UV-Vis quantification of hydroxyl radical concentration and dose using principal component analysis

Recent advances in catalyst design, performance, and challenges of metal-heteroatom-co-doped biochar as peroxymonosulfate activator for environmental remediation

The escalation of global water pollution due to emerging pollutants has gained significant attention. To address this issue, catalytic peroxymonosulfate (PMS) activation technology has emerged as a promising treatment approach for effectively decontaminating a wide range of pollutants. Recently, modified biochar has become an increasingly attractive as PMS activator. Metal-heteroatom-co-doped biochar (MH-BC) has emerged as a promising catalyst that can provide enhanced performance over heteroatom-doped and metal-doped biochar due to the synergism between metal and heteroatom in promoting PMS activation. Therefore, this review aims to discuss the fabrication pathways (i.e., internal vs external doping and pre-vs post-modification) and key parameters (i.e., source of precursors, synthesis methods, and synthesis conditions) affecting the performance of MH-BC as PMS activator. Subsequently, an overview of all the possible PMS activation pathways by MH-BC is provided. Subsequently, Also, the detection, identification, and quantification of several reactive species (such as, •OH, SO4•-, O2•-, 1O2, and high valent oxo species) generated in the catalytic PMS system by MH-BC are also evaluated. Lastly, the underlying challenges associated with poor stability, the lack of understanding regarding the interaction between metal and heteroatom during PMS activation and quantification of radicals in multi-ROS system are also deliberated.

Pyrazine-based iron metal organic frameworks (Fe-MOFs) with modulated O-Fe-N coordination for enhanced hydroxyl radical generation in Fenton-like process

Rational design of coordination environment of Fe-based metal-organic frameworks (Fe-MOFs) is still a challenge in achieving enhanced catalytic activity for Fenten-like advanced oxidation process. Here in, novel porous Fe-MOFs with modulated O-Fe-N coordination was developed by configurating amino terephthalic acid (H2ATA) and pyrazine-dicarboxylic acid (PzDC) (Fe-ATA/PzDC-7:3). PzDC ligands introduce pyridine-N sites to form O-Fe-N coordination with lower binding energy, which affect the local electronic environment of Fe-clusters in Fe-ATA, thus decreased its interfacial H2O2 activation barrier. O-Fe-N coordination also accelerate Fe(II)/Fe(III) cycling of Fe-clusters by triggering the reactive oxidant species mediated Fe(III) reduction. As such, Fe-ATA/PzDC-7:3/H2O2 system exhibited excellent degradation performance for typical antibiotic sulfamethoxazole (SMX), in which the steady-state concentration of hydroxyl radical (OH) was 1.6 times higher than that of unregulated Fe-ATA. Overall, this study highlights the role of O-Fe-N coordination and the electronic environment of Fe-clusters on regulating Fenton-like catalytic performance, and provides a platform for precise engineering of Fe-MOFs.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.

Quantification of peracetic acid (PAA) in the H2O2 + acetic acid reaction by the wavelength shift analysis in near-UV/visible absorption region

In our study, we present an innovative method for the analysis and real-time monitoring of peracetic acid (PAA) formation within the near-UV/Vis (visible) wavelength region. PAA's absorption spectrum, influenced by its presence in a complex quaternary equilibrium mixture with hydrogen peroxide (H2O2), acetic acid, and water, lacks discernible peaks. This inherent complexity challenges conventional analytical techniques like Beer's law, which rely on absorption intensity as a foundation. To address this challenge, we introduce a novel approach that centers on the analysis of blue shifts in absorption wavelengths, particularly at an absorbance of 0.8 a.u. This method significantly enhances the precision of calibration curves for both diluted PAA and H2O2, unveiling an exponential correlation between wavelength and the logarithm of concentration for both components. Significantly, our approach allows for real-time and accurate measurements, especially during the dynamic PAA formation reaction. Our results exhibit excellent agreement with data obtained from Fourier-transform infrared (FT-IR) spectroscopy, validating the reliability of our method. It's noteworthy that under stable PAA concentration conditions (after 12 h of solution interaction), both traditional absorption method and our approach closely align with the FT-IR method. However, in dynamic scenarios (0-12 h), the absorption method exhibits higher error rates compared to our approach. Additionally, the increased concentration of a catalyst, sulfuric acid (H2SO4), significantly reduces the errors in both methods, a finding that warrants further exploration. In summary, our study not only advances our understanding of PAA and its spectral behavior but also introduces innovative and precise methods for determining PAA concentration in complex solutions. These advancements hold the potential to revolutionize the field of chemical analysis and spectroscopy.

