Characterization of La/Fe/TiO₂ and Its Photocatalytic Performance in Ammonia Nitrogen Wastewater

Abstract

La/Fe/TiO₂ composite photocatalysts were synthesized by Sol-Gel method and well characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen-physical adsorption, and UV-Vis diffuse reflectance spectra (UV-Vis DRS). It is interesting that the doped catalysts were in anatase phase while the pure TiO₂ was in rutile phase. In addition, the composites possessed better physical chemical properties in photocatalytic activity than pure TiO₂: stronger visible-light-response ability, larger specific surface area, and more regular shape in morphology. The photodegradation results of ammonia nitrogen indicate that: the La/Fe/TiO₂ had higher catalytic activity to ammonia nitrogen waste water compared pure TiO₂ and the other single metal-doped TiO₂. pH 10 and 2 mmol/L H₂O₂ were all beneficial to the removal of ammonia nitrogen by La/Fe/TiO₂. However, the common inorganic ions of Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻/CO₃²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ in water all inhibited the degradation of ammonia nitrogen. By balance calculation, at least 20% of ammonia nitrogen was converted to N₂ during the 64.6% removal efficiency of ammonia nitrogen.

1. Introduction

In recent years, a large amount of ammonia nitrogen wastewater was discharged into the water with the mining of rare earth metals in Gannan area, China. Excessive amounts of ammonia nitrogen in water would cause many harmful effects, and the treatment of ammonia nitrogen wastewater is a concern [1]. The methods of chemical precipitation, blow-off, and adsorption are commonly used for the treatment of ammonia nitrogen wastewater at low concentrations. Chemical precipitation method intends to reduce the water solubility of ammonia nitrogen by the formation of indissoluble salt; blow-off method is typically used NaOH to adjust pH to basic of wastewater and ammonia nitrogen would exist in the form of free ammonia (NH₃). Then, ammonia nitrogen would escape from aqueous solution to the atmosphere, which might be difficult to recover and cause the secondary pollution in atmospheric. Adsorption method is mainly based on the ion exchange of NH₄⁺ with other cationic ions, which is a reversible process but the exchange capacity is limited [2,3,4]. The ideal treatment result is that ammonia nitrogen would totally convert to nitrogen. Marco [5] reported that partial of ammonia nitrogen was oxidized to nitrogen by photocatalytic oxidation technology using TiO₂ as the catalyst.

As one of the most promising technologies of treating pollutants in water, photocatalytic oxidation process has received intense attention in many fields and has been researched widely on the environmental protection, health care, building materials and other industries, especially on the photodegradation of pollutants [6,7,8,9,10,11,12]. Compared with other advanced oxidation technologies such as Fenton oxidation and ozone oxidation, catalytic wet oxidation, and electrochemical oxidation, photocatalytic oxidation technology is non-toxic and has good stability [13]. Undoubtedly, among the various semiconductor photocatalytic materials, titanium dioxide (TiO₂) is the typical photocatalyst due to its good chemical and biological stability, low cost, ease of availability, and significantly photocatalytic activity under ultraviolet irradiation [14,15,16,17]. However, its application remains limited because of its high electron-hole recombination rate in photocatalytic process; another shortcoming of TiO₂ is that it only absorbs ultraviolet light no longer than 387.5 nm, which only accounts for about 4% of sunlight [18,19,20]. To resolve these problems, one important way is to extend the photoresponse of TiO₂ into visible regions, which has already been studied [11,21,22]; and another important way is to adulterate some amount of metal or nonmetal elements into TiO₂ to increase the migration efficiency of photogenerated electrons and decrease the recombination rate of electron-hole pairs. There are several reports of the photodegradation of ammonia nitrogen by modified TiO₂ [12,23,24,25,26]. TiO₂ doped with Fe could utilize visible light wavelengths and effectively produced hydrogen from the decomposition of aqueous NH₃ while TiO₂ doping with rare earth ions playing the key role in the ammonia photocatalytic decomposition [23,24].

