Degradation of tetracycline by atmospheric-pressure non-thermal plasma: Enhanced performance, degradation mechanism, and toxicity evaluation

CoNi-MOF laccase-like nanozymes prepared by dielectric barrier discharge plasma for treatment of antibiotic pollution

Laccase is a natural green catalyst and utilized in pollution treatment. Nevertheless, its practical application is constrained by limitations including high cost, poor stability, and difficulties in recovery. Herein, with inspiration from catalytic mechanism of natural laccase, we designed and prepared a bimetallic metal-organic framework, namely, CoNi-MOF, using low-temperature plasma (LTP) technology. We employed dielectric barrier discharge (DBD) plasma to prepare CoNi-MOF, and by precisely modulating the N2/O2 gas ratio, we could modulate the distribution concentration of oxygen vacancies in CoNi-MOF. Experimental investigations and density functional theory (DFT) calculations elucidated that the critical role of the oxygen vacancies in enhancing the laccase-like activity, which promoted the activation of molecular oxygen (O2) for generation of reactive oxygen species (ROS). Compared to natural laccase, CoNi-MOF exhibited superior catalytic performance in the degradation of antibiotic tetracycline (TC), along with enhanced resistance to harsh environmental conditions, improved stability, and low biotoxicity. Notably, aeration increased the dissolved oxygen (DO) content, further improving the TC degradation efficiency. As such, this study not only proposes a facile and efficient low-temperature plasma technology for synthesizing high-performance laccase-like nanozymes but also provides a promising and environmentally friendly strategy for the remediation of antibiotic contamination in the environment.

Coal Fly Ash-Based Adsorbents for Tetracycline Removal: Comparative Insights into Modification and Zeolite Conversion

Emerging xenobiotics, such as tetracycline (TC), pose significant risks to both the environment and human health. Adsorption is a recognized method for removing these contaminants, and in this study, fly ash (FA), a by-product of coal combustion, was modified to develop adsorbents. Acid-modified FA (AM-FA) and base-modified FA (BM-FA) were prepared, and zeolite Na-P1 (ZNa-P1) was synthesized via hydrothermal treatment. Adsorption tests revealed that BM-FA and ZNa-P1 removed 76% and 90% of TC, respectively, compared to 35% with unmodified FA. AM-FA had the lowest performance, removing just 11% of TC. ZNa-P1's superior performance was linked to its high zeolite purity, with a cation exchange capacity (CEC) of 6.37 meq/g and a surface area of 35.7 m2/g. Though BM-FA had a larger surface area of 110.8 m2/g, it exhibited a lower CEC of 3.42 meq/g. Adsorption efficiency was more closely related to CEC than surface area. Optimal TC removal with ZNa-P1 was achieved at a 7.5 g/L dosage and pH 5. The process followed pseudo second order kinetics and the Langmuir isotherm, with a maximum capacity of 46.34 mg/g at 30 °C. The adsorption thermodynamics indicated that the adsorption was endothermic and spontaneous. The adsorption mechanism of tetracycline on ZNa-P1 involved electrostatic attraction, hydrogen, and ion exchange. This study aligns with SDGs 6 (Clean Water and Sanitation) and 12 (Responsible Consumption and Production).

Vacuum ultraviolet radiation from gaseous plasma for destruction of water contaminants

Innovative technological solutions are needed for water decontamination to combat the diverse pollutants present in water systems, as no single optimal decontamination technique is appropriate for all circumstances. Vacuum-ultraviolet (V-UV) radiation is a source of energetic photons that break molecular bonds, producing a plethora of chemically reactive agents, most notably OH● radicals, which can cause the degradation of harmful pollutants. Low-pressure gaseous plasma is a good source of V-UV radiation; however, its application to liquid water poses challenges. We constructed an inductively coupled radiofrequency plasma to produce high-intensity V-UV radiation, which was applied to contaminated water via a V-UV-transparent window. Plasma was sustained in hydrogen, as it produces the highest V-UV intensity among all gases at selected discharge parameters. Bacteriophage MS2 was used as an indicator of microbial decontamination efficiency. Reactive oxygen and nitrogen species were measured at various treatment setups to quantify their effect on MS2 inactivation and elucidate the primary inactivation factors. At optimal conditions, the concentration of active virus dropped by 9 log10 PFU/mL in 60 s. The optimal experimental setup was then used to treat bacteria E. coli, S. aureus, antibiotic tetracycline, and synthetic dye methylene blue as representatives of other types of pollutants, all of which were effectively removed/degraded within 10 min of treatment. A comparison of energy efficiency (EEO) to other disinfection setups was made for bacteriophage inactivation. With a low EEO value, we showcase the potential of this technique for further work in this field.

