Size and elemental analyses of nano colloids in deep granitic groundwater: Implications for transport of trace elements

Nano colloids in granitic groundwater obtained from a borehole located at the −300-m access/research gallery of the Mizunami Underground Research Laboratory in Japan were studied in terms of their size distributions by flow-field flow fractionation (Fl-FFF). Colloids were enriched first by ultrafiltration, and then by in-situ enrichment using the focusing technique with a large injection loop and the slot flow technique because of the relatively low concentrations of colloids in the groundwater. The slot flow technique is a colloid enrichment technique inherent to Fl-FFF, where colloids are concentrated inside the channel of Fl-FFF by removing supernatant. Underestimation of colloid sizes by this technique was addressed and corrected, based on empirical linear relationship between the sizes of protein standards with and without the slot flow. The size distributions of organic constituents of colloids were measured by UV/vis and fluorescence detectors and those of inorganic constituents by inductively coupled plasma mass spectrometry (ICP-MS). By comparing the size distributions, possible association of trace elements with host colloidal phases consisting of major elements and organic materials was discussed. Trace elements, namely, Sr, lanthanides (La, Ce, Eu, Lu), actinides (U, Th) and heavy metals (Cu, Zn and W), exhibited distinct size distributions, reflecting their preferential association with certain host colloidal phases.