Size characterization of bentonite colloids by different methods

The size and shape of colloids released from a natural bentonite into a low -mineralized groundwater are investigated using various colloid characteriz ation methods. For the applied methods such as atomic force microscopy (AFM ) laser-induced breakdown detection (LIBD), photon correlation spectroscopy (PCS), and flow field-flow fractionation coupled to ICP-mass spectrometric detection (FFFF-ICPMS), the respective raw size data have to be corrected in order to consider chemical composition and shape of the colloids as well as instrumental artifacts. Noncontact mode AFM of the bentonite colloids s hows disklike shapes of stacked smectite platelets with a mean height-to-di ameter proportion (aspect ratio) of similar to1/10. A broad particle number size distribution is determined by image processing with a mean particle d iameter of 73 mn. In agreement with AFM, a broad size distribution is also found by PCS and FFFF-ICPMS. Likewise, mean particle sizes found by (LIBD) (67 +/- 13 nm) and FFFF-ICPMS (maximum in the number size distribution, sim ilar to 70 nm) are in fair agreement with the AFM data. Somewhat higher val ues are obtained by PCS, where mean particle diameters of the intensity-wei ghted size distributions of larger than 200 nm are found (depending on the algorithm used for data processing). The influence of the disklike particle shape on the results of the individual methods is discussed. As a conclusi on, the application of different colloid characterization methods is a prer equisite to get complementary information about colloid size and shape, whi ch is essential for the understanding of natural colloidal systems.