TEMPERATURE-DEPENDENT RESTRUCTURING OF FRACTAL HUMIC ACIDS - A PROTON-DEPENDENT PROCESS

Humic acid particles stored at low temperature (4 degrees C) show a fr actal dimension of 1.8. When the temperature is increased to 22 degree s C the fractal dimension ((D) under bar) of the particles in solution increases with time and becomes constant at (D) under bar = 2.3 after 34 h. This is shown by small angle neutron scattering (SANS) on D2O s olutions with an acidity corresponding to pH 5.0 and 0.1 M ionic stren gth. This observation can be explained by the assumption that the low- temperature aggregation proceeds via cluster-cluster interaction and t hat the clusters formed restructure at higher temperature to form more compact particles. Electromotive force measurements show that this re structuring 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 t o 63.2% transformation. The implication of the present results for the transport of water and ions in the soil is discussed.