Start Submission

Reading: Photocatalytic degradation of high ammonia concentration wastewater by TiO2

Download

A- A+
Alt. Display

Technical Articles

Photocatalytic degradation of high ammonia concentration wastewater by TiO2

Authors:

Xue Gong ,

The College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China, CN
X close

Haifeng Wang,

The College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China, CN
X close

Chun Yang,

The College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China, CN
X close

Quan Li,

The College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China
X close

Xiangping Chen,

The Electrical Engineering College, Guizhou University, Guizhou, China, CN
X close

Jin Hu

Institute of Thermal Engineering, University of Applied Sciences and Arts of Western Switzerland, Yverdon-les-Bains, Switzerland, CH
X close

Abstract

This study explored the effect on photocatalytic degradation in aqueous solution with high- concentration ammonia where immobilized TiO2 on glass beads was employed as the photocatalyst. TiO2 film was prepared via deep coating TiO2 in a sol–gel system of tetrabutyl titanate precursor and calcinating it at the temperature between 400 to 650 °C. Several crucial factors affecting on the rate of removal of ammonia were investigated. These factors included annealing temperature, catalyst composition, coated times of TiO2 film, aqueous initial pH, UV exposure time, repetitions times, etc. The results validate the effectiveness as TiO2 glass beads was employed for photocatalytic degradation treatment in high concentration ammonia solutions.
How to Cite: Gong, X., Wang, H., Yang, C., Li, Q., Chen, X. and Hu, J., 2015. Photocatalytic degradation of high ammonia concentration wastewater by TiO2. Future Cities and Environment, 1, p.12. DOI: http://doi.org/10.1186/s40984-015-0012-9
2918
Views
74
Downloads
4
Citations
  Published on 21 Aug 2015
 Accepted on 26 Nov 2015            Submitted on 3 Nov 2015

Introduction

Early examples of research into the cleaning properties of TiO2 include those carried out by Frank and Bard. They found that TiO2 powder in contaminated water were able to photocatalyze the conversion of cyanide into cyanate and thus detoxified the water [15, 16].

One of the advantages of TiO2 photocatalysis for water decontamination is that only the TiO2 photocatalyst (immobilized or suspended) and UV light, either from solar light or artificial light sources, are needed and its cost can thus be lower than other kinds of advanced oxidation techniques (UV/O3; UV = H2O2, photo-Fenton). Moreover, no toxic intermediate products are generated in the photocatalytic decontamination process. As a result, TiO2 photocatalysis has been attracting a lot of interest in the areas of water detoxification or drinking water purification [6, 9, 36].

The main potentially-polluting nutrients in relation to water are nitrogen, ammonia, phosphorus and sulphur. Ammonia is a common water contaminant that has notable effects upon the environment and human health when it is in the presence of excess amounts. They arise from the natural breakdown of crop residues and soil organic matter, rainfall, wastewater and industrial effluents…etc. Free ammonia (NH3) and ionized-ammonia (NH4+) represent two forms of reduced inorganic nitrogen. The free ammonia is a gaseous chemical, whereas the NH4+ in the reduced nitrogen form exists in ionized form and remains soluble in water [40]. Ammonia is one of the key matters causing eutrophication pollution. Ammonia brings injurious effects on human and animal health in forms of gas and particle phase [2, 3].

In most cases, contamination of groundwater by chemicals derived from urban and industrial activities, modern agricultural practices, or waste disposal takes place almost imperceptibly. The concentration of nitrogen of the regional groundwater rises along with over fertilization in agriculture [1, 23, 45]. Solid waste produced by both municipal and industrial sectors increased the organic, heavy metals, and inorganic groundwater ions [34, 41, 49]. Excess discharge of domestic and industrial wastewater has also polluted the groundwater [32]. In general, ammonia level in ground waters is below 0.2 mg per litre. Higher natural contents (up to 3 mg/litre) are found in strata rich in humic substances, iron or in forests [12]. Surface waters may contain up to 12 mg/litre [4]. Ammonia may be present in drinking-water as a result of disinfection with chloramines. The maximum limit of ammonia set by the European Association for drinking water is approximately 0.5 mg/l (AWWA, [5]).

Currently, the most popular treatments for ammonia removal from wastewater are air stripping, ionic exchange and nitrification denitrification techniques where however, ammonia cannot be removed completely in most cases. For this reason in the recent years advanced oxidation processes have been investigated as the alternatives to conventional water treatment. [29] used the photocatalytic reduction method of Pt / TiO2 nano-photocatalysts with high platinum loading of 0.5 % to achieve the best efficiency of nitrogen removal. [47] conducted TiO2 photocatalyst doped with metal ions for treating ammonia nitrogen wastewater from coal gasification process. The ammonia nitrogen removal rate was approximately 79 %. Songhua River water treatment using catalyzed ozonation were investigated by [50]. In these cases, Nano TiO2 coated haydite, silica-gel and zeolite were used as the catalyst. The ammonia removal efficiency was close to 80 % after 30 min reaction.

Typically, the studies with respect to photocatalytic reactors adopted either TiO2 powder or suspended counterpart in the water being treated and then went through further process to remove the TiO2. The primary aim of doing so is to avoid the post separation difficulties associated with the powder form of the TiO2 catalyst. Immobilization of TiO2 on various substrates is an important issue in the water treatment applications associated with photocatalytic technologies [43]. There are several advantages in these researches, including higher surface area, superior adsorption properties [24, 53], surface hydroxyl groups increase, and charge recombination reduction [46] in immobilized systems.

