NEW INSIGHTS INTO THE IDENTITY, CHARACTERISTICS, AND TRANSPORT OF SMALL BIOTIC AND ABIOTIC PARTICLES IN THE CRITICAL ZONE

Advances in sample mounting and data interpretation techniques for atomic force microscopy (AFM) allow for the direct measurement of approach and adhesion forces that play a key role in the transport of micro-organisms and colloids in porous media. Small abiotic and biotic particles, including individual, viable bacterial cells, can be attached to the tip of an AFM cantilever. As the mounted object is moved towards a surface in air or solution with sub-nanometer control, forces in the sub-nanoNewton regime can be measured as a function of particle-collector separation. In conjunction with a mathematical construct capable of utilizing experimentally derived force measurements, such as the Interaction Force Boundary Layer (IFBL) model, sticking efficiencies can be obtained and compared with those estimated from column and field studies, as well as DLVO theoretical models.

To date, we have determined sticking efficiencies for 2 micron carboxylated polystyrene microspheres and Enterococcus faecalis cells against a silica glass surface, the latter which simulates course quartz grains. Sticking efficiencies for the bacteria are smaller than those determined from column and field studies in comparable systems; however, sticking efficiencies derived from AFM data and the IFBL model more closely represent field data than those calculated via DLVO models. A comparison with different methods of calculating sticking efficiencies suggests that reversible adhesion may be significant in larger-scale transport studies.

Finally, various transmission electron microscope (TEM) sample preparation techniques have been developed to better search for nanoparticles in surface water, groundwater, and drinking water distribution systems, and to determine their role in the transport of toxic heavy metals. For example, Mn- and Fe-oxide nanoparticles and various sulfide nanoparticles, as small as 1.5 nm in diameter and all containing heavy metals, have been discovered in the Clark Fork Superfund Complex in Montana, USA. These nanoparticles are clearly important in transporting heavy metals tens to hundreds of kilometers down hydrologic gradient.