Effect of dissolved organic matters and inorganic ions on TiO2 photocatalysis of diclofenac: mechanistic study and degradation pathways

Adsorption effects of electron scavengers and inorganic ions on catalysts for catalytic oxidation of sulfamethoxazole in radiation treatment

This study aimed to investigate adsorption effects of electron scavengers (H2O2 and S2O82-) on oxidation performance for mineralization of sulfamethoxazole (SMX) in radiation treatment using catalysts (Al2O3, TiO2). Hydrogen peroxide (H2O2, 1 mM) as an electron scavenger showed weak adsorption onto catalysts (0.012 mmol g-1-Al2O3 and 0.004 mmol g-1-TiO2, respectively), leading to an increase in TOC removal efficiency of SMX within the absorbed dose of 30 kGy by 12.3% with Al2O3 and by 8.0% with TiO2. The weak adsorption of H2O2 onto the catalyst allowed it to act as an electron scavenger, promoting indirect decomposition reactions. However, high adsorption of S2O82- (1 mM) onto Al2O3 (0.266 mmol g-1-Al2O3) showed a decrease in TOC removal efficiency of SMX from 76.2% to 30.2% within the absorbed dose of 30 kGy. The high adsorption of S2O82- onto the catalyst inhibited direct decomposition reaction by reducing adsorption of SMX on catalysts. TOC removal efficiency for Al2O3 without electron scavengers in an acidic condition was higher than that in a neutral or alkaline condition. However, TOC removal efficiency for Al2O3 with S2O82- was higher in a neutral condition than in other pH conditions. This indicates that the pH of a solution plays a critical role in the catalytic oxidation performance by determining surface charges of catalysts and yield of reactive radicals produced from water radiolysis. In the radiocatalytic system, H2O2 enhances the oxidation performance of catalysts (Al2O3 and TiO2) over a wide pH range (3-11). Meanwhile, S2O82- is not suitable with Al2O3 in acidic conditions because of its strong adsorption onto Al2O3 in this study.

Complete Biodegradation of Diclofenac by New Bacterial Strains: Postulated Pathways and Degrading Enzymes

The accumulation of xenobiotic compounds in different environments interrupts the natural ecosystem and induces high toxicity in non-target organisms. Diclofenac is one of the commonly used pharmaceutical drugs that persist in the environment due to its low natural degradation rate and high toxicity. Therefore, this study aimed to isolate potential diclofenac-degrading bacteria, detect the intermediate metabolites formed, and determine the enzyme involved in the degradation process. Four bacterial isolates were selected based on their ability to utilize a high concentration of diclofenac (40 mg/L) as the sole carbon source. The growth conditions for diclofenac degradation were optimized, and bacteria were identified as Pseudomonas aeruginosa (S1), Alcaligenes aquatilis (S2), Achromobacter spanius (S11), and Achromobacter piechaudii (S18). The highest percentage of degradation was recorded (97.79 ± 0.84) after six days of incubation for A. spanius S11, as analyzed by HPLC. To detect and identify biodegradation metabolites, the GC-MS technique was conducted for the most efficient bacterial strains. In all tested isolates, the initial hydroxylation of diclofenac was detected. The cleavage step of the NH bridge between the aromatic rings and the subsequent cleavage of the ring adjacent to or in between the two hydroxyl groups of polyhydroxylated derivatives might be a key step that enables the complete biodegradation of diclofenac by A. piechaudii S18, as well as P. aeruginosa S1. Additionally, the laccase, peroxidase, and dioxygenase enzyme activities of the two Achromobacter strains, as well as P. aeruginosa S1, were tested in the presence and absence of diclofenac. The obtained results from this work are expected to be a useful reference for the development of effective detoxification bioprocesses utilizing bacterial cells as biocatalysts. The complete removal of pharmaceuticals from polluted water will stimulate water reuse, meeting the growing worldwide demand for clean and safe freshwater.