This study attempted to implant immobilized TiO2 onto the small glass beads which were placed into the photocatalytic reactor to investigate and analyze its performance of treating ammonium-rich wastewater. Moreover, we aim to develop a simple, energy efficient, less expensive to build and operate photocatalytic reactor to handle ammonium-rich wastewater. This low cost solution will provide effective and successful experiences for large-scale industrial applications in the future.

Experimental setup and procedure

Experimental apparatus

Experimental apparatus is shown in Fig. 1; it mainly consists of an inner tube and outer borosilicate glass reactor of 500 mL capacity. The inner tube is equipped with a UV lamp (Xin Guang Yuan Lighting, 36 W, with an emission spectrum of 200–600 nm, and λmax at 365 nm), which was mounted axially in the reactor inside a cylindrical, double walled lamp jacket. Instead of ordinary, the glass lamp jacket is made of quartz glass in order to reduce the absorption of ultraviolet light. The outer glass reactor as a photocatalytic reactor is to accommodate the inner tube where TiO2 coated glass beads as the photocatalytic constituent, also as the aqueous samples were placed in. In this study, UV protective enclosure was used to bring out protection effect against exposure to UV radiation.

Fig. 1 

Experimental apparatus: schematic of setup

TiO2 thin film preparation

The TiO2 films were prepared via sol-gel process. Fig. 2 illustrates this procedure in detail. Tetrabutyl titanate ((C4H9O) 4Ti) was used as a precursor to prepare TiO2 sol. CH3CH2OH, ((HOCH2CH2)3 N) and distilled water were added to ((C4H9O) 4Ti) under continuous magnetic agitation at room temperature to form a mixture according to volume ratio of 3: 12: 1: 1. All chemicals used for this study were of analytical grade and from Chengdu Kelong Chemical Co., Ltd. Taking their high mechanical and chemical stability into account, this study selected the glass beads (diameter 5 mm) from Yongqing Ziguang Glass Beads Co., Ltd, as the substrate to be coated with nano-TiO2. Before coating, the glass beads were etched for 24 h in diluted hydrofluoric acid and subsequently were sonicated (DH-120DT, Shanghai Dihoo Instrument) in acetone, absolute ethanol and distilled water for 10 min in order to remove all the organic contaminants. After that, they were dried out at 100 °C by DZF-6050 vacuum dryer (Shanghai Jinghong Laboratory Instrument).

Fig. 2 

The process of fabrication of TiO2 thin films

TiO2 thin films were deposited on glass slides by dip coating method at two different vertical withdrawal speeds of 1 cm/min. TiO2 films were deposited several layers on substrates. During successive coatings, each layer was applied vacuum drying process for 30 min at 100 °C. After multiple dip coating was finished, the films were annealed at various temperatures in the range of 400 ~ 650 °C for 2 h at the heating rate 6 °C/min in an atmosphere.

Determination of the analyte concentration in a solution after degradation

In this study, the Beer–Lambert law was applied to predict the linear relationship between the absorbance of the solution and the concentration of the analyte (assuming all other experimental parameters invariant). A series of ammonium chloride NH4Cl standard solutions were prepared. The absorbance of NH4Cl for various concentrations was measured with a UV-vis spectrophotometer (Shanghai Puyuan Instrument) at a wavelength 425 nm, which offered an effective way to engender a calibration curve to show the absorbance variation along with the concentration as shown in Fig. 3. The slope and the intercept of the line provided a relationship between absorbance and concentration:

Fig. 3 

The standard/calibration curve of absorbance for different concentrations of NH4Cl

(1)

Experimental procedures followed this way: the process began with degradation before 300 ml aqueous sample was added to the outer container. The UV light was then turned on to start UV photolysis for a period of time. The absorbance of the sample was re-measured when the experiment was over. The degradation rate can be calculated with:

(2)

Where η represents degradation rate; A0 represents the initial absorbance of the solution sample; A represents absorbance of the solution sample after a certain time of photocatalytic reaction; represents the initial concentration of solution sample; C represents the concentration of solution sample after degradation.

To ensure reproducibility and decrease the uncertainty, each measurement was repeated 3 times to take the average value. The nonlinear regression analysis was performed to best-fit the response function (photocatalytic degradation efficiency) with the actual experimental data.

Results and discussion

Structural properties of TiO2 thin film

To determine the intrinsic properties of the TiO2 layers, various techniques were used. X-ray diffraction (XRD, model D8 ADVANCE, Bruker. Ltd) analysis of the films on the glass bead substrate was performed to determine the crystalline phase and to estimate the amendment of TiO2 as heating temperature varied. The XRD patterns were recorded by step scanning in the 2θ scan range of 5–80° where a step size was of 0.01°. In Fig. 4, anatase phase and rutile phase are denoted as A and R, respectively.

