Concentration and characterization of organic colloids in deep granitic groundwater using nanofiltration membranes for evaluating radionuclide transport

Abstract

Abstract Understanding the properties of organic colloids is important for geological disposal of high-level radioactive waste in terms of radionuclide transport. To analyze organic colloids in deep groundwater, concentration techniques using adsorption resins and reverse osmosis (RO) membranes have been widely applied, because their concentrations in deep groundwater are very low and detection of the organic colloids in raw groundwater is difficult. However, these techniques have respective disadvantages such as chemical disturbance and membrane fouling caused by cations. To overcome their disadvantages, we propose a new concentration method using nanofiltration (NF) membranes to concentrate organic colloids rapidly without chemical disturbance and to selectively remove monovalent and divalent ions, which may cause inorganic and/or organic fouling. Concentration performance of the NF and RO membranes for aqueous solutions for humic acids was evaluated using a laboratory-scale membrane test unit. The time course of permeate flux and concentration of humic acids were measured. These membranes were applied to the concentration of actual groundwater obtained at a depth of 300 m at the Mizunami Underground Research Laboratory in Japan. The permeate flux and concentration of major ions and organic colloids were measured. The organic colloids concentrated by the NF membrane were successfully analyzed using pyrolysis gas chromatography coupled with mass spectrometry (Py-GC/MS) owing to their high concentrations and low concentrations of salts. The NF membrane was useful for the concentration of organic colloids and rare earth elements (REEs) in deep groundwater, and the findings of the organic colloid structures revealed by Py-GC/MS provided valuable information for evaluating the effect of organic colloids on radionuclide transport.