Simultaneous tartrazine-tetracycline removal and hydrogen production in the hybrid electrocoagulation-photocatalytic process using g-C 3 N 4 /TiNTAs

This study aimed to investigate the removal of tartrazine dye & tetracycline antibiotic and hydrogen (H 2 ) production simultaneously through the hybrid electrocoagulation-photocatalytic process using g-C 3 N 4 /TiO 2 nanotube arrays (TiNTAs) nanocomposite. The g-C 3 N 4 /TiNTAs was used as the photocatalyst. The melamine as the precursor of g-C 3 N 4 was varied to obtain the optimal loading on the removal of tartrazine dye & tetracycline antibiotic and hydrogen (H 2 ) production simultaneously. The integrated acrylic photoreactor was equipped with two 250-W mercury lamps. The nanotubular morphology of TiNTAs and nanostructure features of g-C 3 N 4 /TiNTAs were examined using FESEM/EDX and HR-TEM/SAED. The XRD patterns indicated the composition of TiNTAs, confirming the presence of anatase and rutile crystalline phases. UV-Vis DRS also showed a redshift in the composite absorbance and a reduced bandgap with g-C 3 N 4 introduction. The results showed that when tartrazine and tetracycline were treated simultaneously, tartrazine was more dominantly degraded compared to tetracycline. In mixed pollutant system condition, the H 2 production increased by 17.0% and 41.1% compared to single pollutant system of tartrazine and tetracycline, respectively. The photocatalyst used in the hybrid process was the g-C 3 N 4 /TiNTAs (3 g) which provide the optimum H 2 production.


