Unveiling formation mechanism of carcinogenic N-nitrosodimethylamine in ozonation of dimethylamine: a density functional theoretical investigation

Enhancement of hydrazine accumulation in anammox bioreactors

The role of hydrazine (N2H4) in anammox metabolism has been widely studied; however, studies on N2H4 biosynthesis by anammox bacteria are limited in the literature. In this context, the current research aims to investigate the enhancement of biological N2H4 production in the anammox process in a long-term manner. The experimental studies started with the optimization of the operating conditions to achieve maximum N2H4 accumulation. Under favorable conditions (pH = 8.97 ± 0.08; T = 35.5 ± 0.5 °C; initial hydroxylamine dose = 1.46 ± 0.01 mM), 17.16 ± 0.64 mg L-1 of N2H4 accumulated in the batch systems. The continuity of N2H4 bioproduction was then evaluated by long-term observations. A continuous flow bioreactor was operated in four consecutive manipulated periods under optimized conditions. In the long-term operated bioreactor, 55.10 ± 0.30 mg L-1 N2H4 was accumulated at optimal conditions, which was 2.5 times higher than reported in the literature. Although manipulation of the bioreactor operating conditions initially resulted in a significant increase in N2H4 bioaccumulation, it subsequently caused a severe deterioration in anammox activity. However, this could be mitigated by increasing the biomass concentration in the anammox systems. In addition, the relative abundance of Candidatus Kuenenia decreased by 1.88% throughout the long-term operation.

Oxygen vacancy based WO3/SnO2-x promote electrochemical H2O2 accumulation by two-electron water oxidation reaction and toxic uniform dimethylhydrazine degradation

The key to constructing an anodic electro-Fenton system hinges on two pivotal criteria: enhancing the catalyst activity and selectivity in water oxidation reaction (WOR), while simultaneously inhibiting the decomposition of hydrogen peroxide (H2O2) which is on-site electrosynthesized at the anode. To address the issues, we synthesized novel WO3/SnO2-x electrocatalysts, enriched with oxygen vacancies, capitalize on the combined activity and selectivity advantages of both WO3 and SnO2-x for the two-electron pathway electrocatalytic production of H2O2. Moreover, the introduction of oxygen vacancies plays a critical role in impeding the decomposition of H2O2. This innovative design ensures that the Faraday efficiency and yield of H2O2 are maintained at over 80 %, with a noteworthy production rate of 0.2 mmol h-1 cm-2. We constructed a novel electro-Fenton system that operates using only H2O as its feedstock and applied it to treat highly toxic uniform dimethylhydrazine (UDMH) from rocket launch effluent. Our experiments revealed a substantial total organic carbon (TOC) removal, achieving approximately 90 % after 120 mins of treatment. Additionally, the toxicity of N-nitrosodimethylamine (NDMA), a byproduct of great concern, was shown to be effectively mitigated, as evidenced by acute toxicity evaluations using zebrafish embryos. The degradation mechanism of UDMH is predominantly characterized by the advanced oxidative action of H2O2 and hydroxyl radicals, as well as by complex electron transfer processes that warrant further investigation.

N-Nitrosation in the absence of nitrosating agents in pharmaceuticals?

The possibility of N-Nitrosation in the absence of nitrosating agents was studied on model solutions and film coated tablets containing metformin. N-nitrosodimethylamine (NDMA) and N-nitrosation precursors (dimethylamine and nitrites) were determined using previously published fully validated analytical methods. Alternative routes to N-nitrosation were found. Dimethylamine can undergo an oxidation to nitrite in the presence of strong oxidants (e.g., H₂O₂), as was observed during wastewater treatment in several published works. The resulting nitrite can consecutively act as a nitrosating agent. We proved that the described reaction indeed leads to N-nitrosation (NDMA formation in case of dimethylamine precursor) in model solutions made of dimethylamine and H₂O₂. An experiment was designed in order to prove those reactions take place in dosage forms. Film coated tablets present a highly heterogenous system with several solid phases and low water activity, which is in stark contrast to the liquid wastewater, where this reaction was originally studied. Despite that, the described reaction took place even in the tablets, but only to a small degree. The amount formed via this alternative route corresponds to less than 10 % of the total formed NDMA. The pH optimum of this alternative route lies in the alkaline range which was confirmed by the determined NDMA concentration in model solutions. The solid phase system (i.e., tablets) was found to behave differently. The addition of Na₂CO₃ into the tablets during manufacture resulted in tablets without NDMA (c_NDMA < LOQ) even in batches spiked with both dimethylamine and H₂O₂. Thus, adjusting the pH of the solid dosage forms remains a sufficient measure of controlling N-nitrosamines in the product, even in product with limit amounts of oxidating agent (H₂O₂) and N-nitrosation precursor (dimethylamine).

