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Chemical Oxidation of Dissolved Organic Matter by Chlorine
Dioxide,Chlorine, And Ozone: Eects on Its Optical and
AntioxidantPropertiesJannis Wenk,,, Michael Aeschbacher, Elisabeth
Salhi, Silvio Canonica, Urs von Gunten,,,,*and Michael
Sander,*Eawag, Swiss Federal Institute of Aquatic Science and
Technology CH-8600, Dubendorf, SwitzerlandInstitute of
Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092,
Zurich, SwitzerlandSchool of Architecture, Civil and Environmental
Engineering (ENAC), Ecole Polytechnique Federale de Lausanne
(EPFL),CH-1015 Lausanne, Switzerland
*S Supporting Information
ABSTRACT: In water treatment dissolved organic matter(DOM) is
typically the major sink for chemical oxidants. Theresulting
changes in DOM, such as its optical properties havebeen measured to
follow the oxidation processes. However,such measurements contain
only limited information on thechanges in the oxidation states of
and the reactive moieties inthe DOM. In this study, we used
mediated electrochemicaloxidation to quantify changes in the
electron donatingcapacities (EDCs), and hence the redox states, of
threedierent types of DOM during oxidation with chlorine
dioxide(ClO2), chlorine (as HOCl/OCl
), and ozone (O3). Treat-ment with ClO2 and HOCl resulted in
comparable andprominent decreases in EDCs, while the UV light
absorbancesof the DOM decreased only slightly. Conversely,
ozonation resulted in only small decreases of the EDCs but
pronouncedabsorbance losses of the DOM. These results suggest that
ClO2 and HOCl primarily reacted as oxidants by accepting
electronsfrom electron-rich phenolic and hydroquinone moieties in
the DOM, while O3 reacted via electrophilic addition to
aromaticmoieties, followed by ring cleavage. This study highlights
the potential of combined EDC-UV measurements to monitor
chemicaloxidation of DOM, to assess the nature of the reactive
moieties and to study the underlying reaction pathways.
INTRODUCTIONDrinking water and wastewater treatment facilities
often have achemical oxidation step for disinfection, the removal
of organicmicropollutants, color removal, and taste and odor
control.Among the most commonly used oxidants are chlorine
dioxide(ClO2), chlorine (as hypochlorous acid, HOCl and OCl
), andozone (O3).
1 For a number of reasons, the eciency of theoxidation step and
the quality of the treated water largelydepend on the reaction of
the chemical oxidant with dissolvedorganic matter (DOM). First, DOM
is a major contributor todrinking water color, which negatively
aects the acceptance ofthe water among consumers.2 Second, the
reaction of DOMwith the chemical oxidants accelerates their
consumption and,thus, may reduce the eciency of the oxidation step
fordisinfection and micropollutant oxidation.3,4 Third, the
reactionof the oxidants with DOM may result in the formation
ofpotentially harmful disinfection/oxidation byproducts.57
Fourth, chemical DOM oxidation results in the generation oflow
molecular weight assimilable organic carbon (AOC).8,9
Following the oxidation step, the AOC needs to be removed
bybiological ltration to improve the biological stability of
drinking waters.4,10,11 For these reasons, information on theDOM
concentration and its reactivity is indispensable to ndthe
appropriate dose of an oxidant to meet the variousrequirements of
oxidative water treatment processes and toavoid underperformance,
higher costs, and undesired byproductformation during the oxidation
step.As a consequence, there is considerable interest in simple
and
readily measurable parameters that provide information on
theconcentration and reactivity of the DOM in the water.12,13
Twocommonly measured parameters are the dissolved organiccarbon
(DOC) content, which captures the concentration ofDOM, and the
specic UV absorbance of the water at thewavelength of 254 nm
(SUVA254, expressed in L mgC
1 m1),which is a proxy for DOM aromaticity.14 Previous work
showedthat both the consumption of chemical oxidants by DOM andthe
occurrence of some disinfection/oxidation byproducts are
Received: June 7, 2013Revised: August 22, 2013Accepted: August
26, 2013Published: August 26, 2013
Article
pubs.acs.org/est
2013 American Chemical Society 11147
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positively correlated to SUVA254.1519 These correlations
suggest activated aromatic moieties as major
oxidizablefunctional groups in DOM, consistent with the high
reactivityof low-molecular weight activated aromatic moieties,
includingphenols, methoxybenzenes, and anilines, with ClO2,
chlorine,and O3.
