Fe bimetal

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Selective reduction of nitrate into nitrogen using Cu/Fe bimetal combined with sodium sulfite Yanlan Liu, Xiaobo Gong, Wenjing Yang, Bingqing Wang, Zhao Yang, and Yong Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05721 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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Industrial & Engineering Chemistry Research

Selective reduction of nitrate into nitrogen using Cu/Fe bimetal combined with sodium sulfite Yanlan Liua, Xiaobo Gong*a, b, Wenjing Yanga, Bingqing Wanga, Zhao Yanga, Yong Liu*a, b a

College of Chemistry and Material Science, Sichuan Normal University, Chengdu,

Sichuan 610066, China b

Key Laboratory of Special Waste Water Treatment, Sichuan Province Higher

Education System, Chengdu, Sichuan 610066, China

*Corresponding authors: Jingan Road 5#, Jinjiang District, Chengdu, Sichuan, 610066, China. Tel: +86-028-84760802; Fax: +86-028-84760802 E-mail: [email protected] (X.B. Gong); [email protected] (Y. Liu)

1

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Abstract: A novel two-step reduction process was constructed to removal nitrate selectively in aqueous solution. Nitrate was preliminary reduced into nitrite with low yield of ammonia by micro-scale Cu/Fe bimetal and then the accumulated nitrite was converted into N2 by sodium sulfite. Cu was uniformly dispersed on the surface of Fe by a simple displacement reaction. Role of major iron components and fate of nitrogen species were investigated. The selectivity for N2 was over 90 ％ and the yield of ammonia was below 10%. In the system, accumulation of nitrite resulted from the lower reduction rate of nitrite by Fe2+ in solution and the Cu/Fe coated by oxidized iron layer than the generation rate of nitrite via the reduction of nitrate. The generation of ammonia was caused by the reduction of nitrate/nitrite by Fe2+ adsorbed on the surface of oxidized iron layer coated on the bimetal at low reaction rate. Keywords: nitrate reduction; bimetal; nitrite accumulation; denitrification

2


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1. INTRODUCTION Nitrate is one of the most common chemical contaminants in ground and surface waters as a result of the over-fertilization, discharge of poorly treated industrial wastewater and concentrated animal feeding operations1. High level of nitrate concentration has attracted considerable attention due to its health risks and adverse environmental effects2. Biological denitrification process, the most widely used method, was generally slow and sometimes incomplete compared to chemical reductions3. Among various chemical reduction methods proposed for the effective removal of nitrate-contaminated water, zero valent iron (Fe0) has been demonstrated to reduce nitrate efficiently as electron donor4-10. The direct reduction of nitrate by Fe0 formed through acidic corrosion of iron contribute to nitrate removal, accompanied by passive oxide layers on the surface of iron and a negligible nitrate reduction11. Recently, the introduction of a second metal onto Fe0 such as Ni12、Al13、Mn14、Pd15 can avoid the rapid oxidation of Fe0 and enhance the nitrate reduction rate due to the high potential. Many research indicated that main final products of nitrates was ammonium (NH4+), which itself was of concern13, 16. It is commonly known that the nitrogen gas (N2) is the ideal product for nitrate reduction. Nevertheless, the activity and selectivity for reduction to N2 have not been satisfied simultaneously17. Even if noble metal or photocatalysis were used to assist the reduction of nitrate by bimetal, the N2 selectivity was still as low as 60%

18and

40.3%13. Therefore, improving the

selectivity of N2 is essential for the nitrate reduction. In recent, nitrate could be 3



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high-selectively reduced to N2 by ion exchange/catalysis integrated process and nanocrystalline materials supported bimetallic catalyst reduction process

19-22,

however, these process were relatively expensive for practical application. Thus, simplification of the denitrification process with high N2 selectivity of nitrate reduction have many advantages in economic and efficient aspects. In addition, nitrite was the intermediate product during the process of nitrate reduction by bimetal. The concentration of nitrite could reach over 40% during the nitrate reduction by the nano-bimetal Cu/Fe23-26. Several researches have described that nitrite could be converted to N2 by reducing agent such as sodium sulfite and sulfamic acid

27, 28.

