Triplet state dissolved organic matter in aquatic
photochemistry: reaction mechanisms, substrate
scope, and photophysical properties
Kristopher McNeill
*
a
and Silvio Canonica
b
Excited triplet states of chromophoric dissolved organic matter (
3
CDOM*) play a major role among the
reactive intermediates produced upon absorption of sunlight by surface waters. After more than two
decades of research on the aquatic photochemistry of
3
CDOM*, the need for improving the knowledge
about the photophysical and photochemical properties of these elusive reactive species remains
considerable. This critical review examines the efforts to date to characterize
3
CDOM*. Information
on
3
CDOM* relies mainly on the use of probe compounds because of the difficulties associated with
directly observing
3
CDOM* using transient spectroscopic methods. Singlet molecular oxygen (
1
O
2
),
which is a product of the reaction between
3
CDOM* and dissolved oxygen, is probably the simplest
indicator that can be used to estimate steady-state concentrations of
3
CDOM*. There are two major
modes of reaction of
3
CDOM* with substrates, namely triplet energy transfer or oxidation (via electron
transfer, proton-coupled electron transfer or related mechanisms). Organic molecules, including several
environmental contaminants, that are susceptible to degradation by these two different reaction modes
are reviewed. It is proposed that through the use of appropriate sets of probe compounds and model
photosensitizers an improved estimation of the distribution of triplet energies and one-electron
reduction potentials of
3
CDOM* can be achieved.
Environmental impact
Photochemical processes are critically important in driving biogeochemical element cycling and in the breakdown of contaminants in surface waters. One of the
most important and least understood sets of photochemical pathways involves triplet chromophoric dissolved organic matter (
3
CDOM*), a form of electronically
excited DOM. There has been a recent surge in the study of the properties and reactivity of
3
CDOM* and this review article is an attempt to organize and
synthesize what has been discovered about
3
CDOM* over the past few decades.
Introduction
Sunlight-driven processes are central to both the buildup of
complex molecules through photosynthesis and their break-
down through photodegradation reactions. These photo-
degradation processes may be initiated not only directly by the
absorption of light, but also indirectly through reactions
involving a menagerie of exotic chemical species such as free
radicals and electronically excited molecules, referred to here
collectively as photochemically produced reactive intermediates
(PPRI).
1–4
Triplet excited states of chromophoric dissolved
organic matter (
3
CDOM*) are an important subset of the larger
pool of PPRI formed in sunlit waters that also include singlet
oxygen (
1
O
2
,
1
D
g
), superoxide (O
2
c), hydrogen peroxide,
hydroxyl radical (OHc), and others.
1–4
3
CDOM* has been impli-
cated in the degradation of contaminants, such as pesticides
5
and pharmaceuticals,
6,7
and holds a special position among the
PPRI for at least two reasons. First,
3
CDOM* is known or sus-
pected to be a precursor of other PPRI.
1–4
For example,
3
CDOM*
is the primary source for
1
O
2
in sunlit natural waters.
8,9
Second,
unlike other PPRI,
3
CDOM* is not a well-dened species; rather,
it is an infamously ill-dened mixture of triplet states, which
vary in their excited state energies and excited state redox
potentials.
The goal of this review article is to outline the reactivity
modes of
3
CDOM* and to summarize what is known or can be
reasonably inferred about both the triplet energy and redox
potential of
3
CDOM*. In addition, some back-of-the-envelope
calculations are presented that give rough answers to questions
that oen arise when discussing
3
CDOM*: why are triplet states
more important than singlet states in CDOM-sensitized
processes? And, what is the steady-state concentration
of
3
CDOM*?
a
Institute for Biogeochemistry and Pollutant Dynamics, ETH Zurich,
Universitaetstrasse 16, 8092 Zurich, Switzerland. E-mail: kris.mcneill@env.ethz.ch
b
Eawag, Swiss Federal Institute of Aquatic Science and Technology,
¨
Uberlandstrasse
133, 8600 D
¨
ubendorf, Switzerland
Cite this: Environ. Sci.: Processes
Impacts,2016,18,1381
Received 14th July 2016
Accepted 21st September 2016
DOI: 10.1039/c6em00408c
rsc.li/process-impacts
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Steady-state concentration of
3
CDOM*
in natural waters
Attempting to quantify the steady-state concentration of
3
CDOM* in an aquatic system would seem to be more chal-
lenging than other PPRI, due to the abovementioned problem
that
3
CDOM* is a mixture of triplet states of diverse molecules.
