Chemical Weathering of Pyrite in Soils

Author

Aaron D. Brown, Utah State University

Date of Award:

5-1985

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Soil Science and Biometeorology

Committee Chair(s)

J. J. Jurinak

Committee

J. J. Jurinak

Abstract

The products of pyrite oxidation, including solution phase Fe²⁺, Fe³⁺, S₂O₃^2-, S₄O₆^2-, SO₃^2- and SO₄^2- and solid phase Fe(OH)₃, were measured under controlled conditions in order to investigate the behavior of pyrite in calcareous and alkaline soils.

The distribution of sulfur oxidation products is pH dependent and can be interpreted in terms of metastable equilibrium among thiosulfate, disulfane disulfonate and sulfite. Thiosulfate and sulfite predominate in the pH range greater than about pH 7 or 8. Sulfane disulfonates are more predominant at more acid pH.

Solution concentration data were consistent with the presence of Fe(OH)₃. Concentrations of thiosulfate and sulfane disulfonate were consistent with a redox equilibrium among solution iron and sulfur species at pH 6 to 9.

Linear or zero-order kinetics were found to be sufficient for description of pyrite oxidation in this study. Linear kinetics were observed as electrical conductivity, solution sulfur products and solution plus solid phase iron products. The measurement of solution iron plus solid-phase iron oxide is a more rigorous approach to the extent of reaction than the measurement of sulfate.

The rate of pyrite oxidation is pH dependent, increasing from 10 - 20 pmol(Fe) m^-2 s^-1 to 40 - 60 pmol(Fe) m^-2 s^-1 between pH 5 and 9. This is consistent with an oxidation mechanism involving the reoxidation of solution Fe²⁺ via a reaction between an iron hydroxide complex and hydrated oxygen as the rate-determining step. The effect of background electrolytes on oxidation rates at low pH also supports this interpretation.

Pyrite oxidation rates in the presence of calcium carbonate, sodium bicarbonate, sodium thiosulfate and calcium-saturated bentonite can be related to the pH effect. Sodium thiosulfate and DTPA appeared to have specific inhibitory effects.

Column studies show that the disposal of pyritic mine spoils or tailings by mixing with calcareous material may produce thiosulfate, a good reducing agent for toxic metals. Burial of lime below pyritic materials may protect groundwater quality more effectively than application of lime to the surface.

Checksum

1d4a64ffab386ef670bdbe92236e58bb

Recommended Citation

Brown, Aaron D., "Chemical Weathering of Pyrite in Soils" (1985). All Graduate Theses and Dissertations, Spring 1920 to Summer 2023. 4345.
https://digitalcommons.usu.edu/etd/4345