Molecular-size-distribution-dependent aggregation of humic substances by Na(I), Ag(I), Ca(II), and Eu(III)

• Molecular-size-distribution (MSD)-dependent aggregation of humics was investigated.
• Larger molecular size fractions selectively coagulated by Na and Ca.
• The MSD dependence was attributed to non-specific binding of Na and Ca to humics.
• Large and small molecular size fractions concurrently coagulated by Ag and Eu.
• The reduced MSD dependence was caused by specific binding of Ag and Eu to humics.

Molecular-size-distribution (MSD)-dependent aggregation of Fluka and International Humic Substance Society humic acids (HAs) in the presence of Na(I), Ca(II), Ag(I), and Eu(III) were studied using high-performance size-exclusion chromatography. Larger molecular size fractions (>8000 Da) rich in aliphatic carbons were observed to aggregate in the presence of Na(I) and Ca(II), whereas both large and small molecular size fractions concurrently aggregated in the presence of Ag(I) and Eu(III). In view of the non-specific binding of Na(I) and Ca(II) with HAs, the electric-field screening effect is probably the main cause of HA aggregation by Na(I) and Ca(II), preserving the intrinsic hydrophobicity of the HAs. Reduced surface charges and the hydrophobic heterogeneities of different MSD fractions of HAs by direct binding of Ag(I) and Eu(III) are postulated to induce insignificant dependence of aggregation on the different MSD fractions.

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