Therefore, it may seem surprising that we can actually estimate
the steady-state concentration of
3
CDOM* within about a factor
of two with a high degree of condence. This is thanks to the
inextricable link between
1
O
2
and
3
CDOM*.
To understand this, it is helpful to consider the simplied
kinetic scheme that connects
3
CDOM* and
1
O
2
(Fig. 1). CDOM is
excited by the absorption of a photon (symbolized by hn)toform
the excited singlet state of CDOM,
1
CDOM*. Under optically thin
conditions, the rate of light absorbance (R
abs
), in units of M s
1
,is
given by the product of the irradiance (mmol photons cm
2
s
1
),
theNaperianabsorptioncoefficient of CDOM (natural log-based
absorption coefficient in units of cm
1
), and a conversion factor
(mol L
1
(mmol cm
3
)
1
¼ 1). The efficiency of the conversion of
1
CDOM* to
3
CDOM* (i.e., the intersystem crossing efficiency)
is given by F
ISC
. The rate constants for the O
2
-independent and
O
2
-dependent deactivation pathways of
3
CDOM* are given by
k
T
d
and k
O
2
[O
2
], respectively. Under normal air-saturated surface–
water conditions, O
2
-dependent relaxation almost certainly
dominates over O
2
-independent relaxation. Sharpless has esti-
mated the O
2
-independent lifetime of triplets through O
2
-
dependent formation kinetics of
1
O
2
using Suwannee River and
Pony Lake isolates, and determined a lifetime around 20 ms
(k
T
d
z 5 10
4
s
1
).
10
Zepp has made a reasonable estimate of
k
O
2
¼ 2 10
9
M
1
s
1
,basedonO
2
quenching rate constants of
well-dened sensitizers.
9
While there is certainly some variation
in the individual k
O
2
values among the numerous sensitizers that
comprise
3
CDOM*, they are all expected to be quite high and near
the diffusion-controlled limit. For air-saturated freshwater at
25
C (258 mMO
2
), k
O
2
[O
2
] is thus approximately 5 10
5
s
1
(s ¼ 2 ms), which suggests that the O
2
-dependent relaxation
pathway is an order of magnitude more important than the O
2
-
independent pathway.
The quenching of triplet states by O
2
produces
1
O
2
, but the
yield for this process (f
D
)isdifferent for each sensitizer. It has
oen been assumed that f
D
is close to unity,
9
but studies with
a range of well-dened triplet sensitizers have shown that this
value can vary from near 0 (e.g., coumarin
11
) to near 1 (e.g. ,
perinaphthenone
12
), depending on the sensitizer.
11
Indeed, the
value of f
D
varies with the sensitizer's triplet energy and sensi-
tizer's excited state oxidation potential (i.e., how strong
a reductant the sensitizer is in the excited state), with high
energy and strongly reducing triplet species generally being
poorer
1
O
2
sensitizers.
13,14
Once formed,
1
O
2
mainly undergoes
unimolecular deactivation, k
D
d
.
15
Expressions for the steady-state concentrations of
3
CDOM*
and
1
O
2
based on the scheme depicted in Fig. 1 are given by eqn
(1) and (2).
h
3
CDOM*
i
ss
¼
R
abs
F
ISC
k
O
2
½O
2
þk
T
d
(1)
h
1
O
2
i
ss
¼
h
3
CDOM*
i
ss
k
O
2
½O
2
f
D
k
D
d
(2)
One can rearrange eqn (2) to arrive at an expression for the
ratio of the steady-state concentrations of
1
O
2
and
3
CDOM* (eqn
(3)).
