Evaluation of hydroxyl radical and reactive chlorine species generation from the superoxide/hypochlorous acid reaction as the basis for a novel advanced oxidation process

Insights into the inhibitory effects of trichloroisocyanuric acid disinfectant on the phototransformation of polypropylene microplastics

The chemical components in the natural aquatic environment have the potential to be involved in phototransformation of microplastics (MPs). Little information is available regarding the mediation effects of artificially introduced chemicals on MP phototransformation, especially those used in aquaculture water that are vulnerable to human interference. Herein, this study investigated the phototransformation process and mechanism of polypropylene microplastic (PP MPs) in presence of trichloroisocyanuric acid (TCCA) disinfectant with unique properties unlike the conventional inorganic chlorine disinfectants. The results showed that the presence of TCCA inhibited the surface photooxidation of PP MPs. Analysis of PP MP surface and reaction filtrate indicated that the inhibitory effects were likely derived from TCCA derivatives and the weakening in promoting effect of polypropylene microplastic-derived dissolved organic matter (PP-DOM) as photolytic byproducts, with the more important role of free chlorine in initial period and that of other chlorine species (i.e., the adsorbed chloride ions (Cl-), newly formed carbon-chlorine (CCl) bonds, chlorinated cyanurates, and chlorinated products) in middle and later period. The study highlights for the first time the important role of chlorine species derived from TCCA in phototransformation process of co-existed PP MPs and proposes a previously unrecognized phototransformation pathway, which will provide a new understanding and knowledge for the environmental behavior of MPs in aquaculture environment.

Environmental implications of superoxide radicals: From natural processes to engineering applications

The roles of superoxide radical (O2•-) in the domains of physiological, physical, and material chemistry are becoming increasingly recognized. Although extensive efforts have been directed to understand O2•- functions in diverse aquatic systems, there is a lack of systematic and in-depth review for its kinetics and mechanisms in various environmental scenarios. This review aims to bridge this gap through discussion of O2•- generation pathways under both natural and controlled conditions. The merits and limitations of the generation and detection methods under various conditions are compared, with emphasis on different approaches for the determination of O2•--triggered reaction kinetics. We summarize the reaction rate constants of O2•- with organic contaminants covering a wide diversity of structures and reactivity. The comparison indicates that O2•- exhibits weak reactivity with most contaminants and lacks selectivity towards compounds with different functional groups, except with quinones which exhibit higher reactivity compared to non-quinones. Further, the reaction mechanisms, namely single electron transfer, nucleophilic substitution, hydrogen atom abstraction, and radical-adduct formation, are critically evaluated. Various environmental implications of O2•- are highlighted including maintenance of biogeochemical iron cycle, synthesis of nanoparticles for antibacterial purposes, desorption of contaminants from heterogeneous interfaces, and synergetic degradation of contaminants.

One electron oxidation-induced degradation of brominated flame retardants in electroactive membrane filtration system: Vital role of dichlorine radical-mediated process

Reactive chlorine species (RCS) are inevitably generated in electrochemical oxidation process for treating high-salinity industrial wastewater, thereby resulting in the competition with coexisting hydroxyl radicals (•OH) for oxidizing recalcitrant organic compounds. Due to the low redox potentials compared to •OH, the role of RCS has been often overlooked. In this work, we developed an electroactive membrane filtration (EMF) system that had a high removal efficiency (99.1 ± 0.5 %) for tetrabromobisphenol S (TBBPS) at low energy consumption (1.45 kWh m-3). Electron spin resonance spectroscopy and molecular probing tests indicated the predominance of Cl2•-, of which steady-state concentration (2.2 ×10-10 M) was extremely higher than those of ClO• (6.7 ×10-13 M), •OH (0.95 ×10-13 M), and Cl• (2.39 ×10-15 M). The density functional theory (DFT) and intermediate product analysis highlighted that Cl2•- radicals had a higher electrophilic attack efficacy than •OH radicals for inducing changes in the electron density of the carbon atoms around phenolic hydroxyl groups, thus leading to the generation of transition state intermediates and accelerating the degradation of TBBPS. Our work demonstrates the vital role of Cl2•- radicals for pollutant degradation, highlighting the potential of this technology for cost-effective removal of recalcitrant organic compounds from water and wastewater.