Introduction
Medical waste is a significant threat to both human health and the environment, originating from hospital and WWTP effluents, chemical plants, livestock, and aquaculture sectors [1].A common medical waste often found is effluent from antibiotics and medicinal dyes, prevalently consisting of tetracycline (C22H24N2O8) and tartrazine (C16H9N4Na3O9S2).In the environment, the low degradation rate and high resistance of tetracycline can lead to the development of antibiotic-resistant microorganisms [2].Meanwhile, the high solubility of tartrazine in water makes it a common contaminant in hospital effluents, potentially posing adverse effects on human health at high concentrations [3].Tartrazine dye and tetracycline have the same dangerous potential, but based on the maximum concentration limits in the environment, tetracycline has a lower maximum concentration limit compared to dyes.It indicates that antibiotic waste must be controlled in such a way as to minimize the amount that exceeds the maximum limit.The maximum concentration of dyes is 7.5 mg/kg [4], and the maximum concentration of antibiotics is 0.001 mg/kg [5].
Various process that has been applied to degrade antibiotics and dye wastes include biological, physical, and chemical treatments [6][7].However, these technologies are only partially effective due to several limitations such as efficiency, energy consumption, and high costs [8].
In addition to medical waste issues, there has been a growing global interest in hydrogen as an alternative energy source.Besides its wide application, hydrogen is an energy source with a significantly high energy density [9][10].Approximately 96% of hydrogen is obtained from natural gasbased raw materials through steam reforming, electrolysis, and liquid reforming processes, which require high costs and are not environmentally friendly [10].Consequently, alternative technologies are needed to produce hydrogen without depending on oil-and natural gas-based raw materials.
A promising technology in response to these problems is photocatalytic oxidation using TiO2 semiconductors.This technology has gained much attention due to its costeffectiveness and environmental friendliness in degrading organic and inorganic pollutants in wastewater [7].TiO2 photocatalysts are also commonly used in photocatalytic water splitting, a process that breaks down water molecules to produce hydrogen and oxygen [11].However, the large band gap energy of 3.2 eV limits TiO2 in absorbing photons from visible light, confirming its photons absorption from the UV light region.This limitation hampers the efficient use of TiO2 photocatalysts [12] due to the significant ban`d gap value, restricting UV light adsorption to just 4% of sunlight [11].
Previous investigations have focused on two methods to minimize the drawbacks of TiO2.These include the use of TiO2 nanoparticles coated with aluminum to increase the photocatalyst surface area, thereby improving the ability to degrade dye pollutants [13].Additionally, TiO2 has been developed in nanotube arrays morphology (TiNTAs), characterized by a larger specific surface area, improved photon energy irradiation adsorption, and enhanced electron transport.This structural modification led to a more efficient photocatalytic degradation of dissolved contaminants [14].In a previous study, the performance of TiNTAs was improved by depositing CuO and other metals, such as Fe, as an electron trapper and reducing the band gap energy of the photocatalyst [15][16].The modifications successfully lowered the recombination rate of electron-hole pairs and made the photocatalyst more sensitive to visible light irradiation.
g-C3N4 is a photocatalyst with good chemical stability and responsive to visible light due to its lower band gap of 2.7 eV compared to TiO2.However, g-C3N4 has the disadvantage of a relatively high recombination rate of electron-hole pairs [17].In this study, g-C3N4/TiO2 composites were fabricated to improve the performance of both photocatalysts.To achieve this, TiO2 will be synthesized with nanotube array morphology, facilitating the formation of an effective composite.
In a previous study, a method was developed for wastewater treatment by combining electrocoagulation and photocatalysis in an integrated reactor.The optimal conditions for electrocoagulation, pH, photocatalyst morphology, and photocatalyst dopants were examined to develop a functional system capable of eliminating dissolved pollutants [13][14][15].
These investigations did not discuss the treatment of more than one type of environmental pollutant.According to [16] the use of the hybrid electrocoagulation-photocatalysis process successfully eliminated methylene blue-ciprofloxacin in a mixed pollutant system.However, other types of multiple pollutants in a mixed system were not investigated extensively.
As electrocoagulation and photocatalytic technologies generate hydrogen during water purification (pollutant removal), efforts to enhance H2 production have led to various modifications.Consequently, this study adds an electrolyte species into the reaction system to evaluate the impact of NaCl on both pollutant removal and hydrogen production within electrocoagulation system.
The electrocoagulation-photocatalysis hybrid process was employed using g-C3N4/TiNTAs to eliminate tartrazinetetracycline simultaneously, as a wastewater model in a mixed pollutant system and produce hydrogen in an integrated reactor.The incorporation of g-C3N4 onto TiNTAs was achieved using the adsorption method and the loading effect was also analyzed.Photocatalyst characterization was conducted using FESEM/EDX, HR-TEM/SAED, XRD, and UV-Vis DRS.The results were used to examine the effect of specific parameters on tartrazine-tetracycline removal and hydrogen production.Based on the descriptions above, this study aimed to investigate the removal of tartrazine dye & tetracycline antibiotic and hydrogen (H2) production simultanously through the hybrid electrocoagulationphotocatalytic process using g-C3N4/TiO2 nanotube arrays (TiNTAs) nanocomposite.The melamine as the precursor of g-C3N4 was varied to obtain the optimal loading the removal of tartrazine dye & tetracycline antibiotic and hydrogen (H2) production.

Synthesis of TiNTAs and g-C3N4/TiNTAs
The titanium plate of 8 cm 4 cm (Shaanxi Yunzhong Metal Technology Co., LTD) underwent mechanical polishing using 1500 CW sandpaper.The titanium plate was treated with a chemical solution consisting of HF (Merck, 40%), HNO3 (Merck, 65%), and distilled water in a volume ratio of 1:3:46 for 1 minute.Subsequently, the titanium plate was washed with distilled water and sonicated for 20 minutes to completely remove existing impurities.
TiNTAs were synthesized through an anodization process using a glycerol electrolyte solution (P&G Chemicals, 98%), containing 0.5% weight NH4F and 25% volume of water.A pre-prepared platinum mesh and the titanium plate were used as the cathode and anode, respectively, with a voltage of 50 V for 2 h.g-C 3 N 4 /TiNTAs were obtained by depositing g-C 3 N 4 on the TiO2 nanotube arrays surface through an adsorption method.Melamine (Merk analytical, EMSURE® ACS) was used as the precursor for g-C 3 N 4 .The TiNTAs plate was sonicated for 30 minutes in a melamine solution with varying concentrations of 1 g, 2 g, and 3 g in 200 mL solvent containing equal parts methanol (Merk analytical, EMSURE® ACS, ISO, Reag.Ph Eur) and water, followed by immersion for 24 h.The samples were air-dried for 30 minutes at room temperature and annealed in an atmospheric furnace at 550°C for 3 h with a heating rate of 3 o C/min to convert melamine into g-C3N4 and enhance its crystallinity.Subsequently, g-C3N4/TiNTAs photocatalyst synthesized using melamine precursors of 1 g, 2 g, and 3 g in 200 mL of solvent were referred to as g-C3N4/TiNTAs (1 g), g-C3N4/TiNTAs (2 g), and g-C 3 N 4 /TiNTAs (3 g), respectively.

