Insights into the underlying mechanisms for integrated inactivation of A. spiroides and depression of disinfection byproducts by plasma oxidation

Integrated inactivation of Microcystis aeruginosa and degradation of microcystin-LR by direct current glow discharge plasma in liquid-phase: Mechanisms and cell deactivation process

The frequent occurrence of blooms of Microcystis aeruginosa (M. aeruginosa) and the subsequent release of microcystin-LR (MC-LR) in eutrophic waters pose a serious threat to aquatic ecosystems. This study investigated the optimal conditions for inactivating M. aeruginosa and the degrading MC-LR using direct current glow discharge plasma in liquid phase (DC-LGDP), analyzed the potential inactivation mechanisms and the cell deactivation process of M. aeruginosa. The results showed that DC-LGDP generated reactive species (i.e., •OH, 1O2, and H2O2), active Cl and electroporation effect collectively contributed to inactivation of M. aeruginosa and degradation of MC-LR. The 97.07 % inactivation efficiency of M. aeruginosa and 94.98 % degradation rate of MC-LR were achieved with higher energy yield and without generating nitrogen oxides. Meanwhile, DC-LGDP destroyed the cell integrity, eliminated their antioxidant capacity and reduced the content of photosynthetic pigments. The transcriptome analysis indicated that the transcripts of genes related to photosynthesis, ribosome biosynthesis, ABC transporters, and nitrogen metabolism pathway in M. aeruginosa were altered by DC-LGDP. This study provides insights into the inactivation of M. aeruginosa by DC-LGDP, while elucidating the potential inactivation mechanisms and the cell deactivation process involved. It may be important for the eco-friendly inactivation of M. aeruginosa blooms in natural water bodies.

Nanoenabled Self-Assembled Metal-Organic Algaecides Generated Photosynthetic Inhibition and Oxidative Stress for Sustainable Food Security

Achieving food sustainability is one of the biggest challenges in the new millennium. Plant factory cultivation systems provide an alternative for food sustainability, while they often suffer from algal blooms. The overuse of conventional algaecides has caused significant environmental pollution and concerns about food security. Here, we design a nanoenabled metal-organic algaecide that is self-assembled from natural polyphenols and two functional metal ions for providing shading effects and delivering active ingredients synergistically to suppress algal blooms. Black wattle tannin (BWT) and Fe3+ ions are utilized to develop self-assembled FeBWT nanoalgaecides with significant shading effects for decreasing light transmission (up to 97 %) and effectively inhibiting algal photosynthesis. Further, the FeBWT is functionalized with Cu2+ ions (bimetallic Cu/FeBWT) to target the algal cells and release Cu2+ ions via phenolic-mediated cell surface interactions, thus enhancing the inhibition efficiency. Importantly, the biosafety of Cu/FeBWT is demonstrated through toxicity tests on zebrafish and NIH3T3 cells. In our real-world field test, the Cu/FeBWT demonstrates high algal inhibition performance (>95 %, over 30 days), and enhances the accumulation of food nutrients in model plant lettuces. Collectively, the supramolecular metal-organic nanoalgaecide provides a promise for nanoagrochemical application and promotes food sustainability and security.

Molecular mechanisms underlying sodium percarbonate treatment suppress the recovery and growth of Pseudanabaena sp. in early spring

Increasing frequency and intensity of cyanobacterial blooms in water sources is a growing global issue. Algicides are usually implemented in summer or autumn when blooms break out, however, the blooms will form again when algicide's concentration declines to a certain extent. Preventing the recovery and growth of cyanobacteria in early spring may be conducive to abatement of the blooms in summer or autumn. In this study solid sodium percarbonate (SPC) was used as an algicide to suppress recovery and growth of Pseudanabaena sp., a common odour-producing cyanobacterium, in early spring (12 °C). Results showed that 3.0 and 6.0 mg/L SPC were able to kill most of the algal cells after 12 h treatment at 12 °C, and the residual cells gradually died during the re-cultivation period at 25 °C. As a control, although SPC also caused most of algal cells to lyse at 25 °C, regrowth of cells was found during the period of re-cultivation at 25 °C. Transcriptomic analysis revealed that the dysregulated genes were strongly associated with translation and photosynthesis after SPC treatment. All differentially expressed unigenes related to translation and photosynthesis were down-regulated after SPC oxidation at 12 °C, whereas key genes associated with translation and photosynthesis were upregulated after SPC treatment at 25 °C.

Advances and challenges in the technologies for cyanobacterial cells removal in drinking water treatment

Harmful cyanobacteria in reservoirs pose a serious threat to drinking water safety due to the intracellular metabolites, such as toxins and unpleasant tastes & odours. Effective removal of harmful cyanobacteria with little to no cell damage is very important to ensure the safety of drinking water. This review first introduced development history of cyanobacterial removal technologies in drinking water treatment. Then, impacts of oxidation, coagulation and pre-oxidation enhanced coagulation processes on cyanobacterial removal and integrity of the cells were comprehensively evaluated and discussed. Oxidation can remove cyanobacteria, but high doses of oxidants can result in significant cell lysis and release of intracellular metabolites, especially when using chlorine or ozone. Although there is practically no cell damage during coagulation, the removal efficiency is low in many cases. Pre-oxidation may improve cyanobacterial removal by the subsequent solid-liquid separation processes, and moderate pre-oxidation with little to no cell lysis is very important. Mechanisms of interface interaction between pre-oxidants and cyanobacteria should be defined in future to ensure moderate pre-oxidation of algal cells. Fate of cyanobacterial cells in sludge is also reviewed because more and more waterworks return sludge supernatant to the inlet of plant. Damage to cyanobacterial cells in sludge depends mainly upon coagulant type and dosage, algal species, and cyanobacteria-containing sludge should be treated before cell lysis. Efficient techniques for harmless disposal of cyanobacteria-containing sludge should be developed in future. This paper will help to better understand the cyanobacterial removal processes and provide improved perspectives for future research in this field.