Formation mechanism and control strategies of N-nitrosodimethylamine (NDMA) formation during ozonation

This review summarizes major findings over the last decade related to N-nitrosodimethylamine (NDMA) formed upon ozonation, which was regarded as highly toxic and carcinogenic disinfection by-products. The reaction kinetics, chemical yields and mechanisms were assessed for the ozonation of potential precursors including dimethylamine (DMA), N,N-dimethylsulfamide, hydrazines, N-containing water and wastewater polymers, dyes containing a dimethylamino function, N-functionalized carbon nanotubes, guanidine, and phenylurea. The effects of bromide on the NDMA formation during ozonation of different types of precursors were also discussed. The mechanism for NDMA formation during ozonation of DMA was re-summarized and new perspectives were proposed to assess on this mechanism. Effect of hydroxyl radicals (•OH) on NDMA formation during ozonation was also discussed due to the noticeable oxidation of NDMA by •OH. Surrogate parameters including nitrate formation and UV₂₅₄ after ozonation may be useful parameters to estimate NDMA formation for practical application. The strategies for NDMA formation control were proposed through improving the ozonation process such as ozone/hydrogen peroxide, ozone/peroxymonosulfate and catalytic ozonation process based on membrane pores aeration (MEMBRO₃X).

NDMA formation during ozonation of metformin: Roles of ozone and hydroxyl radicals

Metformin, a high-consumed pharmaceutical for diabetes, has been reported to generate carcinogenic nitroso-dimethylamine (NDMA) during treatment of its containing wastewater. However, whether it would produce NDMA during ozonation or not is unclear, let alone discriminate roles of ozone (O₃) and hydroxyl radicals (OH). In this paper, effects of ozonation on NDMA formation from metformin were investigated, roles of O₃ and OH were also distinguished by adding tert-butyl alcohol (tBA) as OH scavenger. Moreover, various influencing factors and reaction mechanisms were demonstrated. The results indicated that NDMA could be directly formed from metformin during ozonation, the addition of OH scavenger significantly enhanced its formation (0-46.2 ng/L vs 0-139.1 ng/L). The formation of NDMA by O₃ and OH was more affected by bromide and HCO₃^- than those with only O₃; while the impacts of pH and sulphate on the latter were more notable. No matter without/with tBA in the solution, the formed NDMA during ozonation of metformin increased with raising pH (from 5 to 9) and achieved the maximum 69.6 ng/L and 235.9 ng/L at pH 9, respectively; small amount of bromide (0.1 μM) promoted NDMA production, high levels of bromide (10 μM) inhibited its formation; the existence of HCO₃^- enhanced the amounts of NDMA from 44.5 to 73.5 ng/L (raised by 65.2%) by O₃ and OH and from 102.9 to 130 ng/L with only O₃ (raised by 26.3%); with the addition of sulphate, NDMA concentration raised by 43.8% by O₃ and OH, while the value was high up to 134.6% with only O₃. Based on the result of UPLC-Q-TOF and density functional theory, the oxidation intermediates were identified and possible transformation pathways of metformin during ozonation were proposed. The findings in this paper would provide reference when treating metformin-containing water in future.

Phenylurea herbicide degradation and N-nitrosodimethylamine formation under various oxidation conditions: Relationships and transformation pathways

