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Removal of hexavalent chromium using mackinawite (FeS)-coated sand
Journal of Hazardous Materials 360 (2018) 17–23
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Removal of hexavalent chromium using mackinawite (FeS)-coated sand a
a
b
c
Minji Park , Jiwon Park , Jungchun Kang , Young-Soo Han , Hoon Young Jeong a b c
a,⁎
T
Department of Geological Sciences, Pusan National University, Busan, 46241, South Korea Korea Chemical management Association, Seocho-daero 88, Seocho-gu, Seoul, 06673, South Korea Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Deajeon, 34132, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromate FeS Coating X-ray absorption near-edge structure (XANES) Permeable reactive barrier (PRB)
This study investigates the feasibility of mackinawite (FeS)-coated sand in permeable reactive barrier applications to treat Cr(VI)-contaminated groundwater under anoxic conditions. For this, Cr(VI) sorption experiments were conducted using both coated and uncoated sands. Solution-phase Cr speciation and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated the complete reduction of Cr(VI) to Cr(III) by coated sand. At pH 4.7, substantial amounts of Cr(III) remained in solution due to its unfavorable cationic adsorption at acidic pH. At pH 7.1 and 9.8, it was quantitatively immobilized by forming Cr(III)-bearing precipitates. In contrast, uncoated sand showed the decreasing Cr(VI) sorption with pH. In uncoated sand, magnetite impurities would mediate the partial reduction of Cr(VI). Thus, the pH-dependent sorption by uncoated sand was due to both unfavorable anionic Cr(VI) adsorption and its lesser reduction to Cr(III) with pH. Compared to uncoated sand, coated sand showed significantly increased Cr(VI) sorption at neutral to basic pH. By Fe K-edge XANES analysis, FeS was mainly responsible for Cr(VI) reduction by coated sand, with a green rust-like phase being the major Fe product. Since Fe(OH)3 is not thermodynamically stable under the redox conditions favoring formation of green rust, Fe(III)-substituted Cr(OH)3 likely represents a Cr(III)-bearing phase.
1. Introduction Groundwater contamination with chromate Cr(VI) results from metallurgy, leather tanning, pigment production, and metal plating [1–3]. Chromium in groundwaters and soils occurs mainly as two oxidation states: Cr(III) and Cr(VI) [1–5]. While Cr(III) is an essential nutrient to organisms, Cr(VI) is toxic, posing serious environmental concerns [1–5]. Although Cr(VI) is sequestered by adsorption to minerals, the adsorbed one is readily remobilized as geochemical conditions change [6]. On the other hand, Cr(III) is more strongly bound to minerals via adsorption, precipitation, and coprecipitation [[2]–[2][3] [4]5]. In this regard, the sequestration of Cr(VI) typically involves its reduction to Cr(III), which is then immobilized by reaction with minerals [[2]–[2][3][4]5]. Thus, it is critical to develop the remediation technologies capable of reducing Cr(VI) to Cr(III). Bond and Fendorf [1] found the reduction of Cr(VI) to Cr(III) by green rusts. In their study, the reduced Cr(III) was subsequently incorporated into the oxidation products of green rusts (e.g., lepidocrocite and magnetite) or sorbed to such Fe (oxyhydr)oxides via surface complexation. In column tests using zeorovalent iron (ZVI), Jeen et al. [7] reported that Cr(VI) was immobilized by forming Fe(III)-Cr(III)
(oxyhydr)oxides. During the groundwater remediation using ZVI, mackinawite, siderite, and green rust, which were produced as corrosion products, enhanced Cr(VI) removal [8]. By reaction with FeS, Cr (VI) was found to be immobilized by forming [Crx, Fe1-x](OH)3-like coprecipitates [3,9]. Permeable reactive barriers (PRBs) have been installed in the direction perpendicular to contaminated groundwater plumes to treat them while crossing the barriers [8,10,11]. Several materials have been assessed for the potential use in PRBs using batch, column, and field studies [8,10,11]. Among them, ZVI has been most widely employed in PRB applications [8,10]. Nonetheless, its use has been often limited since it is thermodynamically unstable in aquatic environments, thus causing gradual losses in its reduction capacity via anoxic corrosion [10,12,13]. In contrast, the aforementioned problem is not encountered in mackinawite (FeS)-based remediation technologies [11–13]. Due to the great reducing capability for redox-active metals (e.g., Cr, As, and Se) as well as the high sorption capacity, FeS is a promising candidate for PRBs [10–[12]13]. Furthermore, its reactivity may be regenerated as a result of the activity of sulfate-reducing bacteria (SRB) [10,12,13]. Yet, due to its nanoparticulate nature [14], FeS may be washed away by groundwater flows at low ionic strengths or clog pore spaces by forming
⁎
Corresponding author. E-mail addresses: [email protected] (M. Park), [email protected] (J. Park), [email protected] (J. Kang), [email protected] (Y.-S. Han), [email protected] (H.Y. Jeong). https://doi.org/10.1016/j.jhazmat.2018.07.086 Received 22 February 2018; Received in revised form 21 July 2018; Accepted 23 July 2018 Available online 24 July 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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0.068( ± 0.002) mmol per g sand at the optimal coating pH 5.4 [12]. The coated sand generated at this pH was used for all the subsequent experiments.
aggregates at high ionic strengths [11–13]. To facilitate its PRB applications, thus, FeS can be deposited on larger particles using coating methods [11–13]. In this study, sand was chosen as a supporting matrix for FeS coating since it is cheap, abundant in soils and aquifers, and widely utilized for treating heavy metal ions in wastewaters [15,16]. Iron oxide-coated sand has been widely utilized for the treatment of polluted waters [15,17,18], but it is only recently that FeS-coated sand has drawn attentions [10–12,16]. This study aims to investigate the feasibility of FeS-coated sand as a potential reagent in PRBs for chromate remediation under anoxic conditions. To this end, FeS-coated sand was assessed for its ability to remove Cr(VI) under varying pH conditions. Also, the reactivity of coated sand was compared to that of uncoated sand to assess the role of FeS deposits on sand. Furthermore, both Cr K-edge and Fe K-edge X-ray absorption near-edge structure (XANES) analyses were performed to determine the oxidation state of Cr and the composition of Fe phases, respectively, in the solid.
2.3. Cr sorption by FeS-coated and uncoated sands To assess the reactivity of FeS deposits toward Cr(VI) removal, a series of batch experiments were performed using either FeS-coated sand or uncoated sand inside the glove box. First, one gram of coated or uncoated sand was weighed into a 15 mL centrifuge tube. Then, 2 mL of 0.5 M acetate, morpholinopropanesulfonate (MOPS), and N,N,N',N'tetraethylethlenediamine (TEEN) buffer solutions were used to maintain the designated pH at 4.7( ± 0.1), 7.1( ± 0.1), and 9.8( ± 0.1), respectively. These buffers were chosen based on their weak complexation with metals [20,21] and redox inertness [22–24]. Subsequently, Cr(VI) stock solutions, NaCl solutions, and deoxygenated water were added to the tubes to produce 10 mL of the bulk solutions having the initial Cr(VI) concentration at 0.02–0.1 mM, the buffer concentration at 0.1 M, and the ionic strength at 0.2 M. Finally, the resultant batches were allowed to react on a shaker for 3 d. This was sufficient to reach the equilibrium in Cr(VI) removal by FeS [3]. Following the reaction period, aliquots of the suspensions were filtered through 0.20 μm nylon filters. The filtrates were then acidified using 10% HNO3 solutions. A portion of the acidified solutions were measured for the dissolved Cr(VI) and Fe(II) concentrations via spectrophotometric methods. While Cr(VI) was analyzed at 540 nm using 1,5-diphenylcarbazide as a developing reagent [25], Fe(II) was analyzed at 508 nm using a 1,10-phenanthroline method [19]. Also, the other portion of solutions were analyzed for the total dissolved Cr and Fe on an inductively coupled plasma optical emission spectroscopy (ICP-OES; HORIBA Jobin Yvon ULTIMA 2C). The remaining suspensions were centrifuged at 8000 rpm for 10 min to have the solid-phase products. Finally, the collected slurries were air-dried inside the glove box and tightly sealed till XANES analyses. In addition, Cr(III) sorption experiments using uncoated sand were performed to compare to Cr(VI) sorption by coated sand in efforts to examine the role of FeS deposits in immobilizing Cr(VI).
2. Experimental 2.1. FeS synthesis and sand preparation FeS was synthesized inside a glove box according to Jeong et al. [14]. First, FeS precipitates were produced by mixing 2.0 L of 0.6 M FeCl2 solution with 1.2 L of 1.1 M Na2S solution for 3 d. Subsequently, the precipitates were collected by decanting the supernatant after centrifuging at 10,000 rpm for 15 min. To remove the residual salts, they were rinsed at least 5 times by adding the deoxygenated, deionized water and then centrifuging. After freeze-dried, the resultant powder was stored in 200 mL glass vials and sealed with Teflon-coated rubber septa to reduce the potential of hydration and oxidation. When brought outside the glove box for centrifuging and freeze-drying, samples were placed inside polycarbonate centrifuge bottles with sealing closure (Thermo Scientific) to minimize air exposure. The produced FeS was nanoparticulate mackinawite (see Fig. S1 in Supporting material for its diffraction pattern), with the specific surface area of 276–345 m2/g [14]. A beach sand (Hama Industry, Korea), the supporting matrix used for coating, was sieved to obtain a fraction of 106–150 μm and then repeatedly rinsed with deionized water. Since no intensive treatment was applied, the resultant sand, mainly composed of quartz, contained mineral impurities such as Fe (oxyhydr)oxides [12]. By BET analysis, its specific surface area was 1.13( ± 0.01) m2/g. Prior to use, sand particles were let sit for at least one day inside the glove box where oxygen concentration was kept below 1 ppm.
