Temperature-dependent restructuring of fractal humic acids: A proton-dependent process

Humic acid particles stored at low temperature (4°C) show a fractal dimension of 1.8. When the temperature is increased to 22°C the fractal dimension (D) of the particles in solution increases with time and becomes constant at D=2.3 after 34 h. This is shown by small angle neutron scattering (SANS) on D₂O solutions with an acidity corresponding to pH 5.0 and 0.1 M ionic strength. This observation can be explained by the assumption that the low-temperature aggregation proceeds via cluster-cluster interaction and that the clusters formed restructure at higher temperature to form more compact particles. Electromotive force measurements show that this restructuring is accompanied by a fast release of protons followed by a slow consumption of protons. In agreement with the SANS data, the slow uptake of protons relaxes with a value of tau = 22 h, corresponding to 63.2% transformation. The implication of the present results for the transport of water and ions in the soil is discussed.