Journal of Hazardous Materials 285 (2015) 140–147
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Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis Mayumi Hori ∗ , Katsumi Shozugawa, Motoyuki Matsuo Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan
h i g h l i g h t s • A detailed study of Cr(VI) reduction process by Fe(II), humic acid, their combination, and soil was performed using quick X-ray absorption fine structure • • • •
(QXAFS) spectroscopy. Fe(II) and humic acid contribute to Cr(VI) reduction differently from each other and from their combined effect. Humic acid can reduce Fe(III) back to Fe(II), and the reproduced Fe(II) reduces Cr(VI). Humic acid becomes a promoter of Cr(VI) reduction when both Fe(II) and humic acid are applied. Fe(II) is effective for the remediation of soil polluted by Cr(VI).
a r t i c l e
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Article history: Received 14 August 2014 Received in revised form 21 November 2014 Accepted 27 November 2014 Available online 5 December 2014 Keywords: Hexavalent chromium Reduction QXAFS Fe(II) Humic acid
a b s t r a c t The initial reduction behavior of Cr(VI) to Cr(III) has not been clearly understood due to its rapid reduction reaction. In order to study the reduction process of Cr(VI) in detail, we applied quick X-ray absorption fine structure (QXAFS) analysis to observe how Cr(VI) was reduced to Cr(III) by Fe(II) and humic acid (HA) with time. The Cr(VI) concentration was analyzed every 60 s, and the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) showed a linear relation, whereas reduction by HA showed a nonlinear relation. With combined Fe(II) and HA, the linearity was unlike those of Fe(II) and HA individually. The reduction rate was not constant. The structure of Fe(II) produced by HA during the Cr(VI) reduction was investigated by using Mössbauer spectroscopy, which showed that Fe(II) produced by HA reduction of Fe(III) had the same structure as the initial Fe(II). HA can reduce Fe(III) back to Fe(II), and reproduced Fe(II) reduces Cr(VI). For Cr(VI) reduction by combined Fe(II) and HA, each reductant contribute differently: Fe(II) directly contributes to the Cr(VI) reaction, whereas HA reduces both Cr(VI) and Fe(III). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Chromium is generally present in the trivalent (III) and hexavalent (VI) oxidation states in the environment [1]. Hexavalent chromium (Cr(VI)) is more toxic to humans and has been designated as a carcinogen [2]. Most Cr(VI) compounds are anthropogenic, released into the environment as a result of a broad range of industrial activities and various manufacturing processes. Therefore, environmental pollution caused by Cr(VI) in the soil, groundwater, waste products, and landfills is of significant concern. Cr(VI) has high mobility because of its presence in oxyanions formed in solution: CrO4 2− , HCrO4 − , Cr2 O7 2− . In contrast,
∗ Corresponding author. Tel.: +81 3 5454 6566; fax: +81 3 5454 6566. E-mail address: [email protected] (M. Hori). http://dx.doi.org/10.1016/j.jhazmat.2014.11.047 0304-3894/© 2014 Elsevier B.V. All rights reserved.
Cr(III) rarely migrates in the environment. It rapidly precipitates as Cr(OH)3 or Fex Cr1−x (OH)3 [1,3–7]. Cr(III) in the environment is mainly oxidized by Mn(IV) [1,8], and Cr(VI) is reduced by Fe(II), organic matter (e.g., humic and fulvic acids), and sulfides [9–13]. Cr has a high redox potential. The redox potential of Cr2 O7 2− /Cr3+ is +1.33 V; therefore, Cr(VI) is a strong oxidant. Consequently, it reacts rapidly with numerous reducing agents found commonly in the environment [1]. In the aqueous system such as groundwater or waste water, the effectiveness of Fe(II) and organic matter in Cr(VI) reduction has been well-established [10,14,15]. In addition, this effectiveness to soil is well-known [16–18]. It was reported that the reduction of Cr(VI) to Cr(III) starts immediately when Cr(VI) is released into soil without any additional reducing agents. [1,3,18–20]. However, the initial behavior of Cr(VI) released in soil is not well understood because the Cr(VI) species has high mobility in subsurface soil and its reduction reaction is rapid. Therefore, it
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Table 1 Characteristics of the soil. Soil type
Brown forest soil
Organic matter Humic acid (mg/g)
Fulvic acid (mg/g)
20.8
4.07
is important to trace the reduction reaction of Cr(VI) immediately after its release. In order to observe the process for the rapid reduction of Cr(VI) in detail, high time resolved analytical techniques are required. Despite the importance of this reaction, the time resolution of traditional techniques such as batch or stirred-flow methods was inadequate to investigate this rapid reaction process at the solid–liquid interface. Synchrotron-based X-ray absorption fine structure (XAFS) analysis is an attractive technique for the speciation and quantitative study of Cr [20–29]. XAFS is a useful means of nondestructive analysis that requires no sample pretreatment before measurement, and its simplicity makes it ideal for environmental samples. In particular, Cr(VI) exhibits a distinct pre-edge peak at approximately 5992 eV in the Cr K-edge X-ray absorption near edge structure (XANES) spectrum. This peak is characteristic of Cr(VI) and its intensity/area is directly proportional to the concentration of Cr(VI) in the sample [20]. Therefore, XAFS is generally an effective analytical technique for the speciation of Cr. Moreover, in this study, the quick-XAFS (QXAFS) mode was applied for consecutive high time resolved observations of the reduction reaction immediately after Cr(VI) release. In the QXAFS mode, the quick measurements from several seconds to minutes become possible because of the continuous movement of the monochromator [30–32]. Therefore, we can observe the rapid reduction reaction by analyzing the Cr K-edge XANES spectra. QXAFS essentially provides in situ analysis. In this study, we investigated the initial rapid reduction process of Cr(VI) in detail at high time resolution using QXAFS which has not been performed. The valence of Cr was measured continuously every 60 s for several hours during the Cr(VI) reduction reaction. The purpose of this study is to clarify the reduction process of Cr(VI) in soil. Due to the complexity of soils, a simplified reaction system to evaluate the soil-based reduction is necessary. Therefore, in this paper, we investigated the detailed reduction processes of Cr(VI) by Fe(II) and humic acid (HA), both of which strongly contribute to Cr(VI) reduction in soil. A previous study reported that Cr(VI) reduction may be enhanced by using a combination of Fe(II) and sodium dithionite [33]. However, our attention is paid to evaluate the reduction behavior without any additional reducing agents. Both Fe(II) and HA are found naturally in soil, while sodium dithionite is not in the natural environment. Therefore, in order to study reduction processes in the natural environment, we investigated that how reducing agents contribute to Cr(VI) reduction reaction when both Fe(II) and HA are applied as a simple system, and also studied the reduction of Cr(VI) in natural soil.
Total-Cr (mg/kg)
Total-Fe (%)
Total-Mn (mg/kg)
15.7
6.45
882
2. Experimental 2.1. Materials HA was obtained from the International Humic Substances Society collection (1S102H). All chemicals were reagent grade and all solutions were prepared with ultrapure water (18.4 M) from Milli-Q water system (Millipore, Co.). Soil sample was collected from Cr(VI) non-polluted area at Yakushima, Kagoshima in Japan. The soil types are categorized brown forest soil and alkaline soil, respectively. A surface soil sample (0–10 cm depth) was collected using a plastic spatula. In the laboratory, the soil was freeze-dried, and passed through a 150 m sieve. The sieved soil stored at room temperature in plastic vials until further analysis. Before the Cr(VI) reduction reaction study, the concentrations of total-Cr, Fe and Mn in the soil samples were measured by instrumental neutron activation analysis (INAA), as described by Shozugawa et al. [34] and Moromachi et al. [35]. The organic matter contents of the soil sample were determined by total organic carbon (TOC) analysis. The characteristics of the investigated soil sample are shown in Table 1. We confirmed that Cr(VI) was not present in the soil by XANES measurement before spiking Cr(VI) in this study. 2.2. Sample preparation The Cr(VI) reduction reaction was observed using Fe(II), HA, combined Fe(II) and HA, and soil pellets. Approximately 200 mg of each sample was molded into 10 mm diameter, 1.8 mm thick pellets. The Fe(II) pellet was prepared from 5% FeSO4 ·7H2 O in SiO2 , giving the initial reactant concentration of 10 mg Fe(II)/g pellet. The HA pellet was prepared by mixing HA in SiO2 to contain 10 mg HA/g pellet. The combined Fe(II) and HA pellet was prepared by spiking 25 L of 10 g/L HA solution onto a Fe(II) pellet. The HA solution was prepared by dissolution of HA powder in a minimal amount of 0.1 M NaOH and dilution with ultrapure water, followed by pH adjustment to 6–7 with HNO3 . The Cr(VI) spiking solution was prepared by dissolving K2 Cr2 O7 in ultrapure water to give concentration of 1% as Cr(VI) and pH value of 3.8. Each pellet sample was spiked with 25 L of Cr(VI) spiking solution and the valence of Cr in each sample was continuously measured by QXAFS. When Cr(VI) solution was spiked to pellet sample, Cr(VI) solution was uniformly absorbed immediately in the solid pellet. For the experiments of Fe(II) and combined Fe(II) and HA, the reduction reaction was carried out under Fe(II):Cr(VI) = 7:1 reaction stoichiometry.
