Is Sulfate Radical a ROS?

The generation and transformation mechanisms of reactive oxygen species in the environment and their implications for pollution control processes: A review

Reactive oxygen species (ROS), substances with strong activity generated by oxygen during electron transfer, play a significant role in the decomposition of organic matter in various environmental settings, including soil, water and atmosphere. Although ROS has a short lifespan (ranging from a few nanoseconds to a few days), it continuously generated during the interaction between microorganisms and their environment, especially in environments characterized by strong ultraviolet radiation, fluctuating oxygen concentration or redox conditions, and the abundance of metal minerals. A comprehensive understanding of the fate of ROS in nature can provide new ideas for pollutant degradation and is of great significance for the development of green degradation technologies for organic pollutants. At present, the review of ROS generally revolves around various advanced oxidation processes, but lacks a description and summary of the fate of ROS in nature, this article starts with the definition of reactive oxidants species and reviews the production, migration, and transformation mechanisms of ROS in soil, water and atmospheric environments, focusing on recent developments. In addition, the stimulating effects of ROS on organisms were reviewed. Conclusively, the article summarizes the classic processes, possible improvements, and future directions for ROS-mediated degradation of pollutants. This review offers suggestions for future research directions in this field and provides the possible ROS technology application in pollutants treatment.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Vacuum-ultraviolet based advanced oxidation and reduction processes for water treatment

The use of vacuum-ultraviolet (VUV) photolysis in water treatment has been gaining significant interest due to its efficacy in degrading refractory organic contaminants and eliminating oxyanions. In recent years, the reactive species driving pollutant decomposition in VUV-based advanced oxidation and reduction processes (VUV-AOPs and VUV-ARPs) have been identified. This review aims to provide a concise overview of VUV photolysis and its advancements in water treatment. We begin with an introduction to VUV irradiation, followed by a summary of the primary reactive species in both VUV-AOPs and VUV-ARPs. We then explore the factors influencing VUV-photolysis in water treatment, including VUV irradiation dose, catalysts or activators, dissolved gases, water matrix components (e.g., DOM and inorganic anions), and solution pH. In VUV-AOPs, the predominant reactive species are hydroxyl radicals (˙OH), hydrogen peroxide (H2O2), and ozone (O3). Conversely, in VUV-ARPs, the main reactive species are the hydrated electron (eaq-) and hydrogen atom (˙H). It is worth noting that VUV-based advanced oxidation/reduction processes (VUV-AORPs) can transit between VUV-AOPs and VUV-ARPs based on the externally added chemicals and dissolved gases in the solution. Increase of the VUV irradiation dose and the concentration of catalysts/activators enhances the degradation of contaminants, whereas DOM and inorganic anions inhibit the reaction. The pH influences the redox potential of ˙OH, the speciation of contaminants and activators, and thus the overall performance of the VUV-AOPs. Conversely, an alkaline pH is favored in VUV-ARPs because eaq- predominates at higher pH.

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

Despite the widespread use of photochemical and optical properties to characterize dissolved organic matter (DOM), a significant gap persists in our understanding of the relationship among these properties. This study infers the molecular basis for the optical and photochemical properties of DOM using a comprehensive framework and known structural moieties within DOM. Utilizing Suwannee River Fulvic Acid (SRFA) as a model DOM, carboxylated aromatics, phenols, and quinones were identified as dominant contributors to the absorbance spectra, and phenols, quinones, aldehydes, and ketones were identified as major contributors to radiative energy pathways. It was estimated that chromophores constitute ∼63% w/w of dissolved organic carbon in SRFA and ∼47% w/w of overall SRFA. Notably, estimations indicate the pool of fluorescent compounds and photosensitizing compounds in SRFA are likely distinct from each other at wavelengths below 400 nm. This perspective offers a practical tool to aid in the identification of probable chemical groups when interpreting optical and photochemical data and challenges the current "black box" thinking. Instead, DOM photochemical and optical properties can be closely estimated by assuming the DOM is composed of a mixture of individual compounds.

