Measurement of nitrosamine and nitramine formation from NOx reactions with amines during amine-based carbon dioxide capture for postcombustion carbon sequestration

Unveiling the chemical kinetics of aminomethanol (NH2CH2OH): insights into O . H and O2 photo-oxidation reactions and formamide dominance

Aminomethanol is released into the atmosphere through various sources, including biomass burning. In this study, we have expounded the chemical kinetics of aminomethanol in the reaction pathways initiated by the hydroxyl radical (O ˙ H) with the aid of ab initio//density functional theory (DFT) i.e., coupled-cluster theory (CCSD(T))//hybrid-DFT (M06-2X/6-311++G (3df, 3pd). We have explored various possible directions of theO ˙ H radical on aminomethanol, as well as the formation of distinct pre-reactive complexes. Our computational findings reveal that the H transfer necessitates activation energies ranging from 4.1 to 6.5 kcal/mol from the -CH2 group, 3.5-6.5 kcal/mol from the -NH2 group and 7-9.3 kcal/mol from the -OH group of three rotational conformers. The H transfer from -CH2, -NH2 and -OH exhibits an estimated total rate constant (k OH) of approximately 1.97 × 10-11 cm3 molecule-1 s-1 at 300 K. The branching fraction analysis indicates a pronounced dominance of C-centered NH2C ˙ HOH radicals with a favorability of 77%, surpassing the N-centeredN ˙ HCH2OH (20%) and O-centered NH2CH2O ˙ (3%) radicals. Moreover, our investigation delves into the oxidation of the prominently favored carbon-centered NH2C ˙ HOH radical through its interaction with atmospheric oxygen molecules. Intriguingly, our findings reveal that formamide (NH2CHO) emerges as the predominant product in the NH2C ˙ HOH + 3O2 reaction, eclipsing alternative outcomes such as amino formic acid (NH2COOH) and formimidic acid (HN = C(H)-OH). At atmospheric conditions pertinent to the troposphere, the branching fraction value for the formation of formamide is about 99%, coupled with a rate constant of 5.5 × 10-12 cm3 molecule-1 s-1. Finally, we have scrutinized the detrimental impact of formamide on the atmosphere. Interaction of formamide with atmospheric hydroxyl radicals could give rise to the production of potentially perilous compounds such as HNCO. Further, unreactedN ˙ HCH2OH radicals may initiate the formation of carcinogenic nitrosamines when reacting with trace N-oxides (namely, NO and NO2). This, in turn, escalates the environmental risk factors.

Role of Carbonyl Compounds for N-Nitrosamine Formation during Nitrosation: Kinetics and Mechanisms

N-Nitrosamines are potential human carcinogens frequently detected in natural and engineered aquatic systems. This study sheds light on the role of carbonyl compounds in the formation of N-nitrosamines by nitrosation of five secondary amines via different pathways. The results showed that compared to a control system, the presence of formaldehyde enhances the formation of N-nitrosamines by a factor of 5-152 at pH 7, depending on the structure of the secondary amines. Acetaldehyde showed a slight enhancement effect on N-nitrosamine formation, while acetone and benzaldehyde did not promote nitrosation reactions. For neutral and basic conditions, the iminium ion was the dominant intermediate for N-nitrosamine formation, while carbinolamine became the major contributor under acidic conditions. Negative free energy changes (<-19 kcal mol-1) and relatively low activation energies (<18 kcal mol-1) of the reactions of secondary amines with N2O3, iminium ions with nitrite and carbinolamines with N2O3 from quantum chemical computations further support the proposed reaction pathways. This highlights the roles of the iminium ion and carbinolamine in the formation of N-nitrosamines during nitrosation in the presence of carbonyl compounds, especially in the context of industrial wastewater.

N-Nitrosamine sensing properties of novel penta-silicane nanosheets-a first-principles outlook

CONTEXT

N-Nitrosamine is one of the highly toxic carcinogenic compounds that are found almost in the entire environment. In the present work, novel penta-silicene (penta-Si) and penta-silicane (penta-HSi) are utilised to sense the N-nitrosamine in the air environment. Initially, structural firmness of penta-Si and penta-HSi is confirmed using cohesive energy. Subsequently, the electronic properties of penta-Si and penta-HSi are discussed with the aid of electronic band structure and projected density of states (PDOS) maps. The calculated band gap of penta-Si and penta-HSi is 0.251 eV and 3.117 eV, correspondingly. Mainly, the adsorption property of N-nitrosamine on the penta-Si and penta-HSi is studied based on adsorption energy, Mulliken population analysis along with relative energy gap changes. The computed adsorption energy range is in physisorption (- 0.101 to - 0.619 eV), which recommends that the proposed penta-Si and penta-HSi can be employed as a promising sensor to detect the N-nitrosamine in the air environment.

METHODS

The structural, electronic and adsorption behaviour of N-nitrosamine on penta-Si and penta-HSi are studied based on the density functional theory (DFT) approach. The hybrid generalized gradient approximation (GGA) with Becke's three-parameter (B3) + Lee-Yang-Parr (LYP) exchange correlation functional is used to optimise the base material. All calculations in the present work are carried out in Quantum-ATK-Atomistic Simulation Software.

Targeted electrochemical reduction of carcinogenic N-nitrosamines from emission control systems within CO2 capture plants

N-Nitrosamines are one of the environmentally significant byproducts from aqueous amine-based post-combustion carbon capture systems (CCS) due to their potential risk to human health. Safely mitigating nitrosamines before they are emitted from these CO₂ capture systems is therefore a key concern before widescale deployment of CCS can be used to address worldwide decarbonization goals. Electrochemical decomposition is one viable route to neutralize these harmful compounds. The circulating emission control waterwash system, commonly installed at the end of the flue gas treatment trains to minimize amine solvent emissions, plays an important role to capture N-nitrosamines and control their emission into the environment. The waterwash solution is the last point where these compounds can be properly neutralized before becoming an environmental hazard. In this study, the decomposition mechanisms of N-nitrosamines in a simulated CCS waterwash with residual alkanolamines was investigated using several laboratory-scale electrolyzers utilizing carbon xerogel (CX) electrodes. H-cell experiments revealed that N-nitrosamines were decomposed through a reduction reaction and are converted into their corresponding secondary amines thereby neutralizing their environmental impact. Batch-cell experiments statistically examined the kinetic models of N-nitrosamine removal by a combined adsorption and decomposition processes. The cathodic reduction of the N-nitrosamines statistically obeyed the first-order reaction model. Finally, a prototype flow-through reactor using an authentic waterwash was used to successfully target and decompose N-nitrosamines to below the detectable level without degrading the amine solvent compounds allowing them to be return to the CCS and lower the system operating costs. The developed electrolyzer was able to efficiently remove greater than 98% of N-nitrosamines from the waterwash solution without producing any additional environmentally harmful compounds and offers an effective and safe route to mitigate these compounds from CO₂ capture systems.

