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

Ammonium ( NH +4 ) is an undesirable by-product of photocatalytic nitrate ( NO − 3 ) reduction since it is harmful to aquatic life once it converts into ammonia (NH 3 ). This research investigated the removal e ﬃ ciency of NO − 3 and for the ﬁ rst time quanti ﬁ ed the relationships of initial nitrate concentrations ([ NO − 3 ] 0 ) and photocatalyst dosages on the remaining ammonium ( NH +4 ) in synthetic wastewater using photocatalytic reduction process with either nanoparticle titanium dioxide (TiO 2 ) or 1.0%Ag-TiO 2 under Ultraviolet A (UVA). The experiments were systematically carried out under various combinations of [ NO − 3 ] 0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0g). The NO − 3 removal e ﬃ ciency of both photocatalysts was 98.96-99.98%, and the catalytic selectivity products were nitrogen gas (N 2 ), nitrite ( NO − 2 ), and NH +4 . Of the two photocatalysts under comparable experimental conditions, 1.0%Ag-TiO 2 provided better NO − 3 removal e ﬃ ciency. For both photocatalysts, the remaining NH +4 was predominantly determined by [ NO − 3 ] 0 ; higher [ NO − 3 ] 0 led to higher NH +4 . Multiple linear regression analysis con ﬁ rmed the dominant role of [ NO − 3 ] 0 in the remaining NH +4 . The photocatalyst dosage could play an essential role in limiting NH + 4 in the treated wastewater, with large variation in [ NO − 3 ] 0 from di ﬀ erent sources.


Introduction
Photocatalytic reduction is an effective technology for removal of nitrate (NO − 3 ) in wastewater. The major disadvantage of this process is the ammonium (NH + 4 ), an undesirable by-product, remaining at the end. Many researchers studied NO − 3 removal efficiency and the NO − 3 conversion selectivity [1][2][3]. There are also studies on the influencing factors on the remaining NH + 4 , which investigated the relationships between remaining NH + 4 and those influencing factors in the process.
Previous studies demonstrated that high efficiency nitrate removal by photocatalytic reduction with low remaining [NH + 4 ] could be achieved by silver-(Ag-) doped TiO 2 nano-particles under high-performance light sources (i.e., highpressure Hg lamps and xenon lamps) [4][5][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][10][11].
The influencing factors of the photocatalytic selectivity of NO − 3 conversion include initial nitrate concentration ([NO − 3 ] 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 NH + 4 [14,15]. NH + 4 is harmful to aquatic life once in the natural waterways where it converts into ammonia (NH 3 ). According to the United States Environmental Protection Agency (US EPA), the upper safety limit of total ammonia nitrogen (NH 3 -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][18][19][20][21][22]. Doping of silver (Ag + ) on photocatalysts, especially TiO 2 , to improve the photocatalytic performance was a common practice. Previous studies applied 0.1%-7.0% Ag + loading on TiO 2 photocatalysts and found that 1% Ag + was the most optimum dose to enhance the photocatalytic NO − 3 reduction activity [23][24][25]. This

Materials and Methodology
2.1. Ag-TiO 2 Photocatalyst Preparation and Characteristics. In this research, 1.0%Ag-TiO 2 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 TiO 2 nanopowder was dispersed in 500 mL deionized water and purged with nitrogen gas (N 2 ) for 30 min to remove O 2 . 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%AgNO 3 (w/v; Fluka) was added and stirred in the dark for 30 min before irradiation with UVA (800 μW/cm 2 ) 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-TiO 2 photocatalyst in the form of dried light purple powder.
The experimental TiO 2 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.). − 3 Removal. The photocatalytic reduction to remove nitrate (NO − 3 ) was carried out using TiO 2 and Ag-TiO 2 photocatalysts in 125 mL cylindrical borosilicate glass photoreactor. Figure 1 illustrates the schematic of experimental photocatalytic reduction for NO − and [NH + 4 ]) 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 NO − 3 , NO − 2 , and NH + 4 in the synthesis wastewater were determined using ion chromatography instrument with chemical suppression (Metrohm 882 Compact IC Plus). Nitrogen gas (N 2 ) was calculated by the mass balance of nitrogen of the photocatalytic reduction process.

Photocatalytic Reduction for NO
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-TiO 2 photocatalyst of varying dosages but without UVA irradiation; and (3) with TiO 2 photocatalysts of varying dosages but without UVA irradiation. The measured , and [NH + 4 ] of photocatalytic reduction using Ag-TiO 2 and TiO 2 photocatalysts irradiated with UVA were subsequently compared against the controls. − 3 Removal. The NO − 3 removal efficiency of photocatalytic reduction (η) and the catalytic selectivity

Selectivity of Photocatalytic Reduction for NO
International Journal of Photoenergy mathematically expressed in the following equations.
where ½X 0 is the initial concentration of X and ½X t is the concentration of X at time t.