h
1
O
2
i
ss
h
3
CDOM*
i
ss
¼
k
O
2
½O
2
f
D
k
D
d
(3)
Substituting values for k
D
d
(2.5 10
5
s
1
for H
2
O),
15
k
O
2
(2
10
9
M
1
s
1
),
9
and [O
2
] (258 mM at 298 K), one arrives at eqn (4).
h
1
O
2
i
ss
h
3
CDOM*
i
ss
z 2f
D
; 25
C; air-saturated water (4)
This result indicates that for 25
C, air-saturated water, the
ratio of
1
O
2
to
3
CDOM* is linearly dependent on the yield of
1
O
2
from the O
2
-dependent quenching of
3
CDOM* (f
D
)with
amaximum[
1
O
2
]
ss
value of two times [
3
CDOM*]
ss
.Whilewedo
not know the value of f
D
for
3
CDOM*, eqn (4) nevertheless
suggests a useful rule-of-thumb of [
3
CDOM*]
ss
z [
1
O
2
]
ss
.Toreit-
erate, this will hold when the value for k
O
2
is close to the estimate
of 2 10
9
M
1
s
1
and the average f
D
value is near 0.5. Under
noon-time clear summer sky conditions, [
1
O
2
]
ss
,hasbeenfoundto
be between 10
14
and 10
12
M in natural waters, depending on
the concentration of DOM (1–100 mg
C
L
1
;seeforexample
Peterson et al.
16
). Based on the above argumentation, we can
therefore adopt this same concentration range for [
3
CDOM*]
ss
.
Why triplet states and not singlet
states?
Could singlet excited state CDOM moieties (
1
CDOM*) act as
reactive intermediates in a similar manner to
3
CDOM*?Aer
Fig. 1 Kinetic scheme illustrating the connection between
3
CDOM*
and
1
O
2
.Definition of variables and symbols: CDOM, chromophoric
dissolved organic matter;
1
CDOM*, singlet state dissolved organic
matter;
3
CDOM*, triplet state dissolved organic matter;
1
O
2
, singlet
oxygen (
1
D
g
); hn, photon; R
abs
, rate of light absorbance; F
ISC
, inter-
system crossing quantum yield; k
O
2
, bimolecular rate constant for the
quenching of
3
CDOM* by O
2
; f
D
, fraction of O
2
-dependent quenching
that produces
1
O
2
; k
T
d
, rate constant for O
2
-independent relaxation of
3
CDOM*; k
D
d
, rate constant for relaxation of
1
O
2
to O
2
.
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all, the lowest lying singlet excited state of a given sensitizer is
higher in energy than its lowest lying triplet state and would
therefore be expected to be more reactive than the triplet. While
this is true, it is counteracted by the fact that the steady-state
concentration of
1
CDOM* is much lower than that of
3
CDOM*.
To determine exactly how much lower [
1
CDOM*]
ss
is than
[
3
CDOM*]
ss
, we need estimates of the relative formation and
decay rate constants for both species. Formation quantum
yields for
3
CDOM* have been estimated to be in the range of 1–
2%,
9,17
but could be as high as 6% or higher for some DOM
samples, based on
1
O
2
quantum yield measurements.
16,18
This
indicates that
1
CDOM* formation rates are 15–100 times faster
than those for
3
CDOM*. On the decay side, the
1
CDOM* life-
time is much shorter than that of
3
CDOM*, which has a lifetime
of about 2 ms(i.e. , the inverse of k
O
2
[O
2
]; see previous section).
Fluorescence lifetime studies give a direct measurement of the
decay of
1
CDOM*, and, as expected, the mixture of uorophores
do not display a single lifetime. Rather, the data suggest
a dominant pool of short lifetime
1
CDOM* species (s < 150 ps),
with contributions from two other pools of
1
CDOM* (s z 1 and
3 ns).
19
For simplicity, we consider 100 ps to be the typical
lifetime of
1
CDOM*.
