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

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.