Photocatalytic Hydrogen Production from Urine Using Sr-Doped TiO2 Photocatalyst with Subsequent Phosphorus Recovery via Struvite Crystallization

Currently, the discharge of wastewater and utilization of phosphorus (P) in human activities cause some environmental problems, such as high organic pollutants in aquatic environments which results in dirty water sources, and a shortage of phosphate rock reserves due to the high demand of P. Therefore, fuel energy and struvite crystallization from waste sources can be considered interesting alternatives. In this work, the modified catalyst for hydrogen production, along with solving environmental problems, was examined. The strontium (Sr) doped-titanium dioxide (TiO2) nanoparticles were synthesized by wetness impregnation method. The synthesized catalyst was characterized using UV-vis spectroscopy (UV-vis), photoluminescence (PL), X-ray diffraction (XRD), photoluminescence (PL), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The Sr-doped TiO2 catalysts had been utilized as the photocatalyst for the hydrogen production from synthetic human urine (a representative of waste source). The doping content of Sr in TiO2 varied from 0.5, 1, 2, and 4%, and the photocatalytic performances were compared with pristine TiO2 nanoparticles. The results showed that 1% Sr-doped TiO2 had the highest photocatalytic activity for hydrogen production and decreased the amount of chemical oxygen demand (COD) in the synthetic human urine. Subsequently, P could be recovered from the treated human urine in the form of struvite.


Introduction
Currently, a rapidly growing population in the world leads to various environmental problems, especially wastewater. A huge amount of municipal wastewater is difficult to manage and affects aquatic environments. The major sources of municipal wastewater are households, institutions, and commercial buildings which contain organic compounds, heavy metals, and nutrients, especially nitrogen (N) and phosphorus (P). The treatments of municipal wastewater or domestic sewage are necessary for public health and aquatic environments. In general, municipal wastewater has been used in conventional wastewater treatment processes to remove suspended solids and organic compounds before releasing them to rivers and land for reuse [1].
Urine is a waste from living organisms which is rich in nutrients, especially N and P. The largest constituent of urine is urea [2]. In the environment, urine has some negative phosphate concentration [19]. The addition of small amounts of Mg to human urine is the most common method of P recovery and a significant amount of N is precipitated with crystalline struvite. The main factors that control struvite crystallization are the pH and molar ratio of Mg 2+ : NH 4+ : PO 4 3-. Struvite is formed when Mg, ammonia, and phosphate ions exceed the solubility product of struvite, as explained in Equation (1) [21].
Mg 2+ + NH 4+ + PO 4 3-+ 6H 2 Wastewater is the main state problem in the environment. Therefore, the utilization of wastewater as a source for hydrogen energy production and P recovery was studied. In this work, synthetic human urine which contained a high concentration of N and P was utilized for hydrogen energy production and struvite crystallization. Hydrogen gas can be produced via a photocatalytic process with the use of TiO 2 photocatalyst doped with alkaline earth metals (Sr) under visible light irradiation. Hydrogen production efficiencies of Sr-doped TiO 2 photocatalysts at various Sr concentrations were examined. After the photocatalytic process, P was recovered from the synthetic human urine, which has never been reported before. UV-vis diffuse reflectance spectroscopy (DRS) was used for the investigation of an optical property, i.e., light absorption, of photocatalysts ( Figure 1a). It was found that Sr-doped TiO 2 could adsorb more light in the visible region (wavelengths less than 400 nm) compared with pristine TiO 2 . Energy bandgaps (Eg) of pristine TiO 2 and Sr-doped TiO 2 were studied from absorbance spectra using the Tauc plots [22] (Figure 1b). Energy bandgaps of Sr-doped TiO 2 tended to decrease compared with pristine TiO 2 as presented in Table 1, i.e., Sr concentrations in Sr-doped TiO 2 of 0.5, 1, 2, and 4% narrowed the bandgaps of pristine TiO 2 (3.29 eV) to 3.11, 3.11, 3.11, and 3.14 eV, respectively. Light absorption in the visible region and narrowed energy bandgaps by a metal modification will benefit the photocatalytic activity under the visible light irradiation [15,23]. Note that a trendy wider bandgap in 4% Sr-doped TiO 2 compared with those in other Sr-doped TiO 2 samples guides that 4% is too high a concentration for Sr doping.

