2nd ANNUAL RYCKMAN LECTURE
LOPATA HALL, Room 101
November 5, 2004
Time: 3 pm
Aquasols: on the role of secondary
minima
Charles
R. O'Melia
Abel Wolman Professor of Environmental Engineering
Johns Hopkins University
The implications for colloid transport of reversible deposition in secondary minima are very different from those involving irreversible deposition in primary minima. First, particles that are continually captured and released will travel much farther in the subsurface than might be expected if the classic irreversible filtration model is applied. Second, and perhaps more significantly, deposition in the secondary well can increase with increasing particle size. Although particle transport by convective diffusion increases as particle size decreases, particle “attachment” in secondary minima decreases with decreasing particle size. Thus, smaller particles (those with diameters in the order of a few tens of nm) would be more effective in the facilitated transport of highly sorbing contaminants such as hydrophobic organic molecules, metals and radionuclides. Other contaminants are themselves particles, such as viruses (10s of nm in diameter) and bacteria (near 1 µm in diameter). Due to this difference in size, viruses could be transported over much larger distances than bacteria. Third, the transport of colloids and, hence, the transport of contaminants associated with them, depends on the Hamaker constant of the particle-water-aquifer media system. Colloids of lower Hamaker constant are likely to be transported farther than colloids of higher Hamaker constant. The extent of adsorption of specific contaminants and the Hamaker constant for the particle-aquifer system are both characteristics of the particles and contribute to the effectiveness of colloid-facilitated transport. Finally, the solution chemistry of the pore waters, through pH, ionic strength, types of solutes, and the valence of the ions, ultimately controls the deposition and release of colloidal particles in porous media. The pH determines the charge density and surface potential of the surfaces. When the surfaces are similarly charged, their interaction can be unfavorable, with an energy barrier and secondary minimum. The ionic strength and valence of the ions determines the shape of the interaction energy curve, including the presence and height of the energy barrier and the presence and depth of the secondary well. Since the subsequent release of a particle depends on the mode in which the particle is deposited (primary or secondary), these factors are particularly important in determining the extent of colloid transport in the subsurface.
Dr. O'Melia is the Abel Wolman Professor of Environmental Engineering at Johns Hopkins University. His research interests are in aquatic chemistry, environmental colloid chemistry, water and wastewater treatment and modeling of natural surface and subsurface waters. He received his MS and PhD degrees from the University of Michigan, and his BS degree from Manhattan College. He has won numerous awards such as the AP Black Research Award from AWWA in 1990, the Gordon Maskew Fair Medal from the Water Environment Federation in 1993, several Awards from the Association of Environmental Engineering Science Professors. In 1989, he was elected to the National Academy of Engineering.