MXene-based composite aerogels with bifunctional ferrous ions for the efficient degradation of phenol from wastewater

Herein, MXene-based composite aerogel (MXene-Fe2+ aerogel) are constructed by a one-step freeze-drying method, using Ti3C2Tx MXene layers as substrate material and ferrous ion (Fe2+) as crosslinking agent. With the aid of the Fe2+ induced Fenton reaction, the synthesized aerogels are used as the particle electrodes to remove phenol from wastewater with three-dimensional electrode technology. Combined with the dual roles of Fe2+ and the highly conductive MXene, the obtained particle electrode possesses extremely effective phenol degradation. The effects of experiment parameters such as Fe2+ to MXene ratio, particle electrode dosage, applied voltage, and initial pH of solution on the removal of phenol are discussed. At pH = 2.5, phenol with 50 mg/L of initial concentration can be completely removed within 50 min at 10 V with the particle electrode dosage of 0.56 g/L. Finally, the mechanism of degradation is explored. This work provides an effective way for phenol degradation by MXene-based aerogel, which has great potential for the degradation of other organic pollutants in wastewater.

From Theory to Practice: Leveraging Chemical Principles To Improve the Performance of Peroxydisulfate-Based In Situ Chemical Oxidation of Organic Contaminants

In situ chemical oxidation (ISCO) using peroxydisulfate has become more popular in the remediation of soils and shallow groundwater contaminated with organic chemicals. Researchers have studied the chemistry of peroxydisulfate and the oxidative species produced upon its decomposition (i.e., sulfate radical and hydroxyl radical) for over five decades, describing reaction kinetics, mechanisms, and product formation in great detail. However, if this information is to be useful to practitioners seeking to optimize the use of peroxydisulfate in the remediation of hazardous waste sites, the relevant conditions of high oxidant concentrations and the presence of minerals and solutes that affect radical chain reactions must be considered. The objectives of this Review are to provide insights into the chemistry of peroxydisulfate-based ISCO that can enable more efficient operation of these systems and to identify research needed to improve understanding of system performance. By gaining a deeper understanding of the underlying chemistry of these complex systems, it may be possible to improve the design and operation of peroxydisulfate-based ISCO remediation systems.

Recycling of nutrient medium to improve productivity in large-scale microalgal culture using a hybrid electrochemical water treatment system

Recycling and reusing of nutrient media in microalgal cultivation are important strategies to reduce water consumption and nutrient costs. However, these approaches have limitations, e.g., a decrease in biomass production, (because as reused media can inhibit biomass growth). To address these limitations, we applied a novel membrane filtration‒electrolysis‒ultraviolet hybrid water treatment method capable of laboratory-to-large-scale operation to increase biomass productivity and enable nutrient medium disinfection and recycling. In laboratory-scale experiments, electrolysis effectively remove the biological contaminants from the spent nutrient medium, resulting in a high on-site removal efficiency of dissolved organic carbon (DOC; 80.3 ± 5 %) and disinfection (99.5 ± 0.2 %). Compared to the results for the recycling of nutrient medium without water treatment, electrolysis resulted in a 1.5-fold increase in biomass production, which was attributable to the removal of biological inhibitors from electrochemically produced oxidants (mainly OCl-). In scaled-up applications, the hybrid system improved the quality of the recycled nutrient medium, with 85 ± 2 % turbidity removal, 75 ± 3 % DOC removal, and 99.5 ± 2 % disinfection efficiency, which was beneficial for biomass growth by removing biological inhibitors. After applying the hybrid water treatment method, we achieved a Spirulina biomass production of 0.47 ± 0.03 g L-1, similar to that obtained using a fresh medium (0.53 ± 0.02 g L-1). The on-site disinfection process described herein is practical and offers a cost-saving and environmental friendly alternative for nutrient medium recycling and reusing water in mass and sustainable cultivation of microalgae.