Investigation of Dielectric Barrier Discharge Plasma for the Degradation of Erythromycin Solution

Antibiotic contamination constitutes a serious environmental and public health risk. In order to fill the gap in the study of plasma degradation of erythromycin (ERY), this paper systematically investigated the mechanism of ERY degradation by dielectric barrier discharge (DBD) plasma. The underlying reaction mechanisms were investigated by experiments and molecular dynamics simulations. Plasma emission spectra revealed active hydroxyl radicals (·OH) and argon (Ar) spectral lines. The degradation efficiency of plasma treatment for ERY was found to be strongly influenced by treatment parameters, including applied voltage, treatment duration, and gas flow rate. In particular, a maximum degradation of 90% was achieved for a 250 mg/L ERY solution under conditions of 18 kV voltage, 850 sccm gas flow rate, and 60 min of treatment. The presence of ·OH and hydrogen peroxide (H2O2) in the reaction and their important role in the degradation were proved experimentally. Fracture of the ERY lactone ring induced by hydrogen abstraction reactions with reactive oxygen species (ROS) was observed by molecular dynamics simulations. In the in vitro antimicrobial assays targeting Staphylococcus aureus, the treated solutions demonstrated low toxicity, underscoring the practical applicability of dielectric barrier discharge (DBD) plasma technology in addressing antibiotic contamination in aquatic environments.

Essentials and Pertinence of Cold Plasma in Essential Oils, Metal-Organic Frameworks and Agriculture

Cold atmospheric pressure plasma (CAPP) comprises an ensemble of ionized gas, neutral particles, and/or reactive species. Electricity is frequently used to produce CAPP via a variety of techniques, including plasma jets, corona discharges, dielectric barrier discharges, and glow discharges. The type and flow rates of the carrier gas(es), temperature, pressure, and vacuum can all be altered to control the desired properties of the CAPP. Since a few decades ago, CAPP has become a widely used technology with applications in every walk of life. The plasma activated liquid mediums like water, ethanol, and methanol have been merged as novel sterilizers. With recent advancements in material science, particularly work on metal-organic frameworks (MOFs), essential oils, and agricultural technologies, CAPP has become a vital component of these advancements. Likewise, CAPP has been found as a green and benign technology to induce early seed germination and plant development. This review covers the critical components of CAPP, the production of reactive oxygen and nitrogen species, and mechanisms by which CAPP-based technologies are applied to agricultural products, MOFs, and essential oils.

Highly efficient treatment of tetracycline using coupled electro-Fenton and electrocoagulation process: Mechanism and toxicity assessment

In this study, a novel carbon fiber brush (CFB) electrode was designed using carbon fiber filaments and conductive metals. It was used as the cathode to construct an efficient coupled electro-Fenton and electrocoagulation (EF-EC) process for tetracycline (TC) treatment. An optimal 97.9% removal rate of 10 mg L-1 TC was achieved within 20 min. The coupled process is less pH-dependent and more effective in treating TC compared to the traditional individual electro-Fenton (EF) or electrocoagulation (EC) process, achieving efficient TC removal under neutral pH conditions. The removal rate of 10 mg L-1 TC consistently remained above 92% at 20 min after ten cycle experiments using the same electrodes in a Fe-CFB system (92.7-97.9%), indicating excellent reusability and stability of the CFB cathode. Mechanism analysis showed both EF and EC processes were involved in the system. Radicals (such as •OH and SO4-•) generated by EF contributed to the degradation of TC, yielding nine intermediates. Coagulants (such as Fe(OH)3) generated by EC contributed to the removal of TC. Toxicity prediction results indicated that over half of the nine intermediates exhibited lower biotoxicity compared to TC. This study provides a feasible alternative cathode for the efficient treatment of TC using EF-EC process.

