Simultaneous Adsorption and Electrochemical Reduction of N

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Remediation and Control Technologies

Simultaneous Adsorption and Electrochemical Reduction of N-Nitrosodimethylamine using CarbonTiO Composite Reactive Electrochemical Membranes 4

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Soroush Almassi, Zhao Li, WENQING XU, Changcheng Pu, Teng Zeng, and Brian P. Chaplin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05933 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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Environmental Science & Technology

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Simultaneous Adsorption and Electrochemical Reduction of N-

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Nitrosodimethylamine using Carbon-Ti4O7 Composite Reactive

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Electrochemical Membranes

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Soroush Almassi§, Zhao Li†, Wenqing Xu†, Changcheng Pu+, Teng Zeng+, and

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Brian P. Chaplin§*

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§

Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton St., Chicago, IL 60607

†

Department of Civil and Environmental Engineering, Villanova University, 800 E. Lancaster Ave., Villanova, PA 19085

+Department

of Civil and Environmental Engineering, Syracuse University, 151 Link Hall, Syracuse, NY 13244

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*Corresponding author at: Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA

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E-mail address: [email protected] (Brian P. Chaplin)

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Phone No.: +13129960288

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ACS Paragon Plus Environment

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Abstract

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This study focused on synthesis and characterization of Ti4O7 reactive electrochemical

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membranes (REMs) amended with powder activated carbon (PAC) or multi-walled carbon

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nanotubes (MWCNTs). These composite REMs were evaluated for simultaneous adsorption and

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electrochemical reduction of N-Nitrosodimethylamine (NDMA). The carbon-Ti4O7 composite

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REMs had high electrical conductivities (1832 to 2991 S m-1), where carbon and Ti4O7 were in

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direct electrical contact. Addition of carbonaceous materials increased the residence times of

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NDMA in the REMs by a factor of 3.8 to 5.4 and therefore allowed for significant electrochemical

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NDMA reduction. The treatment of synthetic solutions containing 10 µM NDMA achieved > 4-

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log NDMA removal in a single pass (liquid residence time of 11 to 22 s) through the PAC-REM

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and MWCNT-REM with the application of a -1.1 V/SHE cathodic potential, with permeate

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concentrations between 18 and 80 ng L-1. The treatment of a 6.7 nM NDMA-spiked surface water

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sample, under similar operating conditions (liquid residence time of 22 s), achieved 92 to 97%

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removal with permeate concentrations between 16 and 40 ng L-1. Density functional theory

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calculations determined a probable reaction mechanism for NDMA reduction, where the rate-

38

limiting step was a direct electron transfer reaction.

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Keywords: N-Nitrosodimethylamine, Water treatment, Reactive electrochemical membranes,

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Powder activated carbon, Multi-walled carbon nanotubes, density functional theory.


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1. Introduction

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Nitrosodimethylamine (NDMA) is a harmful water contaminant that has been categorized as

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a probable human carcinogen by the U.S. Environmental Protection Agency (EPA). As a result,

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the EPA has established a 10-6 cancer risk at the exposure concentration of 0.7 ng L-1,1 and the

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California Department of Health Services has set a notification level of 10 ng L-1 for NDMA in

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drinking water.2 The occurrence of NDMA is widespread and it has been found in air,3 soil,4 food,5

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and surface water and groundwater.6,7 NDMA can form through various oxidation processes, most

49

notably as a disinfection byproduct during drinking water and wastewater treatment.8,9

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Research has focused on various methods for the removal of NDMA from water and

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wastewater. These methods include separation methods, such as reverse osmosis (RO),10 and

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destructive methods, such as reductive catalysis and zero valent iron (ZVI),11–20 advanced

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oxidation processes (AOPs),21–23 and direct UV photolysis.24,25 All these methods have limitations

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such as poor rejection by RO,10 high operating costs for RO, AOPs, and UV photolysis,8 high

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capital costs and fouling by natural water species for precious metal catalysts,26,27 and low reaction

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rates with ZVI.11,16 Moreover, methods to limit the formation of NDMA by removing NDMA

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precursors has also been extensively studied, with some promising results.28

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Electrochemical oxidative and reductive methods have been shown to be effective for NDMA 29–31

but very long residence times are required to meet the ng L-1 treatment goals.30

59

removal,

60

Recent work has shown that the use of porous electrodes in flow-through mode reactors can greatly

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enhance mass transport rates and achieve complete mineralization of contaminants with very short

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residence times (< 5 s).32–34 This concept has been applied to the development of reactive

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electrochemical membranes (REMs) for both oxidative and reductive destruction of water

64

contaminants.34–36 However, in order to achieve the very low treatment goals (e.g., ng L-1 levels)


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necessary for NDMA and other water contaminants, further innovations are needed for

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electrochemical technologies.

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To that end, this study was focused on the development of carbon-Ti4O7 composite REMs that

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could accomplish simultaneous adsorption and electrochemical destruction of NDMA in single

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pass flow-through mode operation. The REMs were synthesized by blending either powder

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activated carbon (PAC) or multiwalled carbon nanotubes (MWCNTs) with Ti4O7 ceramic

71

material. The carbon-loaded microporous REMs were characterized for their physical and

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electrical properties and tested for the adsorption and electrochemical reduction of NDMA in

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synthetic and surface water solutions. Experimental results were interpreted using density

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functional theory (DFT) simulations and a reaction mechanism for the electrochemical reduction

75

of NDMA was proposed.

