Sunlight-mediated degradation of methyl orange sensitized by riboflavin: Roles of reactive oxygen species

Tungsten disulfide (WS2) is a highly active co-catalyst in Fe(III)/H2O2 Fenton-like reactions for efficient acetaminophen degradation

The most important factor that restricts the decomposition of H₂O₂ in the Fe³⁺/H₂O₂ reaction is the slow cycling efficiency of reducing Fe³⁺ to Fe²⁺. In this study, the addition of tungsten disulfide (WS₂) as a co-catalyst achieved a rapid cycling of the reaction rate-limiting step and a significant enhancement of H₂O₂ decomposition, which resulted in the effective degradation of acetaminophen (APAP). Results show that 99.6% of APAP (5 mg L^-1) could be degraded by H₂O₂/Fe³⁺/WS₂ system within 2.5 min. The conversion of Fe³⁺ to Fe²⁺ occurred mainly on the surface of WS₂ due to the redox reaction of the exposed W⁴⁺ active sites with Fe³⁺ after the unsaturated S atoms were bound to protons. Electron paramagnetic resonance (EPR) and radical quenching experiments evaluated the contribution of hydroxyl radical (•OH) and superoxide radical (O₂^•-) in the degradation of pollutants. WS₂ showed good recoverability after four cycles of the reaction. This study provides a new perspective to improve the efficiency of Fe³⁺/H₂O₂ and provides a reference for the involvement of transition metal sulfides in advanced oxidation processes (AOPs).

Transcriptome Analysis to Identify Crucial Genes for Reinforcing Flavins-Mediated Extracellular Electron Transfer in Shewanella oneidensis

Flavins serve as the electron mediators in Shewanella oneidensis, determining the extracellular electron transfer (EET) rate. Currently, metabolic engineering of flavins biosynthetic pathway has been studied for improving EET. However, the cellular response triggered by flavins that contribute to EET remains to be elucidated. In this study, the riboflavin-overproducing strain C5 (expressing the flavins synthetic genes in plasmid PYYDT) and the PYYDT strain (harboring the empty plasmid PYYDT) in the microbial fuel cells are applied for comparative transcriptomic analyses to investigate beneficial gene targets that could improve EET. From the differentially expressed genes, we select the significantly upregulated and downregulated genes for inverse engineering in S. oneidensis. The results show that overexpression of ahpC and ccpA, and inactivation of pubA, putB, and tonB are able to improve the EET capability. Combinatorial modulation of these five genes results in the recombinant strain CM4, achieving the maximum power density of 651.78 ± 124.60 mW/m², 1.97 folds of the parental strain. These genes modulation is speculated to reduce the ROS damage and to promote cytochrome synthesis and heme accumulation, which coherently enhance EET. Our findings facilitate in-depth understanding of the mechanism of flavins-mediated EET and provide new insights in promoting EET of S. oneidensis for electricity generation.

Activation of peroxymonosulfate by iron oxychloride with hydroxylamine for ciprofloxacin degradation and bacterial disinfection

Iron oxychloride (FeOCl) is a known effective iron-based catalyst and has been used in advanced oxidation processes (AOPs). This study intends to achieve more facile free radicals generation from peroxymonosulfate (PMS) activation by exploring the Fe(III)/Fe(II) cycle of FeOCl in the presence of hydroxylamine (HA). With 0.2 g/L FeOCl, 1.5 mM PMS, and 1 mM HA, the PMS/FeOCl/HA system could effectively achieve 98.88% of the oxidative degradation of 5 mg/L ciprofloxacin (CIP) in 15 min and quickly inactivate 99.99% of E. coli (10⁸ CFU/mL) in 5 min at near-neutral pH. HA played an important role in promoting the Fe(III)/Fe(II) cycle, thereby greatly improving the oxidation activity of the system. The reactive oxygen species (ROS) such as HO, SO₄^- and O₂^- were identified as the dominated free radicals produced in the system. The intermediate products of CIP detected by liquid chromatograph-mass spectrometer (LC-MS) and three possible degradation pathways of CIP were proposed. The presence of common anions in the PMS/FeOCl/HA system, including HCO₃^-, Cl^-, SO₄^2-, and NO₃^-, enhanced the degradation efficiency of CIP to varying degrees at the concentrations of 10 mM. Moreover, FeOCl maintained a high degradation capability for CIP after several recycles. This work offers a new promising means of catalyzing the PMS-based AOPs in the degradation of refractory organics.

Evaluation of practical application potential of a photocatalyst: Ultimate apparent photocatalytic activity

Suffered from rapid recombination of electrons and holes, apparent photocatalytic activity (APA) of all photocatalysts can never achieve their theoretical ultimate values. But the upper limit of practical APA is of great significance to evaluate the practical application potential of a photocatalyst. Thus, in this work, the concept of ultimate apparent photocatalytic activity (UAPA) was firstly proposed and a convenient evaluation method was first established based on the nature that EDTA-2Na can exclusively scavenge photo-excited holes, and methyl orange (MO) is mainly attacked by superoxide radical (O₂^-) which is produced instantly by photo-excited electrons. From a macro perspective, six popular photocatalysts were designedly selected to verify the feasibility and application scope of the proposed UAPA evaluation method. Moreover, O₂^- production rate and photocurrent intensity were measured by spectroscopy and spectrum analyses, and theoretical carrier concentrations were calculated by density functional theory (DFT) to further confirm the rationality and reliability of the proposed method. Positive responses of all the tests guarantee that the proposed UAPA could precisely evaluate the application potential of a photocatalyst and rank the photocatalysts according to their practical potential.

