Mechanistic insights and catalytic enhancement of phenolic wastewater supercritical water gasification: A combined experiment and density functional theory study

Supercritical water gasification technology provides a favorable technology to achieve pollution elimination and resource utilization of phenolic wastewater. In this study, the reaction mechanism of phenolic wastewater supercritical water gasification was investigated using a combination of experimental and computational methods. Five reaction channels were identified to elucidate the underlying pathway of phenol decomposition. Importantly, the rate-determining step was found to be the dearomatization reaction. By integrating computational and experimental analyses, it was found that phenol decomposition via the path with the lowest energy barrier generates cyclopentadiene, featuring a dearomatization barrier of 70.97 kcal/mol. Additionally, supercritical water plays a catalytic role in the dearomatization process by facilitating proton transfer. Based on the obtained reaction pathway, alkali salts (Na2CO3 and K2CO3) are employed as a catalyst to diminish the energy barrier of the rate-determining step to 40.00 kcal/mol and 37.14 kcal/mol. Alkali salts catalysis significantly improved carbon conversion and pollutant removal from phenolic wastewater, increasing CGE from 58.44% to 93.55% and COD removal efficiency from 94.11% to 99.79%. Overall, this study provides a comprehensive understanding of the decomposition mechanism of phenolic wastewater in supercritical water.

Hydrothermal Co-Crystallization of Novel Copper Tungstate-Strontium Titanate Crystal Composite for Enhanced Photocatalytic Activity and Increased Electron–Hole Recombination Time

The development of catalysts continues to have a significant influence on science today since we can utilize them to efficiently destroy some contaminants. A study in this field is justified because there is a dearth of comprehensive literature on the creation of SrTiO3-based photocatalysts. Related to this topic, here we report the facile preparation of a structure-modified SrTiO3 photocatalyst, by incorporating CuWO4. Within the case of the CuWO4-modified samples (0.5–3 wt% nominal CuWO4 content), the photo-oxidation of phenol, as a contaminant, was more than two times higher than the initial SrTiO3. However, the photocatalytic activity does not change linearly with increasing CuWO4 content, and the CWS2.5 (2.5 wt% nominal CuWO4 content and 4.25 wt% measured content) has the highest photo-activity under the applied conditions. The reason for the better activity was the increased recombination time of charge separation on the catalyst surface. Slower recombination can result in more water being oxidized to hydroxyl radicals, leading to the faster decomposition of the phenol.

OH-initiated degradation of 1,2,3-trimethylbenzene in the atmosphere and wastewater: Mechanisms, kinetics, and ecotoxicity

1,2,3-Trimethylbenzene (1,2,3-TMB) is an important volatile organic compound (VOC) present in petroleum wastewater and the atmosphere. This compound can be degraded by OH radicals via abstraction, addition and substitution mechanisms. Results show that the addition mechanism is dominant and H-abstraction is subdominant, while methyl abstraction and substitution mechanisms are negligible in the gas and aqueous phases. Moreover, H-abstraction products undergo further reactions with O₂, NO, NO₂, H₂O, and OH radicals in the atmosphere. Time-dependent density functional theory (TDDFT) calculations show that the degraded products, including 2,3,4-trimethylphenyl-nitroperoxoite, 1,2,3-trimethyl-4-nitrobenzene, 1,2,3-trimethyl-5-nitrobenzene, 2,6-dimethylbenzyl nitroperoxoite, 2,3-dimethylphenyl nitroperoxoite, 2,6-dimethylbenzaldehyde, and 2,3-dimethylbenzaldehyde, can photolyze under the sunlight. Kinetically, the calculated total rate constant is 5.57 × 10^-11 cm³ molecule^-1·s^-1 at 1 atm and 298 K, which is consistent with available experimental values measured in the atmosphere. In addition, the calculated total reaction rate constant in water is close to that in the gas phase. In terms of ecotoxicity, all degradation products are less toxic than the initial reactant to fish, green algae and daphnia. For mammals represented by rats, 1,2,3-TMB and its products are moderately toxic, except for 2,3-dimethylphenol and 2,6-dimethylphenol, which are slightly toxic.

Continuous Fe(II)-dosing scheme for persulfate activation: Performance enhancement mechanisms in a slurry phase reactor

Mechanisms involved in the superior performance of the continuous Fe²⁺ dosing scheme over the single Fe²⁺ dosing scheme was thoroughly investigated. The kinetics and stoichiometry of the phenol removal/persulfate consumption strongly depended on the volumetric or molar Fe²⁺ feeding rate, Fe²⁺ concentration in the feed solution, and Fe²⁺ feeding mode (continuous or single dose). The process performance was determined by the molar Fe²⁺ feeding rate rather than the volumetric Fe²⁺ feeding rate or the Fe²⁺ concentration in the feed solution. The phenol degradation rate increased as the molar Fe²⁺ feeding rate increased to 2.77 mmol/min but decreased as the Fe²⁺ feeding rate increased further. The sulfate radical was predominant radical species formed in continuous Fe²⁺ dosing mode. The hydroxyl and sulfate radicals were both important in single Fe²⁺ dose mode. The presence of hydroxyl radicals in single Fe²⁺ dosing mode decreased the amount of phenol oxidation that occurred, probably because the hydroxyl radicals were readily scavenged by soil organic matter. Continuous Fe²⁺ dosing facilitated phenol mineralization, which was indicated by total organic carbon measurements and toxicity tests performed using Hyalella azteca.