Fig. 4 

XRD patterns of the TiO2 films at various calcination temperatures

Before heat treatment, the TiO2 thin layer consisted of amorphous or poorly crystallized oxide. Figure 4 shows the XRD patterns of the TiO2 films annealed at various temperatures. The line at 400 °C shows a prominent anatase peak at 25.4° (101) due to the transition from the amorphous phase to the anatase phase. When the calcined temperature increased to 450 °C, the crystallinity of TiO2 was improved but still stayed on the anatase phase. As the temperature increased from 400 to 600 °C, there was significant growth for the intensities peaks, which revealed the improvement of the crystallinity. And the outcomes are in agreement with the finding demonstrated by Hasan et al. [19]. When temperature went up to 500 °C, the XRD pattern exhibited a rutile peak at 27.4° (110). At this temperature, it is thought that the transformation from anatase to rutile took place. Anatase/rutile mixture phase appeared when the TiO2 thin film was annealed at 500 °C. At 600 °C, the rutile (110) peak went higher while the counterpart of the anatase (101) dropped. As far as the line of 650 °C, the anatase peak disappeared while the the rutile peak increased considerably, indicating that a complete transformation from the anatase phase to the rutile phase occurred. This phenomenon is in compliance with the work done by [26, 51]. They revealed that TiO2 thin films can be transformed from amorphous phase into high degree crystalline anatase and then converted into rutile by temperature treatment. At above 600 °C, x-ray diffraction pattern showed a peak belonging to the rutile peak. Transformation from anatase to rutile is thought to occur at about 500 °C with the complete transformation at higher temperature. The average crystallite sizes of the samples were estimated by the Debye–Scherrer’s equation as expressed below:

(3)

where D is the diameter of crystalline in nm, λ is the X-ray wavelength of Cu Kα radiation, βhkl is broadening of the hkl diffraction peak width at half height of the maximum intensity (FWHM) in radians and θhkl is the diffraction angle (Bragg’s angle) in degrees.

For TiO2 thin films that contain anatase and rutile phase only, the percentage of mass fraction (XA) for anatase phase in the solution was calculated using the relative intensity of maximum peak of anatase phase (101) and a maximum peak of rutile phase (110). The XA is expressed by the equation below [44];

(4)

Where XA, IA, IR refer to a contents of the anatase phase in the solutions, the intensity at the anatase phase peak as well as the intensity at rutile phase peak, respectively. Meanwhile, the constant 1.265 is the scattering coefficient [54]. The rutile weight fraction XR can be calculated by the empirical equation as followed [38]:

(5)

The influent factors associated to annealing temperature on the structure of TiO2 were summarized in Table 1. As the table has shown, annealing temperature rise results in bigger crystallite size. For instance, when the annealing temperature increased from 400 to 600 °C, the crystallite size on of anatase phase grew from 14.4 to 18.72 nm. Similarly, the crystallite size of rutile phase grew from 37.06 nm to 67.94 nm with the increasing of annealing temperature from 500 to 650 °C. It is observed that the XA of anatase phase was 86.31 % and it decreased to 26.92 % as the annealing temperature increased from 500 to 600 °C. It is noticeable that the amount of rutile phase increased with annealing temperature rise from 500 to 650 °C.

Table 1

The influence of temperature on the grain size and content of TiO2 thin film

T (°C) Phase K λ (nm) β (radian) cosθ hkl D (nm) Mass ratio (%)
400 Anatase 0.89 0.154 0.009546 0.997 101 14.40407 100
450 Anatase 0.89 0.154 0.995986 0.996 101 14.71318 100
500 Anatase 0.89 0.154 0.997976 0.998 101 15.11035 86.31
Rutile 0.89 0.154 0.4324127 0.432 110 37.06334 9.04
550 Anatase 0.89 0.154 0.998008 0.998 101 15.79999 72.88
Rutile 0.89 0.154 0.417477 0.417 110 45.66140 18.87
600 Anatase 0.89 0.154 0.998589 0.998 101 18.72492 26.92
Rutile 0.89 0.154 0.435115 0.435 110 48.12791 62.98
650 Rutile 0.89 0.154 0.385424 0.385 110 67.94198 100

TiO2 exists in the form of three polymorphs: anatase, rutile, and brookite [11]. Although rutile phase has less photocatalytic activity than anatase, Anatase/rutile mixture phase is known to exhibit enhanced photoactivity relative to single-phase titania [13, 21, 37, 42, 48]. It is widely recognized that this is a result of improved charge carrier separation, possibly through the trapping of electrons in rutile and the consequent reduction in electron–hole recombination [8, 21]. Anatase-to-rutile phase transformation is governed by the annealing temperature, compactness of the anatase nanocrystallites, and grain boundary defects [28, 30].

The influence of exposure time on the TiO2 films under UV irradiation

Over the trail test, TiO2 thin films were categorized into 4 groups based on their anneal temperatures of 450 °C, 500 °C, 550 °C and 600 °C under UV irradiation. Comparison among the 4 groups was conducted to check the influence of exposure time on ammonia degradation rate. Ammonia content of wastewater was about 700 mg / L. The volume of reaction solution was 300 ml. The solution was adjusted to a pH of 7. The degradation was carried out at room temperature. The photocatalytic reaction time was 30, 60, 90, 120 and 150 min, respectively. The results are shown in Fig. 5.

