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A kinetic approach on hexavalent chromium removal with metallic iron
Journal of Environmental Management xxx (2017) 1e5
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Research article
A kinetic approach on hexavalent chromium removal with metallic iron M. Gheju a, *, I. Balcu b, A. Enache a, A. Flueras a a
Politehnica University Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, Bd. V. Parvan Nr. 6, 300223, Timisoara, Romania National Institute for Research and Development in Electrochemistry and Condensed Matter, Str. Dr. Aurel Paunescu Podeanu Nr. 144, 300587, Timisoara, Romania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 9 March 2017 Accepted 12 March 2017 Available online xxx
This paper examines the mechanism of Cr(VI) removal with Fe(0), and the possible effect of various experimental parameters, from a kinetic perspective. The experimental data was analyzed using five different kinetic models: three for chemical reactions and two for adsorption processes. It was found that the process fitted well to the zero-order kinetic model for all investigated systems, excepting experiments conducted at 6 C and those with nano-Fe(0), when the process followed the Ho's pseudo secondorder model. Therefore, even though, under acidic conditions, chemical reduction can be generally considered as the main mechanism of Cr(VI) removal with Fe(0), under some experimental conditions (e.g. when working with nano-Fe(0) or at low temperatures), adsorption seems to be the dominant removal path. The enhanced Cr(VI) removal noticed in co-presence of SO2 4 and Cl anions reiterates the significance of the secondary reductant Fe(II) within the process of Cr(VI) removal with Fe(0). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Hexavalent chromium Metallic iron Water pollution Adsorption Reduction
1. Introduction Nowadays, contamination of water environments has become a significant concern, especially in the industrialized countries, due to increasing anthropogenic inputs after the industrial revolution (Chrysochoou and Dermatas, 2015). Because metallic iron (Fe(0)) is a relatively low cost material, readily available, with low toxicity (Btatkeu et al., 2016), important efforts have been focused on the use of Fe(0) for the removal of a wide range of pollutants, both inorganic (e.g. heavy metals (Hashim et al., 2011), metalloids (Vitkova et al., 2017)) and organic (e.g. dyes (Raman and Kanmani, 2016), phenols (Nakatsuji et al., 2015), estrogens (Jarosova et al., 2015)). Heavy metals are particularly problematic contaminants because they are highly toxic, non-biodegradable, and persistent (Pehlivan and Altun, 2008). Chromium is an important metal with widespread use in various industries; as a result, large quantities of this metal have been discharged into the environment due poor storage practices, improper disposal or leakage of chromium waste. In natural environments, chromium can exist mainly in two oxidation states: (þIII) and (þVI). Among these two, Cr(VI) exerts the most toxic effects on living organisms, having also the highest
* Corresponding author. E-mail address: [email protected] (M. Gheju).
mobility in the environment (Gheju, 2011, and references therein). Over the last decades, Fe(0) has been demonstrated to represent a highly efficient reagent for the removal of Cr(VI) from contaminated waters; however, there is yet no consensus at this time in what regards the mechanism of Cr(VI) removal with Fe(0). The first mechanism, proposed in the nineties (the ”reductive precipitation” mechanism) (Cantrell et al., 1995), and widely accepted until our days (Kong et al., 2016), attributed the efficiency of Fe(0)-systems mainly to the direct electron transfer from Fe(0) surface to Cr(VI), coupled with (co-)precipitation of resulted Cr(III). It was probably suggested in agreement with the direct reductive dechlorination mechanism, previously proposed as the most likely cause of chlorinated organics removal with Fe(0) (Gillham and O'Hannesin, 1994). Subsequent studies have, however, acknowledged the importance of another process, Cr(VI) adsorption, as intermediate step within the mechanism of Cr(VI) removal with Fe(0) (Powell et al., 1995). Moreover, it has been indicated that adsorption on some types of Fe(0) (e.g. nano-sized) can be regarded not only as intermediate step, but also as a dominant Cr(VI) removal mechanism by itself (Ai et al., 2008). Recent studies also suggested that, in Fe(0)-H2O systems, along with co-precipitation (Noubactep, 2015a) and size-exclusion (Yoon et al., 2011), adsorption is one of the main contaminant removal mechanisms, while reduction, when possible, occurs mainly indirectly via Fe(0) corrosion products
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Please cite this article in press as: Gheju, M., et al., A kinetic approach on hexavalent chromium removal with metallic iron, Journal of Environmental Management (2017), http://dx.doi.org/10.1016/j.jenvman.2017.03.031
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(Noubactep, 2015b). Even though numerous studies investigated the kinetics of Cr(VI) removal with Fe(0) (Gheju, 2011, and references therein), to authors knowledge, the assessment of the kinetic model was not yet used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). Therefore, the goal of the present paper was to investigate the importance of different possible removal paths within the mechanism of Cr(VI) removal with Fe(0), as well as the effect of several important parameters, by means of kinetic analysis of experimental data. 2. Materials and methods Commercially available Fe(0) from Alfa Aesar (99%, ~1e2 mm) and from Merck (99%, ~10 mm) (hereinafter referred to as milliFe(0) and micro-Fe(0), respectively) was used as received. In addition, nano-Fe(0) was synthesized via the liquid-phase reduction method with sodium borohydride, following a procedure described by Xi et al. (2010). Cr(VI) removal experiments were carried out in a 1.5 L Berzelius flask, by introducing