# Effects of Initial Nitrate Concentrations and Photocatalyst Dosages on Ammonium Ion in Synthetic Wastewater Treated by Photocatalytic Reduction

Authors: Orawan Rojviroon; Sanya Sirivithayapakorn; Thammasak Rojviroon; Chalermraj Wantawin
Journal: International Journal of Photoenergy
 (2020)

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## Abstract

Ammonium (NH4+) is an undesirable by-product of photocatalytic nitrate (NO3−) reduction since it is harmful to aquatic life once it converts into ammonia (NH3). This research investigated the removal efficiency of NO3− and for the first time quantified the relationships of initial nitrate concentrations ([NO3−]0) and photocatalyst dosages on the remaining ammonium (NH4+) in synthetic wastewater using photocatalytic reduction process with either nanoparticle titanium dioxide (TiO2) or 1.0%Ag-TiO2 under Ultraviolet A (UVA). The experiments were systematically carried out under various combinations of [NO3−]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g). The NO3− removal efficiency of both photocatalysts was 98.96-99.98%, and the catalytic selectivity products were nitrogen gas (N2), nitrite (NO2−), and NH4+. Of the two photocatalysts under comparable experimental conditions, 1.0%Ag-TiO2 provided better NO3− removal efficiency. For both photocatalysts, the remaining NH4+ was predominantly determined by [NO3−]0; higher [NO3−]0 led to higher NH4+. Multiple linear regression analysis confirmed the dominant role of [NO3−]0 in the remaining NH4+. The photocatalyst dosage could play an essential role in limiting NH4+ in the treated wastewater, with large variation in [NO3−]0 from different sources.


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## Body

## 1. Introduction

Photocatalytic reduction is an effective technology for removal of nitrate (NO3−) in wastewater. The major disadvantage of this process is the ammonium (NH4+), an undesirable by-product, remaining at the end. Many researchers studied NO3− removal efficiency and the NO3− conversion selectivity [1–3]. There are also studies on the influencing factors on the remaining NH4+, which investigated the relationships between remaining NH4+ and those influencing factors in the process.Previous studies demonstrated that high efficiency nitrate removal by photocatalytic reduction with low remaining [NH4+] could be achieved by silver- (Ag-) doped TiO2 nanoparticles under high-performance light sources (i.e., high-pressure Hg lamps and xenon lamps) [4–6]. However, those light sources have disadvantages that include high energy consumption, potential human health hazard, and generating high heat [7, 8]. For those reasons, the UVA light bulb is chosen for this process because it overcomes those disadvantages and is powerful enough for this process [9–11].The influencing factors of the photocatalytic selectivity ofNO3− conversion include initial nitrate concentration ([NO3−]0), light source intensity, type of photocatalyst, type of dopant, and quantity of photocatalyst dosage [4, 12, 13]. Evidence shows that manipulating the photocatalytic selectivity of photocatalytic reduction helps limit environmentally harmful compounds, particularly NH4+ [14, 15]. NH4+ is harmful to aquatic life once in the natural waterways where it converts into ammonia (NH3). According to the United States Environmental Protection Agency (US EPA), the upper safety limit of total ammonia nitrogen (NH3-N) is 17 mg-N/L (1-hour average) and 1.9 mg-N/L (30-day rolling average) at pH 7.0 and 20°C for acute and chronic criteria, respectively [16]. The reported total ammonia concentrations in treated wastewater from photocatalytic reduction vary between 0.07 and 57.8 mg-N/L [4, 17–22].Doping of silver (Ag+) on photocatalysts, especially TiO2, to improve the photocatalytic performance was a common practice. Previous studies applied 0.1%-7.0% Ag+ loading on TiO2 photocatalysts and found that 1% Ag+ was the most optimum dose to enhance the photocatalytic NO3− reduction activity [23–25].This research investigated the effects of initial nitrate concentrations ([NO3−]0) and photocatalyst dosages on NO3− removal efficiency in synthetic wastewater using photocatalytic reduction. The experiments were carried out under various [NO3−]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g) under UVA irradiation as light source for photocatalytic nitrate reduction. The experimental photocatalysts were TiO2 and 1.0%Ag-TiO2 nanoparticle as photocatalysts [21]. The catalytic selectivity of NO3− conversion was also determined, and the actual concentrations of NH4+ under different experimental NO3− removal conditions were compared. Multiple linear regression was performed to characterize the relationship between the remaining NH4+ and [NO3−]0 and photocatalyst dosages. Essentially, the novelty of this research lies in the use of TiO2 nanopowder as photocatalyst, as opposed to commercial-grade TiO2. By comparison, TiO2 nanopowder possesses larger surface area for adsorption and reaction. Another research novelty is the systematical use of various [NO3−]0 and photocatalyst dosages, unlike previous researches which experimented with specific [NO3−]0 and photocatalyst dosages.


