Binding effects on humic-mediated photoreaction: Intrahumic dechlorination of mirex in water

Article abstract of DOI:10.1021/es950906i

A kinetic model was developed describing the effects of hydrophobic partitioning to humics on second-order humic-mediated photoreactions in aqueous solution. Model development and evaluation were in terms of the humic-mediated dechlorination of the hydrophobic chlorocarbon, mirex. Mirex dechlorination in irradiated (λ ≤ 290 nm) Aldrich humic acid (HA) solution was examined as a function of humic acid and scavenger concentration. Kinetic models in which HA solution is described as a homogeneous or single-phase solution were not able to describe the effects of HA and scavenger concentrations on mirex dechlorination rates. However, these effects were successfully described by including partitioning in the models to characterize the bound phase and dissolved phase mirex reactions separately. The resulting model uses homogeneous phase kinetics to describe reaction in the dissolved phase. Bound phase reaction is described as occurring within individual HA molecules with the distribution of scavengers within HA molecules following a Poisson distribution. Used to assess the relative reactivities of the bound and dissolved phases, the model indicated that the reaction of mirex in HA solution is confined to the humic phase. The general predictions and utility of the model are evaluated both in terms of experimental data and pollutant photodegradation.

Full text of DOI:10.1021/es950906i

Environ. Sci. Technol. 1996, 30, 2934-2941
hydrolysis (1, 2), acceleration and inhibition of photolysis
Binding Effects on
rates (3-5), and changes in reaction products and product
ratios (3, 6). In some cases, reactivity is altered because
sorbed pollutants appear to encounter a solution microen-
vironment that is different from the bulk solution. For
example, the buffer-catalyzed hydrolysis of humic-bound
molecules apparently is analogous to intramicellar hy-
drolysis, with the environment of the sorbed molecules
being buffered by the humic materials (7). In other cases,
the reactivity of sorbed pollutants is altered because humic
materials are themselves reactive. The contribution of
humic reactivity to the transformation of sorbed pollutants
is most apparent in reports of pollutant photochemistry.
Humic-Mediated Photoreaction:
Intrahumic Dechlorination of
Mirex in Water
S U S A N E . B U R N S , *
J O H N P . H A S S E T T , A N D
M A U R A V . R O S S I †
Department of Chemistry, State University of New York
College of Environmental Science & Forestry,
Syracuse, New York 13210
In sunlit surface waters, humic materials either directly
or through secondary photoreactions appear to be capable
of initiating photosensitization reactions, photooxidations,
photoreductions, and radical reactions (8-10). Some
pollutants, such as ethylenethiourea, are resistant to direct
photoreaction in sunlight but nevertheless undergo pho-
totransformations in the presence of humic materials (11).
The ability of molecules to be transformed by humic-
mediated photoreactions can be influenced by binding or
other interactions that bring molecules into close proximity
with humic materials. For example, Blough (12) demon-
strated that EPR signal loss in nitroxide probes was a
function of electrostatic interactions between the probes
and humic materials in solution, with cationic probes
apparently encountering a greater number of humic-
generated reductants or carbon-centered radicals than
neutral and anionic probes. Similarly, Green et al. (13)
found both that cationic nitroxides were more efficient than
neutral nitroxides at quenching the fluorescence of humic
materials and that the quenching of humic fluorescence by
anionic nitroxides was undetectable. These results indicate
that at least some humic reactivity is not homogeneously
distributed in solution. They also indicate that this inho-
mogeneity in reactivity is a function of the ability of
molecules to approach or come into contact with humic
materials. This raises the possibility that pollutants sorbed
to humic materials in water can undergo reactions not
readily available to molecules restricted to the bulk solution.
A kinetic model was developed describing the effects
of hydrophobic partitioning to humics on
second-order humic-mediated photoreactions in
aqueous solution. Model development and evaluation
were in terms of the humic-mediated dechlorination
of the hydrophobic chlorocarbon, mirex. Mirex
dechlorination in irradiated (λ g 290 nm) Aldrich humic
acid (HA) solution was examined as a function of
humic acid and scavenger concentration. Kinetic
models in which HA solution is described as a
homogeneous or single-phase solution were not able
to describe the effects of HA and scavenger
concentrations onmirexdechlorinationrates. However,
these effects were successfullydescribedbyincluding
partitioning in the models to characterize the bound
phase and dissolved phase mirex reactions sepa-
rately. The resulting model uses homogeneous phase
kinetics to describe reaction in the dissolved phase.
Bound phase reaction is described as occurring
within individual HA molecules with the distribution
of scavengers within HA molecules following a
Poisson distribution. Used to assess the relative re-
activities of the bound and dissolved phases, the
model indicated that the reaction of mirex in HA solution
is confined to the humic phase. The general predic-
tions and utility of the model are evaluated both in
terms of experimental data and pollutant photodegra-
dation.
