J. Cent. South Univ. (2018) 25: 2119-2130

DOI: https://doi.org/10.1007/s11771-018-3901-x

Reduction of Cr(VI) with a relative high concentration using different kinds of zero-valent iron powders: Focusing on effect of carbon content and structure on reducibility

LV Jin-fang(吕晋芳)^{1, 2}, TONG Xiong(童雄)^{1, 2}, ZHENG Yong-xing(郑永兴)², XIE Xian(谢贤)^{1, 2}, HUANG Ling-yun(黄凌云)^{1, 2}

1. Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China;

2. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization

(Kunming University of Science and Technology), Kunming 650093, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Reduction of Cr(VI) using zero-valent iron (ZVI) could not only decrease the amounts of chemicals used for reduction, but also decrease the discharge of sludge. In order to find a desirable ZVI material, reduction of Cr(VI) with a relative high concentration using different kinds of ZVI powders (mainly carbon differences) including reduced Fe, grey cast iron, pig iron, nodular pig iron was carried out. Parameters such as ZVI dosage, type and size affecting on Cr(VI) reduction were firstly examined and grey cast iron was selected as a preferable reducing material, followed by pig iron. Additionally, it was found that the parameters had significant influences on experimental kinetics. Then, morphology and composition of the sample before and after reaction were characterized by SEM, EPMA and XPS analyses to disclose carbon effect on the reducibility. In order to further interpret reaction mechanism, different reaction models were constructed. It was revealed that not only the carbon content could affect the Cr(VI) reduction, but also the carbon structure had an important effect on its reduction.

Key words: relative high concentration Cr(VI); reduction; ZVI powder; carbon content; carbon structure

Cite this article as: LV Jin-fang, TONG Xiong, ZHENG Yong-xing, XIE Xian, HUANG Ling-yun. Reduction of Cr (VI) with a relative high concentration using different kinds of zero-valent iron powders: Focusing on effect of carbon content and structure on reducibility [J]. Journal of Central South University, 2018, 25(9): 2119–2130. DOI: https://doi.org/10.1007/s11771-018-3901-x.

1 Introduction

Chromium is a considerable toxic inorganic contaminant and generally entered water body from various industrial processes, such as electroplating, leather tanning, textile dying, metallurgy and mineral processing [1, 2]. As a transition metal, it mainly exists in the form of oxidation states of Cr(VI) and Cr(III) in the natural environment [3, 4]. Cr(VI) compounds such as chromate and bichromate present a high solubility and mobility in the environment and have shown exert toxic effects to human, animals and plants. On the contrary, Cr(III) is less toxic, less mobile and even viewed as an essential nutrient for humans and animals [5].However, exposure to excessive dosage of Cr(III) for a long period can also cause some adverse health effects. Thus, removal of Cr(VI) from industrial effluents polluted prior to discharge is a key step in the pollution control of water body.

Cr(VI) can be removed from aqueous solution by many technologies such as chemical reduction to Cr(III) followed by precipitation [6], adsorption [7], ion exchange [8] and biological separation process [9]. It is well known that the reduction-precipitation process is the most convenient and effective technique. However, this treatment method needs consuming considerable amounts of chemicals used for reduction, e.g., Fe₂SO₄·7H₂O and FeCl₂·6H₂O, and precipitation, resulting in a high running cost. It is more terrible that significant amounts of sludge are generated during this process, which exhibits a potential risk of secondary pollution [10].

In recent years, there has been great interest in using iron-based materials that can substitute traditional ferrous salts. These materials included ZVI, nanoscale zero-valent iron (nZVI) and ZVI coupling with a noble metal. It seems that nZVI has received the most extensive attentions due to its excellent reactivity [11]. However, nZVI particles were easier to be attracted to each other and aggregated into larger particles, which reduced their mobility in water and their effective surface areas. Also, ZVI coupling with a noble metal could enhance the Cr(VI) reduction [12]. In the literature available, they mainly focused on the degradation of organic compounds and few of them involved the reduction of Cr(VI) [13].

