Photocatalytic Simultaneous Removal of Nitrite and Ammonia via a Zinc Ferrite/Activated Carbon Hybrid Catalyst under UV-Visible Irradiation.

Nitrite and ammonia often coexist in waters. Thus, it is very significant to develop a photocatalytic process for the simultaneous removal of nitrite and ammonia. Herein, zinc ferrite/activated carbon (ZnFe2O4/AC) was synthesized and characterized by X-ray diffraction spectroscopy, transmission electron microscopy, Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy. The valence band level of ZnFe2O4 was measured by X-ray photoelectron spectroscopy-valence band spectroscopy, and first-principles calculation was performed to confirm the band structure of ZnFe2O4. The as-synthesized ZnFe2O4/AC species functioned as a photocatalyst to simultaneously remove nitrite and ammonia under anaerobic conditions upon UV-visible light irradiation at the first stage. The results indicated that an average removal ratio of 92.7% with ±0.2% error for nitrite degradation for three runs was achieved in 50.0 mg/L nitrite + 100.0 mg/L ammonia solution with pH 9.5 under anaerobic conditions for 3 h at this stage; simultaneously, the removal ratio of 64.0% with ±0.2% error for ammonia was also achieved. At the second stage, oxygen gas was bubbled in the reactor to photocatalytically eliminate residual ammonia under aerobic conditions upon continuous irradiation. The results demonstrated that the removal ratios for nitrite, ammonia, and total nitrogen reached to 92.0, 90.0, and 90.2% at 12th hour, respectively, and the product released during photocatalysis is N2 gas, detected by gas chromatography, fulfilling the simultaneous removal of nitrite and ammonia. The reaction mechanism was exploited.

Introduction

Nitrite (NO2–) is very toxic to human health. It can combine with hemoglobin to form methemoglobin in the body, decreasing the ability of red blood cells to carry oxygen. Moreover, it can be converted into carcinogenic nitrosamine, which causes hypertension, leukemia, brain tumor, stomach, and bowel cancers.[1,2] Ammonia (NH3 or NH4+) can be easily transferred to NO2– in natural waters because of nitrifying bacteria, resulting in the coexistence of NO2– and NH3 in natural waters. Owing to their hazardous nature, the World Health Organization recommends the limit concentration of 3.0 mg/L for nitrite and 1.5 mg/L for ammonia in drinking water to protect humans from health risks.[3] Therefore, the simultaneous removal of NO2– and NH3 is very significant to human health.

The conventional biological removal of nitrogen can be achieved by nitrification and denitrification processes. The nitrification process is the oxidation of NH3 or NH4+ to nitrate (NO3–) through the pathway of NH4+/NH3 → NO2– → NO3– in the presence of aerobic bacteria. Nitrite could be the ultimate product formed during the nitrification process when the dissolved oxygen is inadequate, for example, at the deep bottom of waters. Subsequently, anaerobic bacteria start the denitrification process, which reduce NO3– or NO2– to nitrogen molecules by the pathway of NO3– → NO2– → NO → N2O → N2 under anaerobic conditions.[4] Both nitrification and denitrification pathways are complicated and time-consuming, in which a carbon source is needed and temperature, pH, and dissolved oxygen are controlled painstakingly and accurately to maintain the activity of bacteria and removal efficiency of total nitrogen.[5,6] Therefore, the development of a simple, economical, time-saving, and easy-going process for the complete removal of nitrogen is very desired.[7,8]

In theory, photocatalysis can realize both oxidization and reduction processes in viewpoint of electrochemistry if the electrode potential is appropriate. The photogenerated electrons on the conduction band of a semiconductor material could reduce a substrate molecule if the conduction band level (Ec) is more negative than the standard electrode potential (E0) of the substrate molecule. On the other hand, the photogenerated holes on the valence band of the semiconductor material could oxidize the other substrate molecule if the valence band level (Ev) is more positive than E0 of the substrate molecule. In the photocatalytic field, Butler and co-workers reported that nitrite was oxidized to nitrate using TiO2 as the photocatalyst.[9] With the assistance of oxalic acid or sodium oxalate as a hole scavenger and Au, Ag, and Pd as cocatalysts on TiO2, nitrite was reduced to ammonia and nitrogen gas or oxidized to nitrate.[10,11] Kominami et al. utilized a metal-free TiO2 suspension to reduce nitrite to N2 using ammonia as a hole scavenger in the atmosphere of argon and investigated the effect of temperature on the reaction of ammonia and nitrite.[12] Other photocatalysts such as CuInS2, KTaO3, LiNbO3, and BaLa4Ti4O15 have been developed to photocatalytically reduce nitrate.[13−16] The studies are very valuable and interesting. However, ZnFe2O4/activated carbon (AC) used as a photocatalyst for simultaneous removal of NO2– and NH3 has not been reported and a mimicking process for removal of nitrogen has not been developed neither.