2029 However, despite the positive correlations withchemical
oxidant consumption, SUVA254 alone was found to bea relatively poor
predictor of DOM reactivity and disinfectionbyproduct formation
with chlorine.14,30 Other methods thathave been used to determine
the concentration and reactivity ofoxidizable moieties in DOM are
dicult to adapt for routinewater analysis or provide only indirect
information on the redoxstates of DOM.3137 Therefore, an analytical
method isdesirable that allows for a direct quantication of changes
inDOM oxidation states caused by reaction with
chemicaloxidants.38
Mediated electrochemical oxidation (MEO), an analyticaltechnique
recently developed in our research group, fulllsthese requirements.
MEO allows for a fast and reliablequantication of the electron
donating capacities (EDCs)(i.e., the number of electrons that are
donated by a givenamount of DOM) of dilute DOM samples in
electrochemicalcells with well-controlled pH and Eh conditions.
39,40 Wepreviously demonstrated that MEO quanties
activatedphenolic moieties in DOM: EDC values of a set of
chemicallydiverse humic substances (HS) were positively correlated
withtheir titrated phenol contents and showed dependencies on Ehand
pH comparable to those of low molecular weight phenolsand
hydroquinones.40 We expect that chemical oxidants oxidizethese
activated phenolic moieties in DOM, resulting indecreasing EDC
values of the DOM during treatment. MEOmay therefore be a powerful
technique to quantify DOMreactivity with chemical oxidants and to
directly monitorchanges in DOM oxidation states during chemical
oxidation inwater treatment.The goal of this study was to explore
the potential of
combined MEO and UVvisible absorbance measurements toselectively
quantify the oxidation states of DOM duringchemical oxidation and
to elucidate the underlying oxidant-dependent reaction pathways. We
measured the UVvisabsorbance spectra and the EDC values of three HS
(SuwanneeRiver Humic and Fulvic Acids (SRHA and SRFA) and PonyLake
Fulvic Acid (PLFA)) during dose-dependent treatmentwith ClO2,
chlorine, and O3. HS, in general, make up the majorfraction of DOM.
We specically chose SRHA, SRFA, andPLFA because these materials are
commercially available andhave been used in previous oxidation
studies,18,41 and their keyphysicochemical properties are known.
Furthermore, SRHA/FA and PLFA represent allochthonous and
autochthonousaquatic HS with terrestrial higher plant-derived and
withmicrobially derived precursor materials, respectively. This
studyaddresses fundamental questions on the changes in
DOMantioxidant properties and reactivities during
chemicaloxidation, and, in the Implication section, highlights
thepotential of combining MEO and SUVA254 measurements tomonitor
chemical oxidant demand in water treatment facilities.
MATERIALS AND METHODSChemicals. All chemicals were from
commercial sources
and used as received: tert-butanol (t-BuOH) (99.7%),
2,2-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) diammoniumsalt
(ABTS) (>99%), potassium peroxodisulfate (99%),sodium chlorite
(NaClO2) (puriss. p.a. 80%), sodium chlorate
(NaClO3) (99%), ortho-phosphoric acid (85%), sodiumdihydrogen
phosphate dihydrate (99%), disodium hydrogenphosphate dodecahydrate
(98.0%) and hypochlorite solution614% were from Sigma-Aldrich,
sodium dihydrogen phos-phate monohydrate (99102%) was from
Merck.Humic Substances. Suwannee River Humic Acid Standard
II (SRHA; catalogue number: 2S101H), Suwannee River FulvicAcid
Standard II (SRFA; 2S101F), and Pony Lake Fulvic AcidReference
(PLFA; 1R109F) were obtained from the Interna-tional Humic
Substances Society (IHSS, St. Paul, MN) andused as received.
Selected physicochemical properties of theHS, including elemental
compositions, aromaticities andtitrated phenol contents, are
provided in Table S1 in theSupporting Information (SI).Preparation
of Aqueous Solutions. Aqueous solutions
were prepared using deionized water either from
Milli-Q(Millipore) or Barnsteadt water purication systems. HS
stocksolutions (100 mg C L1) were prepared in 5 mM phosphatebuer
(pH 8) or in deionized water. The DOC of the HS stocksolutions was
determined after 25-fold dilution on a ShimadzuV-CPH TOC analyzer
(Kyoto, Japan) and used to calculatespecic UV absorbance values and
carbon-normalized EDCvalues.Chlorine dioxide (ClO2) stock solutions
(10 mM) were
produced by mixing potassium peroxodisulfate (K2S2O8, 2 g in50
mL water) with sodium chlorite (NaClO2, 4 g in 50 mL).