## 2. Materials and Methodology

### 2.1. Ag-TiO2 Photocatalyst Preparation and Characteristics

In this research, 1.0%Ag-TiO2 photocatalyst was prepared by composite colloid deposition under alkaline condition, following Doudrick et al. [4, 9] with minor modifications. In the experiment, 12 g of TiO2 nanopowder was dispersed in 500 mL deionized water and purged with nitrogen gas (N2) for 30 min to remove O2. After degassing, 8 mL of methanol was added and stirred prior to adding NaOH to adjust pH of the mixture to 12-13. Afterward, 1.0%AgNO3 (w/v; Fluka) was added and stirred in the dark for 30 min before irradiation with UVA (800 μW/cm2) for 1 h at room temperature.The mixture was then centrifuged at 200 rpm for 2 min to settle the powder, and the supernatant was discarded. Deionized water was added to wash the powder. It was then stirred and centrifuged. The process was repeated until the mixture pH was 7. The washed powder was oven dried at 103°C for 24 h and calcined at 450°C for 1 h for Ag-TiO2 photocatalyst in the form of dried light purple powder.The experimental TiO2 nanopowder was of 15 nm in particle size and 99.5% anatase crystalline phase (US Research Nanomaterials, Inc., USA). The composition and specific surface area of dose photocatalysts were characterized by transmission electron microscopy (TEM, JEM-1400 TEM instrument) and X-ray fluorescence spectrometer (XRF; Bruker model S8 Tiger), Brunauer, Emmett and Teller (BET) analyzer (BELSORP-max Bel Japan Inc.).


### 2.2. Photocatalytic Reduction forNO3− Removal

The photocatalytic reduction to remove nitrate (NO3−) was carried out using TiO2 and Ag-TiO2 photocatalysts in 125 mL cylindrical borosilicate glass photoreactor. Figure 1 illustrates the schematic of experimental photocatalytic reduction for NO3− removal. Synthetic NO3− wastewater of variable initial nitrate concentrations was prepared by dissolving potassium nitrate (KNO3>99%, Fluka) in deionized water. The initial nitrate concentration ([NO3−]0) varied between 10, 25, 50, 80, and 100 mg-N/L.Figure 1
Schematic of photoreactor for nitrate removal.The photoreactor was filled with 100 mL synthetic wastewater of varying [NO3−]0 and 58 mM of formic acid (FA) as hole scavenger, with the mixture pH ranging from 2.28 to 2.42. TiO2 and Ag-TiO2 photocatalysts of varying dosages were independently added into the photoreactor. The photocatalyst dosage was varied from 0.1, 0.5, 1.0, and 2.0 g. The photoreactor condition was anaerobic by purging with N2 for 30 min to remove dissolved oxygen (DO) in the synthetic wastewater.Prior to UVA irradiation, samples were drawn for initial measurement of pH, DO, [NH4+], [NO2−], and [NO3−]. The samples were subsequently irradiated with two UVA light bulbs (300 W, Osram) for 6 h. The UVA light bulbs were located on either side of the photoreactor vessel at a distance of 20 cm. The UVA intensity in the photoreactor vessel was 800 μW/cm2 on average. The concentrations of NO3− and by-products ([NO2−] and [NH4+]) relative to reaction time were periodically measured throughout the experiment, while pH and DO were measured at the beginning and at termination (at 360 min). The concentrations of NO3−, NO2−, and NH4+ in the synthesis wastewater were determined using ion chromatography instrument with chemical suppression (Metrohm 882 Compact IC Plus). Nitrogen gas (N2) was calculated by the mass balance of nitrogen of the photocatalytic reduction process.To verify the experiment, photocatalytic reduction was also carried out under three control conditions: (1) in the absence of photocatalyst but with UVA irradiation; (2) with Ag-TiO2 photocatalyst of varying dosages but without UVA irradiation; and (3) with TiO2 photocatalysts of varying dosages but without UVA irradiation. The measured [NO3−], [NO2−], and [NH4+] of photocatalytic reduction using Ag-TiO2 and TiO2 photocatalysts irradiated with UVA were subsequently compared against the controls.


### 2.3. Selectivity of Photocatalytic Reduction forNO3− Removal

TheNO3− removal efficiency of photocatalytic reduction (η) and the catalytic selectivity (S) of NO3− into NH4+, NO2−, and N2 (denoted by SNH4+, SNO2−, and SN2) [21, 26] are mathematically expressed in the following equations.
(1)η=NO3−0−NO3−tNO3−0×100,(2)%SNO2−=NO2−tNO3−0−NO3−t×100,(3)%SNH4+=NH4+tNO3−0−NO3−t×100,(4)%SN2=NO3−0−NO3−t−NO2−t−NH4+tNO3−0−NO3−t×100,whereX0 is the initial concentration of X and Xt is the concentration of X at time t.


