A Complete Method for Evaluating the Performance of Photocatalysts for the Degradation of Antibiotics in Environmental Remediation

Bing Wang; XuXia Zhang; LeCheng Li; MengTing Ji; Zheng Zheng; ChuanHui Shi; Zhuo Li; Hong Hao

doi:10.3791/64478

JoVE Journal > Environment

Medio ambiente

A Complete Method for Evaluating the Performance of Photocatalysts for the Degradation of Antibiotics in Environmental Remediation

Published: October 06, 2022

doi:

10.3791/64478

Bing Wang¹, XuXia Zhang¹, LeCheng Li¹, MengTing Ji¹, Zheng Zheng¹, ChuanHui Shi¹, Zhuo Li¹, Hong Hao¹

¹School of Chemical Engineering,Northwest University

Summary

Presented here is a protocol to explore a universal set of experimental procedures for comprehensive laboratory evaluation of photocatalysts in the field of environmental purification, using the example of photocatalytic removal of antibiotic organic pollutant molecules from water by phthalocyanine sensitized silver phosphate composites.

Abstract

Various antibiotics such as tetracycline, aureomycin, amoxicillin, and levofloxacin are found in large quantities in groundwater and soil systems, potentially leading to the development of resistant and multi-drug resistant bacteria, posing a threat to humans, animals, and environmental systems. Photocatalytic technology has attracted keen interest due to its rapid and stable treatment and direct use of solar energy. However, most studies evaluating the performance of semiconductor catalysts for the photocatalytic degradation of organic pollutants in water are currently incomplete. In this paper, a complete experimental protocol is designed to comprehensively evaluate the photocatalytic performance of semiconductor catalysts. Herein, rhombic dodecahedral silver phosphate was prepared by a simple solvent phase synthesis method at room temperature and atmospheric pressure. BrSubphthalocyanine/Ag₃PO₄ heterojunction materials were prepared by the solvothermal method. The catalytic performance of as-prepared materials for the degradation of tetracycline was evaluated by studying different influencing factors such as catalyst dosage, temperature, pH, and anions at atmospheric pressure using a 300 W xenon lamp as a simulated solar light source and a light intensity of 350 mW/cm². Compared with the first cycle, the constructed BrSubphthalocyanine/Ag₃PO₄ maintained 82.0% of the original photocatalytic activity after five photocatalytic cycles, while the pristine Ag₃PO₄ maintained only 28.6%. The stability of silver phosphate samples was further tested by a five-cycle experiment. This paper provides a complete process for evaluating the catalytic performance of semiconductor catalysts in the laboratory for the development of semiconductor catalysts with potential for practical applications.

Introduction

Tetracyclines (TCs) are common antibiotics that provide effective protection against bacterial infections and are widely used in animal husbandry, aquaculture, and disease prevention¹^,². They are widely distributed in water due to their overuse and improper application in the past decades, as well as the discharge of industrial wastewater³. This has caused severe environmental pollution and serious risks to human health; for example, the excessive presence of TCs in the aqueous environment can negatively affect microbial community distribution and bacterial resistance, leading to ecological imbalances, mainly due to the highly hydrophilic and bioaccumulative nature of antibiotics, as well as a certain level of bioactivity and stability⁴^,⁵^,⁶. Due to the hyper-stability of TC in the environment, it is difficult to break down naturally; therefore, many methods have been developed, including biological, physicochemical, and chemical treatments⁷^,⁸^,⁹. Biological treatments are highly efficient and low-cost¹⁰^,¹¹. However, because they are toxic to microorganisms, they do not effectively degrade and mineralize antibiotic molecules in water¹². Although physicochemical methods can remove antibiotics from wastewater directly and quickly, this method only converts the antibiotic molecules from the liquid phase to the solid phase, does not completely degrade them, and is too costly¹³.

