Dimethyl phthalate elimination from micro-polluted source water by surface discharge plasma: Performance, active species roles and mechanisms

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.

Effective degradation of lindane and its isomers by dielectric barrier discharge (DBD) plasma: Synergistic effects of various reactive species

Lindane is a broad-spectrum organochlorine insecticide which has been included in the persistent organic pollutants (POPs) list together with its two hexachlorocyclohexane (HCH) isomers. Due to its continuous use in the past decades, the environmental impacts of HCHs are still severe now. Therefore, in the present study, dielectric barrier discharge (DBD) plasma was used as an advanced oxidation process for the destruction of HCHs in water. The result indicated that in air-DBD system, over 95.4% of the initial 5 mg L-1 lindane was degraded within 60 min. Moreover, DBD plasma displayed high degradation efficiencies of other HCH isomers including α, β, and δ-HCH. Electron spin resonance spectra, scavenging experiments and theoretical calculations revealed that the synergistic effects of various reactive species were the main reason for the high efficiency of DBD plasma. For instance, both hydroxyl radicals (•OH) and electrons (e-) could initiate the degradation of HCHs, while other reactive species such as 1O2 and ONOOH played important roles in the decomposition of intermediates. Therefore, the present study not only provided an effective approach for the treatment of HCHs, but also revealed the underlying mechanism based on in-depth experimental investigation and theoretical calculation.

Degradation of methylene blue using a novel gas-liquid hybrid DDBD reactor: Performance and pathways

A novel gas-liquid hybrid double dielectric barrier discharge (DDBD) reactor with coaxial cylinder configuration was developed for the degradation of methylene blue (MB) in this study. In this DDBD reactor, the reactive species generation occurred in the gas-phase discharge, directly in the liquid, and in the mixture of the working gas bubbles and the liquid, which could effectively increase the contact area between the active substance and MB molecules/intermediates, resulting in an excellent MB degradation efficiency and mineralization (COD and TOC). The electrostatic field simulation analysis by Comsol was carried out to determine the appropriate structural parameters of the DDBD reactor. The effect of discharge voltage, air flow rate, pH, and initial concentration on MB degradation was evaluated. Besides, major oxide species, ·OH, the dissolved O₃ and H₂O₂ generated in this DDBD reactor were determined. Moreover, major MB degradation intermediates were identified by LC-MS, based on which, possible degradation pathways of MB were proposed.

Atmospheric plasma-based approaches for the degradation of dimethyl phthalate (DMP) in water

Cold plasma based treatment of contaminated water is becoming a promising novel green remediation option. This study assessed the performance of two different cold plasma reactors, using, respectively, a self-pulsing discharge (SPD) and a multipin corona discharge (MCD), in the degradation of dimethyl phthalate (DMP), a persistent and ubiquitous pollutant of the aquatic environment. The process kinetics and energy efficiency, as well as the main plasma generated reactive species were determined under various operating conditions concerning the plasma feed gas and flowrate, the voltage polarity, the input power, the DMP initial concentration, the liquid conductivity, and the aqueous matrix used to prepare DMP solutions for these experiments. The MCD reactor, operated with air as plasma feed gas and negative voltage polarity, gave the best results in terms of rate and energy efficiency. Moreover, variations in plasma input power and in the liquid conductivity have limited effect on DMP degradation rate, making this reactor suitable for treating liquids with a range of initial conductivities The effects of DMP initial concentration on its rate of degradation and on the process energy efficiency were also investigated. Differences in the efficiency of production and distribution of plasma generated reactive species, notably ^•OH and H₂O₂, observed for the two tested reactors are discussed in terms of different extension of the plasma/liquid interface and diffusion into the bulk solution. It is proposed that among the reactive species, ^•OH foremost, and O₃ to a lesser extent, play a pivotal role in DMP degradation, while the contribution of H₂O₂ appears to be limited. The rate of DMP degradation was not drastically different in Milli-Q water and in tap water, a positive outcome in view of practical applications of the technology. The lower rate observed in tap than in Milli-Q water is attributed to the presence of bicarbonate and carbonate, which are known scavengers of hydroxyl radicals.