42
The stock solution of chlorine (Cl2; 10 mM) was prepared
bydiluting a sodium hypochlorite solution with water. Ozone(O3)
stock solutions (1.3 to 1.5 mM) were prepared bysparging ozone gas
through water cooled in an ice bath.43 TheO3 gas was formed from
pure oxygen with an Apaco CMG 33ozone generator (Grellingen,
Switzerland). The exact concen-trations of oxidants in the stock
solutions were quantiedspectrophotometrically using molar
absorption coecients of = 1200 M1 cm1 at = 359 nm for ClO2,
44 = 350 M1 cm1
at = 290 nm for chlorine (as ClO),45 and = 3000 M1
cm1 at = 258 nm for ozone.46
ClO2, Chlorine, and O3 Oxidation of DOM. Oxidationexperiments
were carried out in a series of identical glassreaction vessels (50
or 100 mL) (Schott, Germany). Thevessels contained either DOM
solutions (nominal concen-trations of 0.83 mmol C L1 (=10 mg C L1)
after reagentmixing) or DOM-free blank solutions at pH 7 (50
mMphosphate buer). Oxidant stock solutions were added to thevessels
under vigorous mixing on a magnetic stirrer plate. Theemployed
oxidant doses were in the range of 00.36 mmolClO2/mmol C, 00.85
mmol chlorine/mmol C, and 01.12mmol O3/mmol C, which cover the
ranges commonly used forwater treatment.28,47,48 Ozonation
experiments were performedin the presence (5 mM) and absence of
t-BuOH as a scavengerfor formed hydroxyl radicals (OH). After
oxidant addition, thevessels were closed, removed from the stirrer
and stored at 22C for 12h for chlorine dioxide, 3 days for
chlorine, and 2 h forozone. Subsequently, unreacted ClO2 and O3
were removedfrom the solution by gently purging with helium for 20
min. Inselected experiments, residual chlorine (max. 0.5 M)
wasmeasured using the DPD colorimetric method.42 The ozoneexposure
in the DOM-containing systems in the presence andabsence of t-BuOH
was measured according to previouslydescribed methods.49,50 Control
experiments in which t-BuOHwas added to the solutions after
depletion of ozone showed thatt-BuOH did not aect UVvisible
absorption and EDCmeasurements.
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UV/Visible Light Absorbance Measurements. Absorb-ance spectra of
untreated and oxidized HS were collected onUvikon 940 (Kontron
Instruments) or Varian Cary 100(Agilent Technologies)
spectrophotometers in quartz glasscuvettes (Hellma) (10 or 100 mm
path lengths). All samplespectra were corrected for the spectrum of
the HS-freephosphate buer (pH 7). The carbon-specic
absorptioncoecients of untreated and treated HS, a() [L/(mg
Cm)],were calculated according to eq 1, where A() is the
sampleabsorbance at a given wavelength , b [m] is the path
length,and CHS [mg C/L] is the organic carbon concentration of
theuntreated HS.
=
aAb C
( )( )
HS (1)
The values a(254 nm) and a(280 nm) are referred to asSUVA254 and
SUVA280, respectively. The spectral slopecoecients of the HS
absorbance spectra, S [1/nm], wereobtained by nonlinear
least-squares tting of DOM absorptiondata from = 300 to 600 nm with
a single exponential decayfunction,51 where a(ref) is the specic
absorption coecient atthe reference wavelength of ref = 350 nm.
52
= a a S( ) ( ) exp[ ( )]ref ref (2)
The parameter S describes the steepness of DOM absorbancespectra
on a logarithmic scale: The relative decrease inabsorbance with
increasing wavelength becomes steeper as Sincreases. Changes in the
spectral slopes were also determinedover narrower wavelength ranges
(i.e., from 275 to 295 nm,S275295, and from 350 to 385 nm, S350385)
following theapproach suggested by Helms and co-workers.53 Data
ttingand integrations were performed using Origin 8.0
software(OriginLab).Quantication of Electron Donating Capacities.