In contrast to conventional methods, semiconductor photocatalysis has been widely used for the degradation of pollutants in the past decades due to its efficient catalytic degradation properties¹⁴. For example, the noble metal-free magnetic Fe_xMn_y catalyst of Li et al. achieved efficient photocatalytic oxidation of a variety of antibiotic molecules in water without the use of any oxidant¹⁵. Yan et al. reported the in situ synthesis of lily-like NiCo₂O₄ nanosheets on waste biomass-derived carbon to achieve efficient photocatalytic removal of phenolic pollutants from water¹⁶. The technology relies on a semiconductor catalyst excited by light to generate photogenerated electrons (e^–) and holes (h⁺)¹⁷. The photogenerated e^– and h⁺ will be converted into superoxide anion radicals (O₂^–) or hydroxyl radicals (OH^–) by reacting with absorbed O₂ and H₂O, and these oxidatively active species oxidize and decompose organic pollutant molecules in water into CO₂ and H₂O and other smaller organic molecules¹⁸^,¹⁹^,²⁰. However, there is no unified field standard for photocatalyst performance evaluation. The evaluation of a material's photocatalytic performance should be investigated in terms of the catalyst preparation process, environmental conditions for optimal catalytic performance, catalyst recycling performance, etc. Ag₃PO_4, with its prominent photocatalytic ability, has triggered substantial concern in environmental remediation. This new photocatalyst achieves quantum efficiencies of up to 90 % at wavelengths greater than 420 nm, which is significantly higher than previously reported values²¹. However, the severe photo corrosion and unsatisfactory electron-hole separation rate of Ag₃PO₄ limit its wide application²². Therefore, various attempts have been made to overcome these drawbacks, such as shape optimization²³, ion doping²⁴, and heterostructure building²⁵^,²⁶^,²⁷. In this paper, Ag₃PO₄ was modified using morphology control as well as heterojunction engineering. First, rhombic dodecahedral Ag₃PO₄ crystals with high surface energy were prepared by solvent phase synthesis at room temperature under ambient pressure. Then, organic supramolecular BrSubphthalocyanine (BrSubPc), which can act as both electron acceptor and electron donor, was self-assembled on the silver phosphate surface by the solvothermal method²⁸^,²⁹^,³⁰^,³¹^,³²^,³³^,³⁴^,³⁵. The photocatalytic performance of the prepared materials was evaluated by investigating the effect of different environmental factors on the photocatalytic performance of the prepared samples to degrade trace amounts of tetracycline in water. This paper provides a reference for the systematic evaluation of the photocatalytic performance of the materials, which is of significance for the future development of photocatalytic materials for practical applications in environmental remediation.

Protocol

1. Preparation of the BrSubPc

NOTE: The BrSubPc sample was prepared according to a previously published work³⁶. The reaction is carried out in a double-row tube vacuum line system, and the reaction process is strictly controlled under water-free and oxygen-free conditions.

Pre-treatment of raw materials
1. Weigh 2 g of o-dicyanobenzene, dry it in a vacuum oven for 24 h, take it out and then grind it carefully in an agate mortar.
2. Put it again in a vacuum oven for 1 week; then, take it out and put it in a desiccator.
3. Measure 50 mL of o-dichlorobenzene, add 1 g of anhydrous magnesium sulfate, and stir the mixture at room temperature (RT) for 24 h at medium speed.
Then, filter the solution under reduced pressure (-0.1-0.09 MPa), collect the filtrate, and set it aside.
Add pretreated o-dicyanobenzene (10 mmol, 1.28 g) into a 100 mL Schlenk bottle, evacuate the system with a double-row tube vacuum line device, and fill the system with nitrogen. Then, inject 50 mL of pretreated o-dichlorobenzene under magnetic stirring at 1,000 rpm for 1 h to disperse o-dicyanobenzene uniformly.
Put the Schlenk bottle into an ice water bath, then add 1.3 mL of Boron tribromide (BBr₃) under magnetic stirring at 1,000 rpm for 120 min, and observe the color of the reaction system change to dark brown.
Then, quickly switch to an oil bath, raise the temperature to 120 °C reflux for 10 h, and observe the color of the reaction system change from dark brown to bright purple.
Stop heating and cool to RT. Filter the solution under reduced pressure (-0.1-0.09 MPa) and collect the filter cake, with the purple solid on the cake being the crude product.
Put the obtained BrSubPc crude product into a vacuum oven for 20 h. Remove and finely grind the product. Then, extract with 200 mL of methanol solution in a Soxhlet extractor until the solution becomes colorless.

2. Preparation of the Rhombic dodecahedral Ag₃PO₄

NOTE: Rhombic dodecahedral Ag₃PO₄ was prepared according to the previously reported literature³⁵.

