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Soil Science Society of America Journal Abstract -

Particle Size of Humic Acid


This article in SSSAJ

  1. Vol. 57 No. 1, p. 283-285
    Received: June 3, 1991

    * Corresponding author(s):
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  1. R. Österberg ,
  2. I. Lindovist and
  3. K. Mortensen
  1. Dep. of Chemistry, Univ. of Agricultural Sciences, P.O. Box 7015, S-75007 Uppsala, Sweden
    Dep. of Physics, Risø National Lab., Roskilde, Denmark



The largest humic acid particles that can exist in solutions of the same acidity as most Scandinavian soil and water systems (but at a higher concentration, 1–4 mg/mL) are ≈ 110 nm in their largest dimension; these particles, thus, are of about the same magnitude as large viruses. This is shown by small-angle neutron scattering at 10°C, pH 5, and 0.10 M ionic strength (NaCl). The mean radii of gyration of humic acids obtained from three different soil samples were found to be of the same magnitude: 21.4, 29.7, and 28.3 nm; the mean molecular masses were 500, 2000, and 1500 kDa. The radius of gyration of the cross section for the particles in solution was found to be constant, 2.1 nm; thus, the humic acid particles might essentially vary only in regard to length. A contrast variation study of one of the samples was found to be consistent with the particles in solution having a cross section with less than average scattering density in its central part. This supports the idea that the particles might contain a central “core” with a different structure than the peripheral parts.

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