EDC
values of untreated and oxidant-treated HS solutions
werequantied by MEO using
2,2-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as
electron transfer mediator.39,40
MEO measurements were conducted in an electrochemical
cellcontaining a reticulated vitreous carbon working electrode(WE),
a Pt counter electrode, and an Ag/AgCl referenceelectrode. The
electrochemical cells were rst lled with 6065mL of buer solution
(0.1 M KCl, 0.1 M phosphate, pH 7) andthe WE was polarized to an
oxidizing potential of Eh = +0.725V vs the standard hydrogen
electrode (SHE), controlled by apotentiostat (either an Autolab
PG302 (EcoChemie B.V.) or a630C instrument (CH Instruments)). A
volume of 2 mL of anaqueous ABTS solution (5 mM) was added to the
cell, resultingin an oxidative current peak due to the oxidation of
ABTS to itsradical cation ABTS+ (standard reduction potentialEh
0(ABTS+/ABTS) = 0.68 V vs SHE54). Upon attainment ofredox
equilibrium between ABTS+/ABTS and the WE (andhence stable current
readings), HS samples (57 mL) weresuccessively spiked to the cell.
Oxidation of electron donatingmoieties in the added HS by ABTS+
resulted in the formationof reduced ABTS, which was subsequently
reoxidized at theWE to ABTS+ to re-establish redox equilibrium. The
resultingoxidative current peak was integrated to yield the EDC
valuesof the added HS:
=
t
mEDC
dIF
HS (Eq. 3)
where I [A] is the baseline-corrected current and F (=96485
sA/mole) is the Faraday constant, and mHS [mgC or mmolC ] isthe
mass/amount of HS analyzed. Most HS samples wereanalyzed in
triplicates and some in duplicates with t = 50 minbetween replicate
analysis to ensure baseline-separation ofindividual current
peaks.
RESULTS AND DISCUSSIONEects of Oxidant Treatments on DOM
Optical
Properties. The specic absorption coecients of theuntreated
samples decreased in the order SRHA>SRFA>PLFAover the entire
measured wavelength range from 220 to 600 nm(SI Figures S1 and S2).
The trend in the absorption coecientsfollows the decrease in HS
aromaticity55 from 31% for SRHAto 22% for SRFA and 12% for PLFA (SI
Table S1).56 Theabsorbance spectrum of untreated SRHA extended
further intothe red than the spectra of both SRFA and PLFA, which
isreected by the smaller S values for SRHA than for SRFA andPLFA.
Longer wavelength absorbance of HS has been ascribedto charge
transfer complexes between electron donor andacceptor pairs in
HS,55,57,58 which may be more abundant inHA than FA.Treatment of
the HS with all oxidants resulted in decreasing
specic absorption coecients at all collected wavelengths(Figure
1ac) and increasing S values (Figure 1df) withincreasing oxidant
doses, consistent with previous re-ports.8,37,59,60 The absorbance
spectra, the dierential spectra,and the spectral slopes S275295 and
S350385 of untreated andoxidant-treated HS are shown in SI Figures
S1S5. Overall, thedecreases in the specic absorption coecients
suggest adecrease in aromaticity of the treated HS. The increase in
Svalues with increasing oxidant dose indicates that
moieties/complexes absorbing at longer wavelengths were
preferentiallyremoved and/or transformed into shorter
wavelength-absorb-ing components. The increase in S and S275295
values withincreasing oxidant doses may also reect decreases in
theaverage molecular weights of the DOM upon reaction with
thechemical oxidants, as detailed in the SI. Consistent
withprevious observations,30,61 the dierential spectra for
HOCltreated HS show a local maximum in absorption loss at
around270272 nm, indicating a selective removal of chromophoresin
this wavelength region by reaction with chlorine under
theassumption that no new chromophores are formed. A similarmaximum
loss in absorbance around 270 nm was also observedfor ClO2-treated
PLFA. This feature was absent from thedierential spectra of
ClO2-treated SRHA and SRFA as well asof the O3-treated SRHA, SRFA,
and PLFA both in absence andpresence of t-BuOH.A detailed analysis
