Complexation of aqueous elements by DOC in a clay aquitard

The extent of partitioning of several elements (Cu, Mn, Mo, Ni, Sr, U, and Zn) on dissolved organic carbon (DOC) was investigated in pore water samples collected from a clay-rich aquitard. High DOC concentrations in the aquitard, ranging from 21 to 143 mg C/L, and natural aqueous metal concentrations higher than in most ground water environments facilitated complexation studies at this site. Analyses were conducted using on-line coupling of asymmetrical flow field-flow fractionation with ultraviolet, total organic carbon, and inductively coupled plasma-mass spectrometry detectors. Of the elements investigated, only U and Zn were complexed with all DOC samples, ranging from 2.2 to 60 microg U/g DOC (0.4% to 3% of the total U in the pore water) and 0.04 to 0.5 microg Zn/g DOC (0.1% to 0.9% of the total Zn in the pore water), respectively. Laboratory experiments conducted over a range in pH (1.3 to 9.7) and geochemical modeling supported the measured complexation of U and Zn on the DOC. The in situ association constant, K(d), for U decreased with depth from 76 mL/g C for pore water samples at 2.2 m below ground (BG) to 24 mL/g C at 9.7 m BG. The decrease was attributed to a decrease in aromaticity of the DOC with depth. Zn K(d)constants ranged from 2 to 12 mL/g C and exhibited no trend with depth. Results of the current study suggest minor masses of U and Zn (less than or equal to 4% of total) complex with this DOC under in situ pH conditions. Our data suggest that competitive complexation by other ligands may limit the importance of DOC-facilitated transport of the elements studied in water of similar chemical composition.