Abstract

In this study, we demonstrated that the choice of precursor has a strong effect on the reduction of nitrate (NO3) using zero-valent copper (Cu0) synthesized by sodium borohydride (NaBH4). Different precursors: CuSO4, CuO, Cu2O, Cu powder, and Cu mesh were used to reduce NO3 at 677 mg-N/L under the reducing conditions of NaBH4. Compared with the prehydrolyzed samples, those prepared without prehydrolysis exhibited lower reduction rates, longer times and higher concentrations of nitrite (NO2) intermediate. It was found that one-time addition of NaBH4 resulted in higher reduction rate and less NO2 intermediate than two-step addition. Results showed that Cu0 from CuSO4 possessed the smallest particle size (890.9 nm), highest surface area (26.0 m2/g), and highest reaction rate (0.166 min−1). Values of pseudo-first-order constant (kobs) were in the order: CuSO4 > CuO > Cu2O > Cu powder >Cu mesh. However, values of surface area-normalized reaction rate (kSA) were approximately equal. It was proposed that NO3 was reduced to NO2 on Cu0, and then converted to NH4+ and N2, respectively; H2 generated from both NaBH4 hydration and Cu (II) reduction contributed to NO3 reduction as well.

INTRODUCTION

Excess amounts of nitrate (NO3) in surface water may result in eutrophication, and its presence in ground water also poses risks to human health. High concentrations of NO3 are found in wastewater from several industries, such as the synthetic fiber, mineral processing, fertilizer, and nuclear industries (De Filippis et al. 2013). Therefore, treatment and disposal of such concentrated NO3 wastewater is a major concern. However, previous research mainly focused on biological treatment processes, and chemical reduction of these industrial wastewaters containing highly concentrated NO3 has not been examined as extensively.

Studies have demonstrated that chemical reduction is one of the effective technologies for removing different contaminants, such as chloroform, NO3, 2,4,6-trichlorophenol, and acetaminophen (Feng & Lim 2005; Kumar & Chakraborty 2006; Choi & Kim 2009; Zhang et al. 2012; Jie et al. 2017). To reduce NO3 from aqueous solutions using chemical reduction, an electron donor has to be provided. Thermodynamically, the reduction of NO3 by zero-valent copper (Cu0) is feasible since its standard electrode potential is lower than NO3 (Snoeyink & Jenkins 1980). Furthermore, studies suggest that precursor type also plays important role in chemical reactions. Panpranot et al. (2005) concluded that palladium silica (Pd/SiO2) catalyst prepared from palladium(II) chloride (PdCl2) precursor resulted in smaller Pd particles, higher dispersion, and consequently higher hydrogenation activities than that prepared from other Pd salts. Lu et al. (2008) pointed out that the activities of copper catalysts prepared from copper(II) nitrate (Cu(NO3)2) showed a better performance than that from copper(II) acetate (Cu(CH3COO)2). Other than precursor types, precursor concentration affects reactions as well. For example, the precursor's concentration of iron strongly affected the NO3 reduction efficiency (Liou et al. 2006).

Despite the growing literature on NO3 reduction by zero-valent metals (ZVMs), there are not many studies that evaluate the application to concentrated systems, and the effect of precursor type. Therefore this work investigates the effect of copper precursor on concentrated NO3 reduction. In this study, five types of copper precursors: copper sulphate (CuSO4), cupric oxide (CuO), cuprous oxide (Cu2O), copper powder (Cu) and copper mesh were examined. The precursor was used to prepare fresh Cu0 surface in situ using sodium borohydride (NaBH4) for the reduction of NO3. In addition, the effects of prehydrolysis and mode of addition of the reducing agent (NaBH4) were investigated. Nitrate concentration (677 mg-N/L) was chosen to prepare synthetic, concentrated wastewater. In addition, to simulate the molybdenum-99 (Mo-99) wastewater from a nuclear facility, the reduction reaction with an initial concentration of NO3 up to 20,322.58 mg-N/L was also examined.

