Particle size of humic acid

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.