Recovery of Organomineral Gel Properties in Dried Soil Samples

Abstract

Based on earlier study results, the drying process changes the soil properties and, in particular, the characteristic features of a specific soil organic material such as a humic substance (HS). HS is the basis of soil organomineral gels that cover and bind soil particles. When water is removed from soil, gels are subjected to hydrophobization and compression resulting in changes in properties of soil samples. The recovery of soil gels of air-dried samples should reduce the discrepancy between the study data obtained on the soil properties of dried and non-dried soil samples. The study objective is to find ways to recover the structure of soil gels. Samples of six soil types were studied. Vibration viscometry, laser diffractometry, scanning electron microscopy (SEM), photocolorimetry, and conductometry were used in this work. The drying of soil samples increases the size of supramolecular formations (SMFs) in the soil and reduces the soil paste viscosity, a parameter characterizing the structure and the ability of gels to swell. To recover the structure of soil gels, it is proposed to reduce the size of SMFs from HSs to the initial level. SMFs of air-dried samples were separated by soil moistening and subsequent treatment with various temperatures, by ultrasound, and by freezing. Based on the SEM data, heating and ultrasound treatment do not reduce, but enlarge SMFs. Humidification of air-dried soils, exposure to moisture for two weeks, and subsequent freezing bring the paste viscosity of a number of studied soils closer to the condition of samples that were not dried. This process is due to the return of SMFs to size values of the initial soils, as evidenced by the laser diffractometer data on the suspended particle size distribution. Hence, a method for recovery of gel structures in dried soils to the initial state is proposed.