a mass of 0.5 g Fe(0) into 1000 mL of Cr(VI) solution. The mixture was stirred (200 rpm) using an overhead Heidolph stirrer and, at preset intervals, samples were withdrawn and filtered. Cr(VI) concentration in the filtrate was analyzed by the 1,5-diphenylcarbazide colorimetric method at 540 nm (APHA, 1995), using a Specord 200 PLUS spectrophotometer. The pH of the solutions was set before the experiments by adding small amounts of concentrated H2SO4 and measured using an Inolab 7320 pH-meter. The morphology of the synthesized nano-Fe(0) was characterized by the use of transmission electron microscopy (TEM), using a FEI e Titan G2 80e200 microscope. The kinetics of Cr(VI) removal was evaluated using five kinetic equations: three suitable for chemical reactions and two for adsorption processes, as shown in the Supplementary material; in addition, the intraparticle diffusion model was also applied when adsorption was found to be the dominant Cr(VI) removal path (Supplementary material). 3. Results and discussion 3.1. Effect of pH The influence of solution pH was investigated at 20 C, within the range of 1.1e3.5, using a 2 mg/L Cr(VI) solution and micro-Fe(0). It is shown that Cr(VI) removal significantly decreased with
increasing pH, being already almost totally inhibited at pH 3.1 (Fig. 1). This observation can be ascribed to involvement of Hþ ions in processes contributing to Cr(VI) removal in Fe(0)-H2O system. Cr(VI) removal with Fe(0) is the result of a complex interplay of processes such as adsorption, reduction and (co-)precipitation (Gheju, 2011). However, under the experimental conditions of the present study (acidic pH), the (co-)precipitation process can be excluded. Hence, in our case, only three pathways may be taken under consideration for the removal of Cr(VI) with Fe(0). The first one is adsorption of Cr(VI) on Fe(0), or onto oxide layers existent at Fe(0) surface (Geen et al., 1994): þ > Fe OH þ CrO2 4 þ H ⇔ > FeCrO4 þ H2 O
(1)
The second pathway is the heterogeneous Cr(VI) reduction, which involves direct electron transfer from Fe(0) surface, or from corrosion products containing Fe(II) existent at Fe(0) surface (Gheju, 2011): 0 þ 2þ 2HCrO þ 2Cr3þ þ 8H2O 4 þ 3Fe þ 14H / 3Fe
(2)
3[FeII4FeIII2(OH)12][SO4$3H2O] þ 4HCrO 4 þ 5H2O / 16 [Fe0,75Cr0,25](OH)3 þ 2Hþ þ 3SO2 4 þ 6Fe(OH)3
(3)
þ 3FeIIFeIII2O4 þ HCrO 4 þ 14H2O þ H / 4 [Fe0,75Cr0,25](OH)3 þ 6Fe(OH)3
(4)
The third pathway is the homogenous Cr(VI) reduction by products of Fe(0) corrosion (Fe2þ and H2) (Gheju, 2011): 2þ þ 7Hþ / 3Fe3þ þ Cr3þ þ 4H2O HCrO 4 þ 3Fe
(5)
þ 3þ 2HCrO þ 8H2O 4 þ 3H2 þ 8H / 2Cr
(6)
It is obvious that, for most of these reactions, an increase in Hþ concentration will lead to an increase in efficiency of Cr(VI) removal ^telier's principle). In the same time, pH is an important (Le Cha parameter controlling adsorption of metal ions, due to its influence on adsorbent surface properties and on ions speciation in solution (Jain et al., 2009). Because adsorption of Cr(VI) is the first step of the heterogeneous reduction mechanism, it is clear that, when discussing the effect of pH on Cr(VI) removal with Fe(0), the influence of pH on the adsorption phase should also be considered. Since surface of Fe(0) corrosion products is mostly positively charged, they are good adsorbents for Cr(VI) anions (Noubactep, 2015a). With increasing concentration of Hþ ions in solution, the number of positively charged centers at Fe(0) surface also increases; as a result, the electrostatic forces of attraction which act between Cr(VI) anions and positively charged Fe(0) surface will also increase. In the same time, the competition for positively charged sites between Cr(VI) anions and HO ions diminishes with decreasing pH, as a result of decreasing HO concentration. The kinetic modeling of experimental data showed that the zero-order kinetic model provided the best match (Table S1 and Figs. S5eS9, Supplementary material), indicating reduction of Cr(VI) to Cr(III) as the main removal mechanism. This is in agreement with previous studies which have shown that, under acidic conditions, chemical reduction was the main pathway of Cr(VI) removal with Fe(0) (83.3%), and dissolved Fe(II) was the major reductant; nevertheless, adsorption was also responsible for the removal of a small amount of Cr(VI) (16.7%) (Gheju et al., 2016). 3.2. Effect of Cr(VI) concentration
Fig. 1. Effect of pH on Cr(VI) removal with Fe(0).
The effect of this parameter was explored at 20 C, by reacting
Please cite this article in press as: Gheju, M., et al., A kinetic approach on hexavalent chromium removal with metallic iron, Journal of Environmental Management (2017), http://dx.doi.org/10.1016/j.jenvman.2017.03.031
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micro-Fe(0) with Cr(VI) solutions with pH 2.1 and an initial concentration that was varied from 1 to 5 mg/L. It was noted that removal efficiency of Cr(VI) decreased upon increasing initial concentration of Cr(VI) (Fig. 2). The modeling of kinetic data showed that zero-order equation was the more appropriate, regardless of the value of initial Cr(VI) concentration (Table S2 and Figs. S10eS14, Supplementary material). Thus, the main mechanism of Cr(VI) removal, reduction to Cr(III), was not affected by the change in Cr(VI) concentration. The observed results can be ascribed to the decrease of the ratio Fe(0) surface area/mass of Cr(VI) with increasing initial concentration of Cr(VI). Less Fe(0) surface available in the system will adversely affect the efficiency of the two mechanisms that contribute to Cr(VI) removal (adsorption and reduction), due to the increase in the number of HCrO 4 ions competing for the available reactive sites on the surface of Fe(0). Hence, the higher the initial concentration of Cr(VI), the faster the saturation of Fe(0) surface. The decrease of Cr(VI) removal with increasing its concentration was also attributed to lower rates of Fe(0) corrosion at higher Cr(VI) concentrations (Melitas et al., 2001). Fig. 3. Effect of temperature on Cr(VI) removal with Fe(0).