Drinking water treatment and associated toxic byproducts: Concurrence and urgence

Reclaimed water is highly required for environmental sustainability and to meet sustainable development goals (SDGs). Chemical processes are frequently associated with highly hazardous and toxic by-products, like nitrosamines, trihalomethanes, haloaldehydes, haloketones, and haloacetic acids. In this context, we aim to summarize the formation of various commonly produced disinfection by-products (DBPs) during wastewater treatment and their treatment approaches. Owing to DBPs formation, we discussed permissible limits, concentrations in various water systems reported globally, and their consequences on humans. While most reviews focus on DBPs detection methods, this review discusses factors affecting DBPs formation and critically reviews various remediation approaches, such as adsorption, reverse osmosis, nano/micro-filtration, UV treatment, ozonation, and advanced oxidation process. However, research in the detection of hazardous DBPs and their removal is quite at an early and initial stage, and therefore, numerous advancements are required prior to scale-up at commercial level. DBPs abatement in wastewater treatment approach should be considered. This review provides the baseline for optimizing DBPs formation and advancements in the remediation process, efficiently reducing their production and providing safe, clean drinking water. Future studies should focus on a more efficient and rigorous understanding of DBPs properties and degradation of hazardous pollutants using low-cost techniques in wastewater treatment.

Energy-Efficient Biphasic Solvents for Industrial Carbon Capture: Role of Physical Solvents on CO2 Absorption and Phase Splitting

Physical solvent is a promising alternative for the phase splitting of solvent to drastically reduce the regeneration energy during CO₂ capture. Here, an aqueous biphasic solvent, optimally composed of 30 wt % polyamine (N, N-dimethylpropylamine, DMPA) and 50 wt % physical solvent (polyethyleneglycol dimethyl ether, NHD), is prepared, which presents high cyclic loading, low regeneration energy, and good stability. L₁₆(4⁵) orthogonal tests are performed to comprehensively evaluate the mass-transfer kinetics and the effect of crucial conditions, verifying the weak effect of NHD solvent on mass transfer. The solvent effect of NHD could decrease the energy barrier of carbamate generation from zwitterions (DMPA⁺COO^-) to enhance chemical absorption. The low polarity of the NHD solvent provides source motivation and accelerates phase splitting. Time-space resolution distribution of CO₂ capacity is established based on a scale-up separator with 5 L solvent, which supports multiscale force analysis for the various stages during phase splitting. The drag force of the homogeneous cluster was first introduced into separation dynamics, referred to as an important reason for the various splitting behaviors of a scale-up separator.

New Mechanistic Insights into Atmospheric Oxidation of Aniline Initiated by OH Radicals

This study theoretically reports the comprehensive kinetic mechanism of the aniline + OH reaction in the range of 200-2000 K and 0.76-7600 Torr. The temperature- and pressure-dependent behaviors, including time-resolved species profiles and rate coefficients, were studied within the stochastic RRKM-based master equation framework with the reaction energy profile, together with molecular properties of the species involved, characterized at the M06-2X/aug-cc-pVTZ level. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH₂ moiety (to form C₆H₅NH (P1)) is found to prevail over the OH-addition on the C atom at the ortho site of aniline (to form 6-hydroxy-1-methylcyclohexa-2,4-dien-1-yl (I2)) with the atmospheric rate expressions (in cm³/molecule/s) as k^abstraction(P1) = 3.41 × 10¹ × T^-4.56 × exp (-255.2 K/T) for 200-2000 K and k^addition(I2) = 3.68 × 10⁹ × T^-7.39 × exp (-1163.9 K/T) for 200-800 K. The U-shaped temperature-dependent characteristics and weakly positive pressure dependence at low temperatures (e.g., T ≤ 800 K and P = 760 Torr) of k_total(T) are also observed. The disagreement in k_total(T) between the previous calculations and experimental studies is also resolved, and atmospheric aniline is found to be primarily removed by OH radicals (τ_OH ∼ 1.1 h) in the daytime. Also, via TD-DFT simulations, it is recommended to include P1 and I2 in any atmospheric photolysis-related model.

An investigation into the adverse effects of O2, SO2, and NOx on polyethyleneimine functional CO2 adsorbents

In this study, we investigated the influence of O2, SO2, and NOx on branched and linear polyethyleneimine (PEI) functional silica CO2 adsorbents (BPEI-SiO2 and LPEI-SiO2, respectively). O2 was much more likely to oxidize BPEI-SiO2, compared with LPEI-SiO2, to form C=O and C=N groups and led to a 23.0% decrease in the CO2 adsorption capacity after 990 min of cumulative contact with 10% O2. In contrast, LPEI-SiO2 lost only approximately 3.6% of its CO2 adsorption capacity, although O2 oxidized LPEI-SiO2 to form C=O groups. SO2 can cause severe degradation of BPEI-SiO2 and LPEI-SiO2 by forming heat-stable NH3+—and/or NH2+—containing adducts and by promoting the formation of urea linkages. After cumulative contact with 10, 50, and 200 ppm SO2 for 990 min, BPEI-SiO2 lost 18.2%, 61.4%, and 89.0% of its CO2 adsorption capacity, and LPEI-SiO2 lost 18.5%, 60.6%, and 78.5% of its CO2 adsorption capacity, respectively. NO2 at 10 ppm and NO at 200 ppm caused almost no loss in CO2 adsorption capacity after cumulative contact for 990 min, but both led to degradation of adsorbents. NO2 can cause irreversible formation of NH3+—and/or NH2+—containing adducts, acid products, N-nitro compounds (N–NO2), C-nitroso compounds (C–N=O), and C-nitro (C–NO2) compounds, and can promote the formation of urea linkages. NO can lead to the formation of NH3+—and/or NH2+—containing adducts and N-nitroso (N–N = O) compounds.