Activation of persulfate with natural organic acids (ascorbic acid/catechin hydrate) for naproxen degradation in water and soil: Mechanism, pathway, and toxicity assessment

In this study, we investigated the effects of different natural organic acids (NOAs), L-ascorbic acid (AA) and (+)-catechin hydrate (CAT), on the activation of persulfate (PDS) for the oxidation of naproxen (NAP) in water and soil. We found that only AA-activated PDS process had a significant degradation efficiency of NAP in water. High AA concentration (500 μM) inhibited the degradation of NAP, whereas high levels of PDS (7.5 mM) and acidic conditions (pH=3-7) were beneficial for NAP degradation. In soil, both CAT and AA promoted PDS activation and NAP degradation. Low soil organic matter and high Fe/Mn-mineral contents were favorable for NAP degradation by AA-activated PDS. Column experiments confirmed that NAP was readily transported and degraded under porous medium conditions using AA-activated PDS. Moreover, we revealed that SO4•- and HO• were the dominant reactive species for NAP degradation by AA-activated PDS. Intermediate products of NAP in the AA-activated PDS process were analyzed and the reactive sites of NAP were predicted. E. coli growth tests verified that the intermediate products in the AA-activated PDS process were less toxic than NAP. Our results highlight the high potential of NOAs-activated PDS process for the remediation of NAP-contaminated water and soil.

A sequential aerated electrocoagulation and peroxicoagulation process for the treatment of municipal stabilized landfill leachate by iron and graphite electrodes

Electrochemical treatment has emerged as a viable technology for the treatment of leachate due to its efficient removal of ammonaical nitrogen and other recalcitrant organics. The main technical issues that prevent its practical deployment are restricted performance of a single electrochemical process and the lengthy tertiary treatment time required to achieve the disposal quality standards. This study demonstrates the performance of electrochemical treatments such as peroxicoagulation (PC) and aerated electrocoagulation (A-EC) separately and also sequentially for the treatment of stabilized leachate. In aerated electro coagulation iron is used as both anode and cathode, whereas in peroxicoagulation, iron is used as anode and graphite as cathode. The area of electrode used for treatments was fixed as 12.5 cm2. The initial concentration of NH4-N, TN, COD, and TOC of the leachate was found to be 480 mg/L, 997 mg/L, 40,200 mg/L, and 9850 mg/L respectively. Removal efficiency of aerated electrocoagulation for NH4-N, TN, COD and TOC were 25.6%, 23.67%, 25.6% and 28.7% respectively, current density of 30 mAcm-2, electrolysis time of 60 min and pH 7.3. Meanwhile for peroxicoagulation, the removal efficiency was found to be 37.2%, 43%, 37.3%, and 45.6% for NH4-N, TN, COD, and TOC respectively, at an current density of 30 mAcm-2, electrolysis time of 120 min and a pH of 3. The sequential aerated electrocoagulation - peroxicoagulation process achieves a maximum removal efficiency of 63%, 68%, 78%, and 75% for NH4-N, total nitrogen, COD, and TOC respectively for a reaction time of 180 min. Removal of NH4-N, total nitrogen, COD and TOC from stabilized landfill leachate with a BOD/COD ratio less than 0.1 was very much effective with the sequential aerated electrocoagulation - peroxicoagulaton treatment. The results also indicate that for the treatment of leachate, a significant synergistic index of 1.22 exists between aerated electrocoagulation and peroxicoagulation.

Simultaneous removal of antibiotics and antibiotic resistant genes using a CeO2@CNT electrochemical membrane-NaClO system

The simultaneous removal of antibiotic and antibiotic resistance genes (ARGs) are important to inhibit the spread of antibiotic resistance. In this study, a coupled treatment system was developed using a CeO2 modified carbon nanotube electrochemical membrane and NaClO (denoted as CeO2@CNT-NaClO) to treat simulated water samples containing antibiotics and antibiotic-resistant bacteria (ARB). As the mass ratio of CeO2 to CNT was 5:7 and the current density was 2.0 mA/cm2, the CeO2@CNT-NaClO system removed 99% of sulfamethoxazole, 4.6 log sul1 genes, and 4.7 log intI1 genes from the sulfonamide-resistance water samples, and removed 98% of tetracycline, 2.0 log tetA genes, and 2.6 log intI1 genes of the tetracycline-resistance water samples. The outstanding performance of the CeO2@CNT-NaClO system for simultaneously removing antibiotic and ARGs was mainly ascribed to the generation of multiple reactive species, including •OH, •ClO, •O2- and 1O2. Antibiotics can undergo efficient degradation by •OH. However, the reaction between •OH and antibiotics reduces the availability of •OH to permeate into the cells and react with DNA. Nevertheless, the presence of •OH enhancd the effects of •ClO, •O2-, and 1O on ARG degradation. Through the coupled action of •OH, •ClO, •O2-, and 1O2, the cell membranes of ARB experience severe damage, resulting in an increase in intracellular reactive oxygen species (ROS) and a decrease in superoxide dismutase (SOD) activity. Consequently, this coordinated mechanism leads to superior removal of ARGs.