Results and Discussion
Photoluminescence spectroscopy (PL) is generally used to determine the charge carrier trapping and to understand the behavior of the electron recombination, i.e., a low PL intensity indicates a low recombination rate [15]. Figure 1c shows PL spectra of TiO 2 and Sr-doped TiO 2 photocatalysts at an excitation wavelength of 320 nm. PL emission peaks corresponding to the inter-band recombination of TiO 2 were observed at 410 nm. Compared with pristine TiO 2 , the emission peak intensity was found to significantly reduce in Sr-doped TiO 2 samples. PL emission peaks are greatly quenched in 0.5% Sr-doped TiO 2 and 1% Sr-doped TiO 2. The result suggested that Sr doping had markedly improved the separation of charge carriers. Great separation of charge carriers (low charge recombination) will further govern high photocatalytic efficiency [14]. The PL quenching in Sr-doped TiO 2 tended to decrease with increasing Sr concentrations to 2% and 4%. This is because the overloaded metal might act as a new recombination center (even though the recombination rates in 2% Sr-doped TiO 2 and 4% Sr-doped TiO 2 are lower than that in pristine TiO 2 ).  X-ray Powder Diffraction (XRD) was performed in order to study the crystalline structures of pristine TiO2 and Sr-doped TiO2 samples ( Figure 2). XRD patterns of all samples show the most intense peak observed at 2θ of ~25°, corresponding to the TiO2 anatase phase (JCPDS No. 21-1272), with the characteristic peaks of the mixed TiO2 rutile phase (JCPDS No. 21-1276), e.g., at 2θ of ~28°. There are small tracks of characteristic peaks of Sr and SrO (at 2θ of ~36.5°) in 0.5% Sr-doped TiO2 and 1% Sr-doped TiO2. It was reported that Sr 2+ has difficulty displacing Ti4+ into the lattice because the ionic radius of Sr 2+ is larger than Ti 4+ [14,24], so the characteristic peak of SrTiO3 (JCPDS card no.35-734, i.e., 32.4°) is seemingly found in 2% Sr-doped TiO2 and stronger in 4% Sr-doped TiO2. This result is in line with the observations in UV-vis and PL spectra and indicates that the Sr-  X-ray Powder Diffraction (XRD) was performed in order to study the crystalline structures of pristine TiO 2 and Sr-doped TiO 2 samples ( Figure 2). XRD patterns of all samples show the most intense peak observed at 2θ of~25 • , corresponding to the TiO 2 anatase phase (JCPDS No. 21-1272), with the characteristic peaks of the mixed TiO 2 rutile phase (JCPDS No. 21-1276), e.g., at 2θ of~28 • . There are small tracks of characteristic peaks of Sr and SrO (at 2θ of~36.5 • ) in 0.5% Sr-doped TiO 2 and 1% Sr-doped TiO 2 . It was reported that Sr 2+ has difficulty displacing Ti4+ into the lattice because the ionic radius of Sr 2+ is larger than Ti 4+ [14,24], so the characteristic peak of SrTiO 3 (JCPDS card no.35-734, i.e., 32.4 • ) is seemingly found in 2% Sr-doped TiO 2 and stronger in 4% Sr-doped TiO 2 . This result is in line with the observations in UV-vis and PL spectra and indicates that the Sr-doped TiO 2 with mixed anatase-rutile crystallite phases is successfully prepared in the 1% Sr-doped TiO 2 sample, which may be beneficial to promote the photon-generated carrier transporting to increase the photocatalysis. Sr-doped TiO2 sample, which may be beneficial to promote the photon-generated carrier transporting to increase the photocatalysis. It was found that Sr-doped TiO2 samples show relatively wider anatase (101) 2θ (see Table 1), implying smaller crystallite sizes compared with pristine TiO2 as shown in [25,26]. The decreases in crystallite sizes of Sr-doped TiO2 indicates that the Sr can penetrate the TiO2 structure, forming some oxide forms (as found in XRD results), but anatase could still be formed, which is beneficial for the photocatalytic applications.

Brunauer-Emmett-Teller (BET) Surface Areas of Sr-Doped TiO2
BET surface area of pristine TiO2 was 46.74 m 2 /g. Doping with 0.5, 1, and 2% Sr could enhance the specific surface area of the photocatalysts to 57.47, 55.36, and 62.73 m 2 /g, respectively. This result is in line with the smaller crystallite sizes of Sr-doped TiO2 as discussed above. Unfortunately, with the too-high doping amount (4%), the specific surface area was found to reduce to 41.51 m 2 /g due to the formation of SrTiO3 as shown in XRD results. Note that the surface areas of the obtained Sr-doped TiO2 samples are relatively lower than those reported by Sood et al. (2015) (100.78 m 2 /g) [14]. This is because of the different Sr doping methods (i.e., wetness impregnation and hydrothermal treatment in this work and the previous report, respectively).

