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Nitrate removal by Fe /Pd/Cu nano-composite in groundwater a
a
Hongyuan Liu , Min Guo & Yan Zhang
b
a
College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China b
Department of Civil Engineering, Zhejiang University, Hangzhou 310058, People's Republic of China Accepted author version posted online: 18 Oct 2013.Published online: 19 Nov 2013.
0
To cite this article: Hongyuan Liu, Min Guo & Yan Zhang (2014) Nitrate removal by Fe /Pd/Cu nano-composite in groundwater, Environmental Technology, 35:7, 917-924, DOI: 10.1080/09593330.2013.856926 To link to this article: http://dx.doi.org/10.1080/09593330.2013.856926
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Environmental Technology, 2014 Vol. 35, No. 7, 917–924, http://dx.doi.org/10.1080/09593330.2013.856926
Nitrate removal by Fe0 /Pd/Cu nano-composite in groundwater Hongyuan Liua∗ , Min Guoa and Yan Zhangb a College
of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China; b Department of Civil Engineering, Zhejiang University, Hangzhou 310058, People’s Republic of China
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(Received 6 May 2013; final version received 14 October 2013 ) Nitrate pollution in groundwater shows a great threat to the safety of drinking water. Chemical reduction by zero-valent iron is being considered as a promising technique for nitrate removal from contaminated groundwater. In this paper, Fe0 /Pd/Cu nano-composites were prepared by the liquid-phase reduction method, and batch experiments of nitrate reduction by the prepared Fe0 /Pd/Cu nano-composites under various operating conditions were carried out. It has been found that nanoFe0 /Pd/Cu composites processed dual functions: catalytic reduction and chemical reduction. The introduction of Pd and Cu not only improved nitrate removal rate, but also reduced the generation of ammonia. Nitrate removal rate was affected by the amount of Fe0 /Pd/Cu, initial nitrate concentration, solution pH, dissolved oxygen (DO), reaction temperature, the presence of anions, and organic pollutant. Moreover, nitrate reduction by Fe0 /Pd/Cu composites followed the pseudo-first-order reaction kinetics. The removal rate of nitrate and total nitrogen were about 85% and 40.8%, respectively, under the reaction condition of Fe–6.0%Pd–3.0%Cu amount of 0.25 g/L, pH value of 7.1, DO of 0.42 mg/L, and initial nitrate concentration of 100 mg/L. Compared with the previous studies with Fe0 alone or Fe–Cu, nano-Fe–6%Pd–3%Cu composites showed a better selectivity to N2 . Keywords: nitrate reduction; nano-Fe0 /Pd/Cu; ammonia; groundwater; anion
1. Introduction With the increasing contamination of groundwater from agriculture and wastewater, the nitrate pollution is being considered as a great threat to the safety of drinking water. Therefore, varieties of technologies, including physical, biological, and chemical removal processes were developed by researchers for nitrate removal from groundwater. The physical–chemical approaches such as reverse osmosis and ion-exchange treatment can effectively remove nitrate from water, but potentially generate concentrated waste-streams.[1,2] Biological denitrification is the most commonly used method, but is difficult to maintain, and needs to polish the effluent through purification and disinfection in order to secure the water quality for potable water purpose.[3,4] Recently, nitrate elimination from groundwater by chemical reduction, especially reduction by zerovalent iron (ZVI), has been received much attention as an efficient and cost-effective method.[5,6] Researches have conducted that nitrate reduction by ZVI is pH dependent and lower pH is favourable, and the main final product is ammonia.[7–10] Zhang et al. [11] reported that nitrate could be completely reduced by nanoscale zero-valent iron (NZVI) within 60 min with the dosage of 1.0 g/L, but about 85% nitrate was reduced to ammonia. The poor stability of NZVI and the generation of ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
by-product ammonia were the main problems to limit its further application. Therefore, NZVI supported on exfoliated graphite or TiO2 [12–14] and some other metal coated on NZVI [15,16] were conducted to decrease the percentage of transformation to ammonia and improve the selectivity to nitrogen. Bimetallic nanoparticles had a higher reaction rate, compared with those observed for NZVI alone.[15–19] Hosseini et al. [16] found that coating of NZVI by copper decreased the aggregation and agglomeration of NZVI, in addition to enhancing nitrate reduction rate in aqueous solution. Kang and his co-workers investigated that Fe/Ni particles showed a much higher nitrate reduction rate and an optimum reduction rate at near-neutral pH, although undesirable transformation of nitrate (91.0 ± 0.37%) to ammonium was observed.[18] Liou et al. [15] evaluated that Pd, Pt, and Cu separately deposited onto nano-Fe0 surface were ranked Cu > Pd > Pt in their promotion on nano-Fe0 reactivity towards nitrate, and coating of NZVI by copper-enhanced nitrate reduction rate and decreased ammonia formation, but resulted in the accumulation of nitrite. Although some improvements were made, the low selectivity to nitrogen in nitrate reduction by NZVI was still a problem. In addition, since Hörold et al. [20] first reported catalytic reduction of nitrate, palladium catalysts such as
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palladium–copper or tin catalysts were developed as the effective catalysts for reducing nitrate.[21–23] In particular, many studies showed that most of nitrate was reduced to nitrogen in catalytic nitrate reduction process over Pd– Cu bimetal catalysts.[24–26] Soares et al. [27] found that high selectivity to nitrogen, between 80% and 89%, was obtained using 1%Pd–1%Cu/Carbon Nanotube catalyst. However, H2 should be used as reducing agent in catalytic nitrate reduction by Pd–Cu catalysts. In addition, hydrogen might be generated in the nitrate reduction by a nano ZVI system. Thus, to combine with the advantages of the two processes mentioned above, Fe0 –Pd–Cu nano-composites were prepared in this study, in an effort to improve the selectivity to nitrogen. Fe0 –Pd–Cu nano-composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), SEM-energydispersive spectrum (EDS), and Brunauer–Emmett–Teller (BET) techniques. Nitrate reduction by Fe0 –Pd–Cu under different experimental conditions was systemically investigated, and the reaction kinetic of nitrate reduction over Fe0 –Pd–Cu nanoparticle was also discussed.
2. Materials and methods All chemical stock solutions were prepared from reagentgrade chemicals using doubled deionized water and stored at 4◦ C unless otherwise specified. Contaminated groundwater was simulated using pure water consisting of a certain concentration of KNO3 . 2.1.