Among them, sodium sulfite (Na2SO3 ) was a high production

volume chemical with relatively low price and was used to reduce the nitrogen oxide in the smoke to N2

28, 29.

Based on the previous researches, it can be an effective

method to increase the selectivity of N2 that nitrate is reduced into production with low yield of ammonia and the accumulated nitrite was converted into N2 by Na2SO3. During the nitrate reduction process, the accumulation of nitrite and the prevention of the production of ammonia play a key role for the reduction of nitrate

18, 30.

It is

necessary to explore the effect factor on the accumulation of nitrite and ammonia in order to control the production composition of nitrate reduction. The reduction mechanism of nitrate had been investigated by the identification of reaction products based on the reaction conditions such as loading rate of metal and pH et al

4, 18, 30, 31.

Furthermore, the roles of major iron components in iron-base bimetal system on 4


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nitrate reduction remain still unclear. Herein, this paper developed a novel two-step reduction method which nitrate was reduced by micro-scale Cu/Fe bimetal into production with low yield of ammonia and then the accumulated nitrite was further reduced by Na2SO3 into N2. The micro-scale Cu/Fe bimetal instead of nano-scale Cu/Fe bimetal was used in our work owing to its convenient to the engineering application. This novel method provides a new pathway of nitrate reduction with high N2 selectivity. The effect factors and the contribution of major iron component including the fresh Cu/Fe bimetal surface (Cu/Fe), Fe2+ in solution (Fe2+aq), Cu/Fe bimetal coated by oxidized iron layer (Cu/Fe@Fe3O4)

and

Fe2+

adsorbed

on

the

surface

of

Cu/Fe@Fe3O4

(Cu/Fe@Fe3O4-Fe2+) were investigated in nitrate-Cu/Fe bimetal system on nitrate/nitrite reduction. As last, a potential mechanism of nitrite accumulation and ammonia formation during the nitrate reduction by Cu/Fe bimetal were also explored.

2. MATERIALS AND METHODS 2.1 Chemicals All chemicals used in this work were of analytical grade and used as received without further purification. Deionized (DI) water was used for preparation of all reagent solutions. Iron sheet with thickness of 0.1 mm was obtained from Harner Metal Hebei Co., Ltd. All the other chemicals including CuSO4•7H2O, Na2SO3, NaNO3, NaNO2, H2SO4 and NaOH were purchased from Kelong Chemical Reagent 5



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Chengdu Co., Ltd. Synthetic nitrate contaminated water was prepared from NaNO3. The iron sheet was cut into small sheets with size of 0.5 cm × 1 cm and pretreated with 10 wt.% H2SO4 to remove any passive oxide layers prior deposition. 2.2 Synthesis of Cu/Fe bimetallic sheets The bimetallic sheets of Cu/Fe were synthesized through a simple substitution method by mixing a solution of copper ion with zero-valent iron sheets. The copper ion stock solution was prepared by dissolving CuSO4•7H2O in deionized water (10 wt.%). Cu/Fe bimetal was prepared using copper bulk loadings of 1 wt.% by diluting the appropriate amount of copper ion stock solution to 100 mL DI water, and then 30 g of fresh iron sheets were added into this solution according to the following redox reaction:

Fe 0  Cu 2  Fe 2  Cu 0

(1)

The samples were shaken for 5 min, and then standing for 5 min at 25 °C to enable the reduction of Cu2+ to Cu0. After discarding the clear aqueous phase, the resulted Cu/Fe sheets were rinsed with purified water until no sulfate ion or copper ions were detected in filtrate. The prepared bimetal sheets were then dried in a vacuum oven at 60 °C for 1 h. The Cu loading on the obtained of Cu/Fe bimetal sheets was measured to be 1.5%(w/w). 2.3 Nitrate reduction by Cu/Fe bimetal combined with Na2SO3 Batch experiments were conducted in a flask with 1000 mL 50 mgN/L stock solution. The initial pH of the solution was adjusted by H2SO4 and NaOH, without 6


