Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation

Insights into the mechanisms of plasma physicochemical characteristics on ultralong-lasting plasma-activated water: the influence of DC power polarity on RONS generation

In our recent work, we successfully developed an innovative method based on pin-water discharge for preparing ultralong-lasting plasma-activated water (PAW) with a lifetime of up to 720 hours at room temperature. However, the impact of power polarity on the preparation method for ultralong-lasting PAW remains unclear. In this study, we discovered that ultralong-lasting PAW could only be achieved with positive polarity. Further analysis of the liquid reactive oxygen and nitrogen species (RONS) revealed that the absence of H2O2 in the discharge chamber was crucial for the failure of ultralong-lasting PAW preparation at negative polarity. To elucidate the mechanism underlying the generation of RONS at different polarities, we conducted plasma feature diagnosis, compared discharge morphologies, and performed theoretical analyses based on chemical reactions. Our results indicated that the introduction of water vapor molecules through intense spraying at positive polarity led to an increase in the generation of H2O2-related source particles, while also interfering with N2-related electron collision reactions and chemical reaction coefficients, ultimately affecting the production of NO2-. Consequently, there was relatively less liquid NO2- and more abundant H2O2 in the discharge chamber at positive polarity, whereas the opposite trend was observed for these two key RONS at negative polarity. Furthermore, the minimal amount of NO2- at positive polarity and the tiny amount of H2O2 at negative polarity in the discharge chamber would be respectively consumed by the relatively abundant H2O2 at positive polarity and NO2- at negative polarity.

Formation of nitrated naphthalene in the sulfate radical oxidation process in the presence of nitrite

Polycyclic aromatic hydrocarbons (PAHs) have become a global environmental concern due to their potential hazardous implication for human health. In this study, we found that sulfate radical (SO4•-) could effectively degrade naphthalene (NAP), a representative PAH in groundwaters, generating 1-naphthol. This intermediate underwent further degradation, yielding ring-opening products including phthalic acid and salicylic acid. However, the presence of nitrite (NO2-), a prevalent ion in subsurface environments, was observed to compete with NAP for SO4•-, thus slowing down the NAP degradation. The reaction between NO2- and SO4•- generated a nitrogen dioxide radical (NO2•). Concurrently, in-situ formed 1-naphthol underwent further oxidization to the 1-naphthoxyl radical by SO4•-. The coupling of 1-naphthoxyl radicals with NO2• gave rise to a series of nitrated NAP, namely 2-nitro-1-naphthol, 4-nitro-1-naphthol, and 2,4-dinitro-1-naphthol. In addition, the in-situ formed phthalic acid and salicylic acid also underwent nitration, generating nitrophenolic products, although this pathway appeared less prominent than the nitration of 1-naphthol. When 10 μΜ NAP was subjected to heat activated peroxydisulfate oxidation in the presence of 10 μΜ NO2-, the total yield of nitrated products reached 0.730 μΜ in 120 min. Overall, the presence of NO2- dramatically altered the behavior of NAP degradation by SO4•- oxidation and contributed to the formation of toxic nitrated products. These findings raise awareness of the potential environmental risks associated with the application of SO4•--based oxidation processes for the remediation of PAHs-polluted sites in presence of NO2-.

Formation of brominated and nitrated byproducts during unactivated peroxymonosulfate oxidation of phenol

Brominated and nitrated byproducts generated from bromide (Br-) and nitrite (NO2-), respectively, by sulfate radical (SO4•-) oxidation have raised increasing concern. However, little is known about the concurrent generation of brominated and nitrated byproducts in the unactivated peroxymonosulfate (PMS) oxidation process. This study revealed that Br- can facilitate the transformation of NO2- to nitrated byproducts during unactivated PMS oxidation of phenol. In the co-existence of 0.1 mM Br- and 0.5 mM NO2-, the total yield of identified nitrated byproducts reached 2.316 μM in 20 min, while none was found with NO2- alone. Nitryl bromide (BrNO2) as the primary nitrating agent was formed via the reaction of NO2- with free bromine in situ generated through the oxidation of Br- by PMS. BrNO2 rapidly reacted with phenol or bromophenols, generating highly toxic nitrophenols or nitrated bromophenols, respectively. Increasing NO2- concentration led to more nitrated byproducts but less brominated byproducts. This study advances our understanding of the transformation of Br- and NO2- in the unactivated PMS oxidation process. It also provides important insights into the potentially underestimated environmental risks when PMS is applied to degrade organic contaminants under realistic environments, particularly when Br- and NO2- co-exist.