Fe0 –Pd–Cu preparation
In this study, one-step and multi-step synthesis processes for preparation of Fe0 /Pd/Cu samples were carried out in advance, and the results showed that there was no obvious difference in the removal rate of nitrate. Therefore, one-step synthesis process by the liquid-phase reduction method was employed in this paper, as shown below: (1) appropriate amounts of FeCl3 · 6H2 O were added in 250 ml alcohol solution, which was purged with purified argon gas to reduce dissolved oxygen (DO); (2) appropriate amounts of 0.08 mol/L sodium borohydride (NaBH4 ) solution were added into FeCl3 · 6H2 O solution drop by drop under constant pressure, and purified argon gas was also purged during the whole process; (3) Fe0 samples were further washed with argon-purged, deoxygenated water several times until supernatant pH reached about 7; (4) appropriate amounts of PdCl2 and CuCl2 · H2 O solution were added into the prepared Fe0 solution drop by drop; and (5) Fe0 /Pd/Cu samples were washed and gathered for further use. During the preparation process, the reactions can be described as the follows: 4Fe3+ + 3BH− 4 + 9H2 O + → 4Fe0 ↓ +3H2 BO− 3 + 12H + 6H2 ↑,
(1)
Fe0 + Pd2+ → Fe2+ + Pd0 ,
(2)
Fe + Cu
(3)
0
2+
→ Fe
2+
+ Cu . 0
2.2. Fe0 –Pd–Cu characterization The morphology of Fe0 /Pd/Cu samples was characterized by SEM (S4800) and TEM(JEM-1200EX). The result could be seen from Figure 1. It shows that the average external diameter of the microsphere was about 20–80 nm, and aggregated nanoparticles could be observed in the formation of chain-like structures, which might result from magnetic property and large specific surface area of Fe0 /Pd/Cu particles.[28] Fe0 /Pd/Cu samples were also analysed by EDS using an Edax Genesis4000 energy dispersive spectrometer. It shows that the three metal elements peaks of Fe, Pd, and Cu were found, and the average contents of Pd and Cu were 5.95% (wt%) and 2.88% (wt%), respectively, which showed that the mass ratios of the metals were almost the same as that prepared. The BET surface area of the prepared Fe0 /Pd/Cu samples was also determined by nitrogen adsorption with a TriStar II 3020 (Micromeritics Instrument Co., USA) surface area and porosity analyser. It was investigated that the BET surface area was 51 m2 /g. 2.3. Batch experiments Batch experiments were carried out in a 1000 ml reactor under constant temperature and atmospheric pressure. Prior to the experiments, predetermined concentration of nitrate solution was filled in the reactor and argon gas was then introduced from the bottom of the reactor through a diffuser in order to strip out dissolved and adsorbed oxygen. Fe0 /Pd/Cu composites were suspended in the solution through stirring, during which predetermined quantities of Fe0 /Pd/Cu composites were added to the reactor. Before the experiments, pH was conducted by adding NaOH or HCl solution, and no buffer was added during experimental periods. The solution temperature was controlled using an electro thermostatic water bath. The experiments were carried out under the amount of Fe–6.0%Pd–3.0%Cu, pH, DO, and initial nitrate concentration of 0.25 g/L, 7.1, 0.42 mg/L, and 100 mg/L, respectively, unless otherwise specified. 2.4. Analysis methods Samples were periodically collected to analyse the concen− − tration of NO− 3 -N, NO2 -N, and NH4 -N. Standard methods GB5750-2006 in China were adopted for the analysis of all the parameters. Ammonia was analysed with the Nessler reagent spectrophotometric method, nitrate was analysed with the thymol metric method, nitrite was analysed with the N -(1-naphthyl) ethylenediamine spectrophotometric method, pH, and DO were periodically monitored with a Hach HQ40d pH and DO analyzers, respectively, and Total
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(a)
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(b)
(c) 6.0 Fe
5.0
Counts
4.0 3.0 2.0 1.0
Fe Cu
Pd
Fe
Cu 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.011.012.0 Energy (keV)
Figure 1.
SEM, TEM, and EDS images of prepared
Fe0 /Pd/Cu.
Organic Carbon (TOC) was measured with a TOC-V CPH analyzer. 3. Results and discussion 3.1. The mass ratio of Pd and Cu According to previous research, the metal Cu can enhance nitrate reduction, especially for the generation of intermediate production (nitrite).[16] In order to examine the role of metal Cu and determine the appropriate mass ratio, various percentages of Cu (0%, 0.75%, 1.5%, 3.0%, and 4.5% w/w) were deposited onto the surface of nano-Fe0
particles. Figure 2 shows the concentration percentage (the current concentration at t time divided by the initial con− centration of nitrate–nitrogen) profiles of NO− 3 -N, NO2 -N, − and NH4 -N during the batch experiments with different ratios of Cu deposited on the ZVI surface. The result shows that nitrate was obviously reduced and then increased with an increase in the mass ratio of Cu. The percentage of nitrate–nitrogen decreased from 25.04% with Fe alone to 0.75% with an extra mass ratio of 1.68% Cu, and then increased to 22.35% with 4.5% Cu, showing that the presence of a certain amount of Cu could promote nitrate reduction.
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Nitrate Nitrite Ammonia
Percentage (%)
40
30
20
10
0
0%Cu
0.75%Cu
1.5%Cu
3.0%Cu
4.5%Cu
50
Nitrate Nitrite Ammonia
Percentage (%)
40
30
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10
0 0%Pd
3.0%Pd
6.0%Pd
9.0%Pd
Mass Fraction of Pd (%)
50
TN Removal perceentage (%)
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Mass fraction of Cu (%)
40.84
40
35.27
30 20
27.61 17.08
10 0
0%Pd
3.0%Pd
6.0%Pd
9.0%Pd
Mass fraction of Pd (%)
of 4.5%. This shows that the excess copper deposited on the ZVI surface resulted in decreasing the surface reaction activated sites, and then decreased the percentage of nitrate removal rate. In addition, the concentration of ammonia decreased with the increasing of mass ratio of Cu, and the percentage of ammonia slightly decreased from 38.8% to 27.0%, with the mass ratio of Cu increasing from 3.0% to 4.5%. Further analysis indicated that the percentages of nitrate and total nitrogen with the Cu mass ratio of 3.0% were lower than that of 4.5%. Therefore, the optimum mass ratio of Cu was 3.0% under the experimental conditions, which was consistent with previous results.[16] As the above mentioned, the concentrations of nitrite and ammonia still remained high when only Cu deposited on the ZVI surface. However, metal Pd showed a high selectivity to nitrogen in catalytic nitrate reduction,[29,30] therefore Pd was further deposited on the Fe0 /3.0%Cu surface, that is to say, nano-Fe0 /Pd/Cu composites were prepared for nitrate removal in the next experiments. Figure 2 illustrates that nitrate removal rate decreased with an increase in the mass ratio of Pd. However, the percentages of nitrite and ammonia did not follow the same trend. The percentage of nitrite decreased obviously, and the percentage of ammonia first decreased and then increased. Moreover, the influence of the mass ratio of Pd on the N2 selectivity exhibited a maximum value over Pd loading of 6% followed by a drop with a further increase in the mass ratio of Pd. Previous literatures reported that nitrate did not be adsorbed at the Pd surface, but could be adsorbed and reduced to nitrite at Cu surface.[29,31] For this reason, the surface active sites decreased with an increase in the mass ratio of Pd, thereby leading to a decrease in nitrate removal rate. On the other hand, nitrite could be adsorbed and reduced at Pd surface, thus the introduction of Pd catalyst was effective for nitrite reduction to nitrogen, thereby resulting in less accumulation of nitrite and ammonia formation. The results show that the introduction of 6% Pd was able to maximize reduction of nitrate into nitrogen gas, and minimize nitrite and ammonia formation, thus maintained a high removal efficiency of nitrate and enhanced the selectivity to N2 . Under the experimental conditions, as much as 40.8% selectivity to N2 was obtained and the generation of ammonia decreased to 33.7%. It should be noted that nano-Fe–6%Pd–3%Cu composites have a better selectivity to N2 , compared with the previous studies with Fe0 alone or Fe0 /Cu reducer.[11,16] Thus, nano-Fe– 6%Pd–3%Cu composites were prepared and employed to the following research in this study.
Figure 2. The effect of mass ratio of Cu and Pd on nitrate reduction.