Integrated assessment of the spatial distribution, sources, degradation, and human risk of tetracyclines in honey in China

Tetracyclines are widely used in Chinese apiculture. However, limited information is available on the presence of tetracycline residues in honey and the sources, degradation patterns, and associated health risks of these compounds. In this study, the presence of tetracyclines in honey samples across China was investigated over a four-year period. Additionally, the risks of dietary intake, as well as the sources and degradation patterns of tetracyclines in honey, were assessed. The three-dimensional spatial distributions (floral region, geographical region and entomological origin) of tetracyclines contamination varied significantly. Tetracycline residues in honey posed a moderate risk to children aged 3-10 years in Northwest China. Source analysis indicated that colony migration serves as the primary source of tetracyclines in honey. Based on the degradation patterns of tetracyclines in honey within colonies and during storage, oxytetracycline is more readily degraded than other tetracyclines. The main degradation products of tetracyclines are epimers and dehydration products, and the effects of these products on human health and the environment should be further evaluated in future studies. This comprehensive investigation provides valuable insights into the safe use and regulation of tetracyclines in Chinese apiculture.

Synergistic effect on simultaneous treatment of Cr(VI) and chloramphenicol using a non-thermal plasma technology

Toxic organic and heavy metal contaminants commonly exist in industrial waste stream(s) and treatment is of great challenge. In this study, a dielectric barrier discharge (DBD) non-thermal plasma technology was employed for the simultaneous treatment of two important contaminants, chloramphenicol (CAP) and Cr(VI) in an aqueous solution through redox transformations. More than 70% of CAP and 20% of TOC were degraded in 60 min, while Cr(VI) was completely removed in 10 min. The hydroxyl radicals were the main active species for the degradation. Meanwhile, the consumption of hydroxyl radicals was beneficial to the reduction of Cr(VI). The synergistic effect was investigated between CAP degradation and Cr(VI) reduction. The reduction of Cr(VI) would be enhanced in the presence of CAP with a low concentration and could be inhibited under a high concentration, because part of hydroxyl radicals could be consumed by the low-concentration CAP and the obtained intermediates with a higher kinetic rate. However, CAP with a high concentration could react with such reductive species as eaq- and •H, which could compete with Cr(VI) and inhibit the reduction. In addition, the presence of Cr(VI) enhanced the degradation and mineralization of CAP; the study of obtained intermediates indicated that the presence of Cr(VI) changed the degradation path of CAP as Cr(VI) would react with reductive species, enhance the generation of hydroxyl radicals, and cause more hydroxylation reactions. Moreover, the mechanism for the simultaneous redox transformations of CAP and Cr(VI) was illustrated. This study indicates that the DBD non-thermal plasma technology can be one of better solutions for simultaneous elimination of heavy metal and organic contaminants in aquatic environments.

The Potential of Cold Atmospheric Pressure Plasmas for the Direct Degradation of Organic Pollutants Derived from the Food Production Industry

Specialized chemicals are used for intensifying food production, including boosting meat and crop yields. Among the applied formulations, antibiotics and pesticides pose a severe threat to the natural balance of the ecosystem, as they either contribute to the development of multidrug resistance among pathogens or exhibit ecotoxic and mutagenic actions of a persistent character. Recently, cold atmospheric pressure plasmas (CAPPs) have emerged as promising technologies for degradation of these organic pollutants. CAPP-based technologies show eco-friendliness and potency for the removal of organic pollutants of diverse chemical formulas and different modes of action. For this reason, various types of CAPP-based systems are presented in this review and assessed in terms of their constructions, types of discharges, operating parameters, and efficiencies in the degradation of antibiotics and persistent organic pollutants. Additionally, the key role of reactive oxygen and nitrogen species (RONS) is highlighted. Moreover, optimization of the CAPP operating parameters seems crucial to effectively remove contaminants. Finally, the CAPP-related paths and technologies are further considered in terms of biological and environmental effects associated with the treatments, including changes in antibacterial properties and toxicity of the exposed solutions, as well as the potential of the CAPP-based strategies for limiting the spread of multidrug resistance.