Morphology of Sr-Doped TiO2
Field Emission Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer (FE-SEM with EDS) was utilized to observe the morphological appearances (Figure 3) and detect elemental compositions in the prepared samples. The FE-SEM images at the low magnification present the aggregated clusters of Sr-doped TiO2 nanoparticles similar to the doped TiO2 reported by Bakhshayesh and Bakhshayesh (2016) while the images at the higher magnification show the nanoparticle structures. Higher concentrations of Sr (2% and 4%) show more irregular clusters on the TiO2 surfaces due to the formation of SrO and SrTiO3. EDS analysis was used to determine the existence of Sr in the doped TiO2. The uniform dispersions of Sr in the samples were found in the elemental mapping results (not shown here). The elemental compositions of Sr in the doped TiO2 reasonably correspond with the Sr loading amounts ( Table 2). It was found that Sr-doped TiO 2 samples show relatively wider anatase (101) 2θ (see Table 1), implying smaller crystallite sizes compared with pristine TiO 2 as shown in [25,26]. The decreases in crystallite sizes of Sr-doped TiO 2 indicates that the Sr can penetrate the TiO 2 structure, forming some oxide forms (as found in XRD results), but anatase could still be formed, which is beneficial for the photocatalytic applications.

Brunauer-Emmett-Teller (BET) Surface Areas of Sr-Doped TiO 2
BET surface area of pristine TiO 2 was 46.74 m 2 /g. Doping with 0.5, 1, and 2% Sr could enhance the specific surface area of the photocatalysts to 57.47, 55.36, and 62.73 m 2 /g, respectively. This result is in line with the smaller crystallite sizes of Sr-doped TiO 2 as discussed above. Unfortunately, with the too-high doping amount (4%), the specific surface area was found to reduce to 41.51 m 2 /g due to the formation of SrTiO 3 as shown in XRD results. Note that the surface areas of the obtained Sr-doped TiO 2 samples are relatively lower than those reported by Sood et al. (2015) (100.78 m 2 /g) [14]. This is because of the different Sr doping methods (i.e., wetness impregnation and hydrothermal treatment in this work and the previous report, respectively).

Morphology of Sr-Doped TiO 2
Field Emission Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer (FE-SEM with EDS) was utilized to observe the morphological appearances ( Figure 3) and detect elemental compositions in the prepared samples. The FE-SEM images at the low magnification present the aggregated clusters of Sr-doped TiO 2 nanoparticles similar to the doped TiO 2 reported by Bakhshayesh and Bakhshayesh (2016) while the images at the higher magnification show the nanoparticle structures. Higher concentrations of Sr (2% and 4%) show more irregular clusters on the TiO 2 surfaces due to the formation of SrO and SrTiO 3 . EDS analysis was used to determine the existence of Sr in the doped TiO 2 . The uniform dispersions of Sr in the samples were found in the elemental mapping results (not shown here). The elemental compositions of Sr in the doped TiO 2 reasonably correspond with the Sr loading amounts (Table 2).

Photocatalytic Hydrogen Production from Synthetic Urine
To study the photocatalytic activities of Sr-doped TiO 2 , hydrogen productions from synthetic urine via photocatalysis using TiO 2 and Sr-doped TiO 2 photocatalysts (at various concentrations of Sr, i.e., 0.5, 1, 2, and 4%) were examined ( Figure 4). The system with methanol as an electron donor (hole or OH radical scavengers) under UV-C irradiation (wavelength = 253.7 nm) at a mild temperature was carried out. COD value was analyzed to measure pollutants in wastewater by close reflux and titrimetric method both before and after the photocatalytic process. The pH values of all samples were found to be in the range of pH 5-7.

