DIFFUSION-LIMITED AND CHEMICAL-INTERACTION-DEPENDENT SORPTION OF SOILBACTERIA AND MICROSPHERES

Our objective was to examine the contribution of cell motility, cell s urface characteristics and hydrodynamic boundary-layer thickness to va riations in sorption rate and transport of bacteria. This was accompli shed by analysing the breakthrough of a H-3-labelled strain of a groun dwater bacterium and carboxylated or plain microspheres in saturated s oil columns. Both transport- and sorption-related processes appeared t o influence the rate of sorption of bacteria and spheres. Motile cells adsorbed faster than non-motile cells, especially at a lower cell den sity (6 x 10(6) cells ml(-1)). The sorption rate increased and the pea k 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 indi cated that sorption was limited by diffusive mass transfer. Microspher e sorption exceeded bacterial sorption, and spheres were more hydropho bic and had a greater electrophoretic mobility than bacteria. Column b reakthrough curves were density-dependent for bacteria but not for mic rospheres, possibly related to the larger degree of hydrophilicity of bacteria. Although the electrophoretic mobilities were similar, sorpti on of the anionic species of carboxylated microspheres exceeded that o f plain spheres by a factor of 3. This indicated that charge interacti ons in addition to hydrophobic interactions were important for the rat e of microsphere sorption in the low organic C soil (0.2% organic C) u sed. The role of charge interactions for sorption was further emphasiz ed as the transport rate of bacteria decreased upon removal of the soi l organic C by H2O2, possibly unblocking inorganic exchange sites on t he sand. It is tempting to suggest an inverse relationship between the retardation of hydrophilic bacteria and the organic C concentration i n aquifer soils with very low C contents.