The previously reported photodechlorination of the
hydrophobic pesticide mirex in humic acid solution (14,
15) appears to exemplify the ability of humic materials to
act as a distinct reactive phase in water. In this work, we
develop a kinetic model describing the effects of sorption
on second-order humic-mediated photoreactions. Ap-
plication of this model to the photodechlorination of mirex
indicates that only sorbed mirex undergoes photodechlo-
rination in humic acid solution and that the reaction can
be described as occurring wholly within humic acid
molecules.
Introduction
Sorption of organic pollutants to humic materials or natural
organic matter affects pollutant reactivity. Reported effects
of sorption on reactivity include acceleration of acid-
catalyzed hydrolysis and retardation of base-catalyzed
Experimental Section
Materials. Solvents used were hexane (Resi-Analyzed, J.
T. Baker Inc.) and methanol (Optima, Fisher Scientific).
Commercial humic acid was obtained from Aldrich Chemi-
cal Company as the sodium salt. Mirex (dodecachloropen-
tacyclo[5.3.0.02,6.03,9.04,8]decane, 100%) was obtained from
the U.S. EPA Pesticides & Industrial Chemical Repository,
and photomirex(1,2,3,4,5,5,6,7,9,10,10-undecachloropenta-
* Corresponding author present address: Department of Chem-
istry, Western Michigan University, Kalamazoo, MI, 49008; phone:
(616) 387-2846; fax: (616) 387-2909.
† Present address: Instituto de Quimica, Universidade de Sao Paulo,
Sao Paulo, Brazil.
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2 9 3 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
S0013-936X(95)00906-0 CCC: $12.00
1996 Am erican Chem ical Society

cyclo[5.3.0.02,6.03,9.04,8]decane, 97%) was obtained from the
Environmental Health Directorate of Health and Welfare
Canada. Compounds used in competition experiments
included 2-chloroethanol (99%, Eastman Kodak), lindane
(99%, Ultra Scientific), pentanol (Aldrich Chemical Co.),
and potassium nitrate (Fisher Scientific). Argon and oxygen
gases were UHP grade. Internal standards used were
octachlorostyrene and PCB congeners (99%+) obtained
from Ultra Scientific.
FIGURE 1. Structures of mirex and photomirex.
Hum ic Acid Solutions. Humic acid (HA) stock solutions
were prepared using a variation on the method of Zepp et
al. (16) by extracting 2 g of solid humic material with 2 L
of 0.1 N sodium hydroxide solution and pressure-filtering
it through a 0.2-µm capsule filter (Gelman Sciences no.
12117) at 4-6 psi. The filtrate was adjusted to pH 6 with
HCl and filtered again. HA working solutions (∼pH 6) were
made by diluting stock with water. All solutions were made
using water purified with a Millipore 4-bowlstandard system
(Milli-Q water). Dissolved organic carbon (DOC) was
routinely measured by absorbance at 355 nm using a Perkin
Elmer Lambda 5 spectrophotometer, following the method
of Zepp et al. (16). This method was calibrated against
potassium persulfate digestion (17) performed in screw-
cap vials, using a Beckman 215A infrared analyzer to
measure evolved carbon dioxide.
capillary column GC-ECD, and compounds were quantified
by comparison to internal standards. Blanks and mirex
and photomirex standards with internal standards were
included in each GC sample run to establish retention times
and response factors.
Two gas chromatographs were used: a Hewlett Packard
5890 and a Varian 3400. Detector signal output on both
instruments was collected on a computer. Both poly-
(dimethylsiloxane) and poly(5% diphenyl-95% dimethyl-
siloxane) capillary columns were used (30 m × 0.25 mm
i.d., 0.25 µm film). All injections were splitless, and various
temperature programs were used.
Rate Constants and Curve Fitting. The transformation
of mirex to photomirex in HA solution has been demon-
strated to follow rate laws for a consecutive, apparent first-
order reaction according to the scheme (14):
Photolysis. Irradiations were performed using Osram
XBO 150W/ 1 xenon or 200 W Hg arc lamps housed in an
Oriel uniform beam illuminator lamp housing (Model 6148)
powered by either an Oriel (Model 6240) or Spectral Energy
universal power supply (Model LPS 251HR). Reaction
vessels were 25 × 150 mm Kimax borosilicate culture tubes
having Teflon-lined caps (Kimble 45066A-25150). Tubes
were filled with 40 mL of solution. Mirex in methanol was
added to these solutions in 5-10-µL aliquots. Final mirex
concentrations in all HA solutions were ∼100 ng/ L, and all
solutions were allowed to equilibrate a minimum of 24 h
before use. HA solutions for competition experiments were
prepared in 4 L batches and amended with mirex in
methanol (∼10-µL aliquots). Aliquots (40 mL) of these
solutions were transferred to culture tubes and other organic
compounds (methanol, 2-chloroethanol, lindane, pentane)
were added and allowed to equilibrate for 24 h. Gases (10
min bubbling at ∼0.1 L/ min) and potassium nitrate were
added immediately prior to irradiation. Culture tubes were
inverted and placed against the condensing lens assembly
of the lamp housing for irradiation. Replicates and dark
and other controls were included in all experiments.