Fig. 1. Schematic figure of the sample preparation.
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2.3. QXAFS measurement and analysis Schematic figure of the sample preparation and QXAFS experiment are shown in Fig. 1. The pellet sample after spiking Cr(VI) solution was packed in polyethylene bag, and then QXAFS measurements were started. QXAFS measurements were conducted at BL-9C, Photon Factory at High Energy Accelerator Research Organization (KEK), Japan. Cr K-edge XANES spectra were measured in fluorescence mode using a Lytle type detector equipped with a double crystal monochromator of Si(111). Less than 2 min were required to spike the Cr(VI) solution, attach the sample to the sample holder, and start the measurement. For several hours, the monochromator was continuously moved from 5953 to 6058 eV in 60 s cycles. The X-ray beam size (1 mm × 2 mm) was much smaller than the pellet sample size. The filter of the Lytle detector used a 3 m vanadium foil and the beam was detuned at 60% from its maximum I0 value. The measurements were conducted at room temperature. It is reasonable to suppose neither oxidation of sample by an inflow of oxygen in air nor a change in the quantity of oxygen occur within QXAFS measurement time, because the polyethylene bag was sealed up tightly. The spectra were treated by the REX2000 program (Rigaku Corporation). The pre-edge peak was approximated by a linear combination of two functions: Gaussian and arctangent. The peak area was determined by the area of the Gaussian from 5979 to 5995 eV. The area of the pre-edge peak of Cr(VI) observed at approximately 5992 eV is strongly correlated with the concentration of Cr(VI) in the sample. The areas of pre-edge peaks and the ratios of Cr(VI)/total-Cr in standard samples were strongly correlated (R2 = 0.9999). The standards were prepared by mixing solid Cr2 O3 (Cr(III)) and K2 Cr2 O7 (Cr(VI)) at various Cr(III)/Cr(VI) ratios (100/0, 75/25, 50/50, 25/75, 0/100). These were mixed with SiO2 to contain 1% (w/w) total-Cr. The ratio of Cr(VI)/total-Cr in each sample was calculated based on the calibration curve of the area of the pre-edge peak observed from standard samples [20]. 2.4.
57 Fe
Mössbauer spectrometry
The structural changes of Fe during its reduction of Cr(VI) were examined using 57 Fe Mössbauer spectrometry. A sample was prepared by mixing FeSO4 ·7H2 O and K2 Cr2 O7 to 10:1 Fe(II):Cr(VI) reaction stoichiometry molar ratio. The sample was spiked with H2 O (40 L) and mixed to initiate the reaction, at which time the Mössbauer measurement started. When the ratio of Fe(II)/Fe(III) remained unchanged, 40 L of 10 g/L HA solution was spiked into the sample and the Mössbauer spectrum was obtained again. The Mössbauer spectra were measured with a TOPOLOGIC SYSTEMS MFD-110D spectrometer using a 1.11 GBq 57 Co/Rh source at room temperature. The Doppler velocity was set to ±10.0 mm/s maximum. Gamma rays at 14.4 keV were collected from each sample for several hours. Curves were fitted to the resulting spectra using a personal computer, assuming that the spectra were composed of peaks with Lorentzian line shapes. The half-widths and peak areas of each quadrupole doublet were constrained to be equal. Isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil.
Fig. 2. Time resolved Cr K-edge XANES spectra for the combined Fe(II) and HA sample. Each spectrum was collected every 60 s for 138 min during the reduction reaction. Intensity of the pre-edge peak at 5992.6 eV decreased with time.
resolved Cr K-edge XANES spectra during the reduction reaction of Cr(VI) by combined Fe(II) and HA for 138 min. For all samples, the intensity/area of the pre-edge peak at 5992.6 eV, which is characteristic of Cr(VI), decreased with elapsed time as shown in Fig. 2, indicating that Cr(VI) was reduced to Cr(III). The ratio of Cr(VI)/total-Cr in the samples was calculated based on the area of the pre-edge peak. The changes in the Cr(VI)/total-Cr ratio with time for each reductant are shown in Fig. 3. The reduction reaction of Cr(VI) was started just after Cr(VI) was spiked into samples.
3. Results and discussion 3.1. Reduction behavior of Cr(VI) After spiking Cr(VI) solution in each pellet sample, we observed how Cr(VI) was reduced to Cr(III) with time by using QXAFS at a time resolution of 60 s per spectrum. We measured continuously until the reaction completed. As an example, Fig. 2 shows the time
Fig. 3. Changes in the Cr(VI)/total-Cr ratio with time during the reduction reaction of Cr(VI) by each reductant (: Fe(II), : HA, : Fe(II) and HA, and ♦: soil).
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Fig. 4. Plots of ln relative concentration (Cr(VI)/Cr(VI)0 ) versus time for the reduction of Cr(VI) in various systems. Reduction by (a) Fe(II); (b) HA;(c) Fe(II) and HA; and (d) soil. [Cr(VI)]0 refers to apparent the initial ratio of Cr(VI) (100%).
Of the total Cr(VI), 86.5% was reduced to Cr(III) within 127 min by Fe(II). The reduction by HA was 68.2% within 95 min, combined Fe(II) and HA was 97.2% within 138 min, and the soil was 86.4% within 123 min. The decrease in Cr(VI) percentage in each reductant within 95 min was following order: combined Fe(II) and HA (84.8%) > Fe(II) (79.3%) > HA (68.2%). These results indicated that the combined Fe(II) and HA is the most effective reducing agent in this system. In order to evaluate the decrease of Cr(VI) in detail, the natural logarithm (ln) of the relative concentration of Cr(VI) (Cr(VI)/Cr(VI)0 ) was calculated based on the ratio of Cr(VI)/total-Cr shown in Fig. 3. The initial ratio of Cr(VI) (Cr(VI)0 ) of each sample was 100%. Fig. 4 shows the plots of ln(Cr(VI)/Cr(VI)0 ) versus time for all samples. In the previous work using XAFS, approximately 10 min was required to collect the Cr XANES spectrum due to measurements using a step scan for the full energy range of the spectrum [20]. In this study, on the other hand, we applied QXAFS mode at a time resolution of 60 s per spectrum and were able to observe rapid initial oxidation state changes in consecutive time scale. Since the many data points were obtained during the reduction reaction of Cr(VI) using QXAFS, we could observe how Cr(VI) was reduced to Cr(III) in detail and discern an inflection point. In the Fe(II) experiment, the shapes of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were linear (Fig. 4a). The rate constant (k) obtained by the least-squares fitting of the data was 1.58 × 10−2 min−1 (R2 = 0.9927). It has been reported that the
Cr(VI) reduction kinetics displayed a linear trend, which shows a first-order reaction with respect to Fe(II) and Cr(VI) [9,36]. When Fe(II) contributes to Cr(VI) reduction, the reduction kinetics were linear. In the HA experiment, in contrast, the shapes of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were nonlinear (Fig. 4b). The decrease of the concentration of Cr(VI) was rapid during the first 20 min of the reaction, and then decreased gradually with time. During the initial period (20 min), 40% of the Cr(VI) was reduced; thereafter, 28% of the Cr(VI) was reduced within approximately 75 min. The results suggested that the reduction of Cr(VI) slowed after 20 min. Wittbrodt and Palmer reported that the reduction exhibited a nonlinear decrease with time for various concentrations of HA, and plots of ln(Cr(VI)/Cr(VI)0 ) versus time showed an initial rapid decrease [37]. The results in our study, which were obtained at a high time resolution (60 s), agreed with this report. It is considered that the reduction kinetics by HA were nonlinear, in which the initial decrease of concentration was relatively rapid and slowed with time. In the reduction by combined Fe(II) and HA, the decrease in Cr(VI) concentration was unlike that with Fe(II) or HA alone (Fig. 4c). The reduction rate was not constant. The reduction of Cr(VI) was especially rapid during the first 2 min of the reaction, in which 13.7% of the Cr(VI) was reduced. The rate then gradually decreased after 3 min, but became rapid again after approximately 95 min. After 3 min of the reaction, the shapes of the plots showed linear relations, though with different rate constants.
