Article pubs.acs.org/est
Antimony and Arsenic Behavior during Fe(II)-Induced Transformation of Jarosite Niloofar Karimian,* Scott G. Johnston, and Edward D. Burton Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia S Supporting Information *
ABSTRACT: Jarosite can be an important scavenger for arsenic (As) and antimony (Sb) in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. When subjected to reducing conditions, jarosite may undergo reductive dissolution, thereby releasing As, Sb, and Fe2+ coincident with a rise in pH. These conditions can also trigger the Fe2+-induced transformation of jarosite to more stable Fe(III) minerals, such as goethite. However, the consequences of this transformation process for As and Sb are yet to be methodically examined. We explore the effects of abiotic Fe2+-induced transformation of jarosite on the mobility, speciation, and partitioning of associated As(V) and Sb(V) under anoxic conditions at pH 7. High concentrations of Fe2+ (10 and 20 mM) rapidly (89%) via an intermediate green rust (GR-SO4) phase. 4263
DOI: 10.1021/acs.est.6b05335 Environ. Sci. Technol. 2017, 51, 4259−4268
Article
Environmental Science & Technology
Information). This suggests that reduction of Sb(V) and As(V) in the experiments described here were primarily controlled by slow redox kinetics.
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DISCUSSION Arsenic and Antimony Incorporation in Jarosite. Selective extraction results suggest that As(V) and Sb(V) were probably initially incorporated as coprecipitates within the synthetic jarosite used in this study. This is consistent with the ability of the jarosite−alunite mineral group to form solid solutions and undergo significant element substitution.6 Arsenic and Sb are group 15 neighbors and are therefore often assumed to behave analogously. However, it is important to recognize that the much larger size of Sb(V) versus As(V) causes the development of contrasting octahedral versus tetrahedral O coordination, respectively.42,43 For this reason, substitution of As(V) and Sb(V) into jarosite cannot involve the same crystallographic sites. For As(V), it is well established that tetrahedral AsVO4 readily occupies the tetrahedral SO42− sites within jarosite.42 Up to 30% replacement of SO42− in the jarosite structure by AsO4 has been reported in previous studies.12,16,42 In contrast, although Sb(V) incorporation in jarosite has not been directly addressed, it is possible that Sb(V) substitutes for Fe(III) within jarosite based on the previously reported Sb(V) incorporation into a range of Fe(III) oxides.44 This is due to the similarity in ionic size for Sb(V) (0.60 Å) and Fe(III) (0.65 Å) and similar Sb−O (1.91 Å) and Fe−O (1.97−2 Å) distances in the SbVO6 and FeIIIO6 octahedra, respectively.43 This similarity also suggests that Sb(V) may substitute for Fe(III) in secondary phases formed via the Fe2+-induced transformation of jarosite. Transformation of Jarosite and Formation of Secondary Fe Phases. Addition of Fe2+(aq) to the As(V)/Sb(V)bearing jarosite at circumneutral pH triggered a rapid transformation of the initial jarosite. Mineralogical transformation enhanced Sb mobilization into the aqueous phase, yet there was no increase in the aqueous concentrations of As. At higher concentrations of Fe2+(aq) (10 and 20 mM), the principal end-product was goethite, while jarosite mainly transformed to lepidocrocite in the presence of lower Fe2+ concentrations (1 and 5 mM). The shifts in mineralogy reflect increasing thermodynamic stability of the solid Fe phases as the experiment progressed (Figure SI2, Supporting Information). The observed transformation of jarosite in our study is also supported by earlier research indicating that reaction of aqueous Fe2+ accelerates the structural transformation of metastable Fe(III) minerals (e.g., schwertmannite and ferrihydrite) to more stable phases, such as goethite and lepidocrocite.26,34,45−48 However, there were considerable variations among Fe2+ treatments in the rates of jarosite transformation and the formation of intermediate phases. For example, addition of 20 mM Fe2+(aq) accelerated the transformation of jarosite to GR-SO4 at an initial rate of ∼74% h−1 while the transformation rate was only ∼35% h−1 for 1 mM Fe2+ treatment (Figure 4b). Variations in the rates of jarosite transformation in different treatments may reflect enhanced Fe2+ surface complexation with a finite number of surface sites as the concentrations of Fe2+(aq) increased, thereby enhancing electron transfer to structural Fe(III). Accelerated electron transfer between the surface-complexed Fe2+ and the structural Fe(III) has been shown to drive fast dissolution and transformation of the host mineral to more thermodynamically
Figure 5. Transmission electron microscopy (TEM) images of green rust: reaction product of As(V)/Sb(V)-jarosite and 20 mM Fe2+(aq) after ≤10 min. Left image shows the GR hexagonal crystals. Right image represents the SAED hexagonal diffraction pattern of the GRSO4 formed in the 20 mM (10 min) sample.
