AGGREGATION OF HUMIC SUBSTANCES IN AQUEOUS-MEDIA AS DETERMINED BY LIGHT-SCATTERING METHODS

Samples of peat humic acid (PHA) and surface water humic (WBHA) and fulvic acids (WBFA) extracted from Whitray Beck in North Yorkshire, UK and previously studied by analytical (UV scanning) ultracentrifugation at low concentrations (10-20 mg dm-3) (Reid et al., 1990) have been investigated by dynamic and static light-scattering methods at concentrations up to approximately 1700 mg dm-3. Dynamic light-scattering gave very low diffusion coefficients for the humic acids (PHA and WBHA) suggesting the presence of aggregates. Static light-scattering from the fulvic acid (WBFA) showed that the second virial coefficients of this material in various media were negative, which is characteristic of reversible aggregation. The aggregates, which consisted of approximately 20 smaller molecules, could not be dissociated completely by the detergents sodium n-dodecylsulphate or Triton X-100, although limited dissociation by the latter may occur. Sedimentation velocity measurements in the same concentration range as the light scattering gave sedimentation coefficients which increased with concentration also consistent with reversible aggregation. The studies illustrate the problems inherent in the characterization of humic substances from natural sources and demonstrate that, at least for lower molecular weight fulvic acids, static light-scattering from solutions of relatively high concentration in combination with sedimentation equilibrium measurements at low concentrations distinguishes a system having a broad molecular weight distribution from one in which aggregation occurs.