of the absorbance and dierential
absorbance spectra revealed that ClO2 and HOCl treatmentshad
dierent eects on DOM optical properties than the O3treatments. The
SUVA254 and SUVA280 values of all three HSdecreased linearly with
increasing doses of ClO2 and HOCl andfollowed similar
dose-dependencies for the two oxidants(Figure 1a-c and SI Figure
S6, respectively). ClO2 and HOCltreatment of SRHA and SRFA also
resulted in comparableincreases in S with increasing oxidant doses,
whereas PLFAshowed larger increases in S upon treatment with ClO2
thanHOCl at the same specic molar oxidant doses (Figure 1df).In
comparison to the ClO2 and HOCl treatments, ozonationresulted in
much larger decreases in the specic absorptioncoecients of the HS,
both in the absence and presence of t-BuOH (Figure 1ac and SI
Figure S6df). The larger
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decreases in SUVA254 and SUVA280 upon treatment with O3than ClO2
and HOCl at the same specic molar oxidant dosesdemonstrates that
UV-light absorbing aromatic moieties in theHS were more eciently
removed (or transformed to lesseciently absorbing moieties) by O3
than by both ClO2 andHOCl. Note that narrower dose ranges were used
for ClO2 andHOCl than for O3 based on the eects of the three
oxidants onthe antioxidant properties of the HS, as detailed
below.Ozonation in the presence of t-BuOH resulted in larger
losses in HS absorbance at wavelengths >315 nm and
largerincreases in S than in the absence of t-BuOH. These eects of
t-BuOH can be ascribed to two factors. First, t-BuOH scavengesOH
which are formed by DOMozone reactions and whichcatalytically
degrade O3.
16,62 Quenching of OH by t-BuOHtherefore enhanced O3 lifetimes
and, hence, resulted in higherO3 exposures of the HS. Enhanced O3
exposure in the presencecompared to the absence of t-BuOH was
veried exper-imentally with PLFA solutions (see SI Figures S7, S8).
Second,by scavenging OH, t-BuOH shifted the overall
oxidationpathway from unselective, diusion-controlled OH
additions,H abstraction (and electron transfer reactions),62 to
moreselective, direct reactions of O3 with moieties such as
olens,activated aromatics, and amines in the DOM.62 The presence
oft-BuOH therefore enhanced O3-reaction induced cleavage oflight
absorbing olenic and aromatic systems,63 resulting inlarger changes
in HS optical properties than in the absence of t-BuOH. The
abatement of SUVA254 by ozone (Figure 1ac)was more ecient for SRHA
in presence than in absence of t-BuOH, whereas the opposite trend
was observed for specicozone doses >0.2 (mmolozone (mmolc)
1) for SRFA and PLFA.
This can be explained by the higher content of
ozone-reactivechromophores in SRHA than in SRFA and PLFA. For
SRFAand PLFA, these moieties are depleted for specic ozone
doses>0.2 (mmolozone (mmolc)
1) in the presence of t-BuOH, andozone-resistant
SUVA254-contributing chromophores can thenonly be further oxidized
by OH, which are present duringozonation in absence of t-BuOH.Eects
of Oxidant Treatments on DOM Antioxidant
Properties. In a rst set of experiments we evaluated
thesensitivity of MEO to detect oxidant-induced changes in theEDCs
of HS by quantifying the kinetics of PLFA oxidation byO3 at a
constant initial dose of 0.5 mmol O3/ mmol C. Wechose PLFA because
it has the lowest EDC values of severalDOMs previously tested.40
Figure 2a shows the evolution ofthe oxidative current responses in
MEO for PLFA samples afterreaction with O3 for various reaction
times. The correspondingEDC values, obtained by integration of the
oxidative currentpeaks (Eq. 3), show fast oxidation of the electron
donatingmoieties in PLFA by O3 (Figure 2b): Within one minute and12
minutes of reaction, the EDC of PLFA decreased toapproximately 50%
and 15% of its original value, respectively.The results of this
experiment demonstrate the suitability ofMEO to quantify changes in
the oxidation states of HS duringtreatments with chemical oxidants.
