CHEMICO-OSMOSIS VERSUS DIFFUSION-OSMOSIS

During the 1960s it became widely recognized that chemico-osmsis is a mechanism by which chemical gradients cause groundwater to move from dilute to more concentrated pore-fluid solutions and is most effective in densely compacted materials of high exchange capacity. Evidence has been accumulating since about 1970 that an additional mechanism may cause groundwater movement in response to chemical gradients and reactions. Some data show that the direction of soil-pore-fluid movement in response to a concentration gradient is opposite to that of chemico-osmosis. Other data suggest that chemically induced groundwater movement may be significant not only in densely compacted materials of high exchange capacity but also in poorly consolidated materials of low exchange capacity. Laboratory evidence is reviewed for the additional mechanism and include recent data on loosely compacted kaolinite and an undistrubed sample of claystone. The additional mechanism appears to be diffusion-osmosis (i.e., the convection, or drag, of bulk pore fluid by the diffusion of solute species). It is suggested that electro-osmosis is a special case of diffusion-osmosis where pore fluid moves in response to the migration of solute species caused by an externally imposed electrical potential gradient.