Step-by-step guide for electrochemical generation of highly oxidizing reactive species on BDD for beginners

Selecting the ideal anodic potential conditions and corresponding limiting current density to generate reactive oxygen species, especially the hydroxyl radical (•OH), becomes a major challenge when venturing into advanced electrochemical oxidation processes. In this work, a step-by-step guide for the electrochemical generation of •OH on boron-doped diamond (BDD) for beginners is shown, in which the following steps are discussed: i) BDD activation (assuming it is new), ii) the electrochemical response of BDD (in electrolyte and ferri/ferro-cyanide), iii) Tafel plots using sampled current voltammetry to evaluate the overpotential region where •OH is mainly generated, iv) a study of radical entrapment in the overpotential region where •OH generation is predominant according to the Tafel plots, and v) finally, the previously found ideal conditions are applied in the electrochemical degradation of amoxicillin, and the instantaneous current efficiency and relative cost of the process are reported.

UV/persulfate processes for the removal of total organic carbon from coagulation-treated industrial wastewaters

Sulfate radical-based oxidation processes were investigated to understand the relationship between persulfate (PS) consumption and total organic carbon (TOC) removal from industrial wastewater under various PS concentrations. First, the degradation and mineralization of Bisphenol A (BPA) (initial concentration: 11 mg/L) were investigated in ultraviolet (UV)/PS systems. Complete degradation was achieved within 30 min of UV irradiation, and 41%-72% TOC removal was achieved at PS concentrations of 200 and 400 mg/L. The consumed concentration of S2O82- and generated concentration of SO42- increased gradually to similar levels. The ratio of the PS consumption to TOC removal based on the mass concentration (mg/L) was 14.5 and 23.2 at 180 min for 200 and 400 mg/L of S2O82-, respectively. Three types of coagulation-treated industrial wastewater from metal-processing, food-processing, and adhesive-producing plants were obtained, and TOC removal was analyzed using the same UV/PS systems (initial TOC concentration: 100 mg/L). The TOC removal rates ranged from 16.9% to 94.4% after 180 min of UV irradiation at PS concentrations of 1,000, 2,000, 4,000, and 8,000 mg S2O82-/L. Despite the higher TOC removal at higher PS concentrations, the PS activation efficiency decreased significantly as the PS concentration increased. Only approximately 30%-40% activation efficiency was achieved at a PS concentration of 8,000 mg/L. In this study, the ratio of PS consumption to TOC removal ranged from 20.6 to 43.9.

Combination of electrochemically activated persulfate process and electro-coagulation for the treatment of municipal landfill leachate with low biodegradability

To handle complex wastewater with limited biodegradability, hybrid treatment systems are necessary. The current study represents the combined effectiveness of sulfate-radical associated electro-chemical advanced oxidation process (SR-EAOP) and electro-coagulation (EC) for the treatment of stabilized landfill leachate. For SR-EAOP, Pt/Ti was employed as the anode and an iron plate as the cathode; while EC treatment was performed by switching the polarity. Hence, both electrochemical treatment was carried out in single reactor. Initially, the effects of pH, applied voltage, persulfate and Fe2+ dosage, on the performance of SR-EAOP was examined. Sulfate radical was generated in the electrolytic system via cathodic reduction of persulfate (PS) and ferrous (Fe2+) ion activation. Auxiliary processes such as anodic oxidation via Pt/Ti anode and indirect electro-chemical oxidation were also contributed for pollutant degradation. Combined process SR-EAOP followed by EC (SR-EAOP + EC) has better leachate treatment efficacy in comparison with EC + SR-EAOPs. The SR-EAOP + EC based combined treatment mechanism achieved an efficient COD reduction of 88.67% than that of EC + SR - EAOP process (74.51% COD reduction). Characterization studies have been carried out for post-treated dried-sludge using Field Emission scanning electron microscope (FE-SEM) and X-ray powder diffraction (XRD) techniques. The combined process treatment (SR-EAOP + EC) can be applied as pre-treatment for leachate decontamination.