Preparation of polyvinylidene fluoride composite ultrafiltration membrane for micro-polluted surface water treatment

Blending modification of graphene oxide (GO) and deposition of silver carbonate (Ag₂CO₃) on the membrane surface by suction filtration was used to prepare polyvinylidene fluoride (PVDF) composite ultrafiltration (UF) membranes (denoted as PGA membranes). The effect of this strategy on the morphology and performance of the pure PVDF membrane was investigated. Owing to an increased hydrophilicity and the formation of a more open pore, the pollution resistance and permeability of the PGA membrane were improved. The pure water flux of the PGA-3 membrane (254 LMH) was increased to more than 2-fold compared to that of the neat PVDF membrane (126 LMH). In addition, the results of antifouling experiments showed that the flux recovery rate, flux decay rate, and antibacterial performance of the PGA-3 membrane was superior to those of the other membranes synthesized in this study. Finally, after conducting multi-cycle filtration experiments with lake water, the flux and recovery rate of the PGA-3 membrane was observed to be the highest, and the water quality of the lake water filtered by the PGA-3 membrane was the best. Thus, the above results indicate that this membrane modification strategy is extraordinarily effective in improving the antifouling properties and permeability of the PVDF UF membranes in practical applications.

Ferric iron reduction reaction electro-Fenton with gas diffusion device: A novel strategy for improvement of comprehensive efficiency in electro-Fenton

Applying the optimal 2-electron oxygen reduction reaction potential in electro-Fenton (2e^-ORR-EF) for degradation has become a common strategy because of the highest H₂O₂ generation rate in such condition. However, in 2e^-ORR-EF system, the Fe(III) ions crystallize on the surface of cathode and form a layer of film according to SEM, XPS, XRD and Mössbauer spectrum resulting in poor reaction rate of EF. Hence, we propose FRR-EF, which is operated by applying the optimal potential of ferric iron reduction reaction (FRR) rather than that of 2e^-ORR on cathode for EF. Gas diffusion device was also carried out to ensure the H₂O₂ generation rate. In this novel strategy, only - 0.1 V was applied on cathode. High H₂O₂ production rate (0.021 ± 0.002 mmol L^-1 min^-1 cm^-2), and slow Fe(II) consumption rate (0.03 min^-1) were achieved. The EIS result showed that at this potential, the formation of the Fe film was effectively alleviated, thus prolonging the degradation life of the cathode. This new strategy can balance both 2e^-ORR and FRR, thus improving the comprehensive efficiency of EF, which provides essential references to the EF not only in potential operation but also in the design of reaction device.

Targeted degradation of dimethyl phthalate by activating persulfate using molecularly imprinted Fe-MOF-74

Adsorptive removal of dimethyl phthalate (DMP) in water combined with advanced oxidation processes (AOPs) has attracted interest. In this work, the adsorptive and catalytic properties of an Fe-based metal-organic framework (Fe-MOF-74) have been improved by molecular imprinting technique. The adsorption behaviors have been evaluated by the Freundlich and pseudo-second-order model. The results have shown that selective adsorption ability of the material for DMP was highly enhanced and chemisorption was dominating. A 1.5-fold increase in catalytic rate after being modified by molecular imprinting indicated that the selective adsorption is crucial. In the synergy of adsorption and catalysis, DMP was first specifically adsorbed on the surface of the material by hydrogen bonds and electrostatic interactions. Then, hydroxyl radicals and sulfate radicals, which were both generated via activation of persulfate (PS), catalytically oxidized DMP. The degradation rate can rapidly reach around 90% in 30 min and three possible degradation pathways were proposed. The molecular imprinting modified catalyst can be used for DMP effective degradation in water.