Mirex KDOC. The dynamic headspace method of Yin
and Hassett (19) was used to measure binding of mirex in
humic acid solution. In this method, KDOC is calculated
from measurements of Henry’s law in pure water and
apparent Henry’s law in HA solution. Measurements were
made using 1.7-L solutions contained in 110 × 285 mm
borosilicate roll bottles (Wheaton-33). HA solutions had
∼10 mg/ L DOC. Gas phase mirex sparged out of solution
was trapped on Tenax and eluted from the Tenax using
acetone and hexane.
kM
M
′8 P
9
kP′8 D
(1)
where kM′ and kP′ are the apparent first-order rate constants
for the reactions of mirex (M) and photomirex (P),
respectively, and D is a dihydro derivative of mirex
tentatively identified elsewhere as 2,8-dihydromirex (14,
18). Structures of mirex and photomirex are given in Figure
1. Mirex apparent reaction rate constants in this work were
determined from single-point irradiations using photomirex
(P) to mirex (M) concentration ratios and the integrated
rate expression:
ln (1 + P/ M) ) kM′t
(2)
This expression has been used previously (14, 15) and was
found to be valid for mirex phototransformations of less
than ∼25%. Data reported here are for experiments meeting
that criterion.
Curve fitting was performed using Scientist Version 2.0,
a MicroMath Scientific Software data fitting program.
Ratios and products of unknown constant terms in models
(eqs 8, 17, 19, and 27) were combined into single parameters.
Boundaryconditions for parameter estimates were specified
when necessary to force parameter estimates to assume
the appropriate sign (e.g., parameter > 0). Additional
boundary conditions were imposed when model outcomes
were unchanged by parameter values outside the boundary
conditions.
Results and Discussion
Analyses. Mirex was extracted from HA solution by an
adaptation of the method of Driscoll et al. (20). Chromic
acid cleaning solution (2 mL) and hexane (3 mL) were added
to 40 mL of the sample in a screw-top culture tube. The
solution was heated for 2 h at 100 °C. After the solution
was cooled, the hexane extract was siphoned offfor analysis.
Solutions in Milli-Q water were extracted using liquid-
liquid extraction into hexane. Extracts were analyzed using
Previous work (14, 15) has demonstrated that mirex is
transformed to photomirex (8-monohydromirex) in Aldrich
humic acid (HA) solution irradiated at sunlight wavelengths.
The same reaction occurs in Fluka HA solution (21) and in
Lake Ontario water (14, 22), where mirex is a pollutant. In
the current work, the transformation ofmirexto photomirex
was studied in irradiated (λ > 290 nm) Aldrich HA solutions.
While Aldrich humic acids are not fully representative of
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natural aquatic organic matter, mirex and photomirex have
similar physicalbehavior in HAsolutions and naturalwaters
(19, 23), and several studies have demonstrated that natural
waters and solutions ofdissolved humic materials represent
similar photochemical environments (cf. refs 24 and 25).
Consistent with previous work (14), dark reaction of
mirex in HA solution was not observed. Reaction of mirex
also was not observed in irradiated controlsolutions (Milli-Q
water) containing no HAs. This result contrasts with
previous work in which mirex transformation was reported
to occur in sunlit glass-distilled water (14). The contradic-
tory findings may reflect the different waters used, with
Milli-Q water presumably being cleaner than distilled water.
HA Concentration Effects. In the absence of direct
photolysis, phototransformation of mirex in HA solutions
must be mediated by HAs. Reports of humic-mediated
photoreactions are numerous (cf. refs, 26 and 27). These
reactions are described as involving intermediate reactive
species or excited states generated when humic materials
absorb light. The reaction of mirex with such an inter-
mediate can be formulated as
FIGURE 2. (a) Change in mirex apparent reaction rate constant (kM′)
with DOC (9) in irradiated Aldrich HA solution (λ g 290 nm). The
dotted and solid lines are the respective curve fits of eqs 8 and 17
to the data. (b) Calculated fraction of mirex bound (fb) in Aldrich HA
solution.
d[M]/ dt ) -kx[X][M]
(3)
where kx is the second-order rate constant for reaction of
mirex with generalized or unknown intermediate X. Under
conditions of constant irradiation, humic-generated in-
termediates typically have been found to occur at steady-
state concentrations, resulting in humic-mediated reactions
that follow first-order kinetics (8, 9). Steady-state con-
centrations of HA-generated intermediates occur when
intermediate production and loss processes are balanced.