Based on the reactionkinetics, the dose-dependent ozonation
experiments were runfor 2 h to guarantee completion of HS-O3
reactions.Figure 3 shows that the EDCs of SRHA, SRFA, and PLFA
decreased with increasing doses of ClO2, HOCl and O3 (bothin the
presence and absence of t-BuOH) and, hence, dose-dependent removal
of electron donating moieties in the HS for
Figure 1. Changes in the optical properties of Suwannee River
Humic Acid (SRHA), Suwannee River Fulvic Acid (SRFA), and Pony Lake
FulvicAcid (PLFA) upon treatment with chlorine dioxide (ClO2),
chlorine (as HOCl), and ozone (O3) (both in the absence and
presence of t-BuOH).Panels (a)-(c): Changes in the specic UV
absorption at 254 nm (i.e., SUVA254) of (a) SRHA, (b) SRFA, and (c)
PLFA as a function of the specicmolar oxidant dose (mmoloxidant
(mmolc)
1). Panels (d)(f): Changes in the spectral slope S (from 300 to
600 nm) of (d) SRHA, (e) SRFA, and (f)PLFA as a function of the
specic molar oxidant dose.
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all three oxidants. Normalized to the same specic molaroxidant
dose, the decreases in EDC were largest for ClO2,intermediate for
HOCl and O3 in the presence of t-BuOH, andsmallest for O3 in the
absence of t-BuOH: Linear ts of thedecreases in EDC values of SRHA
and SRFA at low specicmolar oxidant doses had the steepest slopes
for ClO2 (i.e.,0.69 and 0.46 mmole(mmol ClO2)1), intermediate
slopesfor HOCl (i.e., 0.38 and 0.36 mmole(mmol HOCl)1) andfor O3 in
the presence of t-BuOH (i.e., 0.35 and 0.29mmole(mmol O3)
1), and the shallowest slopes for O3 in theabsence of t-BuOH
(i.e., 0.15 and 0.08 mmole(mmolO3)
1) (SI Figure S9, Table S2)). The PLFA data did not showa linear
decrease in EDC with increasing specic molar oxidantdose and could
therefore not be tted.Treatments with high doses of ClO2 and HOCl
resulted in
complete loss of EDC in some of the systems, including
areplicate SRFA-ClO2 experiment (SI Figure S10), whereas allHS
retained some EDC during ozonation even at the highestO3 doses. The
removal of electron-donating moieties in thetested HS was therefore
more ecient by ClO2 and HOCltreatments than by ozonation. The
larger decreases in EDCs byO3 in the presence than in the absence
of t-BuOH can beassigned to OH quenching by t-BuOH and hence higher
O3exposures of HS and more selective oxidations of electrondonating
moieties by O3.Mechanistic Interpretation. In the following, the
changes
in the optical and the antioxidant properties of the HS will
befurther explored by plotting the oxidant-induced decreases inthe
SUVA254 values versus the corresponding decreases in theEDC values
(Figure 4). Treatments of the HS with ClO2 andHOCl resulted in
comparable SUVA254-EDC dependencies forthese two oxidants with
larger relative decreases in the EDCthan in the SUVA254 values.
This nding implies a moreecient removal of electron donating
phenolic moieties thanUV-light absorbing aromatic moieties upon
treatment of theHS with ClO2 and HOCl. Compared to the ClO2 and
HOCltreatments, ozonation in the presence and absence of t-BuOHled
to distinctly dierent SUVA254-EDC dependencies withlarger relative
losses in the SUVA254 than in the EDC values.Ozonation therefore
caused a more ecient removal of UV-light absorbing aromatic
moieties than electron donatingphenolic moieties. Ozonation of SRHA
in the presence andabsence of t-BuOH resulted in comparable
SUVA254-EDCdependencies. Conversely, ozonation of SRFA in the
presence
of t-BuOH resulted in smaller decreases in SUVA254 and
largerdecreases in the EDC values as compared to ozonation in
theabsence of t-BuOH at the same initial ozone dose. We note
thatthe EDC measurements of PLFA samples at high O3 doseswere close
to the quantication limit of MEO. The apparentincrease in the EDC
value of PLFA at the highest O3 doses(Figure 3c) therefore likely
reected uncertainties in the EDCquantication.The eects on the
optical and antioxidant properties of the
HS shown in Figure 4 can be rationalized on the basis of
knownmajor reaction pathways of ClO2, HOCl, and O3 with
light-absorbing and electron donating phenolic moieties in the
HS(Figure 5). ClO2 reacts as a one-electron transfer oxidant
withlow molecular weight phenols forming chlorite and
thecorresponding phenoxyl radicals.21 At circumneutral pH,
thisreaction proceeds mostly via the phenolate species because
ofits oxidation rate constants with ClO2 that are about 6 orders
ofmagnitude higher than those for the nondissociated
phenolspecies.26 Analogously to low molecular weight
phenols,phenolic moieties in HS are expected to undergo one
electronoxidation by ClO2.We have previously shown that HS contain
electron donating
phenolic and hydroquinone moieties with apparent
oxidationpotentials40,64 much lower than the standard
reductionpotential of ClO2, Eh
