Critical Coagulation Concentration of Oxide Rich Soil Colloids: Implications on the Stability and Transport of Nanoparticles in the Environment.

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 soil dispersions are for the most part colloidal suspensions, coagulation theory has been overlooked. The objective of this research was to investigate the CCC of two iron and aluminum oxides rich Oxisols and its potential implications on the stability of these colloids in soil solution and aqueous environment. 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 h and 24 h. NaOH at concentrations of 1 mmol L^-1, 10 mmol L^-1, 100 mmol L^-1, 200 mmol L^-1 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 at any given time is a function 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. As many of the synthetic and natural nanoparticles are composed of metal oxides, this study shows that the chemical environment where these particles are being deposited plays a major role in the transport or precipitation of these materials.