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consist pH control. The flask with mixture was then stirred on a rotary shaker at 100 rpm and maintained at 25 °C for 120 min. At selected intervals, 10 mL aqueous samples were taken and filtered by syringe filters with pore size of 0.22 μm. The filtrate was analyzed for NO3-, NO2-, NH4+, pH, ferrous, and total iron, respectively. During the experiments for the total nitrogen mass balance, 30 g of fresh Cu/Fe bimetal was added into 1000 mL 50 mgN/L nitrate solution and the reaction was performed under the condition of initial pH 3.0 and temperature 25 °C in the rotary shaker. After 120 min, the Cu/Fe bimetal was removed from the solution and then the pH value was adjusted to 4.0 before 0.6 g Na2SO3 was added to remaining solution. The reduction reaction by Na2SO3 was carried out for 60 min. In order to test the influence of Cu/Fe bimetal surface properties on the nitrate/nitrite reduction, 30 g of fresh Cu/Fe bimetal sheets were added to 1000 mL 50 mgN/L nitrate/nitrite solution and the reaction was done at initial pH 3.0 and temperature 25°C. The ferrous ions in solution influence on nitrate/nitrite reduction was tested in experiments where 1.5 g FeSO4•7H2O was added to 1000 mL 50 mgN/L nitrate/nitrite solution and the initial pH value of solution was adjusted to 3.0 or 6.0. To investigate the role of iron oxide layer on the surface of Cu/Fe sheets for the nitrate/nitrite reduction, the Cu/Fe bimetal sheets with iron corrosion products (denoted Cu/Fe@Fe3O4) was obtained from the reaction of 30 g fresh Cu/Fe bimetal and 1000 mL nitrate (50 mgN/L) under the condition of 25 °C, initial pH 3.0 and reaction time of 120 min in the rotary shaker. To test the influence of adsorbed Fe2+ 7



on the surface of Cu/Fe@Fe3O4 (marked as Cu/Fe@Fe3O4-Fe2+) on nitrate/nitrite reduction, 30 g Cu/Fe@Fe3O4 and 1.5 g FeSO4•7H2O were introduced to 1000 mL 50 mgN/L nitrate/nitrite solution. Because the formation of Cu/Fe@Fe3O4 happened at near neutral environment, the experiments were performed at initial pH 6.0. To test the stability of the Cu/Fe bimetal combined with Na2SO3, the nitrate reduction process was carried out in five consecutive cycles in the same way mentioned at total nitrogen mass balance. During the Cu/Fe bimetal process, at the first cycle, the Cu/Fe bimetal sheets were synthesized freshly, then, the bimetal materials were collected and dried in a vacuum oven at 60 °C for next cycle. 2.4 Analytical method The pH value was measured using a pH meter (pHS-3C+, Leici Instruments Co. Ltd, China). The concentrations of nitrate, nitrite and ammonia in solution were analyzed using ion chromatography (ICS-1100, Dionex, USA). The Fe2+aq and Cu2+ were determined by inductively coupled plasma (ICP, Optima 8000, Germany). The dissolved Fe3+aq concentration was obtained from the difference between the dissolved Fe2+aq concentration and the total dissolved Fe concentration. Scanning electron microscopy (SEM, JSM-7500F, JEOL, Japan) images were obtained in an attempt to investigate the Cu/Fe bimetal morphologies before and after reaction. Power X-ray diffraction (XRD) patterns were measured to analyze the crystal phases of the samples with Cu Kα radiation (Rigaku Corporation, Tokyo, 25kV and 40 mA). The X-ray photo electron spectroscopy (XPS, ESCALABA 250XI, 8


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USA) measurement was used to characterize the chemical state information for iron. A simple first-order kinetic model was used for nitrate reduction. The equations used in this study are as follows 9, 13, 32, 33: dC NO  3

dt

Nitrate reduction

(2)

  k 2C NO 

Nitrite ulterior reduction

(3)

Total nitrogen balance

(4)

3

dC NO  2

dt

  k1C NO 

2

C N 2  C NO  , 0  C NO  - C NO  - C NH  3

3

2

4

Where C NO  , 0 is the initial concentration of nitrate (mgN/L); C NO  , C NO  , C NH  , 3

