Redox reactions in the Fe-As-O2 system.

Area of Science:

• Environmental Chemistry
• Geochemistry
• Inorganic Chemistry

Background:

• Arsenic and iron redox reactions are critical in natural waters and sediments.
• Understanding these reactions is key to predicting arsenic mobility and fate.
• Previous studies often used simplified conditions or overlooked buffer effects.

Purpose of the Study:

• To investigate arsenic(V) reduction by iron(II) under anoxic conditions with goethite.
• To examine arsenic(III) co-oxidation during iron(II) oxygenation at near-neutral pH.
• To evaluate the influence of pH buffers and goethite on these redox processes.

Main Methods:

• Anoxic batch experiments for As(V) reduction by Fe(II) with goethite.
• Aerobic batch experiments for Fe(II) oxygenation and As(III) co-oxidation.
• Varied concentrations of Fe(II), pH buffers (carbonate, phosphate, MES, HEPES), and goethite.

Main Results:

• Fe(II) reduced As(V) in the presence of goethite, with rates increasing with pH and Fe(II) concentration.
• Fe(II) oxygenation and As(III) oxidation kinetics were highly dependent on pH buffer type and concentration.
• Phosphate buffer accelerated Fe(II) oxidation, while MES and HEPES buffers showed slower rates.
• Reactive intermediates, potentially Fe(IV) species, were implicated in As(III) oxidation, not hydroxyl radicals.

Conclusions:

• Goethite facilitates slow As(V) reduction by Fe(II) under anoxic conditions.
• Buffer choice significantly impacts Fe(II) oxygenation mechanisms and rates, affecting As(III) oxidation.
• The oxidation of As(III) during Fe(II) oxygenation involves reactive Fe species other than hydroxyl radicals.