Aggregation of polydisperse soil colloidal particles: Dependence of Hamaker constant on particle size

•The size effect of soil colloidal particles was investigated.•Surface charge properties differed for soil NP and CP.•The critical coagulation concentration (CCC)/mobility of soil NP was higher than CP.•Size effect of soil particle influences CCC through its effect on Hamaker constant. The dispersion and coagulation of soil colloidal particles concern highly with their mobility and activity, as well as the role played in biogeochemical cycle of elements. Particle size is an important factor that affects both the van der Waals potential energy and electrostatic potential energy. However, the size effect of soil particles on surface charge properties and suspension stability has rarely been investigated. Results showed that the zeta potentials (in absolute values) of soil colloidal particles (CP, particle diameter less than 1000 nm) were higher than soil nanoparticles (NP, particle diameter less than 100 nm) for the same solution pH, while the specific surface area of soil NP was 1.6 times of soil CP; taken together, the surface charge density of soil NP was smaller than that of soil CP and the surface charge number of soil NP was slightly higher than soil CP. The stability of soil NP and CP was also different. The critical coagulation concentration (CCC) of soil NP was 1.4 times of soil CP, indicating higher mobility of smaller soil particle in natural conditions. Based on DLVO theory, the Hamaker constants of soil NP and CP were simulated to be 2.06 × 10−20 J and 1.86 × 10−20 J. It could be concluded that the size effect of soil particle influences suspension stability and particle mobility through its effect on Hamaker constant. The results could deepen our understanding for aggregation mechanisms of soil colloid-sized particles and further help in predicting their environmental behaviors.