## 2.1. Ag-TiO2 Photocatalyst Preparation and Characteristics

In this research, 1.0%Ag-TiO2 photocatalyst was prepared by composite colloid deposition under alkaline condition, following Doudrick et al. [4, 9] with minor modifications. In the experiment, 12 g of TiO2 nanopowder was dispersed in 500 mL deionized water and purged with nitrogen gas (N2) for 30 min to remove O2. After degassing, 8 mL of methanol was added and stirred prior to adding NaOH to adjust pH of the mixture to 12-13. Afterward, 1.0%AgNO3 (w/v; Fluka) was added and stirred in the dark for 30 min before irradiation with UVA (800 μW/cm2) for 1 h at room temperature.The mixture was then centrifuged at 200 rpm for 2 min to settle the powder, and the supernatant was discarded. Deionized water was added to wash the powder. It was then stirred and centrifuged. The process was repeated until the mixture pH was 7. The washed powder was oven dried at 103°C for 24 h and calcined at 450°C for 1 h for Ag-TiO2 photocatalyst in the form of dried light purple powder.The experimental TiO2 nanopowder was of 15 nm in particle size and 99.5% anatase crystalline phase (US Research Nanomaterials, Inc., USA). The composition and specific surface area of dose photocatalysts were characterized by transmission electron microscopy (TEM, JEM-1400 TEM instrument) and X-ray fluorescence spectrometer (XRF; Bruker model S8 Tiger), Brunauer, Emmett and Teller (BET) analyzer (BELSORP-max Bel Japan Inc.).


## 2.2. Photocatalytic Reduction forNO3− Removal

The photocatalytic reduction to remove nitrate (NO3−) was carried out using TiO2 and Ag-TiO2 photocatalysts in 125 mL cylindrical borosilicate glass photoreactor. Figure 1 illustrates the schematic of experimental photocatalytic reduction for NO3− removal. Synthetic NO3− wastewater of variable initial nitrate concentrations was prepared by dissolving potassium nitrate (KNO3>99%, Fluka) in deionized water. The initial nitrate concentration ([NO3−]0) varied between 10, 25, 50, 80, and 100 mg-N/L.Figure 1
Schematic of photoreactor for nitrate removal.The photoreactor was filled with 100 mL synthetic wastewater of varying [NO3−]0 and 58 mM of formic acid (FA) as hole scavenger, with the mixture pH ranging from 2.28 to 2.42. TiO2 and Ag-TiO2 photocatalysts of varying dosages were independently added into the photoreactor. The photocatalyst dosage was varied from 0.1, 0.5, 1.0, and 2.0 g. The photoreactor condition was anaerobic by purging with N2 for 30 min to remove dissolved oxygen (DO) in the synthetic wastewater.Prior to UVA irradiation, samples were drawn for initial measurement of pH, DO, [NH4+], [NO2−], and [NO3−]. The samples were subsequently irradiated with two UVA light bulbs (300 W, Osram) for 6 h. The UVA light bulbs were located on either side of the photoreactor vessel at a distance of 20 cm. The UVA intensity in the photoreactor vessel was 800 μW/cm2 on average. The concentrations of NO3− and by-products ([NO2−] and [NH4+]) relative to reaction time were periodically measured throughout the experiment, while pH and DO were measured at the beginning and at termination (at 360 min). The concentrations of NO3−, NO2−, and NH4+ in the synthesis wastewater were determined using ion chromatography instrument with chemical suppression (Metrohm 882 Compact IC Plus). Nitrogen gas (N2) was calculated by the mass balance of nitrogen of the photocatalytic reduction process.To verify the experiment, photocatalytic reduction was also carried out under three control conditions: (1) in the absence of photocatalyst but with UVA irradiation; (2) with Ag-TiO2 photocatalyst of varying dosages but without UVA irradiation; and (3) with TiO2 photocatalysts of varying dosages but without UVA irradiation. The measured [NO3−], [NO2−], and [NH4+] of photocatalytic reduction using Ag-TiO2 and TiO2 photocatalysts irradiated with UVA were subsequently compared against the controls.


## 2.3. Selectivity of Photocatalytic Reduction forNO3− Removal

TheNO3− removal efficiency of photocatalytic reduction (η) and the catalytic selectivity (S) of NO3− into NH4+, NO2−, and N2 (denoted by SNH4+, SNO2−, and SN2) [21, 26] are mathematically expressed in the following equations.
(1)η=NO3−0−NO3−tNO3−0×100,(2)%SNO2−=NO2−tNO3−0−NO3−t×100,(3)%SNH4+=NH4+tNO3−0−NO3−t×100,(4)%SN2=NO3−0−NO3−t−NO2−t−NH4+tNO3−0−NO3−t×100,whereX0 is the initial concentration of X and Xt is the concentration of X at time t.


