Research Accomplishment Reports 2009

Inhibition Of Fe(III) Reduction By Nitrate: Impact Of Anoxic Chemical and Biological Fe(II) Oxidation

C. J. Matocha, M.S. Coyne, A.F. Miller
Department of Plant and Soil Sciences

 

Non-Technical Summary

Elevated levels of nitrate in water supplies are undesirable because of the potential impacts on aquatic life and human health. Nitrate-dependent, iron(II) oxidation is an important process in the removal of elevated nitrate from water and the inhibition of soil iron(III) reduction, yet, the mechanisms are poorly understood. The purpose of this study is to investigate the contribution of chemical and biological Fe(II) oxidation by nitrite and nitrate.

2009 Project Description

Field and laboratory studies have documented that the iron and nitrogen cycles are closely coupled in soil and sediment environments. Nitrate-dependent, iron(II) oxidation is an important process in the inhibition of soil iron(III) reduction and the removal of nitrate from water. However, the mechanisms of this process are poorly understood. Two proposed pathways include chemical reoxidation of iron(II) by nitrite and biological iron(II) oxidation coupled to nitrate reduction by lithotrophic microorganisms.

This past year, activities have included conducting laboratory and field experiments to investigate the contribution of chemical and biological iron(II) oxidation by nitrite and nitrate to the overall process of nitrate-dependent, iron(II) oxidation. The results derived from these activities have been disseminated via two presentations at the annual American Society of Agronomy Meetings in Pittsburgh, PA and a USDA-NRI project director meeting in East Lansing, MI. Two undergraduate research assistants and one Ph.D. graduate student have contributed to the project.

2009 Impact

In laboratory activities, we have shown that a naturally occurring iron(II) mineral, siderite, is involved in chemical nitrite reduction. The nitrogen is lost as nitrous oxide, an important greenhouse gas. Siderite is oxidized by nitrite to form an iron(III) mineral, lepidocrocite. These findings change the way we view the nitrogen cycle because historically iron was assumed to be unimportant in nitrogen transformations. The fundamental rate data generated from this research can be incorporated in models that account for nitrate-dependent iron(II) oxidation in soils and sediments and help optimize nitrogen fertilization practices.

Another potentially reactive pool is surface iron(II). We have explored the chemical structure of surface iron(II)-gibbsite slurries and found that iron(II) sorption were characterized by a fast reaction step followed by a slower reaction with time. An estimated rate coefficient for the rapid sorption step was 0.0075/s at 12 g/L gibbsite and pH 6.5. The sorption data of the slower stage were fitted with pseudo-first order kinetics and the rate coefficient was 0.00221/s. The sorption of iron(II) on gibbsite was strongly pH-dependent and was not affected by ionic strength. This indicates a minimal contribution of coulombic forces to the overall free energy change of Fe(II) sorption. Particle mobilities and optical spectra measured for surface iron(II)-gibbsite slurries suggest that iron(II) binds to gibbsite surfaces as an inner-sphere surface complex. These results will help predict the fate of iron(II) in soils under iron(III)-reducing conditions and its impact on fertilizer nitrate behavior.

2009 Publications

Rakshit, S., Matocha, C. J. and Coyne, M.S. (2008). Nitrite Reduction by Siderite. Soil Science Society of America Journal, 72:1070-1077.