Taken together, we see that while
1
CDOM* is formed 15–100
times faster than
3
CDOM*,itdecaysapproximately20000times
faster, giving 200- to 1300-fold lower steady-state concentrations
than
3
CDOM*. This corresponds to [
1
CDOM*]
ss
of 10
17
to
10
14
M in sunlit surface waters, compared to [
3
CDOM*]
ss
of 10
14
to 10
12
M. To put this into context of another PPRI, [
1
CDOM*]
ss
is
expected to be similar to [OHc]
ss
.Thus,
3
CDOM* is expected
generally to be the more important species, but
1
CDOM* could
also play a role under the right circumstances. For example, we
speculate that this could occur with CDOM samples that have low
intersystem crossing quantum yields (i.e.,lowratesof
3
CDOM*
production) or when the rate constant for reaction with
1
CDOM* is
orders of magnitude faster than with
3
CDOM*.Anothercase
where
1
CDOM* could conceivably participate in bimolecular
reactions despite being so short-lived is when its reaction partner
is already associated with CDOM. Such intra-humic photosensiti-
zation reactions have been proposed for the photoreduction of
mirex,
20,21
reactions involving
1
O
2
with a highly hydrophobic probe
molecule,
22,23
and the
3
CDOM*-sensitized degradation of
amoxicillin.
24
Energy transfer reactions
Triplet excited states of CDOM have been shown to undergo
energy transfer reactions with selected substrates (Fig. 2). The
best studied of these energy transfer processes is the formation
of
1
O
2
from the interaction of triplet ground state O
2
with
3
CDOM*, which was rst reported by Zepp in 1977.
8
The energy required to promote ground state O
2
to
1
O
2
is 94
kJ mol
1
(980 meV).
11
Since most triplet excited states of organic
chromophores are much higher (typically 180–320 kJ mol
1
), O
2
has been proposed to be a universal energy acceptor, capable of
accepting energy from all
3
CDOM* moieties.
9
This is an over-
simplication as discussed above in the section on the
concentration of
3
CDOM* in natural waters, but to a rst
approximation, it is a reasonable statement.
Dienes have also been reported to participate in energy
transfer reactions with
3
CDOM*. Zepp rst demonstrated this
with pentadiene (E
T
¼ 248 kJ mol
1
)
25
and 2,4-hexadien-1-ol
(sorbic alcohol, E
T
¼ 249 kJ mol
1
),
25
showing that various
natural organic matter isolates could sensitize the reversible
photoisomerization of the cis- and trans-forms.
9
Zepp
26
extended this reaction type to include 2,4-hexadienoate (HDA,
also known as sorbic acid, E
T
¼ 239–247 kJ mol
1
).
27
More
recently, Grebel et al.
17
made an in depth study of the reaction of
HDA with
3
CDOM*, and this work has sparked the use of HDA
as both a quencher of
3
CDOM* and a molecular probe to
quantify its concentration.
28–42
In a similar way, isoprene (E
T
¼
251 kJ mol
1
)
25,43
has been effectively used as a triplet quencher,
providing evidence for the involvement of
3
CDOM* in the
oxidation of mefenamic acid,
44
some sulfa drugs,
45,46
and the
amino acids tryptophan, methionine, and tyrosine.
47
Dienes
have not only been used as probe molecules. Domoic acid,
a naturally occurring diene and potent marine toxin, has been
shown to undergo
3
CDOM*-sensitized isomerization, among
other indirect photoprocesses.
48
There are very few well-characterized energy transfer reactions
between
3
CDOM* and non-diene organic substrates. A notable
exception is chlorothalonil, which is promoted to its triplet state
through a CDOM-sensitized process.
49,50
Porras et al. tested for
the involvement of energy transfer between
3
CDOM* and chlor-
othalonil through quenching experiments.
49
In addition, they
determined the triplet energy of chlorothalonil by low tempera-
ture phosphorescence measurements to be 276 kJ mol
1
and
veried that excitation of CDOM with wavelengths longer than
450 nm (<266 kJ mol
1
) gave very little sensitized photoreaction.
49
Triplet energy of
3
CDOM*
Given the heterogeneous nature of the components of
3
CDOM*,
there is no one triplet energy, E
T
, that can be used to describe it.
