Soil particle size and Hofmeister effects on soil colloidal aggregation

Soil aggregates play a vital role in maintaining soil structure. In this study, soil colloid particles (<2000 nm) were further divided into three size fractions (<500 nm, 500–1000 nm, and 1000–2000 nm) to investigate the coupled effects of particle size and cation specificity (Li ⁺, Na ⁺, K ⁺, Cs ⁺) on soil colloid aggregation. The results showed: (1) For each size fraction, the aggregation rate and critical coagulation concentration (CCC) exhibited consistent ion-specific effects that was originated from the asymmetric hybridization of cation outer electron orbitals. (2) Among all size fractions, <500 nm soil colloid particles showed the fastest aggregation rate and required the lowest electrolyte concentration for rapid aggregation, while 1000–2000 nm particles not only had the slowest aggregation rate but also completely failed to achieve rapid aggregation. Although <500 nm colloid particles carried the highest charge quantity, their large specific surface area resulted in the lowest actual surface charge density and weakest interparticle electrostatic repulsion. Moreover, the Brownian motion of <500 nm particles was nearly 100 times that of 1000–2000 nm particles. (3) The main clay mineral affecting charge quantity and specific surface area across different size fractions was montmorillonite, <500 nm particles contained the highest montmorillonite content. Based on these findings, three key conclusions are drawn: (1) <500 nm particles play a crucial role in overall soil particle aggregation, as their rapid Brownian motion enables active collisions with >1000 nm particles, driving collective particle coagulation. (2) Effective soil aggregation occurs exclusively when <500 nm particles interact with cations exhibiting asymmetric outer electron orbital hybridization. (3) For constant-charge soil, montmorillonite-type clay minerals serve as essential material foundations promoting soil particle aggregation.