Overall, ZVI powder was still a promising reduction reagent ascribed to lower cost and easier availability, especially for the waste iron materials [14–16]. However, most of these studies were conducted under low concentrations of chromate. The results obtained might be unable to be applied to treatment of wastewater with a relative high concentration Cr(VI). On the other hand, the reduction differences caused by ZVI itself could not be clearly illustrated, especially for the waste iron materials, which were produced by cutting various billets in mechanical processing plant. Factually, the differences for most of these billets were mainly attributed to the carbon content and structure [17, 18]. It was known that the iron and carbon could form a primary battery in the electrolyte solution. The products of electrode have high activity and cause redox reaction with many components [19]. If the effect of carbon in the ZVI on the Cr(VI) reduction was revealed, the preferable ZVI material would be selected, which was advantageous to enhance the Cr(VI) reduction.

In this work, reduction of Cr(VI) with a relative high concentration using four types of ZVI powders was carried out. They are reduced iron powder nearly containing no carbon and other three kinds of iron powders such as grey cast iron, pig iron and nodular pig iron with similar carbon content (about 3.5%) but with different carbon structures. Firstly, a preferable reduction reagent was selected by examining the Cr(VI) reducibility. Then, morphology changes and surface composition transformations for the ZVI powders before and after reactions were detected by scanning electron microscope (SEM), electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) analyses to disclose carbon effect on the reducibility and finally, different reaction models mainly depending on the differences of carbon content and structure were constructed to further interpret their reaction mechanism. It was believed that this study can provide an excellent reference for seeking existing waste iron material or preparing new iron-based material behaving excellent reducibility.

2 Experimental section

2.1 Materials

Four types of ZVI samples including grey cast iron (represented by HT250), pig iron (represented by Z18), nodular pig iron (represented by Q10) and reduced Fe powder were purchased from a Metal Materials Co., Ltd in Guangdong Province, China. Three of them were strips and the reduced iron was powder with a size less than 74 μm. These strips were separately processed on a lathe and significant amounts of fresh scrap irons were obtained. Then, these fresh particles were grinded to size ranges of <0.074 mm (100%), 0.23–0.71 mm (100%) and 0.71–2.00 mm (100%) in a planetary mill used for Cr(VI) reduction. Their chemical compositions are shown in Table 1, which indicates high Fe and C content samples, except reduced iron powder.

Potassium dichromate (K₂Cr₂O₇), sulfuric acid (H₂SO₄) and other chemical reagents used in this work were of AR grade. Cr(VI) stock solution (10 g/L) was prepared by dissolving 28.29 g of K₂Cr₂O₇ in 1 L of deionised water. Initial solutions containing 100 mg/L of Cr(VI) used for batch experiments were obtained by diluting the stock solution.

2.2 Experimental methods

Batch experiments were carried out in a 0.5 L beaker containing 300 mL of initial solution open to atmosphere. The pH of the standard solutions was adjusted to 2.0 by adding dilute H₂SO₄ solution prior to reduction. This pH is nearly in the effluent pH range for most of electroplating wastewaters [15, 20]. ZVI powders were added in different theoretical ratios and the mixed solution was stirred at a speed of 500 r/min, which was determined by observing the suspended state of ZVI particles. Samples were periodically collected and then separated in a centrifuge, waiting for analyses of Cr(VI) concentration.

2.3 Analytical techniques

The prepared supernatant was used for analyses of Cr(VI) concentration by diphenylcarbazide colorimetric method. The purple was fully displayed after 10 min and part of solutions were transformed to a 1 cm glass cell, followed by a measurement on a JINHUA (JH) 752 spectrophotometer at a wavelength of 540 nm. SEM (JEOL.Ltd, JSM-6360LV, 20 kV) and EPMA (JXA- 8230) were used to determine the morphology and composition of the ZVI powder before and after reaction. The acceleration voltage of the EPMA was set to 20 kV and the probe current to 20 nm using a focused electron beam. XPS (PHI5000 ULVAC- PHI, Japan) equipped with a monochromatic Al K_a X-ray source at 1486.6 eV were applied to identifing the surface components of the sample after reaction.