Hollow NiCo2O4-ZnO-Co3O4-/N-C Micro-Cage for Synergistic Bisphenol A Degradation by Activating Persulfate

The NiCo₂O₄-ZnO-Co₃O₄-/N-C micro-cage was successfully synthesized by calcination of the precursor obtained from a hollow ZIF-8/ZIF-67 with nickel nitrate. The preparation process concerning ion exchange and leaching was illustrated by the investigation of the composition and structure of the composites. As a catalyst for the activation of persulfate (PDS) to degrade bisphenol A (BPA), it was discovered that the BPA degradation in the presence of NiCo₂O₄-Co₃O₄-ZnO/N-C was more efficient than the solids obtained by ZIF-67 with nickel nitrate, indicated by the sevenfold increase of the apparent reaction rate. The further electrochemical analysis evidenced that the electron transfer was more easily accomplished in the system of BPA-PDS-NiCo₂O₄-Co₃O₄-ZnO/N-C. This enhanced activity of NiCo₂O₄-Co₃O₄-ZnO/N-C was mainly due to the hollow structure, the synergistic effect of NiCo₂O₄, as well as the smaller size of the active species, which facilitated the transportation of molecules and ions as well as the activation of PDS.

COVID-19 Spotlights Connections between Disease and Multiple Lifestyle Factors

The SARS-CoV-2 virus (severe acute respiratory syndrome coronavirus 2), and the disease it causes (COVID-19), have had a profound impact on global human society and threaten to continue to have such an impact with newly emerging variants. Because of the widespread effects of SARS-CoV-2, understanding how lifestyle choices impact the severity of disease is imperative. This review summarizes evidence for an involvement of chronic, non-resolving inflammation, gut microbiome disruption (dysbiosis with loss of beneficial microorganisms), and impaired viral defenses, all of which are associated with an imbalanced lifestyle, in severe disease manifestations and post-acute sequelae of SARS-CoV-2 (PASC). Humans' physiological propensity for uncontrolled inflammation and severe COVID-19 are briefly contrasted with bats' low propensity for inflammation and their resistance to viral disease. This insight is used to identify positive lifestyle factors with the potential to act in synergy for restoring balance to the immune response and gut microbiome, and thereby protect individuals against severe COVID-19 and PASC. It is proposed that clinicians should consider recommending lifestyle factors, such as stress management, balanced nutrition and physical activity, as preventative measures against severe viral disease and PASC.

Efficient Activation of Peroxymonosulfate by V-Doped Graphitic Carbon Nitride for Organic Contamination Remediation

Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) activation have been developed as an ideal pathway for completely eradication of recalcitrant organic pollutants from water environment. Herein, the V-doped graphitic carbon nitride (g-C₃N₄) is rationally fabricated by one-step thermal polymerization method to activate PMS for contamination decontamination. The results demonstrate the V atoms are successfully integrated into the framework of g-C₃N₄, which can effectively improve light absorption intensity and enhance charge separation. The V-doped g-C₃N₄ displays superior catalytic performance for PMS activation. Moreover, the doping content has a great influence on the activation performances. The radical quenching experiments confirm •O₂^-, SO₄^•-, and h⁺ are the significant species in the catalytic reaction. This work would provide a feasible strategy to exploit efficient g-C₃N₄-based material for PMS activation.

Degradation of phenolic pollutants by persulfate-based advanced oxidation processes: metal and carbon-based catalysis

Wastewater recycling is a solution to address the global water shortage. Phenols are major pollutants in wastewater, and they are toxic even at very low concentrations. Advanced oxidation process (AOP) is an emerging technique for the effective degradation and mineralization of phenols into water. Herein, we aim at giving an insight into the current state of the art in persulfate-based AOP for the oxidation of phenols using metal/metal-oxide and carbon-based materials. Special attention has been paid to the design strategies of high-performance catalysts, and their advantages and drawbacks are discussed. Finally, the key challenges that govern the implementation of persulfate-based AOP catalysts in water purification, in terms of cost and environmental friendliness, are summarized and possible solutions are proposed. This work is expected to help the selection of the optimal strategy for treating phenol emissions in real scenarios.