METHODS AND MATERIALS

Materials

Sodium nitrate (NaNO3; 99 + %) and other chemicals including NaBH4 (98 + %), CuO (97%), Cu2O (97%), Cu (99%) were obtained from Acros, CuSO4 from Shimakyus Pure Chemicals, copper mesh (0.11 mm) from Alfa Aesar and sulphuric acid (H2SO4; >95%) from Fisher Scientific. The synthetic wastewater containing NO3 was prepared by dissolving NaNO3 in ultrapure deionized water. All chemicals used in the experiments were of analytical reagent grade and used as received without further purification.

Batch experiments

Batch experiments were conducted by using a method mainly adapted from previous work (Belay et al. 2015). For determining the effect of prehydrolysis, the copper precursor was first mixed with 250 mL of NO3 solution in a three-neck glass flask and stirred at 100 rpm and 25 °C for 12 hours as the prehydrolysis period. The suspensions were heated using a water bath and purged with argon gas. Subsequently, NaBH4 was introduced when the temperature reached 60 °C. The same procedures but in the absence of 12 hours stirring of the copper precursor and NO3 solution (without prehydrolysis) was performed to compare the effect of prehydrolysis on the reduction reaction. Regarding the effect of NaBH4 dosing mode, two-step addition of NaBH4 was adopted for comparison, in which one half was added at the beginning, and the remaining half after 1 hour of reaction. In all cases, 3 mL samples were withdrawn periodically, filtered, and analyzed for NO3, NO2, NH4+, and dissolved Cu concentrations. The results are shown as the mean of duplicate experiments.

Sample analysis

Concentration of NO3 and NO2 in solution samples were measured using an ion chromatograph (Dionex ICS-1000, USA). The eluent was a mixture of 3.2 mM sodium carbonate (Na2CO3) and 1.0 mM sodium bicarbonate (NaHCO3). Prior to measurement, the samples were filtered using a 0.45 μm polyvinylidene difluoride membrane (Acrodisc). NH4+ was analyzed using an ultraviolet-visible spectrophotometer (Model V-500, Jasco, Japan) by the indophenol method (APHA 1995). Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Horiba Jobin Yvon, JY 2000, France) was employed to detect the dissolved Cu concentration. The reduction performance of the precursor was evaluated with NO3 conversion which was calculated as follows:  
formula
(1)
where [NO3]0 and [NO3]f are the initial and final concentrations of NO3, respectively.

Characterization

All of the samples for characterization were filtered and repeatedly rinsed with distilled water before analysis. Powder X-ray diffraction (XRD) of samples was assessed by a diffractometer (Bruker, Germany) using CuKα radiation set at 30 kV and 100 mA. Diffraction patterns were collected in the 2θ range from 20° to 80° at a scan rate and sampling interval of 0.5 min−1 and 0.05 min−1, respectively. Morphology of the Cu from different precursors after reduction reactions was examined using a scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS; JSM 6500F, JEOL, USA) at 15 kV. Particle size was measured using Zeta PALS (Brookhaven Instruments, USA). Specific surface area was measured using Brunauer–Emmett–Teller (BET) gas adsorption isotherm with N2 gas (ASAP 2020, Micromeritics, USA).

RESULTS AND DISCUSSION

Effect of prehydrolysis

Experiments were conducted to investigate the prehydrolysis effect, in which initial NO3 concentration was 677 mg-N/L, and the copper and NaBH4 doses were 0.245 g/L and 4.16 g/L, respectively. Results revealed that the prehydrolysis of CuO prior to the addition of NaBH4 was beneficial to NO3 reduction (Figure 1). Compared with the prehydrolyzed copper precursor (kobs = 0.058 min−1), samples prepared without prehydrolysis showed lower reduction rate (kobs = 0.045 min−1). Hence, prehydrolysis improved the efficiency of NO3 reduction by Cu0. Different studies have shown the importance of prehydrolysis for various applications. For example, Chen et al. (2012) observed a well-ordered structure and narrowly distributed pore size by using prehydrolysis during synthesis of amino-functionalized SBA-15 (Santa Barbara Amorphous) material. It has been reported that prehydrolysis promotes homogenous mixing and improved reactivity (Miller et al. 1994a, 1994b). It has also helped the development of surface hydroxyl groups. Prehydrolysis was thus adopted for following experiments.