The secondary outbreak risk and mechanisms of Microcystis aeruginosa after H2O2 treatment

The secondary outbreak of cyanobacteria after algicide treatment has been a serious problem to water ecosystems. Hydrogen peroxide (H2O2) is an algaecide widely used in practice, but similar re-bloom problems are inevitably encountered. Our work found that Microcystis aeruginosa (M. aeruginosa) temporarily hibernates after H2O2 treatment, but there is still a risk of secondary outbreaks. Interestingly, the dormant period was as long as 20 and 28 days in 5 mg L-1 and 20 mg L-1 H2O2 treatment groups, respectively, but the photosynthetic activity was both restored much earlier (within 14 days). Subsequently, a quantitative imaging flow cytometry-based method was constructed and confirmed that the re-bloom had undergone two stages including first recovery and then re-division. The expression of ftsZ and fabZ genes showed that M. aeruginosa had active transcription processes related to cell division protein and fatty acid synthesis during the dormancy stat. Furthermore, metabolomics suggested that the recovery of M. aeruginosa was mainly by activating folate and salicylic acid synthesis pathways, which promoted environmental stress resistance, DNA synthesis, and cell membrane repair. This study reported the comprehensive mechanisms of secondary outbreak of M. aeruginosa after H2O2 treatment. The findings suggest that optimizing the dosage and frequency of H2O2, as well as exploring the potential use of salicylic acid and folic acid inhibitors, could be promising directions for future algal control strategies.

Dielectric barrier discharge plasma promotes disinfection-residual-bacteria inactivation via electric field and reactive species

Traditional disinfection processes face significant challenges such as health and ecological risks associated with disinfection-residual-bacteria due to their single mechanism of action. Development of new disinfection processes with composite mechanisms is therefore urgently needed. In this study, we employed liquid ground-electrode dielectric barrier discharge (lgDBD) to achieve synergistic sterilization through electric field electroporation and reactive species oxidation. At a voltage of 12 kV, Pseudomonas fluorescens (ultraviolet and ozone-resistant) and Bacillus subtilis (chlorine-resistant) were completely inactivated within 8 and 6 min, respectively, surpassing a 7.0-log reduction. The lgDBD process showed good disinfection performance across a wide range of pH values and different practical water samples. Staining experiments suggest that cellular membrane damage contributes to this inactivation. In addition, we used a two-dimensional parallel streamer solver with kinetics code to fashion a representative model of the basic discharge unit, and discovered the presence of a persistent electric field during the discharge process with a peak value of 2.86 × 106 V/m. Plasma discharge generates excited state species such as O(1D) and N2(C3Πu), and further forms reactive oxygen and nitrogen species at the gas-liquid interface. The physical process, which is driven by electric field-induced cell membrane electroporation, synergizes with the bactericidal effects of reactive oxygen and nitrogen species to provide effective disinfection. Adopting the lgDBD process enhances sterilization efficiency and adaptability, underscoring its potential to revolutionize physicochemical synergistic disinfection practices.

Formation and toxicity alteration of halonitromethanes from Chlorella vulgaris during UV/chloramination process involving bromide ion

Frequent algal blooms cause algal cells and their algal organic matter (AOM) to become critical precursors of disinfection by-products (DBPs) during water treatment. The presence of bromide ion (Br-) in water has been demonstrated to affect the formation laws and species distribution of DBPs. However, few researchers have addressed the formation and toxicity alteration of halonitromethanes (HNMs) from algae during disinfection in the presence of Br-. Therefore, in this work, Chlorella vulgaris was selected as a representative algal precursor to investigate the formation and toxicity alteration of HNMs during UV/chloramination involving Br-. The results showed that the formation concentration of HNMs increased and then decreased during UV/chloramination. The intracellular organic matter of Chlorella vulgaris was more susceptible to form HNMs than the extracellular organic matter. When the Br-: Cl2 mass ratio was raised from 0.004 to 0.08, the peak of HNMs total concentration increased 33.99%, and the cytotoxicity index and genotoxicity index of HNMs increased 67.94% and 22.80%. Besides, the formation concentration and toxicity of HNMs increased with increasing Chlorella vulgaris concentration but decreased with increasing solution pH. Possible formation pathways of HNMs from Chlorella vulgaris during UV/chloramination involving Br- were proposed based on the alteration of nitrogen species and fluorescence spectrum analysis. Furthermore, the formation laws of HNMs from Chlorella vulgaris in real water samples were similar to those in deionized water samples. This study contributes to a better comprehension of HNMs formation from Chlorella vulgaris and provides valuable information for water managers to reduce hazards associated with the formation of HNMs.

Changes of characteristics and disinfection by-products formation potential of intracellular organic matter with different molecular weight in metalimnetic oxygen minimum

Metalimnetic oxygen minimum (MOM) occurs in reservoirs or lakes due to stratification and algal blooms, which has low dissolved oxygen (DO) levels and leads to the deterioration of water quality. The transformation mechanism and the impact on the water quality of intracellular organic matter (IOM) derived from algae are poorly understood under MOM conditions. In this study, IOM extracted by Microcystis aeruginosa was divided into five components according to molecular weight (MW), and the changes of characteristics and correlated disinfection by-products formation potential (DBPFP) were analyzed and compared under MOM conditions. The removal efficiency of dissolved organic carbon (DOC) in the <5 kDa fraction (66.6%) was higher than that in the >100 kDa fraction (41.8%) after a 14-day incubation under MOM conditions. The same tendency also occurred in Fmax and DBPFP. The decrease in Fmax was mainly due to the decline in tryptophan-like and tyrosine-like for all IOM fractions. The diversity of microorganisms degrading the MW > 100 kDa fraction was lower than others. Besides low MW fractions, these findings indicated that more attention should be paid to high MW fractions which were resistant to biodegradation under MOM conditions during water treatment.

