Iodine emission in the presence of humic substances at the water's surface

Article abstract of DOI:10.1021/jp2048234

Humic substances that preferentially adsorb at the air/water interfaces of water or aerosols consist of both fulvic and humic acid. To investigate the chemical reactivity for the heterogeneous reaction of gaseous ozone, O ₃(g), with aqueous iodide, I^-(aq), in the presence of standard fulvic acid, humic acid, or alcohol, cavity ring-down spectroscopy was used to detect gaseous products, iodine, I₂(g) and an iodine monoxide radical, IO(g). Fulvic acid enhanced the I₂(g) production yield, but not the IO(g) yield. Humic acid, n-hexanol, n-heptanol, and n-octanol did not affect the yields of I₂(g) or IO(g). We can infer that the carboxylic group contained in fulvic acid promotes the I₂(g) emission by supplying the requisite interfacial protons more efficiently than water on its surface.

Full text of DOI:10.1021/jp2048234

Article
pubs.acs.org/JPCA
Iodine Emission in the Presence of Humic Substances at the Water’s
Surface
Sayaka Hayase, Akihiro Yabushita, and Masahiro Kawasaki*
Department of Molecular Engineering, Kyoto University, Kyoto 615-8510, Japan
ABSTRACT: Humic substances that preferentially adsorb at
the air/water interfaces of water or aerosols consist of both
fulvic and humic acid. To investigate the chemical reactivity for
the heterogeneous reaction of gaseous ozone, O (g), with
3
−
aqueous iodide, I (aq), in the presence of standard fulvic acid,
humic acid, or alcohol, cavity ring-down spectroscopy was used
to detect gaseous products, iodine, I (g) and an iodine
2
monoxide radical, IO(g). Fulvic acid enhanced the I (g)
2
production yield, but not the IO(g) yield. Humic acid, n-
hexanol, n-heptanol, and n-octanol did not affect the yields of
I (g) or IO(g). We can infer that the carboxylic group contained in fulvic acid promotes the I (g) emission by supplying the
2
2
requisite interfacial protons more efficiently than water on its surface.
INTRODUCTION
spectroscopy (CRDS) combined with a gas−liquid interaction
2
■
3−25
cell.
Heterogeneous reaction at the air/water interface plays
ubiquitous and fundamental roles in atmospheric chemistry
involving reactive gas uptake on the aerosol/seawater surface.
Since the hydrophobic components of humic substances are
expected to be preferentially adsorbed at the air/water
interfaces of water or aerosols with their hydrophobic parts
−
−
I + O (g or aq) → IOOO
(1)
(2a)
(2b)
3
−
−
IOOO → IO + O
2
−
IOOO → → IO(g) + products
1
−6
−
+
pointing to the gas phase,
the chemical and physical
IO + H ⇄ HOI(aq) (pK = 10.8)
(3)
a
properties (e.g., uptake, reaction rate, surface tension, and
interfacial composition) of the surface should be affected by the
−
+
HOI(aq) + I + H ⇄ I (aq) + H O
(4)
(5)
2
2
5
−12
surfactants.
Humic substances cannot be described in
I (aq) → I (g)
2
2
specific molecular terms because of their complex, multi-
component nature, but three fractions (humic acid (HUA),
fulvic acid (FA), and humin) have been obtained on the basis of
solubility characteristics. HUA is not soluble in water under
acidic conditions (pH < 2) but is soluble at higher pH values.
FA is soluble in water under all pH conditions. It remains in
solution after removal of HUA by acidification. Humin is not
soluble in either a strong base or a strong acid. The major
functional groups in humic substances are carboxyl, phenolic
In this study, we have studied the chemical reactivity of FA,
HUA, and fatty alcohols at the air/water interface layers for the
−
O
(g) + I (aq) reaction using CRDS detection of gaseous
3
products I
(g) and IO(g). The present results are compared
(g) and
2
with previously proposed production schemes for I
2
23−25
IO(g).
EXPERIMENT
■
13−15
The principle of CRDS and the experimental details, including
the present CRDS setup combined with a gas−liquid
interaction cell, were presented in our previous publica-
hydroxyl, and alcoholic hydroxyl groups.
It is found in
13
rivers, lakes, sea, groundwater, etc. Fine aerosols, including
FA and HUA, are generated by the wind and transported in the
23−26
−1
1
6,17
tions.
Ozone was produced by O with 1 sL min
2
troposphere.
