Reaction kinetics of phenols and p-nitrophenols in flowing aerated aqueous solutions generated by a discharge plasma jet

Analyzing atmospheric plasma's potential for diesel soil remediation: Insightful mechanisms

The remediation of diesel-contaminated soil is a critical environmental concern, driving the need for effective solutions. Recently, the methodology of Non-thermal Atmospheric Plasma (NTAP) technology, which is equipped with a Dielectric Barrier Discharge (DBD) electrode and has become a feasible approach, was proven to be viable. The reactive species from the plasma were exposed to the contaminated soil in this investigation using the NTAP technique. The reacted soil was then extracted using dichloromethane, and the amount of Total Petroleum Hydrocarbon (TPH) removed was assessed. Investigation into varying power levels, treatment durations, and hydrogen peroxide integration revealed significant findings. With an initial concentration of 3086 mg of diesel/kg of soil and a pH of 5.0, 83% of the diesel was removed from the soil at 150 W in under 20 min. Extended exposure to NTAP further improved removal rates, highlighting the importance of treatment duration optimization. Additionally, combining hydrogen peroxide (H2O2) with NTAP enhanced removal efficiency by facilitating diesel breakdown. This synergy offers a promising avenue for comprehensive soil decontamination. Further analysis considered the impact of soil characteristics on removal efficacy. Mechanistically, NTAP generates reactive species that degrade diesel into less harmful compounds, aiding subsequent removal. Overall, NTAP advances environmental restoration efforts by offering a quick, economical, and environmentally benign method of remediating diesel-contaminated soil especially when used in tandem with hydrogen peroxide.

Degradation of phenol in wastewater through an integrated dielectric barrier discharge and Fenton/photo-Fenton process

In this study, a non-thermal dielectric barrier discharge-Fenton/photo-Fenton process was investigated to remove phenol from synthetic wastewater. The changes and optimal values of influencing parameters, including treatment time, iron concentration, phenol initial concentration, and pH, were investigated based on the central composite design (CCD) method. The presence of 0.4 mmol/L of iron in the phenol solution with a concentration of 100 mg/L increased the removal efficiency and pseudo-first-order kinetic constant compared to dielectric barrier discharge cold plasma (DBDP) alone from 0.0824 min-1 and 56.8% to 0.2078 min-1 and 86.83%, respectively. The phenol removal efficiency was reduced to 52.9%, 45.6% and 31.8% by adding tert-butyl alcohol (TBA) with concentrations of 50, 100, and 200 mg/l, respectively. After 12 min of DBDP irradiation, the pH of the sample decreased from 5.95 to 3.42, and the temperature of the sample increased from 19.3 to 37.2 degrees Celsius. The chemical oxygen demand (COD) of the sample containing 100 mg/L phenol under plasma-Fenton/photo-Fenton irradiation decreased from 241 mg/L to 161 mg/L. Phenol removal efficiency after 10 min of treatment in the presence of 0.4 mmol/L of iron with the reactor volume of 50 mL was 87%, but the efficiency decreased to 76%, 47%, and 9% by increasing the volume to 100, 200, and 400 mL, respectively. Reducing the power led to a decrease in the removal efficiency from 56.8% for 100 W power to 10.8% for 40 W. The energy efficiency for 50% removal by DBDP and plasma-Fenton/photo-Fenton systems was 5.86×10-3 kWh/mg and 1.27×10-3 kWh/mg, respectively.

Synergistic effects and mechanisms of plasma coupled with peracetic acid in enhancing short-chain fatty acid production from sludge: Motivation of reactive species and metabolic tuning of microbial communities

Suitable waste activated sludge (WAS) pretreatments that boost short-chain fatty acid (SCFA) production from anaerobic fermentation are essential for carbon emission reduction and sludge resource utilization. This study established an efficient WAS pretreatment process combining atmospheric pressure plasma jet (APPJ) with peracetic acid (PAA). The maximum SCFA production (6.5-fold that of the control) largely increased under the optimal conditions (PAA dosage = 25 mg/g VSS (volatile suspended solids), energy consumption = 20.9 kWh/m3). APPJ/PAA pretreatment enhanced the production of multiple reactive species (including OH, CH3C(O)O, 1O2, ONOO-, O2-, and eaq-) and strengthened the effects of H2O2, heat, and light. This synergistically solubilized WAS and released organic substrates for SCFA-producing microbes. In addition, the enrichment of SCFA-producing bacteria and the decrease in SCFA-consuming bacteria favored SCFA accumulation. The key genes encoding for the main substrate metabolism and SCFA production in the metabolic pathway of fermentation were also enhanced.

