Mechanism of p Substituted Phenol Oxidation at a Ti O ......REM technology, this study investigated the oxidation of phenolic compounds (i.e., p-nitrophenol, p-methoxyphenol, and p-benzoquinone)

Mechanism of p‑Substituted Phenol Oxidation at a Ti4O7 ReactiveElectrochemical MembraneAmr M. Zaky‡ and Brian P. Chaplin*,†

†Department of Chemical Engineering, University of Illinois at Chicago, 810 South Clinton Avenue, Chicago, Illinois 60607, UnitedStates‡Department of Civil and Environmental Engineering and Villanova Center for the Advancement of Sustainable Engineering,Villanova University, Villanova, Pennsylvania 19085, United States

*S Supporting Information

ABSTRACT: This research investigated the removal mechanisms of p-nitrophenol, p-methoxyphenol, and p-benzoquinone at a porous Ti4O7 reactiveelectrochemical membrane (REM) under anodic polarization. Cross-flowfiltration experiments and density functional theory (DFT) calculations indicatedthat p-benzoquinone removal was primarily due to reaction with electrochemi-cally formed OH•, while the dominant removal mechanism of p-nitrophenol andp-methoxyphenol was a function of the anodic potential. At low anodic potentials(1.7−1.8 V/SHE), p-nitrophenol and p-methoxyphenol were removed primarilyby an electrochemical adsorption/polymerization mechanism on the REM.Increasing anodic potentials (1.9−3.2 V/SHE) resulted in the electroassistedadsorption mechanism contributing far less to p-methoxyphenol removalcompared to p-nitrophenol. DFT calculations indicated that an increase inanodic potential resulted in a shift in p-methoxyphenol removal from a 1e− directelectron transfer (DET) reaction that resulted in radical formation and significant adsorption/polymerization, to a 2e− DETreaction that formed nonadsorbing products (i.e., p-benzoquinone). However, the anodic potentials were too low for the 2e−

DET reaction to be thermodynamically favorable for p-nitrophenol. The decreased COD adsorption for p-nitrophenol at higheranodic potentials was attributed to reaction of soluble/adsorbed organics with OH•. These results provide the first mechanisticexplanation for p-substituted phenolic compound removal during advanced electrochemical oxidation processes.

■ INTRODUCTION

Electrochemical advanced oxidation processes (EAOPs) aremethods that generate hydroxyl radicals (OH•) via wateroxidation at an anode surface. Facilitated by the development ofstable anode materials, EAOPs are being increasingly used forwater treatment applications. Numerous studies have shownthe ability of EAOPs to degrade various recalcitrant organiccompounds by a combination of OH•-mediated oxidation anddirect electron transfer (DET) reactions at the anodesurface.1−17

EAOPs primarily use flat plate electrodes operated in flow-bymode. This flow configuration results in a large hydrodynamicdiffuse boundary layer (∼100 μm), which prevents rapidreaction rates of contaminants due to diffusional limitations.Therefore, EAOPs operate at relatively low applied currentdensities (e.g., <5 mA cm−2), and therefore require numerousflat plate electrodes configured in parallel to obtain an adequatespecific surface area for contaminant removal. In response tothis limitation, research is focused on the development of novelporous three-dimensional electrodes that are operated in flow-through mode, which has resulted in significant advection-enhanced mass-transport rates of contaminants relative to flow-by or batch operation.16,18

Recent work has shown that the use of porous substoichi-metric TiO2 (Ti4O7) anodes in flow-through filtration modecreates a reactive electrochemical membrane (REM), whichcombines microfiltration with electrochemical oxidation.16 Themicrometer-sized pores of the REM produced a highelectroactive surface area and advection-enhanced mass transferrates approximately 10-fold higher than those obtained intraditional flow-by mode.16 The water treatment potential ofthe REM is promising, as a high removal (i.e., 99.9%) of p-methoxyphenol, a toxic phenolic compound present inindustrial effluents,19 was achieved by electrochemical adsorp-tion and oxidation by OH•.16 However, a detailed under-standing of these removal processes is still lacking, and furtherresearch is needed to thoroughly understand the proposedremoval mechanisms.Numerous studies have focused on electrochemical oxidation

of substituted phenols at EAOP electrodes, includingchlorinated phenols,8,9,20−31 p-substituted phenols,16,32−35 andmultisubstituted phenols.29,36 These compounds are present in

Received: March 1, 2014Revised: April 24, 2014Accepted: April 27, 2014Published: April 28, 2014

Article

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© 2014 American Chemical Society 5857 dx.doi.org/10.1021/es5010472 | Environ. Sci. Technol. 2014, 48, 5857−5867

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the effluents of numerous industrial processes, including oilrefineries, dyes and textiles, pharmaceuticals, pulp and paper,plastics, and detergents; and conventional biological treatmentis not suitable for the removal of these toxic compounds.19

Phenols are an important class of compounds that require anefficient and effective treatment strategy.To facilitate further understanding and development of the

