Variability in the Properties of Dissolved Organic Matter and its Influence on Speciation of Heavy Metals (Cd, Cu, Pb)EPA Grant Number: R825395
Title: Variability in the Properties of Dissolved Organic Matter and its Influence on Speciation of Heavy Metals (Cd, Cu, Pb)
Investigators: Benoit, Gaboury , Yu-Ping Chin, P. I.
Current Investigators: Benoit, Gaboury , Fisher, Nicholas S , Yu-Ping Chin, P. I.
Institution: Yale University , The Ohio State University
Current Institution: Yale University , The Ohio State University , The State University of New York at Stony Brook
EPA Project Officer: Hiscock, Michael
Project Period: December 5, 1996 through December 4, 1998 (Extended to August 31, 2000)
Project Amount: $253,769
RFA: Exploratory Research - Water Chemistry and Physics (1996) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Engineering and Environmental Chemistry
This project involves research to study factors that control the abundance of free-dissolved heavy metals (Cd, Cu, Pb) in fresh waters. The free dissolved ion (Men+(xH2O) is the most common toxic form of metals and the most chemically reactive species. Furthermore, knowing the concentration of Men+ is necessary in order to carry out exact thermodynamic and kinetic calculations. The proposed research will focus on metal binding by dissolved organic matter (DOM), a poorly-understood process that has a crucial influence in determining the level of free metals and surface complexation on particles, especially colloids.
An important innovation in this research will be direct measurement of the free dissolved metal fraction in fresh waters by means of differential pulse anodic stripping voltammetry (DPASV). So far there have been only a handful of applications of this method in fresh waters, and none in the US. (The method has been used successfully on marine systems by a few US investigators for as long as a decade.) Another novel approach will be extensive use of crossflow ultrafiltration to discriminate between colloidal and truly dissolved metal forms.
Samples collected from each site will be filtered and ultrafiltered (nominal 3,000 MW) to determine the distribution of metals across the full particle size range. Both DPASV analysis and potentiometric metal titration (combined with thermodynamic calculations for inorganic ligand complexes) will yield the proportion of free dissolved metal ions within the truly dissolved fraction, as well as the complexation capacity of the DOM. Conditional stability constants will be calculated from these data. On the same samples DOM will be analyzed for total concentration, elemental composition, molecular weight distribution, total acidity, and functional group content (e.g., carboxyl, phenolic-OH) by u.v.-visible spectroscopy, fluorescence spectroscopy, FTIR, 13C NMR, and high pressure size exclusion chromatography (HPSEC). Measured data on metal complexation will be compared by standard statistical methods to DOM characteristics in order to extract simple predictive relations.
The project involves basic research that will provide fundamental understanding of processes that control the biogeochemical behavior of heavy metals. At the same time, the goal is to provide watershed managers and decision-makers with the tools to quantitatively predict the bioavailable, reactive form of metals based only on knowledge of total dissolved metal levels and readily measurable parameters. These can include information on DOM (concentration, MW, elemental composition, and functional group abundance), colloids (concentration, surface area, and composition), and background solution chemistry (pH, ionic strength, ANC, SPM). As an early step toward that goal, the study will provide some of the first directly measured data on metal complexation by DOM in fresh waters. A second task will be to identify and investigate characteristics of DOM that correlate with its ability to bind heavy metals. A third concurrent part of this proposal is an investigation of metal interactions with naturally occurring colloids.