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doublet is shown by the blue dashed line. As the reaction time progressed, the absorption intensity of the high spin Fe(III) quadrupole doublet increased, but this peak disappeared when HA was spiked. The relative area ratios of each component of Fe(II) and Fe(III) were estimated, assuming that the peak area of a given component is proportional to the amount of Fe present. 29% of Fe(III) was produced 4 h after Cr(VI) and Fe(II) were mixed. The ratio of Fe(III)/total-Fe increased with elapsed time, to 43% after 40 h. Because the ratio of Fe(II)/Fe(III) did not change after 23 h of observation, we determined 40 h as the reaction end point. Then, HA solution was added to the system, and the Fe(III) peak disappeared within 32 h after spiking. This indicated that all of the Fe(III) was reduced to Fe(II) by HA. It is reported that the Cr(VI) reduction rate increases when Fe(III) is added to the Cr(VI)-humic substances (HA or Fulvic acid) reaction system in various Fe(III) concentrations [10,38,39]. Moreover, Fe(III) exists as a cation, whereas Cr(VI) exists as an anion, so it is considered that Fe(III) has a higher affinity for the HA (anion) than does the Cr(VI). Once Fe(III) is produced during the Cr(VI) reduction reaction, HA begins to reduce it in preference to Cr(VI). Thus, HA can reduce Fe(III) to Fe(II), and reproduced Fe(II) can reduce Cr(VI) in turn. Furthermore, the Mössbauer parameters of the Fe(II) did not change from the original Fe(II) (FeSO4 ·7H2 O), as shown in Table 2. The results indicate that not only Fe(III) produced by the reduction of Cr(VI) was reduced to Fe(II) by HA, but also the reproduced Fe(II) had the same structure as the original Fe(II) (Fig. 5, Table 2). Therefore, HA can reduce Fe(III) back to Fe(II) with the same structure as the starting material, and reproduced Fe(II) is available to reduce Cr(VI). 3.3. Reduction process of Cr(VI) by combined Fe(II) and HA
Fig. 5. Mössbauer spectra of the redox reaction of Fe(II) by Cr(VI) and HA. Reduction of Cr(VI) by Fe(II) was carried out, and the oxidation state change of Fe was observed. Then, HA was spiked at the end of the reduction reaction. Black solid lines were fitted to the raw data (dotted). Red and blue dashed lines indicate the Fe(II) and Fe(III) components, respectively. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
The slopes of the plots (rate constant) between 3 and 94 min was 1.75 × 10−2 min−1 (R2 = 0.9980), and the slope of the reaction after 95 min was 3.48 × 10−2 min−1 (R2 = 0.9821), which was twice that of the reaction between 3 and 94 min. It is assumed that the reduction rate accelerated because the ratio of reductant to the residual Cr(VI) increased during the Cr(VI) reduction reaction. 3.2. Role of HA in the reduction of Cr(VI) in the presence of combined Fe(II) and HA The reduction reaction of Fe(III) to Fe(II) by HA during the Cr(VI) reduction reaction has been observed in previous works [10,38,39]. However, the structure of produced Fe(II) by HA is not clear at present. In order to confirm the structure of Fe(II) produced by HA during the Cr(VI) reduction, the oxidation state changes of the Fe components by HA were examined using 57 Fe Mössbauer spectroscopy. The reduction of Cr(VI) was carried out under a stoichiometry of 10:1 Fe(II):Cr(VI), and HA was spiked at the end of the reaction. The Mössbauer spectrometer used in this experiment required at least 4 h to obtain an analyzable spectrum. The results of the structure change of Fe in the Mössbauer spectra are shown in Fig. 5. Two doublet peaks were observed. A high spin Fe(II) quadrupole doublet, indicated by the red dashed line, was assigned to Fe(II) of FeSO4 ·7H2 O, and a high spin Fe(III) quadrupole
In order to clarify how Cr(VI) is reduced to Cr(III) by combined Fe(II) and HA, the role of each reductant was evaluated based on the shape of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time. The plots of ln(Cr(VI)/Cr(VI)0 ) versus time for the Fe(II) experiment exhibited linear relations, whereas the HA experiment exhibited nonlinear relations. These results suggest that the shape of each plot is dependent on which reductant was contributing to the reduction; the results have therefore been used to evaluate the role of each reductant. As it can be seen from the plots of ln(Cr(VI)/Cr(VI)0 ) versus time in the combined Fe(II) and HA experiment (Fig. 4c), the shapes of the plots were linear after 3 min. The reduction reaction was classified into three phases based on the different reduction rate for convenience: phase 1 is the initial rapid decrease (t < 3 min); phase 2 is the gradual decrease (3 min < t < 94 min); and phase 3 is a second rapid decrease (95 min < t < 138 min) (Fig. 6). In the first 2 min of the reaction (phase 1), where the shape of the plots showed nonlinear trends, Cr(VI) was rapidly reduced by both Fe(II) and HA due to the characteristics of the initial rapid reduction reaction of the individual Fe(II) and HA experiments. The linear plots of ln(Cr(VI)/Cr(VI)0 ) versus time were obtained after 3 min of the reaction (phases 2 and 3), suggesting that Fe(II) directly contributes to the reduction reaction. The slope of the plots between 3 and 94 min was 1.75 × 10−2 min−1 (R2 = 0.9980). This value was similar to that from the individual Fe(II) reduction experiment (1.58 × 10−2 min−1 ), which also had a linear relation. Therefore, it is considered that Cr(VI) is mainly reduced by Fe(II) in this phase. The role of HA, on the other hand, is mainly to reduce Fe(III) oxidized by Cr(VI) to Fe(II) because HA can reduce Fe(III) to Fe(II). As the reaction proceeded, the ratio of Fe(II) to residual Cr(VI) increased because of the reproduced Fe(II) by HA. For that reason, the reduction rate accelerated around 95 min into the reaction. Thus, HA strongly contributes to the reduction of Fe(III) to Fe(II) in phase2 and/or phase 3. Even if all the original Fe(II) is oxidized for the reduction of Cr(VI), the reduction of Cr(VI) can continue because produced Fe(III) is reduced to Fe(II) by HA.
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Table 2 Time-scale change of Fe species and Mössbauer parameters of spectra shown in Fig. 5. Sample
Species
Area (%)
I.S.a (mm s−1 )
Q.S.b (mm s−1 )
H.W.c (mm s−1 )
Original Fe(II) (FeSO4 ·7H2 O)
Fe(II)
100
1.25
3.23
0.33
4 h after mixture
Fe(II) Fe(III)
71 28
1.25 0.39
3.22 1.03
0.35 0.56
18 h after mixture
Fe(II) Fe(III)
64 35
1.25 0.38
3.23 1.11
0.33 0.48
23 h after mixture
Fe(II) Fe(III)
59 40
1.25 0.38
3.24 1.11
0.32 0.44
28 h after mixture
Fe(II) Fe(III)
59 40
1.25 0.38
3.24 1.11
0.33 0.40
40 h after mixture
Fe(II) Fe(III)
56 43
1.25 0.38
3.24 1.12
0.32 0.40
5 h after spiking HA solution
Fe(II) Fe(III)
77 22
1.25 0.37
3.23 1.14
0.34 0.95
32 h after spiking HA solution
Fe(II)
100
1.25
3.23
0.37
a b c
Isomer shift. Quadrupole splitting. Half width.
The reduction process of Cr(VI) by the combined Fe(II) and HA consisted of three phases because each reductant contributed differently. The first phase (t < 3 min) affected by both Fe(II) and HA was nonlinear, but linear plots of ln(Cr(VI)/Cr(VI)0 ) versus time were obtained in both phases 2 and 3, where Fe(II) was the principal reductant. When HA contributes directly to the reduction of Cr(VI), the reduction process exhibited a nonlinear relation. In the reduction by the combined Fe(II) and HA, it was found that Fe(II) and HA contributed differently to Cr(VI) reduction. Two kinds of Fe(II) contributed: as the original Fe(II) in phase 2 and the HAreproduced Fe(II) in phase 2 and/or 3. HA contributed in two ways: HA reduced both Cr(VI) and Fe(III). The contribution of Fe(II) is more pronounced than that of HA because Fe(II) directly contributes to the reduction of Cr(VI). On the other hands, HA reduces not only
Cr(VI) but also Fe(III) produced by Cr(VI), while Fe(II) directly contributes to Cr(VI) reduction. Cr(VI) reduction reaction continues even if original-Fe(II) is used up because HA reduces any oxidized Fe(III) back to Fe(II) and the Cr(VI) reduction cycle goes on, as shown in Fig. 6. Thus, HA enhances the reduction of Cr(VI). In other words, HA becomes a promoter of Cr(VI) reduction when both Fe(II) and HA are applied. 3.4. Reduction process of Cr(VI) in soil Based on reduction process by Fe(II) and HA as a simple system, the reduction process in natural soil was evaluated. The changes in the Cr(VI)/total-Cr ratio after Cr(VI) was spiked is shown in Fig. 3. Some data were missing because the measurement was interrupted
Fig. 6. Schema of the contribution of Fe(II) and HA to the reduction of Cr(VI) in each phase.
