Influence of Dissolved Metals on N-Nitrosamine Formation under

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Influence of Dissolved Metals on N-Nitrosamine Formation Under Amine-based CO2 Capture Conditions Zimeng Wang, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03085 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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Environmental Science & Technology

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

Zimeng Wang and William A. Mitch*

Department of Civil and Environmental Engineering, Stanford University, Stanford, CA, United States

*Corresponding author Jerry Yang and Akiko Yamazaki Environment & Energy Building 473 Via Ortega Stanford, California 94305, United States Tel: 650-725-9298 Fax: 650-723-7058 Email: [email protected]

Revised Manuscript Submitted to Environmental Science & Technology

August 2015

ACS Paragon Plus Environment


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Abstract

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As the prime contender for post-combustion CO2 capture technology, amine-based scrubbing

3

has to address the concerns over the formation of potentially carcinogenic N-nitrosamine

4

byproducts from reactions between flue gas NOx and amine solvents. This bench-scale study

5

evaluated the influence of dissolved metals on the potential to form total N-nitrosamines in the

6

solvent within the absorber unit and upon a pressure-cooker treatment that mimics desorber

7

conditions. Among six transition metals tested for the benchmark solvent monoethanolamine

8

(MEA), dissolved Cu promoted total N-nitrosamine formation in the absorber unit at

9

concentrations permitted in drinking water, but not the desorber unit. The Cu effect increased

10

with oxygen concentration. Variation of the amine structural characteristics (amine order, steric

11

hindrance, -OH group substitution and alkyl chain length) indicated that Cu promotes N-

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nitrosamine formation from primary amines with hydroxyl or carboxyl groups (amino acids), but

13

not from secondary amines, tertiary amines, sterically hindered primary amines, or amines

14

without oxygenated groups. Ethylenediaminetetraacetate (EDTA) suppressed the Cu effect. The

15

results suggested that the catalytic effect of Cu may be associated with the oxidative degradation

16

of primary amines in the absorber unit, a process known to produce a wide spectrum of

17

secondary amine products that are more readily nitrosatable than the pristine primary amines,

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and that can form stable N-nitrosamines. This study highlighted an intriguing linkage between

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amine degradation (operational cost) and N-nitrosamine formation (health hazards), all of which

20

are challenges for commercial-scale CO2 capture technology.

21

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Introduction

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Carbon capture and sequestration is one of the necessary strategies to mitigate climate

24

change associated with anthropogenic greenhouse gas emissions to the atmosphere. Fossil-fuel

25

combustion power plants are the largest point sources for CO2 emissions. In the near term, the

26

amine-based scrubbing process remains one of the most mature and economically efficient

27

carbon capture technologies for post-combustion applications.1 Over the last few years, several

28

pilot plants and test facilities have been evaluated in North America, Europe and Australia.2-4 In

29

an amine-based post-combustion CO2 capture process, flue gases (typically 4-12% CO2, 4-10%

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O2, and several hundred ppmv NOx) pass through an absorber column where a countercurrent

31

amine solvent absorbs CO2 by reacting to form a covalent carbamate (for primary and secondary

32

amines) or ionic bicarbonate complex (for tertiary amines and some sterically-hindered amines).

33

The CO2-loaded amine solution then proceeds to a desorber column, where high temperature

34

(usually above 100 °C) reverses the reaction, regenerating lean amine that returns to the absorber 2



35

and releasing CO2 for compression and geological storage. While a number of alternative amine

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solvents have been proposed,5 monoethanolamine (MEA) is the industry benchmark due to its

37

low cost and well-documented physical and chemical properties.

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To improve the economic and environmental sustainability of amine-based carbon capture

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technologies, extensive research6,

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degradation,8-12 (2) metallic leaching from equipment corrosion13-15, fly ash16 and metal-based

41

corrosion inhibitors17,

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amines and reactive nitrogen species (e.g., NOx and nitrite).19, 20 Emerging evidence suggests that

43

the three processes are interdependent in scenarios relevant to carbon capture conditions.

18

7

is being performed in three areas: (1) oxidative solvent

(3) formation of carcinogenic byproducts from the reaction between

44

Amine solvent degradation is a concern due to the high cost of the solvent. Previous research

45

suggests that solvent degradation and metal leaching are synergistic processes. Amine

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carbamates and oxidative degradation products of amines (e.g., heat stable salts, particularly

47

oxalic acid) can exacerbate metal leaching from equipment21 and from fly ash16, 22, 23 by acting as

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conductive electrolytes and chelating agents for metals. Carbon capture facilities may

49

accumulate up to mmol/L concentrations of dissolved Fe, Ni, Cu and Cr over 1000 hours of

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operation.22, 24, 25 On the other hand, it is proposed that oxidative degradation of amines proceeds

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by free radicals, deriving from the decomposition of trace amounts of organic hydroperoxides.26,

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27

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oxidative degradation of amines. Several transition metals that can come from equipment

54

corrosion,14,

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chromium26, 29 and manganese26, were reported to be catalysts for oxidative degradation of MEA.

