Environmental effects on ammonium adsorption onto clay minerals: Experimental constraints and applications

Adsorption of ammonium (NH ₄ ⁺ ) by clay minerals in soil/sediment and altered oceanic crust plays an important role in Earth`s geological nitrogen cycle by transporting nitrogen from the biosphere and hydrosphere into the lithosphere. Clay minerals have also been proposed to play an important role in catalyzing abiotic synthesis of organic compounds (which also requires adsorption enrichment of NH ₄ ⁺ ) for the origin of life. Consequently, nitrogen in clay minerals could be used as a potential tracer for reconstructing paleo-environments and searching for life on extraterrestrial planets and moons. However, the premise of these applications, i.e., how different environmental conditions (e.g., ambient pH, temperature, salinity and aqueous NH ₄ ⁺ concentration) would affect NH ₄ ⁺ adsorption onto clay minerals, is still poorly understood. In this study, we carried out laboratory experiments to examine the NH ₄ ⁺ adsorption behavior of several clay minerals (montmorillonite, vermiculite, illite, chlorite, kaolinite) under conditions varying in aqueous NH ₄ ⁺ concentration (20, 50, 100, 200, 500, 1000 mg/L), pH (2, 5, 7), salinity (fresh water vs. artificial seawater), and temperature (23, 50, 70 °C). Our results show that the NH ₄ ⁺ adsorption on all studied minerals is very fast (reaching equilibrium within a few minutes). The NH ₄ ⁺ adsorption capacity is however strongly mineral-dependent and follows the order of vermiculite ≈ montmorillonite ≫ illite > kaolinite ≈ chlorite. The much higher NH ₄ ⁺ adsorption capacities of vermiculite and montmorillonite can be attributed to the NH ₄ ⁺ adsorption in their interlayer sites in addition to the surface and edge sites that dominate the NH ₄ ⁺ adsorption in illite, kaolinite, and chlorite. But illite contains some frayed edge sites that are accessible to NH ₄ ⁺ adsorption, and thus has higher adsorption capacity than kaolinite and chlorite. The NH ₄ ⁺ adsorption on all these clay minerals is highly susceptible to environmental conditions (e.g., aqueous NH ₄ ⁺ concentration, pH, temperature, and salinity). For example, the NH ₄ ⁺ adsorption capacities of montmorillonite and vermiculite significantly decrease following the increase in salinity and the decreases in pH and aqueous NH ₄ ⁺ concentration. Our experimental data can be best fitted by the Freundlich Model, which suggests that NH ₄ ⁺ adsorption on clay minerals likely occurs in energetically heterogeneous surface sites. Based on these results, we provide new insights into the understanding of NH ₄ ⁺ migration from seawater to sediments and altered oceanic crust, preservation capability of the paleoenvironmental nitrogen signature in clay minerals, and optimal conditions for clay to catalyze organic synthesis toward the origin of life on the early Earth.