(
standard liter per minute) flowing through a high-pressure
Active halogen species in the air, such as halogen atoms and
halogen monoxide radicals, affect the gaseous composition of
the atmosphere, depleting ozone, controlling the HO /NOx
discharge ozonizer. Its concentrations were monitored by UV
absorption with a 253.7 nm Hg lamp prior to the gas−liquid
x
cycle, and producing cloud condensation nuclei in the
1
6,18−22
Special Issue: A. R. Ravishankara Festschrift
atmosphere.
We have proposed on the basis of a
stepwise mechanism of I (g) and the iodine oxide radical,
2
Received: May 24, 2011
Revised: January 16, 2012
Published: January 18, 2012
IO(g), emission at the interface layers via rapid reaction of O3
−
with I (aq) under dark conditions using cavity ring-down
©
2012 American Chemical Society
5779
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The Journal of Physical Chemistry A
Article
interaction cell. Gas flow rates were controlled by mass flow
controllers so that the total flow rate was maintained at 3 sL
−
1
min with N gas acting as a buffer. The typical concentration
2
1
5
−3
of O (g) was 3.0 × 10 molecules cm . The product
concentrations were monitored with a Nd :YAG pumped dye
laser (Lambda Physik, SCANmate) at 435.60 nm for the IO(g)
band head of the A Π ← X Π (v′ = 3, v″ = 0) transition
and absorption cross section = 5.9 × 10 cm molecule .
3
3
+
2
2
3
/2
3/2
−17
2
−1 27
The IO signal baseline was taken at 435.4 nm, a region in which
there was no IO absorption. I (g) concentration was monitored
2
at 435.4 nm for the B−X band and calibrated by introducing a
concentration-known I (g) into the reaction cell with spectral
2
Figure 1. Gaseous iodine concentrations [I (g)] during ozonolysis of
5 mM aqueous NaI solutions in the absence/presence of added fulvic
2
fitting at 430−455 nm for the B−X band. The observed
1
1
−3
concentrations were (1.2−1.5) × 10 molecules cm for
acid or hexanoic acid: dashed line, pH 3.3; solid line, in the presence of
1
3
−3
IO(g) and (1.8−8.2) × 10 molecules cm for I (g).
−1
2
2
.0 g L fulvic acid at pH = 3.3; solid gray line, in the presence of 4
The gas−liquid interaction cell was a Pyrex glass container
mM hexanoic acid at pH = 3.3.
(
21 mm i.d. and 60 cm length). No secondary wall reactions
23,24
occurred.
The cell was maintained at 100 torr by means of a
rotary pump, a mechanical booster pump, and a N (l) trap in
2
tandem. NaI(aq) solution ([NaI] = 5 mM unless otherwise
stated, volume = 20 mL) was poured 5 ± 1 mm above the
bottom of the reaction cell. The CRDS detection region is 6
mm above the solution surface. The gas in the cell was
completely replaced within a time interval of 0.70 s. Hence, the
average contact time of O (g) with the NaI(aq) solution was
3
0.70 s. To minimize possible secondary reactions, a freshly
prepared solution was used to measure each data point, except
for the concentration measurement as a function of reaction
time (see below). FA (Standard II, International Humic
Substances Society), HUA (Wako), malonic acid (Wako),
hexanoic acid (Alfa Aesar), and sodium iodide (Sigma-Aldrich)
were used. The solution or bulk pH was adjusted by adding
HCl/KOH and measured using a pH meter.
Figure 2. Gaseous iodine concentrations, [I (g)], as a function of
fulvic acid concentration in ozonolysis of 5 mM NaI solutions at initial
2
bulk pH 3.3, which are measured 15 s after the introduction of 3.0 ×
15
−3
1
0 molecules cm O (g).
3
−1
The standard concentration of FA was 2.0 g L , except for
FA]-dependence measurements (Figure 2). When HA or
[
phenol were added, the concentrations were 4 and 1 mM,
respectively. Corresponding to the ratio of charge density of the
carboxylic and phenolic groups in FA, a mixture of 4 mM HA
and 1 mM phenol was prepared, the details of which are
described in the Discussion.
