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Metal-silicate Partitioning at High Pressure and Temperature: Experimental Methods and a Protocol to Suppress Highly Siderophile Element Inclusions

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Solubility, speciation and thermodynamics of Fe in reducing aqueous KCl solutions

Paul Q Fürst¹, Neşe Çevirim-Papaioannou¹, Xavier Gaona¹

¹Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology Karlsruhe Germany p.fuerst@kit.edu xavier.gaona@kit.edu.

|November 27, 2025

Abstract:

The solubility of two well-characterized Fe(ii) solid phases (i.e., Fe(OH)₂(cr) and Fe₂(OH)₃Cl(cr)) were investigated in batch undersaturation solubility experiments conducted over a wide range of pH_m (7.5 ≤ pH_m ≤ 10.5) and ionic strength (0.01 M ≤ I ≤ 4.0 M KCl). Solid phase characterization was carried out using XRD including Rietveld analysis, providing key insights into their phase composition and crystallite size. Chemical, thermodynamic and SIT activity models were derived for the system Fe²⁺-K⁺-H⁺-Cl^--OH^--H₂O(l) on the basis of the comprehensive experimental dataset. Solubility constants determined in this work and contribute to improving the description of Fe chemistry under very reducing conditions, and can be implemented in thermodynamic databases and geochemical calculations of relevance, e.g. in the context of nuclear waste disposal.

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