On Critical Coagulation Concentration Theory and Grain Size Analysis of Oxisols

Standard methods for soil grain size analysis are based on the sedimentation velocity of particles, described by Stokes' law. The critical coagulation concentration (CCC) is a concept in wastewater treatment and clay science that states that for a given colloid in suspension there is a concentration of electrolytes at and above which the suspension will begin to undergo rapid coagulation. Despite the fact that dispersions used in soil grain size analysis are for the most part colloidal suspensions, coagulation theory has been overlooked. The objective of this research was to investigate the CCC of two Oxisols and its potential implications in grain size analysis. Soil samples with and without organic matter were dispersed in solution at 0.05% concentration and left to settle in sedimentation cylinders. Solution samples were collected after 3 and 24 h. The NaOH at concentrations of 1, 10, 100, 200, and 400 mmol L–1 was used for evaluating soil dispersion or coagulation behavior. The amount of particles in suspension was evaluated by transmittance at the wavelength of 420 nm. This study shows that the amount of particles in suspension is not a function of time alone, as proposed by Stokes' law, but also of the ionic strength of the solution, which can control fast or slow coagulation, presence of organic matter and type and mineralogical constitution of the soil. Standard grain size analysis need to be adapted based on the effects of the ionic strength on coagulation rates.