Fig. 5 

The influence of exposure time of the TiO2 films under UV irradiation on ammonia degradation rate

The results from Fig. 5 indicate that the photocatalytic activities/degradation rate of the thin TiO2 films prepared at 4 different temperatures displayed the same pattern in function of UV exposure time. As shown in Table 1, TiO2 presented photocatalytic activity on anatase phase without phase conversion at 450 °C. Anatase/rutile mixture phase was seen at 500 °C. At this temperature, anatase is converted to rutile with 9.04 % rutile phase in the mixed phase. This mixed phase has higher photocatalytic activities than single anatase phase. When the sample was prepared at 550 °C (Rutile 18.87 %, Anatase 72.88 %), which has the highest photocatalytic activity among all thin films tested. Sample annealing at 600 °C resulted in 62.98 % rutile phase as a predominantly phase in the mixed phase. At this temperature, TiO2 thin film displayed much less photocatalytic activity than other 3 samples. Ding et al [14] conducted the degradation of organic dye solution using mixed crystal nano TiO2 as a photocatalyst. They found the photocatalytic activity of mixed phase nano-TiO2 crystal samples was significantly higher than that of a single crystal sample or mechanical mixed crystal samples. The photocatalytic activities had the best effect when the mass fraction of rutile phase was about 20 % of mixed crystals. Ji et al [22] prepared anatase - rutile mixed Nano TiO2 crystal samples to test degradation effect of Methyl orange solution. They found when rutile phase is of 18.9 % of mixed crystal, the photo degradation rate of methyl orange can reach the highest 85 %. Degussa P-25 (a mixed-phase titania photocatalyst) is commercially available and was utilized as a reference material in many studies. This nanocrystalline material, formed by flame pyrolysis, consists of 80 wt% anatase and 20 wt% rutile. Our experimental results were in good agreement with other authors’ findings described above.

As can be seen from Fig. 5, over UV exposure time from 30 to 120 min, it is evident that the percentage of photo-degradation for the four samples increases along with irradiation time growth. After 120 min, the growth of the degradation rate of all samples substantially slowed down and almost ceased. This could be attributed to the following reason: in general, the photoactivity of TiO2 is determined by the processes of electron/hole pair generation, recombination, interfacial transfer and by the surface reactions of these charge carriers with the species adsorbed on the surface of the photocatalyst [[10, 17, 18, 31]]. The presence of molecular oxygen in the solution also plays a substantial role in the photo-induced processes on irradiated TiO2 surfaces because that it enables an effective charge carriers separation. Consequently, both superoxide radical anion and hydrogen peroxide as the most important products of the molecular oxygen reduction play an important role in the complex mechanism of Oxygen Species (ROS, e.g., •OH, •O2H or singlet oxygen) generation on the irradiated TiO2 surfaces [[20, 31, 35, 52] ]. Hydroxyl radicals are powerful and indiscriminate oxidizing species. The redox reactions at the surface of the particles lead to the generation of active oxygen species which can attack organic and inorganic species at or near the surface. However, the photocatalytic degradation of ammonia will lead to generate significant number of intermediates species in the reaction mixture which can’t be removed efficiently in the reactor. They gradually accumulated and eventually deposited on the TiO2 film surfaces around reaction time of 2 h which deactivate the active sites and reduce the hydroxyl radicals and active oxygen species generation on thin film surface who play an important role in oxidizing.

The influence of TiO2 layer coated times on ammonia degradation rate

In order to obtain multilayer thin films with controllable thickness, 1-10 TiO2 layers were successively coated on glass beads using dip-coating steps and then thermally treated as aforementioned process.

The photocatalytic degradation of ammonia in water was conducted respectively with the glass beads coated with 1 to 10 layers of TiO2 thin film. The ammonia concentration in wastewater was about 700 mg / L, initial pH of wastewater sample was 7; the degradation was at room temperature for 120 min.

It is clearly seen from Fig. 6 that under the same experimental conditions, the degradation efficiency depends on the film thickness. It was found that the degradation efficiency increased with both TiO2 film layers and film thickness growth but not linearly dependent. It is most likely that more titanium dioxide participated in the photocatalytic reaction. However, the growth of the degradation efficiency over 6 times of coating seemed to slow down or even stayed on the same level while the first 6 coatings greatly improved the degradation efficiency.

Fig. 6 

TiO2 film coated times as function of degradation efficiency

Figure 6 implies that we cannot achieve greater degradation efficiency by unlimited increasing coating layers or film thickness. An optimum value of TiO2 film thickness is presented. Jin et al. [25] showed the transmission spectra of the TiO2 films with various coating times. They demonstrated that the transmittances of the TiO2 thin films coated 5 times and 15 times were higher than that of the thin film coated 30 times in the visible range of 400 nm ~ 800 nm. Moreover, photocatalytic chemical reactions occurred on the surface of TiO2 thin films. Meanwhile, titanium dioxide in the interior region of thin films was not easy to contact the reactant and reaction products were also not easy to enter the solution. Therefore, the photocatalytic activity of titanium dioxide in the interior region of thin films was lower than that of titanium dioxide in the surface of thin films.

The surface topography and the microstructure of the coatings were examined by scanning electron microscopy (SEM). The film thickness was measured by using SEM and weight methods.