The homogeneous solution description for these processes
in HA solution irradiated with monochromatic light of
wavelength λ is (24, 28)
Equation 8 predicts that kx′ will increase with [DOC]
and approach a limiting value. This is qualitatively what
is observed in a plot of kM′ vs [DOC] (Figure 2a), where kM′
is the overall apparent first-order rate constant for mirex
photoreaction. However, as eq 8 indicates, kM′ will ap-
proach a limiting value fastest at the wavelength of greatest
ꢀλ. Under the experimental conditions used, this wave-
length can be no less than ∼290 nm because HA absorbance
increases with decreasing wavelength, and the lower
wavelength cutoff imposed by the borosilicate reaction
vessels is ∼290 nm. Figure 2a shows the fit of eq 8 (dotted
line) to experimental data using a path length of 1 cm and
d[X]/ dt ) Iλ(1 - 10-ꢀ [DOC]l)Φλ,x - kr[X]
(4)
λ
which gives the steady-state concentration of X as
the measured value of ꢀ290 ) 2 × 104 (kg of DOC/ L)-1 cm-1
.
[X] ) [Iλ(1 - 10-ꢀ [DOC]l)Φλ,x]/ kr
(5)
λ
This fit represents an extreme case, with the curvature in
the fit being the maximum possible given the [DOC] range
(<15 mg/ L) and minimum wavelength (290 nm) used.
Figure 2a demonstrates that, even at maximum curvature,
eq 8 is a poor model of the dependence of kM′ on [DOC].
The experimentalresults consequentlyare inconsistent with
a reaction in which photoproduction of X and reaction of
it with mirex occur in a homogeneous solution.
where Iλ is the light flux per unit volume of solution (e.g.,
Einsteins min-1 L-1), ꢀλ is the extinction coefficient of humic
acids expressed in terms of dissolved organic carbon
concentration ([DOC], kg/ L), l is the light path length, Φλ,x
is the quantum efficiency for production of X, and kr is the
combined rate constant for all processes that remove X.
The production rate of X (Pλ,x) is
Dependencies on [DOC] similar to the results in Figure
2a have been observed for the humic-mediated reductive
dissolution of metal oxides complexed to HAs (29) and
postulated for the HA-photosensitized reactions of pol-
lutants bound to HAs (24, 29). What distinguishes these
reactions from the description in eq 8 is binding. Implicit
in eq 8 is the assumption that all X entities have equal
access to all mirex molecules in solution (i.e., solution
homogeneity). That reactions of bound molecules have
been found to deviate from this description indicates that
the HA matrix may represent a separate reactive environ-
ment where bound molecules are exposed to activities of
reactive species different from those experienced by
compounds in the bulk solution. This phenomenon has
been postulated previously (16, 25) and is supported by the
work of Blough (12) and Green et al. (13) showing that
electrostatic interactions influence the access molecules
have to humic-generated excited states and reactive species.
Pλ,x ) AλΦλ,x
(6)
where Aλ is the rate of light absorption by HA solution and
is given by:
Aλ ) Iλ(1 - 10-ꢀ [DOC]l
)
(7)
λ
Substituting eqs 5 and 6 into eq 3 yields an expression for
the system-specific, pseudo-first-order, homogeneous so-
lution rate constant for reaction of mirex with X:
Iλ 1 - 10-ꢀ [DOC]l
λ
kx
∑
(
)
]
[
λ
kx′ ) kxPx(λ)/ kr )
(8)
kr
where Px(λ) is the result of integration of eq 6 across the
spectrum.
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Previous work in our laboratory has shown that mirex
is bound by dissolved HA and aquatic organic matter (19,
23). The DOC-water partition coefficient (KDOC) for the
humic acid used in this study was determined to be 1.82
((0.38) × 106 L/ kg, in good agreement with previous work
(19). The fraction of mirex bound by humic acid (fb) was
calculated as (30)
fb ) KDOC[DOC]/ (1 + KDOC[DOC])
(9)
with (DOC) expressed in kg/ L. A plot of fb vs [DOC] (Figure
2b) follows the same trend as the data for kM′ vs [DOC].
This similarity indicates that photochemicaltransformation
ofmirexis strongly influenced bybinding to humic material.
An alternative to a homogeneous solution reaction of
mirex is a reaction in which the transformation of mirex
molecules bound to HAs must be considered separately
from the transformation of mirex molecules not bound to
HAs. If bound mirex molecules are considered to reside
within the aqueous matrix of individual humic molecules,
then the reaction of mirex with an intermediate within the
humic matrix can be expressed in terms of the concentra-
