Reactivity of Nanoscale Zero-Valent Iron in Unbuffered Systems

Article pubs.acs.org/est

Reactivity of Nanoscale Zero-Valent Iron in Unbuffered Systems: Effect of pH and Fe(II) Dissolution Sungjun Bae and Khalil Hanna* École Nationale Supérieure de Chimie de Rennes, UMR CNRS 6226, 11 Allée de Beaulieu, Rennes 35708 Cedex 7, France S Supporting Information *

ABSTRACT: While most published studies used buffers to maintain the pH, there is limited knowledge regarding the reactivity of nanoscale zerovalent iron (NZVI) in poorly buffered pH systems to date. In this work, the effect of pH and Fe(II) dissolution on the reactivity of NZVI was investigated during the reduction of 4-nitrophenol (4-NP) in unbuffered pH systems. The reduction rate increased exponentially with respect to the NZVI concentration, and the ratio of dissolved Fe(II)/initial NZVI was related proportionally to the initial pH values, suggesting that lower pH (6−7) with low NZVI loading may slow the 4-NP reduction through acceleration of the dissolution of NZVI particles. Additional experiments using buffered pH systems confirmed that high pH values (8−9) can preserve the NZVI particles against dissolution, thereby enhancing the reduction kinetics of 4-NP. Furthermore, reduction tests using ferrous ion in suspensions of magnetite and maghemite showed that surface-bound Fe(II) on oxide coatings can play an important role in enhancing 4-NP reduction by NZVI at pH 8. These unexpected results highlight the importance of pH and Fe(II) dissolution when NZVI technology is applied to poorly buffered systems, particularly at a low amount of NZVI (i.e., 8). The time-dependent UV−vis spectra clearly showed that the peak at 400 nm continuously decreased and completely disappeared with the formation of a new peak at 298 nm, assigned to 4-AP after 40 min of reaction (Figure S2b). We did not observe peaks at 388 and 302 nm, which can correspond to 4-benzoquinone monoxime and 4-nitrosophenol.36 In addition, the two isosbestic points at 280 and 312 nm during the reaction stress that no other byproducts were generated during the reduction of 4-NP to 4-AP by NZVI. HPLC measurements clearly confirmed the degradation of 4NP by NZVI and its stoichiometric conversion to 4-AP during the reaction (i.e., mass balance was achieved) (Figure 1a), indicating that possible losses of 4-NP or 4-AP by sorption on NZVI surfaces are negligible. The rate constant for the kinetics of 4-NP degradation by NZVI can be described by the following pseudo-first-order kinetic model: dC4NP = −kobsd,4NPC4NP (1) dt where C4NP is the concentration of 4-NP at the sampling time (t) and kobsd,4NP is the observed pseudo-first-order rate constant. A linear regression of eq 1 allowed the determination of kobsd,4NP as 0.135 ± 0.009 min−1 with R2 ≈ 0.99. Figure 1a also shows the variation in pH, ORP, and dissolved Fe(II) during (1) the reaction of NZVI with DDW and (2) the transformation of 4-NP in the NZVI suspension. After the addition of NZVI to DDW, the pH and dissolved Fe(II) concentration increased from 6.4 to 8.8 and from 0 to 0.006 mM in 30 min due to the anaerobic corrosion of NZVI (Fe(0) + 2H2O → Fe(II) + 2OH− + H2).5,37 According to the standard reduction potential (E°) of −440 mV for the halfreaction between the Fe2+/Fe0 couple, highly reducing conditions can be expected in the NZVI suspension. Indeed, ORP decreased rapidly from +4 mV to approximately −750 mV in 10 min and seemed to be constant for 30 min. Even 2−3 mg/L NZVI has been reported to be sufficient to achieve a negative ORP solution in 1 h due to the large reactive surface and rapid reaction with water molecules.27 A rapid decrease in pH (from 8.8 to 7.8) and an increase in ORP (from −750 to −465) were observed in 4 min due probably to the acidity of the 4-NP solution, and the pH and ORP became almost constant during the reduction reaction. After the 4-NP reduction was finished (40 min), the concomitant increase in pH and decrease in ORP was observed, indicating that NZVI re-reacts with water molecules to reach the equilibrium condition again. The concentration of dissolved Fe(II) increased to 0.013 mM in 10 min and seemed to fluctuate around this value for 60 min. The increase in dissolved Fe(II) upon adding 4-NP may be caused by the pH decrease. The relationship with dissolved Fe(II) in the pH range of 6−9 will be discussed later. XRD analyses of NZVI before and after the 4-NP reduction showed an increase in the peaks of magnetite versus a decrease C