Among photocatalysts, zinc ferrite (ZnFe2O4) possesses a prominent feature with a direct band gap of 1.98 eV. Ec of ZnFe2O4 was −0.90 V versus normal hydrogen electrode (NHE), and Ev was 1.08 V.[17,18] As a result, it is often utilized for photocatalytic oxidization of organic pollutants,[19−25] reduction of carbon dioxide,[26] nitroaromatic compounds,[27] and hydrogen production.[28−30] Therefore, it is reasonable that the photogenerated electrons on the conduction band of ZnF2O4 could reduce NO2– (Ec is less negative than the standard electrode potential , which is equal to 1.52 V vs NHE) and the photoholes on the valence band of ZnFe2O4 could oxidize NH3 in theory (Ev is more positive than the standard electrode potential , which is equal to 0.057 V vs NHE). According to the theoretical analysis and facts above, we designed the photocatalytic simultaneous removal of nitrite and ammonia via coupling zinc ferrite and AC to form a zinc ferrite/AC (ZnFe2O4/AC) hybrid catalyst, which can mimic the denitrification process under anaerobic conditions, simultaneously removing nitrite and ammonia under UV–visible light irradiation, and the nitrification process under aerobic conditions to eliminate residual ammonia. AC is utilized to enhance the photocatalytic activity because it can adsorb the substrate molecules and promote the separation of photogenerated electron–hole pairs.[31−35] The mimicking process can eliminate two toxic species, without addition of a carbon source, in addition to being cost-effective.

Results and Discussion

X-ray Diffraction Characterization

The X-ray diffraction (XRD) patterns of ZnFe2O4/AC, ZnFe2O4, and AC samples are shown in Figure . The diffraction peaks located at 2θ = 29.7°, 35.0°, 42.7°, 53.0°, 56.5°, 62.0°, 70.4°, and 73.3° corresponded to the Bragg reflections from the (220), (311), (400), (422), (511), (440), (620), and (533) crystal planes of spinel-structured ZnFe2O4 (JCPDS card no. 22-1012),[36−38] respectively. The peak at 26.2° is attributed to the (002) plane of graphene in AC. Compared with curve a, a small peak appeared in curve c, indicating the presence of AC in the ZnFe2O4/AC sample. The average diameters (D) of the samples could be estimated using Scherrer equation.Here, W is the breadth of the observed diffraction peak at its half height, K is the shape factor (usually approximately 0.89), and λ is the wavelength of the X-ray source used (0.154 nm by our measurement). The results displayed that the D values of ZnFe2O4/AC and ZnFe2O4 particles were 6.9 and 7.2 nm, respectively. The size was confirmed by transmission electron microscopy (TEM) observations.

Figure 1

XRD patterns of AC (a), ZnFe2O4 (b), and ZnFe2O4/AC (c) samples.

Morphological Observation

The morphological and structural characteristics of AC, ZnFe2O4, and ZnFe2O4/AC species were analyzed through TEM observation. Figure A,B shows AC images. Figure C displays the ZnFe2O4 particles clearly; the size is very uniform, ranging from 6 to 9 nm, also consistent with those measured by XRD. Compared with Figure B (there is only AC), black ZnFe2O4 particles can been seen in Figure D, and the indistinct interface between ZnFe2O4 and AC shows their compact bonding. It is reasonable that Fe(III) and Zn(II) ions were adsorbed to porous caves of AC material during continuous stirring. AC functioned as templates by incorporating ZnFe2O4 precursors, yielding ZnFe2O4 in porous caves of AC. The high-resolution transmission electron microscopy (HRTEM) image in Figure E shows the distinct lattice fringes; the interplanar spacing between the adjacent lattice fringes is 0.25 nm, which is assigned to plane (311) of ZnFe2O4.[38−40] A 0.29 nm d-spacing is attributed to plane (220) of ZnFe2O4 particles in Figure F,;the particles were embedded in AC, showing that the ZnFe2O4 particles were grown in caves of AC.

