Size, shape, and stability of organic particles formed during freeze-thaw cycles: Model experiments with tannic acid

Freeze-thaw cycles (FTC) in soils can cause the aggregation of dissolved organic matter but controlling factors are little understood.

In freeze-thaw experiments with tannic acid (TA) as model substance, we studied the effect of TA concentration, pH, electrolytes (NaCl, CaCl₂, AlCl₃), and number of FTC on particle formation. Tannic acid (0.005 to 10 g L^-1) was exposed to 1-20 FTC at pH 3 and 6. The size and shape of particles was determined by confocal laser scanning microscopy. Particle stability was deduced from the equivalent circle diameter (ECD) obtained in dry state and the hydrodynamic diameter measured in thawing solutions.

Tannic acid particles occurred as plates and veins, resembling the morphology of ice grain boundaries. Low pH and presence of electrolytes favored the formation of large particles. The freeze-concentration effect was most intense at low TA concentrations and increased with the number of FTC. While ECD of particles formed at low TA concentrations were smaller than at high concentrations, it was vice versa in the thawed state. At low TA concentrations, higher crystallization pressure of ice caused enhanced stability of large particles. We conclude that FTC can strongly alter the physical state of dissolved organic matter, with likely consequences for its bioavailability.