Fig. 7a showed that the glass beads surfaces presented inhomogeneous surfaces after two times coating without full coverage of nanoparticles to provide more reaction sites on the surface over the photocatalytic reaction. Nanoparticles are in irregular shapes and majority of the nanoparticles formed into a monolayer with a few bare or multilayered patches. The film thickness is ~230 nm. Fig. 7b illustrated that the layer consisting of four films yielded totally smooth surface. The full coverage of nanoparticles was observed. When the coating became thicker, it became denser. The film thickness is ~410 nm. The surface particles are in good contact with low surface roughness. Fig. 7c revealed that the surface of the six films layer appeared to be wrinkled while the good crystallinity and homogeneous nanostructure surface area were obtained after the 6 times of coating. Both the boundaries between particles and the grain sizes were clearly observed. These surface particles were in close contact and they formed the blocks with higher density where the film thickness is ~570 nm. Fig. 7c suggested a link between the grain size growth and the increase in roughness. Fig. 7d provided the image of the surface of ten layers films under higher magnification. The surface particles were piled up in layers and the grains agglomeration formed clusters. The film thickness is ~900 nm.

Fig. 7 

SEM images of (a) 2 (b) 4 (c) 6 and (d) 10 layers of TiO2 thin film

The influence of aqueous initial pH on degradation efficiency

The pH condition has been reported to affect the photocatalytic reaction rate. Before turning on the UV lamp, the pH of the aqueous solution was adjusted to the desired value by dropwise addition, 0.5 mol HCl solution or NaOH solution (Chengdu Kelong Chemical Co., Ltd). The concentration of ammonia was about 700 mg/L. The suspension was placed in the dark, shielded with aluminum foil and stirred until the pH stable.

The degradation efficiencies of ammonia in aqueous wastewater during UV illumination for pH from 1 to 13 were investigated as presented in Fig. 8. It was observed that the initial pH values of water samples had significant impacts on the photocatalytic degradation rate. In the pH range 1 – 3.4, the photocatalytic degradation efficiency increased with time. pH =3.4 had the maximum degradation of ammonia, removal efficiencies of ammonia in wastewater were above 70 %. When pH values exceeded 3.4, the photocatalytic degradation rate decreased dramatically. It is concluded that the overall degradation rate under strong acidic or mildly acidic were higher than those under neutral or mildly alkaline or strongly alkaline conditions. The possible reason is that the surface charge of TiO2 particles was influenced by the pH value in the solution. [7] already reviewed that acid-base properties of the metal oxide surfaces could considerably affected their photocatalytic activity. The point of zero charge (pzc) of the TiO2 (Degussa P25) was at pH 6.8 [39]. Thus, the TiO2 surface was positively charged in acidic media (pH < 6.8), whereas it is negatively charged under alkaline conditions (pH > 6.8). At the acidic condition, TiO2 surface was positively charged while the wastewater contained negatively charged groups to improve the absorbance and the photocatalytic effect. In contrast, Sun et al. [47] suggested that the ammonia removal rate was larger in the alkaline condition than that in acidic condition. He also pointed out that an optimum removal rate was obtained at pH 9.1 when the concentration of ammonia was 546 mg/L. Murgia et al. [33] carried out a kinetic study of photo oxidation over TiO2 of NH3/NH4 in the high concentration range of 26 – 214 mg/l. They found that the degradation decreased from 50 % to 17 % at catalyst concentration of 0,008 % and meanwhile the pH value increasing from 10.7 to 9.5.

Fig. 8 

Relationship between aqueous initial pH and degradation efficiency

The influence of repetitions times on degradation rate and restoration of photocatalytic properties of TiO2

The surface modified glass beads were immersed in the TiO2 sol for 10 min. Subsequently, they were dried at 100 °C in a vacuum dryer and calcinated at 550 °C for 2 h. The procedure was repeated until the glass beads were coated with 6 layers of TiO2. The degradation was conducted at room temperature and degradation time was 120 min with ammonia content about 700 mg / L in wastewater. The same glass beads were reused for 4 cycles to check the degradation efficiencies. Restoration of photocatalytic properties of TiO2 thin film was executed as following: after reusing for 4 cycles, TiO2 film coated samples were rinsed with distilled water and dried in the vacuum oven. After that, they were placed in a resistance furnace to calcinate at 550 °C for 2 h. Under the same experimental conditions, the degradation efficiencies were re-measured. The results are presented in Fig. 9.

Fig. 9 

Influence of repetitions times on degradation rate and restoration efficiency

From the above chart, we can conclude that repetitions result in degradation efficiency reduction to some extent. After 4 cycles of usages, the degradation efficiency of the sample reclined ~34 % comparing to the original value. This implies that the TiO2 films can be used continuously for water treatment. Fortunately, this study did not find any deposited thin films peeling off from the glass beads.

This can be explained from the perspective of TiO2 photocatalytic mechanism. Oxidation and decomposition occurred on the surface of TiO2. After photocatalytic reaction, more or less residuals remained on the surface, which weakened the participation of TiO2 in the next photocatalytic reactions and therefore resulted in a steady decline in the degradation efficiency. During the restoration process, calcination at 550 °C for 2 h physically removed residuals adsorbed on TiO2 surface. Restoration of photocatalytic properties of TiO2 thin film had significant effect and ~93 % efficiency was restored in the recycled sampled in contrast to a new sample as shown on Fig. 9.

Conclusion

This project was undertaken to embed the TiO2 thin film on the glass beads via a deep coating in a sol–gel system and the performance of our lab-scale photocatalytic reactor where the volume of reactant solution is <1 L was evaluated on the basis on change in concentration of high ammonia content with respect to time. Immobilization of TiO2 on rigid glass beads surface is due to the main advantage that the glass substrate is the transparent even after the immobilization. This allows the penetration of light resulting in improved photocatalysis. Several suggestions on the basis of the results from the trail tests can be summarized, which contributes to the continue research in the future.