REM technology, this study investigated the oxidation ofphenolic compounds (i.e., p-nitrophenol, p-methoxyphenol,and p-benzoquinone) using a porous, tubular Ti4O7 REMoperated in filtration mode. These compounds were chosenbecause of their environmental relevance and unique electro-chemical properties. The latter allowed them to be used asmodel contaminants to investigate the effect of potential on theprimary removal mechanism (i.e, adsorption versus oxida-tion).16 A specific focus of this work was to determine themechanisms associated with their removal, which wasaccomplished through a combination of linear scan voltamme-try, electrochemical oxidation experiments, and density func-tional theory (DFT) modeling. The data were interpreted toprovide a more detailed understanding of the removalmechanisms of phenolic compounds in the novel REM reactor.

■ MATERIALS AND METHODSReagents. Chemicals were reagent-grade and obtained from

Fisher Scientific and Sigma-Aldrich, and were used withoutadditional purification. Solutions were made from Milli-Qultrapure water (18.2 MΩ cm at 21 °C).Reactive Electrochemical Membrane. The REM was a

20 cm long Ebonex one-channel tubular electrode, with a 28mm outer diameter and 20 mm inner diameter (VectorCorrosion Technologies, Inc.). Ebonex is a Magneli phasesuboxide of TiO2, which consists primarily of Ti4O7.

37 Mercuryporosimetry characterization of the Ti4O7 electrode determineda bimodal pore size distribution; pores <10 nm accounted for>90% of the surface area and pores between 1 and 2 μmcontributed to nearly the entire pore volume.16 The Ti4O7electrode had a median pore diameter of 1.7 μm, porosity of31%, specific surface area of 2.78 m2 g−1, and a roughness factorof 619.16

Cross-Flow Filtration Setup. The cross-flow filtration unithas been reported elsewhere,16 and is described in theSupporting Information (SI) (Figure S-1). The setup containsan Ebonex REM as anode and 3.18 mm diameter 316 stainlesssteel rod as cathode. The solution was pumped through thecenter of the REM at a constant flow rate (Q = 36 L h−1) andback-pressure (10 psi). Experiments were conducted at aconstant current density (0−3.5 mA cm−2) using a directcurrent (DC) power supply (Proteck P6035). Potentials weremeasured versus a Ag/AgCl reference electrode (WarnerInstruments LLC) using a Gamry Reference 600 potentio-stat/galvanostat. All potentials were reported versus thestandard hydrogen electrode (SHE).Electrochemical Experiments. Oxidation experiments

were performed in cross-flow filtration mode in the reactordescribed above and at a temperature of 21 ± 2 °C. Organiccompounds (p-methoxyphenol (p-MP), p-nitrophenol (p-NP),and p-benzoquinone (p-BQ)) were individually added at 1 mMconcentration to a 10 mM NaClO4 supporting electrolyte. TheNaClO4 electrolyte was chosen because it is nonreactive underanodic and cathodic conditions.38,39 Duplicate experimentswere performed with 100% recycle of both the feed andpermeate solutions to the feed container. Electrode potentials

and permeate fluxes were monitored at regular time intervals,and feed and permeate samples were collected and analyzed fororganic compound concentration and chemical oxygen demand(COD). After each experiment, the REM was cleaned toremove and quantify adsorbed compounds, as describedelsewhere.16 The data for oxidation of p-MP at currentdensities between 0 and 1.0 mA cm−2 were adapted withpermission from Zaky and Chaplin.16

Linear scan voltammetry (LSV) experiments were conductedusing the tubular REM immersed in a 100 mL 1 M NaClO4background electrolyte in batch mode at T = 20 °C and a scanrate of 5 mV s−1. Organic compounds were added to thebackground electrolyte in order to determine if DET reactionswere occurring at the REM surface.

Analytical Methods. Flux measurements were madevolumetrically. Concentrations of p-MP, p-NP, and p-BQwere measured using HPLC with a C18 column andphotodiode array detector (wavelength = 314 nm) (ShimadzuSPD-M20A). COD concentrations were determined by Hachmethod 8000.

Kinetic Analysis. On the basis of prior work, the masstransfer rate constant (km) was set equal to the permeate flux(J) in units of L m−2 h−1.16 The units of km are converted to mh−1 by multiplying J by 1000 m3 L−1. The normalized rateconstants for parent compound removal (kN,p‑X) and CODremoval (kN,COD) were determined by regression of the feedconcentration (CF,i) versus time profile, where i representseither parent compound or COD. The percent removal in thepermeate (RP,i) was calculated as the difference between CF,iand the permeate concentration (CP,i), according to eq 1.

=−

×RC C

C

( )100i

i i

iP,

F, P,

F, (1)

The initial normalized removal rates of COD from thepermeate (rN,COD) were calculated by regression of theCF,COD versus time data, divided by the specific surface areaof the REM (30.3 L m−2). The percent of COD oxidized in agiven experiment (%CODox,f) was determined by a massbalance of COD concentration in the reactor according to eq 2.