Role of various factors affecting the photochemical treatment of N-nitrosamines related to CO2 capture

Post-combustion CO₂ capture using amine solvents is the most feasible method of reducing anthropogenic CO₂ emissions, which are the largest contributor to global warming. The formation of carcinogenic N-nitrosamines (i.e. by-products) can hinder the industrial application of this technology. In this study, the effects of direct UV photolysis (N-nitrosamine concentration and amines) and advanced oxidation processes (UV/H₂O₂ and UV/O₃) on the three specific N-nitrosamines that are commonplace in amine-based CO₂ capture (i.e. N-nitrosodiethylamine (NDEA), N-nitrosodiethanolamine (NDELA), and N-nitrosomorpholine (NMOR)) were examined. A significant decrease in the photodegradation rate constants was observed for NDEA (1.02 × 10⁰ to 2.94 × 10^-1 min^-1), NDELA (1.52 × 10⁰ to 3.32 × 10^-1 min^-1), and NMOR (1.93 × 10⁰ to 2.20 × 10^-1 min^-1) as their concentrations increased within 1-50 mg/L. This is the first report of a significant increase in the degradation rate constants of N-nitrosamine with an increase in amine concentrations (i.e. monoethanolamine, diethanolamine, and morpholine) within 10-200 mM. The photodegradation rate constants increased as the molar ratio of H₂O₂ to N-nitrosamine increased to 20, but then decreased at molar ratios beyond this. O₃ had a negligible effect on the photodegradation of N-nitrosamines.

Role of absorber and desorber units and operational conditions for N-nitrosamine formation during amine-based carbon capture

The formation of carcinogenic N-nitrosamines from reactions between solvent amines and flue gas NO_x is an important concern for the application of amine-based processes to capture CO₂ post-combustion. Using an advanced test rig with interconnected absorber and desorber units, we evaluated the importance for N-nitrosamine formation of the desorber relative to the absorber, and any synergism between the two units. Variations in desorber temperature and in flue gas composition indicated that N-nitrosamine formation from fresh monoethanolamine (MEA) occurred predominantly in the absorber. N-nitrosamine formation was driven by high NO₂ and O₂ flue gas concentrations, although NO also contributed. In contrast, N-nitrosamine formation from piperazine (PZ) was driven by reactions with nitrite in the heated desorber, and accelerated concurrent with nitrite accumulation. A complementary experiment simulating aged MEA solvent (high nitrite, 1.5% sarcosine as a proxy of secondary amine degradation products) suggested the desorber becomes an order of magnitude more important than the absorber for N-nitrosamine formation. For fresh MEA solvent, increasing the desorber temperature from 110 °C to 130 °C promoted thermal decomposition of N-nitrosamines, reducing N-nitrosamine accumulation rates two-fold. Compared to the test rig, the prevailing practice of using separate absorber columns and autoclave-like treatments to mimic desorber units predicted the direction, but underestimated the magnitude of N-nitrosamine formation. Because N-nitrosamine accumulation rates are the net result of competing formation and thermal decomposition processes, use of continuously cycling test rigs may be necessary to understand the impacts of different operating conditions.

Multiphase carbon mineralization for the reactive separation of CO2 and directed synthesis of H2

There is a need to capture, convert and store CO₂ by atom-efficient and energy-efficient pathways that use as few process configurations as possible. This need has motivated studies into multiphase reaction chemistries and this Review describes two such approaches in the context of carbon mineralization. The first approach uses aqueous alkaline solutions containing amine nucleophiles that capture CO₂ and eventually convert it into calcium and magnesium carbonates, thereby regenerating the nucleophiles. Gas-liquid-solid and liquid-solid configurations of these reactions are explored. The second approach combines silicates such as CaSiO₃ or Mg₂SiO₄ with CO and H₂O from the water-gas shift reaction to give H₂ and calcium or magnesium carbonates. Coupling carbonate formation to the water-gas shift reaction shifts the latter equilibrium to afford more H₂ as part of a single-step catalytic approach to carbon mineralization. These pathways exploit the vast abundance of alkaline resources, including naturally occurring silicates and alkaline industrial residues. However, simple stoichiometries belie the complex, multiphase nature of the reactions, predictive control of which presents a scientific opportunity and challenge. This Review describes this multiphase chemistry and the knowledge gaps that need to be addressed to achieve 'step-change' advancements in the reactive separation of CO₂ by carbon mineralization.

Chemiresistive Carbon Nanotube Sensors for N-Nitrosodialkylamines

N-Nitrosamines are environmental genotoxicants that are widely encountered in air, water, and food. Contamination of indoor and outdoor air with N-nitrosamines has been reported on many occasions. Conventional detection of airborne N-nitrosamines requires sophisticated instrumentation, field sampling, and laboratory analysis. Herein, we report ultrasensitive carbon nanotube based chemiresistive sensors utilizing a cobalt(III) tetraphenylporphyrin selector element for the detection of N-nitrosamines. Concentrations as low as 1 ppb N-nitrosodimethylamine, N-nitrosodiethylamine, and N-nitrosodibutylamine were detected. We also demonstrate the integration of these sensors with a field deployable sensing node wherein the sensor response can be read online remotely.

N-nitrosomorpholine behavior in sewage treatment plants and urban rivers

The seasonal and diurnal patterns of N-nitrosomorpholine (NMOR) and its formation potential (NMOR FP) were examined with water samples taken from five outlets of four sewage treatment plants (STPs), seven main stream sites, and five tributary sites in the Yodo River basin. STPs were shown to be the main sources of downstream NMOR load. The highest NMOR levels were found in the discharge from one STP (26.4-171 ng/L). Continuous sequential samplings over a period of 24 h at this STP revealed that NMOR flux at the influent point fluctuated in both summer (0.4-3.2 g/h) and winter (0.3-5.4 g/h), while it was steady in the effluent. In addition, levels of NMOR remained stable during the biological treatment and disinfection processes. The present research demonstrated that NMOR could be formed from morpholine (MOR) in raw sewage treated by this STP, with a possible mechanism being formaldehyde-catalyzed nitrosation of MOR by nitrites, prior to raw sewage entering the STP. This implies that the NMOR detected here might not be a disinfection byproduct per se under low-chlorine disinfection (around 1.0 mg/L), but is primarily a contaminant that is difficult to remove during sewage treatment. NMOR attenuated significantly in the rivers in the daytime with production of MOR, but persisted during nights, which demonstrated the importance of monitoring NMOR levels in the water environment during periods of low UV intensity, especially nights.