Combined with the advantage of iron and rare earth doped TiO₂, La-Fe-codoped TiO₂ was chosen to prepare and the main objectives of this paper are in four aspects: firstly, to investigate the physics chemical properties of La-Fe-codoped TiO₂ prepared by Sol-Gel method and characterized by XRD, SEM, EDS, and UV-Vis DRS; secondly, to study the photocatalytic activity of prepared doped TiO₂ to ammonia nitrogen; thirdly, to discuss the effect of reaction solution pH, H₂O₂, and common inorganic ions on the degradation of ammonia nitrogen; finally, to disclose the conversion products of ammonia nitrogen during the photodegradation process.

2. Materials and Methods

2.1. Reagents

Tetrabutyl titanate (Ti(OC₄H₉)₄), lanthanum nitrate (La(NO₃)₃), ferric nitrate (Fe(NO₃)₃) and ammonium chloride (NH₄Cl) were of analytical grade and purchased from National Medicine Group Chemical Reagent Co., Ltd., Shanghai, China. Anhydrous ethanol (CH₃CH₂OH) was purchased from Shanghai Zhan Yun Chemical Co., Ltd., Shang hai, China.

2.2. Modification Methods

The photocatalysts were prepared by Sol-Gel method. The mixture solution of 8.5 mL tetrabutyl titanate dissolved in 20 mL anhydrous ethanol with stirring for 30 min was noted as solution A. Another solution containing 20 mL ethanol, 1.5 mL deionized water, and metal salts (La(NO₃)₃ and/or Fe(NO₃)₃) in the required stoichiometry was noted as solution B. Solution B was pumped into solution A by half drop (ca. 0.05 mL) per second at ca. 30 °C. The mixture was hydrolyzed at room temperature for a period of time under vigorous stirring and finally the translucent sol was formed. The gel was prepared by aging the sol for two days at room temperature. The dry gel was gained after drying at 80 °C for 2 h. Finally, the gel was calcined at 500 °C at the heating rate of 2.5 °C/min in the muffle furnace for 2 h and was ground into powders for use.

2.3. Characterization

Powder X-ray diffraction (XRD) data were recorded on a D/Max-3c X-ray diffraction meter at 40 kV and 40 mA for monochromatized Cu Kα (λ = 1.5418 Å) radiation. Scanning electron microscopy (SEM) measurements were carried out on S-4800 type of field emission scanning electron microscope with energy dispersive spectrometer (EDS). The BET surface areas of the samples were obtained from the automatic analyzer (JW-004A, Beijing JWGB Sci.&Tech. Co.,Ltd, Beijing, China). UV-Vis diffuse reflectance spectra were achieved using a UV-Vis spectrophotometer (UV-2550, Shimadzu China Co., Ltd., Japan), and the absorption spectra were referenced to BaSO₄.

2.4. Photocatalytic Removal Experiments and Analytical Methods

The photocatalytic degradation experiments were carried out in a XPA-7 photochemical reactor (Xujiang Electrical Mechanical Plant, Nanjing, China). The irradiation was provided by a 500 W Mercury lamp (Institute of Electric Light Source, Beijing, China), which mainly radiated 365 nm wavelenght of light and was positioned in the cylindrical quartz cold trap. The system was cooled by circulating water and maintained at room temperature. Before the irradiation, the suspension was magnetically stirred for 30 min in the dark to ensure adsorption equilibrium of ammonia nitrogen on the catalysts. For all the reactions, the irradiation lasted for 300 min. Approximately 5 mL of reaction solution was taken at given time intervals and centrifuged. The supernatant was analyzed by Nessler’s reagent spectrophotometry [27] and the removal efficiency (R) was calculated by Formula (1) as follows:

$$R=(C_0-C)/C_0\times 100\%$$

(1)

where C₀ is the initial concentration of ammonia nitrogen and C is the concentration at reaction time t (min). In order to study the conversion of ammonia nitrogen, NO₂-N and NO₃-N were also detected by spectrophotometry methods [28,29]. In addition, the effects of H₂O₂ and common ions (Cl⁻, HCO₃⁻/CO₃²⁻, NO₃⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺) in natural waters on ammonia nitrogen removal were investigated. Besides, all the experiments were performed at least twice and the mean values were reported.