Fig. 2 Compounds that have been shown to act as energy acceptors
with
3
CDOM*. Triplet energies (E
T
) are given for each compound,
except in the case of O
2
, where the lowest singlet energy (E
S
) is given.
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Rather, there is a distribution of triplet energies. Using the energy
transfer reactions between
3
CDOM* and either O
2
(E
S
¼ 94 kJ mol
1
)
11
or dienes 1,3-pentadiene (E
T
¼ 248 kJ mol
1
)
25
and 2,4-hexadien-1-ol (E
T
¼ 249 kJ mol
1
),
25
Zepp concluded that
3
CDOM* comprised both high-energy triplets (E
T
$ 250 kJ mol
1
)
and low-energy triplets (94 # E
T
# 250 kJ mol
1
).
9
The high-energy
triplets were able to sensitize the isomerization of the 1,3-penta-
diene and produce
1
O
2
, while the low energy triplets could only
produce
1
O
2
. One conclusion of this study was that the high-energy
triplets accounted for about 15–53% (mean ¼ 37%) of the total
triplet pool, depending on the DOM sample.
9
To visualize this
result, a hypothetical normal (Gaussian) distribution of triplet
energies with 37% of the triplet energies being greater than or
equal to 250 kJ mol
1
isshowninFig.3.AlsoplottedinFig.3are
ranges of triplet energies found for representative compounds
(Table 1) that contain chromophoric functional groups believed to
be present in DOM. The data in Fig. 3 suggest that PAH-like
moieties and quinones are most likely not major contributors to
the high-energy triplet pool, whereas aromatic ketones and other
carbonyl-containing compounds (e.g., coumarins and chromones)
are better candidates for high-energy triplets. However, it is not
only the triplet energy that is important, but also the triplet yield
(i.e., intersystem crossing quantum yield). For example, aromatic
ketones have triplet yields near unity,
51,52
while coumarins typically
have poor triplet yields.
53
Another piece of information that could be obtained by Zepp
and coworkers in the CDOM-sensitized isomerization of 1,3-
pentadiene was the apparent E
T
of CDOM from the nal cis–
trans ratio, or the photostationary state, of 1,3-pentadiene.
9
This
photostationary state was shown to reect the sensitizer's E
T
,
and the values obtained for CDOM solutions were consistent
with an apparent E
T
of 250 kJ mol
1
.
9
Similar experiments
conducted with functionalized carbon nanotubes
41
and petro-
leum
54
found the apparent E
T
values to be lower and higher than
CDOM, respectively. The petroleum value was estimated to be
288–303 kJ mol
1
, suggesting that the triplet photochemistry
relevant to oil spills may differ substantially from CDOM-based
photochemistry.
54
The high average E
T
value found in petro-
leum stands in contrast to the low average triplet energies of
PAH molecules in Table 1 and Fig. 3. This may mean that the
small selection of PAHs (triphenylene, phenanthrene, naph-
thalene, pyrene, and anthracene) is not representative of the
PAH mixture in petroleum or that other higher E
T
species
present in petroleum (e.g., ketones formed from oxidation)
55,56
are dominating the sensitization of the diene probes.
At least two spectroscopic estimates of the E
T
value of
3
CDOM* have been made. Bruccoleri et al. applied magnetic
circular dichroism (MCD) spectroscopy to an organic matter
isolate and assigned an absorbance transition as S
0
/ T
1
, and
the wavelength for this transition (714 nm; 14 000 cm
1
) cor-
responded to an energy of 170 kJ mol
1
.
57,58
Mazhul et al. used
room temperature phosphorescence spectroscopy to identify
the opposite transition (T
1
/ S
0
), with an onset near 405 nm,
corresponding to the highest energy (phosphorescing) triplets
having an E
T
value of 300 kJ mol
1
.
59
In both of these studies,
the estimates of E
T
must be viewed with caution, as both tech-
niques are almost certainly confounded by the complex mixture
of DOM. Indeed, Mazhul et al. explicitly point out that they
believe they are only observing phosphorescence from
a minority of the
3
CDOM* components in their mixture.