Wastewater treatment in amine-based carbon capture

Amine-based CO₂ capture (ACC) has become one cost-effective method for reducing carbon emissions in order to mitigate climate changes. The amine-rich wastewater (ARWW) generated from ACC may contain a series of degradation products of amine-based solvents (ABSs). These products are harmful for ecological environment and human health. Effective and reliable ARWW treatment methods are highly required for mitigating the harmfulness. However, there is a lack of a comprehensive review of the existing limited methods that can guide ARWW-related technological advancements and treatment practices. To fill this gap, the review is achieved in this study. All available technologies for treating the ARWW from washwater, condenser, and reclaimer units in ACC are examined based on clarification of degradation mechanisms and ARWW compounds. A series of significant findings and recommendations are revealed through this review. For instance, ARWW treatment methods should be selected according to degradation conditions and pollution concentrations. UV light can be only used for treating wastewater from washwater and condenser units in ACC. Biological activated carbon is feasible for removing nitrosamines from washwater and condenser units. Sequence batch reactors, microbial fuel cells, and the other techniques for removing amines and similar degradation products are applicable for treating ARWW. This review provides scientific support for the selection and improvement of ARWW treatment techniques, the mitigation of ACC's consequences in environment, health and other aspects, and the extensive development and applications of ACC systems.

Formation of N-nitrosamines during the analysis of municipal secondary biological nutrient removal process effluents by US EPA method 521

US EPA Method 521 employs activated carbon-based solid phase extraction (SPE) cartridges for analyzing N-nitrosamines. The analysis of N-nitrosamines and their chloramine-reactive and ozone-reactive precursors in nitrified municipal secondary effluent revealed the potential for NDMA to form as an artefact during the analysis. As samples passed through the SPE cartridge, the activated carbon mediated the reaction of nitrite with dimethylamine to form NDMA. The reaction was not significant with tertiary amines. Artefactual NDMA formation was important for nitrite concentrations >0.2 mg/L as N in the Biological Nitrogen Removal (BNR) process effluent. However, it is difficult to define a general threshold for nitrite concentrations, because the importance of the reaction also depends on secondary amine concentrations, which are usually poorly characterized. Pre-treatment of samples with sulfamic acid to destroy nitrite mitigated the artefact. This artefact did not affect NDMA analysis in a nitrified effluent from another facility, likely due to low dimethylamine concentrations. This artefact also did not affect the analysis of primary effluent, due to the lack of nitrite, or the analysis of other N-nitrosamines, likely due to the lack of their secondary amine precursors. Because chloramination does not significantly degrade nitrite, this artefact could affect the analysis of chloramine-reactive N-nitrosamine precursors. Because ozonation rapidly degrades nitrite, it should not affect the analysis of ozone-reactive precursors. However, ozonation at 0.8 mg ozone/mg dissolved organic carbon resulted in significant degradation of all N-nitrosamines, even though simultaneous NDMA formation from ozone-reactive precursors resulted in a net increase in NDMA concentration.

Solvent-Free CO2 Capture Using Membrane Capacitive Deionization

Capture of CO₂, originating from both fossil fuels, such as coal combustion, and from renewables, such as biogas, appears to be one of the greatest technological challenges of this century. In this study, we show that membrane capacitive deionization (MCDI) can be used to capture CO₂ as bicarbonate and carbonate ions produced from the reaction of CO₂ with water. This novel approach allows capturing CO₂ at room temperature and atmospheric pressure without the use of chemicals. In this process, the adsorption and desorption of bicarbonate ions from the deionized water solution drive the CO₂(g) absorption-desorption from the gas phase. In this work, the effects of the current density and the CO₂ partial pressure were studied. We found that between 55 and 75% of the electrical charge of the capacitive electrodes can be directly used to absorb CO₂ gas. The energy requirement of such a system was found to be ≈40 kJ mol^-1 at 15% CO₂ and could be further improved by reducing the ohmic and non-ohmic energy losses of the MCDI cell.

Recent Progress in the Chemistry of Pyridazinones for Functional Group Transformations

While N-hetereocycles have received significant attention in organic synthesis and other research fields, the chemistry of pyridazine, a six-membered aromatic ring with two adjacent nitrogen atoms, and its derivatives has been relatively little understood. This Synopsis describes recent progress made in the synthesis of pyridazine derivatives-particularly, pyridazin-3(2H)-ones-and their utility as efficient and recyclable functional group carriers for various important organic reactions.

Computational investigation of the nitrosation mechanism of piperazine in CO2 capture

Quantum chemistry calculations and kinetic modeling were performed to investigate the nitrosation mechanism and kinetics of diamine piperazine (PZ), an alternative solvent for widely used monoethanolamine in postcombustion CO₂ capture (PCCC), by two typical nitrosating agents, NO₂^- and N₂O₃, in the presence of CO₂. Various PZ species and nitrosating agents formed by the reactions of PZ, NO₂^-, and N₂O₃ with CO₂ were considered. The results indicated that the reactions of PZ species having NH group with N₂O₃ contribute the most to the formation of nitrosamines in the absorber unit of PCCC and follow a novel three-step nitrosation mechanism, which is initiated by the formation of a charge-transfer complex. The reactions of all PZ species with NO₂^- proceed more slowly than the reactions of PZ species with ONOCO₂^-, formed by the reaction of NO₂^- with CO₂. Therefore, the reactions of PZ species with ONOCO₂^- contribute more to the formation of nitrosamines in the desorber unit of PCCC. In view of CO₂ effect on the nitrosation reaction of PZ, the effect through the reaction of PZ with CO₂ shows a completely different tendency for different nitrosating agents. More importantly, CO₂ can greatly accelerate the nitrosation reactions of PZ by NO₂^- through the formation of ONOCO₂^- in the reaction of CO₂ with NO₂^-. This work can help to better understand the nitrosation mechanism of diamines and in the search for efficient methods to prevent the formation of carcinogenic nitrosamines in CO₂ capture unit.