Preparation of the reaction solution
1. For NH₄NO₃ solution (0.05 M) named Solution 1, dissolve 6 g of ammonium nitrate (NH₄NO₃, 99%) in 200 mL of deionized water, and treat with ultrasonic waves at 40 Khz frequency, 300 W power for 5 min in one cycle to dissolve it completely. Then, put it into a 500 mL volumetric flask to fix the volume.
2. For NaOH solution (0.2 M) named Solution 2, dissolve 4 g of sodium hydroxide (NaOH, 99%) in 200 mL deionized water in a glass beaker, and sonicate for 5 min at 40 Khz frequency, 300 W power in one cycle to dissolve it fully. Then, put it into a 500 mL volumetric flask to fix the volume.
3. For AgNO₃ solution (0.05 M) named Solution 3, dissolve 4.25 g of silver nitrate (AgNO₃, 99.8%) in 200 mL deionized water in a glass beaker, and sonicate for 5 min at 40 Khz frequency, 300 W power in one cycle to dissolve it fully. Then, put it into a 500 mL volumetric flask to fix the volume.
4. For the K₂HPO₄ solution (0.1 M) named Solution 4, dissolve 11.41 g of potassium hydrogen phosphate (K₂HPO₄, 99.5%) in 400 mL deionized water in a glass beaker, and sonicate for 5 min to dissolve it fully. Then, put it into a 500 mL volumetric flask to fix the volume.
Add 2526 mL of deionized water to a beaker, and then add 180 mL of NH₄NO₃ solution (0.4 M), 54 mL of NaOH solution (0.2 M), and 120 mL of AgNO₃solution (0.05 M) sequentially to the beaker.
Stir the solution vigorously for 10 min to prepare the [Ag(NH₃)₂]⁺ complex. Finally, add 120 mL of K₂HPO₄ solution (0.1 M) to the complex and stir for 5 min. After the color of the solution changes from colorless to light yellow, the precipitate obtained is Ag₃PO₄ rhombic dodecahedral.
Separate the resulting precipitate by centrifugation at 7155.5 x g for 10 min at RT and subsequently centrifuge it three times with 50 mL of deionized water in the same conditions. Store the rhombic decahedral Ag₃PO₄ at RT in a dry environment away from light.

3. Preparation of BrSubPc/Ag ₃PO₄

NOTE: Four different composite ratios of BrSubPc to Ag₃PO₄ were prepared according to the mass ratios of 1:25, 1:50, 1:75, and 1:100.

Dissolve 5.77 mg of BrSubPc in 50 mL of ethanol in a glass beaker. Dissolve the BrSubPc completely by sonication at 40 Khz frequency, 300 W power in one cycle for 30 min at RT.
Then, add 144.25 mg of Ag₃PO₄ to the above solution and sonicate at 40 kHz frequency, 300 W power in one cycle for 30 min at RT.
Stir the above solution in an 80 °C water bath to allow complete evaporation of the ethanol.
Dry the resulting brownish-yellow powder overnight in an oven at 60 °C. The prepared sample is named as BrSubPc/Ag₃PO₄ (1:25).
For the other composite ratio samples (1:50, 1:75, and 1:100), follow the same preparation procedure (steps 3.1-3.4) as BrSubPc/Ag₃PO₄ (1:25) but change the amount of BrSubPc to 2.94 mg, 1.97 mg, and 1.49 mg, and the corresponding amount of Ag₃PO₄ to 147.0 mg, 147.75 mg, and 149.0 mg, respectively.

4. Characterization of the samples

Perform X-ray diffraction analysis of powdered materials using a monochromatic Cu-Kα light source, λ = 0.15418 nm, operating at 30 kV and 15 mA.
Use Fourier transform infrared spectroscopy (FT-IR) to characterize the structural features of the as-prepared materials; the measurement wavelength range is 500-4000 cm^-1.
Measure the absorption properties of the as-prepared materials by solid ultraviolet-visible (UV-vis) absorption spectroscopy in the range of 200-800 nm.
Determine the particle size, microstructure, and morphology of the prepared samples by scanning electron microscopy at 5.00 KV accelerating voltage, InLens detector, magnification 500-13000, working distance 7.4-7.7 mm.
Take 5 mL of the reaction solution after 5 cycles, and fix the volume to 10 mL using concentrated HNO₃. Digest the reaction solution with an inductively coupled plasma-optical emission spectrometer (ICP-OES) at a pump rate of 100 r/min, a nebulizer flow of 28.0 psi, auxiliary gas of 0.5 mL/min and a sample flush time of 20 s.

