Importance of choosing the refractive index for determining soil granulometric composition by laser diffraction

Abstract

The laser diffraction method is becoming increasingly widespread in the world due to its speed of execution, versatility and reproducibility of results. That is, it is a modern alternative to the classical sedimentation method, but less expensive and more accurate, which makes it very relevant in determining the nature of land damage as a result of military operations. But it requires verification by comparison with standard methods used in Ukraine. The article highlights the results of improving some aspects of the procedure for determining the granulometric composition of soil by the method of laser diffraction of samples using the Mastersizer 3000E laser particle analyzer from Malvern Instruments with a Hydro EV liquid dispersion module. The method of preparing a soil sample for analysis has been improved, the procedure for setting the optical parameter - the refractive index - has been developed, the obtained data have been compared with the results of analysis using the standardized method (DSTU 4730:2007) and statistical analysis of the compared data has been performed. The study was conducted on soil samples of the Forest-Steppe zone of Ukraine and chernozem of the typical Steppe northern under-moistened subzone, which were collected during 2021–2023. Based on the results of the study, an improved method of preparing soil samples for measurement on a laser particle analyzer was proposed, which consists in creating a homogeneous paste after removing carbonates and taking a subsample for further disaggregation by boiling in a sodium hydroxide solution. A method was developed to determine the refractive index for a specific soil by comparing data obtained by the laser diffraction method for different refractive index parameters with data from a standardized method. The optimality criterion is the smallest sum of differences across all fractions. For soils of the Forest-Steppe zone, the refractive index is not the same: 1.39 for typical chernozem and podzolized chernozem of the Forest-Steppe moderately humid subzone and 1.40 for podzolized, dark-gray podzolized, gray forest and light-gray forest chernozem of other subzones of the Forest-Steppe. Analysis of soil sample fractions with the same refractive index showed that not every fraction has a linear relationship, that is, it is not possible to build a reliable regression model in the form of an equation for each fraction, which connects the content of each fraction isolated by the laser diffraction method with the content of each fraction by the DSTU 4730:2007 method. Nevertheless, for most fractions we have good and high quality regression models.Therefore, the laser diffraction method can be used as an alternative method for determining the grain size distribution of the soil.