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The earliest report of MEA oxidative degradation catalyzed by transition metals was during the

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1960s in the context of a submarine CO2 scrubber, in which the catalytic activity of Cu was

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higher than for iron, nickel and chromium.30, 31 Field tests also found a correlation between MEA

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degradation (represented by NH3 emission rates) and dissolved metal concentrations in the

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solvent.32

Hydroperoxide decomposition can be catalyzed by certain transition metals, accelerating

17

fly ash,16 and corrosion inhibitors18, including iron21,

28, 29

, copper21,

28, 29

,

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An emerging concern for amine-based post-combustion CO2 capture is the formation of

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potentially carcinogenic N-nitrosamines and N-nitramines from reactions between the amine

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solvent and flue gas NOx.6,

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nitrosamines was orders of magnitude higher than N-nitramines.19,

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indicated that N-nitrosamines were more mutagenic than their N-nitramine analogues.34 Previous

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biomedical literature on N-nitrosamine formation from the reaction between nitrite and amines

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under acidic conditions has suggested that N2O3 is the responsible nitrosating agent35, and our

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recent results also indicated that N2O3 (formed by the reaction of NO with NO2) is the dominant

7

We reported that within the capture unit, formation of N-

4


33

In vitro toxicity tests


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nitrosating agent within the absorber.33 Within the desorber, where gaseous species are depleted,

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the high temperature conditions promote nitrosation of the amines by nitrite despite the high

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pH.36-38 The amine order dictates the rate and pathways of N-nitrosamine formation.39 Only

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secondary amines can form N-nitrosamines. Nitrosation of tertiary amines forms an unstable N-

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nitrosamine intermediate that de-alkylates to form nitrosatable secondary amines. Nitrosation of

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primary amines also forms a highly unstable N-nitrosamine intermediate, which decays nearly

75

instantaneously to nitrogen gas and a carbocation. Under both absorber and desorber conditions,

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the rates of total N-nitrosamine formation were in the order: secondary ≈ tertiary >> primary

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amines.

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Recent research suggests a connection between oxidative amine degradation and formation

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of N-nitrosamine byproducts. The small but appreciable formation of N-nitrosamines from

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primary amine solvents is believed to result from the formation of secondary amines as

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degradation products of the primary amines.38-40 Relevant degradation pathways of primary

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amines that lead to secondary amines include oxidation (probably the primary pathway in the

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absorber41) and carbamate polymerization (occurring at desorber temperatures6). Thus, for

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primary amines there is a potential coupling between N-nitrosamine formation and amine

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degradation.

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The effect of transition metals on N-nitrosamine formation under amine-based post-

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combustion carbon capture conditions has not been evaluated. Because transition metals promote

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amine solvent degradation, and formation of secondary amines from degradation of primary

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amine solvents enables N-nitrosamine formation, transition metals may enhance N-nitrosamine

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formation. Additionally, transition metals may oxidize NO to form nitrosyl cation (+NO), a

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potent but short-lived nitrosating agent42. Although readily hydrolyzed to nitrite at carbon

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capture relevant pH, +NO may react with nitrite to form the nitrosating agent, N2O3.43 Moreover,

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transition metals can form complexes with +NO that also are nitrosating agents.44 Previous

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biomedical literature has characterized metal-catalyzed formation of N-nitrosamines in vivo,45 in

95

which the coordination46,

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nitrosation reactions. A catalytic effect of cupric ion was previously documented for

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nitrosodimethylamine (NDMA) formation from 1,1-dimethylhydrazine oxidation by oxygen.49, 50

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The goal of this research was to evaluate the effect of transition metals on N-nitrosamine

99

formation during amine-based carbon capture. Bench-scale experiments were performed with the

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following objectives: (1) to screen the transition metals that may promote N-nitrosamine

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formation from the benchmark MEA solvent, (2) to understand the effects of amine structural

102

characteristics, O2 and metal concentrations on N-nitrosamine formation, 3) to compare the

47

and redox capabilities48 of transition metals contributed to the

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importance of transition metal promotion of N-nitrosamine formation within the absorber and

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desorber, and 4) to evaluate the effectiveness of a transition metal chelating agent for mitigating

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N-nitrosamine formation.

