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Understanding the chemistry of cation leaching in illite/water interfacial system using reactive molecular dynamics simulations and hydrothermal experiments

Large-scale ReaxFF MD of illite/water interfaces is paired with hydrothermal leaching experiments to resolve proton-mediated pathways for K⁺, Al, and Si release and to compare time scales with continuum leaching kinetics.

Summary

Simulations show K⁺ leaching earlier and at higher concentration than network Al and Si. Trajectory analysis identifies proton attack on Al–O–Si linkages via NBO protonation, silanol formation after Al–O cleavage, K⁺ migration to the surface, KOH formation and outward diffusion, and later Al(OH)₃- or Si(OH)₄-like release contributing to Al/Si loss. MD surface reactivity is compared to bulk hydrothermal leach curves; the authors report orders-of-magnitude gaps between the two, motivating scale-bridging. Structural metrics indicate more than 20% cation loss is needed for strong illite distortion in the model. The work situates illite-rich clays in geothermal and barrier contexts; authoritative tables and numbers are in the PDF/SI, not recopied exhaustively on this page.

Methods

Experiment (hydrothermal leaching). Natural or prepared illite-bearing material is leached in autoclave hydrothermal conditions; aqueous analytics follow K, Al, and Si over laboratory (days–months) time scales. P, T, and exact sample preparation are in the version-of-record at pdf_path (not in the short local p1-2 extract).

1 — MD application (illite / water, ReaxFF). The study uses large-scale ReaxFF reactive MD of illite in water (code name: read the Acta Methods—often LAMMPS in this line of work). System size and slab vs interlayer construction are given in the article and figures; the abstract alone only states large-scale ReaxFF MD, not N atoms. PBC: 3D periodic slab-in-water (as in VOR). Ensemble: NVT at target temperatures in K (see VOR; not only p1-2). Timestep: 0.25 fs; integrator: velocity Verlet (Methods). Thermostat: Berendsen with 0.1 ps damping (temperature coupling). Duration / equilibration and production (ps / ns): N/A in the indexed extract—transcribe from pdf_path. Barostat & hydrostatic pressure (NPT): N/A for the NVT stages summarized here. Shear / shock: N/A. External electric field: N/A. Replica / umbrella / metadynamics: N/A; any NEB-style or reaction-path postprocessing is post-trajectory (see VOR). QEq update and cutoffs (ReaxFF): N/A in the p1-2 snippet—see SI / full text.

2 — Force-field training. N/A—the work applies a cited ReaxFF for aluminosilicate / illite-relevant chemistry; it is not a de novo QM-driven ReaxFF parameterization paper.

3 — Static QM / DFT-only (headline result). N/A—the lead evidence is ReaxFF MD paired with experiments; any auxiliary QM (if listed) is not the p1-2 excerpt** focus.

Findings

  • K⁺ is mobilized before substantial Al/Si network dissolution under the modeled neutral-water chemistry.
  • Protonation sequence rationalizes KOH-mediated potassium transport and later Al/Si hydroxide release.
  • MD-derived surface kinetics do not directly match bulk experimental leaching rates without additional transport or scaling arguments.
  • Structural degradation becomes pronounced only after substantial cumulative leaching in the MD-based distortion metric.

Limitations

Reactive FF accuracy for aluminosilicate clay edge/interlayer chemistry; accessible MD times vs laboratory leaching times; experiments on natural/powder samples may include mineral admixtures not in the idealized model.

Relevance to group

van Duin (corresponding, Penn State Mechanical Engineering); collaboration with University of Wyoming geochemistry and hydrothermal experiments.

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