0(ClO2 (aq)/ClO2) = 0.954 V.65 SRHA
and SRFA are derived from higher-plant precursor
materials,including lignin, which is rich in methoxylated
phenols.66
Generally, methoxylation activates phenols for
electrophilicattack and leads to faster oxidation kinetics.26
Phenoxyl radicalsresulting from a rst one electron
oxidation26,67,68 may eitherbe further oxidized by reacting with
another ClO2 to formortho- or para-quinones or undergo irreversible
couplingreactions. Hydroquinone moieties present in the untreatedHS
are expected to be oxidized by ClO2 to semiquinoneintermediates and
subsequently to the respective quinonemoieties. These reaction
pathways involving ClO2 as theoxidant have in common that electron
donating phenolicmoieties are oxidized, whereas their UV-light
absorbingaromatic structure is preserved. In fact, based on the
highermolar absorption coecient of benzoquinone than hydro-quinone
at 254 nm, the oxidation of hydroquinone to quinonemoieties in the
DOM may have resulted in higher SUVA254values than measured if no
hydroquinone moieties had beenoxidized. These pathways are
therefore fully consistent with the
Figure 2. Ozonation of Pony Lake Fulvic Acid (PLFA). Eects of
the reaction time on (a) the oxidative current responses in
mediatedelectrochemical oxidation (MEO) and (b) the corresponding
electron donating capacities (EDC) of PLFA. Experimental
conditions: 0.5 mmol O3/mmol C; 5 mM t-butanol; 50 mM PO4-buer, pH
7.0. The samples were quenched with 1 mM maleic acid at selected
reaction times.
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pronounced decreases in the EDC and the relatively smalllosses
in SUVA254 values observed for HS treatment with ClO2.Phenolic
moieties in HS may react with HOCl in an
electrophilic substitution reaction (Figure 5). At
circumneutralpH, this reaction proceeds via the phenolate due to
its muchhigher reactivity compared to the phenol.28 In this
reaction,HOCl attacks at the ortho and para positions to the
hydroxylsubstituent, resulting in the formation of
(poly)-chlorinatedphenols. Such an initial chlorination should not
lead to adecrease in the electron donating capacities of the
phenolicmoieties.69,70 The reaction of low molecular weight
phenolswith HOCl has been demonstrated to proceed via
polychlori-nated phenols which ultimately undergo ring cleavage to
formnonaromatic, chlorinated products (Figure 5).28 However,
thesmall changes in the SUVA254 values of HS upon HOCltreatment do
not support ring cleavage as a signicant reactionpathway for
phenolic, or more general, aromatic moietiespresent in HS.
Alternatively, the smaller relative decreases inthe SUVA254 than
EDC values upon HOCl treatment areconsistent with the two-electron
oxidation of hydroquinoneand/or catechol moieties by HOCl to form
the respectivequinone moieties and chloride. These reactions are
thermody-namically favorable given that the standard reduction
potentialsfor the two electron reductions of HOCl and OCl
(pKa(HOCl) = 7.54 at 25 C) (i.e., HOCl + H+ + 2e
Cl + H2O: Eh0 = 1.48 V71 and OCl + H2O + 2e
Cl + 2OH; Eh
0 = 0.84 V71) are much higher than the oxidationpotentials of
hydroquinones. This is in agreement with the highsecond order rate
constants for the reaction of HOCl with
hydroxyphenols.72 This reaction pathway may therefore resultin
similar changes in the optical and antioxidant properties asClO2,
which acts almost exclusively by an electron transfermechanism.The
reaction of phenolic moieties with O3 at circumneutral
pH is dominated by phenolate and initiated by an ozoneadduct,
which may react further by (i) loss of ozonide, O3
, toform a phenoxyl radical, (ii) loss of H2O2 to form an
orthobenzoquinone, (iii) loss of singlet oxygen, 1O2, to form
acatechol-type compound, and (iv) a Criegee-type reaction witha
cleavage of the aromatic ring.23,24,62 The formations ofphenoxyl
radicals (pathway (i)) and catechols (pathway (ii))are important
but minor pathways for the oxidation of phenolwith ozone.23 If
these would be the dominant reactionpathways of phenolic moieties
during ozonation, this wouldlead to comparatively large decreases
in the EDC and smalldecreases in the SUVA254 values, whereas the
opposite eectwas observed experimentally (Figure 4). Instead,
thepronounced decreases in SUVA254 support ring cleavage ofphenols
and hydroquinones via the Criegee mechanism(pathway (iv)) to form
muconic-type compounds andeventually aliphatic aldehydes (Figure
5). Ring cleavagereactions may have involved nonphenolic aromatic
moietiessuch as anisoles and polymethoxybenzenes, as demonstrated
forlow-molecular weight methoxylated compounds.22 The loss ofthese
moieties would have resulted in decreasing SUVA254without aecting
the EDC values of the HS, as both the targetcompounds and products
would not be oxidizable in MEO.