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2. Materials and Methods

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2.1. Reagents. All reagents were obtained from Fisher Scientific (Pittsburgh, PA) or Sigma-

78

Aldrich (St. Louis, MO) and were used without additional purification. All solutions were prepared

79

with deionized (DI) water, which was obtained from a Barnstead NANO pure water system (18.2

80

MΩ cm at 25 ºC).

81

2.2. Synthesis of TinO2n-1 Electrodes. Conductive Ti4O7 powder was synthesized from TiO2

82

anatase powder (≥ 99% purity, particle diameter ~ 32 nm). The composite REMs were synthesized

83

from conductive Ti4O7 powder and modified with either MWCNTs (Cheap Tubes Inc. (Grafton,

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VA); 20-30 nm, SKU:030104) or Norit D10 PAC. The electrodes used in this study were defined

85

as follows: 100% Ti4O7 pellet (REM), 10 wt% PAC 90 wt% Ti4O7 pellet (PAC-REM), and 10

86

wt% MWCNT 90 wt% Ti4O7 pellet (MWCNT-REM). More details are included in the Supporting

87

Information (SI).


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2.3 Physical Characterization. Crystallography of powder and REM samples was determined

89

by X-ray diffraction (XRD) (D-5000, Siemens) with Cu-Kα radiation (λ = 1.5418 Å). Thermal

90

gravimetric analysis (TGA) was performed on the REM, MWCNT-REM, and PAC-REM samples

91

using a Mettler thermogravimetric analyzer (Pyris 1 TGA, Perkin-Elmer, Waltham, MA) in the

92

presence of air. The carbonaceous materials in the REMs were characterized using confocal

93

Raman spectroscopy (alpha 300 ra, WITec, Ulm, Germany) with a laser wavelength of 532 nm.

94

All Raman spectral analysis was performed twice on two different 720 nm diameter spots on each

95

sample. The conductivity of the REMs was measured by electrochemical impedance spectroscopy

96

(EIS) using a Gamry Reference 600 potentiostat/galvanostat (Gamry Instruments, Warminster,

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PA) and equation (1):

98

!=$

#

(1)

%&

99

where σ is the conductivity (S m-1), L is the REM thickness (m), A is electrode cross-sectional area

100

(m2), and Rm is the measured material resistance (Ω). The specific surface area and pore size

101

distribution of the powder samples were determined using BET analysis (Nova 2000e,

102

Quantachrome, Boynton Beach, FL) from the nitrogen adsorption data.

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2.4 REM Flow-through Reactor. An upflow, electrochemical, flow-through reactor was used

104

for adsorption and electrochemical oxidation or reduction of NDMA. The solution first passed

105

through the working electrode followed by the counter electrode. A schematic of the reactor setup

106

is shown in Figure S-1. All experiments employed a three-electrode setup and were carried out

107

with either the REM, MWCNT-REM, or PAC-REM with 0.5 cm2 surface area as the working

108

electrode (cathode), a 0.33 cm2 surface area 316 stainless steel tube as the counter electrode

109

(anode), and a leak-free 1 mm diameter Ag/AgCl as the reference electrode (LF-100, Warner

110

Instruments, Hamden, CT). All potentials were reported versus the standard hydrogen electrode

111

(/SHE) and were corrected for the solution resistance between the working and reference


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electrodes, which was determined by EIS. The permeate flux was held constant at either 100 or

113

200 L m-2 h-1 (LMH), which gave a liquid residence time in the REMs of either 22 or 11 s,

114

respectively.

115

Both adsorption and reduction experiments of NDMA were conducted using all REMs. Feed

116

solutions with either 10 µM or 150 µM NDMA were prepared in 10 mM NaH2BO3 buffer (pH =

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8, ionic strength = 4.7 mM) to approximate the pH of natural waters. The NaH2BO3 buffer was

118

used as a nonreactive electrolyte, as typical environmental buffers (e.g., HCO3-) can cause

119

carbonate scale on the cathode and also react on the down-stream anode to form radicals that may

120

react with NDMA. The higher feed concentration was used to increase the concentration of the

121

NDMA reduction products in order to aid in the determination of the reaction mechanism and close

122

the N mass balance. Additional experiments were conducted with a surface water sample spiked

123

with of 6.7 nM NDMA. For adsorption experiments, each REM was tested under open circuit

124

potential (OCP) conditions for 20 hours. These experiments were carried out to achieve NDMA

125

saturation on the sorbents and to monitor the breakthrough curves. After which, either an anodic

126

potential in the range of 2.45 to 2.65 V/SHE or a cathodic potential of approximately -1.1 V/SHE

127

was applied and NDMA in the permeate was monitored with time. These potentials were chosen

128

based on previous work that indicated they were sufficient for NDMA destruction.29,30 The

129

reduction and oxidation experiments were conducted for between 50 to 70 hours and 30 to 56

130

hours, respectively. Adsorption experiments were conducted in duplicate, and all experiments

131

were performed at room temperature (~22 ºC). All the reported errors in this study were the 95%

132

confidence intervals about the mean values.