3. Results and Discussion

3.1. Characterization

3.1.1. XRD

The XRD patterns of P25, pure TiO₂, La/TiO₂ and La/Fe/TiO₂ composites are shown in Figure 1. The major peaks at 2θ values of 25.3°, 37.9°, 48.0°, 53.8°, 54.9°, and 62.5° corresponded to diffractions of the (101), (004), (200), (105), (211), and (204) planes of anatase TiO₂ while the major peaks at 27.5°, 36.1°, 39.2°, 41.3°, 44.1°, 54.4°, 56.7°, 62.8°, 64.1°, 65.6°, 69.1° corresponded to diffractions of the (110), (101), (111), (210), (211), (220), (002), (310), and (112) planes of rutile TiO₂. This showed that the pure TiO₂ prepared existed in the rutile phase while the doped catalysts in the anatase phase whether La/TiO₂ or La/Fe/TiO₂. It is interesting to find that the doped rare earth Lanthanum changed the crystal structure of TiO₂ from rutile to anatase. Compared to P25, the crystallinity of doped TiO₂ decreased as shown in Figure 1. Besides, there are no peaks for the formation of composite metal oxides such as La₂O₃ or Fe₂O₃ in doped TiO₂, which might be ascribed to the fact that the concentration of La-doping and/or Fe-doping was so low and the overlapping of diffraction peaks due to TiO₂, La₂O_3, and/or Fe₂O₃.

3.1.2. UV-Vis DRS

The UV-Vis DRS spectra of P25, TiO₂, La/TiO₂, Fe/TiO_2, and La/Fe/TiO₂ are depicted in Figure 2A. All the doped powders showed a redshift compared to P25 while undoped TiO₂ exhibits an absorption edge to the visible light region due to the rutile phase. Besides, there is an obvious change of light absorption of La/Fe/TiO₂ from ultraviolet to visible light due to the La-Fe-codoping. The redshift phenomenon indicates that the modified TiO₂ broaden the scope of light response as anatase phase. In other words, the visible-light-response catalysts overcome the disadvantage of the broadband gap to a certain extent [30]. In addition, the absorption data were analyzed using the following well-known formula for near-edge optical absorption of semiconductors [31].

α = A (hυ − E_g)ⁿ/hυ

(2)

where α is the absorption coefficient, (hυ) is the photon energy, A is a constant, E_g is the optical gap, and the value of n is 1/2 for TiO₂ [32]. To estimate the optical band gap, the plot of (αhυ)² versus (hυ) is shown in Figure 2B. The E_g values are 3.22, 2.98, 3.01, 3.18, and 2.68 eV for P25, TiO₂, La/TiO₂, Fe/TiO₂ and La/Fe/TiO₂, which indicates that the doped material did broaden the scope of light response of compared with pure TiO₂ and P25.

Figure 1. XRD patterns of P25, TiO₂, La/TiO₂ and La/Fe/TiO₂.

Figure 1. XRD patterns of P25, TiO₂, La/TiO₂ and La/Fe/TiO₂.

Figure 2. UV-Vis DRS of P25, TiO₂, La/TiO₂ Fe/TiO₂, and La/Fe/TiO₂ ((A) Absorbance; (B) Plots of (αhυ)² versus (hυ) for catalysts).

Figure 2. UV-Vis DRS of P25, TiO₂, La/TiO₂ Fe/TiO₂, and La/Fe/TiO₂ ((A) Absorbance; (B) Plots of (αhυ)² versus (hυ) for catalysts).

3.1.3. Surface Morphology Analysis

As seen from Figure 3, pure TiO₂ (Figure 3A) exhibits irregular shape and is agglomerated badly. However, after La and Fe co-doped, the reunion phenomenon is abated and is a relatively flat surface. Compared to Figure 3A and 3B, the more serious reunion phenomenon of pure TiO₂ might be based on the large amounts of hydroxyl groups on the surface of pure TiO₂ which would result in the strong hydrogen bonding between particles [33].

Figure 3. SEM images of TiO₂ (A) and La/Fe/TiO₂ (B).

Figure 3. SEM images of TiO₂ (A) and La/Fe/TiO₂ (B).