Four phenylurea herbicides (PUHs) were assessed for degradation and transformation into N-nitrosodimethylamine (NDMA) under three oxidation conditions (chlorine (Cl₂), chlorine dioxide (ClO₂), and ozone (O₃)) from an aqueous solution. Removal ratios correlated with the numbers of halogen elements contained in PUHs (isoproturon ₍₀₎ > chlorotoluron _{(1 Cl)} > diuron _{(2 Cl)} > fluometuron _(3 F)), and the degradation efficiencies of oxidants from fastest to slowest were: O₃, ClO₂, and Cl₂. NDMA can be generated directly from the ozonation of PUHs. Further, compared with chloramination alone, ozonation prominently promoted NDMA formation potential (NDMA-FP) during post-chloramination, and NDMA-FPs increased approximately 23-68 times than those during ozonation only at 2.5 mg/L O₃ over 10 min; molar yields of NDMA from highest to lowest were 11.1% (isoproturon), 1.17% (chlorotoluron), 1.0% (diuron), and 0.73% (fluometuron). The PUH degradation kinetics data during ozonation agreed with the pseudo-first-order model. The rate constant k_obs were 0.31 × 10^-3-19.8 × 10^-3 s^-1. The k_obs and removal ratios of PUHs during ozonation partially scaled with the mass, LogK_ow, and Henry's constants of PUHs. Comparisons of measured NDMA-FPs with calculated NDMA-FPs from residual PUH after oxidation showed that the intermediates produced during ozonation facilitated NDMA-FPs; this contribution was also observed for chlorotoluron and isoproturon during ClO₂ oxidation. Examination of reaction mechanisms revealed that tertiary amine ozonation, N-dealkylation, hydroxylation, the cleavage of N-C bonds, ammonification, and nitrification occurred during the ozonation of PUHs, and the dimethylamine (DMA) functional groups could be decomposed directly and transformed into NDMA via the formation of the intermediate unsymmetrical dimethylhydrazine. NDMA is also formed from the reaction between DMA and phenylamino-compounds. Clarifying primary degradation products of PUHs and transformation pathways of NDMA during oxidation processes is useful to optimize treatment processes for water supplies.

QSAR modeling for the ozonation of diverse organic compounds in water

The ozonation-based advanced oxidation process is a promising treatment technology for wastewater with micropollutants. The second-order reaction rate constant (k_O3) of ozone (O₃) with organic compounds is an important index for estimating removal efficiency of organic pollutants in engineered treatment; however, the experimental k_O3 values are currently only available for hundreds of chemicals. In this study, two quantitative-structure activity relationship (QSAR) models were developed to predict k_O3 of various organic chemicals with multiple linear regression (MLR) and support vector machine (SVM) methods. The built QSAR models cover a large dataset (136 chemicals) and more structurally diverse chemicals as compared to the existing models. The MLR model possesses satisfactory goodness-of-fit (R²_tr = 0.734), robustness (Q²_LOO = 0.700, Q²_BOOT = 0.772) and predictive ability (R²_ext = 0.797, Q²_ext = 0.794), and the SVM model also has good fitness (R²_tr = 0.862) and predictability (R²_ext = 0.782, Q²_ext = 0.775). The applicability domain of the models has been extended and includes chemicals (especially some emerging pollutants) that are rarely covered in many previous models. The underlying molecular structural factors influencing ozonation are revealed. The energy of the highest occupied molecular orbital (E_HOMO) and the phenol/enol/carboxyl OH group (O-057) are the two most important molecular structural factors governing the reactivity of organic compounds with ozone. The developed models can serve as a prescreening tool for the removal prediction of organic pollutants by ozone.

Investigation on the compositions of unsymmetrical dimethylhydrazine treatment with different oxidants using solid-phase micro-extraction-gas chromatography-mass spectrometer

The majority of unsymmetrical dimethylhydrazine (UDMH) treatments produce lots of toxic by-products, among which N-Nitrosodimethylamine (NDMA) is a strong carcinogen. The compositions of the by-products are important for evaluating the treatment efficiency and understanding the UDMH degradation mechanism to achieve UDMH mineralization. The intermediate and end products of UDMH treatment with different oxidants were investigated by using a simple and fast method, solid-phase micro-extraction (SPME) in combination with gas chromatography-mass spectrometry (GC-MS). The effects of several parameters (coating fibre, salt addition, pH, sampling time and desorption time) were studied to optimize analyte recovery. The best response can be attained by the 65 µm PDMS/DVB fibre at pH 7 during 10 min after desorption of 1 min in the GC inlet. The intermediate and final oxidative products of UDMH wastewater treatment with different oxidants (O₃, Mn²⁺/O₃, Fe²⁺/H₂O₂) were investigated. The results showed that the UDMH treatment with O₃ could lead to high yields of NDMA. Metal catalytic ozonation could largely minimize the formation of NDMA. No NDMA was observed in the final decontaminated samples after treatment with Fe²⁺/H₂O₂. The NDMA formation and degradation mechanism were discussed based on the intermediates. This study is expected to provide useful information for controlling NDMA formation during UDMH wastewater treatment.

