Quantifying uranium complexation by groundwater dissolved organic carbon using asymmetrical flow field-flow fractionation

The long-term mobility of actinides in groundwaters is important for siting nuclear waste facilities and managing waste-rock piles at uranium mines. Dissolved organic carbon (DOC) may influence the mobility of uranium, but few field-based studies have been undertaken to examine this in typical groundwaters. In addition, few techniques are available to isolate DOC and directly quantify the metals complexed to it. Determination of U–organic matter association constants from analysis of field-collected samples compliments laboratory measurements, and these constants are needed for accurate transport calculations. The partitioning of U to DOC in a clay-rich aquitard was investigated in 10 groundwater samples collected between 2 and 30 m depths at one test site. A positive correlation was observed between the DOC (4–132 mg/L) and U concentrations (20–603 μg/L). The association of U and DOC was examined directly using on-line coupling of Asymmetrical Flow Field-Flow Fractionation (AsFlFFF) with UV absorbance (UVA) and inductively coupled plasma-mass spectrometer (ICP-MS) detectors. This method has the advantages of utilizing very small sample volumes (20–50 μL) as well as giving molecular weight information on U–organic matter complexes. AsFlFFF-UVA results showed that 47–98% of the DOC (4–136 mg C/L) was recovered in the AsFlFFF analysis, of which 25–64% occurred in the resolvable peak. This peak corresponded to a weight-average molecular weight of about 900–1400 Daltons (Da). In all cases, AsFlFFF-ICP-MS suggested that ≤ 2% of the U, likely present as U(VI), was complexed with the DOC. This result was in good agreement with the U speciation modeling performed on the sample taken from the 2.3 m depth, which predicted approximately 3% DOC-complexed U. This good agreement suggests that the AsFlFFF-ICP-MS method may be very useful for determining U–organic matter association in small volume samples. Because the pH (7.0–8.1) and carbonate concentrations of these waters are typical of many groundwaters, these data suggested that facilitated transport of U by DOC may be limited in its importance in many groundwater systems.