Multi-level FeCo/N-doped carbon nanosheet for peroxymonosulfate oxidation and sterilization inactivation

Magnetic carbon-based catalysts with environmental friendliness have exhibited prominent effects on advanced oxidation processes. Herein, a multi-level FeCo/N-doped carbon nanosheet (FeCo/CNS) was synthesized by facile impregnation iron-cobalt salt onto cotton and followed by confined pyrolysis. We identified excellent advantages of the modified FeCo/CNS materials: (i) The convenience of the synthesis method and (ii) The dual effect of sterilization and contaminant degradation achieved through the FeCo/CNS-activated Peroxymonosulfate (PMS). The comparative experimental showed that FeCo/CNS could provide favorable catalytic performance, completely removing bisphenol A (BPA) and tetracycline (TC) within 5 min. Moreover, the potent sterilization properties against Staphylococcus aureus and Escherichia coli were also verified. Analysis of the degradation pathway confirmed the existence of intermediates, and toxicological research demonstrated that the toxicity of the degradation intermediates of BPA gradually decreased over time. Our research provided an excellent application of FeCo/CNS in PMS oxidation and sterilization inactivation.

Mechanistic explanation and influence of molecular structure on chemical degradation and toxicity reduction by hydroxyl radicals

This study explored the influence of structural characteristics of organic contaminants on the degradation during an advanced oxidation process (AOP). The target contaminants were acetaminophen (ACT), bisphenol A (BPA), and tetracycline (TC). The Fenton process was selected as the model process in which major reactive species of hydroxyl radicals in most AOPs are generated for target compound degradation. The optimal reagent concentration ratio was [Fe2+]/[H2O2] = 0.5 mM/0.5 mM in an acidic condition, resulting in 83.49%, 79.01%, and 91.37% removals of ACT, BPA, and TC, respectively. Contrarily, the mineralization rates were apparently lower compared to their respective removal efficiencies. Experimental observation also suggested that the aromatic structure was rather difficult to degrade since their unsaturated electron clouds would hinder the attack of hydroxyl radicals due to electric repulsion. The preferred attacking sites of an aromatic ring differ due to the functional groups and structure symmetry. However, the electrophilic attack of the hydroxyl radical is the major reaction for decomposing aliphatic structures of cyclic or branched organics, resulting in the highest removal and mineralization of TC among these three tested chemicals. In addition, an apparent removal of a contaminant may not necessarily reduce its toxic impact on the environment.

Cold plasma turns mixed-dye-contaminated wastewater bio-safe

The excessive and uncontrollable discharge of diverse organic pollutants into the environment has emerged as a significant concern, presenting a substantial risk to human health. Among the advanced oxidation processes used for the purification of wastewater, cold plasma technology is superior in fast and effective decontamination but often fails facing mixed pollutants. To address these issues, here we develop the new conceptual approach, plasma process, and proprietary reactor that ensure, for the first time, that the efficiency of treatment (114.7%) of two mixed organic dyes, methylene blue (MB) and methyl orange (MO), is higher than when the two dyes are treated separately. We further reveal the underlying mechanisms for the energy-efficient complete degradation of the mixed dyes. The contribution of plasma-induced ROS and the distinct degradation characteristics and mechanism of pollutants in mixed treatment are discussed. The electron transfer pathway revealed for the first time suggest that the mixed pollutants reduce the overall redox potentials and facilitate electron transfer during the plasma treatment, promoting synergistic degradation effects. The integrated frameworks including both direct and indirect mechanisms provide new insights into the high-efficiency mixed-contaminant treatment. The degradation products for mixed degradation are revealed based on the identification of intermediate species. The plasma-treated water is proven safe for living creatures in waterways and sustainable fishery applications, using in vivo zebrafish model bio-toxicity assay. Overall, these findings offer a feasible approach and new insights into the mechanisms for the development of highly-effective, energy-efficient technologies for wastewater treatment and reuse in agriculture, industry, and potentially in urban water networks.