Effects of Halides on Organic Compound Degradation during Plasma Treatment of Brines

Plasma has been proposed as an alternative strategy to treat organic contaminants in brines. Chemical degradation in these systems is expected to be partially driven by halogen oxidants, which have been detected in halide-containing solutions exposed to plasma. In this study, we characterized specific mechanisms involving the formation and reactions of halogen oxidants during plasma treatment. We first demonstrated that addition of halides accelerated the degradation of a probe compound known to react quickly with halogen oxidants (i.e., para-hydroxybenzoate) but did not affect the degradation of a less reactive probe compound (i.e., benzoate). This effect was attributed to the degradation of para-hydroxybenzoate by hypohalous acids, which were produced via a mechanism involving halogen radicals as intermediates. We applied this mechanistic insight to investigate the impact of constituents in brines on reactions driven by halogen oxidants during plasma treatment. Bromide, which is expected to occur alongside chloride in brines, was required to enable halogen oxidant formation, consistent with the generation of halogen radicals from the oxidation of halides by hydroxyl radical. Other constituents typically present in brines (i.e., carbonates, organic matter) slowed the degradation of organic compounds, consistent with their ability to scavenge species involved during plasma treatment.

Efficient degradation of imidacloprid by surface discharge cold plasma: Mechanism of interaction between ROS and molecular structure and evaluation of residual toxicity

Incorrect use of neonicotinoid pesticides poses a serious threat to human and pollinator health, as these substances are commonly present in bee products and even drinking water. To combat this threat, the study developed a new method of degrading the pesticide imidacloprid using surface discharge cold plasma oxidation technology. The study showed that this method achieved a very high efficiency of imidacloprid degradation of 91.4%. The main reactive oxygen species (H2O2, O3, ·OH, O2-, 1O2) effectively participated in the decomposition reaction of imidacloprid. Reactive oxygen species were more sensitive to the structure of the nitroimine group. Density functional theory (DFT) further explored the sites of reactive oxygen species attack on imidacloprid and revealed the process of energy change of attacking imidacloprid. In addition, a degradation pathway for imidacloprid was proposed, mainly involving reactive oxygen species chemisorption, a ring-opening intermediate, and complete cleavage of the nitroimine group structure. Model predictions indicated that acute oral and developmental toxicity were significantly reduced after cold plasma treatment, as confirmed by insect experiments. Animal experiments have shown that plasma treatment reduces imidacloprid damage to mice hippocampal tissue structure and inhibits the reduction of brain-derived neurotrophic factor content, thus revealing the detoxification mechanism of the body.

Cell-Targeted Metal-Phenolic Nanoalgaecide in Hydroponic Cultivation to Enhance Food Sustainability

The growing global population necessitates substantial increases in food production. Hydroponic cultivation systems afford a critical alternative for food sustainability and enable stable annual production regardless of the climatic and geographical variations. However, the overgrowth of harmful algal blooms significantly threatens the crop yield by competing with nutrition in the solution and producing contaminants. The conventional practice of algaecides fails to control algal proliferation due to the limited efficiency and food safety concerns. Nanopesticides can deliver active ingredients responsively to suppress crop diseases and offer solutions to current practical challenges and difficulties. Inspired by prospects of nanotechnology for agricultural applications, we have utilized natural polyphenols and copper ions (Cu2+ ions) to develop self-assembled nanoalgaecides referred to as CuBes. The nanoalgaecide attached to algal cells via phenolic surface interactions, enabling localized Cu2+ ion release. This cell-targeted delivery suppressed Chlorella vulgaris for over 30 days (99% inhibition). Transcriptomics revealed that the nanoalgaecide disrupted algal metabolism by downregulating photosynthesis and chlorophyll pathways. In a solar-illuminated plant factory, the nanoalgaecide showed higher algal inhibition and lettuce biosafety versus the commercial Kocide 3000. Notably, the use of nanoalgaecide can enhance the nutrient value of lettuces, which meets the daily supply of Cu for adults. By integrating smart nanotechnology design with selective delivery mechanisms, this metal-phenolic nanoalgaecide provides a nanoenabled solution for controlling harmful algal blooms in hydroponics to advance food production.

Toxicological effects and underlying mechanisms of chlorination-derived metformin byproducts in Escherichia coli

Chlorination-derived byproducts of the emerging contaminant metformin, such as (3E)-3-(chloroimino)-N,N-dimethyl-3H-1,2,4-triazol-5-amine (3,3-CDTA) and N-cyano-N,N-dimethylcarbaminmidic chloride (NCDC), occur in global waters and are toxic to organisms, from bacteria to mice. However, the mechanisms underlying their toxicity remain unknown. Here, we explored the toxicological effects and potential molecular mechanisms of 3,3-CDTA and NCDC at milligram concentrations, using Escherichia coli as a model organism. Compared with metformin (>300 mg/L), 3,3-CDTA and NCDC exerted stronger toxicity to E. coli, with a 4-h half maximal inhibitory concentration of 2.97 mg/L and 75.7 mg/L, respectively. Both byproducts disrupted E. coli cellular structures and components, decreased membrane potential and adenosine triphosphate (ATP) biosynthesis, and led to excessive reactive oxidative species (ROS), as well as the ROS-scavenging enzymes superoxide dismutase and catalase. Proteomic analysis and molecular docking supported these biomarker responses in the byproduct-treated E. coli, and indicated potential damage to DNA/RNA processes, while also provided novel insights into the toxicological and detoxified-byproduct effects at the proteome level. The toxicity-related events of NCDC and 3,3-CDTA included membrane disruption, oxidative stress, and abnormal protein expression. This study is the first to examine the toxicological effects of chlorination-derived metformin byproducts in E. coli and the associated pathways involved; thereby broadening our understanding regarding the toxicity and transformation risks of metformin throughout its entire life process.