Chemiluminescence quenching capacity as a surrogate for total organic carbon in wastewater

Total organic carbon (TOC) is a valuable indicator to evaluate the degree of organic pollution in wastewater. Real-time analysis of TOC in wastewater can allow the wastewater treatment plants to manage the treatment process efficiently, avoid violations of the discharge regulations, and eliminate overtreatment. However, traditional methods for TOC determination are time-consuming. Benefitting from the rapid generation of SO₄^•- in the iron(II)-activated peroxymonosulfate (Fe(II)/PMS) system and the high reactivity of SO₄^•- towards naproxen as a chemiluminescence (CL) probe, a surrogate for TOC based on the determination of CL quenching capacity (CLQC) of organics in the Fe(II)/PMS-naproxen system was developed. According to the derived equation by considering both non-fluorescent and fluorescent quenching, the CLQC of organics in the Fe(II)/PMS-naproxen system was highly dependent on their TOC, making it to be a potential surrogate for TOC. The interferences of ubiquitous inorganic ions in wastewater on the determination of CLQC were leveled by adjusting electrical conductivity and adding mercury ions. Finally, the feasibility of CLQC as a surrogate for TOC in two real wastewaters containing different concentrations of inorganic anions was confirmed. This work can provide a TOC value within several seconds by determining the CLQC of wastewater with Fe(II)/PMS-naproxen system.

Quantification of the diverse inhibitory effects of dissolved organic matter on transformation of micropollutants in UV/persulfate treatment

Dissolved organic matter (DOM), ubiquitous in natural waters, is known to inhibit the degradation of micropollutants in the advanced oxidation processes such as the UV/peroxydisulfate process. However, the quantitative understanding of the inhibitory pathways is missing. In this study, guanosine, aniline and catechol belonging to amines, purines and phenols were first investigated due to their resistance to UV irradiation at 254 nm and similar reactivity with SO₄^•- and HO^•, respectively. The presence of 0.5 mg_C L^-1 Suwannee River NOM (SRNOM) inhibited their degradation rates by 72.9%, 54.5%, and 32.4%, respectively, despite their similar degradation rates in the absence of SRNOM. The results highlight the importance of reverse reduction of oxidation intermediates to the parent compound by antioxidant moieties in SRNOM besides the inner filtering and radical scavenging effects. The three inhibitory pathways were quantified for 34 common micropollutants. In the presence of 0.5 mg_C L^-1 SRNOM, inner filtering effect was found to contribute less than 2.8% of the inhibitory percentages (IP). Radical scavenging effects contribute between 10.7% and 38.9% and compounds having lower reactivity with SO₄^•- (< 4.0 × 10⁹ M^-1 s^-1) tended to be inhibited more strongly. The IP of reverse reduction effects of SRNOM varied significantly from none up to 70.8%. It was linearly related with a micropollutant's reduction potential. Purines and amines generally exhibited more pronounced reverse reduction inhibition than phenols. The results of this study provide guidance on improving the elimination efficiency of micropollutants.

Sulfide enhances the Fe(II)/Fe(III) cycle in Fe(III)-peroxymonosulfate system for rapid removal of organic contaminants: Treatment efficiency, kinetics and mechanism