Figure 1

Effect of prehydrolysis on nitrate reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 1

Effect of prehydrolysis on nitrate reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Effects of borohydride dosing mode

The dosing mode of NaBH4 was investigated using two ways of dosing. The first mode was done by adding 4.16 g/L of NaBH4 after the solution was heated to 60 °C following prehydrolysis. In the second mode, NaBH4 was added in two steps, one half at the beginning (after heated to 60 °C) and the other half after 1 hour of reaction. As illustrated in Figure 2, the observed rate of both experiments did not show a significant difference. However, for the two-step addition, there was higher amount of NO2 after 20 min and it rapidly decreased when the remaining NaBH4 was added. Since NO2 has a lower affinity to Cu0 (Liou et al. 2005), the presence of NO2 in the system tended to be high if NaBH4 concentration was not high enough. More NaBH4 increased the conversion of copper precursor to Cu0 and facilitated the production of hydrogen (H2) due to hydrolysis of NaBH4 with water (Zhang et al. 2010). Therefore, the two-step addition of NaBH4 resulted in a higher amount of NO2 initially, implying that NaBH4 was responsible for intermediate (NO2) conversion. In addition to NO2 formation, the two-step addition mode took longer for complete reduction. Nitrate was reduced completely (100%) within 55 min when all NaBH4 was added at the beginning, whereas 4% of NO3 remained in the system and complete reduction was found at ca. 60 min in two-step addition mode. This indicated method of NaBH4 addition affected the reactions. Therefore, one-step dosing was chosen as the method for further experiments.

Figure 2

Effect of mode of borohydride addition on nitrate reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 2

Effect of mode of borohydride addition on nitrate reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Effects of precursor type

The particle size of Cu0 synthesized from different precursors was measured and results revealed that particles from CuSO4, CuO and Cu2O had mean diameters (volume-based) of 890, 1,956 and 2,603 nm, respectively. This clearly indicated that the size of particles was highly dependent on the precursor salts used (Mohammed et al. 2012). The Cu0 particles produced from CuSO4 had the largest surface area (26.0 m2/g), followed by those synthesized using CuO and Cu2O. The SEM pictures revealed that Cu0 from different precursors had different structures and sizes. It was observed that the particles from CuSO4 (Figure 3(a)) were smaller and better dispersed when compared with those from CuO (Figure 3(b)). In addition, Cu0 from CuO taken at high magnification ratio showed aggregated particles. In contrast, Cu samples from Cu powder and Cu mesh maintained their shape and size (figures not shown). The EDS analysis of all samples revealed the presence of Cu element only. The appearance of different particles showed the effect of precursor type on structure as reported by previous work (Mohammed et al. 2012; Ana et al. 2016; Laura et al. 2016). XRD patterns of particles from CuSO4, CuO, Cu2O, Cu, and Cu mesh are shown in Figure 4. Three clear peaks were identified as Cu0 after reduction by NaBH4. From the result, it can be concluded that the particles were mainly composed of Cu0. A small peak, which was identified as CuO, was detected in some cases, which may be related to incomplete reduction of the precursor or EDX influence of air during sample preparation and measurement.

