Natural Water Redox Chemistry

Redox chemistry sets the oxidation state of carbon, nitrogen, sulfur, iron, and many other elements in natural waters, structuring aquatic systems from oxic surface waters to anoxic sediments.

Definition

The study of electron-transfer reactions and redox conditions that control the oxidation states of dissolved and particulate species in natural waters.

Scope

This topic covers oxidation-reduction equilibria and kinetics in natural waters, the use of electron activity (pE) and Eh as master variables, the sequence of microbially mediated electron-acceptor use, and the redox transformations of key elements across oxic-anoxic boundaries.

Core questions

• How do pE and Eh describe redox conditions in water?
• Why do electron acceptors get used in a predictable sequence?
• How do redox transitions control iron, manganese, sulfur, and nitrogen chemistry?
• Where do kinetics, rather than equilibrium, govern redox outcomes?

Key theories

Redox ladder of electron acceptors

As organic matter is oxidized, microbial communities use electron acceptors in order of decreasing energy yield, from oxygen through nitrate, manganese and iron oxides, sulfate, and finally carbon dioxide, producing a characteristic vertical redox zonation.

Mechanisms

Electron activity (pE) and pH jointly define stability fields for redox-sensitive species, summarized in pE-pH diagrams. In sediments and stratified waters, microbial respiration consumes oxygen and then successively lower-energy acceptors, driving reduction of nitrate, metal oxides, and sulfate and mobilizing or immobilizing associated elements.

Clinical relevance

Redox conditions control the mobility of iron, manganese, arsenic, and nutrients, affecting groundwater quality, the release of contaminants from sediments, and the design of treatment systems.

History

The pE-pH framework was adapted from geochemistry into aquatic chemistry in the mid-20th century, and microbial control of redox sequencing was clarified through later biogeochemical work.

Key figures

• Werner Stumm
• James J. Morgan

Seminal works

• stumm1996
• vanLoon2017

Frequently asked questions

Why does arsenic sometimes appear in groundwater?

Reducing conditions can dissolve iron oxides that hold arsenic, releasing it to groundwater; redox chemistry therefore controls this important contamination problem.