Size and composition analyses of colloids in deep granitic groundwater using microfiltration/ultrafiltration while maintaining in situ hydrochemical conditions

In studies of colloids in deep groundwater, a serious problem exists because the properties of the colloids are influenced by changes in the chemistry of groundwater upon exposure to the atmosphere and pressure release during sampling. To collect and analyze colloids in deep groundwater without significantly changing the groundwater chemistry, a microfiltration/ultrafiltration technique, while maintaining in situ hydrochemical conditions (mainly hydraulic pressure and anaerobic condition), was developed. The size distributions and compositions of colloids in granitic groundwater obtained at a depth of 300 m at the Mizunami Underground Research Laboratory (MIU) in Japan were studied using the microfiltration/ultrafiltration technique. The shapes and compositions of the fractionated colloids collected on the membrane filter were determined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and attenuated total reflection infrared spectroscopy. Additionally, partitioning of rare earth elements (REEs) based on colloid size was compared with REE concentrations in the filtered and unfiltered groundwater samples using inductively coupled plasma mass spectrometry. Different types of colloids consisting of inorganic and organic substances were observed and complex formation between the colloids and REEs was suggested. Furthermore, characteristic partitioning of REEs depending on colloids size was observed in complex natural environments without chemical disturbance. The current findings are useful for understanding migration of radionuclides.