A novel remediation strategy of mixed calcium peroxide and degrading bacteria for polycyclic aromatic hydrocarbon contaminated water

Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic organic pollutants commonly detected in the aqueous phase. Traditional biodegradation is inefficient and advanced oxidation technologies are expensive. In the current study, a novel strategy was developed using calcium peroxide (CP) and PAH-degrading bacteria (PDB) to effectively augment PAH degradation by 28.62-59.22%. The PDB consisted of the genera Acinetobacter, Stenotrophomonas, and Comamonas. Applying the response surface model (RSM), the most appropriate parameters were identified, and the predictive degradation rates of phenanthrene (Phe), pyrene (Pyr), and ΣPAHs were 98%, 76%, and 84%, respectively. The constructed mixed system could reduce 90% of Phe and more than 60% of ΣPAHs and will perform better at pH 5-7 and lower salinity. Because PAHs tend to bind to dissolved organic matter (DOM) with larger molecular weights, humic acid (HA) had a larger negative effect on the PAH-degradation efficiency of the CP-PDB mixed system than fulvic acid (FA). The proposed PAH-degradation pathways in the mixed system were based on the detection of intermediates at different times. The investigation constructed and optimized a novel environmental PAH-degradation strategy. The synergistic application of PDB and oxidation was extended for organic contaminant degradation in aqueous environments.

Advances in the Removal of Organic Pollutants from Water by Photocatalytic Activation of Persulfate: Photocatalyst Modification Strategy and Reaction Mechanism

Environmental pollution caused by persistent organic pollutants has imposed big threats to the health of human and ecological systems. The development of efficient methods to effectively degrade and remove these persistent organic pollutants is therefore of paramount importance. Photocatalytic persulfate-based advanced oxidation technologies (PS-AOTs), which depend on the highly reactive SO4 - radicals generated by the activation of PS to degrade persistent organic pollutants, have shown great promise. This work discusses the application and modification strategies of common photocatalysts in photocatalytic PS-AOTs, and compares the degradation performance of different catalysts for pollutants. Furthermore, essential elements impacting photocatalytic PS-AOTs are discussed, including the water matrix, reaction process mechanism, pollutant degradation pathway, singlet oxygen generation, and potential PS hazards. Finally, the existing issues and future challenges of photocatalytic PS-AOTs are summarized and prospected to encourage their practical application. In particular, by providing new insights into the PS-AOTs, this review sheds light on the opportunities and challenges for the development of photocatalysts with advanced features for the PS-AOTs, which will be of great interests to promote better fundamental understanding of the PS-AOTs and their practical applications.

Bubble Turbulent Gas-Permeable Membrane for Ammonia Recovery from Swine Wastewater: Mass Transfer Enhancement and Antifouling Mechanisms

Recovering ammonium from swine wastewater employing a gas-permeable membrane (GM) has potential but suffers from the limitations of unattractive mass transfer and poor-tolerance antifouling properties. Turbulence is an effective approach to enhancing the release of volatile ammonia from wastewater while relying on interfacial disturbance to interfere with contaminant adhesion. Herein, we design an innovative gas-permeable membrane coupled with bubble turbulence (BT-GM) that enhances mass transfer while mitigating membrane fouling. Bubbles act as turbulence carriers to accelerate the release and migration of ammonia from the liquid phase, increasing the ammonia concentration gradient at the membrane-liquid interface. In comparison, the ammonium mass transfer rate of the BT-GM process applied to real swine wastewater is 38% higher than that of conventional GM (12 h). Through a computational fluid dynamics simulation, the turbulence kinetic energy of BT-GM system is 3 orders of magnitude higher than that of GM, and the effective mass transfer area is nearly 3 times that of GM. Seven batches of tests confirmed that the BT-GM system exhibits remarkable antifouling ability, broadens its adaptability to complex water quality, and practically promotes the development of sustainable resource recycling.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.

pH-dependent bisphenol A transformation and iodine disinfection byproduct generation by peracetic acid: Kinetic and mechanistic explorations

Peracetic acid (PAA) is regarded as an environmentally friendly oxidant because of its low formation of toxic byproducts. However, this study revealed the potential risk of generating disinfection byproducts (DBPs) when treating iodine-containing wastewater with PAA. The transformation efficiency of bisphenol A (BPA), a commonly detected phenolic contaminant and a surrogate for phenolic moieties in dissolved organic matter, by PAA increased rapidly in the presence of I-, which was primarily attributed to the formation of active iodine (HOI/I2) in the system. Kinetic model simulations demonstrated that the second-order rate constant between PAA and HOI was 54.0 M-1 s-1 at pH 7.0, which was lower than the generation rate of HOI via the reaction between PAA and I-. Therefore, HOI can combine with BPA to produce iodine disinfection byproducts (I-DBPs). The transformation of BPA and the generation of I-DBPs in the I-/PAA system were highly pH-dependent. Specifically, acidic conditions were more favorable for BPA degradation because of the higher reaction rates of BPA and HOI. More iodinated aromatic products were detected after 5 min of the reaction under acidic and neutral conditions, resulting in higher toxicity towards E. coli. After 12 h of the reaction, more adsorbable organic iodine (AOI) was generated at alkaline conditions because HOI was not able to efficiency transform to IO3-. The presence of H2O2 in the PAA solution played a role in the reaction with HOI, particularly under alkaline conditions. This study significantly advances the understanding of the role of I- in BPA oxidation by PAA and provides a warning to further evaluate the potential environmental risk during the treatment of iodine-bearing wastewater with PAA.