NDMA formation mechanisms from typical hydrazines and hydrazones during ozonation: A computational study

N-nitrosodimethylamine (NDMA) as the most frequently detected disinfection by-product has aroused widespread concern due to its unusually high carcinogenicity, however, there is still limited understanding of its formation mechanisms. In this study, the formation mechanisms of NDMA from some typical hydrazines and hydrazones with high NDMA conversion yields (60%∼90%) during ozonation, i.e., unsymmetrical dimethylhydrazine (UDMH), 1-formyl-2,2-dimethylhydrazine (FDMH), formaldehyde dimethylhydrazone (FDH) and acetone dimethylhydrazone (ADMH), were investigated by using DFT method with the M05 functional. A new NDMA formation mechanism from hydrazines during ozonation was proposed, in which the initial step is hydrogen abstraction rather than previously reported oxygen addition. For hydrazones, the C atom of the -N = C moiety in hydrazones is preferred to be attacked by ozone to generate N,N-dimethylaminonitrene (DMAN), which is an important intermediate in NDMA formation during ozonation. Moreover, the reactivity order of the following H atoms in hydrogen/hydride ion abstraction (HA) by ozone is -NH₂ > -N(CH₃)₂ > -CO-NH ∼ =C(CH₃)₂ > =CH-. Additionally, formation pathways of some experimentally detected compounds, i.e., HOOOH, HOOH and HCOH, in the ozonation of hydrazine were elucidated in this study. The results are expected to expand our understanding of NDMA formation mechanisms and ozone reaction characteristics.

Products and Mechanistic Investigations on the Reactions of Hydrazines with Ozone in Gas-Phase

The toxic transformation products of hydrazines are of great concern. These products’ properties combined with their formation mechanisms are needed to assess their potential environmental and human impacts. In this study, the gas-phase reaction of hydrazine (N2H4), monomethyldrazine (MMH) and unsymmetrical dimethyhydrazine (UDMH) with O3 have been studied at varying reactant ratios, both in the presence and absence of a radical trap. Gas chromatography-mass spectroscopy (GC-MS) has been implied to follow reactant consumption and product formation. Apart from the reported products detected by Fourier transform infrared spectroscopy (FT-IR), the newly found compounds (hydrazones, formamides, dimethylamine, 1,1,4,4-tetramethyl-1,2-tetrazene,dimethylamino-acetonitrile, N2, H2O, et al.) are identified by GC-MS. The relative yields of the organic products vary considerably at different O3/MMH or UDMH ratios. UDMH and MMH are confirmed as high potential precursors of N-nitrosodimethylamine (NDMA). The presence of hydroxyl radicals (HO·) hinders NDMA formation in MMH-O3 system. Meanwhile, it increases NDMA formation in UDMH-O3 system. The suggested reaction mechanisms which account for the observed products are discussed.

First principles computational study on hydrolysis of hazardous chemicals phosphorus trichloride and oxychloride (PCl3 and POCl3) catalyzed by molecular water clusters

Using first principles calculations we unveil fundamental mechanism of hydrolysis reactions of two hazardous chemicals PCl₃ and POCl₃ with explicit molecular water clusters nearby. It is found that the water molecules play a key role as a catalyst significantly lowing activation barrier of the hydrolysis via transferring its protons to reaction intermediates. Interestingly, torsional angle of the molecular complex at transition state is identified as a vital descriptor on the reaction rate. Analysis of charge distribution over the complex further reinforces the finding with atomic level correlation between the torsional angle and variation of the orbital hybridization state of phosphorus (P) in the complex. Electronic charge separation (or polarization) enhances thermodynamic stability of the activated complex and reduces the activation energy through hydrogen bonding network with water molecules nearby. Calculated potential energy surfaces (PES) for the hydrolysis of PCl₃ and POCl₃ depict their two contrastingly different profiles of double- and triple-depth wells, respectively. It is ascribed to the unique double-bonding O=P in the POCl₃. Our results on the activation free energy show well agreements with previous experimental data within 7kcalmol^-1 deviation.