Further data for the composition of La/Fe/TiO₂ photocatalysts were obtained by EDS: La accounted for 3.62% while Fe accounted for 0.62%. The result is consistent with the UV-Vis DRS and proves that elements lanthanum and iron were all loaded on the surface of TiO₂.

3.1.4. Specific Surface Area Analysis

The specific surface area of P25, TiO₂, La/TiO₂, and La/Fe/TiO₂ is 48.12, 65.57, 78.36, and 120.74 m²/g respectively. A significant increase in specific surface area of the doped samples was observed. The increase in specific surface area after doping may be caused by the decrease in the crystallite size of TiO₂, as described in the XRD and SEM part, which is in agreement with Anandan’s report that doping of rare earth could increase the surface area of TiO₂ [34].

3.2. Degradation Performance of Ammonia Nitrogen Wastewater

A series of control experiments were designed to investigate the photocatlytic activity of prepared doped composites, and all the experiments were carried out with the same conditions of pH (ca. 10), catalyst amount (1 g/L), and 500 W mercury lamp. Figure 4 shows the result. After 5 h, about 15% of ammonia nitrogen was removed by direct photolysis or escaping from the reaction solution by magnetic stirring, while above 50% was removed by photocalytic degradation with catalyst. Furthermore, the doped catalysts showed higher photocatalytic activity on ammonia nitrogen removal than pure TiO₂ by the analysis of first-order reaction kinetics as shown in Table 1. The best removal efficiency of ammonia nitrogen reached 64.6% with La/Fe/TiO₂.

Figure 4. The curves of photocatalysis of NH₄⁺-N at different conditions.

Figure 4. The curves of photocatalysis of NH₄⁺-N at different conditions.

Table 1. Reaction kinetics constant of catalysts.

Catalyst	k	R2
TiO2	0.137	0.990
Fe/TiO2	0.150	0.988
La/TiO2	0.163	0.976
La/Fe/TiO2	0.196	0.984

Table 1. Reaction kinetics constant of catalysts.

Catalyst	k	R2
TiO2	0.137	0.990
Fe/TiO2	0.150	0.988
La/TiO2	0.163	0.976
La/Fe/TiO2	0.196	0.984

Anandan [34] reported that small particle size, high surface area, high surface roughness, and porous surface of La-doped TiO₂ and the suppression of electron-hole recombination by La³⁺ were the reasons for the high photocatalytic activity of La-doped TiO₂, the characterization result of this study is consistent with their conclusion. Besides, Fe³⁺ is born of the electron capture trap [23]. Under the synergy of La³⁺ and Fe³⁺, the electron-hole pairs produced from catalyst under irradiation could be effectively separated and the catalytic activity of La/Fe/TiO₂ was improved.

3.2.1. Effect of Different pH

The initial pH of the reaction solution might influence the surface charge of La/Fe/TiO₂ and the existing form of ammonia nitrogen in water and finally affect the ammonia nitrogen removal efficiency. Firstly, the number of OH⁻ increases with the pH increases gradually, and more ·OH would be generated induced by La/Fe/TiO₂, resulting in promoting the removal rate of ammonia nitrogen. Secondly, there are two forms of ammonia nitrogen in water: NH₃·H₂O and NH⁴⁺. Proportion of NH₃·H₂O molecules increases as the pH increases in the solution. Thirdly, the space steric hindrance of NH₃ is smaller than that of NH₄⁺, which is more conducive to the reaction of NH₃ with ·OH. At last, the pH_PZC (point of zero charge) of La/Fe/TiO₂ is about 6.4 by analysis of Zeta potential. So, it is difficult for the attraction of ammonia molecules onto the surface of the catalyst in an acidic condition. All the analysis above demonstrates that ammonia nitrogen was removed rapidly in alkaline environment [35]. However, as shown in Figure 5, it did not favor the catalytic reaction at pH 10.9, which might be due to that excessive OH⁻ in the solution. The following experiments would be performed at pH ca. 10.

Figure 5. The effect of pH on the degradation of NH₄⁺-N with La/Fe/TiO₂.

Figure 5. The effect of pH on the degradation of NH₄⁺-N with La/Fe/TiO₂.

