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

✍ Scribed by H.Kipton J. Powell; Eric Fenton

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 990 KB
• Volume: 334
• Category: Article
• ISSN: 0003-2670
• DOI: 10.1016/s0003-2670(96)00324-8

✦ Synopsis

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 Na4P207. 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. 5.243 meq -COOHg-'). However, little variation in Cu(I1) binding strength or complexation capacity (gHS)-' 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 (i) a lower Cu(I1) binding strength, (ii) a lower complexation capacity per mol titratable COOH acidity (when batch extracted at pH >6), (iii) a higher titratable acidity (ca. 7.2 (vs. 4.7)meq -COOH g-l), (iv) a lower proportion of polyprotic (n>4) binding sites and (v) a larger fraction of strongly dissociating -COOH groups (42-58% with pK, ca. 2).