Aqueous solutions of two fulvic acids (FA) extracted from sludge‐soil mixtures and converted to K‐fulvates were titrated potentiometrically at 25°C with HCl in ionic media of 0.1M and 0.2M KCl. Data were obtained for a range of concentrations of FA, although during a titration experiment both FA concentration and ionic strength were rigorously held constant. A simple model was employed to interpret the formation functions calculated using the titration data. This model ascribes the acidic properties of a FA solution to an assembly of hypothetical macromolecules (“mean fulvic acid units”) that contain several distinct classes of functional group and are able to associate through hydrogen bonding. Conditional protonation constants were computed for each class of acidic functional group that protonates in the pH range 3–11. The common logarithms of the average protonation constants at 25°C were found to be 3.9, 6.5, and 9.4 for the three classes of weakly acidic functional groups studied. These values were independent (within experimental error) of FA composition and concentration, and of ionic strength in the range investigated.
The distribution of Al, Cd, Cu, Fe, Ni, and Zn among the humic acid, fulvic acid, and precipitable (by adjustment of pH) fractions extracted with 0.5N NaOH from four representative sewage sludges was determined by atomic absorption spectrophotometry. It was found that most of the Al and, with one exception, most of the Fe and Zn were precipitable from the extracted solutions by adjustment of pH in unit intervals between 2.5 and 10.5. Almost all of the Cu was associated with the humic acid fraction. Cd and Ni tended to be associated with the precipitable fraction and with the fulvic acid fraction. Data on the amount of each metal remaining in the extracted solutions as a function of pH suggested that Cd, Ni, and Zn would tend to be the more mobile trace metals in soils affected by application of the sludges investigated.
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