Fractal humic acids in aqueous suspensions at various concentrations, ionic strengths, and pH values

Abstract

The fractal nature and fractal dimension of soil and peat humic acids were measured in dilute aqueous suspensions at various concentrations, ionic strengths, and pH values. The turbidimetric technique was used, in association with particle size analysis and scanning electron microscopy (SEM). The main objectives of the work were to relate the fractal dimension to the underlying morphological features and types of aggregation process that characterize humic acids in aqueous suspensions under various conditions. Analysis of the power-law dependence of the turbidity on the wavelength revealed that: (a) at pH ⪯ 5 in the absence of salt, one soil humic acid assumed a mass fractal nature and the other a non-fractal nature, whereas the peat humic acid had a surface fractal nature; (b) at pH ⪰ 6 in the absence of salt, and at all pH values in the presence of NaCl, the three humic acids exhibited a mass fractal nature; and (c) in systems where the ionic strength was achieved by the addition of CaCl 2, one soil humic acid maintained a mass fractal regime, whereas the other soil humic acid and the peat humic acid assumed a surface fractal regime, at all pH values. A non-fractal response reflected the existence of humic acid particles having compact, space-filled structures with smooth surfaces; a surface fractal regime implied compact structures with corrugated surfaces; and a mass fractal with decreasing dimension suggested increasingly porous, fragmented, and elongated structures having increasingly rougher surfaces. These interpretations were supported by SEM observations of humic acid particles obtained from the various suspensions studied. Low values of the mass fractal dimension measured at near neutral pH reflected an underlying aggregation process for humic acid particles, which could be described by a cluster-cluster reaction-limited aggregation (RLA) model. High values of the mass fractal dimension measured at acidic pH values suggested the occurrence of extended restructing and/or reconformation of humic acid macromolecules with an underlying diffusion-limited aggregation (DLA) model. The fractal dimension thus provides a numerical parameter that enables a quantitative description of the morphological features and aggregation processes of humic acid in aqueous suspension.