¹Hiroto Tokumon, ¹Yohei Noji, ²Keisuke Fukushi, ^2,3Yasuhito Sekine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.07.021]
¹Division of Natural System, Graduate School of Natural Science, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan
²Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan
³Earth-Life Science Institute (ELSI), Institute of Science Tokyo, Meguro, Tokyo 152-8550, Japan
Copyright Elsevier

A key step in understanding prebiotic chemistry in the Solar System is to predict and reconstruct the speciation and solid–liquid partitioning of inorganic nitrogen species, such as ammonium and ammonia, in icy planetesimals—including C-type asteroids, the dwarf planet Ceres, and Saturn’s moon Enceladus. Smectite, a common constituent of these bodies, can regulate the chemical behavior of NH₄⁺ through cation exchange reactions. Accurate reconstruction of ammonium and ammonia speciation and distribution therefore requires appropriate selectivity coefficients for these exchange processes. In this study, we measured the NH₄⁺–Na⁺ selectivity coefficients (K_Na-NH4) of montmorillonite and saponite under varying initial NH₄⁺ and Na⁺ concentration, solid concentration, and pH. Cation exchange was confirmed by stoichiometric NH₄⁺ uptake and Na⁺ release. Montmorillonite exhibited log K_Na-NH4 ranging from −0.06 to 0.41, while saponite showed systematically lower values, from −0.46 to 0.07, likely reflect a difference in hydration retention capacity between the two smectites. Selectivity coefficients for both smectites showed a pH dependence with a maximum around pH 8, and well-described by second-order polynomial fits. Speciation modeling incorporating these coefficients demonstrates that NH₄⁺ interlayer occupancy and the aqueous concentrations of NH₄⁺ and NH₃ are highly sensitive to pH, salinity, and water–rock ratio under plausible geochemical conditions. Modeling results suggest that the aqueous solutions surrounding the Ryugu and Bennu samples during aqueous alteration were highly alkaline (pH > 9.5), favoring NH₃ over NH₄⁺ in solution and resulting in limited NH₄⁺ retention on solids. In the ancient Ceres ocean, NH₄⁺ was abundant in solution due to moderately alkaline conditions (pH ∼ 8) and a high water–rock ratio. For Enceladus, the results indicate that its rocky core may serve as a reservoir of NH₄⁺, with up to 60–70 % of total NH₃ in Enceladus present as interlayer NH₄⁺. These findings provide a quantitative framework for interpreting nitrogen speciation in icy Solar System bodies, including Europa, and their returned or observed materials.