133

2.5 Adsorption Isotherm Analysis. The Ti4O7, PAC-Ti4O7, and MWCNT-Ti4O7 powders were

134

tested in batch experiments to construct adsorption isotherms of NDMA and other nitrosamines,


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which were analyzed according to EPA method 521.37 The solid phase concentrations of

136

nitrosamines were calculated using a mass balance approach as follows:

137

.

q = ()* − ), ) /

(2)

138

where q is the amount of nitrosamines adsorbed by the composite powders (mg g-1), C0 is the initial

139

aqueous nitrosamine concentration (mg L-1), Ce is the aqueous nitrosamine concentration at

140

equilibrium (mg L-1), V is the solution volume (L), and m is the mass of powder (g). The adsorption

141

isotherms for all nitrosamines were constructed by plotting q versus Ce at equilibrium. Additional

142

details are provided in the SI.

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2.6 Batch Experiments. To determine the enthalpy of activation (∆1‡ ) for electrochemical

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NDMA reduction at -1.12 and -1.47 V/SHE, batch experiments were carried out in a jacketed 100

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mL divided cell reactor using a rotating disk electrode (RDE). A Nafion A115 membrane (Ion

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Power, Inc., New Castle, DE) was used to separate anode and cathode components. Experiments

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were conducted using a three-electrode setup with 0.35 cm2 Ti4O7 REM as working electrode

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(cathode), Pt wire as counter electrode, and Ag/AgCl reference electrode. To clarify the reduction

149

mechanism of NDMA, an additional experiment was carried out with an Argon gas purge to

150

remove dissolved oxygen. More details are provided in the SI.

151

2.7 Analytical Methods. The NDMA concentrations were measured using liquid

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chromatography with a photodiode array detector (254 nm). The NDMA concentrations of select

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samples were analyzed using gas chromatography-mass spectroscopy (GC-MS) or liquid

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chromatography-mass spectroscopy (LC-MS) The concentration of NO3-, NH4+, and

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dimethylamine (DMA) were determined by ion chromatography (IC). For all liquid

156

chromatographic methods, the analytical standards were prepared in the background electrolytes

157

used in experiments.


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The total nitrogen analysis was performed according to standard methods.38,39 For evaluating

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the adsorbed nitrogen species on the REMs after flow-through experiments, all REMs (pristine

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and used samples) were crushed using mortar and pestle and 0.1 g of the crushed REMs were

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analyzed for total N in triplicate samples. More details of the analytical methods are provided in

162

the SI.

163 164 165

2.8 Energy Calculations. Electrical energy per order metric (EOE) (kWh m3) were calculated using the following equation:

345 = 1089 x

.;

EF,HIJK

? ABC DE

L,HIJK

(3)

M

166

where Vcell is the cell potential (V), I is current (A), and Q is the volumetric permeate flow rate (m3

167

hr-1).

168

2.9 Quantum Mechanical Simulations. Density functional theory (DFT) simulations were

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performed using Gaussian 16 software.40 Unrestricted spin, all-electron calculations were

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performed using the 6-31G++(d) basis set for frequency and geometry optimizations and the 6-

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311G++(3df, 2p) basis set for energy calculations. The M06-2X hybrid meta exchange-correlation

172

functional was used for all calculations.41 Implicit water solvation was simulated using the SMD

173

model.42 Individual explicit water molecules were incorporated into simulations to accurately

174

simulate the effect of hydrogen bonding from the solvent at polar functional groups.43

175 176 177

The E0 values for a given direct electron transfer reaction were calculated by the following equation:

3* = −

∆N O P QR

* − 3STU (V13)

(4)

178

where DrG0 is the free energy for the reduction reaction, F is the Faraday constant, n is the number

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* of electrons transferred, and 3STU (V13) is a reference value for the absolute standard reduction

180

potential of the SHE ( = 4.28 eV).43,44 Calculations indicated that ∆1W ≈ 3 * , due to small entropic


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contributions to the Gibbs reaction energy. Therefore, the enthalpy of activation (∆1‡ ) as a

182

function of electrode potential for a direct electron transfer reduction reaction was determined

183

using a Marcus-type relationship, as follows:45

184

‡

∆1 =

YZ \1 [

+

^_.a(484 P ) b Y

c

(5)

185

where E is the applied electrode potential and lH is the enthalpic contribution to the total

186

reorganization energy of the reduction reaction. The effect of the aqueous electrolyte solution on

187

lH was not considered based on previous research that showed negligible effects in polar

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solvents.46 Marcus theory assumes that the reactant and product have similar potential energy

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shapes, which was checked by calculating the ratio of the reorganization energy for the forward

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and reverse reactions (L = lH,f/lH,r), which should be close to 1.0.

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3. Results and Discussion

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3.1 Physical Characterization. The XRD patterns of the three REMs are shown in Figure S-2.