At 550 °C, TiO2 sample presented the highest photocatalytic activity among all counterparts, where the mass fraction on the rutile phase accounted for around 18.87 % in the mixed crystals; the relationship between film thickness and degradation efficiency is worth noticing. The thicker films brought out higher degradation. However, degradation went slower when TiO2 film was over 6 times of coating; it was observed that the optimum removal rate is obtained at pH 3.4 when aqueous initial pH changed from 1 to 13; the outcomes supported the finding that better degradation rate was achieved for high concentration ammonia in strong acidic or mildly acidic wastewater rather than neutral, mildly alkaline and strongly alkaline conditions; the influence of exposure time of the TiO2 films under UV irradiation was examined. Within time range of 30 ~ 120 min of UV exposure, the percentage of photo-degradation of the samples increases with irradiation time growth. After 120 min, as time proceeds, the growth of the degradation rate of all samples substantially slowed down and almost unchanged; one of the more significant findings to emerge from this study is that repetitions led to degradation efficiency reduction to some level; after 4-cycles re-utilization, the degradation efficiency reduced ~34 % compared to the original value. However, ~93 % efficiency was restored in the recycled sampled in contrast to a new sample and the TiO2 thin film could recover its photocatalytic properties effectively.

The photocatalytic degradation treatment in this study has proved to be very effective to removal of high concentration ammonia from its solutions. In addition, this photocatalytic reactor seems to be a simple, energy efficient, eco-friendly, less expensive to build and operate photocatalytic reactor to handle ammonium-rich wastewater. This study provides good reference data for the design of pilot plant-scale reactors for treating ammonium-rich wastewater.

Acknowledgements

This study is funded by the Projects of Education Department of Sichuan Province (14CZ0004), China.

References

    References

    1. Aguilar, JB, Orban, P, Dassargues, A and Brouyère, S Identification of groundwater quality trends in a chalk aquifer threatened by intensive agriculture in BelgiumHydrogeology J200715816151627https://doi.org/10.1007/s10040-007-0204-y 

    2. Alonso, A and Camargo, JA Short-term toxicity of ammonia, nitrite, and nitrate to the aquatic snail Potamopyrgus antipodarum (Hydrobiidae, Mollusca)Bull Environ Contam Toxicol20037010061012https://doi.org/10.1007/s00128-003-0082-5 

    3. Alonso, A and Camargo, JA Toxic effects of unionized ammonia on survival and feeding activity of the freshwater amphipod Eulimnogammarus toletanus (Gammaridae, Crustacea)Bull Environ Contam Toxicol20047210521058https://doi.org/10.1007/s00128-004-0350-z 

    4. Ammonia. Geneva (1986) World Health Organization, (Environmental Health Criteria, No. 54) 

    5. Water quality and treatment, A handbook of Community Water Supplies2000Fifth EditionNew YorkMc Graw Hill Co 

    6. Bahnemann, D Photocatalytic water treatment: solar energy applicationsSolar Energy200477445459https://doi.org/10.1016/j.solener.2004.03.031 

    7. Bahnemann, DW, Cunningham, J, Fox, MA, Pelizzetti, E, Pichat, P, Serpone, N, Zepp, RG and Heltz, GR Aquatic Surface Photochemistry1994Boca RatonLewis Publishers261 

    8. Batzill, M, Morales, EH and Diebold, U Influence Of Nitrogen Doping On The Defect Formation and Surface Properties of TiO2 Rutile And AnatasePhys Rev Lett20069626103https://doi.org/10.1103/PhysRevLett.96.026103 

    9. Blanco-Galvez, J, Fernández-Ibáñez, P and Malato-Rodríguez, S Solar Photocatalytic Detoxification and Disinfection of Water: Recent overviewJ Solar Energy Eng2007129415https://doi.org/10.1115/1.2390948 

    10. Carp, O, Huisman, C and Reller, A Photoinduced reactivity of titanium dioxideProg Solid State Chem20043233177https://doi.org/10.1016/j.progsolidstchem.2004.08.001 

    11. Coronado, DR, Gattorno, GR, Pesqueira, ME, Cab, C, Coss, R and Oskam, RG Phase pure TiO2 nanoparticles: anatase, brookite and rutileNanotechnology200819145605https://doi.org/10.1088/0957-4484/19/14/145605 

    12. Dieter, HH and Möller, R Aurand, K ed. AmmoniumDie Trinkwasser verordnung, Einführung und Erläuterungen. [The drinking-water regulations, introduction and explanations]1991BerlinErich-Schmidt Verlag362368 

    13. Ding, K, Miao, Z, Hu, B, An, G, Sun, Z, Han, B and Liu, Z Study on the anatase to rutile phase transformation and controlled synthesis of rutile nanocrystals with the assistance of ionic liquidLangmuir2010261029410302https://doi.org/10.1021/la100468e 

    14. Ding, S, Wang, L, Ding, Y, Zhang, M and Wang, Z Microwave Synthesis and Photocatalysis of Nano-TiO2 Mix-crystalsChinese J Appl Chem2006236 

    15. Frank, SN and Bard, AJ Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solutions at titanium dioxide powderJ Am Chem Soc197799303304https://doi.org/10.1021/ja00443a081 