= − −%COD 100 %COD %CODox,f w,f ad,f (2)

where %CODw,f and %CODad,f are the final percentages ofCOD remaining in solution and adsorbed to the REM at theend the experiment, respectively. The current efficiency forCOD removal (CECOD (%)) was calculated by eq 3.

=Δ

×FV

jA tCE

[COD ]

8100COD

ox,f

(3)

where CODox,f is the concentration of COD oxidized for agiven experiment (g L−1), V is the solution volume (L), Δt isthe time of the experiment (s), j is the current density, A is thesurface area, F is the Faraday constant (96485 C mol−1), and 8is a dimensional conversion factor.

Quantum Mechanical Simulations. Density functionaltheory (DFT) simulations were performed to investigatepossible reaction mechanisms of p-substituted phenols. DFTcalculations were performed using Gaussian 09 software.40

Unrestricted spin, all-electron calculations were performedusing 6-31G(d) and 6-311G(3df,pd) basis sets for geometryoptimization and energy calculations, respectively, and thegradient corrected Becke, three-parameter, Lee−Yang−Parr(B3LYP) functional for exchange and correlation. Implicit

Environmental Science & Technology Article

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water solvation was incorporated using the polarizablecontinuum model.Activation energies (Ea) for DET as a function of electrode

potential were calculated by the method of Anderson andKang.41 Reactant energies were calculated as a function of thereaction coordinate, defined as the incremental change ingeometry the reactant experiences upon electron transfer.Product energies were calculated using the atomic positions ofthe optimized reactant structures, followed by self-consistentfield optimization of the electronic configurations. Calculatedenergies were plotted versus the COH bond length forgraphical display of the data. The energy of the free electron onthe vacuum scale was adjusted to the SHE scale by subtracting4.6 eV.41 Product energies were a function of the electrodepotential, where a 1.0 V increase in electrode potential shiftedthe energy profile of the product downward by 96.5 kJ mol−1

(i.e., 1.0 eV). Intersection of the product and reactant energyprofiles yields the transition state structure and Ea at a givenelectrode potential. The identical geometries of the reactantand product are justified by the Born−Oppenheimerapproximation that states that electron transfer occurs muchmore rapidly than changes in atomic configuration.42 Tosimulate sequential electron transfer, the geometric optimized+1 radical structure generated by the first electron transfer wasused as the reactant for the second electron transfer. Thisapproach was justified by the fact that the redox potential of thesecond electron transfer was much more anodic than that of thefirst electron transfer. Therefore, it was assumed that sufficienttime existed for geometric relaxation of the +1 species prior tothe second DET, as bond lengths and angles did not change bymore than ±3.5% from initial values. Previous work indicatesthat errors in the calculation of the reaction energies by DFTmethods may be up to 16 kJ mol−1.43 In order to identify theatoms in a molecule most likely to react with attackingelectrophiles (e.g., OH•) or nucleophiles (e.g., H2O),condensed Fukui functions were calculated for individualatoms using Hirshfeld atomic charges determined frompopulation analysis data.44,45

■ RESULTS AND DISCUSSIONLSV Results. Results from LSV experiments with and

without p-NP and p-MP are shown in Figure 1. During the firstLSV scan containing 25 mM p-NP, an increased current isobserved at potentials >1.75 V relative to the blank electrolytesolution, and a distinct current peak is observed at 1.88 V(Figure 1a). During the second LSV scan, the peak isattenuated, and during the third scan, a peak is no longerobserved (Figure 1a). This behavior is attributed to electro-chemical adsorption of p-NP, which prevents access of aqueousp-NP to the electrode.29,46,47 The LSV scans with 25 mM p-MPshowed increased current relative to the blank electrolyte atpotentials >0.8 V and a current peak at ∼2.0 V (Figure 1b).Successive scans were also performed with 25 mM p-MP(results not shown), but decreased current was not observed,indicating that adsorption was minimal. LSV experiments werealso conducted with p-BQ, but current peaks were not observed(results not shown).Oxidation Experiments. Various studies have investigated

the oxidation of phenolic compounds at EAOP anodes (e.g.,boron-doped diamond (BDD), doped-SnO2, PbO2, andTi4O7).

1,8,9,16,20−31,36 These studies suggest three oxidationpathways, as shown in Scheme 1. The pathways consist of thefollowing: (A) 1e− DET reaction resulting in polymer

formation, which is predominant at low anodic potentials;(B) 2e− DET reaction forming p-BQ, which occurs at higheranodic potentials; and (C) OH•-mediated pathway resulting incleavage of the phenolic ring, which occurs at potentials ≥2.0 V.Oxidation experiments were conducted to gain information