3.3. Effect of temperature The effect of temperature was studied in the range of 6e33 C, by using a 2 mg/L Cr(VI) solution with pH 2.5, and micro-Fe(0). It was revealed that removal of Cr(VI) with Fe(0) was favored by an increase in temperature (Fig. 3), indicating an endothermic nature of the process. However, the most notable improvement in Cr(VI) removal was achieved by increasing temperature from 20 to 26 C; a further increase of temperature to 33 C led only to a minor enhancement. Temperature had an important influence not only on efficiency of Cr(VI) removal, but also on the kinetics of the process. The kinetic modeling of experimental data showed that, while the zero-order kinetic model provided the best match over the temperature range of 20e33 C, at 6 C the Ho's pseudo second-order equation was the more appropriate (Table S3 and Figs. S15eS19, Supplementary material). Since this equation is a kinetic model developed for sorption processes (Ho et al., 2000), it may indicate that, at 6 C, Cr(VI) reduction was significantly inhibited and the major pathway that contributed to removal of Cr(VI) with Fe(0) was adsorption. The results of Weber and Morris modeling (Table S6 and Fig. S30, Supplementary material) reveal that, in spite of a relative good linearity of the plots, they do not pass through the
Fig. 2. Effect of initial Cr(VI) concentration on Cr(VI) removal with Fe(0).
origin. This inadequacy of the intraparticular diffusion model implies that a combined effect of the film diffusion and intraparticle diffusion mechanisms is probable (Ho et al., 2000). Even though Fe(0) particles are not porous, they have, however, a degree of roughness on their surfaces. Therefore, while intraparticular diffusion is possible, it is reduced and limited only to the external thin layer of oxides existent at Fe(0) surface.
3.4. Effect of Fe(0) size In order to study the influence of Fe(0) size, three different types of Fe(0) were reacted at 20 C with a 2 mg/L Cr(VI) solution having pH 2.5: milli-Fe(0), micro-Fe(0), and nano-Fe(0). The synthesized nano-Fe(0) has a size in the range of 5e50 nm (Figs. S1eS4, Supplementary material). The present experiments have shown that efficiency of Cr(VI) removal with Fe(0) followed the series: milli-Fe(0) > micro-Fe(0) > nano-Fe(0) (Fig. 4), in good agreement with previous findings of Comba et al. (2011). It is generally accepted that the smaller the particle size, the larger the specific
Fig. 4. Effect of Fe(0) type on Cr(VI) removal with Fe(0).
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surface area and the greater the Fe(0) reactivity (Gheju, 2011). If this assumption would have been valid also in our study, than Cr(VI) removal efficiency should decrease in the order: nanoFe(0) > micro-Fe(0) > milli-Fe(0). In fact, we did notice this trend, but only during the first minute of the experiment. After the first minute, the mixing was stopped for about 10 s in order to extract the first sample; afterwards, the mixing was resumed and this procedure was repeated several times during the course of the experiment. These results may be justified by the tendency of Fe(0) powders to agglomerate, behavior that increases with decreasing particle size (Walter, 2013). This is in accord with previous reports indicating that some Fe(0) powders were less reactive than Fe(0) fillings, as a results of agglomeration (Noubactep et al., 2009). On the other hand, because of their higher reactivity, nano-Fe(0) particles have also a lower selectivity, and thus, a higher tendency to react with non-target substances, including dissolved oxygen and water (Tratnyek and Johnson, 2006). Fig. 4 also shows that Cr(VI) removal with nano-Fe(0) proceeds in two stages: high rates are observed within the first time interval, when most of the Cr(VI) was removed, whereas a strong decrease in the Cr(VI) removal occurs in the second one, when the process gradually saturate up, reaching equilibrium. This observation is typical for adsorption processes and corroborates the results of experimental data kinetic modeling which indicate that, for nano-Fe(0), the Ho's pseudo second-order model fitted better among all the investigated kinetic models (Table S4 and Figs. S20eS24, Supplementary material). Instead, for micro-Fe(0) and milli-Fe(0) the zero-order kinetic model provided the highest correlation coefficients. The results of Weber and Morris modeling (Table S6 and Fig. S31, Supplementary material) suggests, following a similar rationale to the one presented in Section 3.3, that both intraparticular and film diffusion are factors that control Cr(VI) adsorption on nano-Fe(0). Consequently, it seems that adsorption was the main mechanism of Cr(VI) removal with nano-Fe(0); this is in accordance with previous reports who indicated that adsorption of Cr(VI) onto nano-Fe(0) wires was the dominant removal path (Ai et al., 2008). In contrast, chemical reduction was the main pathway that contributed to removal of Cr(VI) with micro-Fe(0) and milli-Fe(0) under the experimental conditions applied in the present work. The observed results obtained for nano-Fe(0) can be ascribed to presence of iron oxides at surface of nano-Fe(0), which are known to be very good adsorbents for negatively charged pollutants (Noubactep, 2015b). Even though the freshly-obtained nano-Fe(0) was washed with ethanol and stored under vacuum, its color changed rapidly from black to brown as a result of Fe(0) oxidation and generation of iron oxides (Fig. S3, Supplementary material). 3.5. Effect of nature and concentration of co-present anions 2þ 2 The effect of numerous ions (Ca2þ, Mg2þ, HCO 3 , CO3 , Fe , Cu2þ) has been investigated up to this day (Gheju, 2011, and references therein); however, to the best of our knowledge, the influence of two important anions, commonly found in natural water environments, namely SO2 4 and Cl , was not studied yet. In order to research the effect of these anions, Cr(VI) solution 2 mg/L with pH 2.5 was reacted with micro-Fe(0) at room temperature (20 C), in the co-presence of 0.01 and 0.05 M NaCl and Na2SO4 as background electrolytes. As revealed by Fig. 5, the addition of Cl and SO2 4 led to an increase in Cr(VI) removal efficiency; the higher the concentration of co-present anion, the higher the efficiency of Cr(VI) removal. In the same time, Fig. 5 also shows that co-presence of SO2 induced a more pronounced positive effect than co4 presence of Cl. These observations are consistent with previous studies which have shown that Cl and SO2 4 may accelerate Fe(0) corrosion, by forming soluble complexes with Fe(II); this way, Fe(II)
Fig. 5. Effect of nature and concentration of co-present anion on Cr(VI) removal with Fe(0).