3

2

4

C NOx and C NH 3 are the concentrations of the respective species (mgN/L) in the

solution after reaction; N2 selectivity ( C N 2 ) was calculated according to nitrogen mass balance (Eq.4). k1 is the nitrate reduction constant, and k2 is the nitrite ulterior reduction constant. 3. RESULTS AND DISCUSSION 3.1 Characteristics of Cu/Fe bimetal The morphology of the fresh and used Cu/Fe bimetals are shown in Figure 1a and 1b, which were obvious different. Particles with tight chain net and attracted spherical shape were adhered on the surface of fresh Cu/Fe, suggesting that the preparation procedure resulted in an extensively bound of Cu0 island (spherical particles) on the host iron sheet surfaces. However, there was relative loose and roll slice-like structure build up by flake particles in the used Cu/Fe bimetals (Figure 1b), which can be assumed to the corrosion of Fe0 and the formation of iron oxides on the 9



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Cu/Fe surface during the nitrate reduction reaction34. The transformation of iron species during nitrate reduction and surface chemical species in spent Cu/Fe bimetal was determined by XPS analysis (Figure 1c and Figure S1). The compositions of Fe species were shown in Table S1. The Fe 2p3/2 peaks at binding energies of 709.4 eV and 710.7 eV followed XPS spectra indicates that Fe is presented as Fe0

35, 36.

In

addition, the binding energies of Fe 2p3/2 electron at 713.0 eV and Fe 2p1/2 electron at 719.1 eV can be attributed to Fe2O3, the binding energies of Fe2p1/2 electron at 724.3 can be attributed to FeOOH (22.4%)

36, 37

. It indicated that the partial Fe0

species have been oxidized during the preparation of Cu/Fe bimetal as the evidenced by XPS. The Fe2p1/2 located at respectively 723.5eV and 725.6eV can be assigned to Fe3O4 (44.0%) in used Cu/Fe

36, 38,

indicating that Fe3O4 (magnetite) was the

predominant iron reaction product. XRD patterns of fresh Cu/Fe and used Cu/Fe are shown in Figure 1d. The characteristic peaks of metallic Fe appeared at 44.56°, 64.94°, and 82.23° (JCPDS 99-0064).39 The peaks at 50.39° could be assigned to Cu0 (JCPDS 99-0034),22 suggesting that the Cu0 were successfully loaded on the iron sheet surfaces. The XRD patterns of used Cu/Fe showed that the characteristic peaks of crystalline magnetite (Fe3O4) at 43.03° and 73.98° (JCPDS 99-0073) were present on the used Cu/Fe,

39, 40

which indicated that the iron oxide was covered on the Cu/Fe

bimetal surface during the nitrate reduction. This observation determined the core-shell structure of Fe3O4/Cu on the used Cu/Fe and was consistent with the result of Tasuma Suzuki 31. However, there was no obvious characteristic peak of Fe2O3 and 10


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FeOOH, which have been presented in XPS results, due to the small amount and cover of Cu. Moreover, there were no characteristic peaks of copper oxide in the used Cu/Fe, which revealed that copper plays for the adsorption and reaction interface of nitrogen instead of direct reaction with nitrogen. Figure 1 3.2 The reduction of nitrate by Cu/Fe bimetal combined with Na2SO3 The obtained Cu/Fe bimetal was used for nitrate reduction combined with depth reduction by Na2SO3. The influences of initial solution pH, temperature and dosage of Cu/Fe bimetal sheets on nitrate reduction were investigated and shown in Figure 2. It can be seen from Figure 2a that low pH was favored for the nitrate reduction by Cu/Fe bimetal sheets. The residual nitrate concentration was only 7.71 mgN/L at an initial pH of 3.0 and sharply increased to 31.01 mgN/L, 42.75 mgN/L, 45.75 mgN/L and 47 mgN/L at an initial pH of 5, 7, 9, and 11, respectively. The results are in good consistent that nitrate tended to be reduced easily at lower pH

33.

The maximum

concentration of produced ammonia was 4.4 mgN/L under the condition of initial pH 3.0. As shown in Figure 2b, high temperature was benefit for the nitrate reduction, which can effectively break the barrier of the reduction of energy and increase the average energy of reactant molecules

41.

