The Department of Defense (DoD) is responsible for managing thousands of sites with metal contamination, of which copper (Cu), lead (Pb), and zinc (Zn) are among the most frequently targeted for cleanup due to their ecological and human health risk. Existing methods to determine metal accessibility or availability to organisms are often too uncertain to enable site management on the basis of metal bioavailability. Effective alternatives to costly site-specific empirical assays are needed.
The objective of this limited-scope project was to measure the chemical stability and diffusivity of sub-micromolar concentrations of dissolved Cu(II), Pb(II), and Zn(II) in model estuarine sediment pore waters using scanned stripping chronopotentiometry (SSCP).
This project sought to determine the capability of SSCP to measure the strength of aqueous Cu, Pb, and Zn complexes with dissolved organic matter (DOM) and sulfide in model aqueous systems. SSCP measures the stability of aqueous metal-ligand complexes by chemical dissociation with an electrochemical force spanning the range of binding strengths of the complexes. SSCP measurements were performed in synthetic, chemically defined media in the laboratory to (1) determine the method’s capabilities and refine its execution, (2) ensure that qualitatively the relative chemical stabilities of dissolved metal-organic complexes was known, which was achieved by using well-characterized organic matter isolated from natural waters as well as synthetic organic compounds, and (3) examine trends across a defined sulfide gradient.
The SSCP results confirmed the relative chemical stabilities of the metal-DOM complexes and discerned these stabilities across conditions that subtly varied with respect to aromaticity and reduced sulfur content. Dissolved metal speciation at the 0.25 µmol/L (16 µg Cu and Zn/L and 52 µg Pb/L) concentration level was measured. Researchers also designed a custom procedure within commercially available electrochemistry software to automate SSCP analysis and clarify which physicochemical model to use to derive chemical stability coefficients and diffusivity from SSCP data.
This project evaluated a tool that has the potential to provide a self-contained, nonempirical surrogate for the potential bioaccessibility of Cu, Pb, and Zn in aquatic sediments. SSCP is inexpensive, can quantify metals down to 0.4 nM, requires minimal sample handling, prevents matrix interference, can be interpreted with a minimum number of assumptions, and is complementary to bioassays. Follow-on work is recommended to compare metal speciation parameters derived from SSCP to biological indicators of metal bioavailability under controlled laboratory conditions and to test the method under natural conditions.