Complexation of aqueous elements by DOC in a clay aquitard

Area of Science:

• Environmental Chemistry
• Geochemistry
• Environmental Science

Background:

• Clay-rich aquitards can contain high concentrations of dissolved organic carbon (DOC).
• High DOC and natural metal concentrations in aquitard pore water create unique conditions for studying element complexation.
• Understanding element partitioning on DOC is crucial for predicting contaminant transport in subsurface environments.

Purpose of the Study:

• To investigate the complexation of various elements (Cu, Mn, Mo, Ni, Sr, U, Zn) with DOC in a clay-rich aquitard.
• To quantify the extent of U and Zn complexation with DOC under in situ conditions.
• To determine factors influencing the complexation of U and Zn with DOC, such as pH and DOC properties.

Main Methods:

• Analysis of pore water samples from a clay-rich aquitard using asymmetrical flow field-flow fractionation (AF4) coupled with multiple detectors (UV, TOC, ICP-MS).
• Laboratory experiments to assess the effect of pH on U and Zn complexation with DOC.
• Geochemical modeling to support experimental findings and interpret complexation behavior.

Main Results:

• Only Uranium (U) and Zinc (Zn) showed significant complexation with DOC across all samples.
• Complexation of U ranged from 0.4% to 3% of total U, while Zn complexation was 0.1% to 0.9% of total Zn.
• Uranium's in situ association constant (Kd) decreased with depth, linked to decreasing DOC aromaticity, while Zn's Kd showed no depth trend.

Conclusions:

• Dissolved organic carbon in this aquitard weakly complexes with U and Zn, with less than or equal to 4% of total U and Zn bound to DOC.
• The complexation of U and Zn with DOC is influenced by factors like DOC aromaticity and potentially other competing ligands.
• DOC-facilitated transport of U and Zn in similar subsurface environments may be limited due to competitive complexation effects.