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The formation of Ti4O7 was confirmed by identification of the characteristic peak at 20.8°.47 Peaks

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for other Magnéli phases were not observed in XRD patterns, indicating a high purity of Ti4O7 and

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that the carbonaceous materials did not oxidize nor reduce Ti4O7 during synthesis. The Ti4O7

196

crystallite domain size was calculated as 59 nm for Ti4O7, 50 nm for MWCNT-REM, and 53 nm

197

for PAC-REM using the Scherrer equation (SI, Table S-1). Details of the various crystal lattice

198

planes detected by XRD are shown in the SI (Figure S-3). The lattice parameters were obtained

199

using the XRD data and MDI JADE 9 software and were in accordance with a triclinic Ti4O7

200

structure (a = 5.6, b = 7.1, c = 12.4, α = 95.2, β = 95.3, and γ = 108.9).33,48

201

TGA analysis was performed on the REMs to determine the carbon decomposition temperature

202

and oxidation temperature of Ti4O7 in each sample. The TGA results are shown in the SI (Figure

203

S-4). For REM, an increase in weight of ~ 5% was observed in the temperature range of 420 to


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660 ºC, which was due to the oxidation of Ti4O7 to TiO2.33 For carbon-Ti4O7 composite REMs, a

205

weight loss associated with oxidation of the carbon content was observed in the temperature range

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of 550 to 650 ºC for PAC-REM and 650 to 720 ºC for MWCNT-REM.

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Raman spectroscopy analysis was performed to characterize the carbonaceous material in the

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REMs. The results are shown in Figure 1a, which contain the characteristic D and G peaks at

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Raman shift values of 1332 and 1567 cm-1 for MWCNT-REM and 1348 and 1580 cm-1 for PAC-

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REM, respectively. An additional 2D peak for MWCNT-REM was observed at 2673 cm-1. The

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D/G intensity ratios were 1.03 for MWCNT-REM and 1.01 for PAC-REM, which were similar to

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literature data for MWCNT and PAC samples, respectively.49–51 Additional analysis of a pure PAC

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pellet that was not exposed to heat treatment yielded a D/G intensity ratio value of 1.03, indicating

214

that the PAC material was not significantly altered during the REM synthesis process. Fabrication

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of a MWCNT pellet was not achieved, but the thermal stability of MWCNTs have been reported

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to be ~ 2100 oC,52 so it was assumed that it was stable during our synthesis method.

217

The average DI water permeability was measured as 806 ± 14 LMH bar-1 for REM, 589 ± 16

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LMH bar-1 for PAC-REM, and 290 ± 13 LMH bar-1for MWCNT-REM. The effective pore size (r)

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of each REM was measured using the Hagen-Poiseuille equation (SI, Figure S-5). The r values for

220

REM, PAC-REM, and MWCNT-REM were calculated as 0.35 ± 0.05 µm, 0.30 ± 0.04 µm, and

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0.20 ± 0.04 µm, respectively. These results indicated that the addition of the MWCNTs to the REM

222

significantly reduced the average pore size, which is likely a result of segregation of the MWCNTs

223

in the pore walls.

224

Conductivity measurements indicated that MWCNT-REM had the highest conductivity value

225

of 2991 ± 37 S m-1 and the PAC-REM and REM had values of 1832 ± 19 and 935 ± 14 S m-1,

226

respectively. The results showed that the addition of carbonaceous materials produced composite

227

REMs with high conductivity, where carbonaceous materials and Ti4O7 were in direct electrical


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contact. The specific surface area of the Ti4O7, MWCNTs/Ti4O7, and PAC/Ti4O7 powders were

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3.5, 25, and 43 m2 g-1, respectively. The average pore size of Ti4O7, PAC/Ti4O7, and

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MWCNT/Ti4O7 powders were 3.3, 3.7, and 4.4 nm, respectively, obtained from the Barrett, Joyner,

231

and Halenda (BJH) method.53 Pore volume values, determined by the same method, were 8.3 x 10-

232

3

233

powders.

mL g-1 for Ti4O7, 4.0 x 10-1 mL g-1 for PAC/Ti4O7, and 1.3 x 10-1 mL g-1 for MWCNT/Ti4O7

234

3.2 Adsorption Isotherms. The adsorption isotherms of the nitrosamines in the presence of

235

PAC/Ti4O7 and MWCNT/Ti4O7 powders exhibited non-linear sorption behavior, and the

236

Freundlich model was used to fit the sorption data:

237

e = fR ),Q

(6)

238

where q (mg/g) and Ce (mg/L) are the adsorbed and aqueous concentrations of nitrosamines at

239

equilibrium, respectively. The NDMA isotherm results, including Freundlich linearity coefficient

240

(n), Freundlich adsorption constant (KF), and R2 value, are shown in Figure 1b and other

241

nitrosamine isotherms are shown in the SI (Figure S-6 and Table S-2). In general, the nitrosamines

242

exhibited higher sorption capacities with PAC/Ti4O7 powder (NDMA: KF = 0.134 + 0.011)

243

followed by Ti4O7 (NDMA: KF = 0.026 + 0.005) and MWCNTs/Ti4O7 (NDMA: KF = 0.024 +

244

0.005) powders. The n values were all significantly less than 1.0, indicating heterogeneous

245

adsorption sites. NDMA is nonionic at this pH value, so these results were explained by the

246

hydrophobic interaction between the -CH3 groups of NDMA and the carbon materials. The

247

adsorption of NDMA to the Ti4O7 powder was attributed to the hydrophilic interaction between

248

the -N=O group of NDMA and the -OH groups of Ti4O7.

249

3.3 Flow-through Adsorption Experiments. Results for NDMA breakthrough curves for the

250

REMs for 10 and 150 µM NDMA feed concentrations are shown in Figure 1c and 1d, respectively.

