Fractal Dimension of Humic Acids in Aqueous Suspension as a Function of pH and Time

Abstract

AbstractThe fractal dimensions of soil and peat humic acids equilibrated for various lengths of time in aqueous suspension at different pH values were determined by measuring their turbidity as a function of wavelength, with collaboration by scanning electron microscopy (SEM). Particle‐size distributions of the humic acid systems were also determined. The analysis of the power‐law dependence of the turbidity on the wavelength revealed that soil and peat humic acids may exhibit either a nonfractal or a mass fractal nature. Peat humic acid could also be described as a surface fractal. The passage from one regime to another depended on the pH and equilibration time. With an increase of either factor, the mass fractal dimension of humic acids decreased from about 2.8 to values close to 1.0. This trend, supported by SEM observations, suggested that humic particles in suspension evolve from compact, almost space‐filled structures with smooth surfaces, to less compact, fragmented and elongated structures with increasingly rough and irregular surfaces as the pH or time increases. Values of the mass fractal dimension close to 2.5 would reflect diffusion‐limited aggregation (DLA), whereas values close to 2.1 would suggest a reaction‐limited cluster‐cluster (RLA) aggregation model for humic acid. The fractal dimension thus represents a numerical parameter able to describe quantitatively the morphological features assumed by humic particles in suspension and to provide information on the underlying aggregation processes.