Complete removal of Tetracycline by sonophotocatalysis using ultrasound-assisted hierarchical graphitic carbon nitride nanorods with carbon vacancies

Green synthesis of rectangular hollow tubular carbon nitride via in-situ self-assembly strategy to enhance the degradation of tetracycline hydrochloride under visible light irradiation

It has been an urgent requirement for materials with remarkable performance in the photocatalytic degradation of organic contaminants by photocatalytic technology. Limited surface area and speedy recombination rate of photogenerated charge carriers seriously restrain the application of g-C3N4. Morphology control is a powerful approach to enhance the photocatalytic efficiency of g-C3N4. Herein, we reported a method to attain graphitic carbon nitride with rectangular hollow tubular morphology and asperous surface (TUM-CN-2) which is prepared from urea-melamine hydrothermal products and trithiocyanuric acid by self-assembling without using organic solvents or template agents. The specific surface area, photocatalytic activity, and photo-generated carriers migration and separation rate of the obtained photocatalyst TUM-CN-2 are vastly improved. Contrasted with pure g-C3N4, the degradation rate of tetracycline hydrochloride (TCH) and Rhodamine B (RhB) was enhanced about 3.04 and 13.96 times in visible light irradiation, respectively. Moreover, the interference parameters, active free radicals, potential degradation mechanism, and degradation paths of TCH were researched systematically. This work provides a green way to acquire the modified g-C3N4 with splendid catalytic activity through the self-assembly method.

Nitrogen-Doped Graphene Quantum Dot-Passivated δ-Phase CsPbI3: A Water-Stable Photocatalytic Adjuvant to Degrade Rhodamine B

Inorganic halide perovskite CsPbI3 is highly promising in the photocatalytic field for its strong absorption of UV and visible light. Among the crystal phases of CsPbI3, the δ-phase as the most aqueous stability; however, directly using it in water is still not applicable, thus limiting its dye photodegradation applications in aqueous solutions. Via adopting nitrogen-doped graphene quantum dots (NGQDs) as surfactants to prepare δ-phase CsPbI3 nanocrystals, we obtained a water-stable material, NGQDs-CsPbI3. Such a material can be well dispersed in water for a month without obvious deterioration. High-resolution transmission electron microscopy and X-ray diffractometer characterizations showed that NGQDs-CsPbI3 is also a δ-phase CsPbI3 after NGQD coating. The ultraviolet-visible absorption spectra indicated that compared to δ-CsPbI3, NGQDs-CsPbI3 has an obvious absorption enhancement of visible light, especially near the wavelength around 521 nm. The good dispersity and improved visible-light absorption of NGQDs-CsPbI3 benefit their aqueous photocatalytic applications. NGQDs-CsPbI3 alone can photodegrade 67% rhodamine B (RhB) in water, while after compositing with TiO2, NGQDs-CsPbI3/TiO2 exhibits excellent visible-light photocatalytic ability, namely, it photodegraded 96% RhB in 4 h. The strong absorption of NGQDs-CsPbI3 in the visible region and effective transfer of photogenerated carriers from NGQDs-CsPbI3 to TiO2 play the key roles in dye photodegradation. We highlight NGQDs-CsPbI3 as a water-stable halide perovskite material and effective photocatalytic adjuvant.

Preparation of high-crystalline and non-metal modified g-C3N4 for improving ultrasound-accelerated white-LED-light-driven photocatalytic performances

As a non-metallic organic semiconductor, graphitic carbon nitride (g-C3N4) has received much attention due to its unique physicochemical properties. However, the photocatalytic activity of this semiconductor faces challenges due to factors such as low electronic conductivity and limited active sites provided on its surface. The morphology and structure of g-C3N4, including macro/micro morphology, crystal structure and electronic structure can affect its catalytic activity. Non-metallic heteroatom doping is considered as an effective method to tune the optical, electronic and other physicochemical properties of g-C3N4. Here, we synthesized non-metal-doped highly crystalline g-C3N4 by one-pot calcination method, which enhanced the photocatalytic activity of g-C3N4 such as mesoporous nature, reduced band gap, wide-range photousability, improved charge carrier recombination, and the electrical conductivity was improved. Hence, the use of low-power white-LED-light illumination (λ ≥ 420 nm) and ultrasound (US) irradiation synergistically engendered the Methylene Blue (MB) mineralization efficiency elevated to 100% within 120 min by following the pseudo-first-order mechanism under the following condition (i.e., pH 11, 0.75 g L-1 of O-doped g-C3N4 and S-doped g-C3N4, 20 mg L-1 MB, 0.25 ml s-1 O2, and spontaneous raising temperature). In addition, the rapid removal of MB by sonophotocatalysis was 4 times higher than that of primary photocatalysis. And radical scavenging experiments showed that the maximum distribution of active species corresponds to superoxide radical [Formula: see text]. More importantly, the sonophotocatalytic degradation ability of O-doped g-C3N4 and S-doped g-C3N4 was remarkably sustained even after the sixth consecutive run.