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Specifically, PAC-REM increased the adsorption capacity of NDMA by 3.8-fold and the


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MWCNT-REM enhanced the adsorption capacity of NDMA by 5.4-fold relative to the REM when

253

10 µM NDMA feed solution was used. These results were determined by analyzing the number of

254

bed volumes for permeate NDMA concentration to reach 50% of the feed concentration. The

255

average number of bed volumes for 50% breakthrough of the replicate experiments were 597,

256

2323, and 3252 for REM, PAC-REM, and MWCNT-REM, respectively. Replicate experiments

257

are shown in the SI (Figure S-7). Comparing the NDMA breakthrough curves to those for chloride,

258

which was used as a conservative tracer (Figure 1c and Figure S-8), allowed for calculation of the

259

retardation factor for NDMA on the REM, PAC-REM, and MWCNT-REM, which were 3.6, 7.7,

260

and 11.5, respectively.

261

The results for the 150 µM NDMA feed solution are shown in Figure 1d, which indicated that

262

the permeate stream reached 50% of the feed concentration value for the REM, PAC-REM, and

263

MWCNT-REM at approximately 464, 1924, and 2290 bed volumes, respectively. Taken together,

264

these results indicated the PAC-REM increased the adsorption capacity of NDMA by 4.1-fold and

265

the MWCNT-REM increased the adsorption capacity of NDMA by 4.9-fold relative to the REM.

266

Retardation factors of 2.5 for REM, 6.4 for PAC-REM, and 8.1 for MWCNT-REM were

267

determined by comparison to chloride data.

268

Overall, the MWCNT-REM showed a higher adsorption capacity than the PAC-REM in the

269

flow-through experiments. These results are contradictory to results from adsorption studies from

270

batch reactors, which showed that the PAC/Ti4O7 powder had a much higher adsorption capacity

271

than the MWCNT/Ti4O7 powder (Figure 1b). The contradictory results are not related to faster

272

mass tranport, as an analysis of the adsorption kinetics (ka) of the breakthrough curves for both the

273

PAC-REM and MWCNT-REM showed the rate constant for NDMA adsorption was higher for

274

the PAC-REM (6.1 to 6.6 x 10-2 h-1) than the MWCNT-REM (0.8 to 1.9 x 10-3 h-1)) (Figure S-9).

275

It is therefore likely that the MWCNTs were more exposed in the pore walls of the MWCNT-


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276

REM, which allowed it to be more accesibile to the fluid stream then the carbon content in the

277

PAC-REM. This hypothesis is supported by the smaller pore size measured for the MWCNT-REM

278

relative to either the PAC-REM or REM.

279

3.4 Electrochemical NDMA Destruction. After obtaining the breakthrough curves for NDMA

280

on the REM materials, oxidation and reduction experiments were conducted in the flow-through

281

reactor with a feed concentration of 10 µM. Results for oxidation experiments at an anodic

282

potential of 2.7 V/SHE showed only partial removal of NDMA, with higher removal for the REM

283

(10% - 20%) compared to the PAC-REM (0 - 10%) (SI, Figure S-10). These results indicated that

284

the anodic potential was ineffective for NDMA oxidation, and the addition of PAC to the REM

285

inhibited NDMA oxidation, likely due to parasitic oxidation of the carbon material. The MWCNT-

286

REM was not tested due to these unfavorable results with the PAC-REM.

287

The results for NDMA reduction with 10 and 150 µM NDMA feed concentrations are shown

288

in Figure 2. The permeate pH was stable during all experiments (pH = 8.0

289

first saturated with NDMA under OCP conditions. After obtaining NDMA saturation, a -1.1

290

V/SHE cathodic potential was applied to begin the reduction experiments. For the REM, NDMA

291

reduction was 61 ± 2% (38 hr reduction experiment; n = 7; rate = 1.22 ± 0.02 mmoles m-2 h-1) for

292

the 10 µM feed solution and 40 ± 1% (37 hr reduction experiment; n = 8; rate = 12.0 ± 0.1 mmoles

293

m-2 h-1) for the 150 µM feed solution (Figure 2a). By contrast, the average NDMA removal for the

294

150 µM feed solution was 70 ± 1% (57 hr; n=15; rate = 21.0 ± 0. 2 mmoles m-2 h-1) for PAC-

295

REM (Figure 2b), and 82.5 ± 1% (51 hr; n=10; rate = 24.8 ± 0.3 mmoles m-2 h-1) for MWCNT-

296

REM (Figure 2c). For the 10 µM feed solution NDMA was below the HPLC method detection

297

limit (0.1 µM) the permeate (57 hr, n = 8) for either the PAC-REM or MWCNT REM (Figure 2b

298

and 2c). Supplementary analysis of select permeate samples using GC-MS showed permeate

299

NDMA concentrations between 59 to 89 ng L-1. These results corresponded to an approximate 4-


0.1). The REMs were

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300

log removal (99.99%; rate ~ 2.0 mmoles m-2 h-1) of NDMA, with permeate concentrations less

301

than the 300 ng L-1 response level for the state of California.2 These results indicated that the

302

MWCNT-REM and PAC-REM have the capability for effective removal of NDMA under

303

cathodic potential.

