Photochemical reactivity of nitrogen-doped biochars under simulated sunlight irradiation: Generation of singlet oxygen

New Insights into Aggregation Behaviors of the UV-Irradiated Dissolved Biochars (DBioCs) in Aqueous Environments: Effects of Water Chemistries and Variation in the Hamaker Constant

The aggregation behavior of ubiquitous dissolved black carbon (DBC) largely affects the fate and transport of its own contaminants and the attached contaminants. However, the photoaging processes and resulting effects on its colloidal stability remain yet unknown. Herein, dissolved biochars (DBioCs) were extracted from common wheat straw biochar as a proxy for an anthropogenic DBC. The influences of UV radiation on their aggregation kinetics were systematically investigated under various water chemistries (pH, electrolytes, and protein). The environmental stability of the DBioCs before and after radiation was further verified in two natural water samples. Hamaker constants of pristine and photoaged DBioCs were derived according to Derjaguin-Landau-Verwey-Overbeek (DLVO) prediction, and its attenuation (3.19 ± 0.15 × 10-21 J to 1.55 ± 0.07 × 10-21 J after 7 days of radiation) was described with decay kinetic models. Pearson correlation analysis revealed that the surface properties and aggregation behaviors of DBioCs were significantly correlated with radiation time (p < 0.05), indicating its profound effects. Based on characterization and experimental results, we proposed a three-stage mechanism (contended by photodecarboxylation, photo-oxidation, and mineral exposure) that DBioCs might experience under UV radiation. These findings would provide an important reference for potential phototransformation processes and relevant behavioral changes that DBC may encounter.

Chitin-derived biochar with nitrogen doping to activate persulfate for phenol degradation: Application potential and electron transfer pathway in system

The fast and efficient removal of organic pollutants (e.g., phenolics) remains one of the focus problems in environment pollution. Thus, a chitin-derived biochar with nitrogen doping (N-BC) was successfully prepared at a lower calcination temperature of 600 °C, which is environmentally friendly and energy saving. The N-BC was analyzed by SEM, FTIR, BET, XRD, XPS and Raman spectroscopy to confirm that the doping of nitrogen element provided sufficient defect sites to promote the activation of persulfate (PDS). Quenching experiments and EPR results revealed the presence of •OH and •O₂^- contributed to phenol degradation in N-BC 600/PDS system. In addition, the linear sweep voltammogram experiments also demonstrated the existence of electron transfer pathway. The electrons were donated from phenol and shifted to PDS through N-BC. The graphitic N and carbon defects in N-BC served as the active sites of the reaction and involved absorption and transfer of electrons as the key character. Moreover, the removal rates of phenol and TOC reached 98.8% and 58.2% within 2 h, indicating that N-BC effectively activated the persulfate to degrade phenol. This study provides the theoretical support and potential applications for the activation of persulfate by nitrogen-doped biochar to degrade other phenolic compounds.

Nitrogen-doped carbon materials prepared using different organic precursors as catalysts of peroxymonosulfate to degrade sulfamethoxazole: First-time performance leading to the incorrect selection of the best catalyst

Nitrogen-doped carbon materials are effective catalysts for peroxymonosulfate (PMS) activation to eliminate organic contaminants. In this research, the activity of nitrogen-doped carbon materials was significantly improved by optimizing the carbon source, and the reusability of the catalyst is used to select the best catalyst instead of depending on the performance in the first use, for avoiding the "short-life" catalyst with great initial activity. Fixing ferric nitrate nonahydrate and melamine as the metal and nitrogen sources, four catalysts were prepared using glucose, glucosamine hydrochloride, dopamine, and trimesic acid as the carbon sources, respectively. Based on the performance in PMS activation for sulfamethoxazole (SMX) removal, in the first use, the activity was Fe-DA-CN (carbon source: dopamine) > Fe-BTC-CN (carbon source: trimesic acid) > Fe-GLU-CN (carbon source: glucosamine) > Fe-DGLU-CN (carbon source: glucose). With no washing for the second time use, the activity was Fe-BTC-CN (0.135 min^-1) ≫ Fe-DA-CN (0.037 min^-1) > Fe-GLU-CN (0.032 min^-1) > Fe-DGLU-CN (0.017 min^-1). The large specific surface area, superior graphitization, and high CO/C-N group content endow Fe-BTC-CN with high ability in PMS activity. Surface-bound radicals are responsible for SMX elimination, and most of the SMX degradation intermediates have lower ecotoxicity than SMX.