Chemical scanning of atomic oxygen at the gas-liquid interface of a NaCl solution via quantum mechanics/molecular mechanics molecular dynamics simulations

Atmospheric pressure plasmas can serve as double phase reactors to produce plasma activated water for water treatment. However, the physical-chemical processes involving plasma-supplied atomic oxygen and reactive oxygen species in an aqueous solution remain unclear. In this work, quantum mechanics (QM)/molecular mechanics (MM) molecular dynamics simulations (MDs) have been performed to directly observe the chemical reactions occurring between atomic oxygen and a NaCl solution at the gas-liquid interface using a model containing 10,800 atoms. During simulations, the atoms in the QM and MM Parts are dynamically adjusted. To examine the effects of local microenvironments on the chemical processes, atomic oxygen is used as a chemical probe to scan the gas-liquid interface. The excited atomic oxygen reacts with water molecules and Cl- ions to produce H2O2, OH, HOCl, ClO-, and HO2-/H3O+ species. The ground-state atomic oxygen is significantly more stable than the excited atomic oxygen, although it can react with water molecules to produce OH radicals. However, the branch ratio of ClO- computed for triplet atomic oxygen is significantly larger than that determined for singlet atomic oxygen. This study can help achieve a better understanding of the fundamental chemical processes during plasma-treated solution experiments and promotes advances in applications of QM/MM calculations at the gas-liquid interface.

Non-thermal plasma activated peroxide and percarbonate for tetracycline and oxytetracycline degradation: Synergistic performance, degradation pathways, and toxicity evaluation

Tetracycline (TC) and Oxytetracycline (OTC) are common antibiotics increasingly detected in the environment, posing a potential risk to human and aquatic lives. Although conventional methods such as adsorption and photocatalysis are used for the degradation of TC and OTC, they are inefficient in removal efficiency, energy yield, and toxic byproduct generation. Herein, a falling-film dielectric barrier discharge (DBD) reactor coupled with environmentally friendly oxidants (hydrogen peroxide (HPO), sodium percarbonate (SPC), and HPO + SPC) was applied, and the treatment efficiency of TC and OTC was investigated. Experimental results showed that moderate addition of the HPO and SPC exhibited a synergistic effect (SF > 2), significantly improving the antibiotic removal ratio, total organic removal ratio (TOC), and energy yield by more than 50%, 52%, and 180%, respectively. After 10 min of DBD treatment, the introduction of 0.2 mM SPC led to a 100% antibiotic removal ratio and a TOC removal of 53.4% and 61.2% for 200 mg/L TC and 200 mg/L OTC, respectively. Also, 1 mM HPO dosage led to 100% antibiotic removal ratios after 10 min of DBD treatment and a TOC removal of 62.4% and 71.9% for 200 mg/L TC and 200 mg/L OTC, respectively. However, the DBD + HPO + SPC treatment method had a detrimental effect on the performance of the DBD reactor. After 10 min of DBD plasma discharge, the removal ratios for TC and OTC were 80.8% and 84.1%, respectively, when 0.5 mM HPO +0.5 mM SPC was added. Moreover, principal component and hierarchical cluster analysis confirmed the differences between the treatment methods. Furthermore, the concentration of oxidant-induced in-situ generated ozone and hydrogen peroxide were quantitatively determined, and their indispensable roles during the degradation process were established via radical scavenger tests. Finally, the synergetic antibiotic degradation mechanisms and pathways were proposed, and the toxicities of the intermediate byproducts were evaluated.