## 3. Results and Discussion

### 3.1. Ag-TiO2 Nanopowder Photocatalyst Characteristics

The results of transmission electron microscopy were used to examine the particle size and morphology of Ag nanoparticles on TiO2 as well as the lattice information of both photocatalysts investigated by XRF. In comparison, the TEM image of nano-TiO2 and nano-Ag-TiO2 photocatalysts (Figure 2) showed a similar particle size and morphology with average particle size (in diameter) of approximately 15 nm (as the result of the average TiO2 particle size before Ag doping). The results of the TEM analysis were not clearly distinguishable in terms of particle size and morphology although it was reported that Ag doping would slightly decrease the particle size of the larger TiO2 powders [27].Figure 2
TEM image of (a) TiO2 photocatalyst and (b) 1.0%Ag-TiO2 photocatalyst.
(a)
TiO2(b)
Ag-TiO2The XRF patterns of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts showed strong peaks of Ti, as shown in Figure 3. The XRF pattern of 1.0%Ag-TiO2 photocatalyst indicated that Ag+ dopant was effectively doped onto TiO2 which is the same with the theoretical adding. The Ag dopant in Ag-TiO2 phase was 0.99% for theoretical doping of 1.0% (Table 1). It was confirmed that Ag was effectively deposited on the surface of TiO2 nanopowder. Specifically, Ag+ ions were adsorbed onto the crystal structure of TiO2 and subsequently formed Ag-TiO2 [28, 29].Figure 3
The XRF patterns of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts.Table 1
Composition of TiO2 and 1.0%Ag-TiO2 photocatalysts.
PhotocatalystComposition (%)BET surface area (m2/g)Pore volume (m3/g)TiO2AgSiO2P2O5Nb2O5OtherTiO296.8—0.440.150.112.501.164×1024.727×10−71.0%Ag-TiO296.10.990.400.170.112.231.124×1023.951×10−7The BET specific surface areas of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts were 1.164×102 and 1.124×102m2/g, and the corresponding pore volumes were 4.727×10−7 and 3.951×10−7m3/g. It was found that the specific surface area and pore volume of Ag-TiO2 were decreased after doping. The findings were consistent with previous studies which doped TiO2 nanopowder photocatalysts with varying dopants [4, 30, 31].The slightly decreased BET surface areas and pore volume were due to the interference of Ag dopant on the formation of anatase crystallization [32, 33], and a marked influence on the microstructures was exhibited by calcination temperature [34]. However, the advantage of metal doping on semiconductor particles, Ag-TiO2, was the prevention of recombination between electron and hole by trapping the electron on the metal surface resulting in increasing the lifetime of electron in conduction band, thus enhancing the efficiency of photocatalytic nitrate reduction [34].


