggregation experiments and, ultimately, microaggregate building unit (BU)
Author contributions
Simon Zech: Software, Data Curation and Visualization; Stefan Dultz, Alexander Prechtel and Nadja Ray: Conceptualization and Methodology. All authors contributed to writing and reviewing the manuscript.
Declaration of Competing Interest
None.
Acknowledgments
This study was performed within the framework of the research unit RU 2179 “MAD Soil – Microaggregates: Formation and turnover of the structural building blocks of soils” (DFG RU 2179) through projects PR 1610/2-2, RA 2740/1-2 and GU 406/29-1,2 of the Deutsche Forschungsgemeinschaft.
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