RESULTS
■
In the measurements of I (g) concentration as a function of the
measurement time, t, after introducing O into the cell, the
2
3
enhancement of I (g) emission in the presence of FA or HA
2
was observed during ozonolysis of a NaI solution, initially at pH
3
.3, as shown in Figure 1. The I (g) concentrations promptly
2
increased upon introducing O into the cell and then decayed
3
asymptotically. The I (g) concentration increased with [FA] at
2
−1
least from 0.5 up to 2.0 g L at the initial pH 3.3 when I (g)
2
signals were monitored at t = 15 s, as shown in Figure 2. At t =
Figure 3. Effects of bulk pH on the yields of I (upper panel) and IO
(lower panel) in ozonolysis of 5 mM NaI solutions, which are
3
min, the pH of the reacted solution slightly changed from the
2
−1
initial value of 3.3 to an end value of 3.7 (with 2.0 g L FA),
appreciably from 3.3 to 5.1 with 4 mM HA, and greatly from
measured 15 s after O (g) introduction as a function of initial bulk pH.
3
−1
[
I2] and [IO] in the presence of 2.0 g L fulvic acid (open circle) or 4
3.3 to 10.3 without acids. In the case of 3 min ozonolysis of FA
mM hexanoic acid (solid circle); [I ] and [IO] in the absence of
2
0
0
solution without NaI, any 434−435 nm light-absorbing species
did not appear, and a pH change from the initial value did not
occur.
added acids.
concentrations remain constant (within 20%) in the entire pH
Figure 3 shows the I (g) concentrations (monitored at t = 15
2
range 3−10.
s) with/without FA or HA as a function of initial pH. The I (g)
2
Figure 4(A−D) shows the ratios of IO(g) and I (g)
concentrations increased up to 5-fold below pH ∼ 5 in the
presence of FA or HA. On the other hand, the IO(g)
2
concentrations after ozonolysis of NaI solution in the
5
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The Journal of Physical Chemistry A
Article
and alcohols. Phenols suppressed the I (g) and IO(g) emission,
2
and weak acids enhanced it.
−
−
−
To investigate the reduction of I /I and I /I by FA, we
2
3
prepared a 2.0 g L− solution and an I saturated NaI solution.
1
29
2
Three hours after mixing the two solutions at pH 5.8, no
appreciable change in the pH was found.
DISCUSSION
■
Our previous study reported suppression of I (g) and IO(g)
emission in the presence of phenols at pH > 3 as a result of fast
2
24
proton transfer at the air/water interface. Interfacial reactions
−
of I (aq) + O (g) and subsequent reactions consume protons,
3
resulting in a changing interfacial pH from acidic/neutral to
basic via reactions 3 and 4. Phenol (pKa = 10.0), p-
methoxyphenol (10.3), or p-cresol (10.2) suppressed the
I2(g) and IO(g) emission as a result of the acid dissociation
of phenols and rapid reaction of phenolates with O at the
3
3
0,31
interface layer.
−
+
C H OH → C H O + H
(6)
(7)
(8)
6
5
6 5
C H O + O (g or aq) → C H O + O
3
Figure 4. Effects of acids and bulk pH on the yields of I (black bar)
and IO (white bar) in ozonolysis of 5 mM NaI solutions, measured 15
2
−
−
6
5
3
6
5
s after O (g) introduction at an initial pH of 3.3 (upper panel) and 7.4
3
(
L
lower panel). [X] represents [I ] or [IO] in the presence of (A) 2.0 g
2
C H OH + O (g or aq) → products
−1
6
5
3
fulvic acid, (B) 4 mM hexanoic acid and 1 mM phenol mixture,
(
C) 4 mM hexanoic acid, and (D) 1 mM phenol; [X] 0 represents
By contrast, the I (g) emission is enhanced via the efficient
2
[
I ] or [IO] in the absence of added acids and phenols.
2
0
0
interfacial proton transfer in the presence of organic weak acids
25
when the initial pH is adjusted to the individual pKa
absence/presence of (A) FA, (B) a mixture of HA and phenol,
C) HA, and (D) phenol at pH 3.3 and pH 7.4. Under acidic
−
−
(
IO + HA ⇄ HOI(aq) + A
(9)
conditions, the I (g) emission was enhanced in the cases of A−
C, but was suppressed by the addition of phenol in D. The
IO(g) emission was suppressed in B and D, but not in A or C.
