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Antimony and arsenic behavior during Fe(II)-induced transformation of jarosite. Niloofar Karimian, Scott G Johnston, and Edward D. Burton Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05335 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
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Environmental Science & Technology
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Antimony and arsenic behavior during Fe(II)-induced
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transformation of jarosite.
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Niloofar KarimianA*, Scott G. JohnstonA, Edward D. BurtonA
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*Corresponding author (email:
[email protected] 5
~4600 words, 6 Figures, Supporting information available
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A
Southern Cross Geoscience
Southern Cross University, Lismore, NSW 2480, Australia
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Graphical abstract
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ABSTRACT
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Jarosite can be an important scavenger for arsenic (As) and antimony (Sb) in acid mine
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drainage (AMD) and acid sulfate soil (ASS) environments. When subjected to reducing
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conditions, jarosite may undergo reductive dissolution, thereby releasing As, Sb and Fe2+ and
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causing a rise in pH. These conditions can also trigger the Fe2+-induced transformation of
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jarosite to more stable Fe(III) minerals, such as goethite. However, the consequences of this
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transformation process for As and Sb are yet to be methodically examined. We explore the
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effects of abiotic Fe2+-induced transformation of jarosite on the mobility, speciation and
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partitioning of associated As(V) and Sb(V) under anoxic conditions at pH 7. High
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concentrations of Fe2+ (10 and 20 mM) rapidly (89%) via an intermediate green rust (GR-SO4) phase.
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Transformation of jarosite in both 10 and 20 mM Fe2+ treatments was noticeably more rapid
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than in the lower Fe2+(aq) concentration treatments. At a reaction time of just 10 minutes,
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jarosite had decreased to ~81% of solid-phase Fe in the 1 mM Fe2+ treatment and steadily
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declined towards the end of the experiment. In comparison, jarosite decreased to ~33% of
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total Fe in the 20 mM Fe2+ treatment within 10 minutes, and disappeared entirely during the
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mid-stages of the experiment in both the 10 and 20 mM Fe2+ treatments (Fig. 1 and Fig. 4a,
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b). In the higher Fe2+ treatments (10 and 20 mM), jarosite rapidly (70% of the total solid-phase Fe for up to 8 hours
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in the 20 mM treatment. The formation of GR-SO4 was verified by TEM which revealed the
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presence of distinctive hexagonal crystals in the 20 mM Fe2+ treatment, with the mineralogy
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being confirmed by SAED (Fig. 5). For the 1 and 5 mM Fe2+ treatments, lepidocrocite
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(~65%), goethite (~27%) and jarosite (~7%) were the main phases present after a reaction
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period of 24 h. This contrasts markedly with the 10 and 20 mM Fe2+ treatments, in which the
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final dominant Fe phases were goethite (>88-98%) with minor GR-SO4 (~11%) and
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lepidocrocite (60% of the total) and
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SO42-(aq) (~85-90% of the total) to the aqueous phase in lower Fe2+(aq) concentration
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treatments. The increase in K+(aq) and SO42-(aq) concentrations following addition of Fe2+(aq) is
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consistent with the observed dissolution and transformation of jarosite to new Fe(III) phases
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such as lepidocrocite and goethite (Fig. 1 and Fig. 4). At lower concentrations of Fe2+(aq),
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jarosite rapidly (~1 h) transformed to mainly lepidocrocite, followed by a gradual
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transformation
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transformation of ferrihydrite at higher concentrations of Fe2+(aq) has previously been reported
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by Hansel et al, 2005.47 They suggest that formation of goethite or magnetite hindered the
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continued precipitation of lepidocrocite at higher initial concentrations of Fe2+(aq). Although
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lepidocrocite formation via the Fe2+- induced transformation of jarosite at pH 6.5 over a
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period of 7 days has been reported previously,26 the present study is the first to describe very
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rapid (~1 h) formation of lepidocrocite via this pathway.
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In this study, GR-SO4 was an important intermediate phase, which formed very rapidly in the
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10 mM and 20 mM Fe2+ treatments and subsequently decreased in abundance towards the
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end of the experiment. Formation of a GR-SO4 intermediary has been widely observed during
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the Fe2+-catalyzed transformation of ferrihydrite46,50,51 and has also been previously reported
to
goethite.