Table 1 Chemical composition of four kinds of ZVI materials (Mass fraction, %)

3 Results and discussion

3.1 Influence of ZVI amount

The Cr(VI) reduction was nearly instantaneous and little Cr(VI) was detected at a pH of 2.1 while great amounts of the scrap irons were added (mass ratio of Cr(VI)/iron=1:3333.33) [21]. However, it seemed that the effect of ZVI dosage on the Cr(VI) reduction was not illustrated clearly. So it was initially investigated to select a moderate dosage of ZVI powder, as shown in Figure 1 and Table 2. It can be seen that the reaction rate increases with increasing amount of Z18 (Figure 1(a)), ascribed to the fact that the more particle was added under the same particle size, the more reactive surface was provided [22].

Experimental kinetic results evaluated by the integral method indicated first-order kinetics at a pH of 2.0 (Figure 1(b)) and the apparent rate constant (k_obs) increases with increasing the ZVI dosage (Table 2). It appeared that this result was not consistent with the previous studies, which have reported the zero-order kinetic [21, 23]. Factually, it can be deduced that the result will be close to the zero-order kinetic when the amount of Z18 increased above the given dosage (n(Fe)/n(Cr(VI))=40), which was in accordance with the literatures mentioned. On the other hand, a long reaction time was needed when a small amounts of Z18 powder (n(Fe)/n(Cr(VI))=10) was added. Accordingly, the desirable mole ratio of ZVI/Cr(VI) was determined to be in the range of 20–40 for this study.

3.2 Influence of ZVI type and size

The effect of ZVI powder type on the Cr(VI) reduction was investigated under these conditions: initial Cr(VI) concentration of 100 mg/L, mole ratio of 20 (ZVI/Cr(VI)), room temperature and pH of 2.0. In addition, particle size also significantly affected its reduction [14, 24]. So they were simultaneously examined and the results are shown in Figure 2 and Table 3. It can be observed that the reduction reaction ended until the time increased to 80 min for the three types of materials at size fraction of 0.71–2.00 mm, and the reduction rate for the HT250 powder was faster than the others (Figure 2(a)). The time of reduction reaction decreased to 50–60 min for the three ones when their size decreased to 0.23–0.71 mm and the HT250 sample still exhibited excellent reducibility than the others, especially for the Q10 sample (Figure 2(b)). With further decreasing particle size less than 0.074 mm (Figure 2(c)), reaction time decreased to within 7 min for them and it was obvious that the HT250 sample behaved preferable reducibility.

Figure 1 Cr(VI) concentration (a) and lnCr(VI) (b) vs time at different dosages of Z18 powder (C₀: 100 mg/L; size fraction: 0.23–0.71 mm; pH: 2.0; room temperature)

Table 2 k_obs and R² values for experimental data

Experimental kinetic results obtained by regression analyses revealed first-order kinetics at a pH of 2.0 when particle size was fixed in the range of 0.23–2.00 mm and regardless of any ZVI material (Figures 2(a′) and (b′)). When particle size decreased to less than 0.074 mm, zero-order kinetics was presented (Figure 2(f)). Combining with the apparent rate constant (k_obs) obtained (Table 3), it can be concluded that the preferable ZVI material was the HT250, followed by the Z18.

3.3 Morphology and composition changes

3.3.1 SEM analyses

Figure 3 shows the BSE images of different kinds of ZVI powders before and after reduction at a size less than 0.074 mm. It can be seen that the HT250 occurred as flaky particles, but microstructure of graphite was not obvious (Figure 3(a)). Both Q10 and Z18 sample existed in the form of irregular polyhedron and graphite was distributed in their iron particles. By comparison, it was found that the graphite in Q10 and Z18 samples occurred as spheres and flakes, respectively. Additionally, the graphite seemed to be more evenly scattered for the Z18 sample, resulting in more galvanic cells formed. This may explain the differences of their reducibility (Figure 2(b)). The reduced Fe powders occurred as spherical grains and little graphite was observed, which agreed with the chemical composition (Table 1). After reaction with Cr(VI) (Figures 3(a′), (b′), (c′) and (d′)), it can be seen that some gray compounds covered on the surface of Fe powders and these compounds may be viewed as Fe–Cr oxy-hydroxides [25, 26], which would prevent the further Cr(VI) reduction. This observation was reasonable due to the final pH increasing to about 4.0.