will be carried away from the metal surface (Traubenberg and Foley, 1971) and the formation of oxide scales on Fe(0) will be et al., 2015). Therefore, our results are retarded (Tepong-Tsinde attributable to higher rates of Fe(0) corrosion induced by the copresence of the two anions, hence, to increased concentrations of soluble Fe(II) in solution. In order to investigate whether Fe(0) corrosion was really enhanced in the co-presence of Cl and SO2 4 , the final pH was analyzed at the end of experiments with and without Cl and SO2 4 . It was noticed that final pH was greater for experiments conducted in co-presence of SO2 4 and Cl than for 2 experiments in the absence of SO4 and Cl (data not shown). Since the processes of Fe(0) corrosion and of Cr(VI) reduction (Eqs. (2), (5) and (6)) take place with consumption of protons and increase of pH, it may be concluded that Fe(0) corrosion was more intense when SO2 4 and Cl were added in the system. These results emphasize the importance of Cr(VI) indirect reduction with Fe(II) within the process of Cr(VI) removal with Fe(0). All these observations are in concordance with the results of kinetic modeling, which reveals that Cr(VI) removal becomes more rapid in the presence of Cl and SO2 4 (Table S5 and Figs. S25eS29, Supplementary material). Based on regression analysis of kinetic data, it was concluded that the process was best described by the zero-order model, regardless of the nature and concentration of the co-anion. This indicates that, even in the co-presence of Cl and SO2 4 anions, the main removal mechanism was reduction of Cr(VI) to Cr(III). Normally, Cl and SO2 4 could compete with HCrO4 for the available adsorption sites on Fe(0). It was previously shown that, in multiple-ion mixtures, each anion added decreased Cr(VI) adsorption on amorphous iron oxyhydroxides (Zachara et al., 1997). In addition, SO2 4 could have a much stronger competitive effect than Cl because divalent anionic species are more strongly attracted to positively charged surface sites than monovalent species (Brown et al., 2001); therefore, when both monovalent and divalent anions are present in excess of the available surface sites, divalent anions dominate the sorption behavior (Brown et al., 2001). Accordingly, if heterogeneous reduction of HCrO 4 at Fe(0) surface would have been the dominant removal mechanism, then rates of Cr(VI) removal should decrease with increasing ionic strength given by Cl and SO2 4 , which hinder adsorption of HCrO 4 ; instead, we have noticed exactly the opposite: Cr(VI) removal increased in the presence of Cl and SO2 4 ; moreover, the higher the Cl and SO2 4 concentrations, the higher the removal rates. Therefore, even though Cl and SO2 co4
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presence has probably inhibited adsorption of HCrO 4 onto Fe(0), the overall process of Cr(VI) removal was enhanced. This reiterates the significance of the secondary reductant Fe(II) within the process of Cr(VI) removal with Fe(0). 4. Conclusion In this work, the variation of the kinetic model was used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). The reported results have shown that temperature and nature of Fe(0) can significantly affect both kinetics and mechanism of Cr(VI) removal with Fe(0). While over the temperature range of 20e33 C the kinetics was described by a zero-order model, at 6 C the Ho's pseudo second-order model exhibited the highest correlation with experimental data. With respect to the effect of the nature of Fe(0), it was found that rates of Cr(VI) removal increased in the order: nano-Fe(0) < microFe(0) < milli-Fe(0); additionally, it was also observed that kinetic rates deviated from the zero-order model for micro-Fe(0) and milliFe(0), to Ho's pseudo second-order model for nano-Fe(0). While zero-order kinetics indicates chemical reduction as main removal mechanism of Cr(VI), the Ho's pseudo second-order model suggest that adsorption was the predominant mechanism. Co-presence of 2 Cl and SO2 4 had a positive influence on Cr(VI) removal, with SO4 having a stronger enhancing effect than Cl. This effect was attributed to increased Fe(0) corrosion rates, recognizing thus the importance of Cr(VI) indirect reduction with Fe(II) within the mechanism of Cr(VI) removal with Fe(0). Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-0508. The manuscript was improved by the insightful comments of anonymous reviewers from JEMA. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2017.03.031. References Ai, Z., Cheng, Y., Zhang, L., Qiu, J., 2008. Efficient removal of Cr(VI) from aqueous solution with Fe and Fe2O3 core-shell nanowires. Environ. Sci. Technol. 42, 6955e6960. APHA, AWWA, WEF, 1995. Standard Methods for the Examination of Water and Wastewater, nineteenth ed. United Book Press, Inc., Baltimore. , S., 2016. Designing metallic iron Btatkeu, B.D., Tchatchueng, J.B., Noubactep, C., Care based water filters: light from methylene blue discoloration. J. Environ. Manage. 166, 567e573. Brown Jr., G.E., Chambers, S.A., Amonette, J.E., Rustad, J.R., Kendelewicz, T., Liu, P., Doyle, C.S., Grolimund, D., Foster-Mills, N.S., Joyce, S.A., Thevuthasan, S., 2001. Interaction of water and aqueous chromium ions with iron oxide surfaces. In: Eller, P.G., Heineman, W.R. (Eds.), Nuclear Site Remediation e First Accomplishments of the Environmental Management Science Program, pp. 212e246. Cantrell, K.J., Kaplan, D.I., Wietsma, T.W., 1995. Zero-valent iron for the in situ
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Please cite this article in press as: Gheju, M., et al., A kinetic approach on hexavalent chromium removal with metallic iron, Journal of Environmental Management (2017), http://dx.doi.org/10.1016/j.jenvman.2017.03.031