(∼27%), and jarosite (∼7%) were the main phases present after a reaction period of 24 h. This contrasts markedly with the 10 and 20 mM Fe2+ treatments, in which the final dominant Fe phases were goethite (>88−98%) with minor GR-SO4 (∼11%) and lepidocrocite (70% was incorporated within the structure of the poorly crystalline (As1M HCl) or crystalline minerals (AsResidual). In contrast to As, there was negligible surface-complexed Sb in any of the treatments. This behavior is consistent with the differing size and coordination environment of tetrahedral AsVO4 versus octahedral SbVO6. About 75% of Sb was incorporated into the residual phase of the neo-formed lepidocrocite, GR-SO4, and goethite, an observation that is consistent with SbVO6 octahedra being structurally incorporated during mineral transformation. Our findings are in agreement with Mitsunobu et al.44 who showed ∼80% structural incorporation of Sb(V) into ferrihydrite, goethite, and magnetite. Competitive adsorption of As(V) and blocking the reactive sites on the newly formed host phase surface may also help explain the negligible repartitioning of Sb to the exchangeable phase observed in our study. Generally, As(V) has a higher affinity than Sb(V) for Fe(III) oxide and hydroxide adsorption sites.66,67 The results also indicate that the Sb(V) species persisted during the transformation period in all treatments. This contrasts with the thermodynamic modeling for the speciation of Sb in the Sb−H2O system (Figure SI5b) which shows Sb(III) as the thermodynamically stable Sb species under our experimental conditions. This may simply reflect slow electron transfer kinetics between Sb(V) from surface-complexed Fe2+ or alternatively may be due to the fact that amorphous Fe oxyhydroxides can efficiently oxidize Sb(III) (if any did form) into more mobile and soluble Sb(V) in a short period at circumneutral pH.68−70 In contrast, As K-edge XANES spectra (Figure 6) reveal that the absorption edge of As in 20 mM Fe2+ treatments shifted to a lower energy over time. Abiotic electron transfer both to As(V) and from As(III) has been reported in previous studies in Fe-rich systems. For example, abiotic reduction of As(V) to As(III) occurred during precipitation reactions following sulfide-induced transformation of As(V)jarosite,20,65 whereas abiotic oxidation of As(III) to As(V) has been reported to occur by Fe2+ activated goethite.27 As and Sb Mobilization. Fe2+-induced transformation of As(V)/Sb(V)-bearing jarosite triggered rapid, substantial Sb(V) mobilization into the aqueous phase in all the treatments. In
stable phases (e.g., ferrihydrite and schwertmannite to goethite).45,48,49 Addition of Fe2+(aq) to jarosite triggered significant release of + K (aq) (>60% of the total) and SO42−(aq) (∼85−90% of the total) to the aqueous phase in lower Fe2+(aq) concentration treatments. The increase in K+(aq) and SO42−(aq) concentrations following addition of Fe2+(aq) is consistent with the observed dissolution and transformation of jarosite to new Fe(III) phases such as lepidocrocite and goethite (Figures 1 and 4). At lower concentrations of Fe2+(aq), jarosite rapidly (∼1 h) transformed to mainly lepidocrocite, followed by a gradual transformation to goethite. Limited lepidocrocite formation during Fe2+-induced transformation of ferrihydrite at higher concentrations of Fe2+(aq) has previously been reported by Hansel et al.47 They suggest that formation of goethite or magnetite hindered the continued precipitation of lepidocrocite at higher initial concentrations of Fe2+(aq). Although lepidocrocite formation via the Fe2+-induced transformation of jarosite at pH 6.5 over a period of 7 days has been reported previously,26 the present study is the first to describe very rapid (∼1 h) formation of lepidocrocite via this pathway. In this study, GR-SO4 was an important intermediate phase, which formed very rapidly in the 10 