Application of biochar-based photocatalysts for adsorption-(photo)degradation/reduction of environmental contaminants: mechanism, challenges and perspective

The fast increase of population results in the quick development of industry and agriculture. Large amounts of contaminants such as metal ions and organic contaminants are released into the natural environment, posing a risk to human health and causing environment ecosystem problems. The efficient elimination of contaminants from aqueous solutions, photocatalytic degradation of organic pollutants or the in-situ solidification/immobilization of heavy metal ions in solid phases are the most suitable strategies to decontaminate the pollution. Biochar and biochar-based composites have attracted multidisciplinary interests especially in environmental pollution management because of their porous structures, large amounts of functional groups, high adsorption capacities and photocatalysis performance. In this review, the application of biochar and biochar-based composites as adsorbents and/or catalysts for the adsorption of different contaminants, adsorption-photodegradation of organic pollutants, and adsorption-(photo)reduction of metal ions are summarized, and the mechanism was discussed from advanced spectroscopy analysis and DFT calculation in detail. The doping of metal or metal oxides is the main strategy to narrow the band gap, to increase the generation and separation of photogenerated e−-h+ pairs, to produce more superoxide radicals (·O2−) and hydroxyl radicals (·OH), to enhance the visible light absorption and to increase photocatalysis performance, which dominate the photocatalytic degradation of organic pollutants and (photo)reduction of high valent metals to low valent metals. The biochar-based composites are environmentally friendly materials, which are promising candidates in environmental pollution cleanup. The challenge and perspective for biochar-based catalysts are provided in the end.

Graphical Abstract

S-scheme BiVO4/CQDs/β-FeOOH photocatalyst for efficient degradation of ofloxacin: Reactive oxygen species transformation mechanism insight

Photocatalysis technology exhibited promising application for advanced treatment of wastewater. Nevertheless, the design of efficient photocatalyst and the mechanism of free radicals in pollutant degradation still remained to be further investigated. Herein, BiVO₄/CQDs/β-FeOOH photocatalyst was fabricated by electrostatic self-assembly method, which exhibited the excellent photocatalytic performance. Under visible-light irradiation, the removal rate of ofloxacin by BiVO₄/CQDs/β-FeOOH (0.25 min^-1) was 1.93 times than pristine BiVO₄, and the removal efficiency in 15 min reached 99.21%. The perfect reusability of BiVO₄/CQDs/β-FeOOH was ascribed to the persistent catalytic active centers provided by the renewable surface oxygen vacancies on the β-FeOOH. As electron transfer channels, CQDs facilitated the transfer of BiVO₄ photogeneration electrons. The matched band structure allowed the construction of S-scheme heterojunctions, and the higher conduction band position was retained while the carrier separation was promoted. More importantly, this work firstly reported the phenomenon that the main reactive groups in the photocatalysis process would be directionally transformed with the change of pH conditions. Based on the analysis of capture and electron paramagnetic resonance experiments, ·O₂^- was the main free radicals to photodegrade OFL in neutral and alkaline conditions. However, when the solution pH turned into acidic, the photodegradation of OFL was dominated by ¹O₂. This innovative phenomenon was due to that acidic condition accelerated the reaction kinetics of spontaneous transformation of ·O₂^- to ¹O₂ and inhibited the direct oxidation of pollutants by ·O₂^-. Accordingly, this research could inspire theoretical study of free radical reaction and the design of S-scheme heterojunction photocatalysts.

Insights into the Photodegradation of the Contact Allergen Fragrance Cinnamyl Alcohol: Kinetics, Mechanism, and Toxicity

Fragrances can cause general health issues, and special concerns exist surrounding the issue of skin safety. Cinnamyl alcohol (CAL) is a frequent fragrance contact allergen that has various toxic effects on indiscriminate animals. In the present study, the photodegradation transformation mechanism of CAL and toxicity evolution during this process were examined. The results showed that CAL (50 μM) can be completely degraded after 90-min ultraviolet (UV) irradiation with a degradation rate of 0.086 min^-1 . Increased toxicity on bioluminescent bacteria was observed during this process, with lethality increasing from 10.6% (0 min) to 50.2% (90 min) under UV light irradiation. Further, the photodegradation mechanisms of CAL were explored to find the reason behind the increased toxicity observed. Laser flash photolysis and quenching experiments showed that O₂^•- , ¹ O₂ , and ^• OH were mainly responsible for CAL photodegradation, together with ³ CAL* and e_aq^- . The 5 main photodegradation products were cinnamyl aldehyde, benzaldehyde, benzenepropanal, cinnamic acid, and toluene, as identified using gas chromatography-mass spectrometry and liquid chromatography-quadrupole-time-of-flight-mass spectrometry. Once exposed to air, CAL was found to be easily oxidized to cinnamyl aldehyde and subsequently to cinnamic acid by O₂^•- - or ¹ O₂ -mediated pathways, leading to increased toxicity. Benzaldehyde exhibited bioreactive toxicity, increasing the toxicity through ^• OH-mediated pathways. Theoretical prediction of skin irritation indicated that cinnamyl aldehyde (0.83), benzenepropanal (0.69), cinnamyl aldehyde (0.69), and benzaldehyde (0.70) were higher than CAL (0.63), which may cause a profound impact on an individual's health and well-being. Overall, the present study advances the understanding of the photodegradation processes and health impacts of fragrance ingredients. Environ Toxicol Chem 2021;40:2705-2714. © 2021 SETAC.