Nitrosamines and Nitramines in Amine-Based Carbon Dioxide Capture Systems: Fundamentals, Engineering Implications, and Knowledge Gaps

Amine-based absorption is the primary contender for postcombustion CO₂ capture from fossil fuel-fired power plants. However, significant concerns have arisen regarding the formation and emission of toxic nitrosamine and nitramine byproducts from amine-based systems. This paper reviews the current knowledge regarding these byproducts in CO₂ capture systems. In the absorber, flue gas NO_x drives nitrosamine and nitramine formation after its dissolution into the amine solvent. The reaction mechanisms are reviewed based on CO₂ capture literature as well as biological and atmospheric chemistry studies. In the desorber, nitrosamines are formed under high temperatures by amines reacting with nitrite (a hydrolysis product of NO_x), but they can also thermally decompose following pseudo-first order kinetics. The effects of amine structure, primarily amine order, on nitrosamine formation and the corresponding mechanisms are discussed. Washwater units, although intended to control emissions from the absorber, can contribute to additional nitrosamine formation when accumulated amines react with residual NO_x. Nitramines are much less studied than nitrosamines in CO₂ capture systems. Mitigation strategies based on the reaction mechanisms in each unit of the CO₂ capture systems are reviewed. Lastly, we highlight research needs in clarifying reaction mechanisms, developing analytical methods for both liquid and gas phases, and integrating different units to quantitatively predict the accumulation and emission of nitrosamines and nitramines.

Selective Removal of Nitrosamines from a Model Amine Carbon-Capture Waterwash Using Low-Cost Activated-Carbon Sorbents

Nitrosamines generated in the amine solvent loop of postcombustion carbon capture systems are potent carcinogens, and their emission could pose a serious threat to the environment or human health. Nitrosamine emission control strategies are critical for the success of amine-based carbon capture as the technology approaches industrial-scale deployment. Waterwash systems have been used to control volatile and aerosol emissions, including nitrosamines, from carbon-capture plants, but it is still necessary to remove or destroy nitrosamines in the circulating waterwash to prevent their subsequent emission into the environment. In this study, a cost-effective method for selectively removing nitrosamines from the absorber waterwash effluent with activated-carbon sorbents was developed to reduce the environmental impact associated with amine-based carbon capture. The results show that the commercial activated-carbon sorbents tested have a high capacity and selectivity for nitrosamines over the parent solvent amines, with capacities up to 190 mg/g carbon, under simulated amine waterwash conditions. To further reduce costs, an aerobic thermal sorbent regeneration step was also examined due to the low thermal stability of nitrosamines. To model the effect of oxidation on the sorbent performance, thermal- and acid-oxidized sorbents were also prepared from the commercial sorbents and analyzed. The chemical and physical properties of nitrosamines, the parent amine, and the influence of the physical properties of the carbon sorbents on nitrosamine adsorption was examined. Key sorbent properties included the sorbent hydrophilicity and hydrophobicity, surface pK_a of the sorbent, and chemical structure of the parent amine and nitrosamine.

Nitrosamine Formation in Amine-Based CO2 Capture in the Absence of NO2: Molecular Modeling and Experimental Validation

A computational chemistry approach was used to elucidate and verify the different nitrosamine formation mechanisms and pathways. These included nitrosamine formation under acid or basic environments in the presence of NO, O₂, SO₂ and CO₂ without NO₂. The results clearly showed that nitrosamine could be formed without NO₂ via 2 different types of mechanisms, namely, addition and elimination forming N-N bond before proton transfer and proton transfer before N-N bond formation, respectively. The essence of these mechanisms identified in this work was that two reaction steps were required to complete both reaction mechanisms with different nitrosating agents. Two steps were both necessary neither of which could be neglected, if the nitrosamine formation reaction was to be completed. Computational simulation performed on the reactant, intermediate, transition state, and product for each set of reactions also validated the proposed mechanisms. Experiment also detected nitrosamine from the reaction of diethylamine and NO, SO₂, O₂, and CO₂ in both liquid and gas phase. Thus, NO₂ is not necessary for nitrosamine formation to occur in the CO₂ capture system.

Safe Synthesis of Alkylhydroxy and Alkylamino Nitramines

Three different protocols for the syntheses of hydroxyalkylnitramines are presented and compared. Safety issues regarding the synthesis of nitramines are also discussed.

Carbon-Based Adsorbents for Postcombustion CO2 Capture: A Critical Review

The persistent increase in atmospheric CO2 from anthropogenic sources makes research directed toward carbon capture and storage imperative. Current liquid amine absorption technology has several drawbacks including hazardous byproducts and a high-energy requirement for regeneration; therefore, research is ongoing to develop more practical methods for capturing CO2 in postcombustion scenarios. The unique properties of carbon-based materials make them specifically promising for CO2 adsorption at low temperature and moderate to high partial pressure. This critical review aims to highlight the development of carbon-based solid sorbents for postcombustion CO2 capture. Specifically, it provides an overview of postcombustion CO2 capture processes with solid adsorbents and discusses a variety of carbon-based materials that could be used. This review focuses on low-cost pyrogenic carbon, activated carbon (AC), and metal-carbon composites for CO2 capture. Further, it touches upon the recent progress made to develop metal organic frameworks (MOFs) and carbon nanomaterials and their general CO2 sorption potential.

Quantum Chemical Study on ·Cl-Initiated Atmospheric Degradation of Monoethanolamine

Recent findings on the formation of ·Cl in continental urban areas necessitate the consideration of ·Cl initiated degradation when assessing the fate of volatile organic pollutants. Monoethanolamine (MEA) is considered as a potential atmospheric pollutant since it is a benchmark and widely utilized solvent in a leading CO2 capture technology. Especially, ·Cl may have specific interactions with the N atom of MEA, which could make the MEA + ·Cl reaction have different pathways and products from those of the MEA + ·OH reaction. Hence, ·Cl initiated reactions with MEA were investigated by a quantum chemical method [CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p)] and kinetics modeling. Results show that the overall rate constant for ·Cl initiated H-abstraction of MEA is 5 times faster than that initiated by ·OH, and the tropospheric lifetimes of MEA will be overestimated by 6-46% when assuming that [·Cl]/[·OH] = 1-10% if the role of ·Cl is ignored. The MEA + ·Cl reaction exclusively produces MEA-N that finally transforms into several products including mutagenic nitramine and carcinogenic nitrosamine via further reactions with O2/NOx, and the contribution of ·Cl to their formation is about 25-250% of that of ·OH. Thus, it is necessary to consider ·Cl initiated tropospheric degradation of MEA for its risk assessment.