Maternal exposure to sunlight-irradiated graphene oxide induces neurodegeneration-like symptoms in zebrafish offspring through intergenerational translocation and genomic DNA methylation alterations

The physiochemical properties of graphene oxide may be affected by sunlight irradiation. However, the underlying mechanisms that alter the properties and subsequent intergenerational effects are not sufficiently investigate. Epigenetics is an early sensitive marker for the intergenerational effects of nanomaterial exposure due to the epigenetic memory. In this study, we investigate changes in the physicochemical properties and the intergenerational effects of maternal exposure to simulated sunlight-irradiated polyethyleneimine-functionalized graphene oxide (SL-PEI-GO). Results show that the physicochemical properties of polyethyleneimine-functionalized graphene oxide (PEI-GO) can be altered significantly by the oxidation of carbon atoms with unpaired electrons present in the defects and on the edges of PEI-GO by sunlight. First, the positive charges, sharp edges, defects and disordered structures of SL-PEI-GO make it translocate from maternal zebrafish to offspring, thus catalyzing the production of reactive oxygen species and damaging mitochondria directly. In addition, changes in DNA methylation reduce the expression of protocadherin1a, protocadherin19 and cadherin4, thus destroying cell membrane integrity, cell adhesion and Ca2+ binding. The alteration of DNA methylation induced by maternal exposure activates the Ca2+-CaMKK-brsk2a pathway, which catalyzes the phosphorylation of Tau and eventually results in the appearance of neurodegeneration-like symptoms, including the loss of neurons and neurobehavioral disorders. This study demonstrates that maternal exposure to SL-PEI-GO induces clear neurodegeneration-like symptoms in offspring through both the intergenerational translocation of nanomaterials and differential DNA methylation. These findings may provide new insights into the health risks of nanomaterials altered by nature conditions.

Fluorescent Dissolved Organic Matter Components as Surrogates for Disinfection Byproduct Formation in Drinking Water: A Critical Review

Disinfection byproduct (DBP) formation, prediction, and minimization are critical challenges facing the drinking water treatment industry worldwide where chemical disinfection is required to inactivate pathogenic microorganisms. Fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC) is used to characterize and quantify fluorescent dissolved organic matter (FDOM) components in aquatic systems and may offer considerable promise as a low-cost optical surrogate for DBP formation in treated drinking waters. However, the global utility of this approach for quantification and prediction of specific DBP classes or species has not been widely explored to date. Hence, this critical review aims to elucidate recurring empirical relationships between common environmental fluorophores (identified by PARAFAC) and DBP concentrations produced during water disinfection. From 45 selected peer-reviewed articles, 218 statistically significant linear relationships (R2 ≥ 0.5) with one or more DBP classes or species were established. Trihalomethanes (THMs) and haloacetic acids (HAAs), as key regulated classes, were extensively investigated and exhibited strong, recurrent relationships with ubiquitous humic/fulvic-like FDOM components, highlighting their potential as surrogates for carbonaceous DBP formation. Conversely, observed relationships between nitrogenous DBP classes, such as haloacetonitriles (HANs), halonitromethanes (HNMs), and N-nitrosamines (NAs), and PARAFAC fluorophores were more ambiguous, but preferential relationships with protein-like components in the case of algal/microbial FDOM sources were noted. This review highlights the challenges of transposing site-specific or FDOM source-specific empirical relationships between PARAFAC component and DBP formation potential to a global model.

Potential of non-thermal discharge plasmas for activated sludge settling: effects and underlying mechanisms

With increase in the construction of urban sewage treatment plants, the output of sludge also surges. Therefore, it is highly important to explore effective ways to reduce the production of sludge. In this study, non-thermal discharge plasmas were proposed to crack the excess sludge. High sludge settling performance was obtained, and the settling velocity (SV₃₀) dramatically decreased from the initial value of 96% to 36% after 60 min of treatment at 20 kV, accompanied by 28.6%, 47.5%, and 76.7% decreases in mixed liquor suspended solids (MLSS), sludge volume index (SVI), and sludge viscosity, respectively. Acidic conditions improved the sludge settling performance. The presence of Cl^- and NO₃^- slightly promoted the SV₃₀, but CO₃^2- has adverse effects. ·OH and O₂˙^- in the non-thermal discharge plasma system contributed to the sludge cracking, especially for ·OH. These reactive oxygen species destroyed the sludge floc structure; as a result, the total organic carbon and dissolved chemical oxygen demand obviously increased, the average particle size of the sludge decreased, and the number of coliform bacteria was also reduced. Furthermore, the microbial community abundance and diversity both decreased in the sludge after the plasma treatment.

Role and Mechanism of Cold Plasma in Inactivating Alicyclobacillus acidoterrestris in Apple Juice

A. acidoterrestris has been identified as the target bacterium in fruit juice production due to its high resistance to standard heat treatment. Multiple studies have shown that cold plasma can effectively inactivate pathogenic and spoilage microorganisms in juices. However, we are aware of only a few studies that have used cold plasma to inactivate A. acidoterrestris. In this study, the inactivation efficacy of cold plasma was determined using the plate count method and described using a biphasic model. The effects of the food matrix, input power, gas flow rate, and treatment time on inactivation efficacy were also discovered. Scavenging experiments with reactive oxygen species (•OH, •O2−, and 1O2), scanning electron microscopy (SEM), Raman spectra, as well as an in vitro toxicology assay kit, were used to determine the inactivation mechanism. According to the plate count method, a maximum reduction of 4.14 log CFU/ mL could be achieved within 7 s, and complete inactivation could be achieved within 240 s. The scavenging experiments showed that directly cold plasma-produced singlet oxygen plays the most crucial role in inactivation, which was also confirmed by the fluorescence probe SOSG. The scanning electron microscopy (SEM) and Raman spectra showed that the cold plasma treatment damaged the membrane integrity, DNA, proteins, lipids, and carbohydrates of A. acidoterrestris. The plate count results and the apple juice quality evaluation showed that the cold plasma treatment (1.32 kV) could inactivate 99% of A. acidoterrestris within 60 s, with no significant changes happening in apple juice quality, except for slight changes in the polyphenol content and color value.