Under neutral conditions, the I (g) and IO(g) emission were
not affected in A and C, but were suppressed in B and D. In
2
−
−
HOI(aq) + I + HA ⇄ I (aq) + H O + A
(10)
2
2
The surface-active acids, for example, HA or octanoic acid
2
(
OA), by situating their C(O)OH groups closer to the
5−11
−
interface
provide accessible proton donors to IO /HOI
addition, the unchanged concentrations for I (g) and IO(g)
2
at the air/water interface and more effectively enhance the I (g)
2
were observed at pH = 10.0 in A. The addition of HUA (∼3.0 g
emission. Hayase et al. measured ratios of I (g) concentrations
2
−1
L ) or fatty alcohols (∼10 mM) to the NaI solution also did
in the presence of 4 mM HA, OA, and acetic acid (AA) over
25
not affect the I (g) emission in the pH range of 3.3−10. Table
those measured in their absence. They found that these ratios,
R, follow in the order of R(OA) > R(HA) ≫ R(AA), which
represents direct evidence that the heterogeneous ozonation of
2
1
summarizes the effects of various cosolutes on the formation
of IO(g) and I (g) during ozonolysis of NaI solutions in the
absence/presence of humic substances, phenols, weak acids,
2
−
I proceeds mainly in the air/water interfacial layers, since the
−
Table 1. Effects of Cosolutes on the Formation Rates of IO(g) and I (g) in the Reaction of I with O (g or aq)
2
3
2
,28
−1 −130,44
a
a
cosolute
fulvic acid
concn (mM)
pKa
k /M
s
IO
I2
ref
O
3
−1
b
humic substances
< pH < 10
phenols
0.5−2.0 g L
3.0 g L−
0.2−1.0
0.5
d
N
N
S
E
this work
1
3
humic acid
N
S
this work
phenol
10
1.3 × 103
3.0 × 104
3.0 × 104
≤0.48 × 10
≤2.5 × 10−
7
24
3
< pH < 11
p-cresol
10.3
10.2
4.8
4.8
2.9
4.8
17.1
17.2
f
S
S
24
p-methoxyphenol
hexanoic acid
octanoic acid
malonic acid
acetic acid
0.5
S
S
24
−
3
acids
1−25
4−10
4−10
4−10
5
N
N
N
N
N
N
N
N
N
E
E
25
3
3
< pH < 5
25
e
c
E
25
≤3 × 10−
0.58
3
N
N
N
N
N
N
25
alcohols
n-butyl alcohol
tert-butyl alcohol
n-hexyl alcohol
n-heptyl alcohol
n-octyl alcohol
24
3
3
< pH < 10
100
3.0 × 10−
24
1−10
1−10
1−10
this work
this work
this work
f
f
≤0.8
a
b
c
d
E, S, and N stand for enhancement, suppression, and no effect, respectively. 3.3 < pH < 4.8. 3 < pH < 6. pK ∼ 1.8, pK ∼ 3.4, pK ∼ 4.2, and
1
2
3
e
f
pK ∼ 5.7. pK is 5.7. These values are estimated to be larger than 17.2 on the basis of the fact that acids with the larger alkyl groups weaken
4
a2
acidity.
5
781
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The Journal of Physical Chemistry A
Article
more tensioactive acids, by placing their CO(O)H groups
closer to the interface and providing proton donors accessible
to IO /HOI at the air/water interfacial layers in the pH range
IO(g) concentrations remain constant (within 20%), as
shown in Figure 3. The results imply that the proton, H , is not
+
−
involved in IO(g) emission, since reaction 2b may involve
23
where water itself is a poorer donor, should enhance I (g)
several steps.
2
concentrations.
Unchanged I (g) and IO(g) concentrations caused by HUA
2
In the presence of FA, the enhancement of I (g) emissions
addition were observed in the pH range of 3.3−10. The
dissolution rate of HUA is strongly dependent on the pH. The
dissolution rate, even at pH 4 and 5, is very slow, and it may
2
was also observed during ozonolysis of a NaI solution via a
similar reaction mechanism in the presence of organic weak
acids.
37
take several years to achieve equilibration. HUA does not
promptly provide a proton.
−
−
IO + FA ⇄ HOI(aq) + FA
(11)
(12)
−1
With the addition of 2.0 g L FA, the bulk pH slightly
changed from the original 3.3 to the final 3.7 during the
−
−
HOI(aq) + I + FA ⇄ I (aq) + H O + FA
+
2
2
experiment because the acid provides H in the I formation
2
processes to prevent proton depletion. The bulk pH of the NaI
solution mixed with FA is supposed to be maintained by the
rich amount of carboxylic groups, apparent in the charge
FA and HUA have a range of apparent molecular weights,
1
4,15,32,33
solubilities and acid strengths.