59
Additionally, these spectroscopic values do not seem reasonable
as average or representative values, since one is at the extreme
low end and one is at the far high end of the range of triplet
energies normally found for organic sensitizers.
3
CDOM* oxidation reactions
Redox reactions are the dominant reaction type between
organic substrates and
3
CDOM*, with
3
CDOM* primarily acting
as the oxidant. The oxidation reactions have been reviewed
elsewhere
60
and the discussion here will be mostly conned to
the substrate scope and the reduction potential of
3
CDOM*.
Some patterns are revealed by examining the structures of
compounds for which triplet states have been established as
playing a role in their organic matter-sensitized degradation. In
Fig. 4, selected structures of compounds are presented that have
been shown to react with
3
CDOM*. Excluded from this group
are compounds that are suspected to be reactive toward
3
CDOM*, based on their reactivity toward model sensitizers
(e.g., anthraquinone-2-sulfonate; AQ2S), but that have not yet
been investigated with CDOM.
61–65
Examining the structures in Fig. 4, one can see that anilines
and phenols are well represented. Anilines and compounds
containing aniline substructures are especially susceptible to
oxidation by
3
CDOM*. This includes simple aniline structures,
such as N,N-dimethylaniline,
66
p-aminobenzoic acid,
67
and
Fig. 3 Distributions of triplet energies found for different classes of
organic molecules containing functional groups that are thought to be
present in CDOM and a hypothetical normal distribution of
3
CDOM*
triplet energies that fit the observation of 37% having E
T
$ 250 kJ
mol
1
.
9
The data used for this figure is compiled in Table 1.
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Table 1 Ground-state reduction potentials (E
), triplet energies (E
T
), triplet state reduction potentials (E
*), and singlet oxygen quantum yields (F
D
) for selected DOM model sensitizers and
other widely used sensitizers
Entry Sensitizer (S) E
(S/S
) V (SHE) Solvent
a,b
E
T
(kJ mol
1
) Matrix
b
E
*(
3
S*/S
) V (SHE) F
D
Solvent
b
Quinones
1 Benzoquinone 0.099 (ref. 139) H
2
O 224 (ref. 140) Ne solid 2.42 0.13 (ref. 141) H
2
O (pH 7)
2 Naphthoquinone 0.12 (ref. 142) H
2
O 241 (ref. 143) MCIP 2.38 0.27 (ref. 141) H
2
O (pH 7)
3 Anthraquinone 0.52 (ref. 144) H
2
O 265 (ref. 145 and 146) EPA 2.23 0.62 (ref. 147) CH
3
CN
4 Duroquinone 0.24 (ref. 139) H
2
O 235 (ref. 148) PMMA 2.19 0.89 (ref. 147) CH
3
CN
Aldehydes and ketones
5 Benzil 0.47 (ref. 149) 50% EtOH 230 (ref. 150) EtOAc 1.92 0.58 (ref. 151) C
6
H
6
6 CBBP
b
1.13 (ref. 152) H
2
O (pH 11) 286 (ref. 152) EA 1.84
d
—
7 Acetophenone 1.42 (ref. 149) 50% EtOH 308 (ref. 153) MTHF 1.77 0.33 (ref. 151) C
6
H
6
8 Biacetyl 0.79 (ref. 149) 50% EtOH 239 (ref. 146) EPA 1.69 0.29 (ref. 151) C