3.3.2 EPMA analyses

In order to clarify the reaction mechanism for the HT250, EPMA measurements were carried out and mapping images before and after reduction of Cr(VI) are shown in Figure 4. From Figures 4(a) and (b), it can be seen that pearl-like graphite grains were filled in the matrix of the sample and parts of vermicular graphite were evolved. These observations were consistent with the previous literature reported [17, 18]. Dispersive graphite and ZVI formed infinite micro galvanic cells, which accelerated electrons transfer during corrosion of ZVI, resulting in the reduction rate of Cr(VI) enhanced (Figure 2). From Figures 4(c)–(f) and Table 4, it can be known that the Fe concentration decreased while the O and Cr concentration increased after reaction with Cr(VI). This can be explained that the precipitation of Fe and Cr ions occurred when the pH increased [25]. Additionally,it was observed that many graphite grains still presented their fresh surfaces, on which the Cr(VI) and H⁺ could continuously gain the electrons from inner iron, making the lifetime of the HT250 prolong. On the other hand, the hydrophilic Fe–Cr oxy-hydroxides would be difficult to deposit on the surface of graphite due to its strongly hydrophobic nature [27, 28]. In other words, the graphite could alleviate the passivation of ZVI to some extent.

Figure 2 Cr(VI) concentration (a–c) and lnc(Cr(VI)) (a′–c′) vs time under different ZVI types and different size fraction:

Table 3 k_obs and R² values for experimental data obtained under different ZVI powder types and different size fractions

Figure 3 BSE images of ZVI material before (a, b, c, d) and after (a′, b′, c′, d′) reactions with size less than 0.074 mm:

Figure 4 Series of elemental maps showing distribution of key elements for HT250 powders before (a, b) and after (c, d, e, f) reduction of Cr(VI) at size less than 0.074 mm

Table 4 Relative concentration of key elements for representative area in Figure 4 (Mass fraction, %)

3.3.3 XPS analyses

In order to further identify surface components of the HT250 sample after reaction, XPS measurements were carried out and the C 1s, Cr 2p_3/2, Fe 2p_3/2 and O 1s photoelectron spectra were obtained, as shown in Figure 5. It can be seen that the C1s peak was fitted into three separated peaks at 284.8, 286.08 and 288.77 eV, corresponding to carbon atoms in different functional groups: non-oxygenated ring C (C—C), C in C—O bonds (C—OH) and carbonyl C (C=O), which arised from hydrocarbon contamination or some compounds on the graphite surface [29, 30]. The Cr 2p_3/2 spectrum shows three well separated peaks at 576.93, 577.74 and 578.81 eV, attributing to the Cr (III) forming Cr–Fe oxides, Cr–Fe hydroxides and Cr₂(SO₄)₃, respectively [31, 32]. O 1s signals at 530.37, 531.35 and 532.35 eV were associated with Fe–Cr oxides, Fe–Cr hydroxides and Fe–Cr sulfates, respectively [33] and finally, two peaks at 711.23 and 713.38 eV were found to fit the Fe 2p_3/2 spectrum. Combining with the Cr 2p_3/2 and O 1s spectrums, it can be deduced that the one at 711.23 eV was assigned to FeO(OH) or Fe₂O₃ and the other at 713.38 eV was attributed to Fe₂(SO₄)₃ [34, 35]. The detected signals of Cr₂(SO₄)₃ and Fe₂(SO₄)₃ were ascribed to the pollution of sulfate radical.

Figure 5 Photoelectron spectra for HT250 after reaction:

3.4 Reaction mechanism for different types of ZVI materials

Schematic diagram of the Cr(VI) reduction using different ZVI materials was summarized in Figure 6 and their reactions were usually considered to be an electrochemical process, as listed in Eqs. (1)–(6) [11, 36]. The anodic process is Fe⁰ corrosion while the cathodic process is the liberation of H⁺ as hydrogen gas, Cr(VI) reduction as Cr(III) and oxygen reduction under aerobic conditions. In this study, H⁺ should be the dominant electron acceptor contributing to iron corrosion at the beginning due to the lower pH (Eq. (2)), followed by the interaction of Cr(VI) and Fe²⁺ generated (Eq. (4)) [14, 37]. With increasing reaction time, Cr(VI) became the dominant electron acceptor attributed to the pH increased (final pH: 4.0), as shown in Eq. (1) [14, 37]. Meantime, Fe²⁺ generated is very sensitive to molecular oxygen (O₂) and their reaction rates increased with elevating pH [38]. The resulting Fe³⁺ readily hydrolyzed, precipitated and transformed oxides (Eqs. (5) and (6)).

