FRACTAL HUMIC ACIDS IN AQUEOUS SUSPENSIONS AT VARIOUS CONCENTRATIONS,IONIC STRENGTHS, AND PH VALUES

The fractal nature and fractal dimension of soil and peat humic acids were measured in dilute aqueous suspensions at various concentrations, ionic strengths, and pK values. The turbidimetric technique was used, in association with particle size analysis and scanning electron micr oscopy (SEM). The main objectives of the work were to relate the fract al dimension to the underlying morphological features and types of agg regation 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 less than or equa l to 5 in the absence of salt, one soil humic acid assumed a mass frac tal nature and the other anon-fractal nature, whereas the peat humic a cid had a surface fractal nature; (b) at pH greater than or equal to 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 sys tems where the ionic strength was achieved by the addition of CaCl2, o ne soil humic acid maintained a mass fractal regime, whereas the other soil humic acid and the peat humic acid-assumed a surface fractal reg ime, at all pH values. A non-fractal response reflected the existence of humic acid particles having compact, space-filled structures with s mooth surfaces; a surface fractal regime implied compact structures wi th corrugated surfaces; and a mass fractal with decreasing dimension s uggested increasingly porous, fragmented, and elongated structures hav ing increasingly rougher surfaces. These interpretations were supporte d by SEM observations of humic acid particles obtained from the variou s suspensions studied. Low values of the mass fractal dimension measur ed at near neutral pH reflected an underlying aggregation process for humic acid particles, which could be described by a cluster-cluster re action-limited aggregation (RLA) model. High values of the mass fracta l dimension measured at acidic pH values suggested the occurrence of e xtended restructuring and/or reconformation of humic acid macromolecul es with an underlying diffusion-limited aggregation (DLA) model. The f ractal dimension thus provides a numerical parameter that enables a qu antitative description of the morphological features and aggregation p rocesses of humic acid in aqueous suspension. (C) 1997 Elsevier Scienc e B.V.