2.4. XANES analyses Cr K-edge and Fe K-edge XANES spectra were collected at beamlines 8C and 1D, respectively, at the Pohang accelerator laboratory (PAL) using an unfocused beam with a Si(111) double-crystal monochromator. The storage ring was operated with ∼150 mA at 2.5–3.0 GeV. The incoming beam was detuned by ∼40% to reject higher-order harmonic components. While a 7-elements Ge array detector was used to collect fluorescence signals, high precision ionization chambers (IC-SPEC) were employed for transmission measurements. At least four scans were collected for each sample to enhance the signal-tonoise ratios in XANES spectra. Energy calibration was internally made for each scan using the transmission spectra of a Cr foil (K edge at 5989 eV) or a Fe foil (K edge at 7112 eV) placed between I1 and I2 chambers. During the data collection, all samples were placed in a Hepurged box to minimize beam-catalyzed redox transformation. Due to the poor data quality, Cr K-edge XANES analysis was limited to Cr(VI) sorption samples having the higher Cr(VI) initial concentration (5 mM) than those used in batch experiments (0.02–0.1 mM). Also, the spectra were collected for Cr model compounds including Cr(0), Cr2O3, Cr(OH)3, [Crx, Fe1-x](OH)3, and K2CrO4. Among them, two hydroxides were synthesized as follows: while Cr(OH)3 was obtained by raising the pH of a Cr(NO3)3 solution above 10 using NaOH solutions, [Crx, Fe1-x](OH)3 was prepared by slowly titrating 1.6 L of the solution containing both Cr(NO3)3 and Fe(NO3)3 to pH ∼7 [26]. The resultant precipitates were thoroughly rinsed, air-dried, and stored inside the glove box until XANES analyses. Also, to determine the Cr(III) to Cr(VI) ratios in Cr(VI) sorption samples, a series of K2CrO4–Cr(NO3)3·9H2O
2.2. FeS coating Inside the glove box, 10 g of the rinsed sand particles were weighed into 0.1 M NaCl solutions and then mixed with 20 mL of 2.0 g/L FeS suspensions under agitation for 48 h. To optimize the coating, the pH of the above mixtures was adjusted to range from 4.0 to 10.0 by adding 0.1 M HCl or NaOH solutions during agitation. Following this, FeScoated sand particles were collected using filter papers having the pore size of 110 μm (CHMLAB GROUP). Then, the collected particles were allowed to dry inside the glove box. To determine the amount of FeS deposits, 1.0 g of coated sand was added into 40 mL of 1.0 M HCl solutions and agitated for 48 h. After the acid extraction, aliquots of the solutions were filtered through a 0.20 μm nylon membrane filter (CHMLAB GROUP). Then, 5 mL of the filtrates were mixed with 10% HNO3 solutions for Fe(II) analysis using the Ferrozine method [19]. Since the Fe(II) liberated during the extraction would come from both FeS deposits and Fe impurities in sand, 1.0 g of uncoated sand was also subjected to the above acid-extraction to determine the amount of Fe(II) originating from such impurities. Based on the results of both extracts, the amount of FeS deposits was 18
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Fig. 2a–c, the dissolved Cr at pH 4.7 is predominantly in a trivalent state. Thus, although not completely sequestered at pH 4.7, the initially added Cr(VI) was at least reduced to a less toxic Cr(III). Previously, Cr (III) removal at acidic pH occurred to less extents due to both unfavorable cationic adsorption [29] and the increased solubility of Cr (OH)3 and/or [Crx, Fe1-x](OH)3 under such pH conditions [2,8]. In Fig. 1, the sorption isotherms were fitted into Langmuir models via nonlinear regressions:
qe =
qe,m KCe 1 + KCe
(1)
where Ce is the total dissolved Cr concentration; qe is the solid-phase Cr concentration; qe,m is the complete monolayer coverage; and K is the sorption constant. The fitting results are summarized in Table 1. For Cr (VI) sorption by coated sand at pH 4.7, qe shows a gradual increase with Ce, and it is far lower than qe,m, both suggesting the dominance of surface-mediated sorption at acidic pH. Despite the comparable experiment conditions (e.g., the initial Cr(VI) to FeS concentration ratios), Patterson and Fendorf [3] have proposed that coprecipitation as [Crx, Fe1-x](OH)3 is responsible for Cr(VI) uptake in FeS suspensions at pH 5. Such a difference may result from additional surfaces (e.g., quartz and Fe (oxyhydr)oxides in sand) available for adsorption of the once-reduced Cr(III) in this study. Also, the Fe(III) species generated during Cr (VI) reduction may adsorb to or interact with Fe (oxyhydr)oxides impurities rather than coprecipitating with Cr(III) under our experimental conditions. Cr(VI) sorption by uncoated sand is also characterized by a pH-dependent behavior (see Fig. 1b). Unlike coated sand, however, uncoated sand shows the decreasing sorption behavior with pH. Over the pH range examined, the dissolved Cr(VI) is present as anionic species (e.g., HCrO4− and CrO42-) (see Fig. S2). Consistent with our finding, the anionic Cr(VI) species tend to sorb to lesser extents with pH due to the development of more negative charges on mineral surfaces [3]. Yet, as shown in Fig. 2d–f, the initially added Cr(VI) was partially reduced to Cr(III) even without FeS deposits. According to the analysis of acid extracts, uncoated sand contained a substantial amount of Fe(II) (e.g., 0.033 mmol Fe(II) liberated per g sand). As further discussed in Section 3.4, magnetite (Fe3O4) accounted for ∼30% of the total Fe phases in uncoated sand. In fact, magnetite commonly occurs as beach placers [30]. Magnetite has been shown to be capable of reducing Cr(VI) to Cr(III) [31–33]. Notably, its ability to reduce Cr(VI) becomes kinetically limited and less extensive with pH [31,32]. Before being heterogeneously reduced, Cr(VI) should first adsorb to the magnetite surface [34,35]. Thus, lesser adsorption of anionic Cr(VI) with pH results in its decreased reduction [34,35]. Also, the formation of a passivation layer of Fe (oxyhydr)oxides (e.g., maghemite and goethite) on the magnetite surface can further retard Cr(VI) reduction at neutral to basic pH [31]. With the increasing contact time, thus, Cr(VI) is more likely to adsorb to the magnetite surface without being reduced to Cr(III) [33]. Besides, given the low solubility of magnetite at neutral to basic pH, the homogenous reduction of Cr(VI) by the dissolved Fe(II) may be not significant under such pH conditions. Indeed, the concentration ratios of Cr(III) to Cr(V) in this study decrease with pH (see Fig. 2d–f). Taken together, the observed pH-dependency in Cr(VI) removal by uncoated sand is likely due to both unfavorable anionic adsorption of Cr(VI) and its lesser reduction to Cr(III) with pH. By comparing Fig. 1a and b, Cr(VI) removal by coated sand is more extensive than that by uncoated sand, pointing to the significance of FeS deposits in immobilizing Cr(VI). Importantly, the enhanced sequestration by FeS deposits is more evident at pH 7.1 and 9.8. In contrast, Cr(VI) removal by Fe(0) and Fe(II)-bearing (oxyhydr)oxides is limited due to its lower tendency to be reduced to Cr(III) under such pH conditions [31,32,36].
Fig. 1. Isotherms of Cr(VI) sorption by FeS-coated sand (a) and uncoated sand (b) at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentrations were 0.02–0.1 mM at the loading of 100 g sand/L. Dashed lines correspond to the data fits by Langmuir isotherms.
mixtures were subjected to XANES measurements [3]. In parallel, Fe Kedge XANES spectra were collected to determine the solid-phase Fe speciation in uncoated sand, FeS-coated sand, and Cr(VI) sorption samples. The sorption samples subjected to Fe K-edge XANES analysis were prepared by reacting coated sand with 5 mM Cr(VI) solutions. The collected spectra were processed using SixPACK [27]. Individual scans were averaged and then the background absorbance was subtracted using a linear fit over the pre-edge region. Subsequently, XANE spectra were extracted by normalizing the signal to its edge-jump height. In case of Cr K-edge XANES spectra, the first derivative spectra were obtained after smoothing them to better compare the absorption edges. For Fe K-edge XANES spectra, linear combination fitting (LCF) analysis was performed with Fe model compounds at different oxidation states.
3. Results and discussion 3.1. Cr(VI) sorption by FeS-coated and uncoated sands Cr(VI) sorption experiments were performed using both FeS-coated and uncoated sands. In Fig. 1a, Cr(VI) removal by coated sand shows a pH-dependent behavior. The initially added Cr(VI) was quantitatively removed at pH 7.1 and 9.8. At these pH, the vertical isotherms, as featured by the near constant dissolved Cr concentrations, suggests the bulk-phase (co)precipitation [28]. Previously, Cr(VI) was proposed to be immobilized by forming coprecipitates as [Crx, Fe1-x](OH)3 in aqueous FeS suspensions [3]. In contrast, at pH 4.7, significant amounts of Cr remained in solution. From the solution-phase Cr speciation in 19
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Fig. 2. Dissolved Cr(VI) and Cr(III) concentrations as a function of the initial Cr(VI) concentration: Cr(VI) sorption by FeS-coated sand at pH 4.7 (a), 7.1 (b), and 9.8 (c) and Cr(VI) sorption by uncoated sand at pH 4.7 (d), 7.1 (e), and 9.8 (f). Table 1 Langmuir isotherm parameters for Cr(VI) and Cr(III) sorption by FeS-coated and uncoated sands. Parametersa
Batch
R2
Sorbate
Sorbent
pH
qe,m (mmol g−1)
K (L mmol−1)
Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(III) Cr(III) Cr(III)
Coated sand Coated sand Coated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand
4.7 7.1 9.8 4.7 7.1 9.8 4.7 7.1 9.8
3.6( ± 1.3)×10−3 Unable to fit Unable to fit 2.5( ± 0.1) ×10−4 1.4( ± 0.0) ×10−4 5.0( ± 0.2) ×10−5 6.8( ± 2.7) ×10−4 Unable to fit Unable to fit
3.08( ± 1.25)
0.9978
31.7( ± 1.7) 56.5( ± 3.6) 56.3( ± 9.6) 7.51( ± 4.27)
0.999 0.998 0.971 0.967
a
Errors represent one standard deviation. Fig. 3. Comparison between Cr(VI) sorption by FeS-coated sand and Cr(III) sorption by uncoated sand at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentrations were 0.02–0.1 mM at the loading of 100 g sand/L. Dashed lines correspond to the data fits by Langmuir isotherms.
3.2. Cr(III) sorption by uncoated sand Fig. 3 shows Cr(III) sorption isotherms by uncoated sand. At pH 4.7, Cr(III) removal by uncoated sand was not complete as characterized by a Langmuir-type isotherm. At this pH, surface-mediated sorption would be responsible for Cr(III) removal by uncoated sand. On the other hand, at pH 7.1 and 9.8, the initially added Cr(III) was quantitatively removed from the solution, indicating the dominance of (co)precipitation at neutral to basic pH. Such a pH-dependent pattern is similar to that observed in Cr(VI) sorption by FeS-coated sand. Despite the aforementioned similarity, Cr(III) removal by uncoated sand at pH 4.7 is noticeably lower than Cr(VI) removal by coated sand, implying that the significance of FeS deposits at acidic pH. The unreacted FeS in coated sand can provide surface sites for Cr(III) adsorption. Given the point of zero-charge (PZC) of mackinawite lower than 4.0 [9], its surface becomes negatively charged over the pH range examined, making cationic Cr(III) adsorption favorable. However, as discussed in Section 3.4, FeS deposits at pH 4.7 were completely depleted due to a combination of the consumption during Cr(VI) reduction and H+-promoted dissolution. Instead, the Fe(III) species produced during FeS-mediated Cr(VI) reduction would form amorphous to poorly
crystalline precipitates, thus making additional adsorption sites available for Cr(III) adsorption. At pH 7.1 and 9.8, it is not possible to differentiate the isotherms between Cr(VI) removal by coated sand and Cr(III) removal by uncoated sand (see Fig. 3). As discussed in Section 3.4, FeS deposits in coated sand play a primary role in the reduction of Cr(VI) to Cr(III), which is then removed via (co)precipitation. Although less effective than FeS deposits, magnetite impurities in uncoated sand can partially mediate the coupled reduction-precipitation of Cr(VI) at neutral to basic pH. 3.3. Cr K-edge XANES analysis Cr K-edge XANES analysis provides the information on the oxidation state of Cr in solid products. In Fig. 4, the XANES spectra of Cr(VI) sorption samples by both coated and uncoated sands are compared to those of Cr model compounds. While Cr(VI) exhibits a strong pre-edge at 5993.8 eV due to the electron transitions from 1s to 3d orbitals 20
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Fig. 4. Cr K-edge XANES spectra (a) and their first derivatives (b) of Cr(VI) sorption samples by FeS-coated and uncoated sands at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentration was 5 mM at the loading of 100 g sand/L. In part (a), pre-edges are indicated by dashed lines. In part (b), absorption edges are indicated by dashed lines. C/S and N/S correspond to FeS-coated sand and uncoated sand, respectively. The spectra of Cr model compounds are also included.
