Thermostability and solubility of ammonium illite and ammonium montmorillonite: Implication for inorganic nitrogen preservation

• Published in: Geoderma Vol. 452; p. 117097
• Main Authors: He, Ni, Cheng, Hongfei, Du, Peixin, Chen, Aiqing, Han, Yutong, Li, Shangying
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-12-2024; Elsevier
• Subjects: Illite; Interlayer; Montmorillonite; NH4; Solubility; Thermostability; Thermostability; NH4; Interlayer; Montmorillonite; Illite; Solubility
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2024.117097

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•Highly purified ammonium illite and ammonium montmorillonite were characterized.•The deamination of interlayered NH4+ is estimated to be above 320 °C.•Interlayered NH4+ could react with structural hydroxyl to produce NH3 and H2O.•Interlayered NH4+ is hardly released into fluids with a broad pH value range.•2:1 type clay minerals have a potential for preserving inorganic nitrogen in soil.
Investigation of the thermostability and solubility of ammonium-bearing clay minerals is essential for assessing the inorganic nitrogen preservation by minerals and revealing the fate of inorganic nitrogen in soil. In this study, natural ammonium illite and laboratory-prepared ammonium montmorillonite were systematically characterized to explore their mineralogical characteristics, thermostability, and solubility. For ammonium illite, an increase of d001-value to 10.334 Å in the X-ray diffraction (XRD) pattern and an appearance of strong absorption bands at 3309, 3043, and 1432 cm−1 as well as blue shifts of structural hydroxyls vibrations in the Fourier transform infrared (FTIR) spectrum. The d001-value of ammonium montmorillonite is ∼12.05 Å, and the FTIR spectra show NH4+ absorption bands at 3120, 3005, and 1402 cm−1. Thermostability analyses indicate that the presence of interlayered NH4+ reduces the structural stability of both illite and montmorillonite during heating, but the deamination temperatures of ammonium illite and ammonium montmorillonite are estimated conservatively to be above ∼320°C, which is higher than the deamination temperatures of common inorganic ammonium compounds. The result of dissolution experiments shows that the release efficiency of NH4+ from ammonium illite does not exceed ∼4 % under a pH value range of 1–14 at room temperature, while that in ammonium montmorillonite does not exceed ∼8 % at pH 2–11. These findings help to determine the existence of NH4+ in the interlayers of 2:1 type clay minerals and suggest that 2:1 type clay minerals can effectively preserve inorganic nitrogen in soil under moderate field biomass burning or moist conditions.