Quantitatively distinguishing colloidal composition, surface property of differently sized particles for permanently and variably charged soils

The soil can be classified into two categories according to its charge properties: permanently charged (PE) soil and variably charged (VA) soil. This study distinguished the colloidal compositions and surface properties of differently sized particles for PE and VA soils, in order to clarify the relationship between particle diameter distribution, mineral/organic composition and surface properties, reveal the relationship between the colloidal properties of each particle diameter range and the colloidal properties of overall soil, and evaluate the influence of soil particles with different diameters on overall soil properties. The findings indicated that: (1) In the case of PE soil, the content of permanent-charge minerals (montmorillonite, illite) is 29.9 %, constituted 93 % of the total secondary minerals (montmorillonite, illite, kaolinite, hematite, goethite). While in VA soil, the content of permanent-charge minerals is 23.1 %, accounting for 48 % of the total secondary minerals. Additionally, the surface charge number of PE soil was 2.2 times higher than that of VA soil. (2) For PE and VA soils, the distribution of clay minerals and organic matter was predominantly observed in particles <500 nm, followed by a relatively lower concentration in particles ranging from 500 to 2000 nm. Lowest amounts were found in particles >2000 nm. (3) <500 nm particles contributed the majority of surface charges to the bulk soil: 57 % and 51 % for PE and VA soils, respectively. Simultaneously, these particles accounted for 70 % in PE soil and 59 % in VA soil of the specific surface area. (4) The contributions of organic matter to the specific surface area and surface charge of PE soil were 11 % and 23 %, respectively, while for VA soil they were 8 % and 11 %. Subsequently, upon removal of organic matter, there was a reduction in both the specific surface area and surface charge of the PE and VA soils. (5) With the reduction in particle diameter, there was an observed increase in the levels of organic matter and secondary minerals (particularly clay minerals) within the soil. Additionally, there was an upward trend in both specific surface area and surface charge for VA soil; however, no significant changes were noted for PE soil regarding specific surface area and surface charge. Organic matter and kaolinite contents were identified as the primary influencing factors on surface area and surface charge of VA soil, while montmorillonite content was identified as the primary influencing factors on surface area and surface charge of PE soil.