Platymonas helgolandica-driven nitrogen removal from mariculture wastewater under different photoperiods: Performance evaluation, enzyme activity and transcriptional response

The nitrogen removal performance and biological mechanism of Platymonas helgolandica var. Tsingtaoensis (P. helgolandica) were investigated in treating mariculture wastewater under different light: dark (L:D) photoperiods. The growth of P. helgolandica was positively correlated with the photoperiods from 6L:18D to 15L:9D, and the highest photosynthetic activity appeared under 6L:18D photoperiod on day 3. P. helgolandica exhibited the highest removal efficiencies of total nitrogen and COD at 89 % and 93 % under 15L:9D photoperiod, respectively. NH₄⁺-N assimilation was proportional to the photoperiods from 6L:18D to 15L:9D and longer illumination promoted NO₂^--N removal. However, the highest NO₃^--N reduction rate was achieved under 12L:12D photoperiod. The different nitrogen-transformed enzymatic activities were affected by photoperiod. Transcriptome revealed that unigenes were enriched in nitrogen metabolism and photosynthesis pathways, of which the functional gene expression was up-regulated significantly. This study provides insights into the optimization of photoperiod for mariculture wastewater treatment by P. helgolandica.

Formation of emerging disinfection byproducts from agricultural biomass-derived DOM: Overlooked health risk source

Carbon-derived dissolved organic matter (CDOM) are inevitably released to surface water during returning agricultural biomass carbon to farmland, which are potential precursors of disinfection byproducts (DBPs). In this study, CDOM was extracted from aerobic incineration ("OX") and anoxic pyrolysis ("PY") of three kinds of straw (wheat, corn, and rice), and the emerging DBPs from them were deciphered. The CDOM with molecular weight < 1 kDa in the OX and PY groups accounted for 53-87%, and it was higher in the PY group. A total 1343-2107 of CHO and 641-1761 of CHNO formulas were detected in the CDOM derived from the OX group, among which 74%-83% contained aromatic structures rich in oxygen containing groups. 1919-3289 of CHO and 785-1954 of CHNO formulas were observed in the PY group, and 77%-86% of them were lignins/CRAM-like compounds. Surprisingly, 765-2158 and 895-1648 of emerging DBPs were identified in the OX and PY groups, and the proportions of N-DBPs were 20.3-54.8% and 2.8-4.8%, respectively. Based on HOCl addition and Cl substitution mechanisms, the H/C ratios of the DBP precursors in the OX and PY groups were in the range of 0.2-1.5 and 0.6-2.0, respectively. The DBPs derived from the OX group exhibited higher cytotoxicity and genotoxicity due to the higher aromaticity and more N-DBPs. Thus, returning agricultural biomass carbon, particularly that produced by direct combustion, to farmland brought potential threat to drinking water safety.

Removal of toxic Microcystis aeruginosa using discharge plasma: mechanism and potential environmental risks

Discharge plasma can remove Microcystis aeruginosa quickly and effectively, and reduce the potential environmental risk caused by the released DOM.

Photocatalytic reduction of Cr(VI) via surface modified g-C3N4 by acid-base regulation

Cr(VI) is a class of highly toxic heavy metals. In this study, alkali-modified g-C₃N₄ (cOH-CN) and acid-modified g-C₃N₄ (cH-CN) materials were successfully synthesized, and their photocatalytic activities for Cr(VI) reduction under visible light irradiation were tested. Owing to defect structures by cH-CN and -OH group introduction by cOH-CN, the modified materials exhibited a larger surface area, more abundant pore structures, a wider visible light absorption range, higher energy gap values, and a stronger capacity for electron-hole pair separation. As a result, satisfactory Cr(VI) reduction performance was gained by these two photocatalysts. Almost all Cr(VI) was converted to Cr(III) after 60 min of treatment in the presence of these two catalysts, while it was only 30% for the pristine g-C₃N₄ materials. Relatively higher dosages of cH-CN and cOH-CN and acidic conditions both improved Cr(VI) reduction in the cH-CN and cOH-CN photocatalytic systems. Cr(VI) reduction was mainly initiated by free electrons in the photocatalytic system of the modified materials. Finally, Cr(VI) in the photocatalytic system was almost completely converted to Cr(III). Furthermore, the stability and recycling of the cH-CN and cOH-CN catalysts were evaluated.

Intrinsic mechanisms underlying the highly efficient removal of bacterial endotoxin and related risks in tailwater by dielectric barrier discharge plasma

Endotoxin is widely present in aquatic environments and can induce adverse health effects. In this study, dielectric barrier discharge (DBD) plasma was used to remove bacterial endotoxin from the tailwater of a wastewater treatment plant. The removal efficiency of total endotoxin activity was up to 92% with low electrical energy consumption (0.43 J mL^-1%^-1) after 180 s of the DBD plasma treatment, which was better than other previously reported methods. In the early stage of DBD plasma oxidation, the expression of genes related to cell morphology and bacterial antioxidant enzyme synthesis was distinctly down-regulated, suggesting that cell integrity was destroyed, leading to endotoxin release into the solution. Additionally, endotoxin synthesis in the cells was suppressed. The endotoxin in the solution was effectively removed by ·OH, ¹O₂, and O₂·^-generated by the DBD plasma, with second-order reaction rates of 2.69 × 10¹⁰, 2.20 × 10⁷, and 8.60 × 10⁸ mol^-1 L s^-1, respectively. The core toxic component of endotoxin (lipid A) was attacked by these strong oxidative species, generating smaller molecular fragments with low toxicity. Consequently, the inflammatory factors IL-6, IL-β, and TNF-α of endotoxin decreased by 3.4-4.8 folds after the DBD plasma treatment, implying that the health risks posed by endotoxin were greatly reduced. This study revealed the intrinsic mechanisms of the highly efficient removal of bacterial endotoxin by DBD plasma oxidation.