5. Photocatalytic activity test

NOTE: The light source is a 300 W xenon lamp, and a 400 nm filter is used to remove ultraviolet light from the light source. The xenon lamp was mounted 15 cm above the solution, and the light intensity was determined to be 350 mW/cm².

For the test solution, 10 mg of tetracycline (TC) was dissolved in 500 mL of distilled water to obtain a 20 ppm solution.
Then, transfer 50 mL of the test TC solution to a glass photocatalytic reactor. Stir the solution thoroughly with a magnetic stirrer at 1000 rpm and maintain the temperature at 25 °C. Then, turn the air pump switch on and add the air to the solution at a rate of 100 mL/min to maintain air saturation.
Add 50 mg of the prepared photocatalyst to the test solution to reach a concentration of 1 g/L.
Take the first sample (3 mL) immediately using a glass syringe. After stirring for 30 min in the dark, take the second sample and turn on the light source.
After irradiation for 5 min, 10 min, 15 min, 20 min, and 30 min, take liquid samples (3 mL). Filter all the extracted samples through a 0.22 µm nylon membrane to remove solid particles before analysis. Store the filtered samples away from light in 5 mL centrifuge tubes until analysis.
Measure the concentration of TC with a UV-Vis spectrophotometer at 356 nm. Evaluate the photocatalytic effect by the degradation rate; the specific calculation formula of the degradation rate is as follows (Eq. (1)).
(1)
Where, A₀ is the absorbance of the sample before illumination, A is the absorbance of the sample at illumination time of t min.
Use the same experimental procedures for different catalyst dosages, with starting catalyst amounts as 30 mg, 40 mg, 50 mg, 60 mg, and 70 mg.
For experiments with different pHs, adjust the pH of the tetracycline solution (50 mL, 20 mg/L) between 2.0 and 9.0 with 0.01 mol/L HCl and NaOH solution. Use BrSubPc/Ag₃PO₄ as the catalyst with a catalyst dosage of 50 mg. For other photocatalytic experimental procedures, follow the previously described steps 5.2-5.6.
Investigate the effect of reaction temperature on the photodegradation of tetracycline by using BrSubPc/Ag₃PO₄ as the catalyst with a catalyst dosage of 50 mg and solution pH = 6; the temperature range is 10-50 °C. Other photocatalytic experimental procedures are the same as the previously described steps 5.2-5.6.
Investigate the effects of different anions on the photocatalytic performance of the catalysts by adding 5 mmol/L Na₂SO₄, 5 mmol/L Na₂CO₃, 5 mmol/L NaCl, and 5 mmol/L NaNO₃ to 50 mL of tetracycline solution, respectively. Use BrSubPc/Ag₃PO₄ as the catalyst with a catalyst dosage of 50 mg and solution pH = 7. Other photocatalytic experimental procedures are the same as the previously described steps 5.2-5.6.
After each cycle of photocatalytic degradation reaction, centrifuge the reacted solution at 7155.5 x g for 10 min at RT, and then centrifuge it three times with 10 mL of deionized water in the same conditions (3 x 10 mL). Dry the solid at 120 °C for 1 h. Perform five consecutive photodegradation experiments using photocatalysts that were recovered after each step with no change in the overall concentration of the catalyst to evaluate the stability of the BrSubPc/Ag₃PO₄photocatalyst.