3.2. The percentage of nitrite generation increased to the maximum value of 39.5% with Cu mass ratio of 3.0%, and then decreased slightly to 36.7% with Cu mass ratio
Effect of the amount of Fe0 /Pd/Cu
Figure 3 illustrates the effect of the amount of Fe0 /Pd/Cu on nitrate reduction. An increase in the Fe0 /Pd/Cu amount improved nitrate removal rate. When Fe0 /Pd/Cu dosage
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0.125g 0.250g 0.500g
Nitrate (mgN/L)
20 16 12 8 4 0 0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min)
Ammonium (mgN/L)
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of Fe0 /Pd/Cu at 0.250 g was selected as the operating parameter in this study, unless otherwise specified. 3.3. Effect of initial concentration of nitrate Figure 4(a) illustrates that nitrate removal rate showed a significant decrease with the reaction time going. It can be explained that iron oxides and hydroxides were formed with the reaction time going, which reducing the nitrate removal rate. It was further confirmed that pH value increased from 7.10 to 9.64, 10.56, and 10.65 under the initial concentration of nitrate of 50, 100, and 200 mg/L, respectively, at the reaction time of 60 min. The reactions were shown as following: 0 − 2NO− 3 + 5Fe + 6H2 O → 5Fe(OH)2 + N2 + 2OH , (4)
0.125g 0.250g 0.500g
− 0 NO− 3 + Fe + H2 O → Fe(OH)2 + NO2 ,
16
NO− 3
8
4
5
10 15 20 25 30 35 40 45 50 55 60
Time (min)
Figure 3. removal.
+ 4Fe + H2 O → 4Fe3 O4 + 0
NH+ 4
(5) −
+ 2OH ,
(6)
+ 0 − NO− 3 + 4Fe + 7H2 O → 4Fe(OH)2 + NH4 + 2OH . (7)
12
0 0
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The effect of the amount of Fe0 /Pd/Cu on nitrate
was 0.125, 0.250, and 0.500 g, nitrate removal rate was 61.73%, 82.26%, and 100% with the reaction time of 60 min, respectively. It can be explained that Fe0 /Pd/Cu surface area increased with the increasing amount of Fe0 /Pd/Cu, thereby resulting in more contact chance between nitrate and Fe0 /Pd/Cu, which was beneficial for nitrate reduction.[11,32] Moreover, similarly as nitrate reduction over ZVI or nano ZVI,[33] nitrate reduction over Fe0 /Pd/Cu can also be described by pseudo-first-order reaction kinetics. The apparent surface reaction rate constants kobs (kobs = ln(C0 /C)/t, C0 and C represent the initial and current concentrations of nitrate, respectively) of nitrate reduction increased from 0.0124 to 0.0964 min−1 with the amount of Fe0 /Pd/Cu increasing from 0.125 to 0.500 g. In addition, the concentration of ammonia increased with an increase in the amount of Fe0 /Pd/Cu. Above 83% nitrate transformed to ammonia with the amount of nano-Fe0 /Pd/Cu 0.500 g, while only 19.16% and 35% of nitrate transformed to ammonia with that of 0.125 and 0.250 g, respectively. Therefore, in order to effectively remove nitrate and reduce ammonia formation, an amount
In addition, a high initial concentration of nitrate showed a high initial reduction rate of nitrate. The apparent surface reaction rate constants kobs increased from 0.0115 to 0.0418 min−1 with the initial nitrate concentration increasing from 50 to 200 mg/L. However, nitrate removal rates were 96.99%, 82.26%, and 56.75% at the reaction time of 60 min, when the initial concentrations of nitrate were 50, 100, and 200 mg/L, respectively. It can be explained that the amount of nano-Fe0 /Pd/Cu of 0.25 g was not enough to react with the initial nitrate concentration of 200 mg/L, according to the above chemical reaction equation (4). While the reaction followed Equation (6) or (7), the amount dosage of 0.25 g was not enough for initial concentration of 100 and 200 mg/L. Therefore, the initial concentration of nitrate 100 mg/L was selected as the operating parameter in this study, unless otherwise specified. 3.4. Effect of pH No buffer was added during reaction periods. The effect of pH on nitrate reduction was investigated with different initial pH values of 3.5, 5.0, 7.1, 9.0, and 10.5. Figure 4(b) shows that an increase in pH value resulted in a decrease in nitrate removal rate. As described in Equations (4)–(7), when nitrate was reduced, hydroxide ions were formed. Because no additional acid was added, the OH− accumulated, thus restrained the nitrate reduction reactions. Further analysis indicated that, while pH value decreased from 7.1 to 5.0, nitrate removal rate did not increase but slightly decreased, and pH value continued to decrease to 3.5, nitrate removal rate slightly improved again. This can be explained as following, first, when pH value decreased, the concentration of H + increased, which could accelerate nitrate
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(a) 1.2 50 mg/L 100 mg/L 200 mg/L
1.0
Nitrate (mgN/L)
C/C0
20 16
0.8 0.6 0.4
12 8 4
0.2 0.0
pH = 3.5 pH = 5.0 pH = 7.10 pH = 9.0 pH = 10.5
0
5
0
10 15 20 25 30 35 40 45 50 55 60
0
5
Time (min)
(c)
T = 10ºC T = 15ºC T = 20ºC T = 25ºC T = 30ºC
16 12 8
0.42mg/L 6.05mg/L 10.41mg/L
20 16 Nitrate (mgN/L)
Nitrate (mgN/L)
20
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Time (min)
(d) 24
24
12 8 4
4 0 0
5
10 15 20 25 30 35 40 45 50 55 60
0
0
Time (min)
Figure 4. removal.
5
10 15 20 25 30 35 40 45 50 55 60 Time (min)
The effect of (a) the initial concentration of nitrate, (b) initial pH value, (c) temperature, and (d) initial DO value on nitrate
removal and hinder the formation of iron oxide layers covering at the same time. On the other hand, the reaction between iron and H + happened at the surface of nanoFe0 /Pd/Cu, which would consume a certain amount of nano-Fe0 /Pd/Cu, thereby hindering nitrate reduction.[34] Meanwhile, nitrate could be reduced over Pd and Cu catalyst with H2 as reducer, which improving the nitrate reduction. Due to the dual functions mentioned above, nitrate removal rate has no obvious difference at pH 3.5 and 7.1. Therefore, a pH value of 7.1 was selected as the operating parameter in this study, unless otherwise specified. 3.5.
10 15 20 25 30 35 40 45 50 55 60
Effect of reaction temperature
Reaction temperature plays an important role on the chemical reaction rate. In order to determine the effect of the temperature on nitrate removal rate, the experiments were carried out at the temperature of 10◦ C, 15◦ C, 20◦ C, 25◦ C, and 30◦ C, respectively. Figure 4(c) shows that nitrate removal rate increased with the temperature rising. At the reaction time of 60 min, nitrate removal rates were about 58.76%, 73.85%, 81.81%, 82.88%, and 86.15%, respectively, with the reaction temperature of 10◦ C, 15◦ C, 20◦ C, 25◦ C, and 30◦ C. kobs of nitrate reduction was found to follow a linear relationship with the temperature, and increased from 0.012 to 0.029 min−1 with the temperature raising from 10◦ C to 30◦ C. Using the Arrhenius equation,[35] the apparent
activation energy for nitrate reduction can be calculated from the change of temperature and apparent rate constant: ln kobs = −
E + const, RT
(8)
where kobs is the apparent rate constant, min−1 ; E is the apparent activation energy, kJ/mol; T is the reaction temperature, K; and R is the universal gas constant, R = 8.314 J/(mol.K). Under the experimental conditions, E value was equal to 30.50 kJ/mol. The low value of apparent activation energy obtained in this work further confirmed that nitrate reduction by Fe0 /Pd/Cu was an effective technique, and followed a surface-controlled reaction.[36] 3.6.
Effect of DO value
Similarly, DO may have an important influence on nitrate reduction by Fe0 /Pd/Cu. Experiments under initial DO values of 0.42, 6.05, and 10.41 mg/L were carried out to study the effect of DO on nitrate reduction. As shown in Figure 4(d), nitrate removal rate significantly declined with an increase in DO value. Nitrate removal rate decreased from 82.26% to 20.35%, with the DO value increasing from 0.42 to 10.41 mg/L. It could be explained that Fe0 /Pd/Cu particles were oxidized by oxygen, and formed a passive iron oxide layer at the surface of Fe0 /Pd/Cu particles.