DOI: 10.1021/acs.est.5b01298 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology in the peaks of α-Fe (Figure S1c,d). This prompt formation of magnetite suggests that 4-NP can be converted to 4-AP through the following reaction: 9Fe0 + 4H2O + 4C6H5NO3 → 3Fe3O4 + 4C6H7NO. Although oxidation of Fe(0) to Fe(II) is most often assumed in the literature,38,39 the oxidation of NZVI to magnetite (Fe3O4) seems to be more thermodynamically favorable during the decontamination at pH above 6.1.26,40 Effect of the Injection Order on the Reduction of 4-NP by NZVI in Unbuffered pH Systems. The reduction of 4-NP by NZVI was investigated as previously explained but with a change in the injection order, i.e., NZVI into a pre-equilibrated 4-NP solution instead of 4-NP into an NZVI suspension (Figure 1b). After addition of the desired volume of 4-NP stock solution into DDW, the pH decreased rapidly from 6.0 to 4.8 in 1 min with an increase in ORP from +7 to +236 mV because of the acidic character of the 4-NP solution, and those values seemed to become constant in 10 min. The reduction of 4-NP to 4-AP was initiated by the addition of NZVI into a preequilibrated 4-NP solution. The kobsd,4NP was 0.041 ± 0.004 min−1, which is 3.3 times lower than the value obtained by injecting 4-NP into an NZVI suspension. The injection order of NZVI into a pre-equilibrated 4-NP solution can apparently and significantly slow the reduction rate of 4-NP by NZVI. This inhibition effect may be induced by a competitive electron transfer from NZVI to water (i.e., anaerobic corrosion of NZVI) and 4-NP (i.e., reduction of 4-NP). In the case of 4-NP injection into an NZVI suspension, the electron transfer from NZVI to a water molecule first occurred when the equilibrium state was reached. Therefore, an electron transfer process from NZVI to 4-NP could be a dominant reaction when 4-NP is injected into an NZVI suspension. However, both the equilibrium reaction with water and the 4-NP reduction can simultaneously consume the electron from NZVI when NZVI is injected into a pre-equilibrated 4-NP solution. This phenomenon may kinetically decrease the reduction rate of 4-NP. The pH increased to 7.8 in 10 min and seemed to be constant in 60 min, while a decrease of ORP to −410 mV was observed. The equilibrium values (pH and ORP) during the 4NP reduction were very similar for both injection modes (Figure 1), except that a difference of 55 mV in the ORP value was observed. The concentration of dissolved Fe(II) increased to 0.036 mM, which may result from the low initial pH value (i.e., 4.8) (Figure 1b). However, the concentration of dissolved Fe(II) continued to decrease to 0.020 mM in 60 min due probably to (1) precipitation of Fe as a form of hydroxide at pH 7.8 and/or (2) readsorption of Fe(II) on the surface oxide coating. Although the precipitation of iron hydroxide under anaerobic conditions has normally been reported in the pH range of 8.5−10,41,42 a lower pH (approximately 8) can also precipitate iron hydroxides.43 Adsorption of aqueous Fe(II) on iron oxides (e.g., magnetite and hematite) has been reported to be easily observed at high pH values.44,45 Therefore, both reactions may be occurring until the concentration of aqueous Fe(II) reaches a steady state (i.e., approximately 0.013 mM in Figure 1a). The results obtained from this study revealed that the injection order can significantly influence the key reaction parameters, such as pH, ORP, and dissolved Fe(II), and, finally, the reduction rate constant of 4-NP. Effect of the NZVI Concentration on the Reduction of 4-NP, pH, ORP, and Dissolved Fe(II) in Unbuffered pH Systems. To investigate the effect of the NZVI concentration on kobsd,4NP, the suspension pH, ORP, and dissolved Fe(II), the

reduction of 4-NP with three NZVI concentrations (0.18, 0.45, and 0.90 mM) was conducted and compared with the data obtained at 1.35 mM (Figure 2 and Figure S3). To compare the