Figure 2

TEM images of AC (a,b), ZnFe2O4 (c), and ZnFe2O4/AC (d) and HRTEM images of ZnFe2O4 (e) and ZnFe2O4/AC (f).

Raman Spectroscopic Characterization

The Raman spectra of AC, ZnFe2O4, and ZnFe2O4/AC samples are presented in Figure . Well-resolved Raman scattering peaks of ZnFe2O4 and ZnFe2O4/AC were observed at 337, 478, and 650 cm–1. The Raman peak at 650 cm–1 is attributed to the movement (mode A1g) of oxygen in tetrahedral AO4 groups, while the two low-frequency peaks are ascribed to metal ion vibration (mode F2g) involved in octahedral sites (BO6).[41−43] The peaks at around 1320 and 1600 cm–1 correspond to the D band and G band of AC, respectively, in which the D band was caused by disordered sp3-bonded carbon atoms in the graphitic structure and the G band was caused by sp2-bonded carbon atoms in the graphitic structure.[44] The large intensity ratio of ID/IG (round 1.3:1) for AC verified the amorphous nature of carbon.[45]

Figure 3

Raman spectra of AC (a), ZnFe2O4 (b), and ZnFe2O4/AC (c).

Band Gap and Band Structure

The optical absorption property of a semiconductor material reflects the ability of harvesting photos, thereby playing a significant role in determining the photocatalytic activity. The diffuse reflectance spectroscopy (DRS) spectra of the ZnFe2O4/AC and ZnFe2O4 samples have been measured and are shown in Figure A. Compared with curve b in Figure A, an enhanced absorption portion appeared in the wavelength region of more than 530 nm as displayed in curve a, implying that AC on ZnFe2O4 enhanced the absorption efficiency of incident photons. It will improve the use of solar irradiation.

Figure 4

UV–vis diffuse reflectance absorption spectra (A) of ZnFe2O4/AC (a) and ZnFe2O4 (b) samples. Tauc plots for direct transition of ZnFe2O4/AC (B).

The direct and indirect band gaps of a semiconductor material can be determined by Tauc relationwhere A is a constant, hv is the photon energy, and α is the absorption coefficient, while n = 2 for direct and n = 1/2 for indirect transition. A Tauc plot for direct transition was obtained by transforming data in Figure A and is shown in Figure B. The as-obtained direct band gaps for ZnFe2O4 and the ZnFe2O4/AC composite are 2.1 eV and 2.0 eV, respectively. The band gap for ZnFe2O4 was also confirmed by the first-principles calculation as shown in Figure A. As seen in Figure A, the direct transition at G point is equal to 1.968 eV, which is close to the measured value of 2.1 eV. In addition, the band gap is related to particle size when the diameter of particles falls in the nanometer scale. The larger the band gap is, the smaller the particle size is due to the quantum confinement effects, although the band gap is not inversely proportional to the size of particles.[46,47] ZnFe2O4 particles with approximate 10 nm size showed 1.78 eV of band gap,[24] while the present ZnFe2O4 with approximate 7 nm displayed 2.1 eV of band gap, which confirmed the existence of quantum confinement effects in the system. Also, as seen in Figure B and Figure S1 in Supporting Information, the valence band is majorly composed of O 2p orbitals and the conduction band is majorly composed of Fe 3d orbitals; thus, the electron transition takes place from O 2p orbitals to Fe 3d orbitals upon UV–visible light irradiation.

Figure 5

Band structure (A) and density of states (DOS) (B) of ZnFe2O4.

Photocatalytic Removal of Nitrite and Ammonia

The concentrations of nitrite and ammonia were measured using ultraviolet visible spectroscopy based on Schemes and 2 during the photocatalytic process under anaerobic conditions.[48]

