Evaluation of different clay minerals as additives for soil water repellency alleviation

The objective of this study was to evaluate the effectiveness of kaolinite, illite, Na- and Ca-montmorillonite in alleviating water repellency for a simple model soil material of known composition before and after wetting and drying phases. Sand was rendered water repellent by adding 10 and 30 g kg− 1 stearic acid followed by adding different amounts (1, 2 and 3 mass %) of respective clays. Treated and untreated control sand were wetted and exposed to prolonged drying phase at 50 °C to simulate the effects of wetting followed by drying under a hot spell. The persistence of water repellency was measured with the water drop penetration time (WDPT) test. During the wetting/drying cycle, the control samples (stearic acid treated sand) and clay treated sands behaved in a fashion typically observed in water repellent soils: they were wettable above and water repellent below a critical water content, with repellency persistence (WDPT) increasing with decreasing water content. Kaolinite and Na-montmorillonite were found to be the only clay minerals able to lower the persistence of repellency of the stearic acid sand. The difference between Ca- and Na-montmorillonite ability to alleviate water repellency is explained by the differences in inter-particle forces in the clay–exchangeable cation–water system. During the prolonged drying phase at 50 °C, the persistence of water repellency increased with the duration of heating at 50 °C, with the increase being greatest during the first 48 h in the majority of the samples. While kaolinite and Na-montmorillonite addition resulted in a reduction in the persistence of water repellency of the stearic acid sand, Ca-montmorillonite and illite addition increased the persistence of water repellency of the stearic acid sand. The net effect of clay mineral addition is dependent on the amount of Ca2+ ions occurring at the surface of clay minerals. Pools of available (exchangeable) calcium decrease in the order as follows: Ca-montmorillonite >> illite >> Na-montmorillonite ≈ kaolinite.