Effect of ultrasound power on HCl leaching kinetics of impurity removal of aphanitic graphite

This study aimed to investigate the effect of ultrasonic power and temperature on the impurity removal rate during conventional and ultrasonic-assisted leaching of aphanitic graphite. The results showed that the ash removal rate increased gradually (∼50 %) with the increase in ultrasonic power and temperature but deteriorated at high power and temperature. The unreacted shrinkage core model was found to fit the experimental results better than other models. The Arrhenius equation was used to calculate the finger front factor and activation energy under different ultrasonic power conditions. The ultrasonic leaching process was significantly influenced by temperature, and the enhancement of the leaching reaction rate constant by ultrasound was mainly reflected in the increase of the pre-exponential factor A. Ultrasound treatment improved the efficiency of impurity mineral removal by destroying the inert layer formed on the graphite surface, promoting particle fragmentation, and generating oxidation radicals. The poor reactivity of hydrochloric acid with quartz and some silicate minerals is a bottleneck limiting the further improvement of impurity removal efficiency in ultrasound-assisted aphanitic graphite. Finally, the study suggests that introducing fluoride salts may be a promising method for deep impurity removal in the ultrasound-assisted hydrochloric acid leaching process of aphanitic graphite.

Efficient photocatalytic remediation of typical antibiotics in water via Mn3O4 decorated carbon nitride nanotube

Antibiotic abuse will seriously affect the ecology and environment. Photocatalytic oxidation technology based on carbon nitride (g-C₃N₄) has been widely adopted to treat wastewater containing antibiotics. Here, a novel composite photocatalyst MCNT was prepared by loading manganese oxide (Mn₃O₄) on the surface of g-C₃N₄ nanotubes (CNT). Three typical antibiotics, trimethoprim (TMP), norfloxacin (NOR), and tetracycline (TC) were used as model contaminants to evaluate the oxidative properties of prepared materials. Compared with bulk g-C₃N₄, the degradation rates of TMP, NOR, and TC catalyzed by MCNT-5 were increased by 2, 3, and 1.4 times, respectively, mainly due to 1) the larger specific surface area of the nanotube structure of CNT, which provides abundant active sites for antibiotic adsorption and catalytic oxidation, and 2) the loading of Mn₃O₄, which promotes the directional migration of photogenerated charges and improves the separation efficiency of photogenerated electrons and holes. The free radical capture and quenching experiments confirmed that MCNT degraded the target organic pollutants with hydroxyl radical (·OH) and singlet oxygen (¹O₂) as the main active oxidants. This catalyst maintained 80% photocatalytic oxidation performance after five cyclic experiments. This study provides new insights into developing efficient, stable, and environmentally-friendly photocatalysts and provides a new dimension to mitigate the antibiotic pollution problem.

Combined conversion of lignocellulosic biomass into high-value products with ultrasonic cavitation and photocatalytic produced reactive oxygen species - A review

The production of high-value products from lignocellulosic biomass is carried out through the selective scission of crosslinked CC/CO bonds. Nowadays, several techniques are applied to optimize biomass conversion into desired products with high yields. Photocatalytic technology has been proven to be a valuable tool for valorizing biomass at mild conditions. The photoproduced reactive oxygen species (ROSs) can initiate the scission of crosslinked bonds and form radical intermediates. However, the low mass transfer of the photocatalytic process could limit the production of a high yield of products. The incorporation of ultrasonic cavitation in the photocatalytic system provides an exceptional condition to boost the fragmentation and transformation of biomass into the desired products within a lesser reaction time. This review critically discusses the main factors governing the application of photocatalysis for biomass valorization and tricks to boost the selectivity for enhancing the yield of desired products. Synergistic effects obtained through the combination of sonolysis and photocatalysis were discussed in depth. Under ultrasonic vibration, hot spots could be produced on the surface of the photocatalysts, improving the mass transfer through the jet phenomenon. In addition, shock waves can assist the dissolution and mixing of biomass particles.

Oxidation of micropollutants by visible light active graphitic carbon nitride and ferrate(VI): Delineating the role of surface delocalized electrons

The treatment of recalcitrant micropollutants in water remains challenging. Ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) has emerged as a green oxidant to oxidize organic molecules, however, its reactivity with recalcitrant micropollutants are sluggish. Our results demonstrate enhanced oxidation of carbamazepine (CBZ) by three types of visible light-responsive graphitic carbon nitride (g-C₃N₄) photocatalyst in absence and presence of ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) under mild alkaline conditions. The g-C₃N₄ photocatalysts were prepared by thermal process using urea, thiourea, and melamine and were named as CN-U, CN-T, and CN-M, respectively. The degradation efficiency of CBZ, in both visible light-g-C₃N₄ and visible light-g-C₃N₄-Fe^VIO₄^2- systems followed the order of CN-U > CN-T > CN-M. The mechanisms for this trend was elucidated by measuring physiochemical properties of the microstructures with various surface and analytical techniques. Results suggest the dominating role of specific surface area and surface delocalized electrons of microstructures in degrading CBZ. Crystallinity, morphology, and surface functional groups may not directly associate with CBZ degradation. The CN-U has higher specific surface area and surface delocalized electrons than CN-T and CN-M and therefore the highest degradation efficiency of CBZ. The surface electrons likely generated O₂^●- and ¹O₂ in the visible light-g-C₃N₄ system. The additional oxidants, Fe^V and Fe^IV in the visible light-g-C₃N₄- Fe^VIO₄^2- system led to higher degradation efficiency than the visible light-g-C₃N₄ system. Results suggest that the surfaces of g-C₃N₄ may be prepared preferentially with high levels of delocalized electrons at the surface of microstructures to enhance degradation of micropollutants.