### 3.2.NO3− Removal Using Photocatalytic Reduction

TheNO3− removal efficiency (η) of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts under variable initial NO3− concentrations ([NO3−]0; 10, 25, 50, 80, and 100 mg-N/L) and TiO2 and 1.0%Ag-TiO2 dosages (0.1, 0.5, 1.0, and 2.0 g) was determined by equation (1).Table2 tabulates the NO3− removal efficiency, nitrate concentration at termination ([NO3−]t), ammonium ion selectivity (%SNH4+), and actual ammonium ion concentration ([NH4+]a) of photocatalytic reduction under various [NO3−]0 using TiO2 and 1.0%Ag-TiO2 photocatalysts. The NO3− removal efficiency of TiO2 and 1.0%Ag-TiO2 photocatalysts was 98.96-99.98%. The NO3− removal efficiency increased with the increase in photocatalyst dosage as the surface area for adsorption and reaction increased.Table 2
NO3− removal efficiency, [NO3−]t, %SNH4+, and [NH4+]a of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts at termination.
Photocatalyst[NO3−]0 (mg-N/L)Dosage (g)NO3− removal (%)%SNH4+[NH4+]a (mg-N/L)pHtTiO2100.1-2.098.96-99.4825.81-34.002.55-3.382.41-2.84250.1-2.099.35-99.9220.53-36.55.10-9.122.54-3.11500.1-2.099.84-99.929.53-26.404.76-13.182.80-3.61800.1-2.099.85-99.9816.35-25.5513.06-20.432.9-3.281000.1-2.099.90-99.9517.13-22.5617.12-22.552.93-3.31.0%Ag-TiO2100.1-2.099.60-99.6219.71-64.091.96-6.382.50-2.51250.1-2.099.30-99.8417.59-44.814.39-11.192.52-2.64500.1-2.099.78-99.9014.97-45.987.47-22.972.72-3.03800.1-2.099.85-99.9419.20-38.1515.33-30.503.00-4.021000.1-2.099.82-99.9616.12-38.1016.09-38.103.06-5.50Note:%SNH4+ is NH4+ selectivity (%), [NH4+]a is actual [NH4+], and pHt is pH at the end.The ammonium ion selectivity (SNH4+) and actual ammonium ion concentration ([NH4+]a) increased with the increase in photocatalyst dosage for both TiO2 and 1.0% Ag-TiO2 photocatalysts. In addition, the initial nitrate concentration and [NH4+]a were positively correlated. In other words, low [NO3−]0 resulted in low [NH4+]a and vice versa. Moreover, [NH4+]a of 1.0% Ag-TiO2 photocatalyst was higher than that of TiO2 photocatalyst under all experimental conditions.TheNO3− reduction in photocatalytic process was a stepwise mechanism. When the photoinduced electrons (e-) in valence band were excited onto conduction band, holes (h+) appeared at valence band. This process was called electron-hole pairs photogeneration [32] (equation (5)). The photogenerated holes consumed HCOO-, and CO2·− was generated [35] (equation (6)). The CO2·− is a strong reducing agent to reduce NO3− to either NH4+ or N2 (equations (7)–(12)), in which nitrite (NO2−) was an intermediate product. The results of NO2−, NH4+, and N2 in percent named the selectivity of NO3− reduction.
(5)TiO2+hv→e−+h+(6)HCOO−+h+→H++CO2·−(7)NO3−+2H++2e−→NO2−+H2O(8)2NO3−+12H++10CO2·−→N2+6H2O+10CO2(9)NO3−+10H++8CO2·−→NH4++3H2O+8CO2(10)NO2−+8H++6CO2·−→NH4++2H2O+6CO2(11)2NO3−+12H++10e−→N2+6CO2(12)NO3−+10H2O+8e−→NH4++3H2OIn the nitrate reduction experiments, 58 mmol of formic acid (FA) was used as a hole scavenger to improve the photocatalytic reduction reaction, while the pH of the solution increased from 2.28-2.42 to 2.41-5.5 due to the consumption of formic acid in the reaction and the generation of [NH4+]. In addition, the highly efficient conversion of NO2− to N2 was also related to the initial acidity of the solution [36]. This was probably due to the specific absorption properties of NO3− and NO2− in varying pH solutions. Considering that the point of zero charge of the TiO2 was 6.25 [37], in acidic solution, TiO2 surface accumulated a net positive charge due to the increasing fraction of TiOH2+ sites on the surface and NO2− could be quickly adsorbed.Figure4 compares the nitrate concentrations of photocatalytic reduction under various [NO3−]0 and photocatalytic dosages (TiO2 and 1.0%Ag-TiO2) from the start to end. In Figure 4, the NO3− removal was positively correlated with photocatalyst dosage due to the surface area effect, independent of photocatalyst type. Although at termination there were not much differences in final NO3− removal efficiencies, the NO3− removal rates of 1.0%Ag-TiO2 photocatalyst were faster than those of TiO2 photocatalyst for the comparable conditions. The faster removal rate of photocatalytic reduction activity was attributable to Ag+ doping [33, 38]. The loading of TiO2 with Ag+ reduced the difference between energy levels of the valence and conduction bands, resulting in the extension of light absorption wavelength into the visible light region. Ag+ also acted as a trap site for excited electrons, giving rise to electron-hole separation. In addition, Ag+ doping enhanced charge transport, prolonged the lifetime of electron-hole pairs, and reduced the charge recombination [39–42]. As a result, Ag+ could be adopted for photocatalytic reduction process to improve NO3− removal.Figure 4
The concentrations ofNO3− of the photocatalytic reduction scheme under various [NO3−]0 and TiO2 and 1.0%Ag-TiO2 dosages.Figure5 illustrates the catalytic selectivity (%) of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts in which NO3− was transformed into NO2−, NH4+, and N2. The results showed that overall N2 accounted for the largest proportions of NO3− by-products, followed by NH4+ and NO2−.Figure 5
The catalytic selectivity of the photocatalytic reduction scheme under various [NO3−]0 and TiO2 and 1.0%Ag-TiO2 dosages.In Figure5, the photocatalyst types (TiO2 and 1.0%Ag-TiO2) and dosage played a role in the selectivity of the photocatalytic reduction scheme. This showed that the Ag dopant enhanced the photocatalytic reduction activity, and both the activity and [NH4+] increased with 1.0%Ag-TiO2 dosage increase. However, 1.0%Ag-TiO2 photocatalyst dosage beyond 0.1 g (i.e., 0.5, 1.0, and 2.0 g) contributed to [NH4+] in the treated wastewater exceeding that of TiO2 photocatalyst (Table 2). NH4+ is harmful to aquatic life once in the natural waterways where it converts into NH3.The initial nitrate concentrations also played a role in [NH4+] in the treated wastewater, independent of photocatalyst type (TiO2 and 1.0%Ag-TiO2). Specifically, higher [NO3−]0 resulted in higher [NH4+]. Given 0.1 g of either TiO2 or 1.0%Ag-TiO2 photocatalyst, [NH4+] was 1.96-16.09 mg-N/L, independent of [NO3−]0. Assuming complete NH4+-to-NH3 conversion, these were equivalent to 1.96-16.09 mg-N/L total ammonia nitrogen (NH3-N), which is below 17 mg-N/L NH3-N of the US EPA [16]. Meanwhile, the nitrate concentrations of the three control conditions (i.e., the controls) remained unchanged at the end of the experiment.In Figure5, the catalytic selectivity of NO3− into NO2− could also be observed. The remaining nitrite concentrations ([NO2−]) were negligible as NO2− was converted into NH4+ and N2 during the photocatalytic reduction process [43].To comparatively investigate the effect of initial nitrate concentration and photocatalyst dosage on the concentration of ammonium ion, the relationships between [NH4+] and [NO3−]0 and photocatalyst dosage (TiO2 and 1.0%Ag-TiO2) were established by using statistical multiple linear regression. [NH4+], [NO3−]0, and photocatalyst dosage are denoted by Y, X1, and X2, respectively. The multiple linear regression was expressed in equation (13), and Table 3 tabulates the regression results.
(13)Y=b+β1X1+β2X2+⋯+ε,where b is the linear regression constant, β is the linear regression coefficient, and ε is the error constant.Table 3
Results of multiple linear regression analysis.
PhotocatalystIndependent variablesbβt valuep valueTiO2[NO3−]06.8790.92515.3950.000Dosage0.1840.2884.7870.000Constant-1.489-1.6430.119R=0.969, R2=0.939, F=129.955, and p value < 0.001.1.0%Ag-TiO2[NO3−]00.2740.83811.960.000Dosage6.0640.4075.630.000Constant-3.744-2.160.045R=0.955, R2=0.911, F=87.30, and p value < 0.001.In Table3, β of the initial nitrate concentration ([NO3−]0) was, respectively, 0.925 and 0.838 for TiO2 and 1.0%Ag-TiO2 photocatalysts (p<0.001), indicating that [NO3−]0 played the dominant role in NO3− removal efficiency and the remaining NH4+. Meanwhile, β of photocatalyst dosage was 0.407 and 0.288 for 1.0%Ag-TiO2 and TiO2 photocatalysts, suggesting that the photocatalyst dosage had considerably less effect on the remaining NH4+.In reality, [NO3−]0 varies from area to area. Given diverse [NO3−]0, it is operationally practical to vary the photocatalyst dosage in the photocatalytic reduction scheme. Based on the experimental results, 0.1 g of Ag-TiO2 photocatalyst is advisable due to efficient removal of NO3− (Figure 3).To facilitate estimation of the remaining [NH4+] in treated wastewater using photocatalytic reduction, the multiregression prediction equations of the theoretical remaining ammonium ion ([NH4+]T), as a function of [NO3−]0 and photocatalyst dosage, are expressed in equations (14) and (15), respectively.The prediction equation for TiO2 photocatalyst is
(14)NH4+T=−1.489+0.185NO3−0+2.698dosage.The prediction equation for 1.0%Ag-TiO2 photocatalyst is
(15)NH4+T=−3.744+0.274NO3−0+6.064dosage.The relationships between [NH4+]T, [NO3−], and photocatalyst dosage, as shown in equations (7) and (8), could be further applied to estimate the remaining [NH4+] under various [NO3−]0 and photocatalyst dosages.


