Chemical osmosis in compacted clayey material and the prediction of water transport

Compacted clay membranes are semi-permeable if the double layers of the cla y particles overlap, thereby restricting the passage of ions. Semi-permeabi lity is quantified by the reflection coefficient sigma. In the design of cl ay liners for waste contaminant water, transport as a result of coupled tra nsport is rarely taken into account. Where large salt concentration differe nces exist across natural or man-made clay barriers, water may be transport ed as a result of chemical osmosis. In a flexible wall permeameter two samples of a commercially available Wyom ing Na-bentonite were subjected to a chemical gradient in order to monitor water transport and to obtain values for the reflection coefficient. In bot h experiments water transport by chemical osmosis was observed, and reflect ion coefficients of 0.003 and 0.001 were obtained, which are significantly lower than those predicted by the Fritz-Marine model and values obtained fr om Bolt (1982). However, the values are in good agreement with those obtain ed by Bresler (1973). Both experiments showed a period of 50 h of linear pr essure increase as a result of chemical osmosis, after which the pressure d ifference dropped, i.e. after reaching a maximum pressure difference the wa ter flow was reversed. The reversal of the water flow is consistent with di ffusion osmosis, which is the transport of water as a result of the diffusi on of ions in the absence of an externally applied electrical field. Howeve r, diffusion osmosis is limited to clays of low cation exchange capacity wi th high pore water concentrations and porosities. (C) 1999 Elsevier Science B.V. All rights reserved.