wAbstract
@Microorganisms and higher plants produce biogenic ligands, such
as siderophores, to mobilize Fe that otherwise would be unavailable.
In this paper, we study the stability of arsenopyrite (FeAsS),
one of the most important natural sources of arsenic on earth,
in the presence of desferrioxamine (DFO-B), a common siderophore
ligand, at pH 5. Arsenopyrite specimens from mines in Panasqueira,
Portugal (100-149Κm) that contained incrustations of Pb, corresponding
to elemental Pb as determined by scanning electron microscopy-electron
diffraction spectroscopy (SEM-EDX, were used for this study. Batch
dissolution experiments of arsenopyrite (1 g L-1) in
the presence of 200ΚM DFO-B at initial pH (pHo)
5 were conducted for 110 h. In the presence of DFO-B, release
of Fe, As, and Pb showed positive trends with time; less dependency
was observed for the release of Fe, As, and Pb in the presence
of only water under similar experimental conditions. Detected
concentrations of soluble Fe, As, and Pb in suspensions containing
only water were found to be ca. 0.09}0.004, 0.15}0.003., and 0.01}0.01
ppm, respectively. In contrast, concentrations of soluble Fe,
As, and Pb in suspensions containing DFO-B were found to be 0.4}0.006,
0.27}0.009, and 0.14}0.005 ppm, respectively. Notably, the effectiveness
of DFO-B for releasing Pb was ca. 10 times higher than that for
releasing Fe. These results cannot be accounted for by thermodynamic
considerations, namely, by size-to-charge ratio considerations
of metal complexation by DFO-B. As determined by SEM-EDX, elemental
sample enrichment analysis supports the idea that the Fe-S subunit
bond energy is limiting for Fe release. Likely, the mechanism(s)
of dissolution for Pb incrustations is independent and occurs
concurrently to that for Fe and As. Our results show that dissolution
of arsenopyrite leads to precipitation of elemental sulfur, and
is consistent with a non-enzymatic mineral dissolution pathway.
Finally, speciation analyses for As indicate variability in the
As(III)/As(V) ratio with time, regardless of the presence of DFO-B
or water. At reaction times30 h, As(V) concentrations were found
to be 50-70, regardless of the presence of DFO-B. These results
are interpreted to indicate that transformations of As are not
imposed by ligand-mediated mechanisms. Experiments were also conducted
to study the dissolution behavior of galena (PbS) in the presence
of 200ΚM at pHo 5. Results show that, unlike
arsenopyrite, the dissolution behavior of galena shows coupled
increases in pH with decreases in metal solubility at t80 h.
Oxidative dissolution mechanisms conveying sulfur oxidation bring
about the production oh {H+}. However, dissolution
data trends for arsenopyrite and galena indicate {H+}
consumption. It is plausible that the formation of Pb species
is dependent on {H+} and {OH-}, namely,
stable surface hydroxyl complexes of the form Pb4(OH)44+(pH50 5.8)
and Pb6(OH)84+
for pH values 5.8 or above.x
1. Introduction
2. Materials and methods
@2.1. Materials
@2.2. SEM-EDX analyses
@2.3. Dissolution experiments
@2.4. Adsorption experiments
@2.5. Analytical techniques
@2.6. Determination of total soluble As, Fe, and Pb
@2.7. Arsenic speciation
3. Results and discussion
@3.1. Surface area determination
@3.2. SEM-EDX analyses
@3.3. Variations of pH
@3.4. Release of Fe, As, and Pb
@3.5. Elemental analyses
@3.6. As speciation
@3.7. Arsenic speciation and dissolved oxygen concentration
@3.8. Interaction between DFO-B and arsenopyrite surfaces
@3.9. Release of structural Pb
4. Conclusions
Acknowledgments
References