Highly Efficient Photocatalytic Degradation of Tetracycline by Modifying UiO-66 via Different Regulation Strategies

Wastewater containing organic pollutants cause potential harm to the environment and human health. A series of zirconium-organic frameworks (UiO-66) and their composites were synthesized by solvothermal methods, including band gap adjustment, heterojunction construction, and metal ion doping. For the model pollutant tetracycline (TC), all of the prepared catalysts could achieve effective degradation of it. Therein, the degradation efficiency of tetracycline could reach 95% under the UV irradiation with the aid of the catalyst, in which the UiO-66-NDC was modified with P-C3N4. The free radical capture experiments demonstrated that the superoxide radical (•O2-) was the main oxidizing species for the photodegradation of tetracycline. Hence, the improvement strategy of the catalyst would provide some enlightenment for the development of more efficient photocatalysts for the degradation of organic dyes in wastewater.

CeO2 modified carbon nanotube electrified membrane for the removal of antibiotics

Electrified carbon nanotube membranes (ECM) are used as electroactive porous materials for the degradation of micropollutants. It integrated design of both electrochemical processes and filtration functions. In this study, CeO₂ modified carbon nanotube electrified membrane (CeO₂@CNT membrane) was prepared and activate NaClO towards degradation of antibiotics. As CeO₂ with face-centered cubic (Fcc) fluorite structure was loaded onto the CNT sidewalls, the CeO₂@CNT membrane showed a higher over potential and a smaller equivalent polarization resistance compared to ECM. More reactive oxygen species (ROS) and reactive chlorine species (RCS) were generated by CeO₂@CNT membrane due to faster electron transfer at the solid-liquid interface. Thus, the removal efficiencies of DCF, SMX, CIP, TC and CBZ were more than 91.2%, 91.3%, 94.4%, 99.3% and 89.4% by the CeO₂@CNT membrane with NaClO, respetively. And the apparent reaction rate constant (k) of the CeO₂@CNT membrane was 2.9 times of that of ECM. The selective capping experiments and density functional theory (DFT) calculation showed that the oxygen vacancies of CeO₂ contributed to the generation of ‧OH, and the generation of ClO‧ and ‧O₂^- would mainly occur on Lewis acid sites of CeO₂. In addition, the CeO₂@CNT membrane showed a reasonable stability to treat actual water samples and reduced disinfection byproducts (DBPs) formation, suggesting that it can potentially be combined with the conventional chlorine disinfection to degrade antibiotics in water.

Mechanisms of oxidative removal of 1,4-dioxane via free chlorine rapidly mixing into monochloramine: Implications on water treatment and reuse

Free chlorine (HOCl) and monochloramine (NH₂Cl) are two of the most commonly used water disinfectants in water treatment; however, the capability of rapid mixing of HOCl into NH₂Cl to induce oxidative reactions for efficient removal of contaminants remains largely unknown. In this study, 1,4-dioxane (1,4-D) removal was quantified during the rapid mixing of HOCl into NH₂Cl, to evaluate the effects of solution pH and HOCl-to-NH₂Cl ratio, and to identify mechanisms by which reactive species are generated in the system. Results showed that the highest 1,4-D removal was observed at the near-neutral pH of 6 with the HOCl-to-NH₂Cl molar ratio of 1. Hydroxyl radical (HO^•) contributed to 60-70 % of 1,4-D degradation and its generation was initiated by the hydrolytic decay of NH₂Cl and NHCl₂ upon HOCl addition to NH₂Cl with rapid mixing, and subsequent transformation of peroxynitrite (ONOO^-) and peroxynitrous acid (ONOOH). The results also confirmed that the presence of dissolved oxygen was required to form ONOO^-/ONOOH, and ONOO^- was a crucial precursor for reactive radical generation. These findings provide insight into the reaction mechanism associated with the system of rapidly mixed HOCl into NH₂Cl with the potential optimization and application for efficient trace organics removal in water treatment and reuse.

