The NH4+–N Adsorption Performance Regulated by the External Surface of Water Storage Clay: Particle Size Effect and Path Analysis

Water storage clay is a new type of porous material with a stable structure that can be used as a soil conditioner to enhance the soil's ability to store and transport water and fertilizer. However, its effectiveness in adsorbing NH 4 + –N remains unclear. In order to clarify the adsorption capacity and mechanism of NH 4 + –N by the three-dimensional pore structure of water storage clay, this study employed CT scanning technology to reconstruct the three-dimensional structure of water storage clay of varying particle sizes. Furthermore, the study quantified porosity, fractal dimension, mass specific surface area, and volume specific surface area. The study also proposed a new parameter termed the area coefficient ( AC ). A systematic analysis was conducted of the influence of structural characterization parameters of water storage clay on its NH 4 + –N adsorption behavior, using adsorption kinetics experiments and path analysis. The results showed that the three-dimensional structural characteristic parameters of water storage clay varied significantly with particle size. Small particle size water storage clay has low porosity and fractal dimension, high specific surface area and area coefficient, and dense internal particle structure. Large-sized water storage clay features high porosity and fractal dimension, low specific surface area and area coefficient. Its internal particles are loose and porous, and the pore structure is complex. The adsorption behaviors of water storage clay with different particle sizes for NH 4 + –N all conform to the pseudo-second-order adsorption kinetics model, and the adsorption capacity significantly decreases with the increase of particle size. Path analysis further reveals that the pseudo-second-order kinetic constant (k 2 ) is the main parameter influencing the adsorption capacity. The particle size indirectly regulates the adsorption performance through the volume specific surface area and the mass specific surface area. This study provides a theoretical basis and quantitative support for the application of water storage clay in agricultural nitrogen management. • The first application of CT quantitative analysis of water storage clay structure. • Chemisorption is identified as the rate-limiting step for NH 4 + –N uptake. • Links Volumetric specific surface area to NH 4 + –N adsorption capacity. • Proposes a low-cost, mineral-based alternative for nitrogen management.