 (1)

                (2)

          (3)

 (4)

Figure 6 Reaction models of Cr (VI) reduction for different types of ZVI materials:

       (5)

     (6)

when reduced iron powders were used as reducing agent (Figure 6 (a)), the corrosion of ZVI could only occur at the surface of ZVI [36], which was known as heterogeneous reaction. This became the key step to control the rate of Cr(VI) reduction. Factually, the whole reaction for Cr(VI) reduction should be homogeneous due to the fact that the Fe²⁺ generated interacted with the Cr(VI) (Eq.(4)). With the reaction going on, large quantities of Fe²⁺ were generated and then, inhibited the approach of H⁺ on the surface of ZVI due to the electrostatic repulsion, resulting in the transfer rate of electron decreased. With further increasing amounts of Fe²⁺, Cr(VI) as chromate anion would be close to the surface of ZVI due to the electrostatic attraction and became the dominant electron acceptor, but considerable amounts of Fe–Cr oxy-hydroxides have already been generated due to the pH elevated, resulting in the interrupt of electron transfer. These explained the evolution of Cr(VI) concentration with increasing reaction time when the reduced Fe powders were used as reduction reagent(Figure 2(c)).

When carbon (graphite) was introduced into the pure ZVI, different kinds of galvanic cells were formed (Figures 6(b) and (c)), originating the differences of carbon structure (Figures 3(a)–(c) and 4). The electron transport occurred during the corrosion of ZVI not only from the ZVI surface, but also from its inner parts [39], which could be viewed as homogeneous reaction. In the beginning (pH=2.0), H⁺ could obtain electrons at the surface of graphite, alleviating the repulsion force generated by Fe²⁺ and even in later, Cr(VI) as the dominant electron acceptor could also obtain electrons at the surface of graphite. Obviously, transport rate of the electron for the homogeneous reaction was faster than the heterogeneous reaction during the corrosion of ZVI, making the whole reactions of Cr(VI) reduction accelerated. This explained the lower reducibility of reduced iron powder than other ZVI materials (Figures 2(c) and (c′)).

Comparing Figures 6(b) and (c), it could be known that more electrons would be transferred when the graphite and Fe⁰ formed matrix of the ZVI material (Figure 6(c)). This explained that the HT250 overall behaved the preferable reducibility than the other ZVI materials with similar carbon content (Figure 2). It seemed that the reducibility of Q10 was worse than Z18 (Figures 2(a) and (b)) and a possible explanation was that the spherical graphite in Q10 was easier to be completely liberated during preparing process than the flaky and dispersed graphite in Z18 (Figures 3(b) and (c)). It was understandable that the liberated spherical graphite was difficult to attach to the ZVI forming galvanic cell under the solution stirred.

4 Conclusions

1) ZVI dosage has an important effect on the reduction of Cr(VI) with a relative high concentration at a pH of 2.0 and the desirable mole ratio of ZVI powder to Cr(VI) is determined to be in the range of 20–40. Experimental kinetic results indicate that first-order kinetics would evolve into zero-order kinetics with increasing ZVI amount. Cr(VI) reduction is further carried out under different types of ZVI powders and different size fractions, and the HT250 is selected as a preferable reducing material, followed by Z18. Experimental kinetic results reveal first-order kinetics at a size fraction of 0.23–2.00 mm regardless of any ZVI powder. With decreasing the particle size less than 0.074 mm, zero-order kinetics is presented.

2) The reduced iron powder occurs as spherical grains and little graphite is observed. Both the Q10 and Z18 sample existed in the form of irregular polyhedron and the graphite occur as spheres and flakes, respectively. The flaky graphite is more evenly scattered for the Z18, resulting in more galvanic cells formed. Pearl-like graphite grains were embedded in the matrix of the HT250, forming infinite micro galvanic cells, which improves the reduction rate of Cr(VI). Additionally, the graphite could alleviate the passivation of ZVI ascribed to its strongly hydrophobic nature. After reaction, these compounds such as Fe–Cr oxides, Fe–Cr hydroxides, FeO(OH), Fe–Cr sulfates were formed on the surface of HT250.