Characterizations of g-C3N4/TiNTAs
The morphology and elemental composition of the synthesized photocatalyst samples were analyzed using Field Emission Scanning Electron Microscope (FESEM, Thermo Scientific Quattro S completed with EDS detector) at 30 kV and equipped with Energy Dispersive X-ray spectroscopy (EDX).A Gaussian curve plot was used to determine nanotube average diameter.Meanwhile, the average nanotube length and wall thickness were measured using the integrated program of the instrument.The nanostructural features were investigated using a High-Resolution Transmission Electron Microscope (HRTEM, FEI Tecnai G2 20 S-TWIN) operating at 200 kV, along with selected area electron diffraction (SAED) analysis.
The crystallite properties were determined using an X-ray Diffraction (XRD) analysis (XRD, Empyrean Series 3 Panalytical) with a copper anode tube (λ = 0.15406 nm) operating at 40 kV and 30 mA.XRD scanning was performed in the 2θ range of 10-60° with step size 2θ of 0.013 and scan step time of 37.995 s.The crystallite size was estimated using the Scherrer equation based on the full-width half maximum (FWHM) method of the XRD peaks.The optical properties and band gap values were determined using the Kubelka-Munk functions from UV-Visible Diffuse Reflectance Spectra (UV-Vis DRS, Agilent Cary 60 UV-Vis Spectrophotometer).The data were obtained in the range of 300-500 nm.

Characterizations of g-C3N4/TiNTAs
The performance tests were carried out in an integrated acrylic reactor for the hybrid electrocoagulationphotocatalysis process.The reactor contained a 500 mL solution of tartrazine (TZ, 20 ppm) and tetracycline (TC, 20 ppm) at pH 11.In the photocatalysis test, an 8 cm x 4 cm g-C3N4/TiNTAs photocatalyst was immersed in the pollutant model solution and exposed to illumination from two mercury lamps (17.25% UV and 82.75% visible light, 250 W).Electrocoagulation system was equipped with aluminum (1 mm thick, 8 cm x 4 cm) and stainless steel 316 (2 mm thick, 8 cm x 4 cm) plates as the anode and the cathode, respectively.These electrodes were placed 3 cm apart from each other and connected to a DC power supply (Volomax DC Power Supply KXN-645D).Subsequently, electrocoagulation process performance test was carried out by switching on the power supply at 5 V.
The hybrid electrocoagulation-photocatalytic process performance test was conducted by combining both processes in an integrated acrylic reactor equipped with a stainless-steel mesh to separate electrocoagulation and photocatalysis chamber to prevent the coagulant from entering the photocatalysis chamber and causing a shading effect.The same solution conditions were maintained during this test.Subsequently, the hybrid process experiment was carried out by switching on the mercury lamps and the power supply simultaneously.During the performance test, the solution was stirred continuously, and argon gas flowed through the reactor for 5 min before each experiment to remove oxygen from the system.
Tartrazine-tetracycline samples were taken every 60 minutes throughout the 240-minute experiment.The UV-Vis spectrophotometer (Bell Engineering M51) was used to measure the absorbance of the sample at specific wavelengths.The 392 nm and 237 nm wavelengths were used for measuring the absorbance of tartrazine and tetracycline at pH 11, respectively.To monitor the reduction in concentration of tetracycline and tartrazine, Equation (1) was applied.
where C0 and C represent the concentrations of tartrazine and tetracycline in mg/L (ppm) at the initial and specific time points, respectively.The concentration of hydrogen produced was also analyzed using a Gas Chromatography (Shimadzu GC-2014) equipped with a Molecular Sieve (MS) Hydrogen 5A column, with a known retention time for argon as the carrier gas.