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by experimental error from 10 to 17 min and from 80 to 82 min, but the missing information does not influence the results. The reduction of Cr(VI) started immediately after the measurement began, and 86.4% of the Cr(VI) was reduced to Cr(III) within 124 min. The reduction of Cr(VI) occurred even without addition of any reductant. Thus, the soil used in this study has the ability to reduce Cr(VI). The plots of ln(Cr(VI)/Cr(VI)0 ) versus time shown in Fig. 4d show that the decrease in the concentration of Cr(VI) slowed from 90 min after the reaction started. Because the Cr(VI) percentage was 17% at 90 min, the reaction was incomplete. Therefore, it can be concluded that the reduction rate changed after approximately 90 min, and the reduction process was also nonlinear. These results were similar to the reduction process of Cr(VI) by HA (Fig. 4b), suggesting that HA contributes to the Cr(VI) reduction. Wittbrodt and Palmer reported that the reduction rate increased by increasing the concentration of HA, but the reduction process exhibited nonlinear trends [37]. Because the concentration of HA in the soil used in this study was twice as much as in the experiment with HA, the initial reduction rate was more rapid. The reduction process in soil also showed an initial rapid decrease and nonlinear trend (Fig. 4d), which was similar to that observed for the experiment with HA (Fig. 4b). This indicates that HA directly contributes to the reduction of Cr(VI), and therefore greatly contributes to the reduction of Cr(VI) in soil. The amount of Fe in the soil used in this study was significantly more than the amounts of total-Cr and Cr(VI). However, the major component of the Fe species in soil is Fe(III). Fe(II) in soil is contained in minerals, and reduction is dependent on the dissolution of Fe(II) from the minerals [40]. The Fe(II) dissolution was considered to be little because the volume of the spiked Cr(VI) solution was small (25 L). The Fe(II) concentration in this soil was 1.03% of the total-Fe, which was measured by 57 Fe Mössbauer spectroscopy. Since 57 Fe Mössbauer spectroscopy is one of gamma transmission methods, the concentration of Fe(II) in the soil sample was including inner-sphere Fe(II) in its mineral, where the reduction of Cr(VI) by Fe(II) was not concerned directly. Therefore, the amount of Fe(II) available to reduce Cr(VI) was small, and as a result, the contribution of HA to Cr(VI) reduction was enhanced. If Fe(II) contributes to the reduction of Cr(VI), the reduction rate would become rapid near the reaction’s end, as observed in the combined Fe(II) and HA experiments, because the Fe(III) would be reduced to reactive Fe(II) by HA. Moreover, it is expected that the reduction process show a linear relation. The reduction of Cr(VI) in the soil without any additional reductants was primarily driven by HA, since the reduction process of Cr(VI) in soil was similar to the HA experiment. The reduction of Cr(VI) occurs even with minimal available Fe(II) because HA is originally contained in the soil. However, we suggest that Fe(II) is effective for the remediation of the soil polluted by Cr(VI) because Fe(II) significantly contributes to the Cr(VI) reduction reaction. When Fe(II) is present, HA reduces any oxidized Fe(III) back to Fe(II) and the reproduced Fe(II) reduces Cr(VI) to Cr(III), as in a Cr(VI) reduction cycle (Fig. 6). This cycle can be applied to natural soil. In addition, adding HA may enhance the reduction of Cr(VI) because HA reduces the produced Fe(III) to usable Fe(II).
4. Conclusions The detailed process of the reduction of Cr(VI) by Fe(II) and HA was investigated. The concentration changes of Cr(VI) in the samples were analyzed continuously for 2 h using QXAFS at a time resolution of 60 s per spectrum. Plots of ln(Cr(VI)/Cr(VI)0 ) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) exhibited a linear relation, whereas that by HA exhibited a nonlinear relation. The reduction by combined Fe(II) and HA under stoichiometry of 7:1 Fe(II):Cr(VI) was
unlike the individual Fe(II) or HA experiments. This process consisted of three phases with different reduction rates, suggesting that Fe(II) and HA contribute differently to the reduction of Cr(VI). The results of Mössbauer measurements show that produced Fe(III) by Cr(VI) reduction was reduced back to Fe(II) by HA, which had the same structure as original Fe(II). The results can explain that once Fe(III) was produced, HA began to reduce it in preference to Cr(VI). The reduction rate changed because Fe(II) and HA contributed differently to the Cr(VI) reduction. In the Cr(VI)-Fe(II)-HA system, the direct contribution of Fe(II) was greater than HA because HA reduced Fe(III) in preference to Cr(VI). The reduction of Cr(VI) in soil without sterilization and any additional reductants was driven primarily by HA since the reduction process of Cr(VI) in soil was similar to the HA experiment. However, Fe(II) is effective for the remediation of soil polluted by Cr(VI) because Fe(II) directly contributes to Cr(VI) reduction. In addition, when HA contributes to Cr(VI) reduction by Fe(II), HA becomes a promoter of Cr(VI) reduction since HA has ability to reproduce usable Fe(II) from produced Fe(III) during the Cr(VI) reduction reaction. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 26.9392. This work has been performed with the approval of the Photon Factory Program Advisory Committee, KEK (Proposal No. 2011G251, 2013G563). The neutron activation analysis has been carried out at the Inter-University Laboratory for the Joint Use of JAEA Facilities. References [1] D. Rai, L.E. Eary, J.M. Zachara, Environmental chemistry of chromium, Sci. Total Environ. 86 (1989) 15–23. [2] World Health Organization, Environmental Health Criteria 61 Chromium, World Health Organization, Geneva, 1988. ˇ [3] N. Koˇzuh, J. Stupar, B. Gorenc, Reduction and oxidation processes of chromium in soils, Environ. Sci. Technol. 34 (1999) 112–119. [4] D. Rai, B.M. Sass, D.A. Moore, Chromium(III) hydrolysis constants and solubility of chromium(III) hydroxide, Inorg. Chem. 26 (1987) 345–349. [5] B.M. Sass, D. Rai, Solubility of amorphous chromium(III)–iron(III) hydroxide solid solutions, Inorg. Chem. 26 (1987) 2228–2232. [6] B.R. James, R.J. Bartlett, Behavior of chromium in soils: V. Fate of organically complexed chromium(III) added to soil, J. Environ. Qual. 12 (1983) 169–172. [7] B.R. James, R.J. Bartlett, Behavior of chromium in soils. VI. Interactions between oxidation–reduction and organic complexation, J. Environ. Qual. 12 (1983) 173–176. [8] R. Bartlett, B. James, Behavior of chromium in soils: III. Oxidation, J. Environ. Qual. 8 (1979) 31–35. [9] S.E. Fendorf, G. Li, Kinetics of chromate reduction by ferrous iron, Environ. Sci. Technol. 30 (1996) 1614–1617. [10] P.R. Wittbrodt, C.D. Palmer, Effect of temperature, ionic strength, background electrolytes, and Fe(III) on the reduction of hexavalent chromium by soil humic substances, Environ. Sci. Technol. 30 (1996) 2470–2477. [11] M. Fukushima, K. Nakayasu, S. Tanaka, H. Nakamura, Speciation analysis of chromium after reduction of chromium(VI) by humic acid, Toxicol. Environ. Chem. 62 (1997) 207–215. [12] C. Kim, Q. Zhou, B. Deng, E.C. Thornton, H. Xu, Chromium(VI) reduction by hydrogen sulfide in aqueous media: stoichiometry and kinetics, Environ. Sci. Technol. 35 (2001) 2219–2225. [13] R.J. Bartlett, J.M. Kimble, Behavior of chromium in soils: II. Hexavalent forms, J. Environ. Qual. 5 (1976) 383–386. [14] L.E. Eary, D. Rai, Chromate removal from aqueous wastes by reduction with ferrous ion, Environ. Sci. Technol. 22 (1988) 972–977. [15] W.-W. Tang, G.-M. Zeng, J.-L. Gong, J. Liang, P. Xu, C. Zhang, B.-B. Huang, Impact of humic/fulvic acid on the removal of heavy metals from aqueous solutions using nanomaterials: a review, Sci. Total Environ. 468–469 (2014) 1014–1027. [16] I.J. Buerge, S.J. Hug, Influence of organic ligands on chromium(VI) reduction by iron(II), Environ. Sci. Technol. 32 (1998) 2092–2099. [17] L.E. Eary, D. Rai, Chromate reduction by subsurface soils under acidic conditions, Soil Sci. Soc. Am. J. 55 (1991) 676–683. [18] W. Xiao, Y. Zhang, T. Li, B. Chen, H. Wang, Z. He, X. Yang, Reduction kinetics of hexavalent chromium in soils and its correlation with soil properties, J. Environ. Qual. 41 (2012) 1452–1458. [19] F.C. Richard, A.C.M. Bourg, Aqueous geochemistry of chromium: a review, Water Res. 25 (1991) 807–816.