106 107

Materials and Methods

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Materials. Deionized water was used in all experimental procedures including the

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preparation of amine solvents. MEA and nine other amines varying in amine order, steric

110

hindrance, functional group substitution and alkyl chain length were evaluated (Figure 1).

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Praxair nitrogen (99.995%), carbon dioxide (99.99%), oxygen (99.6%), NO (2580 ppmv in

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nitrogen) and NO2 (2543 ppmv in nitrogen) were used as received. The flow rates of NO and

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NO2 were set by mass flow controllers and those of the other major gases were controlled by

114

volumetric flow meters. The dissolved metal stock solutions (CuCl2, FeCl3, MnCl2, ZnSO4,

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NiSO4, CrCl3) were prepared by dissolution into deionized water. More details about the amines

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and metals are provided in the Supporting Information.

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Bench-scale CO2 absorber column setup. The flue gas-amine reactions were simulated in

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an absorber column (5 cm diameter, 40 cm length), containing 6 mm borosilicate glass Raschig

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rings. A 2.5 M amine stock solution (200 mL) mixed by a magnetic stir bar was recirculated

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through the absorber column at 15 mL/min counter-current to the synthetic flue gas. The

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absorber column and solvent reservoir were maintained at 48 °C, a representative absorber

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temperature during postcombustion carbon capture.24 The amine solutions were self-buffered,

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but amino acid solutions were adjusted to 1 pH unit above the pKa of the amine group using

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sodium hydroxide. The synthetic flue gas (6 L/min) was constituted by N2, O2, CO2, NO, NO2

125

and water vapor. The 12% humidity was set by purging the N2, O2 and CO2 mixture through a

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pre-wetting column maintained at 48 °C. NO and NO2 were added downstream of the pre-

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wetting column. The baseline condition for flue gas composition was 4 % CO2, 14 % O2, 15

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ppmv NO, 3 ppmv NO2 and 6.5 % H2O, with N2 making up the balance of the gas. Due to the

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lack of desorber, the system was pre-equilibrated with the synthetic flue gas without NOx for 1

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hour, which was verified to be sufficient to reach steady CO2 loading (i.e., rich conditions).33

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Then NOx was introduced to the synthetic flue gas. The liquid amine samples were collected at

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various time points after NOx addition and analyzed for nitrite and total N-nitrosamine

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concentrations. Because monitoring of the volume in the amine reservoir indicated that it

134

declined during the experiment due to evaporation, the results are reported as the rate of

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formation of reaction products by mass rather than by concentration. Error bars represent the

136

standard deviation of experimental replicates (n = 2-4). The effect of dissolved metals on N-

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nitrosamine formation was evaluated by adding aliquots of concentrated metal stock solutions

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into the amine reservoir in the midst of the experiments. The pH shift of the amine solution upon

139

addition of dissolved metals was negligible.

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Pressure cooker treatment to mimic desorber conditions. To simulate desorber conditions,

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selected fresh amines were maintained at 48 °C in a water bath and mixed with aliquots of

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sodium bicarbonate. A 1-hour water bath treatment enabled formation of carbamates to simulate

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the loaded amines entering desorber units. Mixing amines with bicarbonate in this fashion was

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previously verified to form carbamates.36 Then the amines were titrated with aliquots of sodium

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nitrite and dissolved metals. To exclude the variation of pH, all the solutions were adjusted to pH

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10.4, representative of desorber conditions.24 Those reactors were heated to 120 °C in a pressure

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cooker at 15 psi for 6 hours. Nitrite and total N-nitrosamines were measured before and after

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pressure-cooker treatment.

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Analyses. Nitrite was measured by the N-(1-naphthyl)ethylenediamine dihydrochloride

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colorimetric method.51 CO2 loadings of solvent amines were measured following a barium

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titration method previously described.52 Among the 10 different amines tested, only morpholine

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(MOR) and diethanolamine (DEA), the secondary amines, react directly to produce N-

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nitrosamines (i.e., N-nitrosomorpholine and N-nitrosodiethanolamine). To form stable N-

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nitrosamines, the other amines, particularly the 7 primary amines, require transformation to

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secondary amines. Because a wide array of potentially nitrosatable amine products may form

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during degradation of these parent amines, a total N-nitrosamine (TONO) analysis was used as

157

the primary analytical method to quantify N-nitrosamine formation. The TONO assay was

158

described previously.19 Briefly, after pretreatment with sulfamic acid to remove nitrite, which

159

interferes with the TONO assay, a sample is injected into a heated reaction chamber containing

160

an acidic tri-iodide solution, which causes cleavage of the N-NO bond in N-nitrosamines. NO is

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purged from the reaction chamber by a stream of nitrogen gas into a chemiluminescence detector