Change of algal organic matter under different dissolved oxygen and pressure conditions and its related disinfection by-products formation potential in metalimnetic oxygen minimum

Most of the reservoirs or lakes will form a metalimnetic oxygen minimum (MOM) with the characterization of a substantial fraction of dissolved oxygen (DO) depleted below the epilimnion. The effect of intracellular organic matter (IOM) of algal cells transformed under MOM conditions is completely different from that of the original IOM on water quality. In this study, the IOM changes of Microcystic aeruginosa under different MOM conditions and its related disinfection by-products formation potentials (DBPFPs) were investigated by changing the pressure and DO concentration of MOM. Total Fmax increased slightly and then decreased under different pressure conditions, finally decreasing by no more than 22.0%. Under aerobic condition, dissolved organic carbon (DOC) and total Fmax decreased significantly, and decreased by 60.4% and 38.8% within the first 2 days. The results of specific UV absorbance (SUVA) and UV₂₅₀/UV₃₆₅ indicated that aromatic compounds and average molecular weight of IOM were gradually increased under different MOM conditions. The total DBPFPs increased firstly and then decreased under different pressure conditions, and finally decreased by 26.2%-33.1%. The decrease of total DBPFPs was significantly higher under aerobic condition than that under anoxic condition, which finally decreased by 64.5%. Redundancy analysis showed that the fluorescence parameter (protein-like and humic-like fluorescence) could be expected as an index to predict the DBPFPs. Moreover, the results revealed that with the decrease of DO, the activity and diversity of natural microbial consortium decreased, which prevented the further degradation and utilization of organic matter by natural microbial consortium. Therefore, lower DO was a key player for the deterioration of water quality under MOM conditions.

p-Nitrophenol contaminated soil remediation in a spray-type coaxial cylindrical dielectric barrier discharge plasma system

In the present work, plasma remediation of p-nitrophenol (PNP) contaminated soil was performed in a novel spray-type coaxial cylindrical dielectric barrier discharge (DBD) system at ambient temperature. This system is capable of generating large-size nonthermal plasma (NTP) and improving the diffusion and transfer of chemical active species around the dispersed soil particles. Several key parameters including plasma treatment time, discharge voltage, soil granular size, the entry speed of soil, PNP initial concentration, gas variety, and gas flow rate were investigated in terms of PNP degradation and energy efficiencies. Under the optimized experimental conditions, 54.2% of PNP was degraded after only 50 s discharge treatment, indicating that the spray-type coaxial cylindrical DBD system can degrade organic pollutants in soil more quickly compared to other plasma systems due to its efficient transfer of reactive oxygen and nitrogen species (RONS) into the contaminated soil. The possible PNP degradation pathways were proposed based on intermediates identification results and the role of reactive species analysis. The toxicological assessment of the PNP decomposition products was conducted by quantitative structure-activity relationship (QASR) analysis. This work is expected to provide a potential plasma technology for rapid and efficient processing of industrial organic pollutants contamination soil.

Underlying mechanisms of promoted formation of haloacetic acids disinfection byproducts after indometacin degradation by non-thermal discharge plasma

Indometacin (IDM), as a kind of non-steroidal anti-inflammatory drugs, has ecological and health risks, which is the potential precursor of chlorination disinfection byproducts (DBPs). Non-thermal discharge plasma was attempted to eliminate IDM and control subsequent DBPs formation. Satisfactory removal performance for IDM was realized by the plasma oxidation; almost 100% of IDM was removed within 2 min. Relatively greater removal efficiency was gained at a higher plasma voltage and a lower pH level. Electron paramagnetic resonance spectrometer revealed that reactive species ·OH, O₂·^-, and ¹O₂ were responsible for IDM decomposition. Based on analyses of Fourier transform infrared spectroscopy, two-dimensional correlation spectroscopy, three-dimensional fluorescence spectrum, and gas chromatography-mass spectrometer, attacks of reactive species resulted in sequence breakages in functional groups of IDM, leading to production of small molecular alcohols, acids, and amines. Possible decomposition pathways of IDM were proposed. The produced acetamide and 1H-indol-5-ol were important precursors of DBPs. Formation and toxicity of nitrogen-containing DBPs were dramatically inhibited after IDM degradation; however, those of haloacetic acids were strengthened. The relevant roadmaps among DBPs and degradation intermediates were figured out. This study revealed the underlying mechanisms of IDM degradation by discharge plasma and its potential risks in chlorination disinfection.

Intensified inactivation of model and environmental bacteria by an atmospheric-pressure air-liquid discharge plasma compared with chlorination

Water-borne pathogenic bacteria are always the top priority to be removed through disinfection process in water treatment due to their threat to human health. It was necessary to develop novel disinfection methods since the conventional chlorine disinfection was inefficient in inactivating chlorine-resistant bacteria, inducing the viable but non-culturable (VBNC) bacteria and forming disinfection by-products (DBPs). In this study, the inactivation of four model strains including Gram-negative (G^-), Gram-positive (G⁺) and environmental samples by atmospheric-pressure air-liquid discharge plasma (ALDP) was assessed systematically. The results showed that ALDP was superior in inactivating all of the samples compared with chlorination. During 10 min ALDP treatment, the G^- bacteria were completely inactivated, and the G⁺ one was inactivated by more than 4.61 logs. The inactivation of bacteria from a campus lake and a wastewater treatment plant effluent exceeded 99.82% and 97.78%, respectively. For G^- bacteria, ALDP resulted in a much lower (10²∼10³ times) levels of VBNC cells than chlorination. ALDP could effectively remove the chlorine-resistant bacteria. More than 96.41% of the intracellular DNA and 99.99% of the extracellular DNA were removed, whereas it was only 56.35% and 12.82% for chlorination. ALDP had a stronger ability to destroy cell structure than chlorination, presumably due to the existence of ROS (·OH, ¹O₂ and O₂^-). GC-MS analysis showed that ALDP produced less DBPs than chlorination. These findings provided new insights for the application of discharge plasma in water disinfection, which could be complemental or alternative to the conventional disinfection methods.