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Research article
A kinetic approach on hexavalent chromium removal with metallic iron M. Gheju a, *, I. Balcu b, A. Enache a, A. Flueras a a
Politehnica University Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, Bd. V. Parvan Nr. 6, 300223, Timisoara, Romania National Institute for Research and Development in Electrochemistry and Condensed Matter, Str. Dr. Aurel Paunescu Podeanu Nr. 144, 300587, Timisoara, Romania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 9 March 2017 Accepted 12 March 2017 Available online xxx
This paper examines the mechanism of Cr(VI) removal with Fe(0), and the possible effect of various experimental parameters, from a kinetic perspective. The experimental data was analyzed using five different kinetic models: three for chemical reactions and two for adsorption processes. It was found that the process fitted well to the zero-order kinetic model for all investigated systems, excepting experiments conducted at 6 C and those with nano-Fe(0), when the process followed the Ho's pseudo secondorder model. Therefore, even though, under acidic conditions, chemical reduction can be generally considered as the main mechanism of Cr(VI) removal with Fe(0), under some experimental conditions (e.g. when working with nano-Fe(0) or at low temperatures), adsorption seems to be the dominant removal path. The enhanced Cr(VI) removal noticed in co-presence of SO2 4 and Cl anions reiterates the significance of the secondary reductant Fe(II) within the process of Cr(VI) removal with Fe(0). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Hexavalent chromium Metallic iron Water pollution Adsorption Reduction
1. Introduction Nowadays, contamination of water environments has become a significant concern, especially in the industrialized countries, due to increasing anthropogenic inputs after the industrial revolution (Chrysochoou and Dermatas, 2015). Because metallic iron (Fe(0)) is a relatively low cost material, readily available, with low toxicity (Btatkeu et al., 2016), important efforts have been focused on the use of Fe(0) for the removal of a wide range of pollutants, both inorganic (e.g. heavy metals (Hashim et al., 2011), metalloids (Vitkova et al., 2017)) and organic (e.g. dyes (Raman and Kanmani, 2016), phenols (Nakatsuji et al., 2015), estrogens (Jarosova et al., 2015)). Heavy metals are particularly problematic contaminants because they are highly toxic, non-biodegradable, and persistent (Pehlivan and Altun, 2008). Chromium is an important metal with widespread use in various industries; as a result, large quantities of this metal have been discharged into the environment due poor storage practices, improper disposal or leakage of chromium waste. In natural environments, chromium can exist mainly in two oxidation states: (þIII) and (þVI). Among these two, Cr(VI) exerts the most toxic effects on living organisms, having also the highest
* Corresponding author. E-mail address: [email protected] (M. Gheju).
mobility in the environment (Gheju, 2011, and references therein). Over the last decades, Fe(0) has been demonstrated to represent a highly efficient reagent for the removal of Cr(VI) from contaminated waters; however, there is yet no consensus at this time in what regards the mechanism of Cr(VI) removal with Fe(0). The first mechanism, proposed in the nineties (the ”reductive precipitation” mechanism) (Cantrell et al., 1995), and widely accepted until our days (Kong et al., 2016), attributed the efficiency of Fe(0)-systems mainly to the direct electron transfer from Fe(0) surface to Cr(VI), coupled with (co-)precipitation of resulted Cr(III). It was probably suggested in agreement with the direct reductive dechlorination mechanism, previously proposed as the most likely cause of chlorinated organics removal with Fe(0) (Gillham and O'Hannesin, 1994). Subsequent studies have, however, acknowledged the importance of another process, Cr(VI) adsorption, as intermediate step within the mechanism of Cr(VI) removal with Fe(0) (Powell et al., 1995). Moreover, it has been indicated that adsorption on some types of Fe(0) (e.g. nano-sized) can be regarded not only as intermediate step, but also as a dominant Cr(VI) removal mechanism by itself (Ai et al., 2008). Recent studies also suggested that, in Fe(0)-H2O systems, along with co-precipitation (Noubactep, 2015a) and size-exclusion (Yoon et al., 2011), adsorption is one of the main contaminant removal mechanisms, while reduction, when possible, occurs mainly indirectly via Fe(0) corrosion products
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(Noubactep, 2015b). Even though numerous studies investigated the kinetics of Cr(VI) removal with Fe(0) (Gheju, 2011, and references therein), to authors knowledge, the assessment of the kinetic model was not yet used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). Therefore, the goal of the present paper was to investigate the importance of different possible removal paths within the mechanism of Cr(VI) removal with Fe(0), as well as the effect of several important parameters, by means of kinetic analysis of experimental data. 2. Materials and methods Commercially available Fe(0) from Alfa Aesar (99%, ~1e2 mm) and from Merck (99%, ~10 mm) (hereinafter referred to as milliFe(0) and micro-Fe(0), respectively) was used as received. In addition, nano-Fe(0) was synthesized via the liquid-phase reduction method with sodium borohydride, following a procedure described by Xi et al. (2010). Cr(VI) removal experiments were carried out in a 1.5 L Berzelius flask, by introducing a mass of 0.5 g Fe(0) into 1000 mL of Cr(VI) solution. The mixture was stirred (200 rpm) using an