162

(EcoPhysics CLD 88sp). The total N-nitrosamine concentration is quantified based upon the

163

moles of NO liberated, using NDMA to construct a standard curve. Interference of dissolved

164

metals with the TONO assay was ruled out by the absence of signal from control samples

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containing dissolved metals only, dissolved metals mixed with amines and nitrite, and samples

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collected during a control experiment involving sparging the synthetic flue gas into a metal

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solution in the absence of amines. Selected samples from MOR experiments were also analyzed

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for nitrosomorpholine concentration by high performance liquid chromatography (HPLC) with

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UV detection using a method described in our previous publication.33

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Results

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Screening tests for metals using MEA. We screened the effects of several metals that are

173

relevant to equipment corrosion and fly ash particulates on N-nitrosamine formation from MEA

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within the absorber column. Under the baseline flue gas conditions (4 % CO2, 14 % O2, 15 ppmv

175

NO, 3 ppmv NO2 and 6.5 % H2O), the total N-nitrosamine formation rate was 0.062 (± 0.026)

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nmol/s. Injection of 5 µM and 100 µM Cu during the experiment promoted total N-nitrosamine

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formation rates instantaneously by factors of 5 and 20, respectively (Figure 2). Note that 5 µM

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Cu is below the 20.5 µM (1.3 mg/L) Primary Maximum Contaminant Level (MCL) for Cu in

179

U.S. drinking waters.53 Although Cu sorption to borosilicate class is possible, the high amine

180

concentration in the solvent would favor complexation with the amines.54 Similar experiments

181

performed with all other metals (Fe, Mn, Ni, Zn and Cr) did not show any appreciable effect of

182

the metals on N-nitrosamine formation rates.

183

Importance of Oxygen. Absorber experiments performed with various combinations of Cu

184

and O2 concentrations suggested that the promotional effect of Cu on total N-nitrosamine

185

formation rates from MEA is dependent on the oxygen content in the flue gas (Figure 3). In the

186

absence of Cu and O2, the total N-nitrosamine formation rate was 0.020 (± 0.001) nmol/s.

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Increasing O2 concentration to 6 % and 14 % increased the N-nitrosamine formation rate by

188

factors of 2 and 3, respectively.

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In the absence of O2 in the flue gas, addition of Cu had a minimal effect. Even though a

190

slight increase of N-nitrosamine formation rate (less than a factor of 4) was suggested at 100 µM

191

Cu, it is probably attributable to the fact that our MEA solvent reservoir was not completely free

192

of O2 uptake from the ambient air as the experiments were conducted in a well-vented fume hood.

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At 6 % O2, 5 µM and 100 µM Cu increased the N-nitrosamine formation rate by factors of 2.7

194

and 15, respectively. These values are lower than the factors of 5 and 20 observed at 14 % O2,

195

yet still substantial.

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The results suggest that the effects of Cu and O2 are synergistic with respect to N-nitrosamine

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formation form MEA. Previous studies reported that MEA degradation kinetics appeared to be

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first order with respect to the concentration of O2.55 The similar dependence of total N-

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nitrosamine formation rates from MEA on O2 concentration (Figure 3) suggests that the N-

200

nitrosamine formation is associated with MEA degradation. In contrast, previous research

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demonstrated that the flue gas O2 concentration in the absorber did not impact N-nitrosamine

202

formation from morpholine (MOR), a secondary amine that can form N-nitrosamines directly.33

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These results suggest that N-nitrosamine formation from MEA derives from secondary amines

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generated as degradation products of MEA38, 39 via reactions promoted by O2 and Cu.

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Importance of amine structure. To evaluate the extent to which the Cu promotion of N-

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nitrosamine formation is specific to MEA, the solvent amine structure was varied to capture

207

differences in (1) amine order (i.e., primary, secondary, and tertiary), (2) steric hindrance of the

208

amino group in primary alkanolamines, (3) substituent functional groups in primary amines, and

209

(4) the alkyl chain length in primary amines (Figure 1). In the absence of Cu, the total N-

210

nitrosamine formation rates for primary amines were one or more orders of magnitude lower

211

than those for two secondary amines (MOR and DEA) and one tertiary amine (TEA) (Figure 4A),

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concurring with the results of previous research.36 For primary amines, substituting the hydroxyl

213

group of MEA with an amine group (EDA), increasing the alkyl chain length (3AP), or

214

introducing steric hindrance (AMP and TRIS) had little effect on total N-nitrosamine formation

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compared with the benchmark MEA in the absence of Cu. The two primary amino acids (GLY

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and ALA) featured the lowest total N-nitrosamine formation rates. The trends of those rates for