Representative Results

The rhombic dodecahedron Ag₃PO₄ was successfully synthesized using this solvent phase synthesis method. This is confirmed by the SEM images shown in Figure 1A,B. According to the SEM analysis, the average diameter of the rhombic dodecahedral structure was found to be between 2-3 µm. The pristine BrSubPc microcrystals show a large irregular flake structure (Figure 1C). In the composite sample, the titanium dioxide still kept the original nanosphere structure, but no phthalocyanine sheet structure was found, which means that the phthalocyanine molecules were uniformly self-assembled on the titanium dioxide surface (Figure 1D). As shown in Figure 2A, all the samples show a characteristic peak located at 20.9°, 29.7°, 33.3°, 36.6°, 42.5°, 47.8°, 52.7°, 55.0°, 57.3°, 61.6°, 65.8°, 69.9°, 71.9°, and 73.8° which were attributed to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (330), (420), (421) and (332) facets of the body-centered cubic structure of Ag₃PO₄ (JCPDS No. 06-0505)²¹. On the other hand, BrSubPc/Ag₃PO₄ samples did not show additional characteristic peaks of BrSubPc, mainly because of the amount of BrSubPc loaded on the surface of Ag₃PO₄ was low and the intensity of the main diffraction peak of Ag₃PO₄ decreased as the amount of BrSubPc increased. The FT-IR spectra of the as-prepared samples are analyzed as shown in Figure 2B. For BrSubPc, the more abundant characteristic peaks in the FT-IR spectrum are peaks at 743 cm^-1, 868 cm^-1, 943 cm^-1, and 1452 cm^-1; this feature is the stretching and bending vibration of the C-C and C-N bonds of the benzene ring backbone. The weak peak at 624 cm^-1 is the characteristic peak of the stretching of the B-Br bond. The symmetric and asymmetric stretching vibrations of P-O-P caused the same FT-IR peaks at 546 cm^-1 and 931 cm^-1 for pristine Ag₃PO₄ and BrSubPc/Ag₃PO₄, respectively. The pristine Ag₃PO₄ can absorb light at wavelengths less than 530 nm, and BrSubPc has two characteristic peaks at 310 nm and 570 nm, respectively (Figure 2C). Compared with pure Ag₃PO₄, the BrSubPc/Ag₃PO₄ composite sample shows significantly increased absorption in the visible region, confirming that the Ag₃PO₄ particles are successfully covered by BrSubPc microcrystals. This can prove that the BrSubPc/Ag₃PO₄ composite is a very promising visible-light-induced photocatalyst.

The photocatalytic activity of the as-prepared materials was assessed following the degradation of the antibiotic TC in pure water under simulated visible light irradiation (λ > 400 nm). As shown in Figure 3A, the photocatalytic performance of pristine Ag₃PO₄ showed only 72.86% degradation of TC after 0.5 h of visible light irradiation. It can be observed that all the composite photocatalysts showed enhanced degradation of TC when BrSubPc supramolecular nanocrystals were loaded on the surface of Ag₃PO₄. In particular, BrSubPc/Ag₃PO₄ (1:50) achieved 94.54% degradation of TC after 0.5 h of visible light illumination, respectively. A pseudo-first-order reaction model (l−ln (C/C₀) = kt)²⁸, where k is the apparent rate constant, was used to fit the kinetics of photodegradation of TC by different samples. As shown in Figure 3B, the apparent rate constant of TC degradation by BrSubPc/Ag₃PO₄ (1:50) composites was 1.69 times higher than that of the pristine Ag₃PO₄. The above results indicate that the photocatalytic performance of Ag₃PO₄ is significantly improved when Ag₃PO₄ is combined with BrSubPc supramolecular nanocrystals.

The photostability and reusability of photocatalysts are important factors affecting their practical applications, and recycling degradation experiments were conducted on the as-prepared pristine Ag₃PO₄ and BrSubPc/Ag₃PO₄ (1:50) composites. Figure 3C shows that after five cycles of the prepared catalysts, the composite still showed a high TC removal rate of 77.5%. However, the TC removal by pristine Ag₃PO₄ decreased from 72.86% to 20.84%. In addition, XRD analysis of the cycled composite BrSubPc/Ag₃PO₄ (1:50) samples showed that the XRD peaks of the cycled samples did not change compared with the XRD of the original samples (Figure 4), which proved the good stability of the composite samples in the photocatalytic reaction. The ICP-OES test results of the reaction solution after five cycles showed that the concentration of elemental silver in the solution after the reaction of pristine Ag₃PO₄ was 1.3 mg/L, while the concentration of elemental silver in the solution after the reaction of the composite sample of BrSubPc/Ag₃PO₄ (1:50) was 0.1 mg/L (Table 1). This indicates that the composite sample photocatalytic reaction has better stability compared to that of pristine Ag₃PO₄.