## 3.1. Ag-TiO2 Nanopowder Photocatalyst Characteristics

The results of transmission electron microscopy were used to examine the particle size and morphology of Ag nanoparticles on TiO2 as well as the lattice information of both photocatalysts investigated by XRF. In comparison, the TEM image of nano-TiO2 and nano-Ag-TiO2 photocatalysts (Figure 2) showed a similar particle size and morphology with average particle size (in diameter) of approximately 15 nm (as the result of the average TiO2 particle size before Ag doping). The results of the TEM analysis were not clearly distinguishable in terms of particle size and morphology although it was reported that Ag doping would slightly decrease the particle size of the larger TiO2 powders [27].Figure 2
TEM image of (a) TiO2 photocatalyst and (b) 1.0%Ag-TiO2 photocatalyst.
(a)
TiO2(b)
Ag-TiO2The XRF patterns of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts showed strong peaks of Ti, as shown in Figure 3. The XRF pattern of 1.0%Ag-TiO2 photocatalyst indicated that Ag+ dopant was effectively doped onto TiO2 which is the same with the theoretical adding. The Ag dopant in Ag-TiO2 phase was 0.99% for theoretical doping of 1.0% (Table 1). It was confirmed that Ag was effectively deposited on the surface of TiO2 nanopowder. Specifically, Ag+ ions were adsorbed onto the crystal structure of TiO2 and subsequently formed Ag-TiO2 [28, 29].Figure 3
The XRF patterns of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts.Table 1
Composition of TiO2 and 1.0%Ag-TiO2 photocatalysts.
PhotocatalystComposition (%)BET surface area (m2/g)Pore volume (m3/g)TiO2AgSiO2P2O5Nb2O5OtherTiO296.8—0.440.150.112.501.164×1024.727×10−71.0%Ag-TiO296.10.990.400.170.112.231.124×1023.951×10−7The BET specific surface areas of TiO2 and 1.0%Ag-TiO2 nanopowder photocatalysts were 1.164×102 and 1.124×102m2/g, and the corresponding pore volumes were 4.727×10−7 and 3.951×10−7m3/g. It was found that the specific surface area and pore volume of Ag-TiO2 were decreased after doping. The findings were consistent with previous studies which doped TiO2 nanopowder photocatalysts with varying dopants [4, 30, 31].The slightly decreased BET surface areas and pore volume were due to the interference of Ag dopant on the formation of anatase crystallization [32, 33], and a marked influence on the microstructures was exhibited by calcination temperature [34]. However, the advantage of metal doping on semiconductor particles, Ag-TiO2, was the prevention of recombination between electron and hole by trapping the electron on the metal surface resulting in increasing the lifetime of electron in conduction band, thus enhancing the efficiency of photocatalytic nitrate reduction [34].