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N-nitrosamine formation, as well as for nitrite formation (Figure 4B), were consistent with

218

previous results.39, 56

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The promotional effect of Cu on total N-nitrosamine formation was observed only for the

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primary amines, MEA, GLY, ALA and 3AP (Figure 4A). Addition of Cu had no impact on

221

nitrite formation rates for any of the tested amines (Figure 4B), indicating that Cu does not

222

impact the absorption of NOx from the gas phase. As a secondary amine, MOR can form stable

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N-nitrosamines directly. Indeed, for samples collected from MOR experiments, the

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concentrations of N-nitrosomorpholine (NMOR) by HPLC matched well with the concentration

225

of total N-nitrosamines.36, 37 For all other amines tested, the original amines must be converted to

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secondary amines to produce stable N-nitrosamines. Transformation of tertiary amines to

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nitrosatable secondary amines occurs through dealkylation of an unstable nitrosated tertiary

228

amine intermediate releasing an aldehyde and a secondary amine.57 The rate of total N-

229

nitrosamine formation from the tertiary amine, TEA, was also not impacted by Cu, suggesting

230

that the dealkylation step was not catalyzed by Cu.

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Sterically hindered primary amines are attractive solvents compared to MEA because they

232

require lower temperatures to regenerate and they are more resistant to oxidative degradation.8,

233

58-60

234

the promotional effect of Cu for total N-nitrosamine formation. The two amino acids (GLY,

235

ALA), which lack steric hindrance and feature substitution of the hydroxyl group of MEA by

Our results showed that introducing steric hindrance to MEA (i.e., AMP, TRIS) eliminated

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carboxyl groups, demonstrated a significant promotional effect of Cu for N-nitrosamine

237

formation. However, substituting the hydroxyl group of MEA by an amine group (EDA)

238

eliminated the Cu effect. The amine 3AP, which retains the hydroxyl group of MEA but bears an

239

additional alkyl carbon, also exhibited a higher total N-nitrosamine formation rate after addition

240

of Cu. Together, the results suggest that promotion of N-nitrosamine formation by Cu is

241

applicable to primary amines with oxygenated functional groups that lack steric hindrance.

242

Interestingly, GLY and ALA are the intuitive oxidation products of MEA and 3AP, respectively

243

(i.e., oxidation of the hydroxyl group to a carboxyl group). While the N-nitrosamine formation

244

rate for GLY was promoted by Cu, it was an order of magnitude less than for MEA exposed to

245

Cu.

246

Test of “NO + Cu” as a potential nitrosating agent. Previous studies reported that NO and

247

Cu react to form +NO, a potent nitrosating agent for secondary amines at alkaline pH,48,

248

following a reaction such as

249

Cu2+ + NO + R2NH → Cu+ + H+ + R2N-N=O

61

(1)

250

This “reductive nitrosation” reaction is particularly appreciated for its biochemical function for

251

NO transport and signaling as part of the mammalian immune response.62 In amine-based carbon

252

capture processes, it is of interest to investigate if the lack of any effect of Cu in the MOR

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experiments was simply due to the presence of NO2 in the synthetic flue gas, which might have

254

masked the impacts, if any, of oxidation of NO by Cu. In authentic flue gas, NO2 is usually

255

easier to remove than NO during pre-treatments due to its higher solubility. Downstream of the

256

absorber column, where most of the NO2 is likely scavenged, NO is probably the dominant

257

reactive nitrogen species in the washwater unit.33 We performed a control experiment using 0.05

258

M MOR (to simulate a diluted amine concentration in the washwater33, 39) in which the flue gas

259

was free of NO2 but contained NO, O2, CO2 and N2. The addition of 100 µM Cu did not induce

260

any change in the N-nitrosamine formation rate (Figure S1). Although the reductive nitrosation

261

mechanism enabled by Cu and NO is physically possible, it does not appear to be as important in

262

post-combustion carbon capture systems as in biomedical systems. Because flue gas always

263

contains unconsumed oxygen which can partially oxidize NO to NO2, N2O3 can form as the

264

predominant nitrosating agent.33, 63 Thus, addition of Cu to initiate reaction 1, and allowing Cu to

265

cycle between the +I and +II oxidation states by oxidation of Cu(I) by oxygen does not

266

contribute appreciably to N-nitrosamine formation compared with N2O3.