Immobilized BiOCl0.75I0.25/g-C3N4 nanocomposites for photocatalytic degradation of bisphenol A in the presence of effluent organic matter

The BiOCl_0.75I_0.25/g-C₃N₄ nanosheet (BCI-CN) was successfully immobilized on polyolefin polyester fiber (PPF) through the hydrothermal method. The novel immobilized BiOCl_0.75I_0.25/g-C₃N₄ nanocomposites (BCI-CN-PPF) were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy EDS, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) to confirm that BCI-CN was successfully immobilized on PPF with abundant oxygen vacancies reserved. Under simulated solar light irradiation, 100 % of bisphenol A (BPA) with an initial concentration of 10 mg·L^-1 was degraded by BCI-CN-PPF (0.2 g·L^-1 of BCI-CN immobilized) after 60 min. A similar photocatalytic efficiency of BPA was obtained in the presence of effluent organic matter (EfOM). The photocatalytic degradation of BPA was not affected by EfOM <5 mg-C/L. In comparison, the photocatalytic performance was considerably inhibited by EfOM with a concentration of 10 mg-C/L. Furthermore, photogenerated holes and superoxide radicals predominated in the photocatalytic degradation processes of BPA. The total organic carbon (TOC) removal efficiencies of BPA and EfOM were 75.2 % and 50 % in the BCI-CN-PPF catalytic system. The BPA removal efficiency of 94.9 % was still achieved in the eighth cycle of repeated use. This study provides a promising immobilized nanocomposite with high photocatalytic activity and excellent recyclability and reusability for practical application in wastewater treatment.

Hydroxylamine promoted hydroxyl radical production and organic contaminants degradation in oxygenation of pyrite

The heterogeneous Fenton-like process using pyrite (FeS₂) is increasingly recognized as a promising advanced oxidation process for removal of organic contaminants. However, the slow regeneration of Fe(II) limits the generation of reactive oxygen species for environment implication. To overcome this drawback, hydroxylamine was applied to enhance the reactivity of FeS₂ to degrade organic contaminants under oxic conditions. Results showed that hydroxylamine facilitated the regeneration of Fe(II) on FeS₂ surface to promote reactive oxygen species generation, thereby efficiently degrading different organic contaminants. The underlying mechanism was further elucidated that the presence of hydroxylamine enhanced electron transfer from FeS₂ to O₂ to produce superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂) and hydroxyl radical (HO^•) via Fenton-like pathways, which induced the rapid degradation of organic contaminants (e.g., sulfamethoxazole (SMX)). The reactivity of FeS₂ for organic contaminant degradation changed negligibly after seven cycles in the presence of hydroxylamine. The effects of pH and inorganic anions on SMX degradation were also clarified in details. The finding of this study would provide a novel strategy to enhance the contaminants degradation by FeS₂-based advanced oxidation technologies for environmental remediation.

Enhanced Radical Generation in an Ultraviolet/Chlorine System through the Addition of TiO2

Ultraviolet (UV)/chlorine draws increasing attention for the abatement of recalcitrant organic pollutants. Herein, it was found that TiO₂ would significantly promote the degradation of dimethyl phthalate (DMP) in the UV/chlorine system (from 19 to 84%). Hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) were the dominant reactive species for DMP degradation in the UV/chlorine/TiO₂ system. Chlorine decayed much faster in UV/chlorine/TiO₂ compared with UV/chlorine, which is possibly because photogenerated electrons (e_cb^-) and superoxide radicals (O₂^•-) have high reactivity with chlorine. As a result, the recombination of photogenerated holes (h_vb⁺) and e_cb^- was inhibited and the accumulation of HO^• and Cl^• was facilitated. A kinetic model was established to simulate the reaction process, and it was found that the concentrations of HO^• and Cl^• were several times to dozens of times higher in UV/chlorine/TiO₂ than that in UV/chlorine. The contributions of HO^• and Cl^• to DMP degradation were 70.3 and 29.7% by model simulation, respectively, and were close to the probe experiment result. In the UV/chlorine/TiO₂ system, the degradation of DMP did not follow pseudo-first-order kinetics but the degradation of benzoate fitted well with pseudo-first-order kinetics. This phenomenon was elucidated by the structure of the pollutant and TiO₂ and further tested by calculating the adsorption energy (E_ads)/binding energy (E_b) with density functional theory. Due to faster decay of chlorine, lower amounts of disinfection byproducts formed in UV/chlorine/TiO₂ compared with UV/chlorine. Adding TiO₂ into the UV/chlorine system can promote the degradation of recalcitrant organic pollutants in an aqueous environment.