about the predominant removal mechanism of each compound.Experiments were conducted with 100% recycle of both thefeed and permeate, and the results are summarized in Table 1.Comparing normalized rate constants for p-MP (kN,p‑MP) and p-NP (kN,p‑NP) with measured mass transfer rate constants (km)indicates that the removal of both compounds was mass-transport controlled at all current densities tested (Table 1).Rate constants for p-BQ were not obtained because the REMwas configured as an undivided cell, and thus p-BQ was easilyreduced to hydroquinone on the cathode. Prior work with theREM tested in this study showed rapid oxidation of p-BQ in adivided cell batch reactor, and was taken as evidence for OH•

production.16

Average values for compound removal in the permeate (RP,i)were calculated by eq 1. Values for RP,p‑MP increased from 92.9± 6.8 to 99.9 ± 6.7% at current densities between 0.2 and 1.0mA cm−2, respectively, and decreased to 59.1 ± 6.4% at anapplied current density of 3.5 mA cm−2 (Table 1). Values forRP,p‑NP were consistently high at all current densities tested, andvaried between 96.9 ± 2.6 and 98.5 ± 1.7% (Table 1). Removalof neither compound was observed without an applied current,indicating that adsorption was negligible under open circuitconditions.

Figure 1. Linear scan voltammetry for (a) p-NP and (b) p-MP at aTi4O7 anode in a 1 M NaClO4 background electrolyte at T = 20 °Cand a scan rate of 5 mV s−1.



Profiles of the COD concentration versus time at a currentdensity of 0.2 mA cm−2 are shown in Figure 2 for the threecompounds tested and a summary of all experiments isprovided in Table 1. Representative concentrations in both thefeed (CF,COD) and permeate (CP,COD) are shown, and thedifference between the two concentrations at a given timerepresents the COD removed upon a single pass through theREM pores. Individual RP,COD values are provided in Table 1.The dashed lines in Figure 2a−c represent the theoreticalmaximum COD removal at 100% anodic current efficiency.The solid red lines in Figure 2a−c represent the maximumCOD removal under mass-transport control, which wasdetermined by the average km value in each experiment.Each compound tested showed a different trend with regard

to COD removal. The removal of both p-NP and p-MP wasmass-transport limited, and therefore COD removal could onlyreach the mass-transfer limit (solid red line) if completeadsorption and/or complete oxidation to CO2 were occurring.At lower current densities, COD removal was often greaterthan the current-limited rate (dashed line), indicating thatcompound adsorption was a dominant removal mechanism(Pathway A, Scheme 1). This scenario was the case for p-NP atcurrent densities between 0.2 and 1.0 mA cm−2, whereadsorption was a significant removal mechanism (Figure 2aand SI Figure S-2). Figure 2d contains a plot of kN,COD valuesnormalized by km (kN,COD/km). At the lowest current density(0.2 mA cm−2), the kN,COD/km value for p-NP was only slightlylower than 1.0 at the 95% confidence interval, indicating that p-NP was removed almost entirely by an electrochemicaladsorption mechanism (Pathway A, Scheme 1). As currentdensity increased, kN,COD/km values for p-NP were much lessthan unity, indicating that the higher anodic potential facilitatedother pathways shown in Scheme 1, which will be discussed inthe DFT Modeling section.Similar trends were observed for p-MP, where the measured

COD removal was greater than the current-limited rates at lowcurrent densities (≤0.5 mA cm−2) (Figure 2b and SI Figure S-3). However, unlike p-NP, kN,COD/km values for p-MP werebetween 0.31 ± 0.02 and 0.44 ± 0.08 (Figure 2d), indicatingthat both Pathways A and B were likely active at the lowercurrent densities tested. At higher current densities, thegeneration of OH• would allow Pathway C to contribute top-MP oxidation and also the oxidation of adsorbed organics, as

was discussed in our prior study.16 The validity of thismechanism will be tested in the DFT Modeling section.The trends for COD removal during p-BQ oxidation differ

from the other organic compounds. The oxidation of p-BQ hasbeen reported to occur via oxidation with OH•,48 and is highlyresistant to DET reactions.15 The COD profiles in Figure 2csupport this mechanism, and show that the p-BQ oxidationkinetics are zeroth order and not significantly different from thekinetic limited oxidation rate (dashed line). Zeroth-orderkinetics for COD removal continues up to current densitiesof 1.0 mA cm−2. The potentials of these experiments were 1.8−2.4 V/SHE, which are in the range associated with the onset ofOH• production.49 Due to the rapid reaction between OH• andp-BQ, OH• production is likely shifted toward lower potentialsand therefore enhances substrate oxidation.50 The rN,COD valuesdetermined for p-BQ were normalized to the theoretical currentcontrolled oxidation rate, assuming 100% current efficiency(rC,COD). The corresponding rN,COD/rC,COD values are plotted inFigure 2d, and are not significantly different from 1.0 forcurrent densities ≤1.0 mA cm−2. Of the organic compoundstested, only p-BQ showed an increasing removal rate uponincreasing current densities, which is consistent with a reactionthat is dependent on OH• concentrations. The COD removalat longer reaction times during the p-BQ experiment conductedat 1.0 mA cm−2 deviates from zeroth-order behavior, as removalis likely affected by oxidation products or mass transfer (SIFigure S-4). At a current density of 3.5 mA cm−2, COD profilesare first-order, suggesting that COD oxidation may be limitedby mass-transport control, the blocking of active sites bykinetically slow reacting intermediates, establishment of asteady-state OH• surface concentration,51 or the blocking ofelectrode sites by O2 bubble formation (SI Figures S-4). ThekN,COD/km value for p-BQ at a 3.5 mA cm−2 current density was0.47 ± 0.01 and was not plotted in Figure 2d due to dataoverlap.The percentage of organic compounds adsorbed to the REM