Efficient degradation of the refractory organic pollutant by underwater bubbling pulsed discharge plasma: performance, degradation pathway, and toxicity prediction

It is essential to develop an efficient technology for the elimination of refractory contaminants due to their high toxicity. In this study, a novel underwater bubbling pulsed discharge plasma (UBPDP) system was proposed for the degradation of Orange II (OII). The degradation performance experiments showed that by enhancing the peak voltage and pulse frequency, the degradation efficiency of OII increased gradually. The removal efficiencies under different air flow rates were close. Reducing OII concentration and solution conductivity could promote the elimination of OII. Compared with neutral and alkaline conditions, acidic condition was more beneficial to OII degradation. The active species including ·OH, ·O2-, 1O2, and hydrated electrons were all involved in OII degradation. The concentrations of O3 and H2O2 in OII solution were lower than those in deionized water. During discharge, the solution pH increased while conductivity decreased. The variation of UV-vis spectra with treatment time indicated the effective decomposition of OII. Possible degradation pathways were speculated based on LC-MS. The toxicity of intermediate products was predicted by the Toxicity Estimation Software Tool. Coexisting constituents including Cl-, SO42-, HCO3-, and humic acid had a negative effect on OII removal. Finally, the comparison with other technology depicted the advantage of the UBPDP system.

DC magnetic field-assisted improvement of textile dye degradation efficiency with multi-capillary air bubble discharge plasma jet

Axial DC magnetic field-assisted multi-capillary underwater air bubble discharge plasma jet has been used to study the productions of reactive oxygen species. Analyses of optical emission data revealed that the rotational (T_r) and vibrational temperatures (T_v) of plasma species slightly increased with magnetic field strength. The electron temperature (T_e) and density (n_e) increased almost linearly with magnetic field strength. T_e increased from 0.53 to 0.59 eV, whereas n_e increased from 1.03 × 10¹⁵ cm^-3 to 1.33 × 10¹⁵ cm^-3 for B = 0 to B = 374 mT, respectively. Analytical results from the plasma treated water provided that the electrical conductivity (EC), oxidative reduction potential (ORP), and the concentrations of O₃ and H₂ O₂ enhanced from 155 to 229 µS cm^-1, 141 to 17 mV, 1.34 to 1.92 mg L^-1, and 5.61 to 10.92 mg L^-1 due to the influence of axial DC magnetic field, while [Formula: see text] reduced from 5.10 to 3.93 for 30 min treatment of water with B = 0 and B = 374 mT, respectively. The model wastewater prepared with Remazol brilliant blue textile dye and the plasma treated wastewater studied by optical absorption spectrometer, Fourier transform infrared spectrometer, and gas chromatography mass spectrometer. The results show that the decolorization efficiency increased ~ 20% after 5 min treatment for the maximum B = 374 mT with respect to zero-magnetic field and, power consumption, and electrical energy cost reduced ~ 6.3% and ~ 4.5%, respectively, due to the maximum assisted axial DC magnetic field strength of 374 mT.

Pathways of organic micropollutants degradation in atmospheric pressure plasma processing - A review

Concern of toxic compounds and their, potentially more harmful degradation products, present in aquatic environment alarmed scientific community and research on the development of novel technologies for wastewater treatment had become of great interest. Up to this date, many papers pointed out the challenges and limitations of conventional wastewater treatment and of some advanced oxidation processes. Advanced technologies based on the use of non-equilibrium or non-thermal plasma had been recognized as a possible solution for, not only degradation, but for complete removal of recalcitrant organic micropollutants. While previous review papers have been focused on plasma physics and chemistry of different types of discharges for few organic micropollutants, this paper brings comprehensive review of current knowledge on the chemistry and degradation pathways by using different non-thermal plasma types for several micropollutants' classes, such as pharmaceuticals, perfluorinated compounds, pesticides, phenols and dyes and points out some major research gaps.