Enhanced decomposition of H2O2 by molybdenum disulfide in a Fenton-like process for abatement of organic micropollutants

Accelerating the rate-limiting step of Fe³⁺/Fe²⁺ conversion is a major challenge for H₂O₂ decomposition in conventional Fenton process. In this study, the catalytic mechanism of H₂O₂ by molybdenum disulfide (MoS₂) nanoparticles and Fe³⁺ ions was revealed and the abatement of organic micropollutants was investigated. The presence of both MoS₂ and Fe³⁺ can efficiently decompose H₂O₂. Reaction system of H₂O₂/MoS₂/Fe³⁺ is found to remove most of the tested pollutants by over 80% (except 65.9% for carbamazepine) within 60 min at pH of 3.0. Effective pH range of this reaction system can be extended to pH of 5.0. Adding MoS₂ to Fe³⁺/H₂O₂ system promotes the Fe³⁺/Fe²⁺ cycle and improves the reaction rate between Fe³⁺ and H₂O₂. The formation of Mo⁶⁺ ions and Mo⁶⁺ peroxo-complexes is beneficial to H₂O₂ decomposition and pollutant degradation. Electron paramagnetic resonance (EPR) measurements and quenching experiments confirm the important role of hydroxyl radicals in H₂O₂/MoS₂/Fe³⁺ system. Chloride ions (Cl^-) promote degradation, while bicarbonate ions (HCO₃^-) inhibit degradation. As H₂O₂ concentration increases from nil to 1.0 mM, the value of total EE/O decreases from 0.083 to 0.003 kWh L^-1, and the most energy efficient condition is determined. This study provides a new pathway for efficient decomposition of H₂O₂ by Fe³⁺ ions in an extended pH range, which is considered a facile and promising strategy for wastewater treatment.

The effect and mechanism of organic pollutants oxidation and chemical energy conversion for neutral wastewater via strengthening reactive oxygen species

Toxic and refractory organic pollutants are continually discharged into the water environment, which has become the crisis for the human living and sustainable development. However, organic pollutants also contain large amounts of chemical energy. In this paper, we studied the effect and mechanism of organic pollutants oxidation and chemical energy conversion for neutral wastewater via strengthening reactive oxygen species (ROS) of HO and O₂^- in a photocatalytic fuel cell (PFC) system, since ROS has the power to oxidize or even mineralize the organics and is environment-friendly to treat refractory organic pollutants. In our PFC system, the HO was enhanced by the cyclic radical chain reaction via the addition of Fe²⁺ and tetrapolyphosphate (TPP), while O₂^- was enhanced by setting an additional bias voltage at the anode which was favorable to O₂ production. The results show that the HO and O₂^- concentration are highly enhanced, showing 8.28 and 8.99 times those of traditional PFC, respectively. Meanwhile, the degradation rate constant is remarkably increased by 6.52 times when methylene blue is used as a model pollutant. Furthermore, the performance of wastewater PFC is so improved that the short-circuit current density (J_sc) and maximum power density (JV_max) have been increased by a factor of 9.05 and 12.67 times in the same experiment, respectively.

Stabilization of Natural Antioxidants by Silk Biomaterials

The stabilities of three natural antioxidants, vitamin C (VC), (-)-epigallocatechin gallate (EGCG), and curcumin, in silk films were examined and mechanisms of stabilization were elucidated. The antioxidants were physically incorporated into three types of silk films: as-cast, dried from hydrogels, and methanol-treated. Films were stored at 4, 37, and 45 °C for 30 days in phosphate-buffered saline, pH 7.4, along with controls consisting of free antioxidants. Incorporation of antioxidants did not significantly change film morphology or secondary structure. When stored at 4 °C, all samples showed similar antioxidant activities (percent scavenging) at different time points, determined by the colorimetric 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. At higher temperatures, VC in the as-cast film, EGCG in the as-cast and dried hydrogel films, and curcumin in the methanol-treated films retained more than 50% scavenging activity after 14 days of storage, significantly higher than the other samples. Interaction between antioxidants and silk, as well as degradation of the antioxidants, was investigated by fast-performance liquid chromatography (FPLC) and high-pressure liquid chromatography (HPLC), with an aim of understanding the mechanisms of silk-based stabilization. Binding of antioxidant molecules to hydrophobic or to hydrophilic/hydrophilic boundary regions of silk, depending on the chemical properties of the antioxidant, may account for the observed stabilization effects. The data can help guide further engineering of antioxidant-functionalized silk biomaterials.

A new trick (hydroxyl radical generation) of an old vitamin (B2) for near-infrared-triggered photodynamic therapy

A new NIR-light-triggered PDT method has been developed using an old vitamin (vitamin B₂) integrated with the upconversion nanotechnology.