Chemical decontamination of foods using non-thermal plasma-activated water

The presence of chemical contaminants in foods and agricultural products is one of the major safety issues worldwide, posing a serious concern to human health. Nonthermal plasma (NTP) containing richly reactive oxygen and nitrogen species (RONS) has been trialed as a potential decontamination method. Yet, this technology comes with multiple downsides, including adverse effects on the quality of treated foods and limited exposure to entire surfaces on samples with hard-to-reach spots, further hindering real-life applications. Therefore, plasma-activated water (PAW) has been recently developed to facilitate the interactions between RONS and contaminant molecules in the liquid phase, allowing a whole surface treatment with efficient chemical degradation. Here, we review the recent advances in PAW utilized as a chemical decontamination agent in foods. The reaction mechanisms and the main RONS contributors involved in the PAW-assisted removal of chemical contaminants are briefly outlined. Also, the comprehensive effects of these treatments on food qualities (chemical and physical characteristics) and toxicological evaluation of PAW (in vitro and in vivo) are thoroughly discussed. Ultimately, we identified some current challenges and provided relevant suggestions, which can further promote PAW research for real-life applications in the future.

Synergistic catalysis induced by a multi-component system constructed by DBD plasma combined with α-Fe2O3/FeVO4/HCP and peroxymonosulfate for gatifloxacin removal

The dielectric barrier discharge (DBD) multi-component system containing plasma, α-Fe₂O₃/FeVO₄, and peroxymonosulfate (PMS) with high catalytic activity was successfully constructed. Thereinto, α-Fe₂O₃/FeVO₄ was loaded on the honeycomb ceramic plate (HCP) surface (α-Fe₂O₃/FeVO₄/HCP) and placed under the water surface below the discharge area. The catalytic activity was evaluated by the removal rate of gatifloxacin (GAT), and the DBD+α-Fe₂O₃/FeVO₄+PMS system exhibited the optimal catalytic activity. The enhanced catalytic activity can be attributed to the fact that the occurrence of synergistic catalysis that simultaneously includes plasma oxidation, photocatalysis, PMS oxidation, O₃ catalysis, and Fenton reaction. The effect of various initial degradation parameters including input power, PMS dosage, pH, etc. On GAT removal was investigated. DBD+α-Fe₂O₃/FeVO₄+PMS system has a significant increase in the concentration of H₂O₂ and O₃, and the role played in the multi-component system was analyzed. The identification and analysis of organic matters during GAT degradation were visualized with the help of 3D EEMs. HPLC-MS and theoretical calculations identified the major intermediates and further deduced the possible GAT degradation pathways. Additionally, the acute toxicity of the major intermediates was predicted by the QSAR model. Finally, the possible mechanisms of synergistic catalysis to enhance catalytic activity were discussed based on the characteristics of several advanced oxidation processes (AOPs) and the results of experimental and characterization. This work provides a feasible technical route and theoretical basis for wastewater treatment by plasma combined with other AOPs.

Bridging the Gap between Charge Storage Site and Transportation Pathway in Molecular-Cage-Based Flexible Electrodes

Porous materials have been widely applied for supercapacitors; however, the relationship between the electrochemical behaviors and the spatial structures has rarely been discussed before. Herein, we report a series of porous coordination cage (PCC) flexible supercapacitors with tunable three-dimensional (3D) cavities and redox centers. PCCs exhibit excellent capacitor performances with a superior molecular capacitance of 2510 F mmol^-1, high areal capacitances of 250 mF cm^-2, and unique cycle stability. The electrochemical behavior of PCCs is dictated by the size, type, and open-close state of the cavities. Both the charge binding site and the charge transportation pathway are unambiguously elucidated for PCC supercapacitors. These findings provide central theoretical support for the "structure-property relationship" for designing powerful electrode materials for flexible energy storage devices.

A novel scheme to improve the photo-Fenton performance of iron oxychloride by carbon: Three existent states and roles of carbon in the degradation of tetracycline in water

The photo-Fenton process is promising for sincerely treating contaminated water. In this work, carbon-decorated iron oxychloride (C-FeOCl) is synthesized as a photo-Fenton catalyst for removing tetracycline (TC) from water. Three actual states of carbon are identified and their different roles in enhancing photo-Fenton performance are revealed. All carbon on/in FeOCl, including graphite carbon, carbon dots and lattice carbon, enhance visible light adsorption. More importantly, a homogeneous graphite carbon on the outer surface of FeOCl accelerates the transportation-separation of photo-excited electrons along the horizontal direction of FeOCl. Meanwhile, the interlayered carbon dots offer a FeOC bridge in helping the transportation-separation of photo-excited electrons along the vertical direction of FeOCl. In this way, C-FeOCl acquires isotropy in conduction electrons to ensure an efficient Fe(II)/Fe(III) cycle. These interlayered carbon dots extend the layer spacing (d) of FeOCl to about 1.10 nm, exposing the internal iron centers. The lattice carbon significantly increases the amounts of coordinatively unsaturated iron sites (CUISs) in activating hydrogen peroxide (H₂O₂) to hydroxyl radical (OH). Density functional theory (DFT) calculations confirm this activation on inner and external CUISs with a significantly low activation energy of about 0.33 eV.