Nevertheless, the accumulated ammonia

increased from 1.84 mgN/L to 11.25 mgN/L when reactional temperature increased from 10 ℃ to 40 ℃. The residual nitrate concentration decreased from 31.77 mgN/L to 9.43 mgN/L when the dosage of Cu/Fe bimetal increased from 5 to 40 g/L (Figure 11



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2c), which was due to the increase of activity sites on bimetal for nitrate reduction. The test of dosage effect of Na2SO3 was also carried out on nitrite reduction at initial solution pH 4.0. During the nitrite reduction process by Na2SO3, about 97.5％ of the intermediate nitrite was removed when the Na2SO3 dosage increased to 0.6 g/L, indicating that nitrate could be efficiently reduced by Cu/Fe bimetal combined with Na2SO3. As mentioned above, the condition of initial pH at 3.0, temperature at 25 °C, Cu/Fe bimetal dosage at 30 g/L and Na2SO3 dosage at 0.6 g/L were chosen to denitrify. The profile of nitrogen species during nitrate reduction by Cu/Fe bimetal combined with Na2SO3 are presented in Table S2. It can be seen that the removal rate of nitrate by whole reduction process was above 80％, the nitrite was not detected (the detect limit, 0.02 mgN/L). Because of the complete reduction of nitrite to nitrogen gas by Na2SO3, and the yield of ammonia was below 10％, the selectivity of N2 was about 90％(Eq.4), which was higher than the results of liou et al 18 (60% with1wt% Pd/Zn), Lubphoo et al

42

(60.05% with 10 wt % Cu-Pd/Fe) and Zhao et al

30

(34.1% with 0.4wt%

Pd-4%Cu/Al). As a result, the Cu/Fe bimetal combined with Na2SO3 depth reduction showed efficient conversion of harmful nitrate to harmless nitrogen, which offered a novel strategy to reduce nitrate to nitrogen. During the two-step reduction process, selectively reduction of nitrate to nitrite was the key step

18, 30.

Therefore, it is

essential to study the mechanism of nitrite accumulation in nitrate-Cu/Fe bimetal system for selectively and effectively reducing nitrate into nitrogen during the 12


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two-step process. Figure2 3.3 Nitrate and nitrite reduction by Cu/Fe bimetal To propose the pathway of nitrate reduction by Cu/Fe bimetal, the contents of nitrogen products in aqueous phase as well as the iron products and dissolved copper irons as a function of reaction time were measured during nitrate reduction by Cu/Fe bimetal under the condition of initial pH 3.0, 25 °C and 30 g/L of Cu/Fe bimetal dosage. In addition, to further elucidate the role of fresh Cu/Fe bimetal for nitrite reduction at acidic condition, experiment was carried out as the same conditions as nitrate reduction. As shown in Figure 3a, NO3− was efficiently reduced by the Cu/Fe bimetal in the first 30 min, where the NO3− concentration decreased from 50 mgN/L to 10.72 mgN/L. After then, the reduction reaction rate declined and less denitrification occurring in the interval from 60 min to 120 min. The final concentration of NO3− was 7.71 mgN/L after 120 min, with cumulative removal efficiency of 84.6%. The Table S3 shows that the good fitting of the linear model to data provides strong evidence that the reaction was pseudo-first order at rate constant of 0.047 min-1 with respect to nitrate concentration. Furthermore, Fan X. et al

43

observed the similar result of the

reaction of micro scale iron powder with nitrate and the values of Kobs ranged from 0.0334 min-1 to 0.0361 min-1 with a mean of 0.0353 min-1. Compared to iron powder, higher reaction rate by Fe/Cu bimetal sheets may be the presence of Cu0 accelerated 13



corrosion rate of iron and acted as a good conductor of electron transfer

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30, 44.