304

A total N balance is provided in Figure 2d for the reductionn experiments with the 150 µM

305

NDMA feed concentration. Analysis of the permeate during the experiments showed that DMA

306

and NO3- were produced at equal molar concentrations, indicating that cleavage of the N-N bond

307

occurred during NDMA reduction. Other products were not detected in the permeate solution, and

308

the total adsorbed N was measured as 0.42 ± 0.2% for REM, 10.2 ± 0.16% for MWCNT-REM,

309

and 20.7 ± 1.1% for PAC-REM of the total NDMA loading during the experiment. Control

310

experiments with pristine REM samples did not detect the presence of N. The total N mass

311

balances were 89.8 ± 3.2% for PAC–REM, 93.8 ± 1.1% for MWCNT-REM, and 96.5 ± 2.2% for

312

REM. The relatively high mass balances indicated that signficant concentrations of unidentified

313

products were not forming and the low residual adsorbed nitrogen concentrations indicated that

314

the simultaneous adsorption and reduction process was an effective treatment strategy for NDMA.

315

In order to assess the feasibility of utlilizing the carbon-Ti4O7 composite REMs for NDMA

316

removal in natural waters, additional experiments with a NDMA-spiked surface water sample were

317

conducted. The composition of the surface water is shown in the SI (Table S-3). Control

318

experiments with the NaH2BO3 buffer were conducted under indentical operating conditions as

319

the other experiments, but with at a flow rate of 100 LMH. The permeate concentrations for the

320

control experiment were 18, 24, and 25 ng L-1 for the MWCNT-REM and 22, 24, 28 ng L-1 for the

321

PAC-REM over three consecutive days of operation, and corresponded to an approximate 4.5-log

322

removal of NDMA. The permeate concentrations for the surface water sample experiment (6.7 nM

323

NDMA feed solution) were 28, 27, and 32 ng L-1 for the MWCNT-REM and 32, 16, 40 ng L-1 for


14

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324

the PAC-REM over the three-day experiment, and corresponded to an approximate 1.3-log

325

removal of NDMA. These permeate concentrations were approaching the 10 ng L-1 notification

326

level for the state of California.2

327

The experiments demonstrated that the carbon-Ti4O7 composite REMs could effectively

328

remove NDMA from water. The carbon-loaded REMs achieved up to a 4.5-log NDMA removal

329

for the 10 µM NDMA concentration in a buffered electrolyte and 1.3-log NDMA removal for the

330

6.7 nM concentration in the surface water sample. These results are contrasted to the carbon-free

331

Ti4O7 REMs, that showed NDMA reduction was 61 ± 2% for the 10 µM feed solution and 40 ±

332

1% for the 150 µM feed solution. The main mechanisms for NDMA removal were simultaneous

333

adsorption and electrochemical reduction on the conductive carbon-Ti4O7 composite REMs.

334

3.5 NDMA Reduction Mechanism. Past work has proposed two primary mechanisms for

335

NDMA reduction, which include direct electron transfer and catalytic H-atom transfer.14–16,54 Both

336

mechanisms are thought to involve cleavage of the N-N bond, yielding DMA and a secondary

337

nitrogen compound. The direct electron transfer mechanism has been reported to produce N2O and

338

N2 as the secondary nitrogen compound, while catalytic H-atom transfer produces primarily

339

NH4+.14,16,54

340

In our experimental work, we observed that NDMA reduction produced statistically identical

341

quantities of DMA and NO3-. Since NH4+ was not detected, DFT simulations were used to first

342

simulate the direct electron transfer reduction reaction for NDMA. The effect of adding explicit

343

water molecules to simulations to account for important hydrogen bonding sites was explored, and

344

results indicated that the addition of two water molecules could account for hydrogen bonding at

345

the -NO functional group of NDMA and still maintain the assumptions of Marcus theory. The

346

assumptions of Marcus theory assume similar potential energy surfaces for the reactant and

347

product, which requires L ~ 1.0. These conditions were approximately met for simulations without


15


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348

water molecules or with 2 water molecules [i.e., L = 1.22 (without water); L = 1.43 (1 water); L =

349

1.25 (2 waters); L = 1.77 (3 waters)]. The DFT geometrically optimized structures for the direct

350

reduction of [NDMA--2H2O] are shown in Figure 3a, and the ∆1‡ versus potential profiles

351

determined by equation (5) are shown in Figure 3b. The coordinates for the optimized structure

352

are provided in the SI (Table S-4). Results indicated that the N-N bond increases from a distance

353

of 1.28 Å for [NDMA--2H2O] to 1.42 Å for [NDMA--2H2O]- (Figure 3a). Values of Eo = -1.75

354

V/SHE, DHo = -1.77 V/SHE, and lH = 106 kJ mol-1 for NDMA and Eo = -1.48 V/SHE, DHo = -

355

1.44 V/SHE, and lH = 102 kJ mol-1 for [NDMA--2H2O] were determined by DFT, and indicated

356

that inclusion of the two explicit water molecules lowered the ∆1‡ for electron transfer (Figure

357

3b). In addition, experimentally determined ∆1‡ values for NDMA reduction, which are also

358

shown in Figure 3b, compared well with the [NDMA--2H2O] direct electron transfer DFT