Simultaneous degradation of p-nitrophenol and reduction of Cr(VI) in one step using microwave atmospheric pressure plasma

Different physicochemical properties between Cr(VI) and phenolic compounds pose serious challenges for the effective treatment of co-contamination. This study developed an electrodeless high-flow microwave atmospheric plasma jet for the single-step simultaneous degradation of p-nitrophenol (PNP) and reduction of Cr(VI). Following a 15 min treatment with microwave atmospheric pressure plasma, the removal efficiency of Cr(VI) and PNP reached 97.5% and 93.6%, respectively, whereas that of total organic carbon reached 30.2%. Adding PNP to the solution significantly improved Cr(VI) reduction, whereas PNP degradation increased slightly with Cr(VI). The results indicate that the PNP intermediates significantly affected Cr(VI) reduction. Additionally, long-lived H₂O₂ and short-lived ^·H aided the reduction of Cr(VI) during plasma treatment. The addition of hydroxyl scavengers during treatment implied that ^·OH was largely responsible for PNP oxidation. High-performance liquid chromatography-mass spectroscopy (HPLC-MS) revealed that PNP intermediates, including p-nitrocatechol and 5-nitrobenzene-1,2,3-triol, function as Cr(VI) reductants. On the basis of the examined intermediate products, the potential PNP degradation pathway was investigated. The factors that could influence simultaneous dehgradation and reduction, including solution pH, gas velocity, and distance between the plasma outlet and the water surface were researched. Low pH supports Cr(VI) reduction, and the promotion of PNP for Cr(VI) reduction applies to all pH values. The degradation of PNP is insensitive to pH values with or without Cr(VI). The optimal gas velocity for PNP degradation and Cr(VI) reduction was revealed to be 6 L/min. The simultaneous removal of PNP and Cr(VI) benefits from a shorter distance between the plasma outlet and the water's surface.

Upcycle hazard against other hazard: Toxic fluorides from plasma fluoropolymer etching turn novel microbial disinfectants

The release of toxic fluoride byproducts is a seemingly unavoidable artifact of surface engineering, causing severe environmental and human health problems. Here we propose and implement a new "upcycle hazard against other hazard" concept in the case study of cold atmospheric plasma surface modification of fluoropolymers such as polytetrafluorethylene (PTFE). Capitalizing on the excellent controllability, precision and energy efficiency of the plasma surface processing, complemented with the recently discovered ability of plasmas to activate water to produce a potent electrochemical disinfectant, referred to as the plasma-activated water (PAW), we demonstrate a radically new solution to capture the hazardous gaseous fluorides into the PAW and use the as-fluorinated PAW (F-PAW) as a very effective antimicrobial disinfectant. A customized surface discharge reactor is developed to evaluate the effects of fluorides released from the plasma etching of PTFE on the chemistries in gas-phase plasmas and F-PAW, as well as the antibacterial effect of F-PAW. The results show that gaseous fluorides, including COF₂, CF₃COF, and SiF₄ are produced in gas-phase plasmas, and the dissolution of thus-generated fluorides into PAW has a strong effect on inactivating catalase and destroying the oxidation resistance of bacterial cells. As a result, the antibacterial effect of PAW-fluorides against the methicillin-resistant Staphylococcus aureus (MRSA) is enhanced by > 5 log reductions, suggesting that otherwise hazardous fluorides from the plasma processing of PTFE can be used to enhance the microbial disinfection efficiency of PAW. The demonstrated approach opens new avenues for sustainable hazard valorization exemplified by converting toxic fluoride-etching products into potent antimicrobial and potentially anti-viral disinfectants.

Reactive Nitrogen Species Generated by Gas-Liquid Dielectric Barrier Discharge for Efficient Degradation of Perfluorooctanoic Acid from Water

Perfluorooctanoic acid (PFOA) poses a serious threat to the ecological environment and biological health because of its ubiquitous distribution, extreme persistence, and high toxicity. In this study, we designed a novel gas-liquid dielectric barrier discharge (GLDBD) reactor which could efficiently destruct PFOA. PFOA removal efficiencies can be obtained in various water matrices, which were higher than 98.0% within 50 min, with energy yields higher than 114.5 mg·kWh^-1. It was confirmed that the reactive species including e^-, ONOOH, •NO₂, and hydroxyl radicals (•OH) were responsible for PFOA removal. Especially, this study first revealed the crucial role of reactive nitrogen species (RNS) for PFOA degradation in the plasma system. Due to the generation of a large amount of RNS, the designed GLDBD reactor proved to be less sensitive to various water matrices, which meant a broader promising practical application. Moreover, influential factors including high concentration of various ions and humic acid (HA), were investigated. The possible PFOA degradation pathways were proposed based on liquid chromatograph-mass spectrometer (LC-MS) results and density functional theory (DFT) calculation, which further confirmed the feasibility of PFOA removal with RNS. This research, therefore, provides an effective and versatile alternative for PFOA removal from various water matrices.