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Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis Mayumi Hori ∗ , Katsumi Shozugawa, Motoyuki Matsuo Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan
h i g h l i g h t s • A detailed study of Cr(VI) reduction process by Fe(II), humic acid, their combination, and soil was performed using quick X-ray absorption fine structure • • • •
(QXAFS) spectroscopy. Fe(II) and humic acid contribute to Cr(VI) reduction differently from each other and from their combined effect. Humic acid can reduce Fe(III) back to Fe(II), and the reproduced Fe(II) reduces Cr(VI). Humic acid becomes a promoter of Cr(VI) reduction when both Fe(II) and humic acid are applied. Fe(II) is effective for the remediation of soil polluted by Cr(VI).
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 21 November 2014 Accepted 27 November 2014 Available online 5 December 2014 Keywords: Hexavalent chromium Reduction QXAFS Fe(II) Humic acid
a b s t r a c t The initial reduction behavior of Cr(VI) to Cr(III) has not been clearly understood due to its rapid reduction reaction. In order to study the reduction process of Cr(VI) in detail, we applied quick X-ray absorption fine structure (QXAFS) analysis to observe how Cr(VI) was reduced to Cr(III) by Fe(II) and humic acid (HA) with time. The Cr(VI) concentration was analyzed every 60 s, and the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) showed a linear relation, whereas reduction by HA showed a nonlinear relation. With combined Fe(II) and HA, the linearity was unlike those of Fe(II) and HA individually. The reduction rate was not constant. The structure of Fe(II) produced by HA during the Cr(VI) reduction was investigated by using Mössbauer spectroscopy, which showed that Fe(II) produced by HA reduction of Fe(III) had the same structure as the initial Fe(II). HA can reduce Fe(III) back to Fe(II), and reproduced Fe(II) reduces Cr(VI). For Cr(VI) reduction by combined Fe(II) and HA, each reductant contribute differently: Fe(II) directly contributes to the Cr(VI) reaction, whereas HA reduces both Cr(VI) and Fe(III). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Chromium is generally present in the trivalent (III) and hexavalent (VI) oxidation states in the environment [1]. Hexavalent chromium (Cr(VI)) is more toxic to humans and has been designated as a carcinogen [2]. Most Cr(VI) compounds are anthropogenic, released into the environment as a result of a broad range of industrial activities and various manufacturing processes. Therefore, environmental pollution caused by Cr(VI) in the soil, groundwater, waste products, and landfills is of significant concern. Cr(VI) has high mobility because of its presence in oxyanions formed in solution: CrO4 2− , HCrO4 − , Cr2 O7 2− . In contrast,
∗ Corresponding author. Tel.: +81 3 5454 6566; fax: +81 3 5454 6566. E-mail address: [email protected] (M. Hori). http://dx.doi.org/10.1016/j.jhazmat.2014.11.047 0304-3894/© 2014 Elsevier B.V. All rights reserved.
Cr(III) rarely migrates in the environment. It rapidly precipitates as Cr(OH)3 or Fex Cr1−x (OH)3 [1,3–7]. Cr(III) in the environment is mainly oxidized by Mn(IV) [1,8], and Cr(VI) is reduced by Fe(II), organic matter (e.g., humic and fulvic acids), and sulfides [9–13]. Cr has a high redox potential. The redox potential of Cr2 O7 2− /Cr3+ is +1.33 V; therefore, Cr(VI) is a strong oxidant. Consequently, it reacts rapidly with numerous reducing agents found commonly in the environment [1]. In the aqueous system such as groundwater or waste water, the effectiveness of Fe(II) and organic matter in Cr(VI) reduction has been well-established [10,14,15]. In addition, this effectiveness to soil is well-known [16–18]. It was reported that the reduction of Cr(VI) to Cr(III) starts immediately when Cr(VI) is released into soil without any additional reducing agents. [1,3,18–20]. However, the initial behavior of Cr(VI) released in soil is not well understood because the Cr(VI) species has high mobility in subsurface soil and its reduction reaction is rapid. Therefore, it
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Table 1 Characteristics of the soil. Soil type
Brown forest soil
Organic matter Humic acid (mg/g)
Fulvic acid (mg/g)
20.8
4.07
is important to trace the reduction reaction of Cr(VI) immediately after its release. In order to observe the process for the rapid reduction of Cr(VI) in detail, high time resolved analytical techniques are required. Despite the importance of this reaction, the time resolution of traditional techniques such as batch or stirred-flow methods was inadequate to investigate this rapid reaction process at the solid–liquid interface. Synchrotron-based X-ray absorption fine structure (XAFS) analysis is an attractive technique for the speciation and quantitative study of Cr [20–29]. XAFS is a useful means of nondestructive analysis that requires no sample pretreatment before measurement, and its simplicity makes it ideal for environmental samples. In particular, Cr(VI) exhibits a distinct pre-edge peak at approximately 5992 eV in the Cr K-edge X-ray absorption near edge structure (XANES) spectrum. This peak is characteristic of Cr(VI) and its intensity/area is directly proportional to the concentration of Cr(VI) in the sample [20]. Therefore, XAFS is generally an effective analytical technique for the speciation of Cr. Moreover, in this study, the quick-XAFS (QXAFS) mode was applied for consecutive high time resolved observations of the reduction reaction immediately after Cr(VI) release. In the QXAFS mode, the quick measurements from several seconds to minutes become possible because of the continuous movement of the monochromator [30–32]. Therefore, we can observe the rapid reduction reaction by analyzing the Cr K-edge XANES spectra. QXAFS essentially provides in situ analysis. In this study, we investigated the initial rapid reduction process of Cr(VI) in detail at high time resolution using QXAFS which has not been performed. The valence of Cr was measured continuously every 60 s for several hours during the Cr(VI) reduction reaction. The purpose of this study is to clarify the reduction process of Cr(VI) in soil. Due to the complexity of soils, a simplified reaction system to evaluate the soil-based reduction is necessary. Therefore, in this paper, we investigated the detailed reduction processes of Cr(VI) by Fe(II) and humic acid (HA), both of which strongly contribute to Cr(VI) reduction in soil. A previous study reported that Cr(VI) reduction may be enhanced by using a combination of Fe(II) and sodium dithionite [33]. However, our attention is paid to evaluate the reduction behavior without any additional reducing agents. Both Fe(II) and HA are found naturally in soil, while sodium dithionite is not in the natural environment. Therefore, in order to study reduction processes in the natural environment, we investigated that how reducing agents contribute to Cr(VI) reduction reaction when both Fe(II) and HA are applied as a simple system, and also studied the reduction of Cr(VI) in natural soil.
Total-Cr (mg/kg)
Total-Fe (%)
Total-Mn (mg/kg)
15.7
6.45
882
2. Experimental 2.1. Materials HA was obtained from the International Humic Substances Society collection (1S102H). All chemicals were reagent grade and all solutions were prepared with ultrapure water (18.4 M) from Milli-Q water system (Millipore, Co.). Soil sample was collected from Cr(VI) non-polluted area at Yakushima, Kagoshima in Japan. The soil types are categorized brown forest soil and alkaline soil, respectively. A surface soil sample (0–10 cm depth) was collected using a plastic spatula. In the laboratory, the soil was freeze-dried, and passed through a 150 m sieve. The sieved soil stored at room temperature in plastic vials until further analysis. Before the Cr(VI) reduction reaction study, the concentrations of total-Cr, Fe and Mn in the soil samples were measured by instrumental neutron activation analysis (INAA), as described by Shozugawa et al. [34] and Moromachi et al. [35]. The organic matter contents of the soil sample were determined by total organic carbon (TOC) analysis. The characteristics of the investigated soil sample are shown in Table 1. We confirmed that Cr(VI) was not present in the soil by XANES measurement before spiking Cr(VI) in this study. 2.2. Sample preparation The Cr(VI) reduction reaction was observed using Fe(II), HA, combined Fe(II) and HA, and soil pellets. Approximately 200 mg of each sample was molded into 10 mm diameter, 1.8 mm thick pellets. The Fe(II) pellet was prepared from 5% FeSO4 ·7H2 O in SiO2 , giving the initial reactant concentration of 10 mg Fe(II)/g pellet. The HA pellet was prepared by mixing HA in SiO2 to contain 10 mg HA/g pellet. The combined Fe(II) and HA pellet was prepared by spiking 25 L of 10 g/L HA solution onto a Fe(II) pellet. The HA solution was prepared by dissolution of HA powder in a minimal amount of 0.1 M NaOH and dilution with ultrapure water, followed by pH adjustment to 6–7 with HNO3 . The Cr(VI) spiking solution was prepared by dissolving K2 Cr2 O7 in ultrapure water to give concentration of 1% as Cr(VI) and pH value of 3.8. Each pellet sample was spiked with 25 L of Cr(VI) spiking solution and the valence of Cr in each sample was continuously measured by QXAFS. When Cr(VI) solution was spiked to pellet sample, Cr(VI) solution was uniformly absorbed immediately in the solid pellet. For the experiments of Fe(II) and combined Fe(II) and HA, the reduction reaction was carried out under Fe(II):Cr(VI) = 7:1 reaction stoichiometry.
