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

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 prevention^1,2. 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 stability^4,5,6. 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 treatments^7,8,9. Biological treatments are highly efficient and low-cost^10,11. 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 molecules^18,19,20. 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 building^25,26,27. 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 method^{28,29,30,31,32,33,34,35}. 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.

1. 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.
2. Then, filter the solution under reduced pressure (-0.1-0.09 MPa), collect the filtrate, and set it aside.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.

1. 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.
2. 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 AgNO₃ solution (0.05 M) sequentially to the beaker.
3. 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.
4. 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.

1. 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.
2. 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.
3. Stir the above solution in an 80 °C water bath to allow complete evaporation of the ethanol.
4. Dry the resulting brownish-yellow powder overnight in an oven at 60 °C. The prepared sample is named as BrSubPc/Ag₃PO₄ (1:25).
5. 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

1. 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.
2. 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.
3. Measure the absorption properties of the as-prepared materials by solid ultraviolet-visible (UV-vis) absorption spectroscopy in the range of 200-800 nm.
4. 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.
5. 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.

1. For the test solution, 10 mg of tetracycline (TC) was dissolved in 500 mL of distilled water to obtain a 20 ppm solution.
2. 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.
3. Add 50 mg of the prepared photocatalyst to the test solution to reach a concentration of 1 g/L.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. 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/Ag₃PO₄ photocatalyst.

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...

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 Ag₃PO...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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