Figure 3

SEM and EDS of particles from different precursors after NO3 reduction: (a) CuSO4; (b) CuO ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 3

SEM and EDS of particles from different precursors after NO3 reduction: (a) CuSO4; (b) CuO ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 4

XRD pattern of particles from different precursors after NO3 reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 4

XRD pattern of particles from different precursors after NO3 reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Previous studies have shown that the reduction rate of different contaminants with zero-valent metals usually follows the pseudo-first-order kinetic model (Feng & Lim 2005; Liou et al. 2005; Wang et al. 2006; Belay et al. 2015). Results of this study indicated that the reduction of NO3 by Cu0 in the presence of NaBH4 could also be described by the pseudo-first-order kinetic model:  
formula
(2)
where [NO3] is nitrate concentration and kobs (min−1) is the observed pseudo-first-order rate constant. Experimental results showed that the removal rates of NO3 were different for different precursors. As shown in Figure 5, the reduction of NO3 by Cu0 from CuSO4 was the fastest among five types of precursors. A much greater BET surface area (26.0 m2/g) and lower particle size (897 nm) for Cu0 from CuSO4 as compared to those from CuO and Cu2O should account for the higher reaction rate for NO3. The effect of Cu0 surface area (CuSO4, CuO, and Cu2O) on the observed rate constant for NO3 reduction is shown in Figure 6. The results indicated that the observed reduction rate constants, kobs, increased with surface area. The kobs were 0.036, 0.058, and 0.166 min−1 with surface areas of 6.1, 9.5, and 26.0 m2/g for Cu0 particles formed from Cu2O, CuO and CuSO4, respectively. NO3 reduction with copper mesh was negligible and it was in good agreement with the finding of Fanning et al. (2000). However, the copper powder showed slow reduction reaction with kobs value of 0.015 min−1. This result was different from previous work, which concludes no reaction when using powder copper (Fanning et al. 2000). This could probably imply that prehydrolysis improved the reduction of NO3 by Cu0. The rate constants (kobs) for the three precursors are summarized in Table 1. These results revealed that the precursor with small particle size and larger surface area resulted in the higher observed reaction rate. Therefore, the degree of effectiveness of the precursor was in the following order: CuSO4 > CuO > Cu2O > Cu powder >Cu mesh. Hence, Cu0 resulting from CuSO4 was used for comparison with other works (Liou et al. 2005; Kumar & Chakraborty 2006; Ahn et al. 2008; Fan et al. 2009) (Table 2). From the table, it may be concluded that this method is efficient for concentrated NO3 reduction.
Table 1

Values of pseudo-first-order reaction rate constants (kobs) and surface-area normalized (kAS) ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, T = 60 °C)

Precursor kobs (min−1kSA (10−3)(L min−1 m−2
CuSO4 0.1660 26 
CuO 0.0580 24 
Cu20.0400 23 
Precursor kobs (min−1kSA (10−3)(L min−1 m−2
CuSO4 0.1660 26 
CuO 0.0580 24 
Cu20.0400 23 
Table 2

Comparison of different zero-valent metals (ZVM) used for reduction of NO3

ZVM T (°C) [NO3]0 (mg-N/L) kobs(10−2) (min−1BET area (m2 g−1kSA(10−3) (L min−1 m−2Reference 
Nano Fe0 25 40 1.37 16.67 2.326 Liou et al. (2005)  
Al0 25 25 6.2 – – Luk & Yeung (2002)  
Mg0 50 100 27.10 – – Kumar & Chakraborty (2006)  
Fe0 75 30 10.90 – – Ahn et al. (2008)  
Fe0 20–22 400 3.61 – – Fan et al. (2009)  
Cu0 60 677 16.6 26.03 26 This study 
ZVM T (°C) [NO3]0 (mg-N/L) kobs(10−2) (min−1BET area (m2 g−1kSA(10−3) (L min−1 m−2Reference 
Nano Fe0 25 40 1.37 16.67 2.326 Liou et al. (2005)  
Al0 25 25 6.2 – – Luk & Yeung (2002)  
Mg0 50 100 27.10 – – Kumar & Chakraborty (2006)  
Fe0 75 30 10.90 – – Ahn et al. (2008)  
Fe0 20–22 400 3.61 – – Fan et al. (2009)  
Cu0 60 677 16.6 26.03 26 This study 
Figure 5