Enhancing Interface Reactions by Introducing Microbubbles into a Plasma Treatment Process for Efficient Decomposition of PFOA

Efficient destruction of perfluoroalkyl compounds in contaminated waters remains a challenge because of highly stable C-F bonds. In this study, mineralization of perfluorooctanoic acid (PFOA) with high concentration (∼30 mg/L) was realized in a needle-plate pulsed discharge reactor integrated with a water jet (NPDW) to which microbubbles (MBs) with different carrier gases (air, N₂, and Ar) were introduced to enhance interfacial reactions. MBs effectively enrich dispersed PFOA from a bulk solution to a liquid surface to allow enhancing contact with reactive species and also expanding the plasma discharge area and channels. The PFOA removal efficiency in air and Ar discharge reached 81.5 and 95.3% in 2 h, respectively, with a defluorination ratio of no less than 50%. Energy requirements (EE/O) ranged from 216.49 to 331.95 kWh/m³. Aside from fluoride, PFOA was degraded to a range of short-chain perfluoroalkyl acids and, to a minor extent, at least 20 other fluorinated transformation products. PFOA degradation mechanisms were proposed, including decarboxylation, hydroxylation, hydrogenation reduction, and defluorination reactions. Real water matrices (groundwater, tap water, wastewater effluent, and surface water) showed moderate impact on treatment outcomes, demonstrating the robustness of the treatment process. The study demonstrated an environmentally friendly nonthermal plasma technology for effective PFOA degradation.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

The Effect of Mass Transfer Rate-Time in Bubbles on Removal of Azoxystrobin in Water by Micro-Sized Jet Array Discharge

In this work, the azoxystrobin removal in water by using a micro-size discharge array was investigated, and the removal efficiency can reach as high as 98.1% after 9 min plasma treatment as well as the energy utilization being only 0.73 g/(kW·h). Based on the relationship between the generation of gas bubbles and parameters of gas-liquid discharge, it was found that the variation of applied voltage, gas flow rate and initial solution temperature could cause particle number change, mass transfer rate change and the mass transfer time change, which significantly affected the practical applications at last. The experimental results indicated that when gas flow rate was 0.7 SLM (Standard Liter per Minute) and the initial solution temperature was 297 K with the applied voltage of 8 kV and discharge frequency of 6 kHz, the removal efficiency of azoxystrobin achieved maximum. Based on the analysis results of liquid mass spectrometry, the removal pathways of azoxystrobin were supposed by the decomposed by-products. Toxicity tests indicated that the decomposed products were safe and non-toxic. So, this study may reveal an azoxystrobin degradation mechanism and provide a safe, reliable and effective way for azoxystrobin degradation.

Catalytic degradation of caffeic acid by DBD plasma and Mn doped cobalt oxyhydroxide catalyst

In this study, caffeic acid (CA) was degraded by electrical discharge plasma combined with Mn doped CoOOH catalyst. Doping of Mn significantly improve the catalytic activity of CoOOH. CA degradation efficiency was 75.6% with dielectric barrier discharge treatment for 10 min, and it reached 97% using CoOOH as the catalyst at the same treatment time. CA was 100% degraded with only 8 min using Mn/CoOOH as the catalyst. The introduction of Mn into the lattice of CoOOH induced the formation of oxygen vacancy, causing part of coordinate number of Co decreased from 6 to 5, and thus produces unsaturated Co to be the Lewis acid sites. Lewis acid sites (unsaturated Co) could coordinate with O₃ and H₂O₂ and break their chemical bonds to form O and -OH. Assisting in the conversion of O₃ to ·OH was the main role of H₂O₂ in the catalytic process. The degradation products and pathway of CA were studied by three-dimensional fluorescence, liquid chromatograph-mass spectrometer and density functional theory calculations.

A microwave atmospheric plasma strategy for fast and efficient degradation of aqueous p-nitrophenol

Plasma technology has received intensive research interest in pollutants degradation. However, conventional plasma generator suffers from erosion of electrodes and consequent short life time and pollution. In this work, an electrodeless high-flow microwave atmospheric plasma jet is developed for fast degradation of p-nitrophenol. With the assistance of injection locking technology, stable plasma is managed to be generated by low-cost magnetron. 100% removal of 100 mg/L PNP is achieved after 12 min, with a TOC removal efficiency of 57.6%. The fast degradation is probably due to the high cross section (around 153 mm²) of plasma gas. Change in the removal efficiency are less than 4% and 5% as the pH of the solution changes from 2.02 to 12.07 and conductivity varies between 5^.38 × 10^-2 and 43.6 mS/cm, respectively. Moreover, optical emission spectroscopy spectra of the microwave plasma and a hydroxyl radical scavenger (t-butanol) are employed to identify the generated oxidizing species, which indicates that ^•OH is the key factor during the degradation process. The hydroxylated intermediates and organic acid transformed from PNP were revealed. Based on the examined intermediate products, the possible degradation mechanism and pathway are analyzed.

