Simultaneous flow of water and solutes through geological membranes-I. Experimental investigation

The relative retardation by geological membranes of cations and anions generally present in subsurface waters was investigated using a high pressure and high temperature ‘filtration cell’. The solutions were forced through different clays and a disaggregated shale subjected to compaction pressures up to 9500 psi and to temperatures from 20 to 70°C.

The overall efficiences 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 slightly at higher temperatures for solutions of the same ionic concentration.

The results further show that geological membranes are specific for different dissolved species. The retardation sequences varied depending on the material used and on experimental conditions. The sequences for monovalent and divalent cations at laboratory temperatures were generally as follows: Li < Na < NH₃ < K < Rb < Cs Mg < Ca < Sr < Ba.

The sequences for anions at room temperature were variable, but at 70°C, the sequence was: HCO₃ < I < B < SO₄ < Cl < Br.

Monovalent cations contrary to some field data were generally retarded with respect to divalent cations. The differences in the filtration ratios among the divalent cations were smaller than those between the monovalent cations. The passage rate of B, HCO₃, I and NH₃ was greatly increased at 70°C.