Reaction rate of NO3 reduction using different precursors. ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 5

Reaction rate of NO3 reduction using different precursors. ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 6

Correlation for kobs and surface area. ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 6

Correlation for kobs and surface area. ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

The observed rate constants could be normalized by taking into account of surface area of Cu0. To describe this, the pseudo-first-order kinetic model used to determine kobs can be modified as:  
formula
(3)
where kSA is the surface area-normalized reaction rate constant (L m−2 min−1), ρa is the surface area of Cu0 (m2 L−1 of solution). The surface area-normalized rate constants (kSA) under current experimental conditions are shown in Table 1. Cu0 from CuSO4 showed slightly higher kSA when compared with those from other precursors. Since kSA is a more general index of reactivity of Cu0, it can be seen from the result that the three precursors showed approximately equal reactivity.

The NO3 reduction profiles with different initial NO3 concentration, [NO3]0, (from 677 to 1,355 mg-N/L) are shown in Figure 7(a) and 7(b). Nitrate was almost completely reduced within 25 min when [NO3]0 was 677 mg-N/L. At higher NO3 loadings the reduction decreased to 97%, 92.9% and 89% at [NO3]0 of 903, 1,129, and 1,355 mg-N/L, respectively. The slower reaction rate at higher [NO3]0 was because of the less active sites of Cu0 available. The pseudo-first-order rate constant (kobs) ranged from 0.166 to 0.146 min−1 with an average value of 0.154 min−1, which indicated that [NO3]0 had an insignificant effect on kobs. This phenomenon was also found in some studies on NO3 reduction using zero-valent iron (Choe et al. 2000; Fan et al. 2009). Therefore, it may be concluded that under the conditions of the experiment, the reaction is a pseudo-first-order reaction within the provided concentration range.

Figure 7

(a) Effect of [NO3]0 on NO3 reduction. ([Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C); (b) plot of ln([NO3]/[NO3]0) versus time for effect of [NO3]0 on NO3 reduction. ([Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

Figure 7

(a) Effect of [NO3]0 on NO3 reduction. ([Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C); (b) plot of ln([NO3]/[NO3]0) versus time for effect of [NO3]0 on NO3 reduction. ([Cu0] = 0.254 g/L, [NaBH4] = 4.16 g/L, pHin = 6.8, T = 60 °C).

To further investigate the efficiency of the method, wastewater at much higher [NO3]0 (2,258.06 mg-N/L and 20,322.58 mg-N/L) was investigated. The results from the highly concentrated systems indicated that the method was applicable by increasing the load of copper and NaBH4, and complete reduction (100%) of NO3 can be achieved. This demonstrated that the method can be used for treating highly concentrated NO3 wastewater, such as the Mo-99 wastewater from nuclear facilities. Even though the accumulation of NO2 was observed in all cases, it may be removed by increasing the reaction time and dose of reducing agents. The amount of Mo-99 nuclear wastewater is ca. 20,000 m3 in stock and the Institute of Nuclear Energy Research preferred batch-type treatment. Indeed, footprint was a concern. However, increasing reagent dose and prolonging reaction time were deemed feasible and chosen for scale-up as a trade-off. It has been reported that NO3 reduction is influenced by the presence of ions and organic compounds, such as Mg2+, Fe3+, Cl, and humic acids (Brian et al. 2006; Wang et al. 2007). Regarding possible interference of the ions and substances in the Mo-99 wastewater containing concentrated NO3, all radioactive compounds were removed in the pretreatment. Thus, there was no concern about interference in this application. However, further investigation is necessary for other applications.

Regarding nitrogen balance, the assumption that the remaining species is nitrogen is based on previous literature which assumed that no other side products are formed other than NH4+ and NO2 (Liou et al. 2005, 2006; Fan et al. 2009). The generation and further removal of NO2 and NH4+ is a topic for further study. As shown in Table 3, CuSO4 as precursor resulted in less NO2 (intermediate) and higher NH4+. Since copper has low affinity to NO2, the high concentration NO2 can be reduced by varying the ratio of Cu to NaBH4. To reduce NH4+, modifying copper by using a second metal or support is an option, which will be investigated in our next work.