Characterization of g-C3N4/TiNTAs
FESEM analysis provided a morphological view of the photocatalyst in the form of nanotube arrays immobilized on the surface of the titanium plate with varying loading of g-C3N4, as presented in Fig. 1.The results showed that there was no change in the morphology with the introduction of g-C3N4 in terms of inner diameter size, wall thickness, and nanotube length.However, nanotube arrays showed very diverse sizes and dimensions.
The calculation of nanotube dimensions, as presented in Table 1, showed an inner diameter ranging from 166 -193 nm, with a wall thickness of 28 -37 nm, and a height of 1094 -1720 nm.These results showed a correlation between g-C 3 N 4 loading and changes in nanotube dimensions.Therefore, the addition of g-C3N4 loading did not affect the morphology of TiNTAs.The absence of changes in nanotube morphology also showed that the synthesis of TiNTAs through electrochemical anodization was a stable process with consistent results.Furthermore, the unaltered morphology suggested that the main factor affecting the dimensional changes of nanotube was anodization conditions [18].These included H2O content influencing nanotube length, fluoride ions affecting diameter, voltage, as well as duration, and stirring conditions during anodization that impacted the homogeneity of the electrolyte solution [19].The EDX results of the photocatalyst showed an increase in the mass percentage of the C element as the melamine concentration in the precursor solution increased.This showed that a greater concentration of melamine precursor solution yielded a higher amount of g-C3N4 that was successfully loaded into TiNTAs.Specifically, when melamine concentrations were 1 g, 2 g, and 3 g in 200 mL of precursor solution, the mass percentages of C were 1.0%, 1.3%, and 1.4%, respectively.The addition of precursor solution ranging from 1 g to 2 g in 200 mL of solvent increased the mass percent of element C by 0.3%, while 2 g to 3 g yielded a 0.1% increase.This showed that the trend of increasing precursor concentration to the mass percent of the component was a saturation-growth-like relationship.In this pattern, the linearity of the relationship only occurred at lower concentrations and gradually reached a saturation point on the surface of TiNTAs at high melamine concentrations.
Based on the EDX analysis, none of the samples showed the presence of the elemental component N on the photocatalyst surface.This phenomenon occurred because EDX results on pure g-C3N4 gave a smaller N element peak compared to the C element.Additionally, in the EDX conducted on g-C3N4/TiNTAs nanocomposite, the Ti and O element peaks were significantly intense compared to the C peak.This suggested that the N element was not detected on g-C3N4/TiNTAs.The non-detection of N was also reported in a previous study related to EDX analysis of g-C3N4/TiO2 nanocomposites [20].The relatively small loading of g-C3N4 on TiNTAs, which remained below 1.5%, contributed to the difficulty in detecting the N element.From the results of FESEM/EDX analysis, g-C3N4 loading on TiNTAs did not cause the formation of agglomerations and clusters of g-C3N4 particles that could interfere with the active sites of TiNTAs.
The nanostructure of the synthesized photocatalyst was further analyzed using HR-TEM and SAED characterization.The presence and location of g-C3N4 loaded on TiNTAs were confirmed.From the TEM analysis results presented in Fig. 2(a), it was discovered that g-C3N4 was successfully deposited on TiNTAs, supporting FESEM/EDX analysis.The TEM image also showed that g-C3N4 was distributed on the outer wall of TiNTAs.
The results of the TEM analysis also showed that the diameter of nanotube was getting larger at the bottom.This phenomenon occurred as the anodization duration progressed, causing more TiF6 2-species to dissolve within nanotube wall, and increasing the size of the inner diameter [15].Consequently, the morphological condition of nanotube becomes a challenge for g-C3N4 precursor to enter the interior of nanotube.Based on the results of TEM, HR-TEM, and SAED characterization, it was concluded that the nanocomposite synthesis between g-C3N4 and TiNTAs was successfully carried out.The XRD patterns in Fig. 3 showed similar diffraction patterns for all photocatalysts.Based on the results, the diffraction peaks of anatase TiO2 crystals were detected at 2 = 25.4°,37.0°, 37.9°, 38.6°, 48.1°, 54.0°, and 55.1°, which corresponded to the (1 0 1), (1 0 3), (0 0 4), ( 1  The presence of the rutile crystal phase of TiO2 was due to the calcination process conducted at a temperature of 550°C for 3 h, with a heating rate of 3°C/minute.The calcination temperature at 550 o C was selected for the optimal conversion of melamine precursor in TiNTAs into g-C3N4 [20,27].The existence of the rutile crystal phase was also in accordance with the previous study.Based on the results, it was discovered that rutile crystals became evident on TiO2 when the calcination temperature used was above 500 o C [24,[28][29].Consequently, calcination temperatures above 500 o C were high enough to promote the growth of rutile crystals. According to JCPDS No. 87-1526, g-C3N4 crystals have two prominent diffraction peaks.Specifically, these peaks are located at 2 = 27.6°,which corresponds to the (0 0 2) plane due to the stacking of conjugated aromatic layers.A smaller peak is also found at 2 = 12.8° for the (1 0 0) plane due to the structure of the layered tri-s-triazine unit [30].However, in all g-C3N4/TiNTAs photocatalysts, no diffraction peaks of g-C3N4 component were detected.This occurred because of the small amount of g-C3N4 loaded into TiNTAs and the low crystallization of melamine, as reported in previous studies [20,27,31].
The small loading of g-C3N4 corresponded to the results of EDX characterization.This showed that the mass percentage of the C element in g-C 3 N 4 /TiNTAs ranged from 1.0 -1.4%.Additionally, the diffraction of g-C3N4 at 2 = 27.6°peak overlapped with rutile crystal (1 1 0) at 2 = 27.37°.This made the diffraction peak of g-C 3 N 4 challenging to be distinguished from that of rutile crystal (1 1 0).However, the diffraction peak at 2 = 27.37° for TiNTAs without g-C3N4 loading confirmed that the peak at 2 around 27° for all photocatalyst samples was dominated by the presence of rutile crystals.
The size of the photocatalyst crystallite was calculated using the Scherrer equation.The analysis was carried out using the FWHM (full width at half maximum) method at the main diffraction peak of the anatase crystal at 2 = 25.4° and the main diffraction peak of the rutile crystal at 2 = 27.37°.Based on the results, anatase crystal size ranged from 33.0 to 35.7 nm, while rutile crystals varied from 21.7 to 30.0 nm.The composition of both parameters in units of mass percentage was also calculated, with values ranging from 80.9 to 93.5% and 6.4 to 19.1%, respectively.The results for the size and mass fraction of the crystallite in the synthesized photocatalyst are presented in Table 2.
The low amount of g-C3N4 loaded on TiNTAs, as evidenced by EDX analysis, suggested insignificant effect on the growth of anatase and rutile crystals during the calcination process.This was shown by the absence of a certain trend between the size and composition of crystals against the amount of g-C3N4 loading.Therefore, it can be concluded that g-C3N4 loading has no effect on the size and composition of the crystals of the synthesized photocatalyst.
The temperature and duration of the calcination process mainly influenced the size and composition of crystals in the photocatalysts.Generally, an increase in the calcination temperature leads to enhanced crystallinity of the photocatalyst, resulting in a more organized crystal structure, as shown by the sharp and narrow XRD peaks.The crystallite size of all crystal phases also tends to increase with a rise in calcination temperature, showing a higher crystallization process.This shows that the calcination temperature promotes crystalline phase transitions, crystal size enlargement, and crystallinity of the synthesized photocatalyst [24,32].The UV-Vis DRS absorbance spectra presented in Fig. 4 showed a significant shift in the absorbance of TiNTAs towards larger wavelengths.This shift showed an enhancement in the visible light absorption of the synthesized photocatalyst.The decrease in band gap energy of the photocatalyst caused the shift in absorbance from TiNTAs towards visible light with g-C3N4 loading.Furthermore, the determination of the band gap energy of the photocatalyst was achieved by processing the DRS absorbance spectra using the Kubelka-Munk equation and Tauc plot.
The band gap calculation results presented in Fig. 4 showed a 3.28 eV value for TiNTAs, as obtained for TiO2 photocatalysts in a previous study [24,32].Furthermore, the loading of g-C3N4 decreased the band gap of the photocatalyst ranging from 3.27 eV to 3.26 eV.This decrease was due to the presence of g-C3N4 semiconductor with a lower band gap (2.7 eV) in the synthesized nanocomposite [33].The band gap energy of g-C3N4/TiNTAs are located between the TiNTAs (3.28 eV) and g-C3N4 (2.7 eV), as expected.However, the low band gap reduction was due to the small amount of g-C3N4 loaded on TiNTAs, as confirmed by EDX characterization.In TiNTAs photocatalyst without g-C3N4 loading, the majority of the excited electrons and hole pairs from the semiconductor material underwent recombination, which reduced the photocatalysis activity.Therefore, the main goal of compositing g-C3N4 with TiNTAs is to suppress the recombination rate of electron-hole pairs through a charge transfer mechanism between the conduction band and valence band of g-C3N4 and TiNTAs.This process effectively prolongs the lifetime of the charge electron holes, thereby enhancing the photocatalysis activity [31].