after each experiment (%CODad,f) showed different trends withrespect to the applied current density for each compound(Table 1). During the oxidation of p-MP, it was observed thatCOD adsorption steadily decreased upon increasing currentdensity, from 47.4 and 44.7% (duplicate experiments) of theinitial COD concentration at a 0.2 mA cm−2 current density, to4.11 and 3.17% (duplicate experiments) of the initial CODconcentration at a 3.5 mA cm−2 current density. The COD

Scheme 1. Proposed Reaction Pathways for p-Substituted Phenol Electrochemical Oxidation



Table

1.Summaryof

theExperim

entalResults

Obtaineddu

ring

theOxidation

ofp-SubstitutedPheno

ls

p-X

COD

compound

(p-X)

currentdensitya

(mA/cm

2 )

measured

anode

potential

(Vvs

SHE)

masstransfer

rate

constat

(km)d

(Lm

−2h−

1 )

norm

alized

rate

constant

(kN,p‑X)

(Lm

−2h−

1 )

removalin

perm

eate

(RP)

(%)

p-X

remaininge

(%)

norm

alized

rate

constant

(kN,COD)

(Lm

−2h−

1 )

initialrate

(rN,COD)

(mgm

−2h−

1 )

removalin

perm

eate

(RP)

(%)

COD

remaininge

(%)

adsorbed

CODe,f

(%)

COD

oxidatione

,g

(%)

current

efficiency

(CE C

OD)e,h(%

)

p-MPc

0.00

0.73b

51.1±

1.9

00

100,100

00

100,100

0,0

0,0

0.20

1.67

45.8±

3.8

42.6±

5.7

92.9±

6.8

3.71,1

0.8

14.4±

0.91

30.1±

3.1

39.8,4

1.2

47.4,4

4.7

12.8,1

4.1

86.8,8

8.8

0.50

1.88

37.2±

2.7

39.5±

1.0

99.9±

0.12

6.98,0

.00

14.4±

1.5

24.5±

9.8

41.3,3

3.5

22.2,2

6.4

36.5,4

0.1

99.0,1

091.00

2.13

43.8±

3.7

47.7±

3.3

99.9±

0.17

5.04,3

.05

14.7±

0.71

25.6±

3.5

39.9,3

4.7

6.05,5

.67

54.1,5

9.6

73.3,8

0.8

3.50

3.12

39.4±

6.4

44.7±

3.0

59.1±

6.4

16.0,9

.06

17.2±

3.3

24.7±

3.8

54.7,3

1.6

4.11,3

.17

41.2,6

5.2

25.9,3

9.3

p-NP

0.00

0.51b

36.0±

1.2

00

100,100

00

100,100

0,0

0,0

0.20

1.83

39.8±

2.5

37.3±

2.8

96.9±

2.6

2.81,9

.94

33.8±

5.4

84.3±

4.4

15.0,1

6.2

81.5,7

1.3

3.5,

12.5

18.6,6

6.5

0.50

1.93

49.6±

5.6

50.4±

4.9

97.9±

1.1

2.76,5

.70

38.4±

4.0

48.3±

149.55,5

.14

73.6,7

0.1

16.8,2

4.7

33.7,4

2.3

1.00

2.27

48.7±

5.2

46.9±

2.0

98.2±

0.73

4.87,2

.86

29.6±

5.1

46.3±

1217.9,8

.64

65.0,6

9.1

17.1,2

2.3

18.3,2

7.3

3.50

3.25

36.1±

9.8

40.6±

3.0

98.5±

1.7

12.1,1

7.3

25.1±

2.4

76.5±

3.9

39.4,2

4.4

42.0,4

5.4

18.6,3

0.2

8.92,1

1.8

p-BQ

0.00

0.77b

40.1±

3.0

00

0100,100

0,0

0,0

0.20

1.81

52.6±

160.21

±0.040

14.7±

4.0

72.4,6

9.5

3.72,6

.98

23.9,2

3.5

109,

113

0.50

1.91

38.7±

3.7

0.40

±0.038

20.4±

3.7

51.9,5

4.2

6.05,7

.20

42.1,3

8.6

76.8,7

8.2

1.00

2.37

38.2±

4.1

0.78

±0.068

25.2±

3.8

44.6,4

5.1

3.81,5

.12

51.6,4

9.8

47.9,5

1.7

3.50

2.92

38.6±

4.6

18.0±

0.57

17.1±

3.0

39.9,3

9.1

4.75,6

.48

55.4,5

4.4

25.7,2

5.2

aCurrent

density

calculated

usingnominalsurfacearea

ofinnerREM

wall.bOpencircuitpotential.c D

ataforcurrentdensities

between0.0and1.0reprinted(adapted)with

perm

ission

from

ref16.