Figure 2. (a) Effect of the NZVI concentration on the reduction of 4NP. The circles, squares, triangles, and tilted squares indicate the experimental data. The solid lines show the pseudo-first-order fits over the first 20 min of reaction. (b) Change in the kinetic rate constants for reduction of 4-NP with respect to the NZVI concentration. The initial concentration of 4-NP was 0.1 mM, and error bars indicate the standard deviation of duplicate samples.

initial rates of 4-NP reduction, the kinetic rate constants were determined by the pseudo-first-order kinetic model using the data collected in the first 20 min. Inevitably, the reduction kinetics of 4-NP was expedited as the NZVI concentration increased. Previous studies using a buffered pH system have shown a linear increase in the degradation rate constant with respect to the NZVI concentration due to the proportional increase in the reactive surface area of NZVI.3,25 However, the rate constant in this study increased exponentially with respect to the NZVI concentration (Figure 2b), indicating that not only the NZVI concentration but also additional factors may contribute to the improvement in the degradation kinetics of 4-NP. The latter were investigated by monitoring the variation in pH and Fe(II) dissolution during each NZVI loading (Figure S3). We observed that the variation in pH after the injection of 4-NP was significantly different under each set of conditions. At 0.18 and 0.45 mM NZVI loading, the pH first dropped, following by a slight increase to reach a steady-state value D

DOI: 10.1021/acs.est.5b01298 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

aqueous dissolution. Many past studies using a high dosage of ZVI (>0.4 g/L) have shown enhanced decontamination as the suspension pH decreased regardless of the size of the ZVI particles (10 nm to 1 mm) and type of contaminant (trichloroethylene, 1,1,1-trichloroethane, Cr(VI), nitrobenzene) (Table 1).3,4,24,26,28 Because our pH range (6−9) was

(Figure S3). However, at 0.9 mM NZVI loading, the pH dropped from 8.7 to 8.0 after the injection of 4-NP and continued to decrease to 7.4 during the 4-NP reduction. In contrast to the highest NZVI loading (1.35 mM) (Figure 1a), the rebound of the pH during the 4-NP reduction was not observed in the range of 0.18−0.90 mM NZVI, probably because (1) the anaerobic corrosion could not be restarted by exhaustion of the NZVI reactivity with the limited amount of NZVI (i.e., at 0.18 and 0.45 mM) or (2) the reduction of 4-NP was still in progress (i.e., at 0.90 mM). The concentration of dissolved Fe(II) at 0.18 and 0.45 mM was 2.27 and 2.43 times higher than that at 1.35 mM after the 4-NP reduction (60 min) (Figure S4). In particular, the ratio of dissolved Fe(II) to initial NZVI after 60 min of reaction with 4-NP was the highest (15%) at 0.18 mM NZVI, followed by 0.45 mM (6.6%), 0.90 mM (2.2%), and 1.35 mM (1.0%) (Figure S5a). We also observed that these values were proportionally related to the suspension pH after the injection of 4-NP (Figure S5b). These findings imply that a lower pH at a low NZVI loading (0.18 and 0.45 mM) resulted in the acceleration of Fe(II) dissolution from the NZVI surface, which may explain the exponential increase in the 4-NP reduction as the NZVI concentration increased. In addition, we observed that ORP reached its equilibrium condition in 30 min (0.45 and 0.90 mM) and 60 min (0.18 mM). The minimum value of ORP at the equilibrium condition was the lowest at 1.35 mM NZVI (−750 mV), followed by 0.90 mM (−732 mV), 0.45 mM (−677 mV), and 0.18 mM (−670 mV), which is consistent with previous findings in which the minimum value of ORP increased as the NZVI concentration increased.46 Effect of the pH on the Reduction of 4-NP by NZVI in Unbuffered pH Systems. To investigate the effect of the pH on the kinetics of 4-NP reduction by NZVI in unbuffered pH systems, a similar experiment was conducted by the injection of 4-nitrophenolate ions prepared at pH 9.87 instead of 4nitrophenol (pH 4.40) (Figure S6). First, we observed that 0.1 mM 4-NP was completely reduced to 4-AP by the NZVI suspension in 30 min, and the kobsd,4NP was 0.203 ± 0.016 min−1, 1.5 times higher than in the previous experiment (Figure 1a). The pH dropped to 8.2, seemed to be constant until the reduction of 4-NP was finished, and then increased gradually to approximately 8.6 (Figure S6). The ORP showed a lower equilibrium value at approximately −485 mV compared to the previous data (−465 mM) (Figure S6 vs Figure 1a). The addition of 4-nitrophenolate ions to the NZVI suspension showed a lower pH drop from 8.8 to 8.2 compared to 4-NP (pH drop to 7.8), which may significantly influence the 4-NP reduction by NZVI. Because the sorption of 4-NP and 4nitrophenolate on the NZVI surface was found to be negligible, the protonation/deprotonation of 4-NP may not significantly affect the interactions with NZVI surfaces. However, no published data are available regarding the effect of the protonation/deprotonation of 4-NP on its reduction potential. The results obtained in this study suggest that a high pH may improve the reduction of 4-NP in the range of pH 6−9. This finding is somewhat unexpected on the basis of the previous studies of NZVI applications, in which high pH lowers the NZVI reactivity due to the surface passivation.3,25,28 This unusual trend cannot be explained by a pH dependence of compound sorption to iron oxide coatings because the sorption was found to be negligible regardless of the pH value. One possible explanation for the enhancement in NZVI reactivity at high pH may be the preservation of ZVI particles against