Fig. 1. Schematic figure of the sample preparation.
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2.3. QXAFS measurement and analysis Schematic figure of the sample preparation and QXAFS experiment are shown in Fig. 1. The pellet sample after spiking Cr(VI) solution was packed in polyethylene bag, and then QXAFS measurements were started. QXAFS measurements were conducted at BL-9C, Photon Factory at High Energy Accelerator Research Organization (KEK), Japan. Cr K-edge XANES spectra were measured in fluorescence mode using a Lytle type detector equipped with a double crystal monochromator of Si(111). Less than 2 min were required to spike the Cr(VI) solution, attach the sample to the sample holder, and start the measurement. For several hours, the monochromator was continuously moved from 5953 to 6058 eV in 60 s cycles. The X-ray beam size (1 mm × 2 mm) was much smaller than the pellet sample size. The filter of the Lytle detector used a 3 m vanadium foil and the beam was detuned at 60% from its maximum I0 value. The measurements were conducted at room temperature. It is reasonable to suppose neither oxidation of sample by an inflow of oxygen in air nor a change in the quantity of oxygen occur within QXAFS measurement time, because the polyethylene bag was sealed up tightly. The spectra were treated by the REX2000 program (Rigaku Corporation). The pre-edge peak was approximated by a linear combination of two functions: Gaussian and arctangent. The peak area was determined by the area of the Gaussian from 5979 to 5995 eV. The area of the pre-edge peak of Cr(VI) observed at approximately 5992 eV is strongly correlated with the concentration of Cr(VI) in the sample. The areas of pre-edge peaks and the ratios of Cr(VI)/total-Cr in standard samples were strongly correlated (R2 = 0.9999). The standards were prepared by mixing solid Cr2 O3 (Cr(III)) and K2 Cr2 O7 (Cr(VI)) at various Cr(III)/Cr(VI) ratios (100/0, 75/25, 50/50, 25/75, 0/100). These were mixed with SiO2 to contain 1% (w/w) total-Cr. The ratio of Cr(VI)/total-Cr in each sample was calculated based on the calibration curve of the area of the pre-edge peak observed from standard samples [20]. 2.4.
57 Fe
Mössbauer spectrometry
The structural changes of Fe during its reduction of Cr(VI) were examined using 57 Fe Mössbauer spectrometry. A sample was prepared by mixing FeSO4 ·7H2 O and K2 Cr2 O7 to 10:1 Fe(II):Cr(VI) reaction stoichiometry molar ratio. The sample was spiked with H2 O (40 L) and mixed to initiate the reaction, at which time the Mössbauer measurement started. When the ratio of Fe(II)/Fe(III) remained unchanged, 40 L of 10 g/L HA solution was spiked into the sample and the Mössbauer spectrum was obtained again. The Mössbauer spectra were measured with a TOPOLOGIC SYSTEMS MFD-110D spectrometer using a 1.11 GBq 57 Co/Rh source at room temperature. The Doppler velocity was set to ±10.0 mm/s maximum. Gamma rays at 14.4 keV were collected from each sample for several hours. Curves were fitted to the resulting spectra using a personal computer, assuming that the spectra were composed of peaks with Lorentzian line shapes. The half-widths and peak areas of each quadrupole doublet were constrained to be equal. Isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil.
Fig. 2. Time resolved Cr K-edge XANES spectra for the combined Fe(II) and HA sample. Each spectrum was collected every 60 s for 138 min during the reduction reaction. Intensity of the pre-edge peak at 5992.6 eV decreased with time.
resolved Cr K-edge XANES spectra during the reduction reaction of Cr(VI) by combined Fe(II) and HA for 138 min. For all samples, the intensity/area of the pre-edge peak at 5992.6 eV, which is characteristic of Cr(VI), decreased with elapsed time as shown in Fig. 2, indicating that Cr(VI) was reduced to Cr(III). The ratio of Cr(VI)/total-Cr in the samples was calculated based on the area of the pre-edge peak. The changes in the Cr(VI)/total-Cr ratio with time for each reductant are shown in Fig. 3. The reduction reaction of Cr(VI) was started just after Cr(VI) was spiked into samples.
3. Results and discussion 3.1. Reduction behavior of Cr(VI) After spiking Cr(VI) solution in each pellet sample, we observed how Cr(VI) was reduced to Cr(III) with time by using QXAFS at a time resolution of 60 s per spectrum. We measured continuously until the reaction completed. As an example, Fig. 2 shows the time
Fig. 3. Changes in the Cr(VI)/total-Cr ratio with time during the reduction reaction of Cr(VI) by each reductant (: Fe(II), : HA, : Fe(II) and HA, and ♦: soil).
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143
Fig. 4. Plots of ln relative concentration (Cr(VI)/Cr(VI)0 ) versus time for the reduction of Cr(VI) in various systems. Reduction by (a) Fe(II); (b) HA;(c) Fe(II) and HA; and (d) soil. [Cr(VI)]0 refers to apparent the initial ratio of Cr(VI) (100%).
Of the total Cr(VI), 86.5% was reduced to Cr(III) within 127 min by Fe(II). The reduction by HA was 68.2% within 95 min, combined Fe(II) and HA was 97.2% within 138 min, and the soil was 86.4% within 123 min. The decrease in Cr(VI) percentage in each reductant within 95 min was following order: combined Fe(II) and HA (84.8%) > Fe(II) (79.3%) > HA (68.2%). These results indicated that the combined Fe(II) and HA is the most effective reducing agent in this system. In order to evaluate the decrease of Cr(VI) in detail, the natural logarithm (ln) of the relative concentration of Cr(VI) (Cr(VI)/Cr(VI)0 ) was calculated based on the ratio of Cr(VI)/total-Cr shown in Fig. 3. The initial ratio of Cr(VI) (Cr(VI)0 ) of each sample was 100%. Fig. 4 shows the plots of ln(Cr(VI)/Cr(VI)0 ) versus time for all samples. In the previous work using XAFS, approximately 10 min was required to collect the Cr XANES spectrum due to measurements using a step scan for the full energy range of the spectrum [20]. In this study, on the other hand, we applied QXAFS mode at a time resolution of 60 s per spectrum and were able to observe rapid initial oxidation state changes in consecutive time scale. Since the many data points were obtained during the reduction reaction of Cr(VI) using QXAFS, we could observe how Cr(VI) was reduced to Cr(III) in detail and discern an inflection point. In the Fe(II) experiment, the shapes of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were linear (Fig. 4a). The rate constant (k) obtained by the least-squares fitting of the data was 1.58 × 10−2 min−1 (R2 = 0.9927). It has been reported that the
Cr(VI) reduction kinetics displayed a linear trend, which shows a first-order reaction with respect to Fe(II) and Cr(VI) [9,36]. When Fe(II) contributes to Cr(VI) reduction, the reduction kinetics were linear. In the HA experiment, in contrast, the shapes of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time were nonlinear (Fig. 4b). The decrease of the concentration of Cr(VI) was rapid during the first 20 min of the reaction, and then decreased gradually with time. During the initial period (20 min), 40% of the Cr(VI) was reduced; thereafter, 28% of the Cr(VI) was reduced within approximately 75 min. The results suggested that the reduction of Cr(VI) slowed after 20 min. Wittbrodt and Palmer reported that the reduction exhibited a nonlinear decrease with time for various concentrations of HA, and plots of ln(Cr(VI)/Cr(VI)0 ) versus time showed an initial rapid decrease [37]. The results in our study, which were obtained at a high time resolution (60 s), agreed with this report. It is considered that the reduction kinetics by HA were nonlinear, in which the initial decrease of concentration was relatively rapid and slowed with time. In the reduction by combined Fe(II) and HA, the decrease in Cr(VI) concentration was unlike that with Fe(II) or HA alone (Fig. 4c). The reduction rate was not constant. The reduction of Cr(VI) was especially rapid during the first 2 min of the reaction, in which 13.7% of the Cr(VI) was reduced. The rate then gradually decreased after 3 min, but became rapid again after approximately 95 min. After 3 min of the reaction, the shapes of the plots showed linear relations, though with different rate constants.