Toward rational design of amines for CO2 capture: Substituent effect on kinetic process for the reaction of monoethanolamine with CO2

Amines have been considered as promising candidates for post-combustion CO2 capture. A mechanistic understanding for the chemical processes involved in the capture and release of CO2 is important for the rational design of amines. In this study, the structural effects of amines on the kinetic competition among three typical products (carbamates, carbamic acids and bicarbonate) from amines+CO2 were investigated, in contrast to previous thermodynamic studies to tune the reaction of amines with CO2 based on desirable reaction enthalpy and reaction stoichiometry. We used a quantum chemical method to calculate the activation energies (Ea) for the reactions of a range of substituted monoethanolamines with CO2 covering three pathways to the three products. The results indicate that the formation of carbamates is the most favorable, among the three considered products. In addition, we found that the Ea values for all pathways linearly correlate with pKa of amines, and more importantly, the kinetic competition between carbamate and bicarbonate absorption pathways varies with pKa of the amines, i.e. stronger basicity results in less difference in Ea. These results highlight the importance of the consideration of kinetic competition among different reaction pathways in amine design.

Influence of Dissolved Metals on N-Nitrosamine Formation under Amine-based CO2 Capture Conditions

As the prime contender for postcombustion CO2 capture technology, amine-based scrubbing has to address the concerns over the formation of potentially carcinogenic N-nitrosamine byproducts from reactions between flue gas NOx and amine solvents. This bench-scale study evaluated the influence of dissolved metals on the potential to form total N-nitrosamines in the solvent within the absorber unit and upon a pressure-cooker treatment that mimics desorber conditions. Among six transition metals tested for the benchmark solvent monoethanolamine (MEA), dissolved Cu promoted total N-nitrosamine formation in the absorber unit at concentrations permitted in drinking water, but not the desorber unit. The Cu effect increased with oxygen concentration. Variation of the amine structural characteristics (amine order, steric hindrance, -OH group substitution and alkyl chain length) indicated that Cu promotes N-nitrosamine formation from primary amines with hydroxyl or carboxyl groups (amino acids), but not from secondary amines, tertiary amines, sterically hindered primary amines, or amines without oxygenated groups. Ethylenediaminetetraacetate (EDTA) suppressed the Cu effect. The results suggested that the catalytic effect of Cu may be associated with the oxidative degradation of primary amines in the absorber unit, a process known to produce a wide spectrum of secondary amine products that are more readily nitrosatable than the pristine primary amines, and that can form stable N-nitrosamines. This study highlighted an intriguing linkage between amine degradation (operational cost) and N-nitrosamine formation (health hazards), all of which are challenges for commercial-scale CO2 capture technology.

Modelling atmospheric oxidation of 2-aminoethanol (MEA) emitted from post-combustion capture using WRF-Chem

Carbon capture and storage (CCS) is a technological solution that can reduce the amount of carbon dioxide (CO2) emissions from the use of fossil fuel in power plants and other industries. A leading method today is amine based post-combustion capture, in which 2-aminoethanol (MEA) is one of the most studied absorption solvents. In this process, amines are released to the atmosphere through evaporation and entrainment from the CO2 absorber column. Modelling is a key instrument for simulating the atmospheric dispersion and chemical transformation of MEA, and for projections of ground-level air concentrations and deposition rates. In this study, the Weather Research and Forecasting model inline coupled with chemistry, WRF-Chem, was applied to quantify the impact of using a comprehensive MEA photo-oxidation sequence compared to using a simplified MEA scheme. Main discrepancies were found for iminoethanol (roughly doubled in the detailed scheme) and 2-nitro aminoethanol, short MEA-nitramine (reduced by factor of two in the detailed scheme). The study indicates that MEA emissions from a full-scale capture plant can modify regional background levels of isocyanic acid. Predicted atmospheric concentrations of isocyanic acid were however below the limit value of 1 ppbv for ambient exposure. The dependence of the formation of hazardous compounds in the OH-initiated oxidation of MEA on ambient level of nitrogen oxides (NOx) was studied in a scenario without NOx emissions from a refinery area in the vicinity of the capture plant. Hourly MEA-nitramine peak concentrations higher than 40 pg m(-3) did only occur when NOx mixing ratios were above 2 ppbv. Therefore, the spatial variability and temporal variability of levels of OH and NOx need to be taken into account in the health risk assessment. The health risk due to direct emissions of nitrosamines and nitramines from full-scale CO2 capture should be investigated in future studies.

Controlling Nitrosamines, Nitramines, and Amines in Amine-Based CO₂ Capture Systems with Continuous Ultraviolet and Ozone Treatment of Washwater

Formation of nitrosamines and nitramines from reactions between flue gas NOx and the amines used in CO2 capture units has arisen as a significant concern. Washwater scrubbers can capture nitrosamines and nitramines. They can also capture amines, preventing formation of nitrosamines and nitramines downwind by amine reactions with ambient NOx. The continuous application of UV alone, or a combination of UV and ozone to the return line of a washwater treatment unit was evaluated to control the accumulation of nitrosamines, nitramines and amines in a laboratory-scale washwater unit. With model secondary amine solvents ranging from nonvolatile diethanolamine to volatile morpholine, application of 272-537 mJ/cm(2) UV incident fluence alone reduced the accumulation of nitrosamines and nitramines by approximately an order of magnitude. Modeling indicated that the gains achieved by UV treatment should increase over time, because UV treatment converts the time dependence of nitrosamine accumulation from a quadratic to a linear function. Ozone (21 mg/L) maintained low steady-state concentrations of amines in the washwater. While modeling indicated that more than 80% of nitrosamine accumulation in the washwater was associated with reaction of washwater amines with residual NOx, a reduction in nitrosamine accumulation rates due to ozone oxidation of amines was not fully realized because the ozonation products of amines reduced nitrosamine photolysis rates by competing for photons.

Amine reclaiming technologies in post-combustion carbon dioxide capture

Amine scrubbing is the most developed technology for carbon dioxide (CO2) capture. Degradation of amine solvents due to the presence of high levels of oxygen and other impurities in flue gas causes increasing costs and deterioration in long term performance, and therefore purification of the solvents is needed to overcome these problems. This review presents the reclaiming of amine solvents used for post combustion CO2 capture (PCC). Thermal reclaiming, ion exchange, and electrodialysis, although principally developed for sour gas sweetening, have also been tested for CO2 capture from flue gas. The three technologies all have their strengths and weaknesses, and further development is needed to reduce energy usage and costs. An expected future trend for amine reclamation is to focus on process integration of the current reclaiming technologies into the PCC process in order to drive down costs.