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0mg/L 0.7mg/L Chlorate 10µg/L Bromate 1mgP/L Phosohate 250mg/L Sulfate 250mg/L Bicarbonate 5mg/L TOC
ln(C0/C)
2
1.5
1
0.5
0 0
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Figure 5. time.
10
20
30 40 Time (min)
50
60
70
The relationship between ln (C0 /C) and the reaction
Table 1. Corresponding kobs and R2 of nitrate reduction under different conditions. Oxyanion or organic pollutant 0 mg/L Chlorate (0.7 mg/L) Bromate (10 μg/L) Phosphate (1 mg/L) Sulphate (250 mg/L) TOC (5 mg/L)
Apparent rate constant kobs (min−1 )
Correlation coefficient R2
.0240 .0206 .0195 .0118 .0082 .0228
.9892 .9890 .9609 .9840 .9416 .9989
That is to say, the competing reaction with oxygen and Fe0 happened at the surface reaction active sites of Fe0 /Pd/Cu, and then resulted in decreasing the nitrate reduction rate. 3.7.
Effect of anions and organic pollutant
At the same time, the influences of the presence of anions and organic pollutant on nitrate removal over nanoFe0 /Pd/Cu were conducted in this study. The concentrations of anions and TOC were chosen basing Drinking Water Standard GB5749-2006 in China. The relationships between ln (C0 /C) and the reaction time were displayed in Figure 5 and Table 1. Figure 5 and Table 1 illustrate that the presence of all anions or TOC decreased the nitrate removal rate. It can be explained that chlorate and bromate competed with nitrate to be adsorbed on the active sites, and their presence made kobs decrease slightly, from 0.0241 to 0.0206 and 0.0195 min−1 , respectively. Phosphate reacted with iron to form complexes, which adsorbed in nano-Fe0 /Pd/Cu surface, then reduced the nitrate reduction. Sulphate might destroy the surface passive film of nano-Fe0 /Pd/Cu, to accelerate its corrosion, and then kobs decreased to 0.0082 min−1 . While the TOC value at 5 mg/L, kobs also decreased to 0.0228 min−1 . Under experimental conditions, the influences of the presence of anions on nitrate removal 3− − − rate followed the order of SO2− 4 > PO4 > BrO3 > ClO3 .
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4. Conclusions In this paper, nano-Fe0 /Pd/Cu composites were prepared, and its application to nitrate reduction was systematically conducted. The results show that nano-Fe0 /Pd/Cu composites had the dual function of catalytic reduction and chemical reduction. The introduction of the metals of Pd and Cu not only improved nitrate removal rate, but also reduced the generation of by-product ammonia. As much as 40.8% total nitrogen removal rate was obtained under the experimental conditions. The nitrate reduction over Fe0 /Pd/Cu can be described by pseudo-first-order reaction kinetics. The low value of apparent activation energy obtained in this work confirmed that nitrate reduction by Fe0 /Pd/Cu followed a surface-controlled reaction and was a promising technique for nitrate removal from water. However, the presence of anions decreased the nitrate removal rate, and the influences of the presence of anions on nitrate removal rate increased as 3− − − follows: SO2− 4 > PO4 > BrO3 > ClO3 under the experimental conditions. Of course, the related research under the real groundwater conditions needs further study. Funding This research was supported by National Natural Science Foundation of China [No. 51108407] and the Program for Zhejiang Leading Team of S&T Innovation [No. 2010R50037]. References [1] Samatya S, Kabay N, Yuksel U, Rda M, Yuksel M. Removal of nitrate from aqueous solution by nitrate selective ion exchange resins. React Funct Polym. 2006;66:1206–1214. [2] Fernández-Nava Y, Maranón E, Soons J, Castrillon L. Denitrification of wastewater containing high nitrate and calcium concentrations. Bioresour Technol. 2008;99:7976–7981. [3] Della RC, Belgiorno V, Meric S. Heterotrophic/autotrophic denitrification (HAD) of drinking water: prospective use for permeable reactive barrier. Desalination. 2007;210:194– 204. [4] Liu H, Jiang W, Wan D, Qu J. Study of a combined heterotrophic and sulfur autotrophic denitrification technology for removal of nitrate in water. J Hazard Mater. 2009;169:23–28. [5] Ahn SC, Oh SY, Cha DK. Enhanced reduction of nitrate by zero-valent iron at elevated temperatures. J Hazard Mater. 2008;156:17–22. [6] Christodoulos PT, Petros GS, Costas NC. Catalytic removal of nitrates from waters in a continuous flow process: the remarkable effect of liquid flow rate and gas feed composition. Appl Catal B: Environ. 2011;102:54–61. [7] Cheng IF, Muftikian R, Fernando Q, Korte N. Reduction of nitrate to ammonia by zero-valent iron. Chemosphere. 1997;35:2689–2695. [8] Huang C-P, Wang HW, Chiu P-C. Nitratereduction by metallic iron. Water Res. 1998;32:2257–2264. [9] Zhang WX. Nanoscale iron particles for environmental remediation: an overview. J Nanopart Res. 2003;5:323–332. [10] Hwang Y-H, Kim D-G, Shin H-S. Mechanism study of nitrate reduction by nano zero valent iron. J Hazard Mater. 2011;185:1513–1521.
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[11] Zhang JH, Hao ZW, Zhang Z, Yang Y, Xu X. Kinetics of nitrate reductive denitrification by nanoscale zero-valent iron. Process Saf Environ Prot. 2010;88: 439–445. [12] Zhang H, Jin ZH, Han L, Qin CH. Synthesis of nanoscale zero-valent iron supported on exfoliated graphite for removal of nitrate. Trans Nonferrous Met Soc China. 2006;16: 345–349. [13] Jiang ZM, Lv L, Zhang WM, Du Q, Pan BC. Nitrate reduction using nanosized zero-valent iron supported by polystyrene resins: role of surface functional groups. Water Res. 2011;45:2191–2198. [14] Pan JR, Huang CP, Hsieh WP, Wu BJ. Reductive catalysis of novel TiO2 /Fe0 composite under UV irradiation for nitrate removal from aqueous solution. Sep Purif Technol. 2012;84:52–55. [15] Liou YH, Lo SL, Lin CJ, Kuan WH, Lin CJ, Weng SC. Chemical reduction of an unbuffered nitrate solution using catalyzed and uncatalyzed nanoscale iron particles. J Hazard Mater. 2005;127:102–110. [16] Hosseini SM, Ashtiani BA, Kholghi M. Nitrate reduction by nano-Fe/Cu particles in packed column. Desalination. 2011;276:214–221. [17] Choi JH, Shin WS, Choi SJ, Kim YH. Reductive denitrification using zero-valent iron and bimetallic iron. Environ Technol. 2009;30:939–946. [18] Kang H, Xiu Z, Chen J, Cao W, Guo Y, Li T, Jin Z. Reduction of nitrate by bimetallic Fe/Ni nanoparticles. Environ Technol. 2012;33:2185–2192. [19] Sparis D, Mystrioti C, Xenidis A, Papassiopi N. Reduction of nitrate by copper-coated ZVI nanoparticles. Desalination Water Treat. 2013;51:2926–2933. [20] Hörold S, Vorlop KD, Tacke T, Sell M. Development of catalyst for a selective nitrate and nitrite removal from drinking water. Catal Today. 1993;17:21–30. [21] Pintar A, Batista J, Levec J, Kajiuchi T. Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions. Appl Catal B. 1996;11:81–90. [22] Yurii MM, Viktor B, Igor Y, Moshe S. Cloth catalysts in water denitrification I. Pd on glass fibers. Appl Catal B. 2000;27:127–135. [23] Zhang Y, Chen YX. Catalytic reduction of nitrate in water over Pd − Cu/γ − Al2 O3 catalyst. Chinese J Catal. 2002;24:270–274.
