Clays and shales serve as semipermeable membranes, retarding by varying degrees the passage of the dissolved species with respect to water. The relative retardation by geologic membranes of cations and anions generally present in waste solutions was investigated using a high temperature filtration cell. The solutions were forced with varying hydraulic gradients through different clays and a disaggregated shale subjected to compaction pressures up to 10,000 psi and to temperatures from 20 to 90°C.

The efficiencies measured increased with increase of exchange capacity of the material used and with decrease in concentration of the input solution. The efficiency of a given membrane increased with increasing compaction pressure, but decreased at higher temperatures and higher hydraulic gradients for solutions of the same ionic concentration.

The results further show that geologic membranes are specific in that the degree of retardation is different in different dissolved species. The retardation sequences obtained varied depending on the material used and on experimental conditions. The retardation sequences for monovalent and divalent cations were generally as follows:

Li < Na < NH₃ < K < Rb < Cs
Mg < Ca < Sr < Ba.

The retardation sequences for anions at room temperature were variable, but at higher temperatures the sequence was:

HCO₃ < I < B < SO₄ < Cl < Br.

Monovalent cations generally were retarded with respect to divalent cations at the higher hydraulic gradients. This trend, however, was reversed at the lower hydraulic gradients. Extrapolation of the results to average hydraulic gradients encountered in subsurface formations indicates agreement with data obtained from field investigations which show that divalent cations generally are retarded with respect to monovalent cations.

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