DIFFUSION-LIMITED AND CHEMICAL-INTERACTION-DEPENDENT SORPTION OF SOIL BACTERIA AND MICROSPHERES

Our objective was to examine the contribution of cell motility, cell surface characteristics and hydrodynamic boundary-layer thickness to variations in sorption rate and transport of bacteria. This was accomplished by analysing the breakthrough of a H-3-labelled strain of a groundwater bacterium and carboxylated or plain microspheres in saturated soil columns. Both transport- and sorption-related processes appeared to influence the rate of sorption of bacteria and spheres. Motile cells adsorbed faster than non-motile cells, especially at a lower cell density (6 x 10(6) cells ml(-1)). The sorption rate increased and the peak effluent concentration decreased as the hydrodynamic boundary layer was compressed by high interstitial water velocities. The influence of variations in motility and boundary-layer thickness on transport indicated that sorption was limited by diffusive mass transfer. Microsphere sorption exceeded bacterial sorption, and spheres were more hydrophobic and had a greater electrophoretic mobility than bacteria. Column breakthrough curves were density-dependent for bacteria but not for microspheres, possibly related to the larger degree of hydrophilicity of bacteria. Although the electrophoretic mobilities were similar, sorption of the anionic species of carboxylated microspheres exceeded that of plain spheres by a factor of 3. This indicated that charge interactions in addition to hydrophobic interactions were important for the rate of microsphere sorption in the low organic C soil (0.2% organic C) used. The role of charge interactions for sorption was further emphasized as the transport rate of bacteria decreased upon removal of the soil organic C by H2O2, possibly unblocking inorganic exchange sites on the sand. It is tempting to suggest an inverse relationship between the retardation of hydrophilic bacteria and the organic C concentration in aquifer soils with very low C contents.