Centrifugal ultrafiltration preconcentration for studying the colloidal phase of a uranium-containing soil suspension

The objective of this work was to explore centrifugal ultrafiltration (UF) to separate and / or preconcentrate natural colloidal particles for their characterization. A soil suspension obtained by batch leaching was used as a laboratory reference sample. It was preconcentrated with concentration factors (CF) varying from 10 to 450. The dimensional analysis of the colloidal phase was carried out by Asymmetric Flow Field-Flow Fractionation (AF4)-multidetection. The colloidal masses were estimated by mass balance of the initial suspension, its concentrates and filtrates. The size-dependent distribution (expressed in gyration radius) and total colloidal mass (especially recovery), as well as chemical composition and concentration (including species partitioning between dissolved and colloidal phases) were determined to assess the effects of UF preconcentration on colloidal particles. The gyration radius of the colloidal particles recovered in these concentrated suspensions ranged from about 20 nm to over 150 nm. Neither de-agglomeration nor agglomeration was observed. However, only (64 ± 4) % (CF = 10) of the colloidal particles initially in the soil suspension were found in the recovered concentrated suspensions, and this percentage decreased as CF increased. The filter membrane trapped all other particles, mainly the larger ones. Whatever the CF, the centrifugal UF did not appear to change the dissolved-colloidal partitioning of certain species (Al, organic carbon); whereas it led to an enrichment of the colloidal phase for others (Fe, U). The enrichment rate was specific to each species (15% for Fe; 100% for U). By fitting the observed trends (i.e. conservation, depletion or enrichment of the colloidal phase in the concentrate) as a function of CF, the colloidal concentrations (total and species) were assessed without bias. This methodology offers a new perspective for determining physicochemical speciation in natural waters, with a methodology applicable for environmental survey or site remediation studies.