Photocatalytic Hydrogen Production from Synthetic Urine
To study the photocatalytic activities of Sr-doped TiO2, hydrogen productions from synthetic urine via photocatalysis using TiO2 and Sr-doped TiO2 photocatalysts (at various concentrations of Sr, i.e., 0.5, 1, 2, and 4%) were examined ( Figure 4). The system with methanol as an electron donor (hole or OH radical scavengers) under UV-C irradiation (wavelength = 253.7 nm) at a mild temperature was carried out. COD value was analyzed to measure pollutants in wastewater by close reflux and titrimetric method both before and after the photocatalytic process. The pH values of all samples were found to be in the range of pH 5-7. Without any photocatalyst, no hydrogen was found to be produced from water in urine, while pristine TiO2 achieved an H2 production yield of 0.8%. The yields of hydrogen with 0.5, 1, 2, and 4% Sr-doped TiO2 photocatalysts were found to be 0.4, 2.7, 1.5, and 1.2%, respectively.
The COD value of synthetic urine before the photocatalytic process was 14,400 mg/L and decreased to 13,600 mg/L after the process with pristine TiO2. The reduction in COD indicated a lower number of organics in urine after photocatalysis, so not only the production of hydrogen but also the quality of water increases after the process. In case of Srdoped TiO2 with various concentrations (0.5, 1, 2, and 4%), the COD values after the photocatalytic process decreased to 5600, 2400, 10,400, and 9600 mg/L, respectively.
It was found that 1% Sr-doped TiO2 presented the highest H2 production yield (2.7%) which is significantly higher than the pristine TiO2 with the lowest COD value (reduction from 14,400 to 2400 mg/L). The result of 1% Sr-doped TiO2 may best describe the properties of Sr-doped TiO2, which can markedly improve the separation of charge carriers (according to PL spectra), resulting in higher photocatalytic efficiency [14].
After the photocatalytic hydrogen production, the photocatalysts as the heterogeneous catalysts were obviously precipitated from the synthetic urine. This showed the potential that the photocatalysts can be regenerated and reused. Without any photocatalyst, no hydrogen was found to be produced from water in urine, while pristine TiO 2 achieved an H 2 production yield of 0.8%. The yields of hydrogen with 0.5, 1, 2, and 4% Sr-doped TiO 2 photocatalysts were found to be 0.4, 2.7, 1.5, and 1.2%, respectively.

Struvite Crystallization from Urine
The COD value of synthetic urine before the photocatalytic process was 14,400 mg/L and decreased to 13,600 mg/L after the process with pristine TiO 2 . The reduction in COD indicated a lower number of organics in urine after photocatalysis, so not only the production of hydrogen but also the quality of water increases after the process. In case of Sr-doped TiO 2 with various concentrations (0.5, 1, 2, and 4%), the COD values after the photocatalytic process decreased to 5600, 2400, 10,400, and 9600 mg/L, respectively.
It was found that 1% Sr-doped TiO 2 presented the highest H 2 production yield (2.7%) which is significantly higher than the pristine TiO 2 with the lowest COD value (reduction from 14,400 to 2400 mg/L). The result of 1% Sr-doped TiO 2 may best describe the properties of Sr-doped TiO 2 , which can markedly improve the separation of charge carriers (according to PL spectra), resulting in higher photocatalytic efficiency [14].
After the photocatalytic hydrogen production, the photocatalysts as the heterogeneous catalysts were obviously precipitated from the synthetic urine. This showed the potential that the photocatalysts can be regenerated and reused.

Struvite Crystallization from Urine
Samples were collected from the prepared synthetic urine and the synthetic urine after the photocatalytic process. The PO4-P concentration was analyzed by the ascorbic acid method before adjusting the pH and adding Mg. Light microscopy was used to study morphology and measure the crystal size of struvite formed from synthetic urine after the photocatalytic process. Figure 5 shows microscopy images of struvite crystals formed from synthetic urines before and after the photocatalytic process using 1% Sr-doped TiO 2 photocatalyst. The typical radiating dendrite-like and hexagon-like shapes of the struvite crystals were observed [27]. The diameter of the struvite crystals was around 0.25-0.31 nm. EDS spectra of struvite crystals formed from synthetic urines before and after the photocatalytic process (using 1% Sr-doped TiO 2 photocatalyst) are shown in Figure 6. The elements of the MgNH 4 PO 4 (i.e., Mg, N, and P) are observed, confirming the formation of the struvite structure. The result observed confirmed that P recovery via struvite crystallization can be successfully carried out using the synthetic urine after the photocatalytic process as the source. Samples were collected from the prepared synthetic urine and the synthetic urine after the photocatalytic process. The PO4-P concentration was analyzed by the ascorbic acid method before adjusting the pH and adding Mg. Light microscopy was used to study morphology and measure the crystal size of struvite formed from synthetic urine after the photocatalytic process. Figure 5 shows microscopy images of struvite crystals formed from synthetic urines before and after the photocatalytic process using 1% Sr-doped TiO2 photocatalyst. The typical radiating dendrite-like and hexagon-like shapes of the struvite crystals were observed [27]. The diameter of the struvite crystals was around 0.25-0.31 nm. EDS spectra of struvite crystals formed from synthetic urines before and after the photocatalytic process (using 1% Sr-doped TiO2 photocatalyst) are shown in Figure 6. The elements of the MgNH4PO4 (i.e., Mg, N, and P) are observed, confirming the formation of the struvite structure. The result observed confirmed that P recovery via struvite crystallization can be successfully carried out using the synthetic urine after the photocatalytic process as the source.