Mechanisms of biochar enhanced Cu2O photocatalysts in the visible-light photodegradation of sulfamethoxazole

Cu₂O nanoparticles are decorated with biochars derived from spent coffee grounds (denoted as Cu₂O/SCG) and applied as visible-light-active photocatalysts in the sulfamethoxazole (SMX) degradation. The physicochemical properties of Cu₂O/SCG are identified by various spectral analysis, electrochemical and photochemical techniques. As a result, the Cu₂O/SCG exhibits the higher removal efficiency of SMX than the pristine Cu₂O under visible light irradiation. We can observe that Cu₂O could be incorporated onto the SCG biochars with rich oxygen vacancies/adsorbed hydroxyl groups. In addition, the Cu₂O/SCG has the lower charge transfer resistance, faster interfacial electron transfer kinetics, decreased recombination of charge carriers and superior absorbance of visible light. The construction of band diagrams for Cu₂O/SCG and pristine Cu₂O via UV-vis spectra and Mott-Schottky plots suggest that the band energy shifts and higher carrier density of Cu₂O/SCG may be responsible for the photocatalytic activity enhancements. From the radical scavenger experiments and electron paramagnetic resonance spectra, the aforementioned energy shifts could decrease the energy requirement of transferring photoinduced electrons to the potential for the formation of active superoxide radicals (·O₂^-) via one and two-electron reduction routes in the photocatalytic reaction. A proposed degradation pathway shows that ·O₂^- and h⁺ are two main active species which can efficiently degrade SMX into reaction intermediates by oxidation, hydroxylation, and ring opening. This research demonstrates the alternative replacement of conventional carbon materials for the preparation of biochar-assisted Cu₂O photocatalysts which are applied in the environmental decontamination by using solar energy.

Revealing the charge transfer mechanism in magnetically recyclable ternary g-C3N4/BiOBr/Fe3O4 nanocomposite for efficient photocatalytic degradation of tetracycline antibiotics

Pharmaceutical compounds in water bodies pose hazards to the ecosystem because of their biotoxicity potency. To eradicate such pharmaceutical compounds, a novel g-CN/BiOBr/Fe₃O₄ nanocomposites was prepared using a simplistic route and appraised for photodegradation of model tetracycline antibiotics. The g-CN/BiOBr/Fe₃O₄ nanocomposites exhibited complete tetracycline degradation in just 60 min exposure of simulated light irradiation, which is 6 times higher than the g-CN. Under the analogous condition, the tetracycline mineralization ability of the g-CN/BiOBr/Fe₃O₄ nanocomposites was evaluated to be 78% of total organic carbon removal. The superior photocatalytic performance is ascribed to the extended visible light harvesting ability and enhanced charge carrier separation/transfer with impeded recombination rate in light of effective indirect Z-scheme heterojunction construction. Based on band-edge potential and radical trapping studies indicate that h⁺ > •O₂^- > •OH are the active species responsible for photodegradation. Furthermore, the ternary nanocomposites are magnetically retrievable and recyclable while retaining their stable photocatalytic performance. This work endows a new perspective on the rational design and construction of magnetically recoverable ternary nanocomposite for environmental remediation.

Sonocatalytic degradation of ciprofloxacin using hydrogel beads of TiO2 incorporated biochar and chitosan

Pharmaceuticals are necessary to be removed from environment. Herein TiO₂ incorporated biochar made from pyrolysis of agricultural wastes was encapsulated into chitosan to obtain a novel hydrogel beads. This hydrogel beads executed a dual role as both adsorbent and sonocatalyst, which proved to be suitable for the removal of antibiotic ciprofloxacin (CIP) from water. The results showed that adsorption of CIP followed pseudo first order kinetics model and Langmuir adsorption isotherm model, having maximum adsorption at pH 9. Whereas the degradation was more efficient at pH 6 due to greater standard potential for ^•OH/H₂O in acidic media. The degradation was maximum at 150 W of ultrasonic power, then decreased in presence of dissimilar electrolytes and even reduced to 0 in presence of Na₃PO₄. Different quenchers such as benzoquinone (BQ), Triethanolamine (TEA) and isopropyl alcohol (IPA) reduced degradation efficiency (DE) and mineralization efficiency (ME). The DE was decreased from 85.23% to 81.50% (BQ), 74.27% (TEA), and 61.77% (IPA) within 25 min. The prepared sonocatalyst was capable of regeneration with DE, remaining sufficiently high (62%) even after four regeneration steps. These results indicate that titanium-biochar/chitosan hydrogel beads (TBCB) are durable and effective for long-term CIP removal.