overhead Heidolph stirrer and, at preset intervals, samples were withdrawn and filtered. Cr(VI) concentration in the filtrate was analyzed by the 1,5-diphenylcarbazide colorimetric method at 540 nm (APHA, 1995), using a Specord 200 PLUS spectrophotometer. The pH of the solutions was set before the experiments by adding small amounts of concentrated H2SO4 and measured using an Inolab 7320 pH-meter. The morphology of the synthesized nano-Fe(0) was characterized by the use of transmission electron microscopy (TEM), using a FEI e Titan G2 80e200 microscope. The kinetics of Cr(VI) removal was evaluated using five kinetic equations: three suitable for chemical reactions and two for adsorption processes, as shown in the Supplementary material; in addition, the intraparticle diffusion model was also applied when adsorption was found to be the dominant Cr(VI) removal path (Supplementary material). 3. Results and discussion 3.1. Effect of pH The influence of solution pH was investigated at 20 C, within the range of 1.1e3.5, using a 2 mg/L Cr(VI) solution and micro-Fe(0). It is shown that Cr(VI) removal significantly decreased with
increasing pH, being already almost totally inhibited at pH 3.1 (Fig. 1). This observation can be ascribed to involvement of Hþ ions in processes contributing to Cr(VI) removal in Fe(0)-H2O system. Cr(VI) removal with Fe(0) is the result of a complex interplay of processes such as adsorption, reduction and (co-)precipitation (Gheju, 2011). However, under the experimental conditions of the present study (acidic pH), the (co-)precipitation process can be excluded. Hence, in our case, only three pathways may be taken under consideration for the removal of Cr(VI) with Fe(0). The first one is adsorption of Cr(VI) on Fe(0), or onto oxide layers existent at Fe(0) surface (Geen et al., 1994): þ > Fe OH þ CrO2 4 þ H ⇔ > FeCrO4 þ H2 O
(1)
The second pathway is the heterogeneous Cr(VI) reduction, which involves direct electron transfer from Fe(0) surface, or from corrosion products containing Fe(II) existent at Fe(0) surface (Gheju, 2011): 0 þ 2þ 2HCrO þ 2Cr3þ þ 8H2O 4 þ 3Fe þ 14H / 3Fe
(2)
3[FeII4FeIII2(OH)12][SO4$3H2O] þ 4HCrO 4 þ 5H2O / 16 [Fe0,75Cr0,25](OH)3 þ 2Hþ þ 3SO2 4 þ 6Fe(OH)3
(3)
þ 3FeIIFeIII2O4 þ HCrO 4 þ 14H2O þ H / 4 [Fe0,75Cr0,25](OH)3 þ 6Fe(OH)3
(4)
The third pathway is the homogenous Cr(VI) reduction by products of Fe(0) corrosion (Fe2þ and H2) (Gheju, 2011): 2þ þ 7Hþ / 3Fe3þ þ Cr3þ þ 4H2O HCrO 4 þ 3Fe
(5)
þ 3þ 2HCrO þ 8H2O 4 þ 3H2 þ 8H / 2Cr
(6)
It is obvious that, for most of these reactions, an increase in Hþ concentration will lead to an increase in efficiency of Cr(VI) removal ^telier's principle). In the same time, pH is an important (Le Cha parameter controlling adsorption of metal ions, due to its influence on adsorbent surface properties and on ions speciation in solution (Jain et al., 2009). Because adsorption of Cr(VI) is the first step of the heterogeneous reduction mechanism, it is clear that, when discussing the effect of pH on Cr(VI) removal with Fe(0), the influence of pH on the adsorption phase should also be considered. Since surface of Fe(0) corrosion products is mostly positively charged, they are good adsorbents for Cr(VI) anions (Noubactep, 2015a). With increasing concentration of Hþ ions in solution, the number of positively charged centers at Fe(0) surface also increases; as a result, the electrostatic forces of attraction which act between Cr(VI) anions and positively charged Fe(0) surface will also increase. In the same time, the competition for positively charged sites between Cr(VI) anions and HO ions diminishes with decreasing pH, as a result of decreasing HO concentration. The kinetic modeling of experimental data showed that the zero-order kinetic model provided the best match (Table S1 and Figs. S5eS9, Supplementary material), indicating reduction of Cr(VI) to Cr(III) as the main removal mechanism. This is in agreement with previous studies which have shown that, under acidic conditions, chemical reduction was the main pathway of Cr(VI) removal with Fe(0) (83.3%), and dissolved Fe(II) was the major reductant; nevertheless, adsorption was also responsible for the removal of a small amount of Cr(VI) (16.7%) (Gheju et al., 2016). 3.2. Effect of Cr(VI) concentration
Fig. 1. Effect of pH on Cr(VI) removal with Fe(0).
The effect of this parameter was explored at 20 C, by reacting
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micro-Fe(0) with Cr(VI) solutions with pH 2.1 and an initial concentration that was varied from 1 to 5 mg/L. It was noted that removal efficiency of Cr(VI) decreased upon increasing initial concentration of Cr(VI) (Fig. 2). The modeling of kinetic data showed that zero-order equation was the more appropriate, regardless of the value of initial Cr(VI) concentration (Table S2 and Figs. S10eS14, Supplementary material). Thus, the main mechanism of Cr(VI) removal, reduction to Cr(III), was not affected by the change in Cr(VI) concentration. The observed results can be ascribed to the decrease of the ratio Fe(0) surface area/mass of Cr(VI) with increasing initial concentration of Cr(VI). Less Fe(0) surface available in the system will adversely affect the efficiency of the two mechanisms that contribute to Cr(VI) removal (adsorption and reduction), due to the increase in the number of HCrO 4 ions competing for the available reactive sites on the surface of Fe(0). Hence, the higher the initial concentration of Cr(VI), the faster the saturation of Fe(0) surface. The decrease of Cr(VI) removal with increasing its concentration was also attributed to lower rates of Fe(0) corrosion at higher Cr(VI) concentrations (Melitas et al., 2001). Fig. 3. Effect of temperature on Cr(VI) removal with Fe(0).