Photocatalytic test on tartrazine degradation and H2 production
Photocatalysis test for 240 minutes showed a significantly low tartrazine degradation, ranging from 2.70 to 3.63%, as presented in Fig. 5 (a).The low level of tartrazine degradation was caused by the high initial concentration of tartrazine, which was 20 ppm, and tartrazine compounds consisting of 16 carbon atoms with 3 carbon rings in their structure.This made tartrazine difficult to degrade through oxidation reactions by photocatalysis.Similarly, Muttaqin et al [16] conducted experiments with an initial concentration of 20 ppm at pH 11 using an optimal photocatalyst of 0.06 M CuO-TNTA at UV lamp irradiation for 12 h and obtained 64% tartrazine degradation.This showed the difficulty of tartrazine compounds to be degraded through photocatalytic reaction.
The performance of H2 production through photocatalysis showed that loading g-C3N4 on TiNTAs significantly increased the amount of H2 gas produced.Photon irradiation for 240 minutes gave H2 gas accumulation of 14.13 μmol/m 2 using TiNTAs, while g-C3N4/TiNTAs (1 g), g-C3N4/TiNTAs (2 g), and g-C 3 N 4 /TiNTAs (3 g) photocatalysts yielded accumulation of 22.30 μmol/m 2 , 22.63 μmol/m 2 , and 26.48 μmol/m 2 , respectively.These results showed that loading g-C 3 N 4 on TiNTAs improved the photocatalysis performance regarding H2 production.The profile of H2 accumulation by photocatalysis with a variation of g-C3N4 loading is shown in Fig. 5 (b).The unchanged tartrazine degradation performance and increased H2 production through photocatalysis with g-C3N4/TiNTAs in comparison to TiNTAs can be explained by the formation heterojunction.The synthesized g-C3N4/TiNTAs photocatalyst is suspected to form a type-II heterojunction.Semiconductor 1 (g-C3N4) has a conduction and valence band with a more negative redox potential position compared to that of semiconductor 2 (TiNTAs).This configuration promotes the migration of electrons from g-C3N4 conduction band to TiNTAs.Similarly, holes in the valence band of TiNTAs tend to migrate to that of g-C3N4.
The accumulation of electrons in the conduction band of TiNTAs and holes in the valence band of g-C3N4 facilitates effective charge separation in semiconductor composites through type II heterojunction.Fig. 6a illustrates the charge separation in type II heterojunction, while Fig. 6b shows the mechanism of the •OH attack on tartrazine.This supports the theory that nucleophilic attacks are the first and require a second attack by sulfur-based or electrophilic free radicals such as superoxide radicals (•O2 -) to break the N=N bond.Production of sulfur-based radicals requires an alkaline solution pH.The limitations of type II heterojunction include low oxidation and reduction ability of the holes and excited electrons.This phenomenon occurs due to charge transfer to the more negative valence band and more positive conduction band.However, the effective charge separation compensates for these limitations, clarifying the absence of a decrease in tartrazine degradation performance and higher photocatalytic H2 production.
The photocatalysis reactions during tartrazine degradation and H2 production are summarized as follows, regardless of the charge separation mechanisms occurring in the test: Excitation of electrons by photon energy: g-C3N4/TiNTAs (h + + e -) → g-C3N4/TiNTAs + energy (3) Formation of radicals and reactive oxygen species: •HO + Intermediate → CO2 (g) + H2O (13) H2 production via reaction of water splitting:

Electrocoagulation Test on Tartrazine Elimination and H2 Production
Based on the results, electrocoagulation test provides a significant percentage of tartrazine elimination compared to the photocatalysis process.This is because the coagulant from the electrocoagulation process can spread freely in the solution as fine solids.Consequently, the surface area of the coagulant for the adsorption of tartrazine becomes enormous compared to the photocatalyst plate.In this study, electrocoagulation test provided tartrazine elimination of 31.82%, which increased to 59.68% due to the addition of NaCl at 0.2 g/L.H2 gas accumulation was obtained at 425.57mmol/m 2 and the incorporation of NaCl at 0.2 g/L caused a 105.03% increase, reaching 872.53 mmol/m 2 , as presented in Fig. 7.The addition of NaCl to tartrazine solution increased the amount of electric current flowing in electrocoagulation circuit [34] and reduced the electrical resistance of the solution.This showed that there was a change in the solution properties to a strong electrolyte [35].The addition of NaCl increased the electrical conductivity of the solution.The increase in electric current in electrocoagulation system promotes more Al 3+ ions to dissolve into tartrazine solution.These Al 3+ ions react with OH -ions to form a coagulant to eliminate pollutants.The high consumption of OH -ions makes the reaction on the cathode shift towards the product, resulting in the generation of abundant electrons.With more electrons on the cathode, a higher rate of water electrolysis reactions occurs, leading to an increase in H2 production [16].
The high conductivity and electric current in electrocoagulation system with the addition of NaCl can be observed from the electric current profile flowing in the system during the test, as presented in Fig. 8. Based on the results, the electric current in electrocoagulation without the addition of NaCl ranged from 0.02 to 0.03 A. Meanwhile, with the addition of NaCl concentration of 0.2 g/L, the electric current flowing ranged from 0.08 to 0.11 A, yielding an average increase of 330%.This corresponds to Ohm's Law, where the electric current flow will be greater when the electrical resistance is minimized at the same voltage.