## 3.2.NO3− Removal Using Photocatalytic Reduction

TheNO3− removal efficiency (η) of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts under variable initial NO3− concentrations ([NO3−]0; 10, 25, 50, 80, and 100 mg-N/L) and TiO2 and 1.0%Ag-TiO2 dosages (0.1, 0.5, 1.0, and 2.0 g) was determined by equation (1).Table2 tabulates the NO3− removal efficiency, nitrate concentration at termination ([NO3−]t), ammonium ion selectivity (%SNH4+), and actual ammonium ion concentration ([NH4+]a) of photocatalytic reduction under various [NO3−]0 using TiO2 and 1.0%Ag-TiO2 photocatalysts. The NO3− removal efficiency of TiO2 and 1.0%Ag-TiO2 photocatalysts was 98.96-99.98%. The NO3− removal efficiency increased with the increase in photocatalyst dosage as the surface area for adsorption and reaction increased.Table 2
NO3− removal efficiency, [NO3−]t, %SNH4+, and [NH4+]a of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts at termination.
Photocatalyst[NO3−]0 (mg-N/L)Dosage (g)NO3− removal (%)%SNH4+[NH4+]a (mg-N/L)pHtTiO2100.1-2.098.96-99.4825.81-34.002.55-3.382.41-2.84250.1-2.099.35-99.9220.53-36.55.10-9.122.54-3.11500.1-2.099.84-99.929.53-26.404.76-13.182.80-3.61800.1-2.099.85-99.9816.35-25.5513.06-20.432.9-3.281000.1-2.099.90-99.9517.13-22.5617.12-22.552.93-3.31.0%Ag-TiO2100.1-2.099.60-99.6219.71-64.091.96-6.382.50-2.51250.1-2.099.30-99.8417.59-44.814.39-11.192.52-2.64500.1-2.099.78-99.9014.97-45.987.47-22.972.72-3.03800.1-2.099.85-99.9419.20-38.1515.33-30.503.00-4.021000.1-2.099.82-99.9616.12-38.1016.09-38.103.06-5.50Note:%SNH4+ is NH4+ selectivity (%), [NH4+]a is actual [NH4+], and pHt is pH at the end.The ammonium ion selectivity (SNH4+) and actual ammonium ion concentration ([NH4+]a) increased with the increase in photocatalyst dosage for both TiO2 and 1.0% Ag-TiO2 photocatalysts. In addition, the initial nitrate concentration and [NH4+]a were positively correlated. In other words, low [NO3−]0 resulted in low [NH4+]a and vice versa. Moreover, [NH4+]a of 1.0% Ag-TiO2 photocatalyst was higher than that of TiO2 photocatalyst under all experimental conditions.TheNO3− reduction in photocatalytic process was a stepwise mechanism. When the photoinduced electrons (e-) in valence band were excited onto conduction band, holes (h+) appeared at valence band. This process was called electron-hole pairs photogeneration [32] (equation (5)). The photogenerated holes consumed HCOO-, and CO2·− was generated [35] (equation (6)). The CO2·− is a strong reducing agent to reduce NO3− to either NH4+ or N2 (equations (7)–(12)), in which nitrite (NO2−) was an intermediate product. The results of NO2−, NH4+, and N2 in percent named the selectivity of NO3− reduction.
(5)TiO2+hv→e−+h+(6)HCOO−+h+→H++CO2·−(7)NO3−+2H++2e−→NO2−+H2O(8)2NO3−+12H++10CO2·−→N2+6H2O+10CO2(9)NO3−+10H++8CO2·−→NH4++3H2O+8CO2(10)NO2−+8H++6CO2·−→NH4++2H2O+6CO2(11)2NO3−+12H++10e−→N2+6CO2(12)NO3−+10H2O+8e−→NH4++3H2OIn the nitrate reduction experiments, 58 mmol of formic acid (FA) was used as a hole scavenger to improve the photocatalytic reduction reaction, while the pH of the solution increased from 2.28-2.42 to 2.41-5.5 due to the consumption of formic acid in the reaction and the generation of [NH4+]. In addition, the highly efficient conversion of NO2− to N2 was also related to the initial acidity of the solution [36]. This was probably due to the specific absorption properties of NO3− and NO2− in varying pH solutions. Considering that the point of zero charge of the TiO2 was 6.25 [37], in acidic solution, TiO2 surface accumulated a net positive charge due to the increasing fraction of TiOH2+ sites on the surface and NO2− could be quickly adsorbed.Figure4 compares the nitrate concentrations of photocatalytic reduction under various [NO3−]0 and photocatalytic dosages (TiO2 and 1.0%Ag-TiO2) from the start to end. In Figure 4, the NO3− removal was positively correlated with photocatalyst dosage due to the surface area effect, independent of photocatalyst type. Although at termination there were not much differences in final NO3− removal efficiencies, the NO3− removal rates of 1.0%Ag-TiO2 photocatalyst were faster than those of TiO2 photocatalyst for the comparable conditions. The faster removal rate of photocatalytic reduction activity was attributable to Ag+ doping [33, 38]. The loading of TiO2 with Ag+ reduced the difference between energy levels of the valence and conduction bands, resulting in the extension of light absorption wavelength into the visible light region. Ag+ also acted as a trap site for excited electrons, giving rise to electron-hole separation. In addition, Ag+ doping enhanced charge transport, prolonged the lifetime of electron-hole pairs, and reduced the charge recombination [39–42]. As a result, Ag+ could be adopted for photocatalytic reduction process to improve NO3− removal.Figure 4
The concentrations ofNO3− of the photocatalytic reduction scheme under various [NO3−]0 and TiO2 and 1.0%Ag-TiO2 dosages.Figure5 illustrates the catalytic selectivity (%) of photocatalytic reduction using TiO2 and 1.0%Ag-TiO2 photocatalysts in which NO3− was transformed into NO2−, NH4+, and N2. The results showed that overall N2 accounted for the largest proportions of NO3− by-products, followed by NH4+ and NO2−.Figure 5