Genotoxic and mutagenic potential of nitramines

Climate change is one of the major challenges in the world today. To reduce the amount of CO2 released into the atmosphere, CO2 at major sources, such as power plants, can be captured. Use of aqueous amine solutions is one of the most promising methods for this purpose. However, concerns have been raised regarding its impacts on human health and the environment due to the degradation products, such as nitrosamines and nitramines that may be produced during the CO2 capture process. While several toxicity studies have been performed investigating nitrosamines, little is known about the toxic potential of nitramines. In this study a preliminary screening was performed of the genotoxic and mutagenic potential of nitramines most likely produced during amine based CO2 capture; dimethylnitramine (DMA-NO2), methylnitramine (MA-NO2), ethanolnitramine (MEA-NO2), 2-methyl-2-(nitramino)-1-propanol (AMP-NO2) and piperazine nitramine (PZ-NO2), by the Bacterial Reverse Mutation (Ames) Test, the Cytokinesis Block Micronucleus (CBMN) Assay and the in vitro Single-Cell Gel Electrophoresis (Comet) Assay. MA-NO2 and MEA-NO2 showed mutagenic potential in the Ames test and a weak genotoxic response in the CBMN Assay. AMP-NO2 and PZ-NO2 significantly increased the amount of DNA strand breaks; however, the level of breaks was below background. Most previous studies on nitramines have been performed on DMA-NO2, which in this study appeared to be the least potent nitramine. Our results indicate that it is important to investigate other nitramines that are more likely to be produced during CO2 capture, to ensure that the risk is realistically evaluated.

Carbon capture and sequestration: an exploratory inhalation toxicity assessment of amine-trapping solvents and their degradation products

Carbon dioxide (CO2) absorption with aqueous amine solvents is a method of carbon capture and sequestration (CCS) from flue gases. One concern is the possible release of amine solvents and degradation products into the atmosphere, warranting evaluation of potential pulmonary effects from inhalation. The CCS amines monoethanolamine (MEA), methyldiethanolamine (MDEA), and piperazine (PIP) underwent oxidative and CO2-mediated degradation for 75 days. C57bl/6N mice were exposed for 7 days by inhalation of 25 ppm neat amine or equivalant concentration in the degraded mixture. The aqueous solutions were nebulized to create the inhalation atmospheres. Pulmonary response was measured by changes in inflammatory cells in bronchoalveolar lavage fluid and cytokine expression in lung tissue. Ames mutagenicity and CHO-K1 micronucleus assays were applied to assess genotoxicity. Chemical analysis of the test atmosphere and liquid revealed complex mixtures, including acids, aldehydes, and other compounds. Exposure to oxidatively degraded MEA increased (p < 0.05) total cells, neutrophils, and lymphocytes compared to control mice and caused inflammatory cytokine expression (statistical increase at p < 0.05). MEA and CO2-degraded MEA were the only atmospheres to show statistical (p < 0.05) increase in oxidative stress. CO2 degradation resulted in a different composition, less degradation, and lower observed toxicity (less magnitude and number of effects) with no genotoxicity. Overall, oxidative degradation of the amines studied resulted in enhanced toxicity (increased magnitude and number of effects) compared to the neat chemicals.

A review of available analytical technologies for qualitative and quantitative determination of nitramines

This review aims to summarize the available analytical methods in the open literature for the determination of some aliphatic and cyclic nitramines. Nitramines covered in this review are the ones that can be formed from the use of amines in post-combustion CO2 capture (PCC) plants and end up in the environment. Since the literature is quite scarce regarding the determination of nitramines in aqueous and soil samples, methods for determination of nitramines in other matrices have also been included. Since the nitramines are found in complex matrices and/or in very low concentration, an extraction step is often necessary before their determination. Liquid-liquid extraction (LLE) using dichloromethane and solid phase extraction (SPE) with an activated carbon based material have been the two most common extraction methods. Gas chromatography (GC) or reversed phase liquid chromatography (RPLC) has been used often combined with mass spectrometry (MS) in the final determination step. Presently there is no comprehensive method available that can be used for determination of all nitramines included in this review. The lowest concentration limit of quantification (cLOQ) is in the ng L(-1) range, however, most methods appear to have a cLOQ in the μg L(-1) range, if the cLOQ has been given.

Potential occupational risk of amines in carbon capture for power generation

PURPOSE

While CO2 capture and storage (CCS) technology has been well studied in terms of its efficacy and cost of implementation, there is limited available data concerning the potential for occupational exposure to amines, mixtures of amines, or degradation of by-products from the CCS process. This paper is a critical review of the available data concerning the potential effects of amines and CCS-degradation by-products.

METHODS

A comprehensive review of the occupational health and safety issues associated with exposure to amines and amine by-products at CCS facilities was performed, along with a review of the regulatory status and guidelines of amines, by-products, and CCS process vapor mixtures.

RESULTS

There are no specific guidelines or regulations regarding permissible levels of exposure via air for amines and degradation products that could form atmospheric oxidation of amines released from post-combustion CO2 capture plants. While there has been a worldwide effort to develop legal and regulatory frameworks for CCS, none are directly related to occupational exposures.

CONCLUSIONS

By-products of alkanolamine degradation may pose the most significant health hazard to workers in CCS facilities, with several aldehydes, amides, nitramines, and nitrosamines classified as either known or potential/possible human carcinogens. The absence of large-scale CCS facilities; absence and unreliability of reported data in the literature from pilot facilities; and proprietary amine blends make it difficult to estimate potential amine exposures and predict formation and exposure to degradation products.