Charge density analysis
enables us to estimate the amounts of dissolved carboxylic and
14
1
4
−1
density study. To investigate the buffering capacity of FA, we
phenolic groups in FA or HUA. Charge densities (meq g
prepared a 2.0 g L− solution and an I -saturated NaI solution.
1
−
1
C ) of both the carboxylic and phenolic groups were
2
−
34
Although the reaction of the I /I reduction by FA was
reported to generate I and H , no appreciable change in the
determined by the titration technique. The charge density
of carboxylic groups was measured at pH 8.0, and that of
phenolic groups were 2 times the change in charge density
between pH 8.0 and 10.0. According to the acidic functional
group analysis of the standard FA, Standard II, charge densities
of the carboxylic and phenolic groups are 11.17 and 2.84 meq
2
3
−
+
29
−
bulk pH was observed by mixing the saturated I /I solution
2
3
with a 2.0 g L− FA solution. FA has many functional groups,
which may act as buffer solutions, to keep a certain pH level
nearly constant during the reduction. No bulk pH change was
1
34
−
1
−1
observed in the heterogeneous experiment of O (g) with 2.0 g
g
C , respectively. I (g) emission from the ozonolysis of NaI
3
2
−1
L
FA only, which means that the reaction products during
solutions in the presence of 4 mM HA and 1 mM phenol,
which is a ratio similar to the charge density of the carboxylic
and phenolic groups in FA, is enhanced in acidic conditions
pH 3.3) because the HA dissociation via reactions 9 and 10 is
preferable to the phenol dissociation via reaction 6 (Figure 4B).
ozonolysis of FA do not change the bulk pH. These results
imply that FA effectively buffers the system in these time scales
under the present experimental conditions.
(
By assuming a universal neutralization rate constant value of
−
+
10
−1 −1
k(X + H ) ∼ 1 × 10
M
s , the rate constants for the
In neutral/basic conditions, the emission of I (g) and IO(g) are
aq
2
10−pKa
−1 −1
reverse acid dissociations become: k ∼ 1 × 10
Acid dissociation rates in the presence of the Armadale
Horizons Bh fulvic acid are 1.6 × 10 s for pK , 4.0 × 10 s
for pK , 6.3 × 10 s for pK , and 2.0 × 10 s for pK ,
which are comparable with that of HA k ∼ 1.6 × 10 s . This
is consistent with the present experimental results that the
presence of FA at the air/water interface effectively buffers the
M
s .
suppressed, since O is consumed by phenolate and phenol via
d
3
reactions 7 and 8 and HA cannot provide accessible interfacial
protons (Figure.4B).
At pH 7.4, there will be a combination of phenol and
8
−1
6
−1
16
1
5
−1
4
−1
2
3
4
5
−1
phenolate, which can react with O . These aqueous rate
d
3
3
−1 −1
constants with O are k = 1.3 × 10 M
s
for phenol and k7
3
8
9
−1 −1
=
1.4 × 10 M
s
for phenolate. The total aqueous rate
7
−1 −1
system to prevent proton depletion and sustain the I (g)
constant at pH 7.4 is ∼10 M s , which depends on how
much of each form is present and the respective rate constants.
The aqueous rate constant of I with O is k = 2 × 10 M
s . Under the conditions of Figure 4, the concentrations of
2
production rate.
−
9
−1
As for atmospheric implications, organic compounds that
coexist with I− in the aqueous phase should be taken into
account for estimation of iodine emission toward the
atmosphere, as summarized in Table 1. The presence of
organic compounds may change the air−water interfacial
property; surface-active organics (e.g., fatty acids and alcohols)
are known to significantly retard the evaporation of water and
3
1
−1
35
−
I and phenol are 5 and 1 mM, respectively. Therefore, the
−
ozone loss rates of O (aq) with I (aq) and phenol(aq) are 1 ×
3
7
4
−1
1
0 and 1 × 10 s at pH 7.4, respectively, and hence, the
reaction of I with O dominates. Note that these are bulk
−
3
phase kinetics.
5
−11
penetration of atmospheric gases through the interface.
