Environ. Sci. Technol. 2010, 44, 1288–1294
Reactivities of Fe(II) on Calcite: Selenium Reduction S U D I P T A C H A K R A B O R T Y , * ,†,§ FABRIZIO BARDELLI,‡ AND LAURENT CHARLET† LGIT-OSUG, University of Grenoble-I, BP 53, F-38041 Grenoble Cedex 9, France, GILDA at the European Synchrotron Radiation Facility, Grenoble, France, and Department of Chemistry, Kanchrapara College, University of Kalyani, West Bengal, India
Received October 8, 2009. Revised manuscript received December 15, 2009. Accepted December 23, 2009.
The reductive immobilization of Se(IV) by micrometer-sized (100-200 µm) calcite containing sorbed or coprecipitated Fe(II) was investigated at pH 7 under anoxic conditions (O2 < 1 ppmv) using X-ray absorption near-edge structure (XANES) spectroscopy. The Se(IV) sorption on calcite increased in the presence of sorbed Fe(II) compared to that of Fe-free pure calcite. XANES spectra of Se K-edge shows that nearly half of the total sorbed Se(IV) is reduced to Se(0) by Fe(II) sorbed on calcite within 24 h. The extent of reduction decreases with increasing equilibration time of calcite with Fe(II) solution before Se(IV) addition. The combined results of field emission scanning electron microscopy and X-ray diffraction have shown that needleshaped red monoclinic elemental Se with diameters of 30-50 nm and lengths of up to 100 nm is precipitated on the calcite surface. Fe(II) coprecipitated calcite does not contribute to Se(IV) reduction within 72 h. Therefore, the reduction capacity of Fe(II) linked to calcite critically depends on its location (either on the surface or in the bulk solid), and less extensively on the pre-equilibration time of calcite with Fe(II) solution. Such understanding is important to predict the transport, transformation, and attenuation of Se in subsurface and in nuclear waste repositories.
Introduction Selenium (Se) has received worldwide attention in the last few decades because of its toxicity, its presence in radioactive waste repositories (79Se is a long-lived fission product with a half-life of 2.95 × 105 years), and increasing applications in nanoengineering technology. Soils with elevated selenium are found in many countries including China, India, Ireland, and the United States (1). The highest abundance of Se is in igneous rocks, but high concentrations are also prevalent in some sedimentary rocks and fossil fuels (2). Natural sources of seleniferous marine sedimentary rocks in San Joaquin Valley (United States) (3) or anthropogenic activities such as coal combustion in Belews Lake (United States) has been responsible for selenium pollution (4). Se exhibits several inorganic forms in the environment which include elemental Se (Se0), selenite (Se(IV), SeO32-), * Corresponding author phone: +33 476 63 5198; fax: +33 476 63 5252; e-mail:
[email protected]. † University of Grenoble-I. ‡ European Synchrotron Radiation Facility. § University of Kalyani. 1288
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selenate (Se(VI), SeO42-), and hydrogen selenide (Se2-, H2Se). The higher oxidation states +6 and +4 are more toxic, soluble, and mobile, whereas the lower oxidation states of 0, -1, and -2 correspond to species with less toxicity and low solubility (5). The chemical forms of Se in sediment and natural water are governed by physicochemical factors including redox conditions, pH (6), and availability of sorbing surfaces. In oxidizing environments Se occurs primarily as Se(VI) and Se(IV) oxyanions that strongly adsorb to mineral surfaces (7, 8). Under reducing conditions, transformations via reduction of Se(VI) or Se(IV) (9) and oxidation of Se2- (10) are reported to form Se(0). The Se(IV) reduction by aqueous Fe(II) does not occur thermodynamically because of the difference in reduction potentials of the redox couples: Fe3+ + e- T Fe2+, E° ) 0.77 V (pH 0, 1 M HCl); HSeO3- + 4e- + 5H+ T Se(0) + 3H2O, E° ) 0.74 V (pH 3-8). The mobility of Se in the environment is controlled by a number of biotic and abiotic reduction pathways. Microorganisms-mediated reduction and subsequent removal of Se(0) by flocculation-sedimentation is considered as an effective bioremediation technique to remove Se from agricultural drainage water (11, 12). However, abiotic options are also extensively