Highly efficient photocatalytic H2O2 generation over dysprosium oxide-integrated g-C3N4 nanosheets with nitrogen deficiency

The increasing global crisis considers energy as the fundamental cause to conduct extensive research work to find clean alternative methods with high capabilities such as H₂O₂ synthesis. Photocatalytic H₂O₂ production can tackle this growing issue by maintaining environmental remediation. In this work, dysprosium oxide (Dy-oxide)-integrated g-C₃N₄ has been synthesized and characterized with XRD, SEM, TEM, XPS, EPR, DRS, PL, and electrochemical analyses. Simulated solar light irradiation implemented photocatalytic H₂O₂ production using the as-prepared catalysts. The facile preparation technique in the Ar atmosphere raises more N deficiency in the g-C₃N₄ matrix. N-deficient g-C₃N₄ nanosheets with an exceptionally high photocatalytic performance can be further enhanced by integrating well-dispersed Dysprosium oxide (Dy₂O₃) particles onto g-C₃N₄. This study reports bandgap narrowing and various surface defects on g-C₃N₄ with trace amounts of Dy₂O₃. Undoped g-C₃N₄ (Dy0) yielded 20.27 mM⋅g^-1⋅h^-1, while the optimized photocatalyst Dy15 showed high performance of H₂O₂ production up to 48.36 mM⋅g^-1⋅h^-1. It is approximately 2.4 times higher than the pristine g-C₃N₄. Dy15 proves the positive impact of Dy-oxide on enhancing the N-deficient g-C₃N₄ performance towards photocatalytic H₂O₂ production. This work highlights the oxygen reduction reaction (ORR) through a mixed pathway of well-known two-step one-electron and one-step two-electron processes in H₂O₂ generation.

Bi@BiOx(OH)y modified oxidized g-C3N4 photocatalytic removal of tetracycline hydrochloride with highly effective oxygen activation

A novel Bi@BiOx(OH)y-modified oxidized g-C₃N₄ photocatalyst was successfully prepared via wet chemical reduction under alkaline conditions for the tetracycline hydrochloride removal. The prepared materials were characterized comprehensively and fully. Sufficient structural representation analyses confirmed the successful loading of Bi in the form of Bi@BiOx(OH)y complex beads. Based on basic photocatalytic studies, 10% (mass percentage) was found to be the best metal Bi loading. DRS, PL, transient photocurrent and EIS have explored the improvement of the photochemical properties of materials by loading Bi@BiOx(OH)y groups, particularly the improvement of photocatalytic properties by the SPR effect and electron traps. 10%Bi-OxCN exhibited the most suitable particle size of nonagglomerated Bi-metal groups, the largest specific surface area (43.53 m² g^-1), the most adsorption sites and the most significant photocurrent (8.694 × 10^-2 mA cm^-2) (7.78 times that of OxCN). This indicated that 10%Bi-OxCN had good adsorption capacity and excellent light response capability. In addition, 10%Bi-OxCN showed the best tetracycline hydrochloride removal efficiency (96.0%), with ∙O₂^- as the main active substance and ¹O₂ as the second most important substance made of ∙O₂^- and h⁺. The excellent photocatalytic effect and good reusability were fundamentally dependent on the modification of OxCN by Bi@BiOx(OH)y groups to produce a large number of active substances (including the separation efficiency of electron-hole pairs and the generation efficiency of ∙O₂^- and ¹O₂). These advantages are all related to the high specific surface area, a large number of active sites, narrow bandgap width, Bi-SPR effect, and BiOx(OH)y electron trap caused by successful loading of Bi groups.

Considering photocatalytic activity of Cu2+/biochar-doped TiO2 using corn straw as sacrificial agent in water decomposition to hydrogen