Comparative in vitro toxicity of nitrosamines and nitramines associated with amine-based carbon capture and storage

Amine-based CO2 capture is a prime contender for the first full-scale implementation of CO2 capture at fossil fuel-fired power plants postcombustion. However, the formation of potentially carcinogenic N-nitrosamines and N-nitramines from reactions of flue gas NOx with the amines presents a potential risk for contaminating airsheds and drinking water supplies. Setting regulatory emission limits is hampered by the dearth of toxicity information for the N-nitramines. This study employed quantitative in vitro bioassays for mutagenicity in Salmonella typhimurium, and chronic cytotoxicity and acute genotoxicity in Chinese hamster ovary (CHO) cells to compare the toxicity of analogous N-nitrosamines and N-nitramines relevant to CO2 capture. Although the rank order was similar for genotoxicity in CHO cells and mutagenicity in S. typhimurium, the Salmonella assay was far more sensitive. In general, mutagenicity was higher with S9 hepatic microsomal activation. The rank order of mutagenicity was N-nitrosodimethylamine (NDMA)>N-nitrosomorpholine>N-nitrodimethylamine>1,4-dinitrosopiperazine>N-nitromorpholine>1,4-dinitropiperazine>N-nitromonoethanolamine>N-nitrosodiethanolamine>N-nitrodiethanolamine. 1-Nitrosopiperazine and 1-nitropiperazine were not mutagenic. Overall, N-nitrosamines were ∼15-fold more mutagenic than their N-nitramine analogues.

Nitrosamine formation in amine scrubbing at desorber temperatures

Amine scrubbing is a thermodynamically efficient and industrially proven method for carbon capture, but amine solvents can nitrosate in the desorber, forming potentially carcinogenic nitrosamines. The kinetics of reactions involving nitrite and monoethanolamine (MEA), diethanolamine (DEA), methylethanolamine (MMEA), and methyldiethanolamine (MDEA) were determined under desorber conditions. The nitrosations of MEA, DEA, and MMEA are first order in nitrite, carbamate species, and hydronium ion. Nitrosation of MDEA, a tertiary amine, is not catalyzed by the addition of CO2 since it cannot form a stable carbamate. Concentrated and CO2 loaded MEA was blended with low concentrations of N-(2-hydroxyethyl) glycine (HeGly), hydroxyethyl-ethylenediamine (HEEDA), and DEA, secondary amines common in MEA degradation. Nitrosamine yield was proportional to the concentration of secondary amine and was a function of CO2 loading and temperature. Blends of tertiary amines with piperazine (PZ) showed n-nitrosopiperazine (MNPZ) yields close to unity, validating the slow nitrosation rates hypothesized for tertiary amines. These results provide a useful tool for estimating nitrosamine accumulation over a range of amine solvents.

Effects of flue gas compositions on nitrosamine and nitramine formation in postcombustion CO2 capture systems

Amine-based technologies are emerging as the prime contender for postcombustion CO2 capture. However, concerns have arisen over the health impacts of amine-based CO2 capture associated with the release of nitrosamines and nitramines, which are byproducts from the reactions between flue gas NOx and solvent amines. In this study, flue gas compositions were systematically varied to evaluate their effects on the formation of nitrosamines and nitramines in a lab-scale CO2 capture reactor with morpholine as a model solvent amine. The accumulation of N-nitrosomorpholine in both the absorber and washwater increased linearly with both NO and NO2 for concentrations up to ∼20 ppmv. These correlations could be extrapolated to estimate N-nitrosomorpholine accumulation at extremely low NOx levels (0.3 ppmv NO2 and 1.5 ppmv NO). NO played a particularly important role in driving N-nitrosomorpholine formation in the washwater, likely following partial oxidation to NO2 by O2. The accumulation of N-nitromorpholine in both the absorber and washwater positively correlated with flue gas NO2 concentration, but not with NO concentration. Both N-nitrosomorpholine and N-nitromorpholine accumulated fastest in the absence of CO2. Flue gas humidity did not affect nitrosamine accumulation in either the absorber or the washwater unit. These results provide a basis for estimating the effects of flue gas composition on nitrosamine and nitramine accumulation in postcombustion CO2 capture systems.

Decomposition of nitrosamines in CO2 capture by aqueous piperazine or monoethanolamine

Amine scrubbing is an efficient method for carbon capture and sequestration, but secondary amines present in all amine solvents can form carcinogenic nitrosamines. Decomposition kinetics for n-nitrosopiperazine (MNPZ), nitrosodiethanolamine (NDELA), and nitroso-(2-hydroxyethyl) glycine (NHeGly) were measured over a range of temperature, base concentration, base strength, and CO2 loading pertinent to amine scrubbing. MNPZ and NDELA decomposition is first order in the nitrosamine, half order in base concentration, and base-catalyzed with a Brønsted slope of β = 0.5. The activation energy is 94, 106, and 112 kJ/mol for MNPZ, NDELA, and NHeGly, respectively. MNPZ readily decomposes at 150 °C in 5 M piperazine, making thermal decomposition an important mechanism for MNPZ control. However, NHeGly and NDELA are too stable at 120 °C in 7 M monoethanolamine (MEA) for thermal decomposition to be important. Base treatment during reclaiming could rapidly and selectively decompose NHeGly and NDELA to mitigate nitrosamine accumulation in MEA.

Atmospheric chemical reactions of monoethanolamine initiated by OH radical: mechanistic and kinetic study

Monoethanolamine (MEA) is a benchmark and widely utilized solvent in amine-based postcombustion CO2 capture (PCCC), a leading technology for reducing CO2 emission from fossil fuel power plants. The large-scale implementation of PCCC would lead to inevitable discharges of amines to the atmosphere. Therefore, understanding the kinetics and mechanisms of the transformation of representative amine MEA in the atmosphere is of great significance for risk assessment of the amine-based PCCC. In this study, the H-abstraction reaction of MEA with ·OH, and ensuing reactions of produced MEA-radicals, including isomerization, dissociation, and bimolecular reaction MEA-radicals+O2, were investigated by quantum chemical calculation [M06-2X/aug-cc-pVTZ//M06-2X/6-311++G(d,p)] and kinetic modeling. The calculated overall rate constant [(7.27 × 10(-11)) cm(3) molecule(-1) s(-1)] for H-abstraction is in excellent agreement with the experimental value [(7.02 ± 0.46) × 10(-11) cm(3) molecule(-1) s(-1)]. The results show that the product branching ratio of NH2CH2 · CHOH (MEA-β) (43%) is higher than that of NH2 · CHCH2OH (MEA-α) (39%), clarifying that MEA-α is not an exclusive product. On the basis of the unveiled reaction mechanisms of MEA-radicals + O2, the proton transfer reaction mass spectrometry signal (m/z 60.044), not recognized in the experiment, was identified.