In the photocatalytic process, the amount of photocatalyst dosage also has an important influence on the photocatalytic effect, too little dosage may lead to lower light utilization efficiency and poor photocatalytic effect, and too much photocatalyst dosage may lead to higher cost and uneconomical. Too little amount of photocatalyst may lead to lower light utilization efficiency and poor photocatalytic effect, while too much amount of photocatalyst may lead to higher cost and uneconomical treatment of wastewater. Therefore, it is important to determine the optimal photocatalyst dosage. As can be seen from Figure 5A, after 30 min of dark reaction, the adsorption and removal of tetracycline increased as the concentration of the photocatalyst in the reaction solution increased (the dosage increased) because the concentration of tetracycline as the adsorbent in the solution remained the same, while the concentration of the photocatalyst as the adsorbent increased, which means that the active point on the surface of the adsorbent in the solution also increased, and the probability of collision adsorption with the adsorbent increased. This means that the probability of collisional adsorption with the adsorbate increases, resulting in a decrease in the concentration of adsorbate in the solution. The degradation rate of TC by photocatalysts at 0.6 g/L, 0.8 g/L, 1 g/L, 1.2 g/L, and 1.4 g/L was 71.6%, 75.0%, 94.5%, 95.7%, and 95.7% after 30 min of light reaction, respectively. When the concentration of the catalyst exceeded 1.0 g/L, the degradation rate of TC could reach more than 90% in 30 min of photoreaction. From the above analysis, it can be seen that when the concentration of photocatalyst is 1.4 g/L, the best removal effect of tetracycline is achieved, and the photocatalytic effect was not greatly improved compared with the catalyst concentration of 1.0 g/L, while the catalyst dosage was 40% higher. The analysis of the degradation kinetic data in Figure 5B also shows that 1.4 g/L and 1.2 g/L are not significantly different compared to 1.0 g/L. From the economic point of view, the optimal dosage of composite material is 1.0 g/L.

As can be seen in Figure 5C, the effect of pH on the photocatalytic degradation of the composite material for the removal of TC is relatively large. The TC aqueous solution pH was detected to be 6, showing the best degradation efficiency. The photocatalytic performance of the composites was slightly reduced in acidic solutions, while the TC degradation efficiency was more attenuated in neutral and alkaline solutions. The maximum kinetic data for degradation TC can also be seen in Figure 5D at solution pH = 6. In alkaline solutions with high pH, tetracycline will be present in the solution in the form of TC^–, which will have electrostatic repulsion with the catalyst, resulting in poor degradation of tetracycline. In acidic solutions with low pH, tetracycline is mainly present in the solution as TC⁺, and H⁺ will compete with TC⁺ in the solution to be absorbed by the photocatalyst, inhibiting the TC⁺ contact with the photocatalyst, thus reducing the photocatalytic activity in the system.

In reality, antibiotic wastewater often also contains some anions (Cl^–, SO₄^2-, NO₃^–, CO₃^2-, etc.), and these common anions may also affect the photocatalytic process. As can be seen in Figure 5E, the addition of SO₄^2- inhibited the adsorption of TC molecules on the catalyst surface during the dark reaction phase. This may be since SO₄^2-, as a negatively charged anion, competes with the tetracycline molecules for the active site on the photocatalyst surface, resulting in a reduction in the number of tetracycline molecules that can undergo catalytic oxidation or the formation of a highly polar environment close to the photocatalyst surface, preventing the expansion of tetracycline to the active site of the photocatalyst³⁷. When the light reaction was carried out for 30 min, the TC degradation rate in the system without the anion was 94.5%, while in the system with the Cl^–, SO₄^2-, NO₃^–, and CO₃^2- anion, the TC degradation rate was 79.2%, 77.3%, 85%, and 80.3%, respectively. TC degradation kinetic data also reflects the inhibition of TC degradation by the addition of all anions (Figure 5F). The addition of all anions had an inhibitory effect on the photocatalytic degradation of TC, but the TC degradation rate was not overly affected.

The results of the effect of temperature on the photocatalytic degradation of TC are shown in Figure 5G. The degradation rates were 35.3%, 70.6%, 94.5%, 96.5%, and 98.0% for 30 min of photoreaction at 10°C, 20°C, 30°C, 40°C, and 50 °C, respectively. The degradation rate of tetracycline gradually increased with the increase in temperature. The degradation kinetic data for TC from Figure 5H also shows that temperature has a large effect on the degradation efficiency. Tetracycline molecules migrate more quickly as a result of the rising temperature of the solution, making them easier to adsorb when in contact with the catalyst surface. Additionally, at higher temperatures, photogenerated electron-hole pairs more actively, allowing electrons to bind to adsorbed oxygen more quickly and holes to produce hydroxyl radicals with ^–OH in water more quickly, which speeds up the destruction of tetracycline³⁸.