studied in the presence of a number of solids like siderite, mackinawite, magnetite (13), pyrite (14), troilite (15), green rust (16, 17), Fe(II)-adsorbed montmorillonite (18), and zerovalent iron (19). The Se(IV) is rapidly reduced within 24 h by nanoparticulate mackinawite and magnetite, whereas the reduction by green rust, pyrite, siderite, and Fe2+ sorbed on montmorillonite is slow (days to weeks) and controlled by kinetically limited redox reactions (13). However, none of these previous studies include the reactivities of Fe(II) sorbed on or coprecipitated with calcite. Depending on Fe(II)-bearing phases and pH, the reduction products can differ with a variety of elemental Se (red, gray) and iron selenides (FeSe, Fe7Se8) (13). Being a common mineral in limestone soils (e.g., calcareous) and sediments, calcite plays an important role in Se(IV) sorption at neutral pH (20). Se(VI) and Se(IV) can also be coprecipitated with calcite (21, 22). In the presence of Fe(II) the sorption/coprecipitation is coupled to oxido-reduction reactions to transform toxic Se(IV) into a less toxic form and to accumulate elemental Se in sediment, the presence of which is being reported in anoxic sediments (23, 24). Recently, Mettler et al. (25) studied the catalytic effect of calcite surface on the oxygenation of sorbed Fe(II). Their results show that the reactivity of sorbed Fe(II) on calcite toward oxygenation decreases with increasing contact time of aqueous Fe(II) with calcite before exposure to oxygen due to partial incorporation of Fe(II) into calcite lattice. On the basis of this hypothesis, the present research investigates the reduction capacity of Fe(II) sorbed on or coprecipitated with calcite toward Se(IV) at pH 7 by solid-phase Se speciation using X-ray absorption near-edge structure (XANES) spectroscopy. The Se redox product is characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM).
Experimental Section Chemicals. Chemicals of analytical grade or higher purity were used without further purification. NaHCO3, CaCl2, and HCl were obtained from Fluka Chemika, Riedel de Hae¨n, and Fixanal (Riedel de Hae¨n), respectively. All the solutions and suspensions were prepared in deionized water (Milli-Q, 18 MΩ) after several hours of boiling and subsequent degassing with argon (99.9992%) until the dissolved O2 concentration dropped down below 1 mg L-1 and immediately transferred to a glovebox (JACOMEX) under N2 10.1021/es903037s
2010 American Chemical Society
Published on Web 01/21/2010
TABLE 1. Experimental Conditions of Fe(II) Coprecipitated Calcite Synthesis units in molarities
range
2+
[Ca ] [NH4+] pH ΣCO2 [Fe2+]
0.025-0.030 1.8-2.0 8.2-8.4 0.0023-0.0031 0.00002-0.00005
atmosphere (CO2, O2 CO3FeIICO3H0 (FeII sorption on calcite) (25) and 4>CO3FeIICO3H0 + HSeIVO3- + 5H+ T Se0 + 4 > CO3FeIIICO3H+ + 3H2O (SeIV sorption-reduction), where > indicates calcite surface groups and >CO3FeCO3H0 is the most probable precursor of a Fe(II)-Ca carbonate phase forming at the calcite surface (25). The overall consumption of proton in sorption-reduction reactions is also experimentally well supported by the observed increase in pH due to addition of Se(IV) (Table 2). Characterization of Elemental Selenium. The SEM images of Se-rich spots on calcite (CA 1) (encircled spots in Figure 3D) upon magnification (Figure 3E) shows clusters of needle-shaped, separate but contacting (aggregated) crystals (crystallites) of elemental Se with diameters of 30-50 nm and lengths of up to 100 nm. The observed clustering is probably linked to the intrinsic property of Se to build chainlike structures and small clusters formed by chemisorption of Se: Sem + Se f Sem+1 is aggregated further with other clusters: Sem + Sen f Sem+n (39). This morphology of elemental Se precipitated on calcite is clearly different from (i) the typical nanospherical (∼300 nm) red monoclinic Se
FIGURE 4. X-ray diffraction patterns of calcite reacted with or without Fe(II) and Se(IV) (dotted lines and red lines are the main peaks of calcite and red β-monoclinic Se8, respectively (inset showing appearance of Se peak). 1292
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formed by anaerobic Se(VI) and Se(IV)-respiring bacteria (40, 41), (ii) spherical red amorphous Se (