The hybrid electrocoagulation-photocatalytic process
In the single process of photocatalysis and electrocoagulation test, tartrazine pollutant model was used to obtain optimal g-C3N4 loading on TiNTAs and electrocoagulation conditions for application in the hybrid process.Subsequently, a mixture of tartrazine dye and tetracycline antibiotic as a wastewater model was used in electrocoagulation-photocatalysis hybrid process test to model the presence of more than one type of pollutant in the environment.The degradation of tartrazine by photocatalysis yielded a very small percentage of degradation, which was 3.47%, while electrocoagulation process produced a relatively large percentage of tartrazine elimination at 50.81%.However, the hybrid process resulted in tartrazine elimination of 67.74%, which was greater than the sum of the photocatalysis and electrocoagulation tartrazine elimination.This increase showed a synergistic effect when electrocoagulation and photocatalysis were combined.A similar profile was also found for H2 gas accumulation in the process variations.Photocatalysis produced H2 gas at 26.48 mmol/m 2 and electrocoagulation at 872.53 mmol/m 2 .Meanwhile, the hybrid electrocoagulation-photocatalysis process was 1,028.16mmol/m 2 .The profiles of tartrazine and tetracycline concentrations and H2 accumulation in the process variations are presented in Fig. 9.
The enhanced performance of tartrazine elimination in the hybrid process was due to removal of pollutants from two different mechanisms simultaneously.In the hybrid process, tartrazine was degraded by •OH radical species, reactive oxygen species, and holes from photocatalysis reactions.Simultaneously, tartrazine was also adsorbed by Al(OH) 3 coagulant from electrocoagulation process.H2 production in the hybrid system increased because the H2 generation process occurred in two places.These included water reduction reaction on the cathode surface and water splitting reaction on the photocatalyst surface.In the hybrid process, there was an extensive consumption of OH -ions for the formation of Al(OH)3 in electrocoagulation process and •OH radicals in the photocatalysis process [36][37].This intense consumption of OH -ions led to a decrease in the concentration of OH -.Based on the principle of equilibrium, there was an increase in the number of H + ions in the reaction system, increasing H2 production [16].
The reaction that occurs in the anode and cathode in electrocoagulation process is expressed as follows.
An illustration of the hybrid electrocoagulation-photocatalysis process system is presented in Fig. 10.The hybrid process test on a mixed pollutant system gave tartrazine and tetracycline elimination of 70.47% and 20.22%, respectively.Meanwhile, the single pollutant system resulted in the elimination of 67.74% and 54.02%.The elimination was consistently higher for tartrazine pollutants compared to tetracycline for the mixed and single pollutant systems.The elimination selectivity toward tartrazine can be explained by the chemical structure of the pollutants.Tartrazine consists of 16 carbon atoms, while tetracycline comprises 22 carbon atoms.Compounds with more carbon atoms tend to have a highly complex molecular structure.Consequently, the accessibility of degrading agents such as radicals and holes is limited, making the degradation process slower.Tartrazine has a negative charge when dissolved in water which makes it easily neutralized by cationic Al 3+ species generated from electrocoagulation process.Tetracycline molecules in water do not have a negative charge tendency which makes this compound more difficult to adsorb.The molecular structure of tartrazine and tetracycline is presented in Fig. 11.The mixed pollutant system provided H2 accumulation of 1,203.38 mmol/m 2 with increased hydrogen production of 17.04% and 41.11% compared to single pollutant system of tartrazine (1,028.16mmol/m 2 ) and tetracycline (852.82mmol/m 2 ), respectively.This higher H2 production in the mixed pollutant system could be due to the formation of a concentrated solution that promoted photocatalytic degradation reactions.These results are in line with research reported by Muttaqin et al [16].Additionally, the adsorption process that occurred in electrocoagulation process was more effective, increasing pollutant elimination and H2 production.The more intense elimination process in the mixed pollutant system made the consumption of OH -ions in the solution higher.This phenomenon caused an abundance of H + ions in the solution, enhancing the possibility of H + ions passing through a reduction reaction to produce H2 gas both on the photocatalyst and the cathode surface.The concentration profile of tartrazine and tetracycline as well as the accumulation of H2 in the mixed pollutant system is presented in Fig. 12.
In previous investigations, different results were obtained, where the use of a mixed pollutant model of methylene blue dye and ciprofloxacin antibiotic increased the total removal of both pollutants.In the present study, the introduction of tetracycline into tartrazine solution was beneficial for tartrazine removal and detrimental for tetracycline.This variation in results, depending on the types of pollutants, showed the need for specified mitigation strategies for environmental However, a mixed pollutants system always provides superior H 2 production compared to a single treatment, as reported in a previous study [16].This variation in results shows the need for further study to optimize electrocoagulation-photocatalysis hybrid process in addressing more than one type of pollutant in the environment.