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doublet is shown by the blue dashed line. As the reaction time progressed, the absorption intensity of the high spin Fe(III) quadrupole doublet increased, but this peak disappeared when HA was spiked. The relative area ratios of each component of Fe(II) and Fe(III) were estimated, assuming that the peak area of a given component is proportional to the amount of Fe present. 29% of Fe(III) was produced 4 h after Cr(VI) and Fe(II) were mixed. The ratio of Fe(III)/total-Fe increased with elapsed time, to 43% after 40 h. Because the ratio of Fe(II)/Fe(III) did not change after 23 h of observation, we determined 40 h as the reaction end point. Then, HA solution was added to the system, and the Fe(III) peak disappeared within 32 h after spiking. This indicated that all of the Fe(III) was reduced to Fe(II) by HA. It is reported that the Cr(VI) reduction rate increases when Fe(III) is added to the Cr(VI)-humic substances (HA or Fulvic acid) reaction system in various Fe(III) concentrations [10,38,39]. Moreover, Fe(III) exists as a cation, whereas Cr(VI) exists as an anion, so it is considered that Fe(III) has a higher affinity for the HA (anion) than does the Cr(VI). Once Fe(III) is produced during the Cr(VI) reduction reaction, HA begins to reduce it in preference to Cr(VI). Thus, HA can reduce Fe(III) to Fe(II), and reproduced Fe(II) can reduce Cr(VI) in turn. Furthermore, the Mössbauer parameters of the Fe(II) did not change from the original Fe(II) (FeSO4 ·7H2 O), as shown in Table 2. The results indicate that not only Fe(III) produced by the reduction of Cr(VI) was reduced to Fe(II) by HA, but also the reproduced Fe(II) had the same structure as the original Fe(II) (Fig. 5, Table 2). Therefore, HA can reduce Fe(III) back to Fe(II) with the same structure as the starting material, and reproduced Fe(II) is available to reduce Cr(VI). 3.3. Reduction process of Cr(VI) by combined Fe(II) and HA
Fig. 5. Mössbauer spectra of the redox reaction of Fe(II) by Cr(VI) and HA. Reduction of Cr(VI) by Fe(II) was carried out, and the oxidation state change of Fe was observed. Then, HA was spiked at the end of the reduction reaction. Black solid lines were fitted to the raw data (dotted). Red and blue dashed lines indicate the Fe(II) and Fe(III) components, respectively. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
The slopes of the plots (rate constant) between 3 and 94 min was 1.75 × 10−2 min−1 (R2 = 0.9980), and the slope of the reaction after 95 min was 3.48 × 10−2 min−1 (R2 = 0.9821), which was twice that of the reaction between 3 and 94 min. It is assumed that the reduction rate accelerated because the ratio of reductant to the residual Cr(VI) increased during the Cr(VI) reduction reaction. 3.2. Role of HA in the reduction of Cr(VI) in the presence of combined Fe(II) and HA The reduction reaction of Fe(III) to Fe(II) by HA during the Cr(VI) reduction reaction has been observed in previous works [10,38,39]. However, the structure of produced Fe(II) by HA is not clear at present. In order to confirm the structure of Fe(II) produced by HA during the Cr(VI) reduction, the oxidation state changes of the Fe components by HA were examined using 57 Fe Mössbauer spectroscopy. The reduction of Cr(VI) was carried out under a stoichiometry of 10:1 Fe(II):Cr(VI), and HA was spiked at the end of the reaction. The Mössbauer spectrometer used in this experiment required at least 4 h to obtain an analyzable spectrum. The results of the structure change of Fe in the Mössbauer spectra are shown in Fig. 5. Two doublet peaks were observed. A high spin Fe(II) quadrupole doublet, indicated by the red dashed line, was assigned to Fe(II) of FeSO4 ·7H2 O, and a high spin Fe(III) quadrupole
In order to clarify how Cr(VI) is reduced to Cr(III) by combined Fe(II) and HA, the role of each reductant was evaluated based on the shape of the plots of ln(Cr(VI)/Cr(VI)0 ) versus time. The plots of ln(Cr(VI)/Cr(VI)0 ) versus time for the Fe(II) experiment exhibited linear relations, whereas the HA experiment exhibited nonlinear relations. These results suggest that the shape of each plot is dependent on which reductant was contributing to the reduction; the results have therefore been used to evaluate the role of each reductant. As it can be seen from the plots of ln(Cr(VI)/Cr(VI)0 ) versus time in the combined Fe(II) and HA experiment (Fig. 4c), the shapes of the plots were linear after 3 min. The reduction reaction was classified into three phases based on the different reduction rate for convenience: phase 1 is the initial rapid decrease (t < 3 min); phase 2 is the gradual decrease (3 min < t < 94 min); and phase 3 is a second rapid decrease (95 min < t < 138 min) (Fig. 6). In the first 2 min of the reaction (phase 1), where the shape of the plots showed nonlinear trends, Cr(VI) was rapidly reduced by both Fe(II) and HA due to the characteristics of the initial rapid reduction reaction of the individual Fe(II) and HA experiments. The linear plots of ln(Cr(VI)/Cr(VI)0 ) versus time were obtained after 3 min of the reaction (phases 2 and 3), suggesting that Fe(II) directly contributes to the reduction reaction. The slope of the plots between 3 and 94 min was 1.75 × 10−2 min−1 (R2 = 0.9980). This value was similar to that from the individual Fe(II) reduction experiment (1.58 × 10−2 min−1 ), which also had a linear relation. Therefore, it is considered that Cr(VI) is mainly reduced by Fe(II) in this phase. The role of HA, on the other hand, is mainly to reduce Fe(III) oxidized by Cr(VI) to Fe(II) because HA can reduce Fe(III) to Fe(II). As the reaction proceeded, the ratio of Fe(II) to residual Cr(VI) increased because of the reproduced Fe(II) by HA. For that reason, the reduction rate accelerated around 95 min into the reaction. Thus, HA strongly contributes to the reduction of Fe(III) to Fe(II) in phase2 and/or phase 3. Even if all the original Fe(II) is oxidized for the reduction of Cr(VI), the reduction of Cr(VI) can continue because produced Fe(III) is reduced to Fe(II) by HA.
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Table 2 Time-scale change of Fe species and Mössbauer parameters of spectra shown in Fig. 5. Sample
Species
Area (%)
I.S.a (mm s−1 )
Q.S.b (mm s−1 )
H.W.c (mm s−1 )
Original Fe(II) (FeSO4 ·7H2 O)
Fe(II)
100
1.25
3.23
0.33
4 h after mixture
Fe(II) Fe(III)
71 28
1.25 0.39
3.22 1.03
0.35 0.56
18 h after mixture
Fe(II) Fe(III)
64 35
1.25 0.38
3.23 1.11
0.33 0.48
23 h after mixture
Fe(II) Fe(III)
59 40
1.25 0.38
3.24 1.11
0.32 0.44
28 h after mixture
Fe(II) Fe(III)
59 40
1.25 0.38
3.24 1.11
0.33 0.40
40 h after mixture
Fe(II) Fe(III)
56 43
1.25 0.38
3.24 1.12
0.32 0.40
5 h after spiking HA solution
Fe(II) Fe(III)
77 22
1.25 0.37
3.23 1.14
0.34 0.95
32 h after spiking HA solution
Fe(II)
100
1.25
3.23
0.37
a b c
Isomer shift. Quadrupole splitting. Half width.
The reduction process of Cr(VI) by the combined Fe(II) and HA consisted of three phases because each reductant contributed differently. The first phase (t < 3 min) affected by both Fe(II) and HA was nonlinear, but linear plots of ln(Cr(VI)/Cr(VI)0 ) versus time were obtained in both phases 2 and 3, where Fe(II) was the principal reductant. When HA contributes directly to the reduction of Cr(VI), the reduction process exhibited a nonlinear relation. In the reduction by the combined Fe(II) and HA, it was found that Fe(II) and HA contributed differently to Cr(VI) reduction. Two kinds of Fe(II) contributed: as the original Fe(II) in phase 2 and the HAreproduced Fe(II) in phase 2 and/or 3. HA contributed in two ways: HA reduced both Cr(VI) and Fe(III). The contribution of Fe(II) is more pronounced than that of HA because Fe(II) directly contributes to the reduction of Cr(VI). On the other hands, HA reduces not only
Cr(VI) but also Fe(III) produced by Cr(VI), while Fe(II) directly contributes to Cr(VI) reduction. Cr(VI) reduction reaction continues even if original-Fe(II) is used up because HA reduces any oxidized Fe(III) back to Fe(II) and the Cr(VI) reduction cycle goes on, as shown in Fig. 6. Thus, HA enhances the reduction of Cr(VI). In other words, HA becomes a promoter of Cr(VI) reduction when both Fe(II) and HA are applied. 3.4. Reduction process of Cr(VI) in soil Based on reduction process by Fe(II) and HA as a simple system, the reduction process in natural soil was evaluated. The changes in the Cr(VI)/total-Cr ratio after Cr(VI) was spiked is shown in Fig. 3. Some data were missing because the measurement was interrupted
Fig. 6. Schema of the contribution of Fe(II) and HA to the reduction of Cr(VI) in each phase.
