The Redox Cycle of Mercury in Natural Waters

EPA Grant Number: R827915
Title: The Redox Cycle of Mercury in Natural Waters
Investigators: Morel, Francois M.
Institution: Princeton University
EPA Project Officer: Hiscock, Michael
Project Period: October 11, 1999 through October 10, 2002 (Extended to October 10, 2003)
Project Amount: $726,318
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals


The objective of this project is to elucidate the parameters that control the flux of elemental mercury from natural waters to the atmosphere. To this end it is proposed to undertake a series of iterative laboratory and field experiments focused on the principal chemical and biological redox mechanisms that transform mercury between its divalent, Hg(II), and elemental, Hg(0), forms. The experimental plan is designed to test three complementary hypotheses based on preliminary data and the literature.

Hypothesis 1. Biological reduction of Hg(II) to Hg(0) is normally effected as a two electron transfer reaction by transmembrane metal reductases in photosynthetic microorganisms, phytoplankton, and cyanobacteria.

Hypothesis 2. Chemical reduction of Hg(II) occurs in two distinct one-electron transfer reactions: i) reduction of Hg(II) to Hg(I) which requires a high energy reductant (typically formed in the light) such as the superoxide anion or an organic radical (probably a semiquinone); ii) reduction of Hg(I) to Hg(0) by organic matter.

Hypothesis 3. Likewise, the oxidation of elemental mercury requires first oxidation of Hg(0) to Hg(I), likely effected by the same radicals, superoxide or semiquinones, and then oxidation of Hg(I) to Hg(II) by oxygen which is facilitated by chloride complexation of the ionic mercury species.


Starting with simple model systems and building up to the complexity of natural waters, the laboratory experiments are designed to establish the mechanisms and the rates of the processes of interest and to provide techniques and probes for field experiments. The field experiments, which will be carried out at a number of sites covering a range of values for the key parameters, are designed in turn to establish the actual occurrence of the mechanisms in nature and provide corresponding kinetic data.

Expected Results:

The results of this project will allow us to understand quantitatively how various parameters such as solar irradiation, dissolved organic matter concentration, pH, or biological productivity control the rate of mercury loss from a particular water body to the atmosphere, and why different water bodies with similar mercury inputs end up with different mercury loadings. This is of course one of the major factors controlling the response of aquatic systems to anthropogenic mercury emissions and modulating the impact of mercury on wildlife and (through bioaccumulation in fish) on humans.

Publications and Presentations:

Publications have been submitted on this project: View all 20 publications for this project

Journal Articles:

Journal Articles have been submitted on this project: View all 7 journal articles for this project

Supplemental Keywords:

photooxidation, photoreduction, volatilization, enzymatic reduction, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Environmental Chemistry, Fate & Transport, Microbiology, Mercury, fate and transport, photosynthetic microorganisms, emissions, phytoplankton, biogeochemical cycling, methylmercury, enzymatic reduction, atmospheric deposition, mercury vapor

Relevant Websites:

Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)

Progress and Final Reports:

  • 2000
  • 2001 Progress Report
  • 2002
  • Final Report