[24] Jung J, Bae S, Lee W. Nitrate reduction by maghemite supported Cu-Pd bimetallic catalyst. Appl Catal B. 2012;127:148–158. [25] Soares OSGP, Órfã JJM, Gallegos-Suarez E, Castillejos E, Rodríguez-Ramos I, Pereira MFR. Nitrate reduction over a Pd-Cu/MWCNT catalyst: application to a polluted groundwater. Environ Technol (United Kingdom). 2012;33:2353– 2358. [26] Al Bahri M, Calvo L, Gilarranz MA, Rodriguez JJ, Epron F. Activated carbon supported metal catalysts for reduction of nitrate in water with high selectivity towards N2 . Appl Catal B. 2013;138–139:141–148. [27] Soares OSGP, Fan X, José JMÓ, Lapkin AA, Pereira MFR. Kinetic modeling of nitrate reduction catalyzed by PdCu supported on carbon nanotube. Ind Eng Chem Res. 2012;51:4854–4860. [28] Wang CY, Cheng L. Surface modification and structure characterization of nano-particles iron prepared with the liquid phase method. J Chem Phys. 1999;12:670–674. [29] Chen YX, Zhang Y, Chen GH. Appropriate conditions or maximizing catalytic reduction efficiency of nitrate into nitrogen gas in groundwater. Water Res. 2003;37:2489– 2495. [30] Chiueh PT, Lee YH, Su CY, Lo S-L. Assessing the environmental impact of five Pd-based catalytic technologies in removing of nitrate. J Hazard Mater. 2011;192:837–845. [31] Prüsse U, Vorlop K-D. Supported bimetallic palladium catalysts for water-phase nitrate reduction. J Mol Catal A: Chem. 2001;173:313–328. [32] Huang Y, Qin Z, Liu D, Wang X. Kinetics and effect factor of reductive denetrification with nanoscale zero-valent iron. Rock Miner Anal. 2011;30:51–58. [33] Hwang YH, Kim DG, Shin HS. Mechanism study of nitrate reduction by nano zero-valent iron. J Hazard Mater. 2011;185:1513–1521. [34] Huang YH, Zhang TC. Effects of low pH on nitrate reduction by iron powder. Water Res. 2004;38:2631–2642. [35] Su C, Puls RW. Kinetics of trichloroethene reduction by zerovalent iron and tin: pretreatment effect, apparent activation energy, and intermediate products. Environ Sci Technol. 1999;33:163–168. [36] Deng B, Burris DR, Campbell TJ. Reduction of vinyl chloride in metallic iron-water systems. Environ Sci Technol. 1999;33:2651–2656.
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0
Nitrate removal by Fe /Pd/Cu nano-composite in groundwater a
a
Hongyuan Liu , Min Guo & Yan Zhang
b
a
College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China b
Department of Civil Engineering, Zhejiang University, Hangzhou 310058, People's Republic of China Accepted author version posted online: 18 Oct 2013.Published online: 19 Nov 2013.
0
To cite this article: Hongyuan Liu, Min Guo & Yan Zhang (2014) Nitrate removal by Fe /Pd/Cu nano-composite in groundwater, Environmental Technology, 35:7, 917-924, DOI: 10.1080/09593330.2013.856926 To link to this article: http://dx.doi.org/10.1080/09593330.2013.856926
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Environmental Technology, 2014 Vol. 35, No. 7, 917–924, http://dx.doi.org/10.1080/09593330.2013.856926
Nitrate removal by Fe0 /Pd/Cu nano-composite in groundwater Hongyuan Liua∗ , Min Guoa and Yan Zhangb a College
of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China; b Department of Civil Engineering, Zhejiang University, Hangzhou 310058, People’s Republic of China
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(Received 6 May 2013; final version received 14 October 2013 ) Nitrate pollution in groundwater shows a great threat to the safety of drinking water. Chemical reduction by zero-valent iron is being considered as a promising technique for nitrate removal from contaminated groundwater. In this paper, Fe0 /Pd/Cu nano-composites were prepared by the liquid-phase reduction method, and batch experiments of nitrate reduction by the prepared Fe0 /Pd/Cu nano-composites under various operating conditions were carried out. It has been found that nanoFe0 /Pd/Cu composites processed dual functions: catalytic reduction and chemical reduction. The introduction of Pd and Cu not only improved nitrate removal rate, but also reduced the generation of ammonia. Nitrate removal rate was affected by the amount of Fe0 /Pd/Cu, initial nitrate concentration, solution pH, dissolved oxygen (DO), reaction temperature, the presence of anions, and organic pollutant. Moreover, nitrate reduction by Fe0 /Pd/Cu composites followed the pseudo-first-order reaction kinetics. The removal rate of nitrate and total nitrogen were about 85% and 40.8%, respectively, under the reaction condition of Fe–6.0%Pd–3.0%Cu amount of 0.25 g/L, pH value of 7.1, DO of 0.42 mg/L, and initial nitrate concentration of 100 mg/L. Compared with the previous studies with Fe0 alone or Fe–Cu, nano-Fe–6%Pd–3%Cu composites showed a better selectivity to N2 . Keywords: nitrate reduction; nano-Fe0 /Pd/Cu; ammonia; groundwater; anion
1. Introduction With the increasing contamination of groundwater from agriculture and wastewater, the nitrate pollution is being considered as a great threat to the safety of drinking water. Therefore, varieties of technologies, including physical, biological, and chemical removal processes were developed by researchers for nitrate removal from groundwater. The physical–chemical approaches such as reverse osmosis and ion-exchange treatment can effectively remove nitrate from water, but potentially generate concentrated waste-streams.[1,2] Biological denitrification is the most commonly used method, but is difficult to maintain, and needs to polish the effluent through purification and disinfection in order to secure the water quality for potable water purpose.[3,4] Recently, nitrate elimination from groundwater by chemical reduction, especially reduction by zerovalent iron (ZVI), has been received much attention as an efficient and cost-effective method.[5,6] Researches have conducted that nitrate reduction by ZVI is pH dependent and lower pH is favourable, and the main final product is ammonia.[7–10] Zhang et al. [11] reported that nitrate could be completely reduced by nanoscale zero-valent iron (NZVI) within 60 min with the dosage of 1.0 g/L, but about 85% nitrate was reduced to ammonia. The poor stability of NZVI and the generation of ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
by-product ammonia were the main problems to limit its further application. Therefore, NZVI supported on exfoliated graphite or TiO2 [12–14] and some other metal coated on NZVI [15,16] were conducted to decrease the percentage of transformation to ammonia and improve the selectivity to nitrogen. Bimetallic nanoparticles had a higher reaction rate, compared with those observed for NZVI alone.[15–19] Hosseini et al. [16] found that coating of NZVI by copper decreased the aggregation and agglomeration of NZVI, in addition to enhancing nitrate reduction rate in aqueous solution. Kang and his co-workers investigated that Fe/Ni particles showed a much higher nitrate reduction rate and an optimum reduction rate at near-neutral pH, although undesirable transformation of nitrate (91.0 ± 0.37%) to ammonium was observed.[18] Liou et al. [15] evaluated that Pd, Pt, and Cu separately deposited onto nano-Fe0 surface were ranked Cu > Pd > Pt in their promotion on nano-Fe0 reactivity towards nitrate, and coating of NZVI by copper-enhanced nitrate reduction rate and decreased ammonia formation, but resulted in the accumulation of nitrite. Although some improvements were made, the low selectivity to nitrogen in nitrate reduction by NZVI was still a problem. In addition, since Hörold et al. [20] first reported catalytic reduction of nitrate, palladium catalysts such as
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palladium–copper or tin catalysts were developed as the effective catalysts for reducing nitrate.[21–23] In particular, many studies showed that most of nitrate was reduced to nitrogen in catalytic nitrate reduction process over Pd– Cu bimetal catalysts.[24–26] Soares et al. [27] found that high selectivity to nitrogen, between 80% and 89%, was obtained using 1%Pd–1%Cu/Carbon Nanotube catalyst. However, H2 should be used as reducing agent in catalytic nitrate reduction by Pd–Cu catalysts. In addition, hydrogen might be generated in the nitrate reduction by a nano ZVI system. Thus, to combine with the advantages of the two processes mentioned above, Fe0 –Pd–Cu nano-composites were prepared in this study, in an effort to improve the selectivity to nitrogen. Fe0 –Pd–Cu nano-composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), SEM-energydispersive spectrum (EDS), and Brunauer–Emmett–Teller (BET) techniques. Nitrate reduction by Fe0 –Pd–Cu under different experimental conditions was systemically investigated, and the reaction kinetic of nitrate reduction over Fe0 –Pd–Cu nanoparticle was also discussed.
