The Use of Centrifugation for the Separation of Suspended and Colloidal Forms of Chemical Elements in the Analysis of River Waters: Possibilities and Limitations

The potential of using centrifugation in the separation of suspended forms from dissolved and colloidal forms in the chemical analysis of river waters is demonstrated in the case of rivers in the south of the Russian Far East. The concentrations of colloidal and suspended particles in nonfiltered water, as well as in centrifugates and filtrates, were characterized with the use of the method of dynamic light scattering (DLS). The comparison of DLS intensity in centrifugates and 0.45-µm filtrates was used to calculate the native density of pelitic and coarse colloidal particles of river suspension, which made it possible to correlate the centrifugation regimes with the size of settling particles within the range from 0.45 to 3 µm. Chemical analysis of supernatants of river water, obtained at different centrifugation regimes was used to evaluate the distribution of chemical elements between dissolved/colloidal (<0.45 µm), coarse-colloidal (0.45–1 µm), and pelite (1–3 µm) fractions. A significant linear relationship was found between the DLS intensity and the concentrations in centrifugates of Fe, Al, Ti, Th, Sc, REE, i.e., chemical elements with a high fraction of coarse colloidal and suspended forms, which confirms the possibility to assess the concentration of colloidal particles in centrifugates by the intensity of DLS. A disadvantage of centrifugation is that it is difficult to use in the field.