Experimental Verification of a Chlorine-37 Minimum in Concentration Polarization Layer Profiles as Predicted by a Hyperfiltration Model

Purdue Research Foundation, West Lafayette IN

Investigators

Abstract

Fritz - 0001054 Hyperfiltration (a.k.a. "reverse osmosis") occurs when solution advects through permselective material like clays. Isotopes of solutes are advected toward the membrane of the same velocity; however, they diffuse back into the high-pressure reservoir of different rates owing to differences of individual isotope's aqueous diffusion coefficients. For a two-isotope solute like NaCl, the developed hyperfiltration model predicts that signfiicant fractionation (>5 per mil relative to SMOC) should occur in pore water of arenaceous and/or carbonate material adjacent to the upgradient side of a clay lens or shale. The easiest way to test the model is to experimentally investigate its most specific prediction: the d37Cl minimum. This aspect of the model is easily testable because solution fluxes of 10-5 to 10-7 cm.sec-1 are easy to replicate in a laboratory. Calculations show that the position of the d37Cl minimum occurs at about 10 of the length of the developed concentration polarization layer (as measured outward from the membrane's high-pressure interface). The predicted magnitude of the d37Cl minimum can attain -5 per mil for a clay membrane of moderate (20%) ideality. The unique aspect of this project is in situ sampling of solutions directly from a CPL. Until now, this has neither been done nor even considered. The proposed experiments will yield show unequivocally that a minimum does (or does not) exist. Should the experiments fail to confirm the theory, then hyperfiltration plays no meaningful role in fractionation of solute isotopes in aqueous systems. If, however, the experimental data support the model, then isotope geochemists may have to reevaluate interpretations of their solute isotope data if their groundwater samples flowed through membrane-functioning units.