Catalyst Preparation and Characterizations
Wetness impregnation was applied to prepare catalysts with a metal ion salt (Sr-doped TiO 2 ). Pristine TiO 2 was used as a catalyst host and Sr (NO 3 ) 2 was used as the precursor for Sr doping by dissolving in distilled water. Accordingly, the solutions with varying concentrations of Sr (%w/w) as 0.5, 1, 2, and 4% were dropped into the pristine TiO 2 powders and stirred on a hotplate. After mixing, the solvent was slowly evaporated in a vacuum oven at 60 • C for 4 h. The samples were dried in an oven at 80 • C overnight and then calcined in a furnace under N 2 condition at 400 • C for 4 h.
UV-vis spectroscopy (DRS, V-550 Jasco, Deutschland GmbH, Pfungstadt, Germany) was used to analyze the optical property of catalysts. Tauc plots were carried out to estimate energy bandgaps (Eg) using indirectly allowed transitions (Equation (2)): where α is absorption coefficient, hν is photon energy, and A is a constant value. Graphs between (αhν) 2 and hν were plotted. An optical energy bandgap was calculated by extrapolating a straight line to intersect the x-axis. Photoluminescence (PL, FP8600 Jasco, Deutschland GmbH) was used for determining the recombination of charge carriers. The X-ray diffraction analysis (XRD, SmartLab Rigaku, Austin, TX, USA) was used to study the relative crystallinity of Sr-doped TiO 2 catalysts to confirm the presence of metal oxide phases in the synthesized nanocatalysts. N 2 adsorption (BEL Japan, BELSORP-mini, Osaka, Japan) was used to determine BET surface area. The morphology of microstructure and mapping for local elements of catalysts were analyzed by a field emission scanning electron microscope (FE-SEM, SU8230 Hitachi, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS).

Synthesis of Urine
In this study, batch experiments were performed using synthetic urine, which was prepared in distilled water according to the composition reported in Table 3 [28,29]. The pH of the obtained urine was 5.8.

Photocatalytic Hydrogen Production
An experimental system for photocatalytic hydrogen production consisted of a photoreactor made from Pyrex glass as shown in Figure 7. The sample volume in the reactor was 600 mL. The reactor height and inner diameters were 30.0 cm and 7.0 cm, respectively. A Phillips TUV PL-S mercury lamp was used as a UV-C light source (λ =253.7 nm) with a power supply of 54W. A vacuum pump was installed to control the cooling water flow inside the photoreactor and maintain the reaction temperature. A gas chromatograph (GC) was connected with the photoreactor for the analysis of hydrogen gas.
An experimental system for photocatalytic hydrogen production consisted of a photoreactor made from Pyrex glass as shown in Figure 7. The sample volume in the reactor was 600 mL. The reactor height and inner diameters were 30.0 cm and 7.0 cm, respectively. A Phillips TUV PL-S mercury lamp was used as a UV-C light source (λ =253.7 nm) with a power supply of 54W. A vacuum pump was installed to control the cooling water flow inside the photoreactor and maintain the reaction temperature. A gas chromatograph (GC) was connected with the photoreactor for the analysis of hydrogen gas. The catalyst powders were suspended in the urine solution and mixed with methanol in the catalyst: urine: methanol ratio of 1:1000:100 [30]. The mixed sample was continuously stirred for a reaction time of 6 h. Ar was purged for 30 min prior to irradiation to remove dissolved oxygen. After 6 h of irradiation time, hydrogen gas was sampled from the headspace of the photoreactor. The amount of hydrogen gas was analyzed using an online GC (Shimadzu GC-2014, Kyoto, Japan) with both thermal conductivity (TCD) and flame ionization (FID) detectors, and a molecular sieve column. The photocatalytic hydrogen production activity was presented in terms of H2 yields. The calculations were defined as Equations (3) The catalyst powders were suspended in the urine solution and mixed with methanol in the catalyst: urine: methanol ratio of 1:1000:100 [30]. The mixed sample was continuously stirred for a reaction time of 6 h. Ar was purged for 30 min prior to irradiation to remove dissolved oxygen. After 6 h of irradiation time, hydrogen gas was sampled from the headspace of the photoreactor. The amount of hydrogen gas was analyzed using an online GC (Shimadzu GC-2014, Kyoto, Japan) with both thermal conductivity (TCD) and flame ionization (FID) detectors, and a molecular sieve column. The photocatalytic hydrogen production activity was presented in terms of H 2 yields. The calculations were defined as Equations (3)-(5). NH 4 COONH 2 (aq) ←→ 2NH 2 (g) + CO 2 (g) 2NH 3(g) + CO 2 (g) → N 2 + 3H 2 + CO 2 (4) H 2 yield (%) = (Actual yield (mol of H 2 )/Theoretical yield (mol of H 2 )) × 100 (5) Finally, liquid samples were filtered using a 0.46-µm PTFE syringe filter (VWR, Amsterdam, Netherlands) to remove Sr-doped TiO 2 catalyst particles prior to the analysis. The common parameters (e.g., COD and pH) of the synthetic urine were measured before and after the photocatalytic process.