Scheme 1

Reactions for the Measurement of Nitrite

Scheme 2

Reaction for the Measurement of Ammonia

The as-recorded ultraviolet visible absorption spectra for nitrite and ammonia are presented in Figure A,B at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 h, respectively. Figure A,B displayed unambiguously that the concentrations of nitrite and ammonia declined simultaneously with irradiation time under anaerobic conditions when ZnFe2O4/AC was utilized as the photocatalyst. The control tests were also conducted in the presence or absence of the ZnFe2O4/AC catalyst upon irradiation or in dark or in the absence of one component; the removal ratios for nitrite and ammonia are presented in Figure C,D, respectively. Curve a in Figure C shows an average removal ratio of 92.7% with ±0.2% error (three times) for nitrite degradation using ZnFe2O4/AC as the photocatalyst in the presence of 50.0 mg/L nitrite–N (NO2––N, NN) + 100.0 mg ammonia–N (NH3–N, AN) under irradiation for 3 h, whereas the removal ratio approaches only 20.5% (curve b) in the absence of the catalyst under similar conditions. Parallel to the case of nitrite, the average removal ratio for degradation of ammonia for three times achieved 64.3% in the presence of ZnFe2O4/AC upon irradiation, whereas the removal ratio was only 18.8% in the absence of ZnFe2O4/AC even upon irradiation as shown in curve a and b in Figure D. It indicates distinctly that ZnFe2O4/AC possessed a photocatalytic feature for the simultaneous removal of nitrite and ammonia.

Figure 6

The as-recorded ultraviolet–visible absorption spectra of nitrite (A) and ammonia (B) under anaerobic conditions in the presence of the ZnFe2O4/AC photocatalyst at various irradiation times. Photocatalytic removal of NN (C) and AN (D) under anaerobic conditions using 1.5 g/L ZnFe2O4/AC (AC 7 wt %) as the catalyst in 250.0 mL of solution with pH 9.5. The initial concentrations of NN and AN are 50.0 and 100.0 mg/L, respectively. (a) NN + AN + ZnFe2O4/AC under light irradiation; (b) NN + AN under light irradiation without ZnFe2O4/AC; (c) NN + ZnFe2O4/AC under light irradiation; (d) NN + AN + ZnFe2O4/AC in dark; and (e) AN + ZnFe2O4/AC under light irradiation.

A control test displayed that the concentration of nitrite almost remained constant in the absence of ammonia even under light irradiation and that only 4.5% of nitrite was adsorbed on the catalyst in 3 h (curve c in Figure C). Nitrite (4.5%, 3 h) and ammonia (16.5%, 3 h) can also be adsorbed simultaneously on the ZnFe2O4/AC catalyst but were not degraded in dark as shown in curve d in Figure C,D. Taking curve d to compare with curve a, light irradiation promotes the reaction of nitrite with ammonia in the presence of the ZnFe2O4/AC catalyst under the lack of dissolved oxygen. Curve e in Figure D shows that ammonia cannot almost be oxidized in the absence of nitrite or dissolved oxygen under irradiation even if the ZnFe2O4/AC catalyst is present in the system.

By analyzing the data above, it can be concluded that ZnFe2O4/AC can remove nitrite and ammonia simultaneously under anaerobic conditions upon UV–visible light irradiation.

Effect of AC Dosage on Photocatalytic Efficiency

Generally, carbon materials can markedly improve the photocatalytic process, largely through the four following mechanisms: (i) minimization of the recombination of photogenerated electron–hole pairs; (ii) modification of the band gap of the photocatalyst to longer wavelengths; (iii) adsorption that accelerate contact between the pollutant and catalyst; and (iv) catalysis of the carboxyl group.[49] Rivera-Utrilla compared the enhanced effect of ordinary AC with that of AC oxidized with H2O2, HNO3, and O3 and concluded that AC with the greatest carboxyl group content showed the highest synergistic activity, whereas the enhanced effect of ordinary AC that had not been oxidized with oxidants was mainly attributed to its high adsorption capacity.[50] In the present case, a series of ZnFe2O4/AC hybrid catalysts containing various weight ratios of AC from 0.0 to 9.0% were synthesized using the abovementioned method; the resulting products were utilized as photocatalysts for the removal of nitrite and ammonia under anaerobic conditions. The removal curves are shown in Figure . The results show that the removal ratios increased as the weight ratio of AC (AC: ZnFe2O4) in the composite rose at the initial stage and that they reached the summit value of 92.7% (±0.2%) for nitrite and 64.3% (±0.2%) for ammonia when the AC content was equal to 7.0%. Subsequently, contrary to what was expected, the removal ratios decreased slightly as the AC content continued to increase; the removal ratio is 90.0% (±0.2%) for nitrite and 63.0% (±0.2%) for ammonia at AC 9.0 wt %. Thus, 7.0% is the optimal weight ratio. The control test indicated that pure ZnFe2O4 (0.1 g) displayed average adsorption percentages of 0.2 and 12.0% for NN and AN in a mixed solution containing 50.0 mg/L NN and 100.0 mg/L AN at 3 h, respectively, whereas the as-prepared ZnFe2O4/AC (0.1 g) displayed adsorption ratios of 4.5 and 16.5% for NN and AN in the same mixed solution at 3 h, respectively. Compared with the adsorption ratios of NN and AN on ZnFe2O4, the adsorption ratios of NN and AN on ZnFe2O4/AC are high, showing the concentration role of AC for NN and AN. Moreover, the photocatalytic removal difference between ZnFe2O4/AC and pure ZnFe2O4 is 12.7% for NN and 14.4% for AN. The differences of photocatalytic removal are much larger than those of adsorption. Therefore, AC improved the adsorption of NN and AN and harvesting of incident light of λ > 530 nm wavelength, resulting in the enhanced photocatalytic activity.[51,52]