    16. Frank, SN and Bard, AJ Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powdersJ Phys Chem19778114841488https://doi.org/10.1021/j100530a011 

    17. Fujishima, A, Zhang, X and Tryk, D TiO2 photocatalysis and related surface phenomenaSurf Sci Rep200863515582https://doi.org/10.1016/j.surfrep.2008.10.001 

    18. Gaya, UI and Abdullah, AH Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problemsJ Photochem Photobiol C: Photochem Rev200891112https://doi.org/10.1016/j.jphotochemrev.2007.12.003 

    19. Hasan, MM, Haseeb, ASMA, Saidur, R and Masjuki, HH Effect of annealing treatment on optical properties of Anatase TiO2 thin filmsWorld Acad Sci, Eng Technol200940221225 

    20. Hirakawa, T, Yawata, K and Nosaka, Y Photocatalytic reactivity for O2• − and •OH radical formation in anatase and rutile TiO2 suspension as the effect of H2O2 additionAppl Catal A2007325105111https://doi.org/10.1016/j.apcata.2007.03.015 

    21. Hurum Deanna, C, Agrios Alexander, G, Gray Kimberly, A, Rajh, T and Thurnauer Marion, C Explaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPRJ Phys Chem B20031071945454549https://doi.org/10.1021/jp0273934 

    22. Ji Chenjing, Zhang Yanfeng, Wei Yu (2010) Microwave Assisted Synthesis and Photocatalysis Property of Nanometer Titania Journal of Hebei Normal University (Natural Science Edition), Vol. 34, No.5 

    23. Jiang, Y and Somers, G Modeling effects of nitrate from non-point sources on groundwater quality in an agricultural watershed in Prince Edward Island, CanadaHydrogeology J2009173707724https://doi.org/10.1007/s10040-008-0390-2 

    24. Jin, L and Dai, B TiO2 activation using acid-treated vermiculite as a support: Characteristics and photoreactivityAppl Surf Sci201225833863392https://doi.org/10.1016/j.apsusc.2011.11.017 

    25. Jin, YS, Kim, KH, Park, SJ, Yoon, HH and Choi, HW Properties of TiO2 Films Prepared for Use in Dye-sensitized Solar Cells by Using the Sol-gel Method at Different Catalyst ConcentrationsJ Korean Physical Society201057410491053 

    26. Kim, D.J., Hahn, S.H., Oh, S.H., Kim, E.J., Materials Letters (2002) Influence of calcination temperature on structural and optical properties of TiO2 thin films prepared by sol–gel dip coating, Volume 57, Issue 2, Pages 355–360 

    27. Lee, DK Mechanism and Kinetics of the Catalytic Oxidation of Aqueous Ammonia to Molecular NitrogenEnviron Sci Technol20033757455749https://doi.org/10.1021/es034332q 

    28. Li, W, Ni, C, Lin, H, Huang, CP and Shah, SI Size dependence of thermal stability of TiO2 nanoparticlesJ Appl Phys20049666636668https://doi.org/10.1063/1.1807520 

    29. Li, X, Liu, L, Yang, F, Zhang, X and Barford, J Nitrogen Removal via Coupled Ammonia Oxidation and Nitrite Reduction Using Pt/TiO2 and PhotocatalysisChinese J Inorg Chem200622711801186https://doi.org/10.1002/cjoc.200690221 

    30. Madras, G, McCoy, BJ and Navrotsky, A Kinetic model for TiO2 polymorphic transformation from anatase to rutileJ Am Ceram Soc200790250255https://doi.org/10.1111/j.1551-2916.2006.01369.x 

    31. Minero, C, Maurino, V and Vione, D Pichat, P ed. Photocatalytic mechanisms and reaction pathways drawn from kinetic and probe moleculesPhotocatalysis and Water Purification: From Fundamentals to Recent Applications20131Weinheim, GermanyWiley-VCH5372 

    32. Mojie, S, Chao, Y, Chong, Z and Chuangjie, Z Influence of TiO2 /PVDF Membrane Catalyzed Ozonation of Ammonia Wastewater, Proceedings of the 2011 International Conference on Informatics, Cybernetics, and Computer Engineering (ICCE2011) November 19–20, 20112012Melbourne, AustraliaAdvances in Intelligent and Soft Computing771778 

    33. Mukhopadhay, A, Akber, A and Al-Awadi, E Evaluation of urban groundwater contamination from sewage network in Kuwait CityWater Air Soil Pollut20112161–4125139https://doi.org/10.1007/s11270-010-0521-y 

    34. Murgia, SM, Poletti, A and Selvaggi, R Photocatalytic degradation of high ammonia concentration water solutions by TiO2Ann Chim2005955335343https://doi.org/10.1002/adic.200590038 

    35. Ngo Michel, J, Gujisaite, V, Latifi, A and Simonnot, MO Parameters describing nonequilibrium transport of polycyclic aromatic hydrocarbons through contaminated soil columns: estimability analysis, correlation, and optimizationJ Contam Hydrol201415893109https://doi.org/10.1016/j.jconhyd.2014.01.005 

    36. Nosaka, Y and Nosaka, AY Pichat, P ed. Identification and roles of the active species generated on various photocatalystsPhotocatalysis and Water Purification: From Fundamentals to recentApplications20131Weinheim, GermanyWiley-VCH324 