359

simulations. For example, at -1.12 V/SHE the experiment value was ∆1‡ = 53.1 ± 3.7 kJ mol-1

360

and the theoretical value was ∆1‡ = 43.0 kJ mol-1, and at -1.47 V/SHE the experiment value was

361

∆1‡ = 17.3 ± 0.5 kJ mol-1 and the theoretical value was ∆1‡ = 23.9 kJ mol-1 (SI, Figures S-11 and

362

S-12). The close agreement between experimental and theoretical results suggested that the

363

probable rate limiting mechanism for NDMA reduction in our experiments was a direct electron

364

transfer reaction. Although the hydrogen atom transfer reaction may also be occurring, it is

365

unlikely to be the rate limiting mechanism for NDMA reduction, since the currents were similar

366

in the experiments conducted at the two potentials tested (e.g., 467 to 495 µA at 20 oC), and

367

therefore the adsorbed H coverage on the electrode should be similar at both potentials. The H

368

atom transfer reaction is not expected to be potential dependent over this potential range.


16

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369

DFT simulations were also conducted to determine a probable mechanism to explain the

370

experimentally determined reaction products. The following H+ transfer reaction was simulated

371

using DFT.

372

NDMA- + H3O+ + 2H2O à DMA + NO + 3H2O

(7)

373

Reaction (7) was thermodynamically feasible, yielding DrG0 = -111 kJ mol-1. Furthermore,

374

simulations indicated that reaction (7) was found to proceed without an activation barrier,

375

indicating it was a probable reaction occurring experimentally. However, the low concentration of

376

protons under the experimental conditions (pH = 8.0) indicates that the electrode surface may also

377

be a source of protons, as DFT simulations indicated that the transfer of H+ from water did not

378

occur. The DFT optimized structures for reaction (7) are shown in Figure 4 and show elongation

379

of the N-N bond, which initiates the production of DMA and NO. The coordinates for the

380

optimized structures are provided in the SI (Table S-4). Simulations were also conducted to

381

provide theoretical evidence for cleavage of the N-N bond. The reaction energy of reaction (7) was

382

considered for the case where the products were separated by an infinite distance, which yielded

383

DrG0 = -254 kJ mol-1. These results indicated that the structure in Figure 4b was a local energy

384

minimum and cleavage of the N-N bond to produce DMA and NO achieved a lower energy.

385

The ultimate formation of NO3- from NO can occur by several different pathways, including

386

reaction with O2(aq), reaction with electrochemically produced species at the cathode (e.g., O2-), or

387

oxidation at the anode. Divided cell batch experiments produced identical products to the flow-

388

through experiments (i.e., DMA and NO3-), ruling out oxidation at the anode as a mechanism for

389

NO3- formation (see SI, Table S-5). An additional set of duplicate experiments were conducted in

390

the divided cell reactor with an Ar gas purge to eliminate dissolved O2(aq). The reaction products

391

from these experiments consisted of near stoichiometric production of DMA and NH4+, where

392

NH4+ concentrations were 89 ± 5% of DMA concentrations (SI, Table S-5). These results


17


Page 18 of 31

393

indicated that O2(aq) or O2(aq) reduction products were involved in the reaction with NO to produce

394

NO3-.55

395

Based on the experimental and theoretical results, a probable reaction mechanism for the

396

electrochemical reduction of NDMA is shown in Scheme 1. The probable rate limiting mechanism

397

for NDMA reduction is a potential-dependent direct electron transfer reaction to form an anionic

398

NDMA- species (see Figure 3). This reaction is followed by an activationless H+ transfer reaction

399

from H3O+, that would be mass transport limited and results in cleavage of the N-N bond to form

400

NO and DMA. The NO product can react with O2 or electrochemically produced O2- to form NO3-

401

under aerobic condition or can undergo a series of H-atom transfer reactions at the cathode surface

402

to form NH4+ under anaerobic conditions. Although it is possible that a catalytic H-atom transfer

403

is also contributing to NDMA reduction, it is not thought to be the primary mechanism. In addition,

404

the NO3- was not electrochemically reduced on the Ti4O7 cathode, which was consistent with

405

previous work.36 The results of this study elucidated a more detailed electrochemical reaction

406

mechanism than previous work.

407

3.6 Technical and Environmental Signficance. The results from this study indicated that > 4-

408

log NDMA removal was achieved in a single pass (liquid residence time of 11 to 22 s) through the

409

PAC-REM and MWCNT-REM with the application of a -1.1 V/SHE cathodic potential. The

410

addition of carbonaceous materials allowed for increased residence times of NDMA in the reactor

411

by a factor of 3.8 to 5.4 and therefore enabled electrochemical reduction to be a feasible approach

412

for NDMA destruction. The reduction products of DMA and NO3-/NH4+ do not pose a health risk

413

at these low levels.56–58 Prior studies required the addition of chemicals (e.g., UV/iodide, H2O2,

414

H2) or precious metal catalysts (e.g., Pd, Pt) for efficient NDMA removal, and signficant removal

415

(e.g., > 99%) was only achieved in batch reactors with reaction times on the order of 10s of minutes

416

to several hours.14,19,20,59 Electrochemical oxidation with the PAC-REM was shown to not be an


18

Page 19 of 31


417

effective remediation method due to slow reaction kinetics and parasitic oxidation of the carbon

418

content.