Figure 7

Effects of AC content on the removal of nitrite (A) and ammonia (B) under anaerobic conditions upon UV–visible light irradiation. A 250.0 mL solution was utilized containing 50.0 mg/L NN + 100.0 mg/L AN + 1.5 g/L ZnFe2O4/AC with pH 9.5.

Effect of pH on the Removal of Nitrite and Ammonia

The hydrogen ion concentration (pH) exerts great impact on both reduction of nitrite and oxidization of ammonia because their reactions involved transfer of hydrogen ions. Thus, pH influence was examined. The results showed that different pHs caused various removal ratios of both NN and AN for three runs shown in Figure .

Figure 8

Effect of pH on the removal of nitrite (A) and ammonia (B) under anaerobic conditions upon UV–visible light irradiation. A 250.0 mL solution was utilized containing 50.0 mg/L NN + 100 mg/L AN + 1.5 g/L ZnFe2O4/AC with various pHs of 8.5, 9.0, 9.5, 10.0, and 10.5.

For the removal of nitrite, 54.7% (±0.2%) of NN removal was achieved in pH 8.5 solutions during UV–visible light irradiation for 180 min. In pH 9.0 solutions, 75.2% (±0.2%) removal ratio was achieved. When pH 9.5 solutions were tested, 92.7% (±0.2%) removal ratio was achieved, which is a higher value of degradation. On the contrary, the removal ratio will decline if pH continues to rise. The removal ratio declined to 81.0% (±0.2%) at pH 10.0, even to 40.0% (±0.2%) at pH 10.5. For the removal of ammonia, a similar event took place as shown in Figure B. At the initial stage, the removal ratio of ammonia went upward as pH rose until a value of 64.0% (±0.2%) was achieved at pH 9.5. After that the removal ratio started to descend, even to 36.5% (±0.2%) at pH 10.5.

According to the ultraviolet–visible spectroscopic measurements, nitrogen gas was formed during the photocatalytic process. As it will be seen below, the oxidization of NH3 will release hydrogen ions out in solution (Scheme ), so the high pH (low concentration of H+ ions) in solution will promote oxidation of NH3 in a viewpoint of chemical equilibrium. On the other hand, hydrogen ions involve in the reduction of nitrite as shown in Scheme . Thus, the extortionate pH value resulted in the declined removal. In this case, the optimal pH value in solution turns out to be equal to 9.5.

Scheme 3

Photogenerated Holes on Ev Oxidize Ammonia

Scheme 4

Photogenerated Electrons on Ec Reduce Nitrite

In addition, it seems that the high concentration of hydrogen ions favors the overall reaction (Scheme ). However, the increase of hydrogen ions favors only the reduction of nitrite, but the oxidization of ammonia will get deteriorated, which is similar to ammonia oxidized photocatalytically by graphene–manganese ferrite.[34] The possible reason is related to NH3 adsorbed on the catalyst. NH3, but not NH4+, is the predominant form in pH > 9.3 solutions because pKa for NH4+ is equal to 9.3. NH3 molecules containing lone pairs of electrons in sp3 hybrid orbitals for donation are more easily adsorbed on the ZnFe2O4 catalyst than NH4+ ions without lone pairs of electrons. Therefore, NH3 adsorbed on the catalyst will sacrifice the photogenerated holes so that the overall reaction continues to occur.