Mechanism of nitro-byproducts formation during persulfate-based electrokinetic in situ oxidation for remediation of anthracene contaminated soil

Persulfate-based electrokinetic (EK) chemical oxidation appears to be a novel and viable strategy for the in situ remediation of polycyclic aromatic hydrocarbons (PAHs) polluted soil; however, the possible toxic byproducts of PAHs have been overlooked. In this study, we systematically investigated the formation mechanism of the nitro-byproducts of anthracene (ANT) during the EK process. Electrochemical experiments revealed that NH₄⁺ and NO₂^- originating from nitrate electrolyte or soil substrates were oxidized to NO₂^• and NO^• in the presence of SO₄^•-. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS/MS) analysis with ¹⁵N labeling revealed the formation of nitro-byproducts (14 kinds), including 1-hydroxy-4-nitro-anthraquinone and its similar derivatives, 4-nitrophenol, and 2,4-dinitrophenol. The nitration pathways of ANT have been proposed and described, mainly including the formation of hydroxyl-anthraquinone-oxygen and phenoxy radicals and the subsequent addition of NO₂^• and NO^•. ANT-based formation of nitro-byproducts during EK, which is usually underestimated, should be further investigated due to their enhanced acute toxicity, mutagenic effects, and potential threat to the ecosystem.

New Insights into the Role of Nitrite in the Degradation of Tetrabromobisphenol S by Sulfate Radical Oxidation

Tetrabromobisphenol S (TBBPS) is a brominated flame retardant and a contaminant of emerging concern. Several studies found that sulfate radical (SO₄^•-) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO₂^-) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO₄^•-. Initially, NO₂^- suppresses the reaction by competing with TBBPS for SO₄^•-. At the same time, SO₄^•- oxidizes NO₂^- to form nitrogen dioxide radicals (NO₂^•), which actively react with some key TBBPS degradation intermediates, thus greatly altering the transformation pathway. As a result, 2,6-dibromo-4-nitrophenol (DBNP) becomes the primary TBBPS product. As TBBPS undergoes degradation, the released bromide (Br^-) is oxidized by SO₄^•- to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO₂^• or NO₂^- to form nitryl bromide (BrNO₂) that in turn attacks the parent TBBPS, resulting in its accelerated degradation and increased formation of toxic nitrophenolic byproducts. These results show that nitryl halides (e.g., BrNO₂ or ClNO₂) are likely formed yet inadequately recognized when SO₄^•- is applied to remediate halogenated pollutants in the subsurface environment where NO₂^- is ubiquitously found. These insights further underscore the potential risks of the application of SO₄^•- oxidation for the remediation of halogenated compounds in realistic environmental conditions.

Nitrophenolic byproducts formation during sulfate radical oxidation and their fate in simulated drinking water treatment processes

Nitrite can be transformed to nitrophenolic byproducts in sulfate radical oxidation processes (SR-AOPs). These nitrophenols are highly mobile in subsurface and can potentially contaminate drinking water sources. However, their fate in a drinking water treatment remains ambiguous. Herein, the removal and transformation of four nitrophenolic byproducts formed during a heat activated peroxydisulfate oxidation process, i.e., 4-nitrophenol, 2,4-dinitrophenol, 5-nitrosalicylic acid, and 3,5-dinitrosalicylic acid, in a simulated drinking water treatment train were comprehensively examined. The removal of these nitrophenolic compounds in coagulation by either aluminum sulfate or ferric chloride ranged from 3.8% to 13.4%. In the chlorination process, 4-nitrophenol was removed only by 45.4% in 24 h at a chlorine dose of 5.0 mg/L. The removal of the other three nitrophenolic byproducts were less than 20%. Reaction between nitrophenolic byproducts and chlorine via electrophilic substitution gave rise to their chlorinated derivatives. Chlorinated nitrophenolic byproducts were more recalcitrant and toxic than their parent compounds, but still a tiny fraction of them could undergo further oxidation to form trichloronitromethane. This work implied that once nitrophenolic byproducts enter water source, they can penetrate the drinking water treatment train and react with the residual chlorine in distribution pipelines to form more hazardous byproducts. The findings raised additional concerns to the potential risk of the nitrophenolic byproducts formed in SR-AOPs.