The degradation mechanism of sulfamethoxazole under ozonation: a DFT study

Sulfamethoxazole (SMX), a kind of antibiotic, remains in the environment and threatens public health. Ozone as a strong and green oxidant was widely used for selective oxidation degradation of residual SMX. However, it is hard to elucidate the detailed oxidation mechanism through current experimental approaches. A theoretical study has been carried out herein for exploring possible ozonation pathways of SMX. Two reaction mechanisms, viz., direct addition (DA) and H atom transfer (HAT), are considered. The results show that the primary oxidation of aromatic rings (benzene or isoxazole rings) of SMX follows the DA mechanism, featuring an electrophilic addition. Whereas, the oxidation of amino and methyl groups of SMX follows the HAT mechanism. Following the proposed mechanisms, the primary oxidation products detected in previous experiments could reasonably be obtained according to the current calculations. More importantly, O₃ molecules as an electrophilic agent feasibly attack the moiety having a large orbital contribution to the highest occupied molecular orbital (HOMO) of sulfonamides. This result suggests that the primary ozonation site of sulfonamides could be theoretically predictable through the information of their frontier molecular orbitals. Meanwhile, a positive correlation between the O₃-mediated HAT energy barriers and bond dissociation energies has been found for N-H and C-H bond oxidations.

Ozonation of indomethacin: Kinetics, mechanisms and toxicity

Ozonation of a commonly used non-steroidal anti-inflammatory drug indomethacin (IM) was studied. Kinetic constants of IM with ozone and hydroxyl radicals were measured at an order of magnitude of 10⁵M^-1s^-1 and 10⁹M^-1s^-1, respectively. IM was degraded within 7min under the lowest ozone dose, but TOC removal was only 50% even under the highest ozone dose used in the experiments. Ozone rather than hydroxyl radicals was found to be the main oxidant during reaction, with a contribution rate of 80% under pH 7. Six intermediates were identified by high resolution mass spectrometer. Nitrogen atom, CC double bond and benzene ring were found to be the main reaction sites. Electrophilic attack or Criegee cyclo-addition were proved to be the most probable pathways at the first step. The formation mechanism of one of the ozone products was first proposed during the experiment, then confirmed by the density functional theory (DFT) calculation. Acetic acid, formic acid and oxalic acid were detected as small molecule organic products. The toxicity change during ozonation was measured by luminescent bacterium with results showing that the toxicity can be reduced to zero when ozone dose was high enough.

Comparison of N-nitrosodimethylamine formation mechanisms from dimethylamine during chloramination and ozonation: A computational study

N-nitrosodimethylamine (NDMA) as a disinfection by-product has recently become the focus of considerable research interest due to its unusually high carcinogenicity. In this study, the formation mechanisms of NDMA from dimethylamine (DMA) during chloramination and ozonation were investigated by using the quantum chemical G4 method. The reactivity of haloamines and hydroxylamine reacting with DMA was found in the order: NHCl₂∼NHBrCl (Br-leaving)>NHBr₂>NH₂Cl∼NH₂Br>>NH₂OH. This offers a theoretical support for the experimentally proposed mechanism that dimethylamine reacts with NHCl₂ rather than NH₂Cl to form chlorinated unsymmetrical dimethylhydrazine intermediate and the existence of bromochloramine in the presence of bromide during chloramination, and explains the observation that NDMA yield during ozonation is much lower than that during chloramination. Importantly, an N,N-dimethylaminonitrene was found to be a significant intermediate to form NDMA in oxidation reactions by molecular oxygen and ozone. Additionally, results suggest that the amines containing the second nitrogen source directly connecting or close to the N-(CH₃)₂ moiety are potential significant NDMA precursors upon ozonation. The findings of this study are helpful for expanding the knowledge of NDMA formation mechanism, and predicting potential NDMA precursors during disinfection.

Assessment of the electronic structure and properties of trichothecene toxins using density functional theory

A comprehensive quantum chemical study was carried out on 35 type A and B trichothecenes and biosynthetic precursors, including selected derivatives of deoxynivalenol and T-2 toxin. Quantum chemical properties, Natural Bond Orbital (NBO) analysis, and molecular parameters were calculated on structures geometry optimized at the B3LYP/6-311+G** level. Type B trichothecenes possessed significantly larger electrophilicity index compared to the type A trichothecenes studied. Certain hydroxyl groups of deoxynivalenol, nivalenol, and T-2 toxin exhibited considerable rotation during molecular dynamics simulations (5 ps) at the B3LYP/6-31G** level in implicit aqueous solvent. Quantitative structure activity relationship (QSAR) models were developed to evaluate toxicity and detection using genetic algorithm, principal component, and multilinear analyses. The models suggest electronegativity and several 2-dimensional topological descriptors contain important information related to trichothecene cytotoxicity, phytotoxicity, immunochemical detection, and cross-reactivity.