Scheme 5

Overall Photocatalytic Reaction for the Simultaneous Removal of Nitrite and Ammonia

Complete Removal of Total Nitrogen

At the first stage, nitrite was reduced to nitrogen gas in the presence of ammonia under anaerobic conditions upon irradiation; the average removal ratio of nitrite for three runs reached to 93.3% with ±0.2% error in 3 h; simultaneously, the average removal ratio of ammonia for three runs approached to 64.5% with ±0.2% error. That is, the remaining ammonia of 35.5 mg/L in the solution needs to be degraded to achieve the complete removal of nitrogen. Thus, at the second stage, the solution containing 35.5 mg/L AN was aerated for 20 min; then, the lamp was switched on to continue the removal of the remaining ammonia. The results are shown in Figure . At the 12th hour during irradiation, the average removal ratios of NN, total nitrogen (TN), and AN for three runs reached to 92.0, 90.2, and 90.0% with ±0.2% error, respectively, removing most of total nitrogen. In addition, the removal ratios of NN and AN at the end of the first stage are about 6.9 and 13.2%, respectively, and the removal of AN is about 20.4% at the end of the second stage, when P25 (TiO2) was used as the photocatalyst. It turns out that ZnFe2O4/AC is very effective in the removal of NN and AN under similar conditions, compared with P25.

Figure 9

Complete removal of ammonia under aerobic conditions by means of photocatalysis.

Identification of Products

To identify the products of nitrite reaction with ammonia during photocatalysis, measurements were performed during the photocatalytic removal of nitrite and ammonia in the sealed photocatalytic reaction system mentioned previously, in which a 100 mL aqueous solution containing 50.0 mg/L NO2––N and 100.0 mg/L NH3–N was irradiated under UV–visible light, the mixed gas of oxygen and argon was cycled, and the produced N2 and others were detected by gas chromatography. The results are displayed in Figure . As seen, the peak of N2 gas was boosting with the irradiation time, whereas the peak of O2 gas in the sealed reaction system was declining with the irradiation time; meanwhile, no other gas peaks such as N2O, NO, and NO2 were observed, indicating that the product N2 gas was formed during the photocatalysis of nitrite and ammonia.

Figure 10

Gas chromatograms during photocatalytic simultaneous removal of nitrite and ammonia via the zinc ferrite/AC hybrid catalyst under UV–visible light irradiation at t = 0.0, 1.0, 2.0, 3.0, 5.0, 7.0, 9.0, 11.0, and 13.0 h.

Photocatalytic Reaction Mechanism

The valence band spectrum of ZnFe2O4 was measured by X-ray photoelectron spectroscopy (XPS) technique as shown in Figure . The measured results indicated that the valence band of ZnFe2O4 is equal to 1.33 eV, which is very close to 1.08 eV,[17] so the conduction band is equal to −0.65 eV. It is well known that photogenerated holes may oxidize water to O2 when Ev is more positve than E(O2/H2O), whereas photogenerated electrons may reduce hydrogen ions to H2 when Ec is more negative than E(H2/H2O).[53]

Figure 11

XPS valence band spectrum of the ZnFe2O4 sample.

For the oxidization of NH3 (Scheme ), the driving power ΔE1 is found to be 1.27 V () because of the standard electrode potential () of 0.057 V versus NHE. For the reduction of NO2– (Scheme ), the drive power ΔE2 is 2.17 V () because of the standard electrode potential () being 1.52 V versus NHE. Here, the oxidization of NH3 to N2 by photogenerated holes is similar to the oxidization of H2O to O2 by photogenerated holes,[54,55] whereas the reduction of NO2– to N2 by photogenerated electrons is similar to the reduction of H+ ions to H2.[56] Reactions 3 and 4 can both occur spontaneously in energy because both ΔE1 and ΔE2 are more than zero. Thus, the overall reaction is denoted in Scheme ; the overall photocatalytic process is presented in Figure .

Figure 12

Reduction of nitrite and oxidization of ammonia to release nitrogen gas by means of photocatalysis.