Limited
lepidocrocite
formation
during
Fe2+-induced
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during the biomineralization and sulfidization of jarosite.20,52-54 However, to the best of our
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knowledge, formation of GR-SO4 intermediary via abiotic Fe2+ - induced transformation of
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jarosite has not been reported previously. Green rust compounds are very reactive Fe2+/Fe3+
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layered double-hydroxides [Fe2+(1-x) Fe3+x(OH)2]x+[(x/n)An-, mH2O]x-.52,
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formation can, therefore, significantly decrease the aqueous concentrations of Fe2+ and Fe3+.52
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This may explain the observed sharp decrease in the Fe2+(aq) concentrations during initial ~10
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min following the addition of Fe2+(aq) to the jarosite suspension. Replacement of the Fe2+ by
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Fe3+ results in a positive layer charge in green rust structure which can be balanced by
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inclusion of SO42- or other anions such as CO32- and Cl-.56 The lower concentrations of SO42-
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in the aqueous phase during the initial ~2-4 h following the addition of 10 and 20 mM Fe2+(aq)
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to the jarosite suspension (Fig. 2) is consistent with SO42- inclusion between green rust layers.
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Changes in solid-phase As and Sb partitioning
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One of the important findings of this study is the substantial repartitioning of the initially co-
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precipitated As to the surface complexed (AsEx) fraction following Fe2+-induced
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transformation of jarosite. Despite this dramatic rapid increase in AsEx, the residual fraction
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persisted as the dominant reservoir of solid-phase As throughout the full experiment duration.
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As discussed above, while the SbVO6 octahedra is capable of isomorphically substituting for
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the FeIIIO6 octahedra in a range of Fe(III) oxides,43 the AsVO4 tetrahedra is unlikely to act as a
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structural substitute within the end-product minerals observed in the present study (i.e.
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goethite and lepidocrocite). However, the persistence of As in the residual phase suggests that
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some As(V) may be occluded within the secondary mineral precipitates as structural defects,
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or alternatively via incorporation of surface-bound species during aggregation-based crystal
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growth. Overall, these considerations are consistent with observed partial repartitioning of
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structural As(V) in the original jarosite to a surface-bound (exchangeable) fraction in the neo-
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formed goethite and lepidocrocite.
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Green rust
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Given the negligible concentrations of As(aq) during the transformation period in all the
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treatments, this suggest that any As released following Fe2+-induced jarosite dissolution
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either rapidly re-sorbed to the surface or was incorporated as structural defects in the newly-
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formed Fe(III) phases. Efficient incorporation of As(V) into more stable Fe(III) phases such
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as goethite during the Fe2+-catalyzed recrystallization of thermodynamically less stable
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Fe(III) oxides, such as lepidocrocite and ferrihydrite, has been reported in previous studies.57-
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on edges of the GR layers, has also been documented in previous studies.60-62
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Decreases in particle size, possible increases in surface area of the neo-formed minerals and
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decreases in As(V) electrostatic activity coefficient by shifting the surface potential to more
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positive values following Fe2+ adsorption, can be considered as possible mechanisms
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contributing to the increase in the AsEx fraction following jarosite dissolution.63-65 The results
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of our study suggest that, for the neo-formed lepidocrocite, GR and goethite, ~26% of total
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As(V)-pool appears to be associated with the iron mineral surface, while >70% was
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incorporated within the structure of the poorly crystalline (As1M HCl) or crystalline minerals
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(AsResidual).