The catalytic selectivity of the photocatalytic reduction scheme under various [NO3−]0 and TiO2 and 1.0%Ag-TiO2 dosages.In Figure5, the photocatalyst types (TiO2 and 1.0%Ag-TiO2) and dosage played a role in the selectivity of the photocatalytic reduction scheme. This showed that the Ag dopant enhanced the photocatalytic reduction activity, and both the activity and [NH4+] increased with 1.0%Ag-TiO2 dosage increase. However, 1.0%Ag-TiO2 photocatalyst dosage beyond 0.1 g (i.e., 0.5, 1.0, and 2.0 g) contributed to [NH4+] in the treated wastewater exceeding that of TiO2 photocatalyst (Table 2). NH4+ is harmful to aquatic life once in the natural waterways where it converts into NH3.The initial nitrate concentrations also played a role in [NH4+] in the treated wastewater, independent of photocatalyst type (TiO2 and 1.0%Ag-TiO2). Specifically, higher [NO3−]0 resulted in higher [NH4+]. Given 0.1 g of either TiO2 or 1.0%Ag-TiO2 photocatalyst, [NH4+] was 1.96-16.09 mg-N/L, independent of [NO3−]0. Assuming complete NH4+-to-NH3 conversion, these were equivalent to 1.96-16.09 mg-N/L total ammonia nitrogen (NH3-N), which is below 17 mg-N/L NH3-N of the US EPA [16]. Meanwhile, the nitrate concentrations of the three control conditions (i.e., the controls) remained unchanged at the end of the experiment.In Figure5, the catalytic selectivity of NO3− into NO2− could also be observed. The remaining nitrite concentrations ([NO2−]) were negligible as NO2− was converted into NH4+ and N2 during the photocatalytic reduction process [43].To comparatively investigate the effect of initial nitrate concentration and photocatalyst dosage on the concentration of ammonium ion, the relationships between [NH4+] and [NO3−]0 and photocatalyst dosage (TiO2 and 1.0%Ag-TiO2) were established by using statistical multiple linear regression. [NH4+], [NO3−]0, and photocatalyst dosage are denoted by Y, X1, and X2, respectively. The multiple linear regression was expressed in equation (13), and Table 3 tabulates the regression results.
(13)Y=b+β1X1+β2X2+⋯+ε,where b is the linear regression constant, β is the linear regression coefficient, and ε is the error constant.Table 3
Results of multiple linear regression analysis.
PhotocatalystIndependent variablesbβt valuep valueTiO2[NO3−]06.8790.92515.3950.000Dosage0.1840.2884.7870.000Constant-1.489-1.6430.119R=0.969, R2=0.939, F=129.955, and p value < 0.001.1.0%Ag-TiO2[NO3−]00.2740.83811.960.000Dosage6.0640.4075.630.000Constant-3.744-2.160.045R=0.955, R2=0.911, F=87.30, and p value < 0.001.In Table3, β of the initial nitrate concentration ([NO3−]0) was, respectively, 0.925 and 0.838 for TiO2 and 1.0%Ag-TiO2 photocatalysts (p<0.001), indicating that [NO3−]0 played the dominant role in NO3− removal efficiency and the remaining NH4+. Meanwhile, β of photocatalyst dosage was 0.407 and 0.288 for 1.0%Ag-TiO2 and TiO2 photocatalysts, suggesting that the photocatalyst dosage had considerably less effect on the remaining NH4+.In reality, [NO3−]0 varies from area to area. Given diverse [NO3−]0, it is operationally practical to vary the photocatalyst dosage in the photocatalytic reduction scheme. Based on the experimental results, 0.1 g of Ag-TiO2 photocatalyst is advisable due to efficient removal of NO3− (Figure 3).To facilitate estimation of the remaining [NH4+] in treated wastewater using photocatalytic reduction, the multiregression prediction equations of the theoretical remaining ammonium ion ([NH4+]T), as a function of [NO3−]0 and photocatalyst dosage, are expressed in equations (14) and (15), respectively.The prediction equation for TiO2 photocatalyst is
(14)NH4+T=−1.489+0.185NO3−0+2.698dosage.The prediction equation for 1.0%Ag-TiO2 photocatalyst is
(15)NH4+T=−3.744+0.274NO3−0+6.064dosage.The relationships between [NH4+]T, [NO3−], and photocatalyst dosage, as shown in equations (7) and (8), could be further applied to estimate the remaining [NH4+] under various [NO3−]0 and photocatalyst dosages.


## 4. Conclusion

This research investigated theNO3− removal efficiency of photocatalytic reduction process under various [NO3−]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g) using nanoparticle TiO2 and 1.0%Ag-TiO2 photocatalysts under UVA. The NO3− removal efficiency of both photocatalysts under experimental [NO3−]0 and photocatalyst dosages was between 98.96 and 99.98%. The catalytic selectivity products were NH4+, NO2−, and N2, with N2 accounting for a significant proportion of the selectivity. The doping of TiO2 with Ag+ improved the removal efficiency of NO3−. It was found that [NO3−]0 played a more important role in the remaining NH4+ than the photocatalyst dosage. Specifically, higher [NO3−]0 led to higher [NH4+] in the treated wastewater. Multiple linear regression analysis confirmed the dominant role of [NO3−]0 in the remaining NH4+.



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