In contrast, the phenolic group of FA does not suppressed
However, the IO(g) and I (g) concentrations were unaffected
IO(g) or I (g) emission under neutral/basic conditions
2
2
by the addition of octanol that coats the solution surface. The
(
Figure.4A), whereas the enhancement effect of the carboxylic
results summarized in Table 1 for organic compounds imply
group is observed at an initial bulk pH < 5. The experimental
results suggest that the enhancement and suppression effects
depend not only on the charge densities of the carboxylic and
phenolic groups, but also on other physical or chemical
−
that the interfacial rapid reaction of I with gaseous O depends
3
on chemical reaction processes such as the competitive reaction
+
with O or H provision for the I (g) formation pathways as
3
2
well as the physical state of the surface.
properties. No suppression effects on IO(g) or I (g) emission
2
by the phenolic groups in FA may be caused by reactivity of
ozone toward aromatic compounds, which is known to be
highly dependent on the charge density of the compounds.
Since the structures of large phenolic compounds in humic
substances vary, unlike simple phenols, their respective
reactivities toward ozone could also vary. Moreover, for large
macromolecules, the reaction rate constants of ozone with
reactive sites are limited because of the steric hindrance effect.
CONCLUSION
■
36
Humic substances, which consist of carboxylic and phenolic
groups, are widely distributed in estuaries and coastal regions
where biogenic-produced iodide is also richly con-
1
5,17,38−40
tained.
Fulvic acid enhances the I (g) concentrations
2
via interfacial proton transfer in the interfacial reaction of
−
I (aq) with O (g or aq). The proposed enhancement
3
5
782
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The Journal of Physical Chemistry A
Article
mechanism is similar to that of the weak organic acids in the
(17) Leenheer, J. A.; Croue, J. P. Environ. Sci. Technol. 2003, 37,
8A−26A.
−
1
heterogeneous reaction of I with ozone in the pH range of 3−
2
5
(18) Saiz-Lopez, A.; Plane, J. M. C. Geophys. Res. Lett. 2004, 31,
5
. However, humic acid does not affect this. This difference is
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2
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M. R.; Colussi, A. J. J. Phys. Chem. A 2010, 114, 6016−6021.
25) Hayase, S.; Yabushita, A.; Kawasaki, M.; Enami, S.; Hoffmann,
concentrations by the undissociated carboxylic group in the pH
3
(
range from 3 to pK and suppression of the IO(g) and I (g)
a
2
emission occur in ambient atmospheric air as a result of the
3
0,31
high reactivity of simple phenols with O at pH > 3.
The
3
(
widely distributed aqueous fulvic acid accelerates the I (g)
2
−
emission in the pH range of 3−5. Thus, the acidity of I
(
containing aerosols and chemical species control iodine transfer
between water and air.
M. R.; Colussi, A. J. J. Phys. Chem. A 2011, 115, 4935−4940.
(26) Ninomiya, Y.; Hashimoto, S.; Kawasaki, M.; Wallington, T. J.
Int. J. Chem. Kinet. 2000, 32, 125−130.
AUTHOR INFORMATION
Corresponding Author
E-mail: kawasaki@moleng.kyoto-u.ac.jp.
(27) Nakano, Y.; Enami, S.; Nakamichi, S.; Aloisio, S.; Hashimoto, S.;
Kawasaki, M. J. Phys. Chem. A 2003, 107, 6381−6387.
■
(28) CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press:
*
Boca Raton, FL, 2010.
Notes
(29) Skogerboe, R. K.; Wilson, S. A. Anal. Chem. 1981, 53, 228−232.
(
(
(
30) Hoigne, J.; Bader, H. Water Res. 1983, 17, 173−183.
31) Hoigne, J.; Bader, H. Water Res. 1983, 17, 185−194.
32) Leenheer, J. A.; Wershaw, R. L.; Reddy, M. M. Environ. Sci.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by grants-in-aid from JSPS (Grants
Nos. 20245005 and 23684045). The authors thank Prof.
■
Technol. 1995, 29, 393−398.
(33) Leenheer, J. A.; Wershaw, R. L.; Reddy, M. M. Environ. Sci.
Technol. 1995, 29, 399−405.
(34) http://www.humicsubstances.org/
(35) Hoigne, J.; Bader, H. Water Res. 1985, 19, 993−1004.
(36) Galapate, R. P.; Baes, A. U.; Okada, M. Water Res. 2001, 35,
́
Michael R. Hoffmann, Drs. Agustin J. Colussi and Shinichi
Enami of California Institute of Technology for stimulating
discussions.
2
(
1
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201−2206.
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