3) Cr(VI) reduction using ZVI powder is considered to be an electrochemical process and different reaction models are constructed, originating from the differences of carbon content and structure. For the reducing material without carbon, the electron transport during the corrosion of ZVI occurred only on ZVI surface. But for the material containing carbon, especially for the HT250 sample, the electron could transport not only from the ZVI surface, but also from its inner parts, which accelerates the electrons transfer, resulting in the reduction rate of Cr(VI) improved.

References

[1] HEMAMBIKA B, KANNAN V R. Intrinsic characteristics of Cr⁶⁺-resistant bacteria isolated from an electroplating industry polluted soils for plant growth-promoting activities [J]. Applied Biochemistry and Biotechnology, 2012, 167: 1653–1667.

[2] LV Jin-fang, ZHANG Han-ping, TONG Xiong, FAN Chun-lin, YANG Wen-tao, ZHENG Yong-xing. Innovative methodology for recovering titanium and chromium from a raw ilmenite concentrate by magnetic separation after modifying magnetic properties [J]. Journal of Hazardous Materials, 2017, 325: 251–260.

[3] SHANKER A K, CERVANTES C, LOZA-TAVERA H, AVUDAINAYAGAM S. Chromium toxicity in plants [J]. Environment International, 2005, 31: 739–753.

[4] ZHENG Lin, ZHOU Kan, WEN Ben-de, QIN Zhi-rong, XU Rong-nian. Chromium and human health [J]. Trace Elements Science, 2003, 10: 11–15.

[5] MA J, YANG B, BYRNE R H. Determination of nanomolar chromate in drinking water with solid phase extraction and a portable spectrophotometer [J]. Journal of Hazardous Materials, 2012, 219–220: 247–252.

[6] PAPASSIOPI N, VAXEVANIDOU K, CHRISTOU C, KARAGIANNI E, ANTIPAS G S E. Synthesis, characterization and stability of Cr(III) and Fe(III) hydroxides [J]. Journal of Hazardous Materials, 2014, 264: 490–497.

[7] TANG Lin, YANG Gui-de, ZENG Guang-ming, CAI Ye, LI Si-si, ZHOU Yao-yu. Synergistic effect of iron doped ordered mesoporous carbon on adsorption-coupled reduction of hexavalent chromium and the relative mechanism study [J]. Chemical Engineering Journal, 2014, 239: 114–122.

[8] ALVARADO L, TORRES I R, CHEN A. Integration of ion exchange and electrodeionization as a new approach for the continuous treatment of hexavalent chromium wastewater [J]. Separation and Purification Technology, 2013, 105: 55–62.

[9] KANMANI P, ARAVIND J, PRESTON D. Remediation of chromium contaminants using bacteria [J]. International Journal of Environmental Science and Technology, 2012, 9: 183–193.

[10] LI Chun-cheng, XIE Feng-chun, MA Yang, CAI Ting-ting, LI Hai-ying, HUANG Zhi-yuan, YUAN Gao-qing. Multiple heavy metals extraction and recovery from hazardous electroplating sludge waste via ultrasonically enhanced two-stage acid leaching [J]. Journal of Hazardous Materials, 2010, 178: 823–833.

[11] GUAN Xiao-hong, SUN Yuan-kui, QIN He-jie, LI Jin-xiang, LO I M C, HE Di, DONG Hao-ran. The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: The development in zero-valent iron technology in the last two decades (1994–2014) [J]. Water Research, 2015, 75: 224–248.

[12] HU C Y, LO S L, LIOU Y H, HSU Y W, SHIH K, LIN C J. Hexavalent chromium removal from near natural water by copper–iron bimetallic particles [J]. Water Research, 2010, 44: 3101–3108.

[13] ZHOU Hai-mei, SHEN Yuan-yuan, LV Ping, WANG Jian-ji, FAN Jing. Degradation of 1-butyl-3-methylimidazolium chloride ionic liquid by ultrasound and zero-valent iron/activated carbon [J]. Separation and Purification Technology, 2013, 104: 208–213.

[14] GHEJU M, BALCU I. Hexavalent chromium reduction with scrap iron in continuous-flow system. Part 2: Effect of scrap iron shape and size [J]. Journal of Hazardous Materials, 2010, 182: 484–493.