267

Cu effects quenched by a chelating agent. Addition of a strong chelating agent could

268

suppress the promotional effect of Cu on N-nitrosamine formation. While spiking 100 µM Cu

269

accelerated the formation of N-nitrosamines in MEA by a factor of 20, adding 1 mM EDTA

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dramatically reduced the N-nitrosamine formation rate (Figure 5). The usefulness of EDTA as an

271

inhibitor of Cu catalysis for MEA oxidative degradation has been recognized since the 1960s

272

with reference to a CO2 scrubber in submarines.30,31 Recent studies performed under post-

273

combustion CO2 capture conditions confirmed the role of EDTA in suppressing Cu- and Fe-

274

catalyzed MEA oxidative degradation, although the inhibition capacity declined over time.28, 29

275

Our results indicate that the inhibitory effect of EDTA also is applicable to Cu-catalyzed N-

276

nitrosamine formation from MEA.

277

Metal effects under desorber conditions. Under desorber conditions, where nitrite is the

278

nitrosating agent,38 the effect of 100 µM metals on N-nitrosamine formation rates was compared

279

between MEA and MOR. Previously, we had demonstrated that nitrosamine formation was more

280

important in the absorber for primary amines, but more important in the desorber for secondary

281

and tertiary amines.39 Six hours of pressure-cooker treatment completely consumed an initial 5

282

mM nitrite when the MOR concentration was 2.5 M as in the absorber experiments. Those

283

experiments produced 5 mM N-nitrosomorpholine, as confirmed by both TONO and HPLC

284

analyses, consistent with previous literature that nitrosation of secondary amines by nitrite is

285

stoichiometric.36, 38 Therefore, a lower MOR concentration (25 mM) was used to examine the

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effect of dissolved metals. No effect of any dissolved metal (100 µM) on the formation of N-

287

nitrosamines was observed for MOR (Figure 6).

288

Compared with the absorber conditions, similar results were obtained from the metal

289

screening tests under the desorber conditions using 2.5 M MEA (Figure 6). None of the metals

290

affected total N-nitrosamine formation rates, except for 100 µM Cu, for which a 2.5-fold increase

291

in total N-nitrosamine formation was observed. Nevertheless, the promotional effect of Cu under

292

desorber conditions, where nitrite was the nitrosating agent, was much less pronounced

293

compared with the 20-fold increase under absorber conditions, where flue gas containing oxygen

294

and NOx was continuously delivered to the MEA solvent. It was also noted that the amount of

295

total N-nitrosamine formation (on the order of µM) was orders of magnitude less than the

296

consumption of nitrite (on the order mM). These results are consistent with nitrosation of MEA

297

leading predominantly to formation of nitrogen gas and a carbocation, with only a small fraction

298

of the nitrite reacting with the traces of secondary amine degradation products of MEA to form

299

stable N-nitrosamines.

300

Interestingly, the pressure cooker treatments generated the same order-of-magnitude total N-

301

nitrosamine concentration from MOR and MEA, even though the MOR concentration was three

302

orders of magnitude lower than MEA. This confirmed again the significantly lower potential for

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303

N-nitrosamine formation from primary amines than secondary amines under desorber

304

conditions.39

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Discussion

306

An indirect role for Cu in catalyzing N-nitrosamine formation. The formation of N-

307

nitrosamines from MEA and other primary amines relies on an initial transformation via

308

oxidative degradation to secondary amines that can be nitrosated.40 Therefore the catalytic effect

309

of Cu may relate to either formation of the secondary amines or nitrosation of the secondary

310

amines. MOR and DEA are among the possible oxidative degradation products of MEA.11, 26

311

Moreover, MOR is resistant to oxidative degradation.64 The lack of Cu effect on total N-

312

nitrosamine formation from MOR and DEA (Figure 4 and S1) indicates that Cu does not

313

catalyze the nitrosation of secondary amines under carbon capture conditions.

314

The correlation of the promotion of N-nitrosamine formation by Cu from MEA with oxygen

315

concentration (Figure 3) suggests that Cu promotes the oxidative degradation of MEA to form an

316

array of products, a portion of which are secondary amines that can react with N2O3 in the

317

absorber column to produce N-nitrosamines. Previous research has indicated that oxygen

318

promotes the degradation of MEA to form an array of secondary amine products.8, 11, 65 The wide

319

array of products that are produced renders the characterization of N-nitrosamine formation

19


Page 21 of 36


320

pathways difficult, and necessitated the use of the total N-nitrosamine assay to quantify N-

321

nitrosamine production. Indeed, the Cu effect was not observed for sterically-hindered primary

322

amines, which resist oxidative degradation because they lack α-hydrogens; α-hydrogens play a

323

role in the formation of an imine, an initial step in an oxidative degradation pathway following

324

electron abstraction from the lone electron pair on the amine nitrogen (see Scheme S1).10, 60

325

Additionally, the Cu effect was only observed for primary amines with oxygenated

326

functional groups. Previous research on MEA oxidative degradation pathways indicated that the

