Water partitioning and migration in unsaturated bentonites by low-field NMR characterization

Water behavior in bentonite clay pores is influenced by soil-water interaction mechanisms such as capillary and adsorptive forces. Quantitative measurement of these water statuses remains challenging, leading to the adoption of advanced techniques. This study uses low-field nuclear magnetic resonance (NMR) technique to investigate water partitioning dynamics and changes in the water state in sodium-rich Wyoming bentonite and calcium-rich Denver bentonite under various humidity conditions. NMR T2 relaxation and T1-T2 mapping techniques, along with a multi-Gaussian decomposition method, enable a quantitative analysis of capillary and adsorptive water in both bentonites. A conceptual water partitioning model is derived to explain water molecule trajectories of water molecules under unsaturated conditions. Our findings indicate distinct transitions in hydrated layers for Na+-smectite and Ca2+-smectite at different relative humidity (RH) ranges. Characteristic T2 ranges are identified for capillary and adsorptive water in both clays and provide valuable insights into their water behavior. This study advances our understanding of soil properties at different RH environments and highlights the potential of low-field NMR techniques in characterizing capillary and adsorptive water in bentonite clays. We utilized low-field NMR techniques to analyze water behavior within bentonite clays, shedding light on the dynamics of moisture within these complex materials.Our research developed a conceptual model that details water partitioning in bentonite clays under unsaturated conditions.We identified distinct transitions in hydrated layers of different types of bentonite clays based on saturation degree, highlighting the unique properties and responses of these clays to varied environmental moisture conditions. .