Removal of phthalates from aqueous solution by semiconductor photocatalysis: A review

While phthalate esters are commonly used as plasticizers to improve the flexibility and workability of polymeric materials, their presence and detection in various environments has become a significant concern. Phthalate esters are known to have endocrine-disrupting effects, which affects reproductive health and physical development. As a result, there is now increased focus and urgency to develop effective and energy efficient technologies capable of removing these harmful compounds from the environment. This review explores the use of semiconductor photocatalysis as an efficient and promising solution towards achieving removal and degradation of phthalate esters. A comprehensive review of photocatalysts reported in the literature demonstrates the range of materials including commercial TiO₂, solar activated catalysts and composite materials capable of enhancing adsorption and degradation. The degradation pathways and kinetics are also considered to provide the reader with an insight into the photocatalytic mechanism of removal. In addition, through the use of two key platforms (the technology readiness level scale and electrical energy per order), the crucial parameters associated with advancing photocatalysis for phthalate ester removal are discussed. These include enhanced surface interaction, catalyst platform development, improved light delivery systems and overall system energy requirements with a view towards pilot scale and industrial deployment.

Catalytic oxidation of dimethyl phthalate over titania-supported noble metal catalysts

Semi-volatile organic compounds (SVOCs) are organic compounds with the boiling point ranging between 240/260 ℃ and 380/400 ℃. Detailed knowledge regarding catalytic removal of SVOCs from indoor environment is very limited as it remains challenge to explore such reaction due to the viscosity nature of target contaminants. Here, we established a facile methodology to explore the heterogeneous catalytic oxidation reaction of dimethyl phthalate (DMP), a model SVOC, over the surface of supported catalyst. DMP was found to be gradually oxidized over the surface of titania supported catalysts including palladium (Pd), platinum and ruthenium with increasing temperature. The cleavage of side chain of DMP occurs at 75 ℃ over the surface of Pd/TiO₂, which is significantly lower than that of the other two catalysts. Carbon dioxide was observed as the main product of the catalytic oxidation reaction. However, aromatic products and small molecule products were still observed as side-product in different temperature range. Density functional theory calculations further show that DMP can react with reactive oxygen species to form phthalic acid. While the cleavage of the DMP side chain occurs to form products such as methyl benzoate. This work thus provides basic knowledge about indoor SVOCs catalytic oxidation removal.

Underwater microplasma bubbles for efficient and simultaneous degradation of mixed dye pollutants

Complete degradation of mixtures of organic pollutants is a major challenge due to their diverse degradation pathways. In this work, a novel microplasma bubble (MPB) reactor was developed to generate plasma discharges inside small forming bubbles as an effective mean of delivering reactive species for the degradation of the target organic contaminants. The results show that the integration of plasma and bubbles resulted in efficient degradation for all azo, heterocyclic, and cationic dyes, evidenced by the outstanding energy efficiency of 13.0, 18.1 and 22.1 g/kWh with 3 min of processing, in degrading alizarin yellow (AY), orange II (Orng-II) and methylene blue (MB), individually. The MPB treatment also effectively and simultaneously degraded the dyes in their mixtures such as AY + Orng-II, AY + MB and AY + Orng-II + MB. Scavenger assays revealed that the short-lived reactive species, including the hydroxyl (OH) and superoxide anion (O₂^-) radicals, played the dominant role in the degradation of the pollutants. Possible degradation pathways were proposed based on the intermediate products detected during the degradation process. The feasibility of this proposed strategy was further evaluated using other common water pollutants. Reduced toxicity was confirmed by the observed increases in human cell viability for the treated water. This work could support the future development of high performance- and energy-efficient wastewater abatement technologies.

Theoretical Design of Biodegradable Phthalic Acid Ester Derivatives in Marine and Freshwater Environments

The biodegradability of phtalic acid esters in marine and freshwater environments was characterized by their binding free energy with corresponding degrading enzymes. According to comprehensive biodegradation effects weights, the binding free energy values were converted into dimensionless efficacy coefficient using ratio normalization method. Then, considering comprehensive dual biodegradation effects value and the structural parameters of PAEs in both marine and freshwater environments, a 3D‐QSAR pharmacophore model was constructed, five PAE derivatives (DBP−COOH, DBP−CHO, DBP−OH, DINP−NH₂, and DINP−NO₂) were screened out based on their environmental friendliness, functionality and stability. The prediction of biodegradation effects on five PAE derivatives by biodegradation models in marine and freshwater environment increased by 15.90 %, 15.84 %, 27.21 %, 12.33 %, and 8.32 %, and 21.57 %, 15.21 %, 20.99 %, 15.10 %, and 9.74 %, respectively. By simulating the photodegradation path of the PAE derivative molecular, it was found that DBP−OH can generate ^.OH and provides free radicals for the photodegradation of microplastics in the environment.

