Size fractionation of humic substances: Effect on protonation and metal binding properties

Humic substances have been extracted from a standard reference peat at pH 2.0, 6.0–6.5, 9.0 and 12.0 using 0.01 M or 0.001 M Na4P2O7. Humic acids extracted at pH 6.0, 9.0 and 12.0 showed a progressive increase in mean molecular size, as determined by gel permeation chromatography. With increasing molecular size there was a significant decrease in titratable acidity for humic acids (ca. 5.2–4.3 meq COOH g−1). However, little variation in Cu(II) binding strength or complexation capacity (g HS)−1 was observed between humic acids (or between fulvic acids) extracted at different pH values. Likewise, there was little variation in the protonation constants when calculated on the basis of a discrete binding site model. The major differences were between fulvic and humic fractions. Fulvic acids had 1. (i) a lower Cu(II) binding strength, 2. (ii) a lower complexation capacity per mol titratable COOH acidity (when batch extracted at pH >6), 3. (iii) a higher titratable acidity (ca. 7.2 (vs. 4.7)meq COOH g−1), 4. (iv) a lower proportion of polyprotic (n>4) binding sites 5. (v) a larger fraction of strongly dissociating COOH groups (42–58% with pKa ca. 2).