Freezing-Enhanced Accumulation of ROS Generated by Fulvic Acid: Implications for As(III) Oxidation

Area of Science:

• Environmental Chemistry
• Geochemistry
• Biogeochemistry

Background:

• Humic substances (HS) are redox-active macromolecules generating reactive oxygen species (ROS).
• Electron transfer pathways of HS redox processes under freezing conditions are poorly understood.
• Cold environments are common globally, impacting geochemical cycles.

Purpose of the Study:

• To investigate the role of freezing on fulvic acid (FA)-mediated oxidation of arsenic (As(III)).
• To elucidate the mechanisms behind enhanced redox reactions in ice.
• To understand the implications for arsenic redox cycling in cold environments.

Main Methods:

• Controlled freezing experiments at temperatures from -5 to -18 °C.
• In situ fluorescence imaging to visualize ROS.
• Spectroscopic (EPR, 3D-EEM) and chromatographic (GPC) analyses.

Main Results:

• Freezing significantly enhanced As(III) oxidation by FA in ice (68% conversion at -18 °C).
• Cryogenic acceleration resulted from freeze-concentration and kinetic decoupling of ROS generation/scavenging.
• ROS accumulated in ice grain boundaries, and freezing suppressed FA humification, favoring As(III) oxidation.

Conclusions:

• A novel pathway for enhanced As(III) oxidation in ice was identified.
• Cryogenic conditions are critical for redox dynamics involving HS and pollutants.
• Findings redefine understanding of arsenic cycling in cold environments.