Results and Discussion
3.1. Ag-TiO 2 Nanopowder Photocatalyst Characteristics. The results of transmission electron microscopy were used to examine the particle size and morphology of Ag nanoparticles on TiO 2 as well as the lattice information of both photocatalysts investigated by XRF. In comparison, the TEM image of nano-TiO 2 and nano-Ag-TiO 2 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 TiO 2 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 TiO 2 powders [27].
The XRF patterns of TiO 2 and 1.0%Ag-TiO 2 nanopowder photocatalysts showed strong peaks of Ti, as shown in Figure 3. The XRF pattern of 1.0%Ag-TiO 2 photocatalyst indicated that Ag + dopant was effectively doped onto TiO 2 which is the same with the theoretical adding. The Ag dopant in Ag-TiO 2 phase was 0.99% for theoretical doping of 1.0% (Table 1). It was confirmed that Ag was effectively deposited on the surface of TiO 2 nanopowder. Specifically, Ag + ions were adsorbed onto the crystal structure of TiO 2 and subsequently formed Ag-TiO 2 [28,29].
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-TiO 2 , 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].    The NO − 3 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 CO ·− 2 was generated [35] (equation (6)). The CO ·− 2 is a strong reducing agent to reduce NO − 3 to either NH + 4 or N 2 (equations (7)-(12)), in which nitrite (NO − 2 ) was an intermediate product. The results of NO − 2 , NH + 4 , and N 2 in percent named the selectivity of NO − 3 reduction.

NO
2NO − 3 + 12H + + 10e − → N 2 + 6CO 2 ð11Þ In 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 [NH + 4 ]. In addition, the highly efficient conversion of NO − 2 to N 2 was also related to the initial acidity of the solution [36]. This was probably due to the specific absorption properties of NO − 3 and NO − 2 in varying pH solutions. Considering that the point of zero charge of the TiO 2 was 6.25 [37], in acidic solution, TiO 2 surface accumulated a net positive charge due to the increasing fraction of TiOH 2 + sites on the surface and NO − 2 could be quickly adsorbed. Figure 4 compares the nitrate concentrations of photocatalytic reduction under various [NO − 3 ] 0 and photocatalytic dosages (TiO 2 and 1.0%Ag-TiO 2 ) from the start to end. In Figure 4, the NO − 3 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 NO − 3 removal efficiencies, the NO − 3 removal rates of 1.0%Ag-TiO 2 photocatalyst were faster than those of TiO 2 photocatalyst for the comparable conditions. The faster removal rate of photocatalytic reduction activity was attributable to Ag + doping [33,38]. The loading of TiO 2 with Ag + reduced the difference between    International Journal of Photoenergy 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][40][41][42]. As a result, Ag + could be adopted for photocatalytic reduction process to improve NO − 3 removal. Figure 5 illustrates the catalytic selectivity (%) of photocatalytic reduction using TiO 2 and 1.0%Ag-TiO 2 photocatalysts in which NO − 3 was transformed into NO − 2 , NH + 4 , and N 2 . The results showed that overall N 2 accounted for the largest proportions of NO − 3 by-products, followed by NH + 4 and NO − 2 .
In Figure 5, the photocatalyst types (TiO 2 and 1.0%Ag-TiO 2 ) 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 [NH + 4 ] increased with 1.0%Ag-TiO 2 dosage increase. However, 1.0%Ag-TiO 2 photocatalyst dosage beyond 0.1 g (i.e., 0.5, 1.0, and 2.0 g) contributed to [NH + 4 ] in the treated wastewater exceeding that of TiO 2 photocatalyst (  3 conversion, these were equivalent to 1.96-16.09 mg-N/L total ammonia nitrogen (NH 3 -N), which is below 17 mg-N/L NH 3 -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 Figure 5, the catalytic selectivity of NO − 3 into NO − 2 could also be observed. The remaining nitrite concentrations ([NO − 2 ]) were negligible as NO − 2 was converted into NH + 4 and N 2 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 [NH + 4 ] and [NO − 3 ] 0 and photocatalyst dosage (TiO 2 and 1.0%Ag-TiO 2 ) were established by using statistical multiple linear regression. [NH + 4 ], [NO − 3 ] 0 , and photocatalyst dosage are denoted by Y, X 1 , and X 2 , respectively. The multiple linear regression was expressed in equation (13), and Table 3 tabulates the regression results.
where b is the linear regression constant, β is the linear regression coefficient, and ε is the error constant. In Table 3 (14) and (15)

Conclusion
This research investigated the NO − 3 removal efficiency of photocatalytic reduction process under various [NO − 3 ] 0 (10,

Data Availability
The analysis data used to support the findings of this study are included within the supplementary information file(s).

Conflicts of Interest
The author(s) declare(s) that they have no conflicts of interest.