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Interference of nitrite with pyrite under acidic conditions – implications for studies of chemolithotrophic denitrification Ruiwen Yan, Andreas Kappler, and Stefan Peiffer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02981 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015
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Environmental Science & Technology
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Interference of nitrite with pyrite under acidic conditions –
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implications for studies of chemolithotrophic denitrification
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Ruiwen Yan a, Andreas Kappler b, Stefan Peiffer a, *
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a
Department of Hydrology, University of Bayreuth, 95440 Bayreuth, Germany
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b
Geomicrobiology Group, Eberhard-Karls-University Tuebingen, 72076 Tuebingen, Germany
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TOC art
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ABSTRACT: Chemolithotrophic denitrification coupled to pyrite oxidation is regarded a key process
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in the removal of nitrate in aquifers. A common product is nitrite which is a strong oxidant under
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acidic conditions. Nitrite may thus interfere with Fe(II) during acidic extraction, a procedure typically
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used to quantify microbial pyrite oxidation, in overestimating Fe(III) production. We studied the
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reaction between pyrite (5-125 mM) and nitrite (40-2000 µM) at pH 0, 5.5 and 6.8 in the absence and
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presence of oxygen. Significant oxidation of pyrite was measured at pH 0 with a yield of 100 µM
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Fe(III) after 24 h of incubation of 5 mM pyrite with 2000 µM nitrite. Dissolved oxygen increased the
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rate at pH 0. No oxidation of pyrite was observed at pH 5.5 and 6.8. Our data imply a cyclic model for
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pyrite oxidation by Fe(III) based on oxidation of residual Fe(II) by NO and/or NO2. Interference by
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nitrite could be avoided if nitrite was removed from the pyrite suspensions through a washing
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procedure prior to acidic extraction. We conclude that such interferences should be considered in
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studies on microbially mediated pyrite oxidation with nitrate.
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Environmental Science & Technology
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INTRODUCTION
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The disappearance of nitrate coupled to sulfate generation as observed in several pyrite bearing
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aquifers has been attributed to microbial chemolithotrophic denitrification linked to pyrite oxidation1-6
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and fueled a series of laboratory studies to resolve the mechanisms underlying this reaction.7-11 A
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common product of denitrification is nitrite.8-11 Significant formation of nitrite has been determined in
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laboratory column experiments with sediments from groundwater aquifers.9, 12 Nitrite was generated as
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a prominent intermediate compound in anaerobic denitrification with pyrite as electron donor in the
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presence of the nitrate-reducing bacterium Thiobacillus denitrificans.9-11
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Pyrite oxidation is typically quantified by acidic extraction of Fe(III) that is assumed to have formed
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upon oxidation of pyrite in experiments under circumneutral conditions.10, 13 Such techniques may,
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however, bear the risk of producing artifacts if nitrite is present because under acidic conditions nitrite
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decomposes into highly reactive compounds after protonation to nitrous acid (HNO2) (equation 1).
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Nitrous acid is unstable at pH