Conclusions

Simultaneous removal of nitrite and ammonia was achieved successfully based on the zinc ferrite/AC catalyst via two steps under UV–visible light irradiation. The detection of products by means of gas chromatography indicated that nitrogen gas was released out during photocatalysis. The XPS measurements and analysis of the band structure for ZnFe2O4 showed that Ec (−0.65 V vs NHE) of ZnFe2O4 is lower than the reduced potential (1.52 V vs NHE) of nitrite and thereby the photogenerated electrons can reduce nitrite to N2 gas under irradiation; while Ev (1.33 V vs NHE) of ZnFe2O4 is higher than the potential (0.057 V vs NHE) of ammonia, as a result, the photogenerated holes can oxidize ammonia to N2 gas. AC caused the red shift of the band gap, separation of the photogenerated electron–hole pairs, and adsorption for nitrite and ammonia, promoting the photocatalytic activity.

Experimental Section

Chemicals

AC and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich LLC (China). Ferric nitrate nonahydrate (Fe(NO3)3·9H2O) was bought from Tianjin Damao Chemical Factory, China. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and ammonium chloride (NH4Cl) were obtained from Nanjing Chemical Reagent Co., Ltd. Sodium nitrate and sodium nitrite were purchased from Tianjin Baodi Chemical Industry Co., Ltd. and Wuxi Jingke Chemical Industry Co., Ltd, respectively. All reagents were of analytical grade and utilized without further purification. All solutions were prepared with 18.2 MΩ·cm deionized Milli-Q water.

Synthesis of ZnFe2O4/AC

A one-step hydrothermal process was adopted.[24,57] First, 1.7850 g of Zn(NO3)2·6H2O (6.0 mmol) and 4.8480 g of Fe(NO3)3·9H2O (12.0 mmol) were dissolved in 20.0 mL of deionized water under vigorous stirring. Second, 0.101 g of AC (after dried at 120 °C for 2 h, 7 wt % of ZnFe2O4) was dispersed in 10.0 mL of deionized water by ultrasonic dispersion for 1 h. The as-prepared solution containing Zn(II) and Fe(III) was added to 10 mL of the AC aqueous suspension; then, it was vigorously stirred for 1 h. Finally, a 10.0 mL solution containing 2.40 g of NaOH was slowly added dropwise to the above brown suspension with continuous stirring for 1.5 h. Deionized water (20.0 mL) was also added to the suspension to obtain a total volume of 60 mL. The suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was sealed and heated at 180 °C for 8 h. Then, it was cooled naturally to room temperature, taken out, and filtered to obtain ZnFe2O4/AC precipitates. The ZnFe2O4/AC precipitates were washed thrice with deionized water to remove excess NaOH and other electrolytes. The precipitates were collected and dried at 60 °C for 24 h in a vacuum chamber for characterization and photocatalytic tests.

Structure Characterization of ZnFe2O4/AC

XRD measurements were performed using an X’Pert-Pro MPD X-ray diffractometer (Panalytical, Netherlands). The X-ray source was Cu Kα radiation with a wavelength of 0.154 nm, tube voltage of 40 kV, and tube current of 40 mA. ZnFe2O4/AC, ZnFe2O4, and AC powder were dispersed in water by using an ultrasonic device, placed on carbon-coated copper grids, and dried under ambient conditions for morphological observations using a TEM system (Tecnai G220, FEI, USA). UV–vis DRS spectra were recorded on a double-beam TU-1901 spectrophotometer (type T1901, Puxi Co., Beijing, China). Raman spectra were recorded on an HR800 micro-confocal Raman spectrometer with a 633 nm laser.

Simultaneous Removal of Nitrite and Ammonia

The experiments for the removal of nitrite and ammonia were performed using a commercial glass photoreactor (Shanghai Binlong Instrument Co. Ltd.) as shown in Figure . A high-pressure mercury lamp (300 W), surrounded by a flow of water to maintain 25 °C (the temperature in the reaction solution is 28 °C as measured by the thermometer), was inserted in a 250 mL suspension solution containing NN (50 mg/L), AN (100 mg/L), and 1.5 g/L ZnFe2O4/AC catalyst. The pH of the suspension was adjusted to 9.5 using 1.0 mol/L NaOH solution. The suspension was bubbled with nitrogen gas at a flow rate of 0.3 L min–1 for 30 min to remove dissolved oxygen for photocatalytic tests under magnetic stirring. Solution (3 mL) was taken out to determine the concentrations of nitrite, ammonia, or nitrate at the interval of 0.5 h.

Figure 13

Experimental apparatus for the simultaneous removal of nitrite and ammonia.

To achieve complete removal of nitrogen, after nitrite was reduced by ammonia photocatalytically, NaOH was added to the suspension solution to maintain pH 9.5 and oxygen gas was also pumped into the suspension solution to provide with dissolved oxygen for degrading the residual ammonia to achieve the complete removal of total nitrogen.