structure (EXAFS) analysis, the similar background scattering between Cr and Fe makes it impossible to discern one from the other in Cr coordination environments [26]. Previously, a transmission electron microscopy coupled with energy dispersive spectroscopy (TEM-EDS) was applied to determine the composition of Cr(III)-bearing hydroxides in Cr(VI) sorption studies by FeS [3,9]. Yet, given the amorphous nature of Fe and Cr precipitates and the strong tendency to form aggregates under intensive vacuum [3,9], it is doubtful that their results reflect the composition of a single Cr phase rather than the averaged composition of multiple phases. Although the composition of Cr(III)-bearing hydroxides has been proposed in several studies, it has not been conclusively identified [2,3,9,39]. In this study, the presence of Fe impurities in sand further hindered the application of TEM-EDS. For Cr(VI) sorption samples by uncoated sand, their XANES spectra appear to have both pre-edges at 5990.5 and 5993.8 eV, indicating the partial reduction of Cr(VI) to Cr(III). As aforementioned, uncoated sand contained magnetite impurities, which could mediate Cr(VI) reduction. Due to the highly diffuse pre-edges in their XANES spectra, it is not possible to accurately determine the Cr(VI):Cr(III) ratios. Nonetheless, by comparing the pre-edge height, Cr(VI) fraction was the greatest at pH 4.7. This supports a favorable anionic Cr(VI) adsorption at acidic pH. Similar results were reported for Cr(VI) reduction by magnetite [31].
[5,8,26], Cr(III) is characterized by very weak pre-edges at 5990.5 and 5993.8 eV due to the transition from 1s to 3d (t2g) and 3d (eg), respectively [37]. Consistently, a strong pre-edge is present in the XANES spectrum of K2CrO4, whereas there are two weak pre-edges for Cr(III)bearing minerals. In case that Cr(VI) and Cr(III) coexist, the height of the pre-edge at 5993.8 eV in background-subtracted spectra is directly proportional to the Cr(VI) fraction [3,37]. In Fig. S3, the pre-edge height increases with Cr(VI) fraction among the mixtures between K2CrO4 and Cr2O3. The regression of the pre-edge height against Cr(VI) fraction using a second-order polynomial [37] produced a good correlation (R2 = 0.9996). Subsequently, this regression was used to determine the relative amounts between Cr(III) and Cr(VI) in solid products. In Fig. 4a, the XANES spectra of all Cr(VI) sorption samples by coated sand show the pre-edges at 5990.5 eV but not the discernible pre-edges at 5993.8 eV, both indicating the dominance of Cr(III) in the solid phase. Consistently, the determination of the Cr(VI):Cr(III) ratios using the above regression points to negligible Cr(VI) fractions in all sorption samples. The absorption edge, which is given by the maximum of the first derivative of a XANES spectrum, can be used to determine the oxidation state of Cr; the absorption edge of a more oxidized species is located at a higher energy than that of a relatively reduced one [8]. In Fig. 4b, the absorption edges of Cr(VI) sorption samples by coated sand are close to those of Cr(OH)3 and [Crx, Fe1-x](OH)3. Note that the absorption edge of Cr2O3, despite the same oxidation state as these hydroxides, is located at a lower energy due to its greater covalent nature [38]. Thus, given the XANES results as well as the solution-phase Cr speciation, nearly all of the initially added Cr(VI) was reduced to Cr(III) by coated sand. In Fig. 4, the XANES spectra of all Cr(VI) sorption sample by coated sand are similar to those of Cr(OH)3 and [Crx, Fe1-x](OH)3, suggesting these hydroxide-like coordination environments around Cr. This is not consistent with the dominance of surface-mediated sorption at pH 4.7 as discussed in Section 3.1. Due to the poor quality of XANES spectra, a much higher Cr(VI) concentration (5 mM) than those used in sorption experiments (0.02–0.1 mM) was applied to prepare XANES samples. At such a high loading, the adsorption capacity would become saturated, thus rendering (co)precipitation take place. Based on the similarity of XANES spectra, Cr(OH)3 and/or [Crx, Fe1-x](OH)3 are likely to be Cr (III)-bearing precipitates formed at pH 7.1 and 9.8. Yet, it is not possible to distinguish them due to the close resemblance in their XANES spectra (see Fig. 4). Even with Cr K-edge extended X-ray absorption fine
3.4. Fe K-edge XANES analysis Fe K-edge XANES spectra were subjected to LCF analysis to determine the solid-phase Fe composition in Cr(VI) sorption samples by coated sand as well as uncoated/coated sands (see Table 2 and Fig. S4). For uncoated sand, ∼30% of the total Fe was accounted for by magnetite, with the remainder being mainly Fe(III) (oxyhydr)oxides (e.g., maghemite, hematite, lepidocrocite, and goethite). In terms of Fe composition, coated sand consisted of ∼14% mackinawite (FeS). Notably, it contained a significant fraction of green rust, a mixed Fe(II)-Fe (III) hydroxide. During the coating step, this phase would form by reaction between FeS and Fe impurities in sand. Compared to the unreacted, coated sand, Cr(VI) sorption samples by coated sand show quite different Fe compositions (see Table 2). First, the FeS fraction decreased dramatically as a result of reaction with Cr (VI), but the magnetite fraction remained relatively constant, both indicating that FeS deposits were mainly responsible for Cr(VI) reduction in coated sand. At neutral to basic pH, FeS can reduce Cr(VI) to Cr(III) 21
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Table 2 Results of linear combination fitting (LCF) analysis for Fe K-edge XANES spectra. Fe model compounds
Uncoated sand
Coated sand
Cr(VI) at pH 4.7a
Cr(VI) at pH 7.1a
Cr(VI) at pH 9.8a
Mackinawite (FeS) Pyrite (FeS2) Greigite (Fe3S4) Green rust (FeII3FeIII(OH)8Cl) Magnetite (Fe3O4) Maghemite (γ-Fe2O3) Hydrous ferric oxide (Fe2O3·0.5H2O) Goethite (α-FeOOH) Lepidocrocite (γ-FeOOH) Hematite (α-Fe2O3) Rf b
0.0( ± 2.6)% 0.0( ± 3.6)% 0.0( ± 3.9)% 0.1( ± 2.9)% 29.7( ± 6.2)% 25.8( ± 4.4)% 6.9( ± 21.1)% 10.1( ± 12.8)% 12.9( ± 6.2)% 14.5( ± 4.8)% 0.0014
14.1( ± 2.6)% 0.0( ± 3.3)% 0.0( ± 3.8)% 12.7( ± 2.7)% 6.8( ± 5.8)% 7.4( ± 4.3)% 5.7( ± 11.7)% 7.0( ± 12.5)% 38.4( ± 6.1)% 8.0( ± 4.6)% 0.0013
0.0( ± 0.9)% 1.9( ± 1.3)% 0.0( ± 1.3)% 41.2( ± 1.0)% 7.2( ± 2.2)% 1.4( ± 2.1)% 4.0( ± 4.9)% 5.3( ± 5.2)% 22.7( ± 2.4)% 16.2( ± 1.7)% 0.0011
1.8( ± 0.9)% 0.5( ± 1.2)% 0.0( ± 1.2)% 42.4( ± 0.9)% 5.4( ± 2.1)% 1.1( ± 2.0)% 7.4( ± 4.5)% 8.5( ± 4.9)% 19.8( ± 2.3)% 13.2( ± 1.2)% 0.0010
5.6( ± 0.8)% 0.2( ± 1.2)% 0.0( ± 1.2)% 39.3( ± 0.7)% 11.0( ± 2.0)% 2.6( ± 1.9)% 7.8( ± 4.3)% 5.3( ± 4.8)% 10.8( ± 2.2)% 17.2( ± 1.4)% 0.0009
a b
All sorption samples were prepared by reacting FeS-coated sand with 5 mM Cr(VI) solutions. Rf indicates the goodness of fitting.
via a surface-mediated redox process [3]. At acidic pH, due to the H+promoted dissolution of FeS [40], the Fe(II) liberated from FeS can homogeneously catalyze Cr(VI) reduction [3]. Second, the fraction of green rust in Cr(VI) sorption samples was three times greater than that in the unreacted, coated sand. This suggests that a green rust-like phase would be the major Fe product in Cr(VI) sorption by coated sand. Under moderately reducing conditions, green rust is thermodynamically more stable than Fe(III) (oxyhydr)oxide [41]. Consistent with our finding, cronstedtite (FeII3FeIIISiO4(OH)5), another mixed Fe(II)-Fe(III) hydroxide, was produced in Cr(VI) reduction by pyrrhotite [42]. At pH 7.1 and 9.8, the dissolved Fe(III) was present at elevated concentrations, but the dissolved Fe(II) was minimal (see Fig. S5). At these pH, thus, the incorporation of Fe(III) into a green rust-like phase may have been limited by the availability of Fe(II). In contrast, at pH 4.7, given the abundance of the dissolved Fe(II), the dissolved Fe(III) was mostly immobilized by forming a green rust-like phase. Taken together, reactions (2) and (3) are proposed to represent the principal oxidation half-reactions at acidic and neutral-to-basic pH, respectively:
Fe(III)-substituted Cr(OH)3 was likely to be a Cr(III)-bearing phase. In constant to coated sand, uncoated sand showed the decreasing Cr(VI) sorption with pH. Magnetite impurities in uncoated sand could mediate the partial reduction of Cr(VI) to Cr(III). Thus, the observed pH-dependent sorption by uncoated sand was due to both unfavorable anionic adsorption of Cr(VI) and its lesser reduction to Cr(III) with pH. Compared to Cr(III) sorption by uncoated sand, Cr(V) sorption by coated sand was significantly enhanced at pH 4.7. Thus, the role of FeS deposits was not restricted to Cr(VI) reduction, but their oxidation products could also provide additional surface sites for cationic Cr(III) adsorption. In this study, FeS-coated sand shows a greater reactivity in Cr(VI) removal than uncoated sand. Of particular, the elevated reactivity is more evident at neutral to basic pH where Cr(VI) removal by Fe(0) and Fe(II)-bearing (oxyhydr)oxides is often limited. Despite the great reactivity of FeS-coated sand, additional studies are needed before its PRB applications, which may include the susceptibility of FeS-coated sand to suboxic/oxic environments, the aging effect of FeS, and the presence of other contaminants.