dDetermined

bymeasuredmem

braneflux

(J).e Finalconcentrationatendof

theexperim

ent(valuesforduplicateexperim

ents).f Finaladsorbed

massremaining

atendof

experim

ent(valuesforduplicate

experim

ents).gCalculatedby

(COD

oxidation=100%

−%

COD

remaining

−%

adsorbed

COD).hCum

ulativecurrenteffi

ciency

forexperim

ent(valuesforduplicateexperim

ents).



adsorption followed a similar trend for p-NP, but values weremuch higher than those for p-MP (Table 1). During theoxidation of p-BQ, it was observed that COD adsorption wasminimal and approximately the same for all oxidationexperiments (3.72−7.20% of initial COD concentration). Theadsorption results suggest that the oxidation mechanisms of thethree organic compounds differ, which will be furtherinvestigated using DFT modeling.The percent COD oxidation values reported in Table 1 are

plotted versus the measured anodic potential in order to moreeasily observe trends in the data (SI Figure S-5). As shown in SIFigure S-5, plateaus in the percent COD oxidation uponincreasing anodic potentials were observed for all threecompounds. This trend is indicative of a process that is limitedby mass transport. The plateaus for COD oxidation occur atsimilar values for p-NP and p-BQ, as both of these compoundsshowed negligible adsorption at anodic potentials greater than2.0 V (Table 1). However, the much lower plateau value forCOD oxidation observed for p-NP is likely due to the presenceof adsorbed polymer on the REM surface, which is supportedby the adsorbed COD values reported in Table 1. Theadsorbed polymer likely blocked reactive sites for wateroxidation to OH• and therefore inhibited COD removal.Data from duplicate experiments showed the largest variabilityfor experiments whose measured anodic potentials were greaterthan 3.0 V (i.e., p-MP and p-NP at 3.5 mA cm−2), which maybe attributed to O2 bubble formation within the REM pores,which blocked reaction sites to different extents in the duplicateexperiments.DFT Modeling. DFT modeling was used to interpret the

experimental results and determine a mechanism for theoxidation of the p-substituted phenolic compounds. Specifically,DFT modeling was used to understand why p-MP and p-NP

apparently reacted by different mechanisms during oxidationexperiments. Simulations were conducted to determine an Eaprofile as a function of anodic potential for the 1e− directoxidation of p-MP (Pathway A, Scheme 1), as shown inreaction 4a. The subsequent deprotonation step is shown in

reaction 4b.The energy profiles of the reactants and products atan anodic potential of 0.85 V are shown in Figure 3a, and the Eaprofile as a function of anodic potential is shown in Figure 3b.The Ea at 0.85 V was 13.0 kJ mol−1, and corresponded to thepotential at which current began to flow in response to theaddition of p-MP in LSV experiments (Figure 1b). The Eaprofile shown in Figure 3b indicates that reaction 4a wasactivationless at potentials ≥1.16 V. This potential is lower thananodic potentials measured during oxidation experiments,indicating that Pathway A is feasible during all experiments.The shaded region in Figure 3b indicates the anodic potentialrange during oxidation experiments.The phenoxy radicals formed in reaction 4b are known to

polymerize.17 Therefore, it was of interest to calculate thepotential at which the 2e− DET reaction shown in Pathway Bwas feasible (Scheme 1). Model simulations were performed to

Figure 2. Profiles of COD concentration versus time where solid squares represent the feed concentration and hollow squares represent thepermeate concentration for (a) p-NP, (b) p-MP, and (c) p-BQ. The dashed lines represent current controlled removal at 100% Faradaic currentefficiency determined by SI eq S-1. The solid red lines represent the mass-transfer controlled rate determined by SI eq S-2. Solid lines through feeddata represent a regression line. Panel (d) shows the ratio of kN,COD/km on the left axis for p-NP and p-MP, and rN,COD/rC,COD on the right axis for p-BQ.