Figure 3. Dependencies of the electron donating capacities
(EDCs) of (a) Suwannee River Humic Acid (SRHA), (b) Suwannee River
Fulvic Acid(SRFA), and (c) Pony Lake Fulvic Acid (PLFA) on the
specic molar doses of the chemical oxidants chlorine dioxide
(ClO2), chlorine (as HOCl),and ozone (in the absence and presence
of t-BuOH).
Figure 4. Eect of chemical oxidant treatments on the specic UV
absorbances (SUVA254) and the electron donating capacities (EDCs)
of (a)Suwanee River Humic Acid (SRHA), (b) Suwannee River Fulvic
Acid (SRFA), and (c) Pony Lake Fulvic Acid (PLFA). The chemical
oxidants usedwere chlorine dioxide (ClO2), chlorine (as HOCl), and
ozone (O3; in the absence and presence of tertiary butanol
(t-BuOH)). The chemical oxidantdose increased in the directions
indicated by the grey arrows.
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IMPLICATIONSThis study establishes that the EDC of DOM is a
parameterthat directly relates to the DOM redox state. The EDC is
highlysensitive to changes in DOM occurring during
chemicaloxidation processes and can be readily quantied by
mediatedelectrochemical oxidation (MEO). If combined with
measure-ments of complementary optical parameters, such as
SUVA254,the changes in the EDC values provide information on
thekinetics and the dose-dependent oxidation of electron
donatingmoieties in DOM. The combined analysis of optical
andantioxidant properties also provides insight into which
moietiesin the DOM react with the chemical oxidants and helps
identifying the major oxidant-dependent reaction pathways
ofDOM.In addition to advancing the fundamental understanding of
chemical DOM oxidation, the results from this study are
alsorelevant from a more applied, water treatment perspective.MEO
has potential to be used in water treatment facilities tomonitor
DOM oxidation during a chemical oxidation step.Combined
determination of changes in the EDC and SUVA254(or other suitable
optical parameters) in close to real-time canbe used to control
chemical oxidant doses. The resultingrened dosing operation can
minimize overdosing which mayhave negative impacts on water
quality, such as the enhancedformation of disinfection/oxidation
byproducts. Future work
Figure 5. Proposed reaction pathways of phenolic moieties in the
humic substances during reaction with chlorine dioxide (ClO2),
chlorine (HOCl),and ozone (O3).
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needs to assess the potential of EDC-SUVA254 measurements asa
new tool to advance the understanding of and the capabilityto
predict other important processes occurring during
chemicaloxidation of DOM, such as the formation of
disinfectionbyproducts, the generation of assimilable carbon, and
theeciency of disinfection.
ASSOCIATED CONTENT*S Supporting InformationAdditional
information as noted in the text. This material isavailable free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATIONCorresponding Authors*(U.v.G.) Phone:
+41-(0)58 765 5270; fax: +41-(0)58 7655210; e-mail:
[email protected].*(M.S.) Phone: +41-(0)44 6328314; fax: +41 (0)44
633 1122;e-mail: [email protected].
Present Address (J.W.) Department of Civil & Environmental
Engineering,University of California at Berkeley, Berkeley,
California 94720,United States and ReNUWIt Engineering Research
CenterNotesThe authors declare no competing nancial interest.
ACKNOWLEDGMENTSJ.W. and M.A. contributed equally to the work.
This work wassupported by the Swiss National Science Foundation
(Beitrage200021-117911 and 200020-134801). We thank Marcel
Burgerfor help on measuring absorbance spectra and
Hans-UlrichLaubscher for technical support.
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