Titration Studies on the Polynuclear, Polyacidic Nature of Fulvic Acid Extracted from Sewage Sludge-soil Mixtures1
- Garrison Sposito and
- Kenneth M. Holtzclaw2
Aqueous solutions of two fulvic acids extracted from sludge-soil mixtures were studied by potentiometric titration with either KOH or HCl in an ionic medium of 0.1 mol/kg KCl. Data were obtained for a range of concentrations of FA at 25°C and for a single concentration at 35°C that duplicated one of those employed at the lower temperature. Formation functions giving the amount of H+ neutralized as a function of the free OH- concentration or of the pH were computed for each set of titration data. Analysis of these functions led to the conclusions that: (i) sludge-derived FA contains functional groups ranging in acidity from very strong (ionized at pH < 2) to very weak (ionized at pH > 10); (ii) the strongly acidic groups are probably sulfonic acid groups and are relatively numerous; (iii) the more weakly acidic groups are probably carboxyls, N-containing groups, phenolic OH, and SH groups; (iv) the strongly acidic groups can bind H+ from solution (counter-ion condensation) when they get close to complete neutralization; (v) the H+ binding effect is enhanced by lowering the concentration of FA or by raising the temperature; and (vi) the reversibility of the titration curves is influenced greatly by the presence of the strongly-acidic functional groups. These conclusions support the conceptualization of sludge-derived FA as a heterogeneous polynuclear polyacid whose intrinsic acidity will show a pronounced dependence on its concentration and on the temperature. Thus the behavior of this material in soil solutions may be expected to be quite complex.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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