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by experimental error from 10 to 17 min and from 80 to 82 min, but the missing information does not influence the results. The reduction of Cr(VI) started immediately after the measurement began, and 86.4% of the Cr(VI) was reduced to Cr(III) within 124 min. The reduction of Cr(VI) occurred even without addition of any reductant. Thus, the soil used in this study has the ability to reduce Cr(VI). The plots of ln(Cr(VI)/Cr(VI)0 ) versus time shown in Fig. 4d show that the decrease in the concentration of Cr(VI) slowed from 90 min after the reaction started. Because the Cr(VI) percentage was 17% at 90 min, the reaction was incomplete. Therefore, it can be concluded that the reduction rate changed after approximately 90 min, and the reduction process was also nonlinear. These results were similar to the reduction process of Cr(VI) by HA (Fig. 4b), suggesting that HA contributes to the Cr(VI) reduction. Wittbrodt and Palmer reported that the reduction rate increased by increasing the concentration of HA, but the reduction process exhibited nonlinear trends [37]. Because the concentration of HA in the soil used in this study was twice as much as in the experiment with HA, the initial reduction rate was more rapid. The reduction process in soil also showed an initial rapid decrease and nonlinear trend (Fig. 4d), which was similar to that observed for the experiment with HA (Fig. 4b). This indicates that HA directly contributes to the reduction of Cr(VI), and therefore greatly contributes to the reduction of Cr(VI) in soil. The amount of Fe in the soil used in this study was significantly more than the amounts of total-Cr and Cr(VI). However, the major component of the Fe species in soil is Fe(III). Fe(II) in soil is contained in minerals, and reduction is dependent on the dissolution of Fe(II) from the minerals [40]. The Fe(II) dissolution was considered to be little because the volume of the spiked Cr(VI) solution was small (25 L). The Fe(II) concentration in this soil was 1.03% of the total-Fe, which was measured by 57 Fe Mössbauer spectroscopy. Since 57 Fe Mössbauer spectroscopy is one of gamma transmission methods, the concentration of Fe(II) in the soil sample was including inner-sphere Fe(II) in its mineral, where the reduction of Cr(VI) by Fe(II) was not concerned directly. Therefore, the amount of Fe(II) available to reduce Cr(VI) was small, and as a result, the contribution of HA to Cr(VI) reduction was enhanced. If Fe(II) contributes to the reduction of Cr(VI), the reduction rate would become rapid near the reaction’s end, as observed in the combined Fe(II) and HA experiments, because the Fe(III) would be reduced to reactive Fe(II) by HA. Moreover, it is expected that the reduction process show a linear relation. The reduction of Cr(VI) in the soil without any additional reductants was primarily driven by HA, since the reduction process of Cr(VI) in soil was similar to the HA experiment. The reduction of Cr(VI) occurs even with minimal available Fe(II) because HA is originally contained in the soil. However, we suggest that Fe(II) is effective for the remediation of the soil polluted by Cr(VI) because Fe(II) significantly contributes to the Cr(VI) reduction reaction. When Fe(II) is present, HA reduces any oxidized Fe(III) back to Fe(II) and the reproduced Fe(II) reduces Cr(VI) to Cr(III), as in a Cr(VI) reduction cycle (Fig. 6). This cycle can be applied to natural soil. In addition, adding HA may enhance the reduction of Cr(VI) because HA reduces the produced Fe(III) to usable Fe(II).
4. Conclusions The detailed process of the reduction of Cr(VI) by Fe(II) and HA was investigated. The concentration changes of Cr(VI) in the samples were analyzed continuously for 2 h using QXAFS at a time resolution of 60 s per spectrum. Plots of ln(Cr(VI)/Cr(VI)0 ) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) exhibited a linear relation, whereas that by HA exhibited a nonlinear relation. The reduction by combined Fe(II) and HA under stoichiometry of 7:1 Fe(II):Cr(VI) was
unlike the individual Fe(II) or HA experiments. This process consisted of three phases with different reduction rates, suggesting that Fe(II) and HA contribute differently to the reduction of Cr(VI). The results of Mössbauer measurements show that produced Fe(III) by Cr(VI) reduction was reduced back to Fe(II) by HA, which had the same structure as original Fe(II). The results can explain that once Fe(III) was produced, HA began to reduce it in preference to Cr(VI). The reduction rate changed because Fe(II) and HA contributed differently to the Cr(VI) reduction. In the Cr(VI)-Fe(II)-HA system, the direct contribution of Fe(II) was greater than HA because HA reduced Fe(III) in preference to Cr(VI). The reduction of Cr(VI) in soil without sterilization and any additional reductants was driven primarily by HA since the reduction process of Cr(VI) in soil was similar to the HA experiment. However, Fe(II) is effective for the remediation of soil polluted by Cr(VI) because Fe(II) directly contributes to Cr(VI) reduction. In addition, when HA contributes to Cr(VI) reduction by Fe(II), HA becomes a promoter of Cr(VI) reduction since HA has ability to reproduce usable Fe(II) from produced Fe(III) during the Cr(VI) reduction reaction. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 26.9392. This work has been performed with the approval of the Photon Factory Program Advisory Committee, KEK (Proposal No. 2011G251, 2013G563). The neutron activation analysis has been carried out at the Inter-University Laboratory for the Joint Use of JAEA Facilities. References [1] D. Rai, L.E. Eary, J.M. Zachara, Environmental chemistry of chromium, Sci. Total Environ. 86 (1989) 15–23. [2] World Health Organization, Environmental Health Criteria 61 Chromium, World Health Organization, Geneva, 1988. ˇ [3] N. Koˇzuh, J. Stupar, B. Gorenc, Reduction and oxidation processes of chromium in soils, Environ. Sci. Technol. 34 (1999) 112–119. [4] D. Rai, B.M. Sass, D.A. Moore, Chromium(III) hydrolysis constants and solubility of chromium(III) hydroxide, Inorg. Chem. 26 (1987) 345–349. [5] B.M. Sass, D. Rai, Solubility of amorphous chromium(III)–iron(III) hydroxide solid solutions, Inorg. Chem. 26 (1987) 2228–2232. [6] B.R. James, R.J. Bartlett, Behavior of chromium in soils: V. Fate of organically complexed chromium(III) added to soil, J. Environ. Qual. 12 (1983) 169–172. [7] B.R. James, R.J. Bartlett, Behavior of chromium in soils. VI. Interactions between oxidation–reduction and organic complexation, J. Environ. Qual. 12 (1983) 173–176. [8] R. Bartlett, B. James, Behavior of chromium in soils: III. Oxidation, J. Environ. Qual. 8 (1979) 31–35. [9] S.E. Fendorf, G. Li, Kinetics of chromate reduction by ferrous iron, Environ. Sci. Technol. 30 (1996) 1614–1617. [10] P.R. Wittbrodt, C.D. Palmer, Effect of temperature, ionic strength, background electrolytes, and Fe(III) on the reduction of hexavalent chromium by soil humic substances, Environ. Sci. Technol. 30 (1996) 2470–2477. [11] M. Fukushima, K. Nakayasu, S. Tanaka, H. Nakamura, Speciation analysis of chromium after reduction of chromium(VI) by humic acid, Toxicol. Environ. Chem. 62 (1997) 207–215. [12] C. Kim, Q. Zhou, B. Deng, E.C. Thornton, H. Xu, Chromium(VI) reduction by hydrogen sulfide in aqueous media: stoichiometry and kinetics, Environ. Sci. Technol. 35 (2001) 2219–2225. [13] R.J. Bartlett, J.M. Kimble, Behavior of chromium in soils: II. Hexavalent forms, J. Environ. Qual. 5 (1976) 383–386. [14] L.E. Eary, D. Rai, Chromate removal from aqueous wastes by reduction with ferrous ion, Environ. Sci. Technol. 22 (1988) 972–977. [15] W.-W. Tang, G.-M. Zeng, J.-L. Gong, J. Liang, P. Xu, C. Zhang, B.-B. Huang, Impact of humic/fulvic acid on the removal of heavy metals from aqueous solutions using nanomaterials: a review, Sci. Total Environ. 468–469 (2014) 1014–1027. [16] I.J. Buerge, S.J. Hug, Influence of organic ligands on chromium(VI) reduction by iron(II), Environ. Sci. Technol. 32 (1998) 2092–2099. [17] L.E. Eary, D. Rai, Chromate reduction by subsurface soils under acidic conditions, Soil Sci. Soc. Am. J. 55 (1991) 676–683. [18] W. Xiao, Y. Zhang, T. Li, B. Chen, H. Wang, Z. He, X. Yang, Reduction kinetics of hexavalent chromium in soils and its correlation with soil properties, J. Environ. Qual. 41 (2012) 1452–1458. [19] F.C. Richard, A.C.M. Bourg, Aqueous geochemistry of chromium: a review, Water Res. 25 (1991) 807–816.
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