Coal Fly Ash Derived NaX Zeolite for Arid-Soil Water Retention: A Kinetic and Performance Study

Water scarcity in arid regions is a pressing challenge, particularly for sandy loam soils with poor water-holding capacity.Zeolite materials are widely recognized for their high surface area and porosity, which enable effective improvement of soil water retention. In this study, we synthesize NaX zeolite (FAU) from Xinjiang coal fly ash (CFA) via alkali fusion–hydrothermal processing and conduct a laboratory-scale evaluation of its effect on soil moisture retention. Under optimal conditions (SiO ₂ /Al ₂ O ₃  = 3.0, NaOH/CFA = 1.0, liquid/solid (l/s) = 6:1, 100°C, 12 h), a pure-phase NaX with a BET surface area of 511.01 m ² ·g ^− 1 was obtained. In five-day laboratory tests with 0–20 wt% zeolite amendments, soil bulk density decreased from 1.44 to 1.06 g·cm ^− 3 and total porosity increased from 46.43% to 58.97%. Maximum water-holding capacity rose from 49.70 ± 0.56 g to 65.89 ± 0.38 g, and day-5 water retention improved from 7.27 ± 3.14% to 44.11 ± 1.55%. Moisture-loss kinetics followed a logarithmic model for the control and a single-exponential model for zeolite-amended soils. Increasing zeolite content extended the water half-life (t _1/2 ) from 2.65 to 3.63 days, thereby prolonging plant-available moisture. Considering performance and material use, a 15 wt% amendment is recommended as a cost-effective dose, while 20 wt% is preferable under severe aridity. This approach simultaneously valorizes CFA and improves water management for arid-land agriculture, as demonstrated by this laboratory-scale evaluation.