Measurements of Nitrite, Nitrate, and Ammonia

A double-beam TU-1901 spectrophotometer was used to probe the concentrations of nitrite and ammonia by using Griess reagents[58,59] and Nessler reagents[60] during photocatalysis, respectively. Griess reagents contain sulphanilamide and N-(1-naphthyl)ethylenediamine as the coupling agent. Five grams (5.0 g) of sulphanilamide was added to a 400 mL solution containing 36.5 wt % HCl of 50 mL; the solution was diluted to 500 mL. Then, 100.0 mg of N-(1-naphthyl)ethylenediamine was dissolved in 50 mL of deionized water; it was diluted to 100 mL. The measurement procedures are as follows: one milliliter (1.0 mL) of the sample solution containing nitrite was taken out to a 50 mL colorimetric cylinder. Then, 1.0 mL of sulphanilamide solution was added to the nitrite solution for 5 min; after that 1.0 mL of N-(1-naphthyl)ethylenediamine solution was also added the solution to form a red azo dye with an absorption maximum at 540 nm. Finally, it was diluted to 50 mL for the measurement at 540 nm. The reactions are presented in Scheme . Ammonia reacts with Nessler reagent to give colored solutions as shown in Scheme . The absorbance at 382 nm approached the peak and was recorded at the wavelength of 382 nm.[48]

Nitrate was determined directly at 203 nm using ultraviolet visible spectroscopy.[61] The removal ratios of NN, AN, and TN were calculated using eqs –5where CNN0 is the initial concentration of NN, CAN0 is the initial concentration of AN, CNN is the concentration of NN at t min during the photocatalytic process, and CAN is the concentration of AN at t min during the photocatalytic process.

Identification of Products

To detect the products of nitrite reaction with ammonia during photocatalysis, a sealed photocatalytic reaction system (Labsolar 6A photocatalytic system, Perfectlight Co. Ltd., Beijing, China) was applied, in which a 100 mL aqueous solution containing 50.0 mg/L NO2––N + 100.0 mg/L NH3–N was irradiated under UV–visible light irradiation after the 465 mL reaction system was evacuated to 0.4 kPa. Subsequently, the first stage of photocatalysis was conducted under 300 W Xe-lamp irradiation for 2 h. After that a mixed gas with the initial ratio of oxygen to argon at 1:2.6 was pumped until the internal pressure reached to 80 kPa and then the second stage of photocatalysis continued to occur. The reaction system was coupled with a GC-7806 gas chromatography (Shiwei Puxin Instruments Co. Ltd., Beijing, China) system. A chromatographic column of 5 m length × 2 mm i.d., filled with a 5 Å molecular sieve was utilized as a separation column. High-purity argon was utilized as the carrier gas. The flow rate of the carrier gas was set at 23 mL/min. A thermal conductivity tank was used to detect N2, O2, and others. The column temperature was set at 80 °C, inlet temperature at 120 °C, and detector temperature at 150 °C.

First-Principles Calculation

The calculations of band structure and DOS were performed by using the CASTEP package of Materials Studio 7.0. In the plane-wave calculations, a cutoff energy of 520 eV was applied. The generalized gradient approximation was adopted with the Perdew–Burke–Ernzerhof functional. The calculations were performed in a ferromagnetic spin-polarized configuration. The self-consistent field convergence criterion and energy tolerance were set as fine levels. A Monkhorst–Pack scheme with a 6 × 6 × 6 k-point grid was employed. Tkatchenko–Scheffler method was used for DFT-D correction. The ZnFe2O4 phase is of the cubic structure with space group Fd3̅m (no. 227) and the lattice parameters a = b = c = 8.4418 Å, and α = β = γ = 90°, in which the Zn, Fe, and O atoms occupy the fractional coordinates of Zn1 (0.00, 0.00, 0.00) with site occupation factor (SOF) 0.89, Fe1 (0.00, 0.00, 0.00) with SOF 0.11, Fe2 (0.625, 0.625, 0.625) with SOF 0.945, Zn2 (0.625, 0.625, 0.625) with SOF 0.055, and O (0.3854, 0.3854, 0.3854), respectively. These accurate data validated the applicability of the CASTEP package for calculating the band structures and DOS of ZnFe2O4.