Struvite Crystallization
Liquid samples of synthetic urine after the photocatalytic process were collected for struvite precipitation (Figure 8). The suitable pH conditions for struvite crystallization are 7.5-9.0 [31]. Therefore, pH 9 was selected for this experiment. NaOH (15%) was added to the sample solution for the increase in pH value [32]. MgCl 2 was added as an Mg source to the solution for adjustment of the molar ratio of Mg:P of 1:1 [29]. Struvite crystals were spontaneously precipitated. The samples were collected and characterized using a light microscope (Nikon Eclipse E200, Tokyo, Japan) and FE-SEM (SU5000 Hitachi, Tokyo, Japan) for the determination of morphologies and structures of the struvite crystals from the synthetic urine. Before morphology characterization, the samples were filtered by a Pall Supor ® -450 (0.45 µm) membrane filter and dried in an oven at 105 • C for 1 h [29]. the sample solution for the increase in pH value [32]. MgCl2 was added as an Mg source to the solution for adjustment of the molar ratio of Mg:P of 1:1 [29]. Struvite crystals were spontaneously precipitated. The samples were collected and characterized using a light microscope (Nikon Eclipse E200, Tokyo, Japan) and FE-SEM (SU5000 Hitachi, Tokyo, Japan) for the determination of morphologies and structures of the struvite crystals from the synthetic urine. Before morphology characterization, the samples were filtered by a Pall Supor ® -450 (0.45 μm) membrane filter and dried in an oven at 105 °C for 1 h [29].

Conclusions
TiO2 photocatalyst modified with an alkaline earth metal ion (i.e., Sr 2+ ) has been synthesized. Prepared Sr-doped photocatalysts were found to have great light absorption, great crystallinity, and quenched PL spectra. Photocatalytic hydrogen production from synthetic urine was carried out. From various doping contents of Sr in TiO2 (0.5, 1, 2, and 4%), it was found that 1% Sr-doped TiO2 presented the highest H2 production yield, which was significantly higher than the pristine TiO2 with the lowest COD value, implying a marked improvement in the separation of charge carriers (according to PL spectra). This suggests that the successfully doped photocatalyst can utilize light for the conversion of water in urine wastewater to hydrogen while also degrading the organic compounds in the wastewater. Moreover, the treated synthetic wastewater can be subsequently used as a resource for phosphorus recovery via the struvite crystallization process.

Conclusions
TiO 2 photocatalyst modified with an alkaline earth metal ion (i.e., Sr 2+ ) has been synthesized. Prepared Sr-doped photocatalysts were found to have great light absorption, great crystallinity, and quenched PL spectra. Photocatalytic hydrogen production from synthetic urine was carried out. From various doping contents of Sr in TiO 2 (0.5, 1, 2, and 4%), it was found that 1% Sr-doped TiO 2 presented the highest H 2 production yield, which was significantly higher than the pristine TiO 2 with the lowest COD value, implying a marked improvement in the separation of charge carriers (according to PL spectra). This suggests that the successfully doped photocatalyst can utilize light for the conversion of water in urine wastewater to hydrogen while also degrading the organic compounds in the wastewater. Moreover, the treated synthetic wastewater can be subsequently used as a resource for phosphorus recovery via the struvite crystallization process.