    37. Ollis, DF, Pelizzetti, E and Serpone, N Photocatalyzed destruction of water contaminantsEnviron Sci Tech19912515221529https://doi.org/10.1021/es00021a001 

    38. Paola, AD, Cufalo, G, Addamo, M, Bellardita, M, Campostrini, R, Ischia, M, Ceccato, R and Palmisano, L Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookite-based) powders prepared by thermohydrolysis of TiCl4 in aqueous chloride solutionsColloids Surf A2008317366376https://doi.org/10.1016/j.colsurfa.2007.11.005 

    39. Porter, JF, Li, YG and Chan, CK The effect of calcination on the microstructural characteristics and photoreactivity of Degussa P-25 TiO2J Mater Sci19993415231531https://doi.org/10.1023/A:1004560129347 

    40. Poulios, I and Tsachpinis, I Photodegradation of the textile dye reactive black 5 in the presence of semiconducting oxidesJ Chem Technol Biotechnol199971349357https://doi.org/10.1002/(SICI)1097-4660(199904)74:4<349::AID-JCTB5>3.0.CO;2-7 

    41. Prajapati, Jignasha C., Syed, Huma S., Chauhan, Jagdiah (2014) Removal of Ammonia from wastewater by ion exchange technology, Int J Innovative Res Techno, Volume 1 Issue 9 

    42. Rao, GT, Rao, VVSG, Ranganathan, K, Surinaidu, L, Mahesh, J and Ramesh, G Assessment of groundwater contamination from a hazardous dump site in Ranipet, Tamil Nadu, IndiaHydrogeology J201119815871598https://doi.org/10.1007/s10040-011-0771-9 

    43. Scotti, R, Bellobono, IR, Canevali, C, Cannas, C, Catti, M, D’Arienzo, M, Musinu, A, Polizzi, S, Sommariva, M, Testino, A and Morazzoni, F Sol–gel pure and mixed phase titanium dioxide for photocatalytic purposes: relations between phase composition, catalytic activity, and charge-trapped sitesChem Mater20082040514061https://doi.org/10.1021/cm800465n 

    44. Shan, AY, Ghazi, TIM and Rashid, SA Immobilisation of titanium dioxide onto supporting materials in heterogeneous photocatalysis: A reviewAppl Catal A201038918https://doi.org/10.1016/j.apcata.2010.08.053 

    45. Spurr, RA and Myers, H Quantitative analysis of anatase-rutile mixtures with an X-ray diffractomerAnal Chem1957295760762https://doi.org/10.1021/ac60125a006 

    46. Stamatis, G, Parpodis, K, Filintas, A and Zagana, E Groundwater quality, nitrate pollution and irrigation environmental management in the Neogene sediments of an agricultural region in central Thessaly (Greece)Environ Earth Sci201164410811105https://doi.org/10.1007/s12665-011-0926-y 

    47. Stathatos, E, Papoulis, D, Aggelopoulos, CA, Panagiotaras, D and Nikolopoulou, A TiO2/palygorskite composite nanocrystalline films prepared by surfactant templating route: Synergistic effect to the photocatalytic degradation of an azo-dye in waterJ Hazard Mater20122116876https://doi.org/10.1016/j.jhazmat.2011.11.055 

    48. Sun, Y, Guo, W, Shan, Y, Lv, H and Song, Y Treatment of ammonia nitrogen wastewater from coal gasification process with TiO2 photocatalysts doped with metal ionsInd Water Treat2011319 

    49. Suzana, M, Francisco, P and Mastelaro, VR Inhibition of the Anatase Rutile Phase Transformation with Addition of CeO2 to CuO-TiO2 System: Raman Spectroscopy, X-ray Diffraction, and Textural StudiesChem Mater2002142514https://doi.org/10.1021/cm011520b 

    50. Vijay, R, Khobragade, P and Mohapatra, PK Assessment of groundwater quality in Puri City, India: an impact of anthropogenic activitiesEnviron Monit Assess20111771–4409418https://doi.org/10.1007/s10661-010-1643-9 

    51. Wang, SJ, Ma, J, Yang, YX, Zhang, J, Qin, QD and Liang, T Influence of nanosized TiO2 catalyzed ozonation on the ammonia concentration in Songhua River waterHuan Jing Ke Xue2007281125202525 

    52. Zainovia, L, Chin Hui, K and Srimala, S Effect Of Annealing Temperature on The Anatase and rutile Tio2 nanotubes formationJ Nucl Related Technol200961 

    53. Zhang, J and Nosaka, Y Mechanism of the OH radical generation in photocatalysis with TiO2 of different crystalline typesJ Phys Chem C20141181082410832https://doi.org/10.1021/jp501214m 

    54. Zhu, B and Zou, L Trapping and decomposing of color compounds from recycled water by TiO2 coated activated carbonJ Environ Manag20099032173225https://doi.org/10.1016/j.jenvman.2009.04.008 

    55. Zhu, J Yang, J Zhen-Fen, B Ren, J Yong-Mei, L Cao, Y He-Xing et al. Nanocrystalline anatase TiO2 photocatalysts prepared via a facile low temperature nonhydrolytic sol-gel reaction of TiCl4 and benzyl alcoholAppl Catal B-Environ2007761–28291https://doi.org/10.1016/j.apcatb.2007.05.017