6
H
6
9 Benzophenone 1.31 (ref. 149) 50% EtOH 288 (ref. 150) EtOAc 1.67 0.37 (ref. 11) CH
3
CN
10 3MAP 1.50 (ref. 149) 50% EtOH 303 (ref. 149)
c
1.64 0.27 (ref. 154) C
6
H
6
11 2-Naphthaldehyde 1.10 (ref. 149) 50% EtOH 249 (ref. 146) MCIP 1.48
d
—
12 9-Fluorenone 0.97 (ref. 149) 50% EtOH 223 (ref. 146) MCIP 1.34 0.82 (ref. 11) C
6
H
6
13 2-Acetylnaphthone (2AN) 1.48 (ref. 149) 50% EtOH 249 (ref. 146) EPA 1.10 0.71 (ref. 11) C
6
H
6
Coumarins, chromones, and related
14 Xanthone 1.21 (ref. 155) 25% EtOH 306 (ref. 150) EtOAc 1.96 0.27 (ref. 11) C
6
H
6
15 Coumarin 1.16 (ref. 156) 75% MeOH 267 (ref. 157) H
2
O 1.61 0.01 (ref. 11) D
2
O
16 Flavone 1.18 (ref. 158) 50% iPrOH 260 (ref. 146) IPMC 1.51 0.16 (ref. 159) MeCf
17 Umbelliferone 1.23 (ref. 156) 75% MeOH 255 (ref. 157) EtOH 1.42
d
—
18 Cinnamic acid 1.14 (ref. 156) 75% MeOH 235 (ref. 160)
c
1.29
d
—
Polycyclic aromatic hydrocarbons
19 Triphenylene 2.22 (ref. 161) DMF 281 (ref. 146 and 162) EPA 0.69 0.40 (ref. 163) C
6
H
6
20 Phenanthrene 2.22 (ref. 164) 75% dioxane 259 (ref. 146 and 162) EPA 0.46 0.33 (ref. 163) C
6
H
6
21 Naphthalene 2.25 (ref. 164) 75% dioxane 253 (ref. 150) CFC–EA 0.37 0.50 (ref. 163) C
6
H
6
22 Pyrene 1.86 (ref. 164) 75% dioxane 204 (ref. 146) MCIP 0.25 0.38 (ref. 163) C
6
H
6
23 Anthracene 1.70 (ref. 164) 75% dioxane 178 (ref. 165) EPA 0.15 0.61 (ref. 163) C
6
H
6
Other sensitizers
AQ2S Anthraquinone-2-sulfonate 0.39 (ref. 166) H
2
O 258 (ref. 167) CH
3
CN 2.28 0 (ref. 125 and 147) H
2
O
LC Lumichrome 0.50 (ref. 168) H
2
O 232 (ref. 169) EM 1.91 0.63 (ref. 170) H
2
O (pH 7.4)
RF Riboavin 0.29 (ref. 171) H
2
O 209 (ref. 169) EM 1.88 0.49 (ref. 11) H
2
O (pH 7.4)
MB Methylene blue 0.024 (ref. 172) H
2
O 142 (ref. 133)
c
1.50 0.37–0.56 (ref. 173) H
2
O
RB Rose bengal 0.54 (ref. 93) H
2
O 171 (ref. 93) EPA 1.23 0.75 (ref. 11) D
2
O (pD 8.2)
PN Perinaphthenone 0.67 (ref. 174 and 175) 50% EtOH 164 (ref. 176) CH
3
CN 1.03 0.98 (ref. 12) H
2
O
a
Balance is H
2
O when a percentage co-solvent is specied.
b
Abbreviations: CBBP ¼ 4-carboxybenzophenone; 3MAP ¼ 3-methoxyacetophenone; EtOH ¼ ethanol; MeOH ¼ methanol; iPrOH ¼
isopropanol; DMF ¼ N,N-dimethylformamide; MCIP ¼ 5 : 1 methylcyclohexane : isopentane; EPA ¼ 5 : 5 : 2 diethyl ether : isopentane : ethanol; PMMA ¼ polymethylmethacrylate; EtOAc ¼
ethyl acetate; EA ¼ 3 : 1 diethyl ether : ethanol; MTHF ¼ 2-methyltetrahydrofuran; IPMC ¼ 5 : 1 isopentane : methylcyclohexane; CFC–EA ¼ mixture of CFC-113 (1,1,2-trichlorotriuoroethane)
and EtOAc; EM ¼ 9 : 1 ethanol : methanol; MeCf ¼ 3 : 1 methanol : CHCl
3
.
c
Matrix not specied.
d
No reported value found.
This journal is © The Royal Society of Chemistry 2016 Environ. Sci.: Processes Impacts,2016,18,1381–1399 | 1385
Critical Review Environmental Science: Processes & Impacts
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