2. Materials and methods All chemical stock solutions were prepared from reagentgrade chemicals using doubled deionized water and stored at 4◦ C unless otherwise specified. Contaminated groundwater was simulated using pure water consisting of a certain concentration of KNO3 . 2.1.
Fe0 –Pd–Cu preparation
In this study, one-step and multi-step synthesis processes for preparation of Fe0 /Pd/Cu samples were carried out in advance, and the results showed that there was no obvious difference in the removal rate of nitrate. Therefore, one-step synthesis process by the liquid-phase reduction method was employed in this paper, as shown below: (1) appropriate amounts of FeCl3 · 6H2 O were added in 250 ml alcohol solution, which was purged with purified argon gas to reduce dissolved oxygen (DO); (2) appropriate amounts of 0.08 mol/L sodium borohydride (NaBH4 ) solution were added into FeCl3 · 6H2 O solution drop by drop under constant pressure, and purified argon gas was also purged during the whole process; (3) Fe0 samples were further washed with argon-purged, deoxygenated water several times until supernatant pH reached about 7; (4) appropriate amounts of PdCl2 and CuCl2 · H2 O solution were added into the prepared Fe0 solution drop by drop; and (5) Fe0 /Pd/Cu samples were washed and gathered for further use. During the preparation process, the reactions can be described as the follows: 4Fe3+ + 3BH− 4 + 9H2 O + → 4Fe0 ↓ +3H2 BO− 3 + 12H + 6H2 ↑,
(1)
Fe0 + Pd2+ → Fe2+ + Pd0 ,
(2)
Fe + Cu
(3)
0
2+
→ Fe
2+
+ Cu . 0
2.2. Fe0 –Pd–Cu characterization The morphology of Fe0 /Pd/Cu samples was characterized by SEM (S4800) and TEM(JEM-1200EX). The result could be seen from Figure 1. It shows that the average external diameter of the microsphere was about 20–80 nm, and aggregated nanoparticles could be observed in the formation of chain-like structures, which might result from magnetic property and large specific surface area of Fe0 /Pd/Cu particles.[28] Fe0 /Pd/Cu samples were also analysed by EDS using an Edax Genesis4000 energy dispersive spectrometer. It shows that the three metal elements peaks of Fe, Pd, and Cu were found, and the average contents of Pd and Cu were 5.95% (wt%) and 2.88% (wt%), respectively, which showed that the mass ratios of the metals were almost the same as that prepared. The BET surface area of the prepared Fe0 /Pd/Cu samples was also determined by nitrogen adsorption with a TriStar II 3020 (Micromeritics Instrument Co., USA) surface area and porosity analyser. It was investigated that the BET surface area was 51 m2 /g. 2.3. Batch experiments Batch experiments were carried out in a 1000 ml reactor under constant temperature and atmospheric pressure. Prior to the experiments, predetermined concentration of nitrate solution was filled in the reactor and argon gas was then introduced from the bottom of the reactor through a diffuser in order to strip out dissolved and adsorbed oxygen. Fe0 /Pd/Cu composites were suspended in the solution through stirring, during which predetermined quantities of Fe0 /Pd/Cu composites were added to the reactor. Before the experiments, pH was conducted by adding NaOH or HCl solution, and no buffer was added during experimental periods. The solution temperature was controlled using an electro thermostatic water bath. The experiments were carried out under the amount of Fe–6.0%Pd–3.0%Cu, pH, DO, and initial nitrate concentration of 0.25 g/L, 7.1, 0.42 mg/L, and 100 mg/L, respectively, unless otherwise specified. 2.4. Analysis methods Samples were periodically collected to analyse the concen− − tration of NO− 3 -N, NO2 -N, and NH4 -N. Standard methods GB5750-2006 in China were adopted for the analysis of all the parameters. Ammonia was analysed with the Nessler reagent spectrophotometric method, nitrate was analysed with the thymol metric method, nitrite was analysed with the N -(1-naphthyl) ethylenediamine spectrophotometric method, pH, and DO were periodically monitored with a Hach HQ40d pH and DO analyzers, respectively, and Total
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(a)
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(b)
(c) 6.0 Fe
5.0
Counts
4.0 3.0 2.0 1.0
Fe Cu
Pd
Fe
Cu 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.011.012.0 Energy (keV)
Figure 1.
SEM, TEM, and EDS images of prepared
Fe0 /Pd/Cu.
Organic Carbon (TOC) was measured with a TOC-V CPH analyzer. 3. Results and discussion 3.1. The mass ratio of Pd and Cu According to previous research, the metal Cu can enhance nitrate reduction, especially for the generation of intermediate production (nitrite).[16] In order to examine the role of metal Cu and determine the appropriate mass ratio, various percentages of Cu (0%, 0.75%, 1.5%, 3.0%, and 4.5% w/w) were deposited onto the surface of nano-Fe0
particles. Figure 2 shows the concentration percentage (the current concentration at t time divided by the initial con− centration of nitrate–nitrogen) profiles of NO− 3 -N, NO2 -N, − and NH4 -N during the batch experiments with different ratios of Cu deposited on the ZVI surface. The result shows that nitrate was obviously reduced and then increased with an increase in the mass ratio of Cu. The percentage of nitrate–nitrogen decreased from 25.04% with Fe alone to 0.75% with an extra mass ratio of 1.68% Cu, and then increased to 22.35% with 4.5% Cu, showing that the presence of a certain amount of Cu could promote nitrate reduction.