327

hydroxyl group of MEA reacts with carboxylic acids (which are known MEA degradation

328

products) to form esters. Ring cyclization followed by an SN2 reaction with a second MEA

329

produces secondary amine products, such as hydroxyethylethylenediamine (HEEDA).8, 11, 41, 59

330

The carboxyl groups of GLY and ALA may play a similar role in producing secondary amines

331

via oxidative degradation. In contrast, EDA which does not contain an oxygenated functional

332

group, and did not exhibit a promotional effect on N-nitrosamine formation by Cu. Nevertheless,

333

definition of the specific reactions involved in Cu promotion of N-nitrosamine formation from

334

primary amines requires further characterization of the oxidative degradation pathways of

335

primary amines, which is beyond the scope of this study.

20



Page 22 of 36

336

While our results indicated the Cu catalysis of total N-nitrosamine formation relates to the

337

promotion of oxidative degradation, Cu likely promotes a specific subset of degradation

338

pathways leading to secondary amine formation. Fe and Mn were also reported to catalyze MEA

339

oxidative degradation, as indicated by MEA loss, and instantaneously enhanced formation rates

340

of organic acids and NH3.21, 26, 28, 29 However, Fe and Mn do not promote total N-nitrosamine

341

formation, suggesting that these metals promote the formation of products other than secondary

342

amines. Formation of N-nitrosamines from tertiary amines proceeds through a nitrosative

343

dealkylation step to produce secondary amines. However, Cu did not promote N-nitrosamine

344

formation from TEA (Figure 4). Previous research indicating similar total N-nitrosamine

345

formation rates between tertiary amines and their corresponding secondary amines suggests that

346

the dealkylation reaction is relatively efficient under absorber conditions,39 such that catalysis by

347

Cu, if any, would not significantly enhance the observed N-nitrosamine formation rates.

348

Implications for carbon capture practice. Cu was the only one of six transition metals that

349

promoted N-nitrosamine formation from primary amines. The low potential for N-nitrosamine

350

formation from primary amines has been considered an attractive feature compared to secondary

351

and tertiary amines. However, the N-nitrosamine formation rates from MEA in the presence of

352

Cu were similar to those of secondary and tertiary amines. Although for secondary and tertiary

21


Page 23 of 36


353

amines, nitrite-induced nitrosamine formation in the desorber is more important than that in the

354

absorber, primary amines were found to form more nitrosamines in the absorber than in the

355

desorber.39,

356

absorber.

66

The catalytic effect of Cu can further enhance nitrosamine formation in the

357

Carbon capture operations employing primary amine solvents will need to beware of

358

potential sources of Cu entering solvents and washwaters, where reactions of residual NOx with

359

amines accumulating in the washwater may also form nitrosamines.33 While Cu is usually

360

considered as a relatively minor component in stainless steel, it is a common element in flue gas

361

particulates from coal combustion processes.22, 67 Although Cu concentrations in carbon capture

362

processes may vary from case to case depending on the specific fuel and flue gas composition,

363

Cu was observed in lean amine and reclaimer waste sludge at appreciable concentrations (ppm

364

level) that may be comparable with Fe.11, 68 Additionally, as Cu-based corrosion inhibitors (e.g.,

365

CuCO3) are widely applied for protecting stainless steel equipment,18 their promotion of

366

oxidative degradation of amine solvents has been recognized,21,

367

catalyze nitrosamine formation as illustrated in the present study. EDTA effectively suppressed

368

the Cu effect. Development of alternative oxidation inhibitors for amine solvents should evaluate

369

the extent to which they can mitigate Cu catalysis for N-nitrosamine formation.70 Lastly, the use

22


28, 29, 69

and they may also


370

of tap water or surface waters to constitute solvents or washwaters may be a concern given that

371

Cu promoted nitrosamine formation from primary amines at concentrations lower than the

372

Maximum Contaminant Level permitted in drinking water.

Page 24 of 36

373 374

Acknowledgements

375

This work was funded by the Stanford Woods Institute for the Environment. Valuable

376

discussions with personnel from the CO2 Capture Mongstad Project and Dr. Ning Dai (currently

377

at the State University of New York at Buffalo) are appreciated. Comments and suggestions of

378

three anonymous reviewers and Associate Editor Rich Valentine improved the quality of an

379

earlier version of this paper.

380 381 382 383

Supporting Information This supporting information (one figure, one table, and additional details about the materials) is available free of charge via the Internet at http://pubs.acs.org.