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In contrast to As, there was negligible surface-complexed Sb in any of the treatments. This
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behavior is consistent with the differing size and coordination environment of tetrahedral
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AsVO4 versus octahedral SbVO6. About 75% of Sb was incorporated into the residual phase of
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the neo-formed lepidocrocite, GR-SO4 and goethite, an observation that is consistent with
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SbVO6 octahedra being structurally incorporated during mineral transformation. Our findings
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are in agreement with Mitsunobu et al.44 who showed ~80% structural incorporation of Sb(V)
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into ferrihydrite, goethite and magnetite. Competitive adsorption of As(V) and blocking the
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reactive sites on the newly-formed host phase surface may also help explain the negligible
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repartitioning of Sb to the exchangeable phase observed in our study. Generally, As(V) has a
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In addition to goethite, the capacity of GR to scavenge As(V), via formation of complexes
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higher affinity than Sb(V) for Fe(III) oxide and hydroxide adsorption sites.66,67
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The results also indicate that the Sb(V) species persisted during the transformation period in
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all treatments. This contrasts with the thermodynamic modeling for the speciation of Sb in
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Sb-H2O system (Fig. SI5b) which shows Sb(III) as the thermodynamically-stable Sb species
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under our experimental conditions. This may simply reflect slow electron transfer kinetics
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between Sb(V) from surface-complexed Fe2+ or alternatively may be due to the fact that
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amorphous Fe oxyhydroxides can efficiently oxidize Sb(III) (if any did form) into more
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mobile and soluble Sb(V) in a short period at circumneutral pH.68-70 In contrast, As K-edge
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XANES spectra (Fig. 6) reveal that the absorption edge of As in 20 mM Fe2+ treatments
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shifted to a lower energy over time. Abiotic electron transfer both to As(V) and from As(III)
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has been reported in previous studies in Fe-rich systems. For example, abiotic reduction of
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As(V) to As(III) occurred during precipitation reactions following sulfide-induced
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transformation of As(V)-jarosite,20,65 whereas abiotic oxidation of As(III) to As(V) has been
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reported to occur by Fe2+ activated goethite.27
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As and Sb mobilization
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Fe2+-induced transformation of As(V)/Sb(V)-bearing jarosite triggered rapid, substantial
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Sb(V) mobilization into the aqueous-phase in all the treatments. In contrast, there was
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negligible mobilization of As(V) into aqueous phase. This contrasting behavior suggests that
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As(V) had a relatively strong affinity for sorption to the secondary Fe phases under the
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conditions examined here, whereas Sb(V) displayed a relatively low sorption affinity. This is
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consistent with studies showing that Sb(V) is much less strongly sorbed to goethite than
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As(V) over a wide pH range.69 The fact that As mobilization was negligible in all the
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treatments here suggests that the initial release of structurally incorporated As(V) was
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followed by very rapid sorption to the surface or incorporation into the structure of the neo21 ACS Paragon Plus Environment
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formed Fe solid-phases (lepidocrocite, GR-SO4 and goethite). Similar trends of attenuated
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mobility of As(V) following the formation of lepidocrocite, ferrihydrite and more stable
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minerals such as goethite and magnetite have been reported in several previous
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studies.20,24,59,71
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Environmental implications
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The observation that Fe2+-induced transformation of As(V)/Sb(V)-bearing jarosite enhanced
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aqueous Sb(V) mobilization and caused As(V) to repartition to an exchangeable phase has
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implications for water-quality in jarosite-rich environments. Prior to the addition of Fe2+(aq),
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As(V) and Sb(V) were both incorporated into the jarosite structure and thus unable to
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participate
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(aq)
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(with 1 and 5 mM Fe2+) and goethite, preceded by formation of a GR-SO4 intermediate phase
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(at Fe2+(aq) concentrations of 10 and 20 mM). Our results reveal a notable increase in As(V)
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extractability during the Fe2+-induced transformation of jarosite. This increase in
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extractability suggests that the Fe2+-induced transformation of jarosite is associated with an
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increased risk of subsequent As mobilization.
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This study also reveals partial abiotic reduction of As(V) to As(III) during the Fe2+-induced
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transformation of jarosite. Although the reduction of As(V) to As(III) is important for the
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toxicology and environmental fate of As, this process was only relevant under high Fe2+ (aq) (≥
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20 mM) conditions. Solid-phase partitioning data indicate that the residual fraction of As and
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Sb remained dominant solid-phase during the transformation period. This suggests that neo-
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formed lepidocrocite, GR-SO4 and goethite will likely act as strong scavengers for both
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As(V) and Sb(V) under Fe2+-rich, circumneutral pH conditions. However, it is important to
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note that in natural systems a wide range of factors can influence Sb and As behaviour during
in
surface-exchange
reactions.
However,
addition
of
Fe2+
triggered a very rapid (