Figure 1: SEM images. (A,B) Ag₃PO₄. The left side shows a low-resolution image, and the right side provides a magnified image. (C) BrSubPc and (D) BrSubPc/Ag₃PO₄. All samples were measured in the powder state. Please click here to view a larger version of this figure.

Figure 2: XRD, FT-IR, and UV-Vis spectra of the samples. (A) XRD patterns. For XRD analysis, the scanning range was 10°-80°, and the scanning speed was 8°/min. The numbers placed vertically at the bottom indicate the corresponding crystal plane. (B) FT-IR spectrum. All samples were tested in the dried powder state. (C) UV-vis spectra of the samples. Solid powders were used for the measurement at a range of 200-800 nm. Please click here to view a larger version of this figure.

Figure 3: TC photocatalytic degradation. (A) TC photocatalytic degradation, the vertical coordinate C₀ indicates the initial absorbance of TC (0.664) measured using a UV-vis spectrophotometer, and C indicates the absorbance of TC at each sampling point. (B) The apparent rate constants k for TC photodegradation of Ag₃PO₄ and BrSubPc/Ag₃PO₄, calculated from the pseudo-first-order reaction model (l-ln(C/C₀) = kt). (C) Cycle experiment of BrSubPc/Ag₃PO₄ (1:50) for TC photocatalytic degradation reaction, the latter reactions are all based on the samples collected after the previous step. Please click here to view a larger version of this figure.

Figure 4: XRD patterns of BrSubPc/Ag₃PO₄. XRD patterns of BrSubPc/Ag₃PO₄ (1:50) before and after the photocatalytic reaction at a scanning range of 10°-80° and a scanning speed of 8°/min. Please click here to view a larger version of this figure.

Figure 5: Exploring TC photocatalytic degradation under the influence of different factors. (A) different catalyst dosages, (C) different pH, (E) different anions and (G) different temperatures. The apparent rate constants k for TC photodegradation using (B) different catalyst dosages, (D) different pH, (F) different anions, and (H) different temperatures. Please click here to view a larger version of this figure.

Sample	Test elements	Sample elemental content (mg/L)
Ag₃PO₄	Ag	1.3
BrSubPc:Ag₃PO₄ (1:50)	Ag	0.1

Table 1: ICP-OES data. Ag elemental concentration data in the reaction solution after five cycles of testing using ICP-OES.

Discussion

In this paper, we present a complete methodology for evaluating the catalytic performance of photocatalytic materials, including the preparation of catalysts, the investigation of factors affecting photocatalysis, and the performance of catalyst recycling. This evaluation method is universal and applicable to all photocatalytic material performance evaluations.

In terms of material preparation methods, many schemes have been reported for the preparation of rhombic dodecahedral Ag₃PO₄ using different precursors²¹^,²². The method we have used is relatively homogeneous in terms of the shape of the Ag₃PO₄ synthesized, the synthesis process is simple, large quantities can be synthesized, and there are fewer factors affecting the experimental process. It should be noted that ammonium nitrate, a raw material for the synthesis of Ag₃PO₄, is an oxidizing agent and is subject to explosive decomposition by violent impact or heat, so it should be stored and used to avoid violent impact. In the synthesis of the composites, BrSubPc was firstly dissolved in sufficient amount of ethanol solution to destroy the weak forces between BrSubPc molecules (hydrogen bonding, π−π interaction), then Ag₃PO₄ was added in an appropriate amount, and the ethanol was evaporated by heating, during which the BrSubPc molecules reassemble themselves on the Ag₃PO₄ surface through intermolecular hydrogen bonding and π−π interaction.

The effect of different catalyst amounts, solution pH, anions in solution, and reaction temperature on the photocatalytic performance of the prepared materials was investigated. The airflow rate, the intensity of the light source, and the distance of the light source from the reactor should be controlled when doing photocatalytic reactions with different influencing factors. When filtering samples using 0.22 µm nylon membrane, it should be noted that not all degradation contaminants are suitable for use with 0.22 µm nylon membrane as some contaminants are inherently blocked by 0.22 µm nylon membrane, in which case centrifugation should be used to separate the catalyst from the reaction solution. Therefore, a 0.22 µm nylon membrane should be used to filter a simple solution of contaminants without a catalyst to exclude the possibility that the contaminants themselves may be blocked by the 0.22 µm nylon membrane.