3.3. Effect of temperature The effect of temperature was studied in the range of 6e33 C, by using a 2 mg/L Cr(VI) solution with pH 2.5, and micro-Fe(0). It was revealed that removal of Cr(VI) with Fe(0) was favored by an increase in temperature (Fig. 3), indicating an endothermic nature of the process. However, the most notable improvement in Cr(VI) removal was achieved by increasing temperature from 20 to 26 C; a further increase of temperature to 33 C led only to a minor enhancement. Temperature had an important influence not only on efficiency of Cr(VI) removal, but also on the kinetics of the process. The kinetic modeling of experimental data showed that, while the zero-order kinetic model provided the best match over the temperature range of 20e33 C, at 6 C the Ho's pseudo second-order equation was the more appropriate (Table S3 and Figs. S15eS19, Supplementary material). Since this equation is a kinetic model developed for sorption processes (Ho et al., 2000), it may indicate that, at 6 C, Cr(VI) reduction was significantly inhibited and the major pathway that contributed to removal of Cr(VI) with Fe(0) was adsorption. The results of Weber and Morris modeling (Table S6 and Fig. S30, Supplementary material) reveal that, in spite of a relative good linearity of the plots, they do not pass through the
Fig. 2. Effect of initial Cr(VI) concentration on Cr(VI) removal with Fe(0).
origin. This inadequacy of the intraparticular diffusion model implies that a combined effect of the film diffusion and intraparticle diffusion mechanisms is probable (Ho et al., 2000). Even though Fe(0) particles are not porous, they have, however, a degree of roughness on their surfaces. Therefore, while intraparticular diffusion is possible, it is reduced and limited only to the external thin layer of oxides existent at Fe(0) surface.
3.4. Effect of Fe(0) size In order to study the influence of Fe(0) size, three different types of Fe(0) were reacted at 20 C with a 2 mg/L Cr(VI) solution having pH 2.5: milli-Fe(0), micro-Fe(0), and nano-Fe(0). The synthesized nano-Fe(0) has a size in the range of 5e50 nm (Figs. S1eS4, Supplementary material). The present experiments have shown that efficiency of Cr(VI) removal with Fe(0) followed the series: milli-Fe(0) > micro-Fe(0) > nano-Fe(0) (Fig. 4), in good agreement with previous findings of Comba et al. (2011). It is generally accepted that the smaller the particle size, the larger the specific
Fig. 4. Effect of Fe(0) type on Cr(VI) removal with Fe(0).
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surface area and the greater the Fe(0) reactivity (Gheju, 2011). If this assumption would have been valid also in our study, than Cr(VI) removal efficiency should decrease in the order: nanoFe(0) > micro-Fe(0) > milli-Fe(0). In fact, we did notice this trend, but only during the first minute of the experiment. After the first minute, the mixing was stopped for about 10 s in order to extract the first sample; afterwards, the mixing was resumed and this procedure was repeated several times during the course of the experiment. These results may be justified by the tendency of Fe(0) powders to agglomerate, behavior that increases with decreasing particle size (Walter, 2013). This is in accord with previous reports indicating that some Fe(0) powders were less reactive than Fe(0) fillings, as a results of agglomeration (Noubactep et al., 2009). On the other hand, because of their higher reactivity, nano-Fe(0) particles have also a lower selectivity, and thus, a higher tendency to react with non-target substances, including dissolved oxygen and water (Tratnyek and Johnson, 2006). Fig. 4 also shows that Cr(VI) removal with nano-Fe(0) proceeds in two stages: high rates are observed within the first time interval, when most of the Cr(VI) was removed, whereas a strong decrease in the Cr(VI) removal occurs in the second one, when the process gradually saturate up, reaching equilibrium. This observation is typical for adsorption processes and corroborates the results of experimental data kinetic modeling which indicate that, for nano-Fe(0), the Ho's pseudo second-order model fitted better among all the investigated kinetic models (Table S4 and Figs. S20eS24, Supplementary material). Instead, for micro-Fe(0) and milli-Fe(0) the zero-order kinetic model provided the highest correlation coefficients. The results of Weber and Morris modeling (Table S6 and Fig. S31, Supplementary material) suggests, following a similar rationale to the one presented in Section 3.3, that both intraparticular and film diffusion are factors that control Cr(VI) adsorption on nano-Fe(0). Consequently, it seems that adsorption was the main mechanism of Cr(VI) removal with nano-Fe(0); this is in accordance with previous reports who indicated that adsorption of Cr(VI) onto nano-Fe(0) wires was the dominant removal path (Ai et al., 2008). In contrast, chemical reduction was the main pathway that contributed to removal of Cr(VI) with micro-Fe(0) and milli-Fe(0) under the experimental conditions applied in the present work. The observed results obtained for nano-Fe(0) can be ascribed to presence of iron oxides at surface of nano-Fe(0), which are known to be very good adsorbents for negatively charged pollutants (Noubactep, 2015b). Even though the freshly-obtained nano-Fe(0) was washed with ethanol and stored under vacuum, its color changed rapidly from black to brown as a result of Fe(0) oxidation and generation of iron oxides (Fig. S3, Supplementary material). 3.5. Effect of nature and concentration of co-present anions 2þ 2 The effect of numerous ions (Ca2þ, Mg2þ, HCO 3 , CO3 , Fe , Cu2þ) has been investigated up to this day (Gheju, 2011, and references therein); however, to the best of our knowledge, the influence of two important anions, commonly found in natural water environments, namely SO2 4 and Cl , was not studied yet. In order to research the effect of these anions, Cr(VI) solution 2 mg/L with pH 2.5 was reacted with micro-Fe(0) at room temperature (20 C), in the co-presence of 0.01 and 0.05 M NaCl and Na2SO4 as background electrolytes. As revealed by Fig. 5, the addition of Cl and SO2 4 led to an increase in Cr(VI) removal efficiency; the higher the concentration of co-present anion, the higher the efficiency of Cr(VI) removal. In the same time, Fig. 5 also shows that co-presence of SO2 induced a more pronounced positive effect than co4 presence of Cl. These observations are consistent with previous studies which have shown that Cl and SO2 4 may accelerate Fe(0) corrosion, by forming soluble complexes with Fe(II); this way, Fe(II)
Fig. 5. Effect of nature and concentration of co-present anion on Cr(VI) removal with Fe(0).