An innovative method to degrade xanthate from flotation tailings wastewater in acid mine drainage (AMD) system: Performance, degradation mechanism and pathways

This study objective is to degrade xanthate from flotation tailings wastewater by using a coagulation-flocculation co-Fenton oxidation process in an acid mine drainage (AMD)-H2O2 system. More than 98% sodium butyl xanthate (SBX) removal rate was achieved by the method under optimal conditions. The acids and Fe2+ in AMD were sufficient to initiate a Fenton reaction at the aid of H2O2. Furthermore, iron ions were reduced to an extremely low level (0.19 mg/L) by participating in an oxidation process. Meanwhile, the Cu2+ ions in AMD facilitated the coagulation-flocculation process. Comparison experiments confirmed that the method was superior to the AMD alone (54.26%) and H2O2 alone (32.23%) in terms of performance in degrading SBX. The kinetic results showed that SBX degradation followed a pseudo first-order kinetic model. Additionally, the electron paramagnetic resonance (EPR) and quenching results suggested that hydroxyl radicals (•OH) were the main active species in AMD-H2O2 system. Degradation products were analyzed, and two possible pathways of SBX degradation were proposed. One pathway displayed that the SBX was first transformed into butyl xanthate peroxide (BPX), CO32- and S2O32-, and then further decomposed into CO2, H2O and SO42- under the ongoing •OH attack. Another pathway showed that precipitates consisting of butyl copper xanthate and iron oxide species were generated during the SBX degradation. This study provides a novel perspective on the innovative application of AMD in Fenton oxidation and provides a strong basis for the green and sustainable treatment of xanthate wastewater in tailings.

High band-width mid-infrared frequency-modulated Faraday rotation spectrometer for time resolved measurement of the OH radical

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O₃) and water (H₂O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 10⁸ molecule/cm³ (1σ, 0.2 ms) after 3 and falling to 8.0 × 10⁷ molecule/cm³ after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (A_min) of 1.9 × 10^-5 and 2.2 × 10^-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

Theoretical insights into the degradation mechanisms, kinetics and eco-toxicity of oxcarbazepine initiated by OH radicals in aqueous environments

As an anticonvulsant, oxcarbazepine (OXC) has attracted considerable attention for its potential threat to aquatic organisms. Density functional theory has been used to study the mechanisms and kinetics of OXC degradation initiated by OH radicals in aqueous environment. A total of fourteen OH-addition pathways were investigated, and the addition to the C8 position of the right benzene ring was the most vulnerable pathway, resulting in the intermediate IM8. The H-abstraction reactions initiated by OH radicals were also explored, where the extraction site of the methylene group (C14) on the seven-member carbon heterocyclic ring was found to be the optimal path. The calculations show that the total rate constant of OXC with OH radicals is 9.47 × 10⁹ (mol/L)^-1sec^-1, and the half-life time is 7.32 s at 298 K with the [·OH] of 10^-11 mol/L. Moreover, the branch ratio values revealed that OH-addition (89.58%) shows more advantageous than H-abstraction (10.42%). To further understand the potential eco-toxicity of OXC and its transformation products to aquatic organisms, acute toxicity and chronic toxicity were evaluated using ECOSAR software. The toxicity assessment revealed that most degradation products such as OXC-2OH, OXC-4OH, OXC-1O-1OOH, and OXC-1OH' are innoxious to fish and daphnia. Conversely, green algae are more sensitive to these compounds. This study can provide an extensive investigation into the degradation of OXC by OH radicals and enrich the understanding of the aquatic oxidation processes of pharmaceuticals and personal care products (PPCPs).