[15] CHEN S S, HSU B C, HUNG L W. Chromate reduction by waste iron from electroplating wastewater using plug flow reactor [J]. Journal of Hazardous Materials, 2008, 152: 1092–1097.

[16] FU F L, DIONYSIOU D D, LIU H. The use of zero-valent iron for groundwater remediation and wastewater treatment: A review [J]. Journal of Hazardous Materials, 2014, 267: 194–205.

[17] WANG Xiao-song, ZHANG Wei-zheng, GUO Bing-bin, ZHAO Wei-mao. The characteristics of microcrack initiation process in cast iron materials under thermal shock test [J]. Materials Science and Engineering A, 2014, 609: 310–317.

[18] PEVEC M, ODER G, POTRC I, SRAML M. Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs [J]. Engineering Failure Analysis, 2014, 42: 221–230.

[19] WANG Yu-ping, WANG Lian-jun, PENG Pan-ying, LU Tian-hong. Treatment of naphthalene derivatives with iron-carbon micro-electrolysis [J]. Transactions of Nonferrous Metals Society of China, 2006, 16: 1442–1447.

[20] CHEN S S, CHENG C Y, LI C W, CHAI P H, CHANG Y M. Reduction of chromate from electroplating wastewater from pH 1 to 2 using fluidized zero valent iron process [J]. Journal of Hazardous Materials, 2007, 142: 362–367.

[21] GHEJU M, IOVI A. Kinetics of hexavalent chromium reduction by scrap iron [J]. Journal of Hazardous Materials, 2006, 135: 66–73.

[22] SUTTIPONPARNIT K, JIANG J, SAHU M, SUVACHITTANONT S, CHARINPANITKUL T, BISWAS P. Role of surface area, primary particle size, and crystal phase on titanium dioxide nanoparticle dispersion properties [J]. Nanoscale Research Letter, 2010, 6: 1–8.

[23] MELITAS N, CHUFFE-MOSCOSO, FARRELL J. Kinetics of soluble chromium removal from contaminated water by zerovalent iron media:Corrosion inhibition and passive oxide effects [J]. Environmental Science Technology, 2001, 35: 3948–3953.

[24] ZHANG Yan-hui, CHEN Zhang, LIU Si-qi, XU Yi-jun. Size effect induced activity enhancement and anti-photocorrosion of reduced graphene oxide/ZnO composites for degradation of organic dyes and reduction of Cr(VI) in water [J]. Applied Catalysis B: Environmental, 2013, 140–141: 598–607.

[25] SUN Yuan-kui, GUAN Xiao-hong, WANG Jian-ming, MENG Xiao-guang, XU Chun-hua, ZHOU Guang-ming. Effect of weak magnetic field on arsenate and arsenite removal from water by zerovalent iron: An XAFS investigation [J]. Environmental Science Technology, 2014, 48: 6850-–6858.

[26] HOU Mei-fang, WAN Hong-fu, LIU Ting-lin, FAN Yan-ning, LIU Xin-ming, WANG Xu-guang. The effect of different divalent cations on the reduction of hexavalent chromium by zerovalent iron [J] Applied Catalysis B: Environmental, 2008, 84: 170–175.

[27] QIU Yang-shuai, YU Yong-fu, ZHANG Lin-yan, PENG Wei-jun, QIAN Yu-peng. Dispersion and agglomeration mechanism of flaky graphite particles in aqueous solution [J]. Journal of Dispersion Science and Technology, 2017, 38: 796–800.

[28] MENG Sheng, GAO Shi-wei. Formation and interaction of hydrated alkali metal ions at the graphite-water interface [J]. The Journal of Chemical Physics, 2006, 125: 014708.

[29] KAZUHIRO S, NORIAKI O, KEI K, KAZUMI M, HIROFUMI Y. Atomic-resolution imaging of graphite– water interface by frequency modulation atomic force microscopy [J]. Applied Physics Express, 2011, 4: 125102.

[30] CANO E, TORRES C L, BASTIDAS J M. An XPS study of copper corrosion originated by formic acid vapour at 40% and 80% relative humidity [J]. Materials and Corrosion, 2001, 52: 667–676.