Table 3

Fraction of NO2 and NH4+ as products of NO3 reduction ([NO3]0 = 677 mg-N/L, [Cu0] = 0.254 g/L, T = 60 °C)

Precursor NO2 (%) NH4+ (%) 
CuSO4 24 65 
CuO 42 43 
Cu254 30 
Precursor NO2 (%) NH4+ (%) 
CuSO4 24 65 
CuO 42 43 
Cu254 30 

Reaction mechanism

The reduction of NO3 can proceed in two ways. It is either reduced to NH4+ or N2 and the reaction intermediate is NO2. The most widely accepted model is that NO3 was reduced to NO2 in the first stage and then to NH4+ and N2 (Prüsse & Vorlop 2001). Similarly, the pathway was a good fit for this study since high a amount of NO2 was found at the beginning of reaction. More than 65% of NO3 ended up as NH4+, while NO2 accounted for 24% when the CuSO4 precursor is used (Table 3). This could be due to rapid conversion of NO2. Electrons from BH4 are responsible for copper reduction and H2 production (Equation (4)):  
formula
(4)
In addition, NaBH4 is a hydrogen-carrying molecule that produces H2 when going through hydrolysis (Equation (5)):  
formula
(5)
The reaction was extremely efficient, because the total H2 produced came both from NaBH4 (one-half) and H2O (the other half) (Guella et al. 2006). The hydrolysis reaction is significantly enhanced in the presence of a catalyst (Yan et al. 2009). Therefore, once Cu0 was produced it could act as a catalyst. Hence, in addition to Cu0, the H2 produced in the system also contributed in the reduction reaction (Fanning et al. 2000). Mass balance on nitrogen showed that the sum of NH4+ and NO2 accounts for 84–89% of the reaction products and implied that N2 should account for the rest. Since insignificant reduction of NO3 was found when NaBH4 was used in the absence of Cu0 (Belay et al. 2015), the contribution of NaBH4 alone on mechanism is negligible. Therefore the following reactions, (6)–(8), can be involved.  
formula
(6)
 
formula
(7)
 
formula
(8)

Regarding possible Cu dissolution in the effluent, the Cu concentration in all experiments was below 0.18 mg/L under Cu0 load of 0.312–1.28 g/L (Belay et al. 2015). This shows dissolved Cu concentrations were lower than the WHO guideline for drinking water (2 mg/L) and it also indicates that the particles (Cu0) are stable. Based on the findings above, possible reaction pathways for NO3 reduction by Cu0 are proposed as shown in Figure 8.

Figure 8

Mechanism of NO3 reduction by zero-valent copper (Cu0) in the presence of BH4.

Figure 8

Mechanism of NO3 reduction by zero-valent copper (Cu0) in the presence of BH4.

CONCLUSIONS

Different precursors of copper were used in the reduction of NO3 in the presence of NaBH4. Based upon experimental results, the study can be summarized as follows:

  • Prehydrolysis of copper precursors was beneficial for NO3 reduction and higher values of the observed rate constant, Kobs, were found.

  • Compared with one-step dosing, the two-step addition of NaBH4 resulted in higher amount of NO2 initially, and it took longer for complete reduction.

  • Cu0 from CuSO4 had the highest reaction rate owing to its smallest size and highest specific surface area. However, when the observed rate constant was normalized taking into account the surface area, the rate constant in fact did not differ much among CuSO4, Cu2O, and CuO as precursors.

  • Results from the highly concentrated systems indicated that the method was applicable by increasing the load of copper and NaBH4. And total reduction (100%) of NO3 can be achieved.

ACKNOWLEDGEMENTS

The authors are grateful for the financial support from the Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan (Contract No. NL-1020163).

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