384 385

23


Page 25 of 36


386

References

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392

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scale postcombustion capture of CO2 with monoethanolamine solvent: key considerations for solvent

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Nielsen, C. J.; Herrmann, H.; Weller, C., Atmospheric chemistry and environmental impact of the Goff, G. S.; Rochelle, G. T., Monoethanolamine degradation: O2 mass transfer effects under CO2 Schallert, B.; Neuhaus, S.; Satterley, C. J., Is fly ash boosting amine losses in carbon capture from Huang, Q.; Thompson, J.; Bhatnagar, S.; Chandan, P.; Remias, J. E.; Selegue, J. P.; Liu, K.,

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Goff, G. S.; Rochelle, G. T., Oxidation inhibitors for copper and iron catalyzed degradation of Sexton, A. J.; Rochelle, G. T., Catalysts and inhibitors for oxidative degradation of 25


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blended amines for CO2 capture. Int. J. Greenh. Gas. Con. 2015, 39, 329-334.

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547 548

28



Monoethanolamine (MEA)

Industry Benchmark Amine Order

Page 30 of 36

Substitution

Steric Hindrance

Chain Length

Ethylenediamine (EDA)

Morpholine (MOR) 2-Amino-2-methyl1-propanol (AMP)

Diethanolamine (DEA)

Triethanolamine (TEA)

Glycine (GLY)

2-Amino-2(hydroxymethyl)-1,3propanediol (TRIS)

3-Aminopropanol (3AP)

β-Alanine (ALA)

549 550 551 552

Figure 1. Structures of the nine model solvent amines exhibiting four different types of structural variations from the benchmark solvent monoethanolamine (MEA).

553

29


Page 31 of 36


Total N-Nitrosamine (µmol)

20

5 µM

No Cu

100 µM

15

10

5

0

0

2

4 Time (h)

6

8

554 555 556 557

Figure 2. Accumulation of total N-nitrosamines in an absorber experiment at 48 °C during which dissolved Cu was spiked into the 2.5 M MEA solvent. The flue gas composition was at its baseline condition.

558 559

30



2 1.5

561 562 563 564

0.041

0.5

0.062

1

0 560

0 µM Cu 5 µM Cu 100 µM Cu

0.020 0.022 0.076

Total N-Nitrosamine Formation Rate (nmol/s)

2.5

Page 32 of 36

0% 6% 14 % Oxygen in Flue Gas (%)

Figure 3. Formation rates of total N-nitrosamines in the absorber experiments at 48 °C as a function of Cu and O2 concentrations. All experiments were performed with 2.5 M MEA and, with the exception of O2, the baseline flue gas composition. Error bars represent standard deviations from replicate experiments (n = 2 – 4).

565 566

31


Page 33 of 36


567

570 571

0.1 0.01

12

(b)

No Cu

100 M Cu

10 8 6 4 2 0

MEA MOR DEA TEA AMP TRIS EDA GLY ALA 3AP

100 M Cu

Nitrite Formation Rate (nmol/s)

No Cu

14

1

0.001

568 569

(a)

MEA MOR DEA TEA AMP TRIS EDA GLY ALA 3AP

Total N-Nitrosamine Formation Rate (nmol/s)

10

Figure 4. Formation rates of total N-nitrosamines (panel a) and nitrite (panel b) in the absorber experiments at 48 °C. All experiments were performed with 2.5 M amine and the baseline flue gas composition. Error bars represent standard deviations from replicate experiments (n = 2 – 4).

572 573 574 575

32



20

578

100 µM Cu

100 µM Cu + 1 mM EDTA

15 10 5 0

576 577

Baseline

Total N-Nitrosamine (µmol)

25

Page 34 of 36

0

2

4 Time (h)

6

8

Figure 5. Formation of total N-nitrosamines over time from 2.5 M MEA with the baseline flue gas composition in an absorber experiment at 48 °C.

579

33


Page 35 of 36


580

Final Concentration of Total N-Nitrosamine (µM)

12

25 mM MOR

10 8 6 4 2 0

581

2.5 M MEA

Blank Cu Fe Mn Zn

Ni

Cr

582

Figure 6. Final concentrations of total N-nitrosamines from the pressure-cooker treatment (2 bar,

583

120 °C, 6 hours) starting with 2.5 M MEA or 25 mM MOR with 100 µM of various metals at pH 10.4. Horizontal lines provide visual guides for the N-nitrosamine formation in the metal-free control experiments. Initial nitrite = 5 mM. Final nitrite = 3.75 mM for MEA and 5 mM for MOR. The CO2 loadings (as NaHCO3 added) were 0.5 and 0.2 for MEA and MOR respectively. Error bars represent the range from duplicate experiments.

584 585 586 587 588

34



589

Table of Contents Art

590

35


Page 36 of 36

Influence of Dissolved Metals on N-Nitrosamine Formation under

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