FT-ICR/MS deciphers formation of unknown macromolecular disinfection byproducts from algal organic matters after plasma oxidation

Algal organic matter (AOM) is a potential precursor of disinfection byproducts (DBPs) in water treatment. It is a major challenge to identify macromolecular DBPs due to the diversity of AOM. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) was applied to diagnose the AOM diversity after algae removal by plasma oxidation and to recognize the macromolecular DBPs in subsequent chlorination. Significant removal of AOM released by M. aeruginosa, C. raciborskii, and A. spiroies was achieved by plasma oxidation, accompanied by decrease in the proportion of CHNO formulas and increase in CHO formulas. Without plasma treatment, chlorination generated approximately 2486 macromolecular carbonaceous DBPs (C-DBPs) and 1984 nitrogenous DBPs (N-DBPs), with C₁₁H_nO_mCl_x and C₁₈H_nN_mO_zCl_x as the most abundant DBPs. The numbers of C-DBPs and N-DBPs decreased by 63.3% and 62.9%, respectively, if plasma treatment was applied prior to chlorination. Network computational analysis revealed that Cl substitution was the main formation pathway of AOM-derived DBP formation rather than HOCl addition. The precursors of macromolecular DBPs contained a characteristic atomic number of C and O (7 ≤ C ≤ 18; 3 ≤ O ≤ 11). This study firstly disclosed the relationship between AOM diversity and novel macromolecular DBPs during algae-laden water treatment.

Bacterial Inactivation and Biofilm Disruption through Indigenous Prophage Activation Using Low-Intensity Cold Atmospheric Plasma

Biofilms can be pervasive and problematic in water treatment and distribution systems but are difficult to eradicate due to hindered penetration of antimicrobial chemicals. Here, we demonstrate that indigenous prophages activated by low-intensity plasma have the potential for efficient bacterial inactivation and biofilm disruption. Specifically, low-intensity plasma treatment (i.e., 35.20 W) elevated the intracellular oxidative reactive species (ROS) levels by 184%, resulting in the activation of prophage lambda (λ) within antibiotic-resistant Escherichia coli K-12 (lambda+) [E. coli (λ+)]. The phage activation efficiency was 6.50-fold higher than the conventional mitomycin C induction. Following a cascading effect, the activated phages were released upon the lysis of E. coli (λ+), which propagated further and lysed phage-susceptible E. coli K-12 (lambda-) [E. coli (λ-)] within the biofilm. Bacterial intracellular ROS analysis and ROS scavenger tests revealed the importance of plasma-generated ROS (e.g., ^•OH, ¹O₂, and ^•O₂^-) and associated intracellular oxidative stress on prophage activation. In a mixed-species biofilm on a permeable membrane surface, our "inside-out" strategy could inactivate total bacteria by 49% and increase the membrane flux by 4.33-fold. Furthermore, the metagenomic analysis revealed that the decrease in bacterial abundance was closely associated with the increase in phage levels. As a proof-of-concept, this is the first demonstration of indigenous prophage activations by low-intensity plasma for antibiotic-resistant bacterial inactivation and biofilm eradication, which opens up a new avenue for managing associated microbial problems.

Mineralization, characteristics variation, and removal mechanism of algal extracellular organic matter during vacuum ultraviolet/ozone process

Extracellular organic matter (EOM) produced by algal blooms in source water is detrimental to drinking water treatment processes and supplied water quality. Ozonation has been used to treat algal EOM, but it could not mineralize EOM effectively. In this study, mineralization and characteristics variation of EOM by vacuum ultraviolet/ozone (VUV/O₃) and its sub-processes were comprehensively investigated. Results showed that EOM removal in different processes followed the order of VUV/O₃ > UV/O₃ > O₃ > VUV > UV. For VUV/O₃ process, removal efficiencies of dissolved organic carbon (DOC), UV₂₅₄, protein, and polysaccharide at 50 min were 75.6%, 80.8%, 80.1%, and 78.0%, respectively, and fluorescence components received very high removal rates (≥92.8%, at 10 min). The yield of trichloromethane dropped from 102.0 to 30.1 μg/L after treating for 50 min by VUV/O₃. Besides, effects of O₃ dosage, initial pH, and water matrices on EOM removal in VUV/O₃ process were investigated. Moreover, fluorescent molecular probe experiments confirmed that hydroxyl radical and superoxide radical were the main reactive oxygen species (ROS) in VUV/O₃ process, and the transformation of ROS was proposed. The mechanism of EOM removal by VUV/O₃ included VUV photolysis, direct O₃ oxidation, and ROS oxidation. Furthermore, the removal of EOM in filtered water by VUV/O₃ was satisfactory. All results indicated that VUV/O₃ process had great application potential in treating EOM-rich filtered water.

Can we use mine waste as substrate in constructed wetlands to intensify nutrient removal? A critical assessment of key removal mechanisms and long-term environmental risks

The utilization of natural ores and/or mine waste as substrate in constructed wetlands (CWs) to enhance nutrient removal performance has been gaining high popularity recently. However, the knowledge regarding the long-term feasibility and key removal mechanisms, particularly the potential negative environmental effects of contaminants leached from mine waste is far insufficient. This study, for the first time, performed a critical assessment by using different CWs with three mine waste (coal gangue, iron ore and manganese ore) as substrates in a 385-day experiment treating wastewater with varying nutrient loadings. The results showed that the addition of mine waste in CWs increased removal of total phosphorus (TP) by 17-34%, and total nitrogen (TN) by 11-51%. The higher removal of TP is mainly attributed to the strong binding mechanism of phosphate with the oxides and hydroxides of Mn, Fe and/or Al, which are leached out of mine waste. Moreover, integration of mine waste in CWs also significantly stimulated biofilm establishment and enriched the relative abundance of key functional genes related to the nitrogen cycle, supporting the observed high-rate nitrogen removal. However, leaching of heavy metals (Fe, Mn, Cu and Cr) from the beded mine waste in the experimented CWs was monitored, which further influenced cytoplasmic enzymes and created oxidative stress damage to plants, resulting in a decline of nutrient uptake by plants.

