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 microg/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 microL) 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<or=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.
Ranville
James F
JF
Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, United States. jranville@mines.edu
Hendry
M Jim
MJ
Reszat
Thorsten N
TN
Xie
Qianli
Q
Honeyman
Bruce D
BD
D001578:Benzopyrans / Q000032:analysis / Q000737:chemistry*; D031521:Fractionation, Field Flow; D005618:Fresh Water / Q000737:chemistry; D013058:Mass Spectrometry; D012525:Saskatchewan; D014501:Uranium / Q000032:analysis / Q000737:chemistry*; D014875:Water Pollutants, Radioactive / Q000032:analysis / Q000737:chemistry*
D001578:Benzopyrans; D014875:Water Pollutants, Radioactive; D014501:Uranium; C005023:fulvic acid