Removal mechanism of tetracycline-Cr(Ⅵ) combined pollutants by different S-doped sludge biochars: Role of environmentally persistent free radicals

In this work, by using sodium thiosulfate as the S source, S-doped biochars were prepared to remove tetracycline/hexavalent chromium (TC-Cr (Ⅵ)) combined pollutants in aqueous solutions. The concentration of environmentally persistent free radicals (EPFRs) was used to directly determine the degradation of TC and the reduction of Cr (Ⅵ). The concentration of EPFRs in S-doped hydrothermal + pyrocarbon (S-HPBC) (3.64 × 10¹⁹ spins/g) was greater than that of S-doped hydrochar (S-HBC) (5.64 × 10¹⁸ spins/g) and S-doped pyrocarbon (S-PBC) (6.53 × 10¹⁸ spins/g). The increase in EPFRs concentration after S doping was positively correlated with the number of defect structures. In the TC-Cr (Ⅵ) system, the reduction efficiency of Cr (Ⅵ) decreased due to competition for electrons, while the TC degradation efficiency remained high. This was likely because Cr (Ⅵ) reduction promoted the degradation of TC. In addition, de-complexation was the primary factor for the removal of TC-Cr (Ⅵ), and some ROS were consumed during this process. The thiophene groups (-C-S-C-) that formed after S-doping of biochar were the main functional groups involved in the catalytic degradation of TC. In the radical pathway, SO₄^•- and •OH provided the greatest contribution to the degradation of TC, while ¹O₂ contributed the most to TC degradation via a non-radical pathway. The regeneration experiment confirmed that S-doped biochar could be reused and maintained a high pollutant removal efficiency. S-HPBC is a promising modified biochar material for removing mixed pollutants.

Preparation of Graphite-UiO-66(Zr)/Ti electrode for efficient electrochemical oxidation of tetracycline in water

Tetracycline (TC) is widely-used antibiotic pollutant with high toxicity, refractory, persistence and bacteriostasis, and its removal from water needs to be enhanced. In this work, a novel Graphite-UiO-66(Zr)/Ti electrode was successfully prepared and evaluated for electrochemical oxidation degradation of TC. The electrochemical performance tests indicate the Graphite-UiO-66(Zr)/Ti electrode had higher electrochemical oxidation activity, which achieved higher TC removal efficiency (98.1% ± 1.5%) than Ti plate (65.2% ± 3.5%), Graphite-MIL-53(Al)/Ti electrode (79.5% ± 2.9%) and Graphite-MIL-100(Fe)/Ti electrode (89.0% ± 2.6%). The influence of operating condition was also systematically studied, and the optimized condition was pH 5.0, 20 mA/cm2 current density and 0.1 M electrolyte (Na2SO4). Through the liquid chromatography mass spectrometry (LC-MS), the TC degradation pathway by Graphite-UiO-66(Zr)/Ti electrode oxidation was proposed. Under the •OH free radical oxidative decomposition effect, the double bond, phenolic group and amine group of TC were attacked. TC was transformed into intermediate product ① (m/z = 447), then was further degraded to intermediates ② (m/z = 401) and ③ (m/z = 417). The latter was fragmented into small fractions ④ (m/z = 194), ⑤but-2-enedioic acid (m/z = 116) and ⑥oxalic acid (m/z = 90, the proposed intermediate). In addition, TC removal remained at 89.6% ± 2.7% in the sixth cycle of operation, which confirmed the efficient reusability and stability for antibiotics removal from water.