4Fe2+ + Cl− + 8H2O → FeII3FeIII(OH)8Cl + 8H+ + e−
Acknowledgments
(2)
6FeS + Cl− + 17H2O → FeII3FeIII(OH)8Cl + 2Fe3+ + 3S2O32- + 26H+ + 27e− (3)
This work was supported by Basic Science Research Program through the National Research Foundation of KoreaNRF2016R1D1A1B04931643 and the "R&D Project on Environmental Management of Geologic CO2 Storage" from the KEITI (Project Number: 2014001810003). Experimental works at Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Education, Science and Technology of the Korean Government and Pohang University of Science and Technology (POSTECH).
Chloride green rust is included given the use of 0.15 M NaCl as a background electrolyte. In reaction (3), thiosulfate (S2O32−) is included given it was reported to be the major oxidation product of sulfur in Cr (VI) reduction by FeS [3]. Yet, considering the formation of sulfate and other oxidized sulfurs [3,9], there are multiple alternatives to reaction (3). Upon receipt of electrons from reactions (2) and (3), Cr(VI) is reduced to Cr(III). As previously discussed, Cr(III) is immobilized mainly via adsorption at pH 4.7 and (co)precipitation at pH 7.1 and 9.8. Given the formation of a green rust-like phase as the major Fe product, Fe(III) may substitute Cr(III) in Cr(OH)3 instead of forming Fe(OH)3. Thus, if a mixed Cr(III)-Fe(III) hydroxide forms, it is likely to be Cr(OH)3-rich. Also, although Cr(III) may occupy Fe(III) sites in green rust, such occupancy is expected to be limited to be consistent with the dominance of surface-mediated sorption at pH 4.7.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.07.086. References [1] D.L. Bond, S. Fendorf, Kinetics and structural constraints of chromate reduction by green rusts, Environ. Sci. Technol. 37 (2003) 2750–2757. [2] L.E. Eary, D. Rai, Chromate removal from aqueous wastes by reduction with ferrous ion, Environ. Sci. Technol. 22 (1988) 972–977. [3] R.R. Patterson, S. Fendorf, Reduction of hexavalent chromium by amorphous iron sulfide, Environ. Sci. Technol. 31 (1997) 2039–2044. [4] O. Ajouyed, C. Hurel, M. Ammari, L.B. Allal, N. Marmier, Sorption of Cr(VI) onto natural iron and aluminum (oxy)hydroxides: effects of pH, ionic strength and initial concentration, J. Hazard. Mater. 174 (2010) 616–622. [5] R.A. Whittleston, D.I. Stewart, R.J.G. Mortimer, Z.C. Tilt, A.P. Brown, K. Geraki, I.T. Burke, Chromate reduction in Fe(II)-containing soil affected by hyperalkaline leachate from chromite ore processing residue, J. Hazard. Mater. 194 (2011) 15–23. [6] E.L. Hawley, R.A. Deeb, M.C. Kavanaugh, J.A. Jacobs, Treatment technologies for chromium (VI), in: J. Guertin, J.A. Jacobs, C.P. Avakian (Eds.), Chromium(VI) Handbook, CRC Press, Boca Raton, FL, 2004, pp. 275–309.
4. Conclusions This study demonstrates that FeS-coated sand can be used for PRB applications to treat Cr(VI) contamination under anoxic conditions. According to the solution-phase Cr speciation and Cr K-edge XANES analysis, Cr(VI) was completely reduced to Cr(III) by coated sand. At pH 4.7, significant amounts of Cr(III) was present in solution due to the unfavorable cationic adsorption. On the other hand, at pH 7.1 and 9.8, it was quantitatively immobilized via (co)precipitation. By Fe K-edge XANES analysis, a green rust-like phase was the major Fe product. Thus, 22
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iron(II), Environ. Sci. Technol. 32 (1998) 2092–2099. [25] American Public Health Association, Standard Methods for Examination of Water and Wastewater, American Public Health Association, Washington, DC, 1985. [26] C.M. Hansel, B.W. Wielinga, S. Fendorf, Structural and compositional evolution of Cr/Fe solids after indirect chromate reduction by dissimilatory iron-reducing bacteria, Geochim. Cosmochim. Acta 67 (2003) 401–412. [27] S.M. Webb, SIXPack: a graphical user interface for XAS analysis using IFEFFIT, Phys. Scr. 115 (2005) 1011–1014. [28] F.M.M. Morel, J.G. Hering, Principles and Applications of Aquatic Chemistry, Wiley-Interscience, New York, 1993. [29] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, third ed., John Wiley & Sons, New York, 1996. [30] M.N. Alam, M.I. Chowdhury, M. Kamal, S. Ghose, M.N. Islam, M.N. Mustafa, M.M.H. Miah, M.M. Ansary, The 226Ra, 232Th and 40K activities in beach sand minerals and beach soils of Cox’s Bazar, Bangladesh, J. Environ. Radioact. 46 (1999) 243–250. [31] Y.T. He, S.J. Traina, Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions: the role of passivation, Environ. Sci. Technol. 39 (2005) 4499–4504. [32] A.F. White, M.L. Peterson, Reduction of aqueous transition metal species on the surfaces of Fe(II)-containing oxides, Geochim. Cosmochim. Acta 60 (1996) 3799–3814. [33] A.H. Meena, Y. Arai, Effects of common groundwater ions on chromate removal by magnetite: importance of chromate adsorption, Geochem. Trans. 17 (2016) 1. [34] J.M. Zachara, D.C. Girvin, R.L. Schmidt, C.T. Resch, Chromate adsorption on amorphous iron oxyhydroxide in the presence of major groundwater ions, Environ. Sci. Technol. 21 (1987) 589–594. [35] B.S. Krishna, Thermodynamics of chromium(VI) anionic species sorption onto surfactant-modified montmorillonite clay, J. Colloid Interface Sci. 229 (2000) 230–236. [36] M.J. Alowitz, M.M. Scherer, Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal, Environ. Sci. Technol. 36 (2002) 299–306. [37] M.L. Peterson, G.E. Brown Jr., G.A. Parks, C.L. Stein, Differential redox and sorption of Cr(III/VI) on natural silicate and oxide minerals: EXAFS and XANES results, Geochim. Cosmochim. Acta 61 (1997) 3399–3412. [38] H.Y. Jeong, D.-C. Koh, K.-S. Lee, H.H. Lee, Characterization of thermally treated Co2+-exchanged zeolite X, Appl. Catal. B-Environ. 127 (2012) 68–76. [39] A.I. Zouboulis, K.A. Kydros, K.A. Matis, Removal of hexavalent chromium anions from solutions by pyrite fines, Water Res. 29 (1995) 1755–1760. [40] D. Rickard, The solubility of FeS, Geochim. Cosmochim. Acta 70 (2006) 5779–578. [41] J.M.R. Génin, G. Bourrié, F. Trolard, M. Abdelmoula, A. Jaffrezic, P. Refait, V. Maitre, B. Humbert, A. Herbillon, Thermodynamic equilibria in aqueous suspensions of synthetic and natural Fe(II)-Fe(III) green rusts: occurrences of the mineral in hydromorphic soils, Environ. Sci. Technol. 32 (1998) 1058–1068. [42] S.A. Baig, Q. Wang, Z. Wang, J. Zhu, Z. Lou, T. Sheng, X. Xu, Hexavalent chromium removal from solutions: surface efficacy and characterizations of three iron containing minerals, Clean Soil Air Water 42 (2014) 1409–1414.
[7] S.W. Jeen, D.W. Blowes, R.W. Gillham, Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions, J. Contam. Hydrol. 95 (2008) 76–91. [8] R.T. Wilkin, C. Su, R.G. Ford, C.J. Paul, Chromium-removal processes during groundwater remediation by a zerovalent iron permeable reactive barrier, Environ. Sci. Technol. 39 (2005) 4599–4605. [9] M. Mullet, S. Boursiquot, J.-J. Ehrhardt, Removal of hexavalent chromium from solutions by mackinawite, tetragonal FeS, Colloids Surf. A Physicochem. Eng. Asp. 244 (2004) 77–85. [10] A.D. Henderson, A.H. Demond, Permeability of iron sulfide (FeS)-based materials for groundwater remediation, Water Res. 47 (2013) 1267–1276. [11] Y.S. Han, T.J. Gallegos, A.H. Demond, K.F. Hayes, FeS-coated sand for removal of arsenic(III) under anaerobic conditions in permeable reactive barriers, Water Res. 45 (2011) 593–604. [12] S. Lee, J.C. Kang, M. Park, K. Yang, H.Y. Jeong, Sorption of arsenite using nanosized mackinawite (FeS)-coated silica sand, J. Miner. Soc. Korea 25 (2012) 185–195. [13] S. Lee, J.C. Kang, M. Park, K. Yang, H.Y. Jeong, Removal of arsenite by nanocrystalline mackinawite (FeS)-coated alumina, J. Miner. Soc. Korea 26 (2013) 101–110. [14] H.Y. Jeong, J.H. Lee, K.F. Hayes, Characterization of synthetic nanocrystalline mackinawite: crystal structure, particle size, and specific surface area, Geochim. Cosmochim. Acta 72 (2008) 493–505. [15] B. Rusch, K. Hanna, B. Humbert, Coating of quartz silica with iron oxides: characterization and surface reactivity of iron coating phases, Colloids Surf. A Physicochem. Eng. Asp. 353 (2010) 172–180. [16] Y.S. Han, H.Y. Jeong, A.H. Demond, K.F. Hayes, X-ray absorption and photoelectron spectroscopic study of the association of As(III) with nanoparticulate FeS and FeScoated sand, Water Res. 45 (2011) 5727–5735. [17] E. Tanis, K. Hanna, E. Emmanuel, Experimental and modeling studies of sorption of tetracycline onto iron oxides-coated quartz, Colloids Surf. A Physicochem. Eng. Asp. 327 (2008) 57–63. [18] J.-S. Yang, M.J. Kwon, Y.-T. Park, J. Choi, Adsorption of arsenic from aqueous solutions by iron oxide coated sand fabricated with acid mine drainage, Separ. Sci. Technol. 50 (2015) 267–275. [19] E.J.P. Phillips, D.R. Lovely, Determination of Fe(III) and Fe(II) in oxalate extracts of sediments, Soil Sci. Soc. Am. J. 51 (1987) 938–941. [20] Q. Yu, A. Kandegedara, Y. Xu, D.B. Rorabacher, Avoiding interferences from Good’s buffers: a contiguous series of noncomplexing tertiary amine buffers covering the entire range of pH 3–11, Anal. Biochem. 253 (1997) 50–56. [21] I.J. Buerge, S.J. Hug, Kinetics and pH dependence of chromium(VI) reduction by Fe (II), Environ. Sci. Technol. 31 (1997) 1426–1432. [22] J.K. Grady, Chasteen N.D, D.C. Harris, Radicals from "Good’s" buffers, Anal. Biochem. 173 (1988) 111–115. [23] M. Kirsch, E.E. Lomonosova, H.-G. Korth, R. Sustmann, H. de Groot, Hydrogen peroxide formation by reaction of peroxynitrite with HEPES and related tertiary amines. Implications for a general mechanism, J. Biol. Chem. 273 (1998) 12716–12724. [24] I.J. Buerge, S.J. Hug, Influence of organic ligands on chromium(VI) reduction by
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Removal of hexavalent chromium using mackinawite (FeS)-coated sand a
a
b
c
Minji Park , Jiwon Park , Jungchun Kang , Young-Soo Han , Hoon Young Jeong a b c
a,⁎
T
Department of Geological Sciences, Pusan National University, Busan, 46241, South Korea Korea Chemical management Association, Seocho-daero 88, Seocho-gu, Seoul, 06673, South Korea Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Deajeon, 34132, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromate FeS Coating X-ray absorption near-edge structure (XANES) Permeable reactive barrier (PRB)
This study investigates the feasibility of mackinawite (FeS)-coated sand in permeable reactive barrier applications to treat Cr(VI)-contaminated groundwater under anoxic conditions. For this, Cr(VI) sorption experiments were conducted using both coated and uncoated sands. Solution-phase Cr speciation and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated the complete reduction of Cr(VI) to Cr(III) by coated sand. At pH 4.7, substantial amounts of Cr(III) remained in solution due to its unfavorable cationic adsorption at acidic pH. At pH 7.1 and 9.8, it was quantitatively immobilized by forming Cr(III)-bearing precipitates. In contrast, uncoated sand showed the decreasing Cr(VI) sorption with pH. In uncoated sand, magnetite impurities would mediate the partial reduction of Cr(VI). Thus, the pH-dependent sorption by uncoated sand was due to both unfavorable anionic Cr(VI) adsorption and its lesser reduction to Cr(III) with pH. Compared to uncoated sand, coated sand showed significantly increased Cr(VI) sorption at neutral to basic pH. By Fe K-edge XANES analysis, FeS was mainly responsible for Cr(VI) reduction by coated sand, with a green rust-like phase being the major Fe product. Since Fe(OH)3 is not thermodynamically stable under the redox conditions favoring formation of green rust, Fe(III)-substituted Cr(OH)3 likely represents a Cr(III)-bearing phase.