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Nitrate Nitrite Ammonia
Percentage (%)
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30
20
10
0
0%Cu
0.75%Cu
1.5%Cu
3.0%Cu
4.5%Cu
50
Nitrate Nitrite Ammonia
Percentage (%)
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10
0 0%Pd
3.0%Pd
6.0%Pd
9.0%Pd
Mass Fraction of Pd (%)
50
TN Removal perceentage (%)
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Mass fraction of Cu (%)
40.84
40
35.27
30 20
27.61 17.08
10 0
0%Pd
3.0%Pd
6.0%Pd
9.0%Pd
Mass fraction of Pd (%)
of 4.5%. This shows that the excess copper deposited on the ZVI surface resulted in decreasing the surface reaction activated sites, and then decreased the percentage of nitrate removal rate. In addition, the concentration of ammonia decreased with the increasing of mass ratio of Cu, and the percentage of ammonia slightly decreased from 38.8% to 27.0%, with the mass ratio of Cu increasing from 3.0% to 4.5%. Further analysis indicated that the percentages of nitrate and total nitrogen with the Cu mass ratio of 3.0% were lower than that of 4.5%. Therefore, the optimum mass ratio of Cu was 3.0% under the experimental conditions, which was consistent with previous results.[16] As the above mentioned, the concentrations of nitrite and ammonia still remained high when only Cu deposited on the ZVI surface. However, metal Pd showed a high selectivity to nitrogen in catalytic nitrate reduction,[29,30] therefore Pd was further deposited on the Fe0 /3.0%Cu surface, that is to say, nano-Fe0 /Pd/Cu composites were prepared for nitrate removal in the next experiments. Figure 2 illustrates that nitrate removal rate decreased with an increase in the mass ratio of Pd. However, the percentages of nitrite and ammonia did not follow the same trend. The percentage of nitrite decreased obviously, and the percentage of ammonia first decreased and then increased. Moreover, the influence of the mass ratio of Pd on the N2 selectivity exhibited a maximum value over Pd loading of 6% followed by a drop with a further increase in the mass ratio of Pd. Previous literatures reported that nitrate did not be adsorbed at the Pd surface, but could be adsorbed and reduced to nitrite at Cu surface.[29,31] For this reason, the surface active sites decreased with an increase in the mass ratio of Pd, thereby leading to a decrease in nitrate removal rate. On the other hand, nitrite could be adsorbed and reduced at Pd surface, thus the introduction of Pd catalyst was effective for nitrite reduction to nitrogen, thereby resulting in less accumulation of nitrite and ammonia formation. The results show that the introduction of 6% Pd was able to maximize reduction of nitrate into nitrogen gas, and minimize nitrite and ammonia formation, thus maintained a high removal efficiency of nitrate and enhanced the selectivity to N2 . Under the experimental conditions, as much as 40.8% selectivity to N2 was obtained and the generation of ammonia decreased to 33.7%. It should be noted that nano-Fe–6%Pd–3%Cu composites have a better selectivity to N2 , compared with the previous studies with Fe0 alone or Fe0 /Cu reducer.[11,16] Thus, nano-Fe– 6%Pd–3%Cu composites were prepared and employed to the following research in this study.
Figure 2. The effect of mass ratio of Cu and Pd on nitrate reduction.
3.2. The percentage of nitrite generation increased to the maximum value of 39.5% with Cu mass ratio of 3.0%, and then decreased slightly to 36.7% with Cu mass ratio
Effect of the amount of Fe0 /Pd/Cu
Figure 3 illustrates the effect of the amount of Fe0 /Pd/Cu on nitrate reduction. An increase in the Fe0 /Pd/Cu amount improved nitrate removal rate. When Fe0 /Pd/Cu dosage
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0.125g 0.250g 0.500g
Nitrate (mgN/L)
20 16 12 8 4 0 0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min)
Ammonium (mgN/L)
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20
of Fe0 /Pd/Cu at 0.250 g was selected as the operating parameter in this study, unless otherwise specified. 3.3. Effect of initial concentration of nitrate Figure 4(a) illustrates that nitrate removal rate showed a significant decrease with the reaction time going. It can be explained that iron oxides and hydroxides were formed with the reaction time going, which reducing the nitrate removal rate. It was further confirmed that pH value increased from 7.10 to 9.64, 10.56, and 10.65 under the initial concentration of nitrate of 50, 100, and 200 mg/L, respectively, at the reaction time of 60 min. The reactions were shown as following: 0 − 2NO− 3 + 5Fe + 6H2 O → 5Fe(OH)2 + N2 + 2OH , (4)
0.125g 0.250g 0.500g
− 0 NO− 3 + Fe + H2 O → Fe(OH)2 + NO2 ,
16
NO− 3
8
4
5
10 15 20 25 30 35 40 45 50 55 60
Time (min)
Figure 3. removal.
+ 4Fe + H2 O → 4Fe3 O4 + 0
NH+ 4
(5) −
+ 2OH ,
(6)
+ 0 − NO− 3 + 4Fe + 7H2 O → 4Fe(OH)2 + NH4 + 2OH . (7)
12
0 0
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The effect of the amount of Fe0 /Pd/Cu on nitrate
was 0.125, 0.250, and 0.500 g, nitrate removal rate was 61.73%, 82.26%, and 100% with the reaction time of 60 min, respectively. It can be explained that Fe0 /Pd/Cu surface area increased with the increasing amount of Fe0 /Pd/Cu, thereby resulting in more contact chance between nitrate and Fe0 /Pd/Cu, which was beneficial for nitrate reduction.[11,32] Moreover, similarly as nitrate reduction over ZVI or nano ZVI,[33] nitrate reduction over Fe0 /Pd/Cu can also be described by pseudo-first-order reaction kinetics. The apparent surface reaction rate constants kobs (kobs = ln(C0 /C)/t, C0 and C represent the initial and current concentrations of nitrate, respectively) of nitrate reduction increased from 0.0124 to 0.0964 min−1 with the amount of Fe0 /Pd/Cu increasing from 0.125 to 0.500 g. In addition, the concentration of ammonia increased with an increase in the amount of Fe0 /Pd/Cu. Above 83% nitrate transformed to ammonia with the amount of nano-Fe0 /Pd/Cu 0.500 g, while only 19.16% and 35% of nitrate transformed to ammonia with that of 0.125 and 0.250 g, respectively. Therefore, in order to effectively remove nitrate and reduce ammonia formation, an amount
In addition, a high initial concentration of nitrate showed a high initial reduction rate of nitrate. The apparent surface reaction rate constants kobs increased from 0.0115 to 0.0418 min−1 with the initial nitrate concentration increasing from 50 to 200 mg/L. However, nitrate removal rates were 96.99%, 82.26%, and 56.75% at the reaction time of 60 min, when the initial concentrations of nitrate were 50, 100, and 200 mg/L, respectively. It can be explained that the amount of nano-Fe0 /Pd/Cu of 0.25 g was not enough to react with the initial nitrate concentration of 200 mg/L, according to the above chemical reaction equation (4). While the reaction followed Equation (6) or (7), the amount dosage of 0.25 g was not enough for initial concentration of 100 and 200 mg/L. Therefore, the initial concentration of nitrate 100 mg/L was selected as the operating parameter in this study, unless otherwise specified. 3.4. Effect of pH No buffer was added during reaction periods. The effect of pH on nitrate reduction was investigated with different initial pH values of 3.5, 5.0, 7.1, 9.0, and 10.5. Figure 4(b) shows that an increase in pH value resulted in a decrease in nitrate removal rate. As described in Equations (4)–(7), when nitrate was reduced, hydroxide ions were formed. Because no additional acid was added, the OH− accumulated, thus restrained the nitrate reduction reactions. Further analysis indicated that, while pH value decreased from 7.1 to 5.0, nitrate removal rate did not increase but slightly decreased, and pH value continued to decrease to 3.5, nitrate removal rate slightly improved again. This can be explained as following, first, when pH value decreased, the concentration of H + increased, which could accelerate nitrate
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(a) 1.2 50 mg/L 100 mg/L 200 mg/L
1.0
Nitrate (mgN/L)
C/C0
20 16
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pH = 3.5 pH = 5.0 pH = 7.10 pH = 9.0 pH = 10.5
0
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(c)
T = 10ºC T = 15ºC T = 20ºC T = 25ºC T = 30ºC
16 12 8
0.42mg/L 6.05mg/L 10.41mg/L
20 16 Nitrate (mgN/L)
Nitrate (mgN/L)
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(d) 24
24
12 8 4
4 0 0
5
10 15 20 25 30 35 40 45 50 55 60
0
0
Time (min)
Figure 4. removal.