In this paper, a simple one-pot thermal synthesis method was used to successfully prepare Cu²⁺/biochar-doped TiO₂ composite catalytic materials. The photocatalytic hydrogen production performance of the composites under different environmental conditions (dark, solar, and visible light irradiation) was analyzed in a biomass photocatalytic system using a corn straw suspension as a sacrificial agent. The Cu²⁺/biochar-doped TiO₂ materials were characterized by SEM, TEM, XRD, FT-IR, XPS, and UV analysis. The photoelectric properties of the Cu²⁺/biochar-doped TiO₂ composites were also analyzed, and the charge separation mechanism of photogenerated carriers under different environmental conditions was investigated. Compared with pure TiO₂, the hydrogen production rate of Cu²⁺/biochar-doped TiO₂ is 23.6 times higher under visible light irradiation and 16.8 times higher under simulated solar irradiation. Using density functional theory, a crystal structure model of Cu²⁺/biochar-doped TiO₂ was established to analyze its energy band structure and density of states. An analysis of the mechanism shows that under simulated sunlight irradiation, the synergistic effect of the TiO₂ doped with Cu²⁺ and biochar causes the formation of a potential Schottky heterojunction on the surface and induces interfacial charge transfer. Furthermore, under visible light irradiation, the photocatalytic production of hydrogen by the Cu²⁺/biochar-doped TiO₂ composite is mainly due to the surface plasmon resonance mechanism of Cu ion-doped TiO₂.

Oxygen doping through oxidation causes the main active substance in g-C3N4 photocatalysis to change from holes to singlet oxygen

Improving the photocatalytic activity of graphitic carbon nitride (g-C₃N₄) for organic pollutant removal from water and in-depth study of the effect of oxygen doping on its photocatalytic mechanism are deserving of research attention. Oxygen-doped g-C₃N₄ with a suitable degree of oxidation was prepared by oxidation using concentrated acid-ultrasound double oxidation. Oxygen doping by oxidation changed the physical and chemical properties of g-C₃N₄, and improved its rhodamine B photocatalytic degradation efficiency. The physical and chemical properties of g-C₃N₄ were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and zeta potential analysis, among other methods. The photocatalytic mechanism was also studied in depth using photoluminescence, electrochemical impedance spectroscopy, transient photocurrent measurements, diffuse reflectance spectroscopy, and Mott-Schottky plots. An appropriate degree of oxidation introduced pit-like defects rich in oxygen-containing functional groups onto the g-C₃N₄ surface, which improves its photocatalytic performance. Furthermore, in the photocatalytic process of oxidized g-C₃N₄, oxygen doping was confirmed to change the main active substance of g-C₃N₄ photocatalysis from holes to singlet oxygen. This study provides an in-depth explanation of the photocatalytic mechanism of g-C₃N₄ doped with oxygen, which provides guidance and reference for the design of photocatalysts for organic pollutant removal from water and the analysis of photocatalytic mechanisms for water treatment.

Efficient photoelectrocatalytic performance of beta-cyclodextrin/graphene composite and effect of Cl− in water: degradation for bromophenol blue as a case study

Photoelectrocatalytic technology has proven to be an efficient way of degrading organic contaminants, including dyes. Graphene (GR) -based catalysts have been frequently used in photoelectrocatalysis, due to their excellent catalytic performances. In this work, the GR/beta-cyclodextrin (GR/β-CD) composite was prepared and used for a widely used triphenylmethane dye (bromophenol blue, BPB) photoelectrocatalytic degradation. The results indicated that the degradation of the prepared GR/β-CD composite for BPB was effective with the combination of external bias voltage and simulated sunlight irradiation. Under optimum conditions, the BPB (10 mg L⁻¹) was completely eliminated by GR/β-CD composite within 120 min. ˙O₂⁻ played a prominent role in the BPB photoelectrocatalytic degradation. The time required for the removal of BPB in water to reach 100% can be reduced to 30 min with the presence of Cl⁻, owing to the generation of ˙Cl. Moreover, the toxicity of the degraded system with Cl⁻, predicted by the QSAR (Quantitative Structure–Activity Relationship) model in ECOSAR (Ecological Structure–Activity Relationships) program, was weaker than that without Cl⁻. The prepared GR/β-CD composite revealed great advantages in photoelectrocatalytic degradation of organic pollutants due to its metal-free, low cost, simplicity, and efficient performance. This work provided new insight into the efficient and safe degradation of organic pollutants in wastewaters.

O₂˙⁻ played a crucial role in the photoelectrocatalytic degradation of BPB by the prepared GR/β-CD. Cl⁻ marginally promoted the degradation of BPB and chlorinated intermediates were generated.