Electrochemical H2O2 sensor based on a Au nanoflower-graphene composite for anticancer drug evaluation

Reliable H₂O₂ sensors for in situ cellular monitoring under drug stimulation can be developed as a powerful and versatile tool for drug evaluation. Herein, a novel electrochemical biosensor capable of detecting and quantifying H₂O₂ was fabricated by graphene and shape-controlled gold nanostructures. With the help of polyelectrolytes, gold exhibited hierarchical flower-like nanostructures. This kind of nanozyme material exhibited a prominent electrochemical response for H₂O₂. Electrocatalytic activity for H₂O₂ reduction with high sensitivity (5.07◊10^-4 mA μmol L^-1 cm^-2) and good detection capability (the lowest detection limit is 4.5 μmol L^-1 (S/N = 3)) were achieved. This electrochemical biosensor was successfully used to measure the concentration of H₂O₂ released from HepG2 hepatoma cells. Ascorbic acid (AA) and Camellia nitidissima Chi saponins (CNCS) were selected as model drugs, and their anticancer activities were compared by in situ monitoring of H₂O₂. Interestingly, the electrochemical sensor showed remarkable sensitivity, accuracy, and rapidity compared with the traditional enzymatic detection kit. In brief, the as-synthesized nanostructured H₂O₂ sensors can be applied to assess the antitumor properties of candidate drugs and inspire developments for personalized health care monitoring and cancer treatment.

Bactericidal Effect and Associated Properties of Non-Electrolytic Hypochlorite Water on Foodborne Pathogenic Bacteria

This study investigated the broad-spectrum bactericidal activity of non-electrolytic hypochlorite water (NEHW) and detected its hydroxyl radical content compared with that of slightly acidic electrolytic water (SAEW). Based on the results of UV scanning and storage stability, higher hypochlorite content and stronger oxidation were found to be responsible for the stronger bactericidal effect of NEHW. NEHW can achieve 99% bacterial disinfection effect by treating with 10 mg/L available chlorine concentration for more than 5 minutes. At the same time, the storage stability of NEHW was higher than that of SAEW. After 20 days of storage under sealed and dark conditions, the pH value only increased by 7.9%, and the effective chlorine concentration remained nearly 80%. The results showed that NEHW had higher germicidal efficacy and storage stability than SAEW.

Hydroxyl Radical Production via a Reaction of Electrochemically Generated Hydrogen Peroxide and Atomic Hydrogen: An Effective Process for Contaminant Oxidation?

An electrochemical advanced oxidation process (EAOP) is demonstrated with a catalytic cathode capable of simultaneously catalyzing the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) with resultant in situ generation of atomic hydrogen (H*) and hydrogen peroxide (H₂O₂). A palladium-coated carbon-PTFE gas diffusion electrode (Pd/C GDE) was used as a catalytic cathode with hydroxyl radical (^•OH) formed as a result of the reaction of electrogenerated H* with H₂O₂. As both the HER and ORR can be induced to occur at the same cathode, the H*/GDE process results in more effective degradation of organic contaminants than can be achieved by a conventional H*/H₂O₂ process involving direct addition of H₂O₂. At circumneutral pH, 82.7% of added formate was degraded after 2 h treatment at an applied potential of -1.0 V vs Ag/AgCl with relatively low concentrations of generated H₂O₂ remaining in the solution. We also show that H* and H₂O₂ (and thus ^•OH) can be electrogenerated effectively over a wide range of pH (3.2-7.0). These results suggest that by in situ generation of H* and H₂O₂, the H*/GDE process is able to produce significant amounts of ^•OH without external chemical addition and thus offers an alternative method for abatement of aqueous organic contaminants.

Generation of hydroxyl radical-activatable ratiometric near-infrared bimodal probes for early monitoring of tumor response to therapy

Tumor response to radiotherapy or ferroptosis is closely related to hydroxyl radical (•OH) production. Noninvasive imaging of •OH fluctuation in tumors can allow early monitoring of response to therapy, but is challenging. Here, we report the optimization of a diene electrochromic material (1-Br-Et) as a •OH-responsive chromophore, and use it to develop a near-infrared ratiometric fluorescent and photoacoustic (FL/PA) bimodal probe for in vivo imaging of •OH. The probe displays a large FL ratio between 780 and 1113 nm (FL₇₈₀/FL₁₁₁₃), but a small PA ratio between 755 and 905 nm (PA₇₅₅/PA₉₀₅). Oxidation of 1-Br-Et by •OH decreases the FL₇₈₀/FL₁₁₁₃ while concurrently increasing the PA₇₅₅/PA₉₀₅, allowing the reliable monitoring of •OH production in tumors undergoing erastin-induced ferroptosis or radiotherapy.