will be carried away from the metal surface (Traubenberg and Foley, 1971) and the formation of oxide scales on Fe(0) will be et al., 2015). Therefore, our results are retarded (Tepong-Tsinde attributable to higher rates of Fe(0) corrosion induced by the copresence of the two anions, hence, to increased concentrations of soluble Fe(II) in solution. In order to investigate whether Fe(0) corrosion was really enhanced in the co-presence of Cl and SO2 4 , the final pH was analyzed at the end of experiments with and without Cl and SO2 4 . It was noticed that final pH was greater for experiments conducted in co-presence of SO2 4 and Cl than for 2 experiments in the absence of SO4 and Cl (data not shown). Since the processes of Fe(0) corrosion and of Cr(VI) reduction (Eqs. (2), (5) and (6)) take place with consumption of protons and increase of pH, it may be concluded that Fe(0) corrosion was more intense when SO2 4 and Cl were added in the system. These results emphasize the importance of Cr(VI) indirect reduction with Fe(II) within the process of Cr(VI) removal with Fe(0). All these observations are in concordance with the results of kinetic modeling, which reveals that Cr(VI) removal becomes more rapid in the presence of Cl and SO2 4 (Table S5 and Figs. S25eS29, Supplementary material). Based on regression analysis of kinetic data, it was concluded that the process was best described by the zero-order model, regardless of the nature and concentration of the co-anion. This indicates that, even in the co-presence of Cl and SO2 4 anions, the main removal mechanism was reduction of Cr(VI) to Cr(III). Normally, Cl and SO2 4 could compete with HCrO4 for the available adsorption sites on Fe(0). It was previously shown that, in multiple-ion mixtures, each anion added decreased Cr(VI) adsorption on amorphous iron oxyhydroxides (Zachara et al., 1997). In addition, SO2 4 could have a much stronger competitive effect than Cl because divalent anionic species are more strongly attracted to positively charged surface sites than monovalent species (Brown et al., 2001); therefore, when both monovalent and divalent anions are present in excess of the available surface sites, divalent anions dominate the sorption behavior (Brown et al., 2001). Accordingly, if heterogeneous reduction of HCrO 4 at Fe(0) surface would have been the dominant removal mechanism, then rates of Cr(VI) removal should decrease with increasing ionic strength given by Cl and SO2 4 , which hinder adsorption of HCrO 4 ; instead, we have noticed exactly the opposite: Cr(VI) removal increased in the presence of Cl and SO2 4 ; moreover, the higher the Cl and SO2 4 concentrations, the higher the removal rates. Therefore, even though Cl and SO2 co4
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presence has probably inhibited adsorption of HCrO 4 onto Fe(0), the overall process of Cr(VI) removal was enhanced. This reiterates the significance of the secondary reductant Fe(II) within the process of Cr(VI) removal with Fe(0). 4. Conclusion In this work, the variation of the kinetic model was used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). The reported results have shown that temperature and nature of Fe(0) can significantly affect both kinetics and mechanism of Cr(VI) removal with Fe(0). While over the temperature range of 20e33 C the kinetics was described by a zero-order model, at 6 C the Ho's pseudo second-order model exhibited the highest correlation with experimental data. With respect to the effect of the nature of Fe(0), it was found that rates of Cr(VI) removal increased in the order: nano-Fe(0) < microFe(0) < milli-Fe(0); additionally, it was also observed that kinetic rates deviated from the zero-order model for micro-Fe(0) and milliFe(0), to Ho's pseudo second-order model for nano-Fe(0). While zero-order kinetics indicates chemical reduction as main removal mechanism of Cr(VI), the Ho's pseudo second-order model suggest that adsorption was the predominant mechanism. Co-presence of 2 Cl and SO2 4 had a positive influence on Cr(VI) removal, with SO4 having a stronger enhancing effect than Cl. This effect was attributed to increased Fe(0) corrosion rates, recognizing thus the importance of Cr(VI) indirect reduction with Fe(II) within the mechanism of Cr(VI) removal with Fe(0). Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-0508. The manuscript was improved by the insightful comments of anonymous reviewers from JEMA. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2017.03.031. References Ai, Z., Cheng, Y., Zhang, L., Qiu, J., 2008. Efficient removal of Cr(VI) from aqueous solution with Fe and Fe2O3 core-shell nanowires. Environ. Sci. Technol. 42, 6955e6960. APHA, AWWA, WEF, 1995. Standard Methods for the Examination of Water and Wastewater, nineteenth ed. United Book Press, Inc., Baltimore. , S., 2016. Designing metallic iron Btatkeu, B.D., Tchatchueng, J.B., Noubactep, C., Care based water filters: light from methylene blue discoloration. J. Environ. Manage. 166, 567e573. Brown Jr., G.E., Chambers, S.A., Amonette, J.E., Rustad, J.R., Kendelewicz, T., Liu, P., Doyle, C.S., Grolimund, D., Foster-Mills, N.S., Joyce, S.A., Thevuthasan, S., 2001. Interaction of water and aqueous chromium ions with iron oxide surfaces. In: Eller, P.G., Heineman, W.R. (Eds.), Nuclear Site Remediation e First Accomplishments of the Environmental Management Science Program, pp. 212e246. Cantrell, K.J., Kaplan, D.I., Wietsma, T.W., 1995. Zero-valent iron for the in situ
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Please cite this article in press as: Gheju, M., et al., A kinetic approach on hexavalent chromium removal with metallic iron, Journal of Environmental Management (2017), http://dx.doi.org/10.1016/j.jenvman.2017.03.031