Table 1. ZVI Studies Showing the Enhanced Decontamination as the Suspension pH Decreased ref

target contaminant

ZVI size

3 4 24

1,1,1-trichloroethane Cr(VI) trichloroethylene

26 28

trichloroethylene nitrobenzene

10−40 nm 25 nm finer than 100 mesh (∼150 μm) ∼100 nm 1 mm

dosage (g/L) 0.4 2 2.5 0.5 13.3

pH range 6−9 6−9 4.9−8 6.5−8.9 3−9

similar to the pH range previously reported, the knowledge to date cannot properly explain our findings at a low dosage of NZVI ( 7.5, the adsorption of Fe(II) reaches 100% regardless of the oxide tested.51,52 Therefore, these results suggest that the enhanced reduction of 4-NP may be partially caused by the increase in electron density of the Fe(II) adsorbed on the oxide layer coating53 of the NZVI such as magnetite and/or maghemite. Environmental Significance. Our findings imply that both the pH and Fe dissolution in NZVI suspensions are of fundamental importance to investigating the kinetics of decontamination by NZVI and its reaction mechanism in the pH range of 6−9. In this study, we have notably demonstrated that the reduction rate of 4-NP can be improved at high pH values in both unbuffered and buffered suspensions in the presence of low amounts of NZVI (0.010−0.075 g/L). These surprising findings are in contrast with numerous previous reports using buffered pH systems and a high NZVI dosage (0.4−20 g/L), which may underestimate the loss of NZVI by Fe dissolution at neutral pH values.3,4,26 These unexpected results may be explained through one and/or a combination of the following processes: (1) relatively less Fe(II) dissolution than at pH 6 and 7, leading to the preservation of solid Fe phases, and/or (2) an increase in the amount of surface Fe(II) complexed with oxide coatings and then in the electron density of the surface-bound Fe(II). The inhibition effect observed at pH 6 and 7 may result from the loss of NZVI particles through rapid iron dissolution. The results obtained from this study can help in the understanding of the removal mechanism of contaminants in the presence of low amounts of NZVI and in poorly buffered systems. Poorly buffered conditions prevail in some contaminated media, e.g., industrial/domestic wastewaters29 and poorly buffered natural environments.30 Our findings suggest that the treatment of such systems using a reasonable amount of NZVI should be carefully reconsidered to economically and effectively remove the target contaminants. In addition, an effort has been started to develop the NZVI-based treatment system as a batch reactor for industrial/domestic wastewaters,54 and we expect that this practical application will be continued as a field study. Therefore, our experimental results can provide a useful guideline to design the NZVI technology effectively in poorly buffered systems.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +33 2 23 23 80 27; fax: +33 2 23 23 81 20; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the “Région Bretagne” for financial support (Contract SAD-ReSolEau (8256)). We thank Dr. M. Pasturel and Dr. V. Dorcet for XRD and TEM analyses, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01298. Details of the chemicals used in this study, material characterization, analytical methods, and additional results for reduction of 4-NP by NZVI (PDF) G

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DOI: 10.1021/acs.est.5b01298 Environ. Sci. Technol. XXXX, XXX, XXX−XXX