Conclusion
In conclusion, this study analyzed the simultaneous removal of tartrazine dye and tetracycline antibiotic in a mixed pollutant system using a hybrid electrocoagulationphotocatalytic process.The hybrid process provides optimal pollutant elimination and H2 production compared to the single process, with tartrazine elimination of 67.74% and H2 production of 1,028.2mmol/m 2 .The significant improvement of the hybrid process suggests a synergetic effect between both processes.When tartrazine and tetracycline were treated simultaneously, the results showed an elimination selectivity towards tartrazine, and the H2 production increased by 17.0% and 41.1% compared to when tartrazine and tetracycline were eliminated separately.The photocatalyst used in the hybrid process was g-C3N4/TiNTAs (3 g), which provides the optimum H2 production.g-C3N4 was successfully composited with TiNTAs, as confirmed by FESEM/EDX and HR-TEM/SAED analysis.Upon g-C3N4 introduction, bandgap lowering, and the redshift of the optical onset were also observed.The NaCl-based electrolyte solution was used in the hybrid process to obtain the optimum electrocoagulation condition.

Fig. 5 .
Fig. 5. Effect of g-C3N4 loading on TiNTAs on the a. tartrazine elimination and b. hydrogen production for 240-minute process

Fig. 7 .
Fig. 7. Effect of NaCl addition on the a. tartrazine elimination and b. hydrogen production for 240-minute electrocoagulation process set at a voltage of 5 V

Fig. 9 .
Fig. 9. (a) elimination of tartrazine and (b) hydrogen production as a function of various processes

Fig. 12 .
Fig. 12.(a) elimination of tartrazine-tetracycline and (b) hydrogen production at various pollutants (single and mixture) with the hybrid electrocoagulationphotocatalytic process

Table 1 .
FESEM/EDX analysis result of the synthesized photocatalysts

Table 2 .
Crystal size and TiO2 Composition of the synthesized photocatalysts.