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Geological Society, London, Special Publications; 2002; v. 198; p. 187-204;
DOI: 10.1144/GSL.SP.2002.198.01.12
© 2002 Geological Society of London

Modelling the oxidation of sulphides in an unsaturated soil

Maarten W. Saaltink1, Cristina Domenech2, Carlos Ayora2 & Jesus Carrera1

1 Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia (UPC), C/ Jordi Girona 1-3, Building D-2, 08034 Barcelona, Spain
2 Institute of Earth Sciences Jaume Almera, CSIC, C/ Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain

(e-mail: maarten.saaltink{at}upc.es)

(e-mail: cdomenec{at}ija.csic.es)

(e-mail: cayora{at}ija.csic.es)

(e-mail: jesus.carrera{at}upc.es)

Within the framework of a study on the impact of a mine-tailing spill at Aznalcollar, SW Spain, we investigated the oxidation of pyrite and other sulphides by means of two column experiments and reactive transport modelling. The columns were filled with pyritic sludge mixed up with a sandy and a clayey soil, respectively. The columns were located outdoors for 15 months and leached 10 times. Prior to simulating reactive transport, a flow model permitted a detailed description of the behaviour of the column at a daily time-scale. The most important parameter extracted was the hydraulic saturation. This parameter controlled the amount of O2 that could diffuse into the soil, which, in its turn, affected the rate of pyrite oxidation. The sandy and clayey columns behaved very differently. In the sandy column, pH dropped due to the oxidation of pyrite. As a result, silicate minerals dissolved, providing Na and/or K that precipitate together with Fe and SO4 as jarosite. The high concentration of Zn in the leachates was consistent with the concentrations predicted from sphalerite oxidation. The low As and Pb concentrations, however, were explained by their coprecipitation in the jarosite. In the clayey column, the dissolution of dolomite kept the pH high, impeding the dissolution of silicate minerals and precipitating amorphous Fe(OH)3 in the place of jarosite. The model also permitted rate laws proposed in the literature for pyrite oxidation to be discussed. We found that the oxidation of pyrite by Fe3+ was not faster than by O2, contrary reports in the literature. Finally, the model was used to predict the behaviour of other soil types and other sludge contents. According to the predictions the dissolution of jarosite was very important to maintaining the pH at a value of approximately 2, even for gravels or low reactive sand.





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P. L. Younger and N. S. Robins
Challenges in the characterization and prediction of the hydrogeology and geochemistry of mined ground
Geological Society, London, Special Publications, 2002; 198: 1 - 16.
[Abstract] [PDF]