Modification of graphite felt doped with nitrogen and boron for enhanced removal of dimethyl phthalate in peroxi-coagulation system and mechanisms

To enhance the generation of hydrogen peroxide (H₂O₂), a modified graphite felt cathode doped with nitrogen and boron was developed and used in peroxi-coagulation system to degrade dimethyl phthalate (DMP). After a simple modification method, the yield of H₂O₂ on cathode increased from 9.39 to 152.8 mg/L, with current efficiency increased from 1.61 to 70.3%. Complete degradation of DMP and 80% removal of TOC were achieved within 2 h at the optimal condition with pH of 5, cathodic potential of - 0.69 V (vs. SCE), oxygen aeration, and electrode gap of 1 cm. Possible mechanism with synergistic effect of electro-Fenton and electrocoagulation process in the peroxi-coagulation system was revealed via quenching experiments. The prospect of this system in the effluent of landfill leachate and domestic sewage was studied, achieving 50% and 61% of DMP removal in 2 h. This efficient system with simple modified cathode had promising prospects in practical applications.

Non-thermal plasma oxidation of Cu(II)-EDTA and simultaneous Cu(II) elimination by chemical precipitation

Cu²⁺ readily complexes with ethylenediaminetetraacetic acid (EDTA) to form a heavy metal complex (Cu-EDTA) that is typical in the effluents from mining and electroplating industries. It was difficult for the classical alkaline precipitation method to eliminate the heavy metal complex due to the strong bonding ability between Cu(II) and EDTA. Cu(II) release and removal performance after Cu-EDTA decomplexation in a non-thermal plasma oxidation system was carried out in this study. The removal process was characterized by chemical oxygen demand, total organic carbon, atomic force microscopy, and electroconductivity analysis. The toxicity effect of the treated Cu-EDTA solution was also tested by photobacterium bioassay. The experimental results showed that 80.2% of Cu was released and removed within 60 min of the non-thermal plasma treatment/alkaline precipitation. Relatively higher energy input, lower Cu-EDTA concentration, and acidic conditions were necessary to obtain greater Cu release and removal performance, and there existed an appropriate air flow rate for high-efficient Cu release and removal. O₂^-, OH, ¹O₂, and O₃ were the main active substances leading to Cu²⁺ release. Its residual toxicity to P.phosphoreum sp.-T3 was significantly reduced after treatment.

Degradation of antibiotic chloramphenicol in water by pulsed discharge plasma combined with TiO2/WO3 composites: mechanism and degradation pathway

Pulsed discharge plasma (PDP) combined with TiO₂/WO₃ composites for chloramphenicol (CAP) degradation was investigated. The prepared TiO₂/WO₃ composites were characterized by scanning electron microscope, transmission electron microscope, nitrogen adsorption apparatus, zeta sizer, X-ray diffraction, Raman spectra, UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy, photocurrent and electrochemical impedance spectroscopy. The degradation performance showed that the addition of TiO₂/WO₃ composites significantly enhanced the removal efficiency of CAP in PDP system. At a peak voltage of 18 kV, the highest removal efficiency of CAP could reach 88.1% in PDP system with 4 wt% TiO₂/WO₃, which was 36.8% and 26.0% higher than that in sole PDP system and PDP/TiO₂ system, respectively. The TiO₂/WO₃ composites significantly accelerated interfacial charge transfer process compared to the pure TiO₂. Besides, the effect of catalyst dosage and peak voltage on CAP removal was evaluated. OH, O₃O₂^-, h⁺ and high-energy electrons contributed to CAP degradation in PDP-TiO₂/WO₃ system. Addition of TiO₂/WO₃ composites can decompose O₃ and produce more OH and H₂O₂. The degradation intermediates were measured by liquid chromatography-mass spectrometry (LC-MS) and ion chromatography (IC). The cycling degradation experiment showed that the TiO₂/WO₃ composites have good reusability as well as stability.