The activation of peroxymonosulfate (PMS) by Fe(II) or Fe(III) for environmental decontamination is severely limited by the low conversion rate from Fe(III) to Fe(II). Here, we found that this puzzling problem could virtually be solved by introducing trace amounts of S^2-. With the addition of 0.2 mM S^2-, the bisphenol A (BPA) degradation efficiency and total organic carbon (TOC) removal in PMS/Fe(III) system were improved by 3.8 and 6.0 times, respectively. Meanwhile, the k_obs and PMS utilization efficiency also markedly increased by 650% and 160%, respectively. The constructed PMS/Fe(III)/S^2- system exhibited a good applicability to a wide pH range (3.2 ~ 9.5) and high resistance to humic acid, Cl^- and NO₃^-. The main reactive oxidant species in PMS/Fe(III)/S^2- system were identified by scavenging experiments, electron paramagnetic resonance measurement, chemical probe approach, and ¹⁸O isotope-labeling technique. The identification results revealed that Fe^IVO²⁺ was the primary reactive oxidant species, while •OH, SO₄^•-, O₂^•- and ¹O₂ were also involved in the degradation of BPA. Finally, the generalizability of PMS/Fe(III)/S^2- system was evaluated by varying the target pollutants, oxidants, and reducing S species. The construction of PMS/Fe(III)/S^2- system provides some insights into the treatment of organic wastewaters containing S^2-, e.g., from refineries and tanneries.

Recent progress on in-situ chemical oxidation for the remediation of petroleum contaminated soil and groundwater

Accidental oil leaks and spills can often result in severe soil and groundwater pollution. In situ chemical oxidation (ISCO) is a powerful and efficient remediation technology. In this review, the applications and recent advances of three commonly applied in-situ oxidants (hydrogen peroxide, persulfate, and permanganate), and the gap in remediation efficiency between lab-scale and field-scale applications is critically assessed. Feasible improvements for these measures, especially solutions for the 'rebound effect', are discussed. The removal efficiencies reported in 108 research articles related to petroleum-contaminated soil and groundwater were analyzed. The average remediation efficiency of groundwater (82.7%) by the three oxidants was higher than that of soil (65.8%). A number of factors, including non-aqueous phase liquids, adsorption effect, the aging process of contaminants, low-permeability zones, and vapor migration resulted in a decrease in the remediation efficiency and caused the residual contaminants to rebound from 19.1% of the original content to 57.7%. However, the average remediation efficiency of ISCO can be increased from 40.9% to 75.5% when combined with other techniques. In the future, improving the utilization efficiency of reactive species and enhancing the contact efficiency between oxidants and petroleum contaminants will be worthy of attention. Multi-technical combinations, such as the ISCO coupled with phase-transfer, viscosity control, controlled release or natural attenuation, can be effective methods to solve the rebound problem.

Kinetics and mechanisms of flumequine degradation by sulfate radical based AOP in different water samples containing inorganic anions

Many studies have reported that hydroxyl radical (HO˙) driven advanced oxidation processes (AOPs) could degrade fluoroquinolones (FQs) antibiotics effectively. Compared with HO˙, sulfate radical (SO₄˙⁻) shows a similar oxidation capacity but a longer half-life. SO₄˙⁻ could cause chain reactions and resulted in the generation of halogen radicals and carbonate radicals from the main anions in sea water including Cl⁻, Br⁻ and HCO₃⁻. However, few studies were focused on the degradation of FQs in marine aquaculture water and seawater, as well as the bioaccumulation of transformation products. As a typical member of FQs, flumequine (FLU) was degraded by UV/peroxodisulfate (PDS) AOPs in synthetic fresh water, marine aquaculture water and seawater. The reaction rate constants in the three water samples were 0.0348 min⁻¹, 0.0179 min⁻¹ and 0.0098 min⁻¹, respectively. The reason was attributed to the inhibition of the anions as they could consume SO₄˙⁻ and initiate the quenching reaction of free radicals. When the pH value increased from 5 to 9, the reaction rate decreased from 0.0197 min⁻¹ to 0.0066 min⁻¹. The energy difference between HOMO and LUMO of FLU was calculated to be 8.07 eV indicating that FLU was a stable compound. The atoms on quinolone ring of FLU with high negative charge would be more vulnerable to attack by free radicals through electrophilic reactions. Two possible degradation pathways of FLU were inferred according to the degradation products. Preliminary bioaccumulation analysis of transformation products by the EPI suite software proved that the values of log K_ow and log BCF of the final product P100 were less than those of FLU and the intermediates.

Many studies have reported that hydroxyl radical (HO˙) driven advanced oxidation processes (AOPs) could degrade fluoroquinolones (FQs) antibiotics effectively.