A catalyst can only be considered to be a promising photocatalyst if it shows good catalytic performance under this evaluation system and not if only a single influencing factor is studied without taking into account environmental factors. In addition, to promote the healthy development of the field of photocatalytic environmental purification, we believe that the same evaluation criteria should be set for the same pollutant, for example, a uniform TC concentration of 20 mg/L, a catalyst dosage of 1 g/L, a light intensity of 350 mW/cm², an airflow rate of 100 mL/min and a temperature of 30 °C should be used for TC degradation, so that the best catalyst for degrading the same pollutant can be selected by comparing different literature reports.

The photocatalytic performance of the photocatalyst is more comprehensive than that reported in some papers³⁹^,⁴⁰^,⁴¹, especially in the laboratory photocatalytic experiments to ensure a stable oxygen content in the water and to take into account the thermal effect. The limitation of this scheme is that it does not consider the effect of reactor optical thickness and catalyst optical properties on photocatalytic performance, both of which are important when performing scale-up labs⁴²^,⁴³^,⁴⁴. This scheme provides a reference for evaluating the removal of antibiotic-like molecules from water by photocatalysts in the laboratory and compensates for the lack of uniform criteria for evaluating the photocatalytic water purification ability of photocatalysts in the field. This research protocol can be extended to other photocatalytic fields, such as photocatalytic hydrogen production and photocatalytic carbon dioxide reduction⁴⁵^,⁴⁶. It is recommended that each field should have a set of strict research protocol criteria for evaluating the catalytic performance of catalysts, which will help to select the best photocatalysts for early experimental industrial applications.

Divulgaciones

The authors have nothing to disclose.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21606180), and the Natural Science Basic Research Program of Shaanxi (Program No. 2019JM-589).

Materials

300 W xenon lamp	CeauLight	CEL-HXF300
AgNO₃	Aladdin Reagent (Shanghai) Co., Ltd.	7783-99-5
Air Pump	Samson Group Co.	ACO-001
BBr₃	Bailingwei Technology Co., Ltd.	10294-33-4
Constant temperature circulating water bath	Beijing Changliu Scientific Instruments Co.	HX-105
Dichloromethane	Tianjin Kemiou Chemical Reagent Co., Ltd.	75-09-2
Ethanol	Tianjin Fuyu Fine Chemical Co., Ltd.	64-17-5
Fourier-transform infrared	Bruker	Vector002
Hexane	Tianjin Kemiou Chemical Reagent Co., Ltd.	110-54-3
HNO₃	Aladdin Reagent (Shanghai) Co., Ltd.	7697-37-2
ICP-OES	Aglient	5110
K₂HPO₄	Aladdin Reagent (Shanghai) Co., Ltd.	16788-57-1
Magnesium Sulfate	Tianjin Kemiou Chemical Reagent Co., Ltd.	10034-99-8
Methanol	Tianjin Kemiou Chemical Reagent Co., Ltd.	67-56-1
NaOH	Aladdin Reagent (Shanghai) Co., Ltd.	1310-73-2
NH₄NO₃	Sinopharm Group Chemical Reagent Co., Ltd.	6484-52-2
o-dichlorobenzene	Tianjin Fuyu Fine Chemical Co., Ltd.	95-50-1
o-dicyanobenzene	Sinopharm Group Chemical Reagent Co., Ltd.	91-15-6
Scanning electron microscopy	JEOL	JSM-6390
Trichloromethane	Tianjin Kemiou Chemical Reagent Co., Ltd.	67-66-3
Ultraviolet-visible Spectrophotometer	Shimadzu	UV-3600
X-ray diffractometer	Rigaku	D/max-IIIA

Referencias

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Wang, B., Zhang, X., Li, L., Ji, M., Zheng, Z., Shi, C., Li, Z., Hao, H. A Complete Method for Evaluating the Performance of Photocatalysts for the Degradation of Antibiotics in Environmental Remediation. J. Vis. Exp. (188), e64478, doi:10.3791/64478 (2022).