419

The energy consumption for the electrochemical REM treatment process was calculated (EEO)

420

using equation (3). For the 10 µM NDMA concentration EEO values were 0.47 ± 0.03 kWh m-3 for

421

REM, and decreased to 0.086 ± 0.035 kWh m-3 for PAC-REM, and 0.12 ± 0.03 kWh m-3 for the

422

MWCNT-REM. For the 150 µM NDMA concentration EEO values were 0.54 ± 0.02, 0.63 ± 0.02,

423

and 0.58 ± 0.02 kWh m-3 for REM, PAC-REM, and MWCNT-REM, respectively. For the surface

424

water sample with 6.7 nM NDMA concentration, EEO values were 0.31 ± 0.10 kWh m-3 for PAC-

425

REM and 0.43 ± 0.03 kWh m-3 MWCNT-REM. Comparing these EEO values with other

426

technologies for NDMA removal, suggest that our results for adsorption/reduction of NDMA using

427

the carbon composite Ti4O7 REMs is an energy efficient treatment method and the cost of adding

428

carbon material is low (e.g., PAC ~ $0.10 g-1 for kg-scale quantities). The EEO for NDMA removal

429

was reported between 0.5 and 0.9 kWh m-3 for ozonation and 0.3 to 1.62 kWh m-3 for UV/H2O2

430

oxidation,60 and approximately 2.0 kWh m-3 for RO.61 The low energy consumption, efficient

431

performance, and selective transformation of contaminants achieved by the carbon-Ti4O7

432

composite REMs make them a promising treatment technology for water treatment. In addition,

433

utilizing the REMs as cathodes avoids the production of halogenated organic compounds

434

associated with electrochemical oxidation and advanced oxidation processes.

435

Supporting Information

436

Additional experiment setup details, analytical methods to analyze NDMA samples, XRD and

437

TGA analyses, pore size measurement, additional isotherms for other nitrosamines, detailed

438

breakthrough curve data, kinetics analysis, oxidation results, surface water analysis, detailed data

439

for calculating activation energy, and additional batch experimental results.

440

Corresponding Author


19


Page 20 of 31

441

*E-mail: [email protected]

442

Acknowledgements

443 444 445 446 447

Funding for this work was provided by the Water Innovation Network for Sustainable Small Systems (WINSSS) to W.X and B.P.C. and a National Science Foundation CAREER award to B.P.C. (CBET-1453081). The authors thank Dr. Sangil Kim for TGA measurements, and Dr. Vikas Berry and Dr. Sanjay Behura for Raman spectroscopy analysis.


20

Page 21 of 31

448 449

450 451 452 453 454 455 456 457 458 459


Figures and Tables

Figure 1. a) Raman spectroscopy results for REM, PAC-REM, and MWCNT-REM. The highlighted area represents the standard regions for D and G peaks. b) Adsorption isotherms for NDMA with Ti4O7, PAC/Ti4O7, and MWCNTs/Ti4O7 powders. Breakthrough adsorption curves for REMs in 10 mM NaH2BO3 background electrolyte with initial feed concentration of c) 10 µM NDMA and d) 150 µM NDMA. Flow-through adsorption experiments were performed in duplicate, and results shown here are the average values from the duplicate experiments (see SI for individual data sets).


21


460 461 462 463 464 465 466 467 468

Page 22 of 31

Figure 2. Adsorption and electrochemical reduction results at -1.1 V/SHE for NDMA (10 and 150 µM) in 10 mM NaH2BO3. a) REM, b) PAC-REM, and c) MWCNT-REM. Each experiment contains average of two OCP monitoring for adsorption followed by cathodic reduction. d) Nitrogen balance for NDMA reduction experiment with PAC-REM, MWCNT-REM, and REM The initial concentration was 150 µM NDMA and 10 mM NaH2BO3.


22

Page 23 of 31


a)

b)

469 470 471 472 473 474 475 476 477

Figure 3. a) DFT geometrically optimized structures for the direct reduction of [NDMA--2H2O] and [NDMA--2H2O]-. b) Enthalpy of activation (∆1‡ ) versus potential profile for the direct reduction of NDMA. Solid black line is the result for the direct reduction of [NDMA--2H2O], and dotted red line is the result for the direct reduction of NDMA. Black squares represent experimentally measured ∆1‡ values. Error bars represent 95% confidence intervals. Atom key: carbon = grey; hydrogen = white; oxygen = red; nitrogen = blue


23


a) Reactants: NDMA- + 2H2O + H3O+

Page 24 of 31

b) Products: DMA + NO + 3H2O

1.97 Å 1.43 Å 1.54 Å

478 479 480 481 482

Figure 4. DFT geometrically optimized structures for the reactants and products for the addition of a proton to NDMA- (reaction (7)). The overall reaction energy was -111 kJ mol-1 and the reaction was activationless. Atom key: carbon = grey; hydrogen = white; oxygen = red; nitrogen = blue


24

Page 25 of 31


+e-

+H+

Aerobic Conditions +O2 or O2-

Anaerobic Conditions +Had

483 484 485

Scheme 1. Proposed pathway for electrochemical NDMA reduction under both aerobic and anaerobic conditions. Atom key: carbon = grey; hydrogen = white; oxygen = red; nitrogen = blue


25


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