3.2.2. Effect of H₂O₂

H₂O₂ is usually applied as a stimulator in TiO₂ photocatalysis system to enhance the rate of photocatalytic oxidation [36,37]. In order to investigate the effect of H₂O₂ addition on ammonia nitrogen degradation by La/Fe/TiO₂, experiments were conducted by varying the initial H₂O₂ concentration in the range of 0.01 to 10 mmol/L (0.01, 0.1, 0.5, 2, 10 mmol/L). As shown in Figure 6, addition of H₂O₂ promoted the removal rate of ammonia nitrogen. The removal rate reached 78.3% with H₂O₂ of 2 mmol/L. It is well known that H₂O₂ has strong absorbance in the range of 200–350 nm and could produce ·OH under UV irradiation (Formula (2)). Besides, as a kind of strong oxidizer, H₂O₂ can effectively capture the photoproduction electrons of TiO₂ conduction belt and be converted to ·OH as Formula (3). So, the degradation was accelerated with the addition of H₂O₂.

$${\text{H}}_2{\text{O}}_2\stackrel{h\upsilon }{\to }2·\text{OH}$$

(2)

$${\text{H}}_2{\text{O}}_2+e^{-}\to ·\text{OH}+{\text{HO}}^{-}$$

(3)

However, excessive H₂O₂ would exhaust the generated ·OH in the reaction solution (Formulas (4) and (5)) [38] to reduce the promotion.

$${\text{H}}_2{\text{O}}_2+·\text{OH}\to {\text{H}}_2\text{O}+·{\text{HO}}_2$$

(4)

$$·{\text{HO}}_2+·\text{OH}\to {\text{H}}_2\text{O}+{\text{O}}_2$$

(5)

Figure 6. The effect of H₂O₂ on the degradation of NH₄⁺-N with La/Fe/TiO₂.

Figure 6. The effect of H₂O₂ on the degradation of NH₄⁺-N with La/Fe/TiO₂.

3.2.3. Effects of Inorganic Ions

There are eight common inorganic ions in natural water [39], including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, NO₃⁻, and HCO₃⁻/CO₃²⁻, and the concentrations are all 0.1 mmol/L. They might affect the removal of pollutants in water. Results of the effects of cations and anions on ammonia nitrogen degradation are shown in Figure 7.

Figure 7. The effect of ions on the degradation of NH⁴⁺-N with La/Fe/TiO₂.

Figure 7. The effect of ions on the degradation of NH⁴⁺-N with La/Fe/TiO₂.

As seen from Figure 7, at the same experimental conditions, all kinds of inorganic ions showed an obvious inhibitory effect on ammonia nitrogen removal. Sörensen indicated that NO₃⁻ acted as an “inner filter” and reduces the UV light intensity in the photoreactor [40]. Thereby, addition of NO₃⁻ decreased the degradation rate of pollutant in the reaction system. Besides, the inhibited effect increased as the reaction went on. The reason might be that excessive NO₃⁻ was produced from the conversion of ammonia nitrogen (the content will be discussed below). It was reported that HCO₃⁻/CO₃²⁻ is an effective ·OH scavenger [40]. It can react with ·OH to produce carbonate radicals, which are weak oxidizing reagents that hardly react with other pollutant molecules. Therefore, HCO₃⁻/CO₃²⁻ displayed distinct inhibition effect on the degradation of ammonia nitrogen by La/Fe/TiO₂. Cl⁻ and SO₄²⁻ also could react with ·OH like HCO₃⁻/CO₃²⁻, but the reaction ability was lower than NO₃⁻, and the inhibited effect was smaller than HCO₃⁻/CO₃²⁻. Since SO₄²⁻ is double charged, it may display higher inhibition ability than Cl⁻.