determine the Ea profile as a function of anodic potential for thedirect oxidation of p-MP•+, as shown in reaction 5a. The

subsequent deprotonation step is shown in reaction 5b. Asshown in Figure 3b, reaction 5a has an Ea = 32.0 kJ mol−1 at ananodic potential of 2.11 V, and the reaction was activationless atpotentials ≥2.72 V. The Ea calculated at 2.11 V is low enoughthat oxidation could occur at room temperature, and thepotential is close to that of the oxidation peak that wasobserved at 2.0 V in LSV experiments containing p-MP (Figure1b). Two separate current peaks for reaction 4a and reaction 5awere not observed in LSV experiments containing p-MP, likelybecause either current generated from the two reactionsoccurred at overlapping potentials and/or dispersion in theanodic current was caused by ohmic drop with depth into theporous REM. Anodic potentials in p-MP oxidation experimentsincreased from 1.7 to 3.12 V at a current density between 0.2and 3.5 mA cm−2. Therefore, DFT results suggest that PathwayB was an increasingly favorable oxidation mechanism atincreasing current densities. This hypothesis is supported bythe observed trend of decreasing compound adsorption at

increasing current densities during p-MP oxidation experiments(Table 1). The phenoxonium ion product formed in reaction5b (p-MP+) is rapidly converted to p-BQ, as shown in reaction6. The increasing formation of p-BQ upon increasing current

density has been previously confirmed experimentally for theoxidation of p-MP in the REM system,16 providing furtherevidence to support DFT modeling results.Reactions 4a and 5a were also simulated for p-NP oxidation.

Energy profiles for reaction 4a are displayed in Figure 3c at anelectrode potential of 2.02 V and Ea values as a function ofanodic potential are shown in Figure 3d. Modeling resultsdetermined Ea = 67.0 kJ mol−1 at 1.76 V for p-NP undergoingreaction 4a, and the reaction was activationless at potentials≥2.35 V. These results compared well with the anodic peakcurrent observed at 1.88 V in LSV experiments (Figure 1a).Anodic potentials during p-NP oxidation experiments werebetween 1.83 and 3.25 V, indicating that reaction 4a wasfeasible. The Ea profile was also calculated for p-NP•+

undergoing reaction 5a, and it was found that Ea = 43.8 kJmol−1 at 3.32 V and was activationless at potentials ≥4.18 V. Asshown in Figure 3d and Table 1, the highest anodic potentialduring p-NP oxidation experiments was 3.25 V, indicating thatreaction 5a was likely not a significant oxidation pathway. Theseresults support the experimental findings that suggested p-NPunderwent electrochemical adsorption (Pathway A) to a greaterextent than p-MP. Reaction 5a was thermodynamicallyfavorable for p-MP, but not for p-NP. The p-NP• formed via

Figure 3. (a) Energy profiles as a function of COH bond length at an electrode potential of 0.85 V/SHE for the reactant (p-MP) and product (p-MP•+ + e−) shown in rxn. 4a. (b) Activation energy calculations for p-MP as a function of electrode potential for reactions shown in rxn. 4a and rxn.5a. (c) Energy profiles as a function of COH bond length at an electrode potential of 2.02 V/SHE for the reactant (p-NP) and product (p-NP•+ +e−) shown in rxn. 4a. (d) Activation energy calculations for p-NP as a function of electrode potential for reactions shown in rxn. 4a and rxn. 5a.Shaded boxes show the range of anodic potentials measured in the experiments.



reactions 4a,b underwent adsorption/polymerization at theREM surface and was retained by the micrometer-sized pores.Theoretical calculations support experimental results that didnot detect the formation of p-BQ during p-NP oxidation. Thelower %CODad,f value observed for p-NP oxidation experimentsat higher current densities suggests that OH• formation wasresponsible for the oxidation of the adsorbed organiccompounds and not a shift to Pathway B.The oxidation of p-NP at other EAOP anodes (i.e., Sb-doped

SnO2 and Bi-doped PbO2) is consistent with the mechanismfound above, where p-BQ was not detected during p-NPoxidation,8,52 but was observed during p-MP oxidation.52

However, studies have shown that the oxidation of p-NP atBDD anodes formed p-BQ, and two separate oxidation peakswere detected in cyclic voltammetry scans,32 suggesting thatPathway B was active during p-NP oxidation at BDD anodes.The second oxidation peak was observed at ∼2.2−2.6 V/SHE,32,33 which is significantly less than potentials calculated inour study for p-NP oxidation by Pathway B (Scheme 1). Theseresults indicate that the BDD surface may catalyze p-NPoxidation. Although prior studies have not investigated thishypothesis, the surficial C atoms of (111) diamond are phenolicin shape and internuclear distances (1.544 Å) are similar toCC bonds in phenolic compounds,53 suggesting catalyticeffects are possible.The direct oxidation of p-BQ was also investigated for

reaction 7. Results determined that p-BQ underwent an

activationless DET reaction at potentials ≥3.11 V, whichsupports LSV experiments that did not observe direct oxidationin the presence of p-BQ, and previous studies that report p-BQis highly resistant to direct oxidation.15 At anodic potentials of∼3.0 V, significant water oxidation occurs, which makes itdifficult to observe oxidation peaks during LSV scans. AlthoughDFT simulations suggest that direct oxidation of p-BQ waspossible at the highest current density used experimentally (3.5mA cm−2 and 2.9 V), at this potential a significant quantity ofOH• is formed, and therefore p-BQ likely reacted with OH•

instead of through the DET pathway. The measured second-order rate constant for the reaction between p-BQ and OH• is1.2 × 109 M−1 s−1,48 which is in the diffusion-limited range.DFT modeling results confirm that Pathway C is the dominantreaction pathway for p-BQ, as previously suggested.15

