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The Mode of Incorporation of As(-I) and Se(-I) in Natural Pyrite Revisited
journal contribution
posted on 2020-02-17, 18:56 authored by Alain Manceau, Margarita Merkulova, Olivier Mathon, Pieter Glatzel, Magdalena Murdzek, Valentina Batanova, Alexandre Simionovici, Stephan N. Steinmann, Dogan PaktuncPyrite
(FeS2) from coal, sedimentary rocks, and hydrothermal
ore deposits generally contains hazardous selenium (Se) and arsenic
(As) that are released in natural waters through oxidative dissolution
of the host. Knowing how As and Se are structurally incorporated into
pyrite has important implications in controlling or preventing their
release because trace metal(loid) substitution accelerates the dissolution
of pyrite. Previous extended X-ray absorption fine structure (EXAFS)
studies have reported that nominally monovalent arsenic clusters at
the sulfur site form As–As pairs at 3.2 Å, whereas monovalent
Se does not form Se–Se pairs at this distance for unknown reasons.
Here, we revisit this question using As and Se K-edge X-ray absorption
near-edge structure (XANES) and EXAFS spectroscopy complemented with
atomistic calculations. We find that neither As nor Se atoms can be
differentiated from a S atom at 3.2–3.3 Å with the cluster
and dilute model-fits to As- and Se-EXAFS data yielding equivalent
least-squares solutions. Thermodynamic calculations of Fe48As3S93 (3.8 wt % As) and Fe48Se3S93 (4.0 wt % Se) structures show that the formation
of As–As pairs is energetically favorable and the formation
of Se–Se pairs is unfavorable. Thus, the equilibrium distribution
of As and Se predicted by calculation agrees with published EXAFS
data. However, this agreement is incidental because EXAFS fits are
ambiguous with the same EXAFS spectra being fit indifferently with
a cluster and a dilute model. Regarding Se, the dilute model-fit is
probably correct since Se–Se pairs are precluded thermodynamically.
The situation is less clear for As. The lowest energy atomic arrangement
of As in Fe48S93As3 is similar to
the local structure of As in arsenopyrite (FeAsS), thus supporting
the cluster model. However, the energy gain to total energy provided
by the formation of As clusters decreases with decreasing As concentration,
making them thermodynamically less favorable below 1.0 wt %.
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monovalent SeFe 48XANESatomistic calculationsThermodynamic calculationsEXAFS spectraNatural Pyrite Revisited PyriteEXAFS dataequivalent least-squares solutionsstructures showequilibrium distributioncluster modelhydrothermal ore depositsSe K-edge X-ray absorption near-edge structureoxidative dissolutionFe 48 S 93monovalent arsenic clustersSe atomsEXAFS spectroscopyFeS 2sulfur site formS atomclusters decreasesX-ray absorptionSe-EXAFS dataenergy gain
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