The effect of temperature and wetting-drying cycles on soil wettability: Dynamic molecular restructuring processes at the solid-water-air interface

摘要

The impact of heat treatment and wetting-drying cycles on the wetting properties of sandy forest soils was explored. Topsoil and upper subsoil were sampled at three beech forest sites in northern Germany. The air-dried soils were treated at 20, 40 and 80 degrees C for 24 h, with materials treated at 20 degrees C serving as reference. After wetting, materials were air-dried or shock-frozen in liquid N-2 and freeze-dried. Interfacial properties were monitored by sessile drop contact angles (CAs) and X-ray photoelectron spectroscopy (XPS), which provide physical and chemical information on the outermost particle interface layer. CAs of reference samples were around 90 degrees and significantly increased after 80 degrees C-treatment to >90 degrees, whereas 40 degrees C-treatment had in comparison to reference soils no distinct impact on CA. Depending on the initial temperature treatment, air-drying after wetting decreased CA to 60-80% and shock-freezing and freeze-drying decreased CA to 10-50% of the reference value. Results suggest that shock-freezing may preserve the organic matter molecular structure that prevails during contact with water at the solid-liquid interface, thus indicating the wettability of the wet surface. Generally, wetting-drying cycles had the least impact on 80 degrees C-treated material. XPS analysis confirmed dynamic interfacial molecular restructuring processes by changes in O and C content and the content of non-polar C compounds. A second heat treatment after two wetting-drying cycles again proved the distinct and pronounced impact of 80 degrees C-treatment on CA, especially with prior shock-freezing and freeze-drying. In conclusion, the findings of our study indicate a sensitive and partly reversible reorganization of the solid interfacial wetting properties. Results may conceptually be used to develop dynamic wettability models, which are needed to simulate the sensitive interplay between wettability and dynamic soil hydraulic functions at sites that are exposed to intensive and periodic moisture fluctuations.