The

decline of nitrate reduction rate after 60 min can be the formation of iron oxide coated on the surface of Fe/Cu bimetal sheets and hindered the reduction reaction. The concentration of nitrite increases sharply, reaches the peak concentration at 60 min and afterwards undergoes a gradual decay. With the decrease of nitrite concentration, ammonia concentration increased gradually, which indicates that nitrate reduction was a step-wise process

45

and the generation of ammonia can be caused by the reduction

of NO2−. Even though, the maximum yield of ammonia was only about 10％. Figure 3 It can be observed from Figure 3a that the solution pH increased rapidly from 3.0 to higher than 6.0 in the first 30 min of reaction and remained constant there after 120 min. The increase of solution pH was attributed to the reduction of nitrate to nitrite and the consumption of H+ according to the following reactions: NO3- + 2H+ + 2e- → NO2- + H2O

（5）

The solution pH increase was responsible for the rapid inactivation after 30 min of Cu/Fe bimetal for the reduction of nitrate because some iron oxide could be formed and covered on the surface of Cu/Fe bimetal at high pH which prohibited further reduction of nitrate. Many research about the nitrate reduction by Fe0 showed that metallic iron was always covered by iron oxides/hydroxide species even after the shortest reaction time

9, 46.

It was also suggested that surface passivation was the

limiting factor for the reduction of nitrate and the reduction of nitrate was an 14


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acid-driven surface-mediated process, which was similar to the nitrate reduction by nano-Fe/Cu 47, nano-Fe 8 and Pd-Cu/Al 30. The nitrogen gas (Figure 3b) during the nitrate reduction was agreed with the results of S.Choe 48. The concentration of N2 first decreased and then increased. The tendency of gradual increase was due to the adsorption/desorption of the bimetallic sheets when reaction time was range from 5-10 min

48.

It could be seem that the

selectivity of N2 from 46.31 ％ to 42.47 ％ after 120 min which indicated that nitrogen gas was one of the main final products of nitrate reduction by Cu/Fe bimetal. Figure 3c shows that concentrations of Fe2+ and Fe3+ in solution also increased over time. Dissolved Fe2+ increased in a similar pattern to pH (Figure 3a) suggesting that the progressive dissolution of the Fe0 in the acidic solute. The maximum concentration of Fe2+ was 238.7 mgFe/L within 120 min. The dissolved Fe3+ increased initially and then decreased slowly during the reaction with a maximum concentration of 74.3 mgFe/L. Dissolved Fe3+ continued to decrease in the last 60 min was due to the conversion of Fe3+ to insoluble iron oxide. The total iron ions in solution had the similar trend of dissolved Fe3+. The extent of the Fe0 dissolution over 120 min based on the final Fe2+ concentration was approximately 70% of all dissolved Fe, with Fe3+ comprising approximately 30%. Total iron ions continued to decrease in the last 60 min indicating the coprecipitation of ferric iron and oxidation of Fe(OH)2 to magnetite 49, which was proved by the XRD (Figure 1b). Furthermore, none copper ions were detected during the whole reduction process. Therefore, it could be 15



Page 16 of 42

proposed that metallic Fe0 rather Cu0 acts as the source of electrons that reduce nitrate to nitrite and further to ammonium, and the presence of Cu0 accelerated corrosion rate of iron and acted as a good conductor of electron transfer 30, 44. According to the results, the fate of nitrogen species during nitrate reduction by Cu/Fe bimetal was decided clearly. That is, nitrate was primary reduced into nitrite. Next, partial nitrite was further reduced to ammonia and nitrogen gas. The nitrate reduction process in solution with the Cu/Fe bimetal could be depicted as  N 2     K1 NO3  NO2   K 2    NH 4

(6)

At the same time, ferrous irons were produced in the solution due to the corrosion of iron, then part of it were attached to the surface of the Cu/Fe bimetal and take place superficial oxidation. To investigate the reduction efficiency of Cu/Fe bimetal on nitrite, only nitrite without nitrate was added to the reduction process. As shown in Figure 3d, the nitrite could be greatly reduced by fresh Cu/Fe bimetal at initial pH 3.0 and more than 77% nitrite reduction removal efficiency was observed after 120 min. In this system, ammonia was generated as a major product with accumulated ammonium accounting for 46% at a reaction time of 120 min. It was also important to note that, in the nitrate-Cu/Fe system (Figure 3a), there was almost none ammonia before the pH was increased to near neutral (

Fe bimetal

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