1. Introduction Groundwater contamination with chromate Cr(VI) results from metallurgy, leather tanning, pigment production, and metal plating [1–3]. Chromium in groundwaters and soils occurs mainly as two oxidation states: Cr(III) and Cr(VI) [1–5]. While Cr(III) is an essential nutrient to organisms, Cr(VI) is toxic, posing serious environmental concerns [1–5]. Although Cr(VI) is sequestered by adsorption to minerals, the adsorbed one is readily remobilized as geochemical conditions change [6]. On the other hand, Cr(III) is more strongly bound to minerals via adsorption, precipitation, and coprecipitation [[2]–[2][3] [4]5]. In this regard, the sequestration of Cr(VI) typically involves its reduction to Cr(III), which is then immobilized by reaction with minerals [[2]–[2][3][4]5]. Thus, it is critical to develop the remediation technologies capable of reducing Cr(VI) to Cr(III). Bond and Fendorf [1] found the reduction of Cr(VI) to Cr(III) by green rusts. In their study, the reduced Cr(III) was subsequently incorporated into the oxidation products of green rusts (e.g., lepidocrocite and magnetite) or sorbed to such Fe (oxyhydr)oxides via surface complexation. In column tests using zeorovalent iron (ZVI), Jeen et al. [7] reported that Cr(VI) was immobilized by forming Fe(III)-Cr(III)
(oxyhydr)oxides. During the groundwater remediation using ZVI, mackinawite, siderite, and green rust, which were produced as corrosion products, enhanced Cr(VI) removal [8]. By reaction with FeS, Cr (VI) was found to be immobilized by forming [Crx, Fe1-x](OH)3-like coprecipitates [3,9]. Permeable reactive barriers (PRBs) have been installed in the direction perpendicular to contaminated groundwater plumes to treat them while crossing the barriers [8,10,11]. Several materials have been assessed for the potential use in PRBs using batch, column, and field studies [8,10,11]. Among them, ZVI has been most widely employed in PRB applications [8,10]. Nonetheless, its use has been often limited since it is thermodynamically unstable in aquatic environments, thus causing gradual losses in its reduction capacity via anoxic corrosion [10,12,13]. In contrast, the aforementioned problem is not encountered in mackinawite (FeS)-based remediation technologies [11–13]. Due to the great reducing capability for redox-active metals (e.g., Cr, As, and Se) as well as the high sorption capacity, FeS is a promising candidate for PRBs [10–[12]13]. Furthermore, its reactivity may be regenerated as a result of the activity of sulfate-reducing bacteria (SRB) [10,12,13]. Yet, due to its nanoparticulate nature [14], FeS may be washed away by groundwater flows at low ionic strengths or clog pore spaces by forming
⁎
Corresponding author. E-mail addresses: [email protected] (M. Park), [email protected] (J. Park), [email protected] (J. Kang), [email protected] (Y.-S. Han), [email protected] (H.Y. Jeong). https://doi.org/10.1016/j.jhazmat.2018.07.086 Received 22 February 2018; Received in revised form 21 July 2018; Accepted 23 July 2018 Available online 24 July 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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0.068( ± 0.002) mmol per g sand at the optimal coating pH 5.4 [12]. The coated sand generated at this pH was used for all the subsequent experiments.
aggregates at high ionic strengths [11–13]. To facilitate its PRB applications, thus, FeS can be deposited on larger particles using coating methods [11–13]. In this study, sand was chosen as a supporting matrix for FeS coating since it is cheap, abundant in soils and aquifers, and widely utilized for treating heavy metal ions in wastewaters [15,16]. Iron oxide-coated sand has been widely utilized for the treatment of polluted waters [15,17,18], but it is only recently that FeS-coated sand has drawn attentions [10–12,16]. This study aims to investigate the feasibility of FeS-coated sand as a potential reagent in PRBs for chromate remediation under anoxic conditions. To this end, FeS-coated sand was assessed for its ability to remove Cr(VI) under varying pH conditions. Also, the reactivity of coated sand was compared to that of uncoated sand to assess the role of FeS deposits on sand. Furthermore, both Cr K-edge and Fe K-edge X-ray absorption near-edge structure (XANES) analyses were performed to determine the oxidation state of Cr and the composition of Fe phases, respectively, in the solid.
2.3. Cr sorption by FeS-coated and uncoated sands To assess the reactivity of FeS deposits toward Cr(VI) removal, a series of batch experiments were performed using either FeS-coated sand or uncoated sand inside the glove box. First, one gram of coated or uncoated sand was weighed into a 15 mL centrifuge tube. Then, 2 mL of 0.5 M acetate, morpholinopropanesulfonate (MOPS), and N,N,N',N'tetraethylethlenediamine (TEEN) buffer solutions were used to maintain the designated pH at 4.7( ± 0.1), 7.1( ± 0.1), and 9.8( ± 0.1), respectively. These buffers were chosen based on their weak complexation with metals [20,21] and redox inertness [22–24]. Subsequently, Cr(VI) stock solutions, NaCl solutions, and deoxygenated water were added to the tubes to produce 10 mL of the bulk solutions having the initial Cr(VI) concentration at 0.02–0.1 mM, the buffer concentration at 0.1 M, and the ionic strength at 0.2 M. Finally, the resultant batches were allowed to react on a shaker for 3 d. This was sufficient to reach the equilibrium in Cr(VI) removal by FeS [3]. Following the reaction period, aliquots of the suspensions were filtered through 0.20 μm nylon filters. The filtrates were then acidified using 10% HNO3 solutions. A portion of the acidified solutions were measured for the dissolved Cr(VI) and Fe(II) concentrations via spectrophotometric methods. While Cr(VI) was analyzed at 540 nm using 1,5-diphenylcarbazide as a developing reagent [25], Fe(II) was analyzed at 508 nm using a 1,10-phenanthroline method [19]. Also, the other portion of solutions were analyzed for the total dissolved Cr and Fe on an inductively coupled plasma optical emission spectroscopy (ICP-OES; HORIBA Jobin Yvon ULTIMA 2C). The remaining suspensions were centrifuged at 8000 rpm for 10 min to have the solid-phase products. Finally, the collected slurries were air-dried inside the glove box and tightly sealed till XANES analyses. In addition, Cr(III) sorption experiments using uncoated sand were performed to compare to Cr(VI) sorption by coated sand in efforts to examine the role of FeS deposits in immobilizing Cr(VI).
2. Experimental 2.1. FeS synthesis and sand preparation FeS was synthesized inside a glove box according to Jeong et al. [14]. First, FeS precipitates were produced by mixing 2.0 L of 0.6 M FeCl2 solution with 1.2 L of 1.1 M Na2S solution for 3 d. Subsequently, the precipitates were collected by decanting the supernatant after centrifuging at 10,000 rpm for 15 min. To remove the residual salts, they were rinsed at least 5 times by adding the deoxygenated, deionized water and then centrifuging. After freeze-dried, the resultant powder was stored in 200 mL glass vials and sealed with Teflon-coated rubber septa to reduce the potential of hydration and oxidation. When brought outside the glove box for centrifuging and freeze-drying, samples were placed inside polycarbonate centrifuge bottles with sealing closure (Thermo Scientific) to minimize air exposure. The produced FeS was nanoparticulate mackinawite (see Fig. S1 in Supporting material for its diffraction pattern), with the specific surface area of 276–345 m2/g [14]. A beach sand (Hama Industry, Korea), the supporting matrix used for coating, was sieved to obtain a fraction of 106–150 μm and then repeatedly rinsed with deionized water. Since no intensive treatment was applied, the resultant sand, mainly composed of quartz, contained mineral impurities such as Fe (oxyhydr)oxides [12]. By BET analysis, its specific surface area was 1.13( ± 0.01) m2/g. Prior to use, sand particles were let sit for at least one day inside the glove box where oxygen concentration was kept below 1 ppm.
