An integrated colloid fractionation approach applied to the characterisation of porewater uranium–humic interactions at a depleted uranium contaminated site

Methods for the fractionation of aquatic colloids require careful application to ensure efficient, accurate and reproducible separations. This paper describes the novel combination of mild colloidal fractionation and characterisation methods, namely centrifugal ultrafiltration, gel electrophoresis and gel filtration along with spectroscopic (UV–visible) and elemental (Inductively Coupled Plasma-Optical Emission Spectroscopy, Inductively Coupled Plasma-Mass Spectrometry) analysis, an approach which produced highly consistent results, providing improved confidence in these methods. Application to the study of the colloidal and dissolved components of soil porewaters from one soil at a depleted uranium (DU)-contaminated site revealed uranium (U) associations with both large (100 kDa–0.2 µm) and small (3–30 kDa) humic colloids. For a nearby soil with lower organic matter content, however, association with large (100 kDa–0.2 µm) iron (Fe)–aluminium (Al) colloids in addition to an association with small (3–30 kDa) humic colloids was observed. The integrated colloid fractionation approach presented herein can now be applied with confidence to investigate U and indeed other trace metal migration in soil and aquatic systems.