and 20 mM Fe2+ treatments and subsequently decreased in abundance toward the end of the experiment. Formation of a GR-SO 4 intermediary has been widely observed during the Fe2+catalyzed transformation of ferrihydrite46,50,51 and has also been previously reported during the biomineralization and sulfidization of jarosite.20,52−54 However, to the best of our knowledge, formation of GR-SO4 intermediary via abiotic Fe2+induced transformation of jarosite has not been reported previously. Green rust compounds are very reactive Fe2+/Fe3+ layered double hydroxides [Fe2+(1‑x) Fe3+x(OH)2]x+[(x/n)An‑, mH2O]x‑.52,55 Green rust formation can, therefore, significantly decrease the aqueous concentrations of Fe2+ and Fe3+.52 This may explain the observed sharp decrease in the Fe2+(aq) concentrations during the initial ∼10 min following the addition of Fe2+(aq) to the jarosite suspension. Replacement of Fe2+ by Fe3+ results in a positive layer charge in the green rust structure which can be balanced by inclusion of SO42− or other anions such as CO32− and Cl−.56 The lower concentrations of SO42− in the aqueous phase during the initial ∼2−4 h following the addition of 10 and 20 mM Fe2+(aq) to the jarosite suspension (Figure 2) is consistent with SO42− inclusion between green rust layers. Changes in Solid-Phase As and Sb Partitioning. One of the important findings of this study is the substantial repartitioning of the initially coprecipitated As to the surface complexed (AsEx) fraction following Fe2+-induced transformation of jarosite. Despite this dramatic rapid increase in AsEx, the residual fraction persisted as the dominant reservoir of solid-phase As throughout the full experiment duration. As discussed above, while the SbVO6 octahedra is capable of isomorphically substituting for the FeIIIO6 octahedra in a range of Fe(III) oxides,43 the AsVO4 tetrahedra is unlikely to act as a structural substitute within the end-product minerals observed in the present study (i.e., goethite and lepidocrocite). However, the persistence of As in the residual phase suggests that some As(V) may be occluded within the secondary mineral precipitates as structural defects or alternatively via incorporation of surface-bound species during aggregation-based crystal growth. Overall, these considerations are consistent with observed partial repartitioning of structural As(V) in the 4265
DOI: 10.1021/acs.est.6b05335 Environ. Sci. Technol. 2017, 51, 4259−4268
Article
Environmental Science & Technology contrast, there was negligible mobilization of As(V) into aqueous phase. This contrasting behavior suggests that As(V) had a relatively strong affinity for sorption to the secondary Fe phases under the conditions examined here, whereas Sb(V) displayed a relatively low sorption affinity. This is consistent with studies showing that Sb(V) is much less strongly sorbed to goethite than As(V) over a wide pH range.69 The fact that As mobilization was negligible in all the treatments here suggests that the initial release of structurally incorporated As(V) was followed by very rapid sorption to the surface or incorporation into the structure of the neo-formed Fe solid phases (lepidocrocite, GR-SO4, and goethite). Similar trends of attenuated mobility of As(V) following the formation of lepidocrocite, ferrihydrite, and more stable minerals such as goethite and magnetite have been reported in several previous studies.20,24,59,71 Environmental Implications. The observation that Fe2+induced transformation of As(V)/Sb(V)-bearing jarosite enhanced aqueous Sb(V) mobilization and caused As(V) to repartition to an exchangeable phase has implications for water quality in jarosite-rich environments. Prior to the addition of Fe2+(aq), As(V) and Sb(V) were both incorporated into the jarosite structure and thus unable to participate in surfaceexchange reactions. However, addition of Fe2+(aq) triggered a very rapid (