5
10 15 20 25 30 35 40 45 50 55 60 Time (min)
The effect of (a) the initial concentration of nitrate, (b) initial pH value, (c) temperature, and (d) initial DO value on nitrate
removal and hinder the formation of iron oxide layers covering at the same time. On the other hand, the reaction between iron and H + happened at the surface of nanoFe0 /Pd/Cu, which would consume a certain amount of nano-Fe0 /Pd/Cu, thereby hindering nitrate reduction.[34] Meanwhile, nitrate could be reduced over Pd and Cu catalyst with H2 as reducer, which improving the nitrate reduction. Due to the dual functions mentioned above, nitrate removal rate has no obvious difference at pH 3.5 and 7.1. Therefore, a pH value of 7.1 was selected as the operating parameter in this study, unless otherwise specified. 3.5.
10 15 20 25 30 35 40 45 50 55 60
Effect of reaction temperature
Reaction temperature plays an important role on the chemical reaction rate. In order to determine the effect of the temperature on nitrate removal rate, the experiments were carried out at the temperature of 10◦ C, 15◦ C, 20◦ C, 25◦ C, and 30◦ C, respectively. Figure 4(c) shows that nitrate removal rate increased with the temperature rising. At the reaction time of 60 min, nitrate removal rates were about 58.76%, 73.85%, 81.81%, 82.88%, and 86.15%, respectively, with the reaction temperature of 10◦ C, 15◦ C, 20◦ C, 25◦ C, and 30◦ C. kobs of nitrate reduction was found to follow a linear relationship with the temperature, and increased from 0.012 to 0.029 min−1 with the temperature raising from 10◦ C to 30◦ C. Using the Arrhenius equation,[35] the apparent
activation energy for nitrate reduction can be calculated from the change of temperature and apparent rate constant: ln kobs = −
E + const, RT
(8)
where kobs is the apparent rate constant, min−1 ; E is the apparent activation energy, kJ/mol; T is the reaction temperature, K; and R is the universal gas constant, R = 8.314 J/(mol.K). Under the experimental conditions, E value was equal to 30.50 kJ/mol. The low value of apparent activation energy obtained in this work further confirmed that nitrate reduction by Fe0 /Pd/Cu was an effective technique, and followed a surface-controlled reaction.[36] 3.6.
Effect of DO value
Similarly, DO may have an important influence on nitrate reduction by Fe0 /Pd/Cu. Experiments under initial DO values of 0.42, 6.05, and 10.41 mg/L were carried out to study the effect of DO on nitrate reduction. As shown in Figure 4(d), nitrate removal rate significantly declined with an increase in DO value. Nitrate removal rate decreased from 82.26% to 20.35%, with the DO value increasing from 0.42 to 10.41 mg/L. It could be explained that Fe0 /Pd/Cu particles were oxidized by oxygen, and formed a passive iron oxide layer at the surface of Fe0 /Pd/Cu particles.
Environmental Technology 2.5
0mg/L 0.7mg/L Chlorate 10µg/L Bromate 1mgP/L Phosohate 250mg/L Sulfate 250mg/L Bicarbonate 5mg/L TOC
ln(C0/C)
2
1.5
1
0.5
0 0
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Figure 5. time.
10
20
30 40 Time (min)
50
60
70
The relationship between ln (C0 /C) and the reaction
Table 1. Corresponding kobs and R2 of nitrate reduction under different conditions. Oxyanion or organic pollutant 0 mg/L Chlorate (0.7 mg/L) Bromate (10 μg/L) Phosphate (1 mg/L) Sulphate (250 mg/L) TOC (5 mg/L)
Apparent rate constant kobs (min−1 )
Correlation coefficient R2
.0240 .0206 .0195 .0118 .0082 .0228
.9892 .9890 .9609 .9840 .9416 .9989
That is to say, the competing reaction with oxygen and Fe0 happened at the surface reaction active sites of Fe0 /Pd/Cu, and then resulted in decreasing the nitrate reduction rate. 3.7.
Effect of anions and organic pollutant
At the same time, the influences of the presence of anions and organic pollutant on nitrate removal over nanoFe0 /Pd/Cu were conducted in this study. The concentrations of anions and TOC were chosen basing Drinking Water Standard GB5749-2006 in China. The relationships between ln (C0 /C) and the reaction time were displayed in Figure 5 and Table 1. Figure 5 and Table 1 illustrate that the presence of all anions or TOC decreased the nitrate removal rate. It can be explained that chlorate and bromate competed with nitrate to be adsorbed on the active sites, and their presence made kobs decrease slightly, from 0.0241 to 0.0206 and 0.0195 min−1 , respectively. Phosphate reacted with iron to form complexes, which adsorbed in nano-Fe0 /Pd/Cu surface, then reduced the nitrate reduction. Sulphate might destroy the surface passive film of nano-Fe0 /Pd/Cu, to accelerate its corrosion, and then kobs decreased to 0.0082 min−1 . While the TOC value at 5 mg/L, kobs also decreased to 0.0228 min−1 . Under experimental conditions, the influences of the presence of anions on nitrate removal 3− − − rate followed the order of SO2− 4 > PO4 > BrO3 > ClO3 .
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4. Conclusions In this paper, nano-Fe0 /Pd/Cu composites were prepared, and its application to nitrate reduction was systematically conducted. The results show that nano-Fe0 /Pd/Cu composites had the dual function of catalytic reduction and chemical reduction. The introduction of the metals of Pd and Cu not only improved nitrate removal rate, but also reduced the generation of by-product ammonia. As much as 40.8% total nitrogen removal rate was obtained under the experimental conditions. The nitrate reduction over Fe0 /Pd/Cu can be described by pseudo-first-order reaction kinetics. The low value of apparent activation energy obtained in this work confirmed that nitrate reduction by Fe0 /Pd/Cu followed a surface-controlled reaction and was a promising technique for nitrate removal from water. However, the presence of anions decreased the nitrate removal rate, and the influences of the presence of anions on nitrate removal rate increased as 3− − − follows: SO2− 4 > PO4 > BrO3 > ClO3 under the experimental conditions. Of course, the related research under the real groundwater conditions needs further study. Funding This research was supported by National Natural Science Foundation of China [No. 51108407] and the Program for Zhejiang Leading Team of S&T Innovation [No. 2010R50037]. References [1] Samatya S, Kabay N, Yuksel U, Rda M, Yuksel M. Removal of nitrate from aqueous solution by nitrate selective ion exchange resins. React Funct Polym. 2006;66:1206–1214. [2] Fernández-Nava Y, Maranón E, Soons J, Castrillon L. Denitrification of wastewater containing high nitrate and calcium concentrations. Bioresour Technol. 2008;99:7976–7981. [3] Della RC, Belgiorno V, Meric S. Heterotrophic/autotrophic denitrification (HAD) of drinking water: prospective use for permeable reactive barrier. Desalination. 2007;210:194– 204. [4] Liu H, Jiang W, Wan D, Qu J. Study of a combined heterotrophic and sulfur autotrophic denitrification technology for removal of nitrate in water. J Hazard Mater. 2009;169:23–28. [5] Ahn SC, Oh SY, Cha DK. Enhanced reduction of nitrate by zero-valent iron at elevated temperatures. J Hazard Mater. 2008;156:17–22. [6] Christodoulos PT, Petros GS, Costas NC. Catalytic removal of nitrates from waters in a continuous flow process: the remarkable effect of liquid flow rate and gas feed composition. Appl Catal B: Environ. 2011;102:54–61. [7] Cheng IF, Muftikian R, Fernando Q, Korte N. Reduction of nitrate to ammonia by zero-valent iron. Chemosphere. 1997;35:2689–2695. [8] Huang C-P, Wang HW, Chiu P-C. Nitratereduction by metallic iron. Water Res. 1998;32:2257–2264. [9] Zhang WX. Nanoscale iron particles for environmental remediation: an overview. J Nanopart Res. 2003;5:323–332. [10] Hwang Y-H, Kim D-G, Shin H-S. Mechanism study of nitrate reduction by nano zero valent iron. J Hazard Mater. 2011;185:1513–1521.
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