2.4. XANES analyses Cr K-edge and Fe K-edge XANES spectra were collected at beamlines 8C and 1D, respectively, at the Pohang accelerator laboratory (PAL) using an unfocused beam with a Si(111) double-crystal monochromator. The storage ring was operated with ∼150 mA at 2.5–3.0 GeV. The incoming beam was detuned by ∼40% to reject higher-order harmonic components. While a 7-elements Ge array detector was used to collect fluorescence signals, high precision ionization chambers (IC-SPEC) were employed for transmission measurements. At least four scans were collected for each sample to enhance the signal-tonoise ratios in XANES spectra. Energy calibration was internally made for each scan using the transmission spectra of a Cr foil (K edge at 5989 eV) or a Fe foil (K edge at 7112 eV) placed between I1 and I2 chambers. During the data collection, all samples were placed in a Hepurged box to minimize beam-catalyzed redox transformation. Due to the poor data quality, Cr K-edge XANES analysis was limited to Cr(VI) sorption samples having the higher Cr(VI) initial concentration (5 mM) than those used in batch experiments (0.02–0.1 mM). Also, the spectra were collected for Cr model compounds including Cr(0), Cr2O3, Cr(OH)3, [Crx, Fe1-x](OH)3, and K2CrO4. Among them, two hydroxides were synthesized as follows: while Cr(OH)3 was obtained by raising the pH of a Cr(NO3)3 solution above 10 using NaOH solutions, [Crx, Fe1-x](OH)3 was prepared by slowly titrating 1.6 L of the solution containing both Cr(NO3)3 and Fe(NO3)3 to pH ∼7 [26]. The resultant precipitates were thoroughly rinsed, air-dried, and stored inside the glove box until XANES analyses. Also, to determine the Cr(III) to Cr(VI) ratios in Cr(VI) sorption samples, a series of K2CrO4–Cr(NO3)3·9H2O
2.2. FeS coating Inside the glove box, 10 g of the rinsed sand particles were weighed into 0.1 M NaCl solutions and then mixed with 20 mL of 2.0 g/L FeS suspensions under agitation for 48 h. To optimize the coating, the pH of the above mixtures was adjusted to range from 4.0 to 10.0 by adding 0.1 M HCl or NaOH solutions during agitation. Following this, FeScoated sand particles were collected using filter papers having the pore size of 110 μm (CHMLAB GROUP). Then, the collected particles were allowed to dry inside the glove box. To determine the amount of FeS deposits, 1.0 g of coated sand was added into 40 mL of 1.0 M HCl solutions and agitated for 48 h. After the acid extraction, aliquots of the solutions were filtered through a 0.20 μm nylon membrane filter (CHMLAB GROUP). Then, 5 mL of the filtrates were mixed with 10% HNO3 solutions for Fe(II) analysis using the Ferrozine method [19]. Since the Fe(II) liberated during the extraction would come from both FeS deposits and Fe impurities in sand, 1.0 g of uncoated sand was also subjected to the above acid-extraction to determine the amount of Fe(II) originating from such impurities. Based on the results of both extracts, the amount of FeS deposits was 18
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Fig. 2a–c, the dissolved Cr at pH 4.7 is predominantly in a trivalent state. Thus, although not completely sequestered at pH 4.7, the initially added Cr(VI) was at least reduced to a less toxic Cr(III). Previously, Cr (III) removal at acidic pH occurred to less extents due to both unfavorable cationic adsorption [29] and the increased solubility of Cr (OH)3 and/or [Crx, Fe1-x](OH)3 under such pH conditions [2,8]. In Fig. 1, the sorption isotherms were fitted into Langmuir models via nonlinear regressions:
qe =
qe,m KCe 1 + KCe
(1)
where Ce is the total dissolved Cr concentration; qe is the solid-phase Cr concentration; qe,m is the complete monolayer coverage; and K is the sorption constant. The fitting results are summarized in Table 1. For Cr (VI) sorption by coated sand at pH 4.7, qe shows a gradual increase with Ce, and it is far lower than qe,m, both suggesting the dominance of surface-mediated sorption at acidic pH. Despite the comparable experiment conditions (e.g., the initial Cr(VI) to FeS concentration ratios), Patterson and Fendorf [3] have proposed that coprecipitation as [Crx, Fe1-x](OH)3 is responsible for Cr(VI) uptake in FeS suspensions at pH 5. Such a difference may result from additional surfaces (e.g., quartz and Fe (oxyhydr)oxides in sand) available for adsorption of the once-reduced Cr(III) in this study. Also, the Fe(III) species generated during Cr (VI) reduction may adsorb to or interact with Fe (oxyhydr)oxides impurities rather than coprecipitating with Cr(III) under our experimental conditions. Cr(VI) sorption by uncoated sand is also characterized by a pH-dependent behavior (see Fig. 1b). Unlike coated sand, however, uncoated sand shows the decreasing sorption behavior with pH. Over the pH range examined, the dissolved Cr(VI) is present as anionic species (e.g., HCrO4− and CrO42-) (see Fig. S2). Consistent with our finding, the anionic Cr(VI) species tend to sorb to lesser extents with pH due to the development of more negative charges on mineral surfaces [3]. Yet, as shown in Fig. 2d–f, the initially added Cr(VI) was partially reduced to Cr(III) even without FeS deposits. According to the analysis of acid extracts, uncoated sand contained a substantial amount of Fe(II) (e.g., 0.033 mmol Fe(II) liberated per g sand). As further discussed in Section 3.4, magnetite (Fe3O4) accounted for ∼30% of the total Fe phases in uncoated sand. In fact, magnetite commonly occurs as beach placers [30]. Magnetite has been shown to be capable of reducing Cr(VI) to Cr(III) [31–33]. Notably, its ability to reduce Cr(VI) becomes kinetically limited and less extensive with pH [31,32]. Before being heterogeneously reduced, Cr(VI) should first adsorb to the magnetite surface [34,35]. Thus, lesser adsorption of anionic Cr(VI) with pH results in its decreased reduction [34,35]. Also, the formation of a passivation layer of Fe (oxyhydr)oxides (e.g., maghemite and goethite) on the magnetite surface can further retard Cr(VI) reduction at neutral to basic pH [31]. With the increasing contact time, thus, Cr(VI) is more likely to adsorb to the magnetite surface without being reduced to Cr(III) [33]. Besides, given the low solubility of magnetite at neutral to basic pH, the homogenous reduction of Cr(VI) by the dissolved Fe(II) may be not significant under such pH conditions. Indeed, the concentration ratios of Cr(III) to Cr(V) in this study decrease with pH (see Fig. 2d–f). Taken together, the observed pH-dependency in Cr(VI) removal by uncoated sand is likely due to both unfavorable anionic adsorption of Cr(VI) and its lesser reduction to Cr(III) with pH. By comparing Fig. 1a and b, Cr(VI) removal by coated sand is more extensive than that by uncoated sand, pointing to the significance of FeS deposits in immobilizing Cr(VI). Importantly, the enhanced sequestration by FeS deposits is more evident at pH 7.1 and 9.8. In contrast, Cr(VI) removal by Fe(0) and Fe(II)-bearing (oxyhydr)oxides is limited due to its lower tendency to be reduced to Cr(III) under such pH conditions [31,32,36].
Fig. 1. Isotherms of Cr(VI) sorption by FeS-coated sand (a) and uncoated sand (b) at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentrations were 0.02–0.1 mM at the loading of 100 g sand/L. Dashed lines correspond to the data fits by Langmuir isotherms.
mixtures were subjected to XANES measurements [3]. In parallel, Fe Kedge XANES spectra were collected to determine the solid-phase Fe speciation in uncoated sand, FeS-coated sand, and Cr(VI) sorption samples. The sorption samples subjected to Fe K-edge XANES analysis were prepared by reacting coated sand with 5 mM Cr(VI) solutions. The collected spectra were processed using SixPACK [27]. Individual scans were averaged and then the background absorbance was subtracted using a linear fit over the pre-edge region. Subsequently, XANE spectra were extracted by normalizing the signal to its edge-jump height. In case of Cr K-edge XANES spectra, the first derivative spectra were obtained after smoothing them to better compare the absorption edges. For Fe K-edge XANES spectra, linear combination fitting (LCF) analysis was performed with Fe model compounds at different oxidation states.
3. Results and discussion 3.1. Cr(VI) sorption by FeS-coated and uncoated sands Cr(VI) sorption experiments were performed using both FeS-coated and uncoated sands. In Fig. 1a, Cr(VI) removal by coated sand shows a pH-dependent behavior. The initially added Cr(VI) was quantitatively removed at pH 7.1 and 9.8. At these pH, the vertical isotherms, as featured by the near constant dissolved Cr concentrations, suggests the bulk-phase (co)precipitation [28]. Previously, Cr(VI) was proposed to be immobilized by forming coprecipitates as [Crx, Fe1-x](OH)3 in aqueous FeS suspensions [3]. In contrast, at pH 4.7, significant amounts of Cr remained in solution. From the solution-phase Cr speciation in 19
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Fig. 2. Dissolved Cr(VI) and Cr(III) concentrations as a function of the initial Cr(VI) concentration: Cr(VI) sorption by FeS-coated sand at pH 4.7 (a), 7.1 (b), and 9.8 (c) and Cr(VI) sorption by uncoated sand at pH 4.7 (d), 7.1 (e), and 9.8 (f). Table 1 Langmuir isotherm parameters for Cr(VI) and Cr(III) sorption by FeS-coated and uncoated sands. Parametersa
Batch
R2
Sorbate
Sorbent
pH
qe,m (mmol g−1)
K (L mmol−1)
Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cr(III) Cr(III) Cr(III)
Coated sand Coated sand Coated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand Uncoated sand
4.7 7.1 9.8 4.7 7.1 9.8 4.7 7.1 9.8
3.6( ± 1.3)×10−3 Unable to fit Unable to fit 2.5( ± 0.1) ×10−4 1.4( ± 0.0) ×10−4 5.0( ± 0.2) ×10−5 6.8( ± 2.7) ×10−4 Unable to fit Unable to fit
3.08( ± 1.25)
0.9978
31.7( ± 1.7) 56.5( ± 3.6) 56.3( ± 9.6) 7.51( ± 4.27)
0.999 0.998 0.971 0.967
a
Errors represent one standard deviation. Fig. 3. Comparison between Cr(VI) sorption by FeS-coated sand and Cr(III) sorption by uncoated sand at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentrations were 0.02–0.1 mM at the loading of 100 g sand/L. Dashed lines correspond to the data fits by Langmuir isotherms.
3.2. Cr(III) sorption by uncoated sand Fig. 3 shows Cr(III) sorption isotherms by uncoated sand. At pH 4.7, Cr(III) removal by uncoated sand was not complete as characterized by a Langmuir-type isotherm. At this pH, surface-mediated sorption would be responsible for Cr(III) removal by uncoated sand. On the other hand, at pH 7.1 and 9.8, the initially added Cr(III) was quantitatively removed from the solution, indicating the dominance of (co)precipitation at neutral to basic pH. Such a pH-dependent pattern is similar to that observed in Cr(VI) sorption by FeS-coated sand. Despite the aforementioned similarity, Cr(III) removal by uncoated sand at pH 4.7 is noticeably lower than Cr(VI) removal by coated sand, implying that the significance of FeS deposits at acidic pH. The unreacted FeS in coated sand can provide surface sites for Cr(III) adsorption. Given the point of zero-charge (PZC) of mackinawite lower than 4.0 [9], its surface becomes negatively charged over the pH range examined, making cationic Cr(III) adsorption favorable. However, as discussed in Section 3.4, FeS deposits at pH 4.7 were completely depleted due to a combination of the consumption during Cr(VI) reduction and H+-promoted dissolution. Instead, the Fe(III) species produced during FeS-mediated Cr(VI) reduction would form amorphous to poorly
crystalline precipitates, thus making additional adsorption sites available for Cr(III) adsorption. At pH 7.1 and 9.8, it is not possible to differentiate the isotherms between Cr(VI) removal by coated sand and Cr(III) removal by uncoated sand (see Fig. 3). As discussed in Section 3.4, FeS deposits in coated sand play a primary role in the reduction of Cr(VI) to Cr(III), which is then removed via (co)precipitation. Although less effective than FeS deposits, magnetite impurities in uncoated sand can partially mediate the coupled reduction-precipitation of Cr(VI) at neutral to basic pH. 3.3. Cr K-edge XANES analysis Cr K-edge XANES analysis provides the information on the oxidation state of Cr in solid products. In Fig. 4, the XANES spectra of Cr(VI) sorption samples by both coated and uncoated sands are compared to those of Cr model compounds. While Cr(VI) exhibits a strong pre-edge at 5993.8 eV due to the electron transitions from 1s to 3d orbitals 20
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Fig. 4. Cr K-edge XANES spectra (a) and their first derivatives (b) of Cr(VI) sorption samples by FeS-coated and uncoated sands at pH 4.7, 7.1, and 9.8. The initial Cr(VI) concentration was 5 mM at the loading of 100 g sand/L. In part (a), pre-edges are indicated by dashed lines. In part (b), absorption edges are indicated by dashed lines. C/S and N/S correspond to FeS-coated sand and uncoated sand, respectively. The spectra of Cr model compounds are also included.