The hydroxyl radical is generated during radiotherapy and ferroptosis and accurate imaging of this reactive oxygen species may permit the monitoring of response to therapy. Here, the authors develop a ratiometric probe for accurate imaging of hydroxyl radical generation in vivo.

Alendronate-Modified Nanoceria with Multiantioxidant Enzyme-Mimetic Activity for Reactive Oxygen Species/Reactive Nitrogen Species Scavenging from Cigarette Smoke

Highly toxic radicals including reactive oxygen species (ROS) and reactive nitrogen species (RNS) in cigarette smoke play an important role in oxidative damage of the lungs, which cannot be efficiently scavenged by current filter techniques. Herein, a novel alendronate-coated nanoceria (CeAL) nanozyme is explored for cigarette filter modification for ROS/RNS scavenging. The CeAL nanozyme with an adjustable oxidation state and high thermal stability exhibits an excellent superoxide dismutase (SOD)-like activity, hydroxyl radical elimination capacity, catalase-mimicking activity, and nitric oxide radical scavenging ability. These synergistic antioxidant abilities make the CeAL nanozyme a lucrative additive for cigarette filters. The filter incorporated with the CeAL nanozyme can efficiently scavenge ROS/RNS in the hot smoke generated by burned commercial cigarettes, resulting in reduction of oxidative stress-induced pulmonary injury and acute inflammation of mice. The developed CeAL nanozyme opens up new opportunities for cigarette filter modification to decrease the toxicity of cigarette smoke and expands the application fields of nanoceria.

Photocatalytic degradation of ibuprofen using titanium oxide: insights into the mechanism and preferential attack of radicals

The present work studied ibuprofen degradation using titanium dioxide as a photocatalyst. Mechanistic aspects were presented and the preferred attack sites by the OH˙ radical on the ibuprofen molecule were detailed, based on experimental and simple theoretical-computational results. Although some previous studies show mechanistic proposals, some aspects still need to be investigated, such as the participation of 4-isobutylacetophenone in the ibuprofen degradation and the preferred regions of attack by OH˙ radicals. The photodegradation was satisfactory using 0.03 g of TiO₂ and pH = 5.0, reaching 100% decontamination in 5 min. The zeta potential curve showed the regions of attraction and repulsion between TiO₂ and ibuprofen, depending on the pH range and charge of the species, influencing the amount of by-products formed. Different by-products have been identified by GC-MS, such as 4-isobutylacetophenone. Ibuprofen conversion to 4-isobutylacetophenone takes place through decarboxylation reaction followed by oxidation. The proposed mechanism indicates that the degradation of ibuprofen undergoes a series of elementary reactions in solution and on the surface. Three different radicals (OH˙, O₂⁻˙ and OOH˙) are produced in the reaction sequence and contribute strongly to the oxidation and mineralization of ibuprofen and by-products, but the hydroxyl radical has a greater oxidation capacity. The simple study using the DFT approach demonstrated that the OH˙ radical attacks preferentially in the region of the ibuprofen molecule with high electronic density, which is located close to the aromatic ring (C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond). The presence of the OH˙ radical was confirmed through a model reaction using salicylic acid as a probe molecule.

The degradation of ibuprofen undergoes a series of elementary reactions, generating different radicals which attack preferentially in the region of the ibuprofen with high electron density.

Degradation of fluorescent dye-Solvent Green 7 (HPTS) in wastewater by advanced oxidation process

Solvent Green 7 (HPTS) is a widely used fluorescent dye. As a kind of polycyclic aromatic hydrocarbon (PAHs) derivative, HPTS would cause pollution when it is discharged into the environment. This study adopted advanced oxidation processes (UV/H₂O₂) to degrade the HPTS in aqueous solution and investigated the effects of various factors on the degradation. The results showed that: the initial concentration and the fluorescence characteristics of HPTS reduced the degradation efficiency. When the oxidant concentration of H₂O₂ was 3 mg/L, the degradation efficiency and cost of HPTS (20 mg/L) were the most appropriate; when there were various inorganic anions in the solution, the degradations were not affected, but when the solution was strong acid and there existed a lot of chloride ions, the degradation of HPTS was inhibited. The degradation pathways indicated HPTS degraded into naphthalene derivatives, benzene derivatives through oxidation and decarboxylation reactions, finally into water and carbon dioxide. Further research for substances similar to HPTS structure will make progress in understanding the degradation process of PAHs.