Ultra-high synergetic intensity for humic acid removal by coupling bubble discharge with activated carbon

Humic acid (HA) removal research focuses on the global water treatment industry. In this work, efficient HA degradation with an ultra-high synergetic intensity is achieved by combined bubble discharge with activated carbon (AC). Adding AC to the discharge greatly improves HA removal efficiency and degradation speed; the synergetic intensity reaches 651.52% in the combined system, and the adsorption residual on AC is 4.52%. After 90 min of treatment, the HA removal efficiency reaches 98.90%, 31.29%, and 7.61% in the plasma-AC combined, solo bubble discharge, and solo AC adsorption systems, respectively. During the plasma process, the number of pore structures and active sites and the amount of oxygen-containing functional groups on the AC surface increase, resulting in a higher adsorption capacity to reactive species (H₂O₂ and O₃) and HA and promoting interactions on the AC surface. For HA mineralization, the presence of AC greatly promotes the destruction of aromatic structures and chromophoric HA functional groups.

Degradation of caffeic acid by dielectric barrier discharge plasma combined with Ce doped CoOOH catalyst

Herein, Ce doped CoOOH was used as the catalyst for caffeic acid (CA) degradation by dielectric barrier discharge (DBD) plasma. The treatment performance and catalytic mechanism were studied by a series of experiments and density functional theory (DFT) simulations. The results show that the doping amounts of Ce significantly influenced the catalytic performance of CoOOH in DBD plasma, and the catalytic effect reached maximum when the molar ratio of Ce to Co was 1:9. CA was 100 % degraded by Ce₁/Co₉OOH/DBD with 10 min treatment, while only 75.6 % of CA was degraded by 10 min DBD treatment. Transformation of O₃ and H₂O₂ to ⋅OH was mainly responsible for the catalytic effect. The content of oxygen vacancies and unsaturated Co (Lewis acid sites) of CoOOH was increased by doping Ce according to the results of experiments and simulations, and the change was conducive to the catalytic reactions. DFT simulations also indicated that DBD generated O₃ and H₂O₂ were decomposed to O atoms, OH groups and free OH by Ce/CoOOH. The presence of reductive species in DBD plasma was confirmed, and ⋅H was a kind of important reactive specie for CA degradation. CA degradation pathway was proposed based on the detected degradation products.

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.

Review on the treatment of organic wastewater by discharge plasma combined with oxidants and catalysts

Discharge plasma technology is a new advanced oxidation technology for water treatment, which includes the effects of free radical oxidation, high energy electron radiation, ultraviolet light hydrolysis, and pyrolysis. In order to improve the energy efficiency in the plasma discharge processes, many efforts have been made to combine catalysts with discharge plasma technology. Some heterogeneous catalysts (e.g., activated carbon, zeolite, TiO₂) and homogeneous catalysts (e.g., Fe²⁺/Fe³⁺, etc.) have been used to enhance the removal of pollutants by discharge plasma. In addition, some reagents of in situ chemical oxidation (ISCO) such as persulfate and percarbonate are also discussed. This article introduces the research progress of the combined systems of discharge plasma and catalysts/oxidants, and explains the different reaction mechanisms. In addition, physical and chemical changes in the plasma catalytic oxidation system, such as the effect of the discharge process on the catalyst, and the changes in the discharge state and solution conditions caused by the catalysts/oxidants, were also investigated. At the same time, the potential advantages of this system in the treatment of different organic wastewater were briefly reviewed, covering the degradation of phenolic pollutants, dyes, and pharmaceuticals and personal care products. Finally, some suggestions for future water treatment technology of discharge plasma are put forward. This review aims to provide researchers with a deeper understanding of plasma catalytic oxidation system and looks forward to further development of its application in water treatment.

Construction of S-doped MgO coupled with g-C3N4 nanocomposites with enhanced photocatalytic activity under visible light irradiation

In the present work, a series of photocatalysts in which S-MgO (SM) was coupled with g-C3N4 (CN) were synthesized and characterized in detail using various characterization techniques.