Theoretical and experimental insights into the mechanisms of C6/C6 PFPiA degradation by dielectric barrier discharge plasma

As an emerging alternative legacy perfluoroalkyl substance, C6/C6 PFPiA (perfluoroalkyl phosphinic acids) has been detected in aquatic environments and causes potential risks to human health. The degradation mechanisms of C6/C6 PFPiA in a dielectric barrier discharge (DBD) plasma system were explored using validated experimental data and density functional theory (DFT) calculations. Approximately 94.5% of C6/C6 PFPiA was degraded by plasma treatment within 15 min at 18 kV. A relatively higher discharge voltage and alkaline conditions favored its degradation. C6/C6 PFPiA degradation was attributed to attacks of •OH, •O₂^-, and ¹O₂. Besides PFHxPA and C2 -C6 shorter-chain perfluorocarboxylic acids, several other major intermediates including C4/C6 PFPiA, C4/C4 PFPiA, and C3/C3 PFPiA were identified. According to DFT calculations, the potential energy surface was proposed for possible reactions during C6/C6 PFPiA degradation in the discharge plasma system. Integrating the identified intermediates and DFT results, C6/C6 PFPiA degradation was deduced to occur by stepwise losing CF₂, free radical polymerization, and C-C bond cleavage. Furthermore, the DBD plasma treatment process decreased the toxicity of C6/C6 PFPiA to some extent. This study provides a comprehensive understanding of C6/C6 PFPiA degradation by plasma advanced oxidation.

Inactivation of Microcystis Aeruginosa by peracetic acid combined with ultraviolet: Performance and characteristics

The inactivation of algae by a combined process of peracetic acid and ultraviolet irradiation (UV/PAA) was systematically investigated by choosing Microcystis aeruginosa as the reference algal species. Both hydroxyl (HO^•) and organic radicals (RO^•) contributed to the cell integrity loss and RO^• played the dominant roles. The algae inactivation kinetics can be well fitted by the typical Hom model, showing that the inactivation kinetic curves followed a type of shoulder and exponential reduction. The initial shoulder might be induced by the protection from the cell wall. Although the results from the cell morphology, UV-vis spectra and fluorescence excitation-emission matrices analysis suggested the cell lysis and the release of algal organic matter (AOM) in the UV/PAA process, the AOM could be subsequently degraded. Humic acid (1 - 5 mg/L) inhibited the algal cell inactivation, and the presence of chloride (0.5 - 2 mM) had little effect on the cell viability reduction. However, the addition of bicarbonate (1 - 5 mM) promoted cell integrity loss. The UV/PAA process displayed better performance under the natural water background, demonstrating the extensive potential for the practical application of this approach. This study suggests that the UV/PAA process is an effective strategy for algae inactivation.

Pyrene contaminated soil remediation using microwave/magnetite activated persulfate oxidation

Polycyclic aromatic hydrocarbons (PAHs) are important mutagen prevalent in the contaminated sites, bringing potential risks to human health. Iron oxides are important natural components in soils. Pyrene removal in soil using persulfate (PS) oxidation activated by microwave (MW) and magnetite (Fe₃O₄) was investigated. Fe₃O₄ significantly promoted pyrene removal in the soil; 91.7 % of pyrene was degraded within 45 min treatment. Pyrene removal rate in the Fe₃O₄/MW/PS system was 5.18 and 3.00 times higher than that in the Fe₃O₄/PS and MW/PS systems. Increasing in Fe₃O₄ dosage, PS concentration, MW temperature, and soil moisture content in the selected range were conducive for pyrene degradation. SO₄^•-, ^•OH, O₂^•-, and ¹O₂ were responsible for pyrene degradation, and the conversion of Fe (Ⅱ) in the Fe₃O₄ to Fe (Ⅲ) contributed to the formation of O₂^•- and ¹O₂. Characteristic bands of pyrene were more obviously destroyed by the Fe₃O₄/MW/PS oxidation, in comparison with MW/PS oxidation. Ring hydroxylation and ring-opening reactions were the main degradation pathways of pyrene. The toxicities of the formed byproducts were significantly reduced after treatment. This study provided a promising option for pyrene contaminated soil remediation.

Inhibited conjugative transfer of antibiotic resistance genes in antibiotic resistant bacteria by surface plasma

Antibiotic resistant bacteria (ARB) and resistance genes (ARGs) are emerging environmental pollutants with strong pathogenicity. In this study, surface plasma was developed to inactivate the donor ARB with Escherichia coli (AR E. coli) as a model, eliminate ARGs, and inhibit conjugative transfer of ARGs in water, highlighting the influences of concomitant inorganic ions. Surface plasma oxidation significantly inactivated AR E. coli, eliminated ARGs, and inhibited conjugative transfer of ARGs, and the presence of NO₃^-, Cu²⁺, and Fe²⁺ all promoted these processes, and SO₄^2- did not have distinct effect. Approximately 4.5log AR E. coli was inactivated within 10 min treatment, and it increased to 7.4log AR E. coli after adding Fe²⁺. Integrons intI1 decreased by 3.10log (without Fe²⁺) and 4.43log (adding Fe²⁺); the addition of Fe²⁺ in the surface plasma induced 99.8% decline in the conjugative transfer frequency. The inhibition effects on the conjugative transfer of ARGs were mainly attributed to the reduced reactive oxygen species levels, decreased DNA damage-induced response, decreased intercellular contact, and down-regulated expression of plasmid transfer genes. This study disclosed underlying mechanisms for inhibiting ARGs transfer, and supplied a prospective technique for ARGs control.