Na⁺, K⁺, Ca²⁺, and Mg²⁺ are all in the highest and stable oxidation state and cannot capture electrons or holes in solution. It is hypothesized that these metal ions would not show significant impacts on the photo degradation of ammonia nitrogen by La/Fe/TiO₂. As shown in Figure 7B, the four metal ions displayed inhibition effects on ammonia nitrogen removal, which could be attributed to the effect of Cl⁻ ions co-present in the solution. The metal ions were used in their chloride salts. As described above, Cl⁻ ions might inhibit the photo degradation due to the reaction with ·OH. Since K⁺ and Na⁺ are in the same elemental main group, they have similar properties. KCl showed similar effect as NaCl. In addition, Ca²⁺ and Mg²⁺ also have similar properties and they (CaCl₂ and MgCl₂) displayed similar trends. Furthermore, Ca²⁺ and Mg²⁺ displayed higher inhibition effects than NaCl and KCl at the same mole concentrations. This is expected since the concentration of Cl⁻ in CaCl₂ and MgCl₂ solutions was twice of that in NaCl and KCl solutions [41]. Besides, Mg²⁺ and Ca²⁺ tend to form precipitation and adhere to the surface of the catalyst to reduce the photocatalytic efficiency in weak alkaline conditions [42].

3.2.4. Analysis of Degradation Products of Ammonia Nitrogen

WuJie [43] mentioned that the process of photodegradation of inorganic nitrogen in water would generate highly reactive ·OH and O₂⁻ and other reactive oxygen species, which have photocatalytic ability. Inorganic nitrogen ions can induce a series of REDOX reactions, mainly containing NH₄⁺ oxidation and NO₃⁻ reduction (Formulas (6–10)). However, the specific mechanism is still not yet confirmed, and it needs further research.

$${\text{NH}}_4^{+}+2{\text{OH}}^{-}\to {\text{NO}}_2^{-}+3{\text{H}}_2$$

(6)

$$2{\text{NO}}_2^{-}+{\text{O}}_2\to 2{\text{NO}}_3^{-}$$

(7)

$${\text{NO}}_3^{-}+2{\text{H}}^{\mathrm{+}}+2e_{ab}^{-}\to 2{\text{ON}}_2^{-}+{\text{H}}_2\text{O}$$

(8)

$${\text{NO}}_3^{-}+10{\text{H}}^{\mathrm{+}}+8e_{ab}^{-}\to {\text{NH}}_4^{+}+3{\text{H}}_2\text{O}$$

(9)

$$2{\text{NO}}_3^{-}+12{\text{H}}^{\mathrm{+}}+10e_{ab}^{-}\to 2{\text{N}}_2+3{\text{H}}_2\text{O}$$

(10)

NO₃-N and NO₂-N were detected as the photocatalytic degradation products of ammonia nitrogen wastewater as the following Figure 8. After 300 min of photocatalytic reaction using La/Fe/TiO₂ as the photocatalyst, the concentration of ammonia nitrogen reached to 34.96 mg/L by the initial concentration of 100.67 mg/L and the conversion rate was 64.6%. During the degradation process, 9.56 mg/L of NO₃-N and 2.07 mg/L of NO₂-N were generated. Considering the escape free ammonia and adsorption part onto the catalyst’s surface, it is proposed that at least 20% of ammonia nitrogen was converted to N₂ according to the mass balance of the total nitrogen [44].

Figure 8. The conversion curves of the NH₄⁺-N during the photodegradation process.

Figure 8. The conversion curves of the NH₄⁺-N during the photodegradation process.

4. Conclusions

La-Fe-codoped catalyst demonstrates better physical chemical properties in photocatalytic activity than pure TiO₂: first of all, the doped catalysts were in anatase phase while the pure TiO₂ was in rutile phase; second, the composites possessed strong visible-light-response ability; third, La/Fe/TiO₂ had larger specific surface area and more regular shape in morphology. Furthermore, the doped catalysts indicated higher photocatalytic degradation ability to ammonia nitrogen wastewater: the removal rate of ammonia nitrogen reached to 78.3% at the conditions of pH 9.9, 100.67 mg/L of ammonia nitrogen, 1 g/L of catalyst, and 2 mmol/L of H₂O₂. Besides, the common inorganic ions in water all inhibited the degradation of ammonia nitrogen. At last, it is proposed that there at least 20% of ammonia nitrogen was converted to nitrogen gas during the photodegradation process with 64.6% removal efficiency of ammonia nitrogen.