Optimization of Plasmonic Copper Content at Copper-Modified Strontium Titanate (Cu-SrTiO3): Synthesis, Characterization, Photocatalytic Activity

Catalyst development still has a major impact on science today, as we can use catalysts to break down certain pollutants in an energy-efficient way. There is no comprehensive literature on the development of SrTiO3-based photocatalysts, so study in this area is justified. Related to this topic, here we report the facile preparation of surface-modified SrTiO3 photocatalyst, performed by plasmonic copper deposition. In the case of the copper-modified samples (0.25–3 wt.% Cu content), the photooxidation of phenol, as model contaminant, was almost 4–5 times higher than the bare SrTiO3. However, the photocatalytic activity was not linearly related to copper content, since the highest photoactivity was achieved at 1 wt.% copper content. The reason for the better activity was the plasmonic effect of copper, which increases the recombination time of charge separation on the catalyst surface. During slower recombination, more water is oxidized to hydroxyl radicals, which can lead to faster degradation of phenol.

Screening of multifunctional fruit carbon dots for fluorescent labeling and sensing in living immune cells and zebrafishes

Nine kinds of carbon dots (CDs) were synthesized by using fruits with different varieties as carbon sources; meanwhile, the fluorescence characteristics, quantum yield, and response ability to different metal ions and free radicals were systematically studied. These CDs showed similar excitation and emission spectral ranges (λ_ex ≈ 345 nm, λ_em ≈ 435 nm), but very different fluorescence quantum yield (QY), in which orange and cantaloupe CDs have the highest QY around 0.25 and green plum CDs showed the lowest quantum yield around 0.1. Interestingly, the fluorescence of all of these CDs can be significantly quenched by hydroxyl radical (•OH) and iron ion (Fe³⁺); however, these CDs showed very different response characteristics to other metal ions (e.g., Co²⁺, Ni²⁺, Cu²⁺, Ce³⁺, Mn²⁺, Ag⁺, and Fe²⁺). Through in-depth analysis, we found some interesting patterns of the influence of carbon sources on the fluorescence characteristics of CDs. Finally, by using white pitaya CDs as fluorescence probe, we realized sensing of Fe³⁺ and •OH with limits of detection (LOD) of 19.4 μM and 0.7 μM, respectively. Moreover, the CDs were also capable for sensitive detection in immune cells and even in zebrafishes. Our work can provide valuable guidance for the rational design of functional CDs for biological applications.

Non-specific degradation of chloroacetanilide herbicides by glucose oxidase supported Bio-Fenton reaction

Bio-Fenton reaction supported by glucose oxidase (GOx) for producing H₂O₂ was applied to degrade persistent chloroacetanilide herbicides in the presence of Fe (Ⅲ)-citrate at pH 5.5. There were pH decrease to 4.3, the production of 8 mM H₂O₂ and simultaneous consumption to produce •OH radicals which non-specifically degraded the herbicides. The degradation rates followed the order acetochlor ≈ alachlor ≈ metolachlor > propachlor ≈ butachlor with the degradation percent of 72.8%, 73.4%, 74.0%, 47.4%, and 43.8%, respectively. During the Bio-Fenton degradation, alachlor was dechlorinated and filtered into catechol via the production of intermediates formed through a series of hydrogen atom abstraction and hydrogen oxide radical addition reactions. The current Bio-Fenton reaction leading to the production of •OH radicals could be applied for non-specific oxidative degradation to various persistent organic pollutants under in-situ environmental conditions, considering diverse microbial metabolic systems able to continuously supply H₂O₂ with ubiquitous Fe(II) and Fe(III) and citrate.

A DFT mechanistic and kinetic study on the reaction of phloroglucinol with •OH in different media: Hydrogen atom transfer versus oxidation

A theoretical study on the reaction of phloroglucinol with •OH has been performed with the aim of elucidating the geometric, energetic and kinetic properties of the reaction as well as identifying the preferred reaction pathway. Three reaction mechanisms have been considered, namely, direct hydrogen atom abstraction, addition–elimination mechanism in the absence and in the presence of a base catalyst and oxidation mechanism in the absence and in the presence of O2. The study has been performed using the DFT/M06[Formula: see text]2X, DFT/BHHLYP and DFT/MPW1K methods in conjunction with either the 6-31++G(d,p) or the 6-311++G(3df,2p) basis set. The energetic parameters are influenced by the type of function utilized and the media in which the calculation is done. The direct hydrogen abstraction mechanism provides the smallest branching ratio with respect to the •OH addition mechanisms. The PG + •OH reaction under atmospheric conditions saturated with O2 would predominantly form tetrahydroxybenzene; the predominant product within the biological system would largely depend on physiological conditions; under pH [Formula: see text] 7 and with oxygen dissolved within the biological system, the preferred product would be tetrahydroxybenzene; however, if the reaction takes place in some part of the biological system where the pH [Formula: see text] 7, the preferred product would be the phenoxyl radical.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

Reactions of hydroxyl radicals with benzoic acid and benzoate

Density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases as well as benzoate with hydroxyl radicals in the aqueous phase at the M06-2X/6-311+G(d,p) level of theory.

Selective hydrogenation of cinnamaldehyde catalyzed by Co-doped Pt clusters: a density functional theoretical study

The reaction mechanism of the selective hydrogenation of cinnamaldehyde catalyzed by pure Pt clusters and Co-doped Pt clusters was studied by the density functional theory.

Free radical nano scavenger based on amphiphilic novolacs

Synthetic amphiphilic novolacs via click chemistry are capable of forming micelles, and interestingly, can act as highly efficient nano-scavengers for hydroxyl free radicals.