[31] ABDEL-SAMAD H, WATSON P R. An XPS study of the adsorption of chromate on goethite (α-FeOOH) [J]. Applied Surface Science, 1997, 108: 371–377.

[32] BIESINGER M C, BROWN C, MYCROFT J R, DAVIDSON R D, MCINTYRE N S. X-ray photoelectron spectroscopy studies of chromium compounds [J]. Surface and Interface Analysis, 2004, 36: 1550–1563.

[33] GALICIA P, BATINA N, GONZALEZ I. The relationship between the surface composition and electrical properties of corrosion films formed on carbon steel in alkaline sour medium:An XPS and EIS study [J]. The Journal of Physical Chemistry B, 2006, 110: 14398–14405.

[34] DESCOSTES M, MERCIER F, THROMAT N, BEAUCAIRE C, GAUTIER-SOYER M. Use of XPS in the determination of chemical environment and oxidation state of iron and sulfur samples: Constitution of a data basis in binding energies for Fe and S reference compounds and applications to the evidence of surface species of an oxidized pyrite in a carbonate medium [J]. Applied Surface Science, 2000, 165: 288–302.

[35] GROSVENOR A P, KOBE B A, BIESINGER M C, MCINTYRE N S. Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds [J]. Surface and Interface Analysis, 2004, 36: 1564–1574.

[36] FIUZA A, SILVA A, CARVALHO G, DELAFUENTE A V, DELERUE-MATONS C. Heterogeneous kinetics of the reduction of chromium (VI) by elemental iron [J]. Journal of Hazardous Materials, 2010, 175: 1042–1047.

[37] GHEJU M, IOVI A, BALCU I. Hexavalent chromium reduction with scrap iron in continuous-flow system: Part 1: Effect of feed solution pH [J]. Journal of Hazardous Materials, 2008, 153: 655–662.

[38] GUAN Xiao-hong, DONG Hao-ran, MA Jun, LO I M C, DOU X M. Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater [J]. Separation and Purification Technology, 2011, 80: 179–185.

[39] BRANSFIELD S J, CWIERTNY D M, LIVI K, FAIRBROTHER D H. Influence of transition metal additives and temperature on the rate of organohalide reduction by granular iron: Implications for reaction mechanisms [J]. Applied Catalysis B: Environmental, 2007, 76: 348–356.

(Edited by FANG Jing-hua)

中文导读

使用不同种类的零价铁粉还原相对高浓度的Cr(VI)：

关注碳的含量和结构对还原性影响的研究

摘要：使用零价铁还原Cr(VI)不仅可以减少还原剂的用量，还能够减少污泥的排放量。为找到一种理想的零价铁基材料，对不同种类的零价铁粉还原相对高浓度的Cr(VI)进行研究。这些零价铁粉是还原铁粉、铸铁粉、生铁和球磨生铁，他们的差异性是由碳的不同所致。首先研究了零价铁粉用量、种类和粒度大小等参数对Cr(VI)还原的影响，筛选出了灰铸铁粉是较为理想的还原性铁基材料，其次是生铁。另外，发现这些参数对试验过程动力学有重要的影响。然后，采用SEM、EPMA和XPS等测试手段，研究了零价铁粉反应前、后形貌和组成的变化，证实了碳差异性对还原性的影响。为进一步理解其反应机理，构筑了不同的反应模型。得出不仅碳的含量对Cr(VI)还原有影响，碳的结构对Cr(VI)还原也有重要的影响。

关键词：Cr(VI)还原；相对高浓度；零价铁粉；碳含量；碳结构

Foundation item: Project(51604131) supported by the National Natural Science Foundation of China; Project(2017FB084) supported by the Yunnan Province Applied Basic Research, China; Project(KKSY201563041) supported by the Talent & Training Program of Yunnan Province, China; Projects(2017T20090159, 2018T20150055) supported by the Testing and Analyzing Funds of Kunming University of Science and Technology, China

Received date: 2017-07-25; Accepted date: 2017-09-14

Corresponding authors: TONG Xiong, PhD, Professor; E-mail: xiongtong2000@yahoo.com; ORCID: 0000-0003-1200-0393; ZHENG Yong-xing, PhD, Associate Professor; E-mail: yongxingzheng2017@ 126.com