The geometry optimized structures for p-MP, p-NP, andtheir oxidation products are shown in Figure 4. Also included inFigure 4 are atomic charges of the C atoms in the phenol ring.Upon the removal of successive electrons from thesecompounds, the charges on the C1 and C4 atoms increasefor both p-MP and p-NP. The largest changes in electrondensity for p-MP occur on the C1 and C4 atoms, and thelargest change in electron density for p-NP occurs on the C4atom.Fukui functions were calculated for the C atoms in the

phenol ring (SI Table S-1), and it was determined for p-MPthat electrophilic attack by OH• was most likely at the C1 andC4 atoms. Although the charges of the other C atoms are moreelectronegative than the C1 and C4 atoms, these atoms hold

their electrons more tightly and are not expected to be sites forOH• attack. Upon removing an electron from p-MP to form p-MP• (reaction 4a,b), the reactive sites change. Fukui functioncalculations indicate that the C2, C6, and C4 atoms are themost likely sites for electrophilic attack by OH•. Uponremoving an additional electron to form p-MP+ (reaction5a,b), the reactive sites change again. Fukui functioncalculations indicate that the C4 atom is the most likely sitefor nucleophilic attack, which supports reaction Pathway Bshown in Scheme 1, and indicates that nucleophilic attack bywater at the C4 atom of p-MP+ is the likely mechanism for p-BQ formation. The C3 and C5 atoms of p-MP+ were shown tobe the most likely sites for electrophilic attack by OH•.Fukui function calculations for p-NP indicate that the C4 and

C6 atoms are the most likely sites for electrophilic attack byOH•. Similarly, a previous study concluded that the C2, C4,and C6 atoms were the likely sites for electrophilic attack basedon calculated C atom charges.54 Upon removing an electronfrom p-NP to form p-NP• (reaction 4a,b), calculated Fukuifunction values indicate that the C2, C4, and C6 atoms are themost likely sites for electrophilic attack. Upon removing anadditional electron to form p-NP+ (reaction 5a,b), Fukuifunction calculations indicate that the C4 atom is the mostlikely site for nucleophilic attack, and electrophilic attack wasnot observed to be favorable at the phenol ring for thiscompound. Results shown in Figure 3 suggest that high anodicpotentials are necessary to form p-NP+, suggesting it was notpresent at significant concentrations in our experiments.However, other studies that oxidized p-NP at BDD electrodesdetected p-BQ as an intermediate, and therefore these resultsindicate that Pathway B (Scheme 1) is feasible for p-NP if theanodic potential is sufficient to allow the 2e− DET reactions.The results presented in this study provide conclusive

experimental and DFT modeling evidence that the dominantmechanism for p-substituted phenolic compound removal at aTi4O7 anode is a function of both the electrode potential andthe substituent type. Electron donating substituents (e.g., OCH3 groups) increase the electron density of the phenolicring, and allow DET reactions to proceed at lower anodicpotentials relative to p-substituted phenolic compounds withelectron withdrawing substituents (e.g.,NO2). Therefore, theanodic potential at which the mechanism for p-substitutedphenolic compound removal switches from the 1e− polymer-ization mechanism to the 2e− oxidation mechanism isdetermined by the electronegativity of the substituent.The results of this study can be used to develop a technology

for the electrochemical adsorption of phenolic compounds,which would mimic packed bed electrode columns that weredeveloped for heavy-metal removal.55 Such a strategy wouldutilize a downstream cathode and, due to ohmic drop withdepth into the electrode, would create an increasing anodicpotential in the direction of flow.55 Therefore, mixtures ofphenolic compounds would be polymerized at different depthswithin the anode pores. Either offline electrochemical treatmentor chemical/electrochemical desorption methods could beexplored as potential regeneration methods. For this strategy tobe a viable remediation technique, the adsorption capacity andenergy consumption of the anode must be determined andcompared to existing adsorption technologies (e.g., activatedcarbon).



■ ASSOCIATED CONTENT*S Supporting InformationExperimental setup, REM filtration experimental results, andFukui Function values. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 312-996-0288; fax: 312-996-0808; e-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFunding for this work was provided by the National ScienceFoundation (CBET-1159764). We thank Mr. Thomas Hoff-man and Mr. Kai Ding for assistance in filtration experiments.

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Figure 4. Geometry optimized structures for (a) p-MP, (b) p-MP•, (c) p-MP+, (d) p-NP, (e) p-NP•, and (f) p-NP+. Hirshfeld atomic charges of Catoms in the phenol ring are shown, and arrows indicate preferred sites for electrophilic attack by OH• and nucleophilic attack by water. Atom key: C= gray; O = red; and H = white.



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Mechanism of p Substituted Phenol Oxidation at a Ti O ......REM technology, this study investigated the oxidation of phenolic compounds (i.e., p-nitrophenol, p-methoxyphenol, and p-benzoquinone)