structure (EXAFS) analysis, the similar background scattering between Cr and Fe makes it impossible to discern one from the other in Cr coordination environments [26]. Previously, a transmission electron microscopy coupled with energy dispersive spectroscopy (TEM-EDS) was applied to determine the composition of Cr(III)-bearing hydroxides in Cr(VI) sorption studies by FeS [3,9]. Yet, given the amorphous nature of Fe and Cr precipitates and the strong tendency to form aggregates under intensive vacuum [3,9], it is doubtful that their results reflect the composition of a single Cr phase rather than the averaged composition of multiple phases. Although the composition of Cr(III)-bearing hydroxides has been proposed in several studies, it has not been conclusively identified [2,3,9,39]. In this study, the presence of Fe impurities in sand further hindered the application of TEM-EDS. For Cr(VI) sorption samples by uncoated sand, their XANES spectra appear to have both pre-edges at 5990.5 and 5993.8 eV, indicating the partial reduction of Cr(VI) to Cr(III). As aforementioned, uncoated sand contained magnetite impurities, which could mediate Cr(VI) reduction. Due to the highly diffuse pre-edges in their XANES spectra, it is not possible to accurately determine the Cr(VI):Cr(III) ratios. Nonetheless, by comparing the pre-edge height, Cr(VI) fraction was the greatest at pH 4.7. This supports a favorable anionic Cr(VI) adsorption at acidic pH. Similar results were reported for Cr(VI) reduction by magnetite [31].
[5,8,26], Cr(III) is characterized by very weak pre-edges at 5990.5 and 5993.8 eV due to the transition from 1s to 3d (t2g) and 3d (eg), respectively [37]. Consistently, a strong pre-edge is present in the XANES spectrum of K2CrO4, whereas there are two weak pre-edges for Cr(III)bearing minerals. In case that Cr(VI) and Cr(III) coexist, the height of the pre-edge at 5993.8 eV in background-subtracted spectra is directly proportional to the Cr(VI) fraction [3,37]. In Fig. S3, the pre-edge height increases with Cr(VI) fraction among the mixtures between K2CrO4 and Cr2O3. The regression of the pre-edge height against Cr(VI) fraction using a second-order polynomial [37] produced a good correlation (R2 = 0.9996). Subsequently, this regression was used to determine the relative amounts between Cr(III) and Cr(VI) in solid products. In Fig. 4a, the XANES spectra of all Cr(VI) sorption samples by coated sand show the pre-edges at 5990.5 eV but not the discernible pre-edges at 5993.8 eV, both indicating the dominance of Cr(III) in the solid phase. Consistently, the determination of the Cr(VI):Cr(III) ratios using the above regression points to negligible Cr(VI) fractions in all sorption samples. The absorption edge, which is given by the maximum of the first derivative of a XANES spectrum, can be used to determine the oxidation state of Cr; the absorption edge of a more oxidized species is located at a higher energy than that of a relatively reduced one [8]. In Fig. 4b, the absorption edges of Cr(VI) sorption samples by coated sand are close to those of Cr(OH)3 and [Crx, Fe1-x](OH)3. Note that the absorption edge of Cr2O3, despite the same oxidation state as these hydroxides, is located at a lower energy due to its greater covalent nature [38]. Thus, given the XANES results as well as the solution-phase Cr speciation, nearly all of the initially added Cr(VI) was reduced to Cr(III) by coated sand. In Fig. 4, the XANES spectra of all Cr(VI) sorption sample by coated sand are similar to those of Cr(OH)3 and [Crx, Fe1-x](OH)3, suggesting these hydroxide-like coordination environments around Cr. This is not consistent with the dominance of surface-mediated sorption at pH 4.7 as discussed in Section 3.1. Due to the poor quality of XANES spectra, a much higher Cr(VI) concentration (5 mM) than those used in sorption experiments (0.02–0.1 mM) was applied to prepare XANES samples. At such a high loading, the adsorption capacity would become saturated, thus rendering (co)precipitation take place. Based on the similarity of XANES spectra, Cr(OH)3 and/or [Crx, Fe1-x](OH)3 are likely to be Cr (III)-bearing precipitates formed at pH 7.1 and 9.8. Yet, it is not possible to distinguish them due to the close resemblance in their XANES spectra (see Fig. 4). Even with Cr K-edge extended X-ray absorption fine
3.4. Fe K-edge XANES analysis Fe K-edge XANES spectra were subjected to LCF analysis to determine the solid-phase Fe composition in Cr(VI) sorption samples by coated sand as well as uncoated/coated sands (see Table 2 and Fig. S4). For uncoated sand, ∼30% of the total Fe was accounted for by magnetite, with the remainder being mainly Fe(III) (oxyhydr)oxides (e.g., maghemite, hematite, lepidocrocite, and goethite). In terms of Fe composition, coated sand consisted of ∼14% mackinawite (FeS). Notably, it contained a significant fraction of green rust, a mixed Fe(II)-Fe (III) hydroxide. During the coating step, this phase would form by reaction between FeS and Fe impurities in sand. Compared to the unreacted, coated sand, Cr(VI) sorption samples by coated sand show quite different Fe compositions (see Table 2). First, the FeS fraction decreased dramatically as a result of reaction with Cr (VI), but the magnetite fraction remained relatively constant, both indicating that FeS deposits were mainly responsible for Cr(VI) reduction in coated sand. At neutral to basic pH, FeS can reduce Cr(VI) to Cr(III) 21
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Table 2 Results of linear combination fitting (LCF) analysis for Fe K-edge XANES spectra. Fe model compounds
Uncoated sand
Coated sand
Cr(VI) at pH 4.7a
Cr(VI) at pH 7.1a
Cr(VI) at pH 9.8a
Mackinawite (FeS) Pyrite (FeS2) Greigite (Fe3S4) Green rust (FeII3FeIII(OH)8Cl) Magnetite (Fe3O4) Maghemite (γ-Fe2O3) Hydrous ferric oxide (Fe2O3·0.5H2O) Goethite (α-FeOOH) Lepidocrocite (γ-FeOOH) Hematite (α-Fe2O3) Rf b
0.0( ± 2.6)% 0.0( ± 3.6)% 0.0( ± 3.9)% 0.1( ± 2.9)% 29.7( ± 6.2)% 25.8( ± 4.4)% 6.9( ± 21.1)% 10.1( ± 12.8)% 12.9( ± 6.2)% 14.5( ± 4.8)% 0.0014
14.1( ± 2.6)% 0.0( ± 3.3)% 0.0( ± 3.8)% 12.7( ± 2.7)% 6.8( ± 5.8)% 7.4( ± 4.3)% 5.7( ± 11.7)% 7.0( ± 12.5)% 38.4( ± 6.1)% 8.0( ± 4.6)% 0.0013
0.0( ± 0.9)% 1.9( ± 1.3)% 0.0( ± 1.3)% 41.2( ± 1.0)% 7.2( ± 2.2)% 1.4( ± 2.1)% 4.0( ± 4.9)% 5.3( ± 5.2)% 22.7( ± 2.4)% 16.2( ± 1.7)% 0.0011
1.8( ± 0.9)% 0.5( ± 1.2)% 0.0( ± 1.2)% 42.4( ± 0.9)% 5.4( ± 2.1)% 1.1( ± 2.0)% 7.4( ± 4.5)% 8.5( ± 4.9)% 19.8( ± 2.3)% 13.2( ± 1.2)% 0.0010
5.6( ± 0.8)% 0.2( ± 1.2)% 0.0( ± 1.2)% 39.3( ± 0.7)% 11.0( ± 2.0)% 2.6( ± 1.9)% 7.8( ± 4.3)% 5.3( ± 4.8)% 10.8( ± 2.2)% 17.2( ± 1.4)% 0.0009
a b
All sorption samples were prepared by reacting FeS-coated sand with 5 mM Cr(VI) solutions. Rf indicates the goodness of fitting.
via a surface-mediated redox process [3]. At acidic pH, due to the H+promoted dissolution of FeS [40], the Fe(II) liberated from FeS can homogeneously catalyze Cr(VI) reduction [3]. Second, the fraction of green rust in Cr(VI) sorption samples was three times greater than that in the unreacted, coated sand. This suggests that a green rust-like phase would be the major Fe product in Cr(VI) sorption by coated sand. Under moderately reducing conditions, green rust is thermodynamically more stable than Fe(III) (oxyhydr)oxide [41]. Consistent with our finding, cronstedtite (FeII3FeIIISiO4(OH)5), another mixed Fe(II)-Fe(III) hydroxide, was produced in Cr(VI) reduction by pyrrhotite [42]. At pH 7.1 and 9.8, the dissolved Fe(III) was present at elevated concentrations, but the dissolved Fe(II) was minimal (see Fig. S5). At these pH, thus, the incorporation of Fe(III) into a green rust-like phase may have been limited by the availability of Fe(II). In contrast, at pH 4.7, given the abundance of the dissolved Fe(II), the dissolved Fe(III) was mostly immobilized by forming a green rust-like phase. Taken together, reactions (2) and (3) are proposed to represent the principal oxidation half-reactions at acidic and neutral-to-basic pH, respectively:
Fe(III)-substituted Cr(OH)3 was likely to be a Cr(III)-bearing phase. In constant to coated sand, uncoated sand showed the decreasing Cr(VI) sorption with pH. Magnetite impurities in uncoated sand could mediate the partial reduction of Cr(VI) to Cr(III). Thus, the observed pH-dependent sorption by uncoated sand was due to both unfavorable anionic adsorption of Cr(VI) and its lesser reduction to Cr(III) with pH. Compared to Cr(III) sorption by uncoated sand, Cr(V) sorption by coated sand was significantly enhanced at pH 4.7. Thus, the role of FeS deposits was not restricted to Cr(VI) reduction, but their oxidation products could also provide additional surface sites for cationic Cr(III) adsorption. In this study, FeS-coated sand shows a greater reactivity in Cr(VI) removal than uncoated sand. Of particular, the elevated reactivity is more evident at neutral to basic pH where Cr(VI) removal by Fe(0) and Fe(II)-bearing (oxyhydr)oxides is often limited. Despite the great reactivity of FeS-coated sand, additional studies are needed before its PRB applications, which may include the susceptibility of FeS-coated sand to suboxic/oxic environments, the aging effect of FeS, and the presence of other contaminants.
4Fe2+ + Cl− + 8H2O → FeII3FeIII(OH)8Cl + 8H+ + e−
Acknowledgments
(2)
6FeS + Cl− + 17H2O → FeII3FeIII(OH)8Cl + 2Fe3+ + 3S2O32- + 26H+ + 27e− (3)
This work was supported by Basic Science Research Program through the National Research Foundation of KoreaNRF2016R1D1A1B04931643 and the "R&D Project on Environmental Management of Geologic CO2 Storage" from the KEITI (Project Number: 2014001810003). Experimental works at Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Education, Science and Technology of the Korean Government and Pohang University of Science and Technology (POSTECH).
Chloride green rust is included given the use of 0.15 M NaCl as a background electrolyte. In reaction (3), thiosulfate (S2O32−) is included given it was reported to be the major oxidation product of sulfur in Cr (VI) reduction by FeS [3]. Yet, considering the formation of sulfate and other oxidized sulfurs [3,9], there are multiple alternatives to reaction (3). Upon receipt of electrons from reactions (2) and (3), Cr(VI) is reduced to Cr(III). As previously discussed, Cr(III) is immobilized mainly via adsorption at pH 4.7 and (co)precipitation at pH 7.1 and 9.8. Given the formation of a green rust-like phase as the major Fe product, Fe(III) may substitute Cr(III) in Cr(OH)3 instead of forming Fe(OH)3. Thus, if a mixed Cr(III)-Fe(III) hydroxide forms, it is likely to be Cr(OH)3-rich. Also, although Cr(III) may occupy Fe(III) sites in green rust, such occupancy is expected to be limited to be consistent with the dominance of surface-mediated sorption at pH 4.7.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.07.086. References [1] D.L. Bond, S. Fendorf, Kinetics and structural constraints of chromate reduction by green rusts, Environ. Sci. Technol. 37 (2003) 2750–2757. [2] L.E. Eary, D. Rai, Chromate removal from aqueous wastes by reduction with ferrous ion, Environ. Sci. Technol. 22 (1988) 972–977. [3] R.R. Patterson, S. Fendorf, Reduction of hexavalent chromium by amorphous iron sulfide, Environ. Sci. Technol. 31 (1997) 2039–2044. [4] O. Ajouyed, C. Hurel, M. Ammari, L.B. Allal, N. Marmier, Sorption of Cr(VI) onto natural iron and aluminum (oxy)hydroxides: effects of pH, ionic strength and initial concentration, J. Hazard. Mater. 174 (2010) 616–622. [5] R.A. Whittleston, D.I. Stewart, R.J.G. Mortimer, Z.C. Tilt, A.P. Brown, K. Geraki, I.T. Burke, Chromate reduction in Fe(II)-containing soil affected by hyperalkaline leachate from chromite ore processing residue, J. Hazard. Mater. 194 (2011) 15–23. [6] E.L. Hawley, R.A. Deeb, M.C. Kavanaugh, J.A. Jacobs, Treatment technologies for chromium (VI), in: J. Guertin, J.A. Jacobs, C.P. Avakian (Eds.), Chromium(VI) Handbook, CRC Press, Boca Raton, FL, 2004, pp. 275–309.
4. Conclusions This study demonstrates that FeS-coated sand can be used for PRB applications to treat Cr(VI) contamination under anoxic conditions. According to the solution-phase Cr speciation and Cr K-edge XANES analysis, Cr(VI) was completely reduced to Cr(III) by coated sand. At pH 4.7, significant amounts of Cr(III) was present in solution due to the unfavorable cationic adsorption. On the other hand, at pH 7.1 and 9.8, it was quantitatively immobilized via (co)precipitation. By Fe K-edge XANES analysis, a green rust-like phase was the major Fe product. Thus, 22
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M. Park et al.
iron(II), Environ. Sci. Technol. 32 (1998) 2092–2099. [25] American Public Health Association, Standard Methods for Examination of Water and Wastewater, American Public Health Association, Washington, DC, 1985. [26] C.M. Hansel, B.W. Wielinga, S. Fendorf, Structural and compositional evolution of Cr/Fe solids after indirect chromate reduction by dissimilatory iron-reducing bacteria, Geochim. Cosmochim. Acta 67 (2003) 401–412. [27] S.M. Webb, SIXPack: a graphical user interface for XAS analysis using IFEFFIT, Phys. Scr. 115 (2005) 1011–1014. [28] F.M.M. Morel, J.G. Hering, Principles and Applications of Aquatic Chemistry, Wiley-Interscience, New York, 1993. [29] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, third ed., John Wiley & Sons, New York, 1996. [30] M.N. Alam, M.I. Chowdhury, M. Kamal, S. Ghose, M.N. Islam, M.N. Mustafa, M.M.H. Miah, M.M. Ansary, The 226Ra, 232Th and 40K activities in beach sand minerals and beach soils of Cox’s Bazar, Bangladesh, J. Environ. Radioact. 46 (1999) 243–250. [31] Y.T. He, S.J. Traina, Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions: the role of passivation, Environ. Sci. Technol. 39 (2005) 4499–4504. [32] A.F. White, M.L. Peterson, Reduction of aqueous transition metal species on the surfaces of Fe(II)-containing oxides, Geochim. Cosmochim. Acta 60 (1996) 3799–3814. [33] A.H. Meena, Y. Arai, Effects of common groundwater ions on chromate removal by magnetite: importance of chromate adsorption, Geochem. Trans. 17 (2016) 1. [34] J.M. Zachara, D.C. Girvin, R.L. Schmidt, C.T. Resch, Chromate adsorption on amorphous iron oxyhydroxide in the presence of major groundwater ions, Environ. Sci. Technol. 21 (1987) 589–594. [35] B.S. Krishna, Thermodynamics of chromium(VI) anionic species sorption onto surfactant-modified montmorillonite clay, J. Colloid Interface Sci. 229 (2000) 230–236. [36] M.J. Alowitz, M.M. Scherer, Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal, Environ. Sci. Technol. 36 (2002) 299–306. [37] M.L. Peterson, G.E. Brown Jr., G.A. Parks, C.L. Stein, Differential redox and sorption of Cr(III/VI) on natural silicate and oxide minerals: EXAFS and XANES results, Geochim. Cosmochim. Acta 61 (1997) 3399–3412. [38] H.Y. Jeong, D.-C. Koh, K.-S. Lee, H.H. Lee, Characterization of thermally treated Co2+-exchanged zeolite X, Appl. Catal. B-Environ. 127 (2012) 68–76. [39] A.I. Zouboulis, K.A. Kydros, K.A. Matis, Removal of hexavalent chromium anions from solutions by pyrite fines, Water Res. 29 (1995) 1755–1760. [40] D. Rickard, The solubility of FeS, Geochim. Cosmochim. Acta 70 (2006) 5779–578. [41] J.M.R. Génin, G. Bourrié, F. Trolard, M. Abdelmoula, A. Jaffrezic, P. Refait, V. Maitre, B. Humbert, A. Herbillon, Thermodynamic equilibria in aqueous suspensions of synthetic and natural Fe(II)-Fe(III) green rusts: occurrences of the mineral in hydromorphic soils, Environ. Sci. Technol. 32 (1998) 1058–1068. [42] S.A. Baig, Q. Wang, Z. Wang, J. Zhu, Z. Lou, T. Sheng, X. Xu, Hexavalent chromium removal from solutions: surface efficacy and characterizations of three iron containing minerals, Clean Soil Air Water 42 (2014) 1409–1414.
[7] S.W. Jeen, D.W. Blowes, R.W. Gillham, Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions, J. Contam. Hydrol. 95 (2008) 76–91. [8] R.T. Wilkin, C. Su, R.G. Ford, C.J. Paul, Chromium-removal processes during groundwater remediation by a zerovalent iron permeable reactive barrier, Environ. Sci. Technol. 39 (2005) 4599–4605. [9] M. Mullet, S. Boursiquot, J.-J. Ehrhardt, Removal of hexavalent chromium from solutions by mackinawite, tetragonal FeS, Colloids Surf. A Physicochem. Eng. Asp. 244 (2004) 77–85. [10] A.D. Henderson, A.H. Demond, Permeability of iron sulfide (FeS)-based materials for groundwater remediation, Water Res. 47 (2013) 1267–1276. [11] Y.S. Han, T.J. Gallegos, A.H. Demond, K.F. Hayes, FeS-coated sand for removal of arsenic(III) under anaerobic conditions in permeable reactive barriers, Water Res. 45 (2011) 593–604. [12] S. Lee, J.C. Kang, M. Park, K. Yang, H.Y. Jeong, Sorption of arsenite using nanosized mackinawite (FeS)-coated silica sand, J. Miner. Soc. Korea 25 (2012) 185–195. [13] S. Lee, J.C. Kang, M. Park, K. Yang, H.Y. Jeong, Removal of arsenite by nanocrystalline mackinawite (FeS)-coated alumina, J. Miner. Soc. Korea 26 (2013) 101–110. [14] H.Y. Jeong, J.H. Lee, K.F. Hayes, Characterization of synthetic nanocrystalline mackinawite: crystal structure, particle size, and specific surface area, Geochim. Cosmochim. Acta 72 (2008) 493–505. [15] B. Rusch, K. Hanna, B. Humbert, Coating of quartz silica with iron oxides: characterization and surface reactivity of iron coating phases, Colloids Surf. A Physicochem. Eng. Asp. 353 (2010) 172–180. [16] Y.S. Han, H.Y. Jeong, A.H. Demond, K.F. Hayes, X-ray absorption and photoelectron spectroscopic study of the association of As(III) with nanoparticulate FeS and FeScoated sand, Water Res. 45 (2011) 5727–5735. [17] E. Tanis, K. Hanna, E. Emmanuel, Experimental and modeling studies of sorption of tetracycline onto iron oxides-coated quartz, Colloids Surf. A Physicochem. Eng. Asp. 327 (2008) 57–63. [18] J.-S. Yang, M.J. Kwon, Y.-T. Park, J. Choi, Adsorption of arsenic from aqueous solutions by iron oxide coated sand fabricated with acid mine drainage, Separ. Sci. Technol. 50 (2015) 267–275. [19] E.J.P. Phillips, D.R. Lovely, Determination of Fe(III) and Fe(II) in oxalate extracts of sediments, Soil Sci. Soc. Am. J. 51 (1987) 938–941. [20] Q. Yu, A. Kandegedara, Y. Xu, D.B. Rorabacher, Avoiding interferences from Good’s buffers: a contiguous series of noncomplexing tertiary amine buffers covering the entire range of pH 3–11, Anal. Biochem. 253 (1997) 50–56. [21] I.J. Buerge, S.J. Hug, Kinetics and pH dependence of chromium(VI) reduction by Fe (II), Environ. Sci. Technol. 31 (1997) 1426–1432. [22] J.K. Grady, Chasteen N.D, D.C. Harris, Radicals from "Good’s" buffers, Anal. Biochem. 173 (1988) 111–115. [23] M. Kirsch, E.E. Lomonosova, H.-G. Korth, R. Sustmann, H. de Groot, Hydrogen peroxide formation by reaction of peroxynitrite with HEPES and related tertiary amines. Implications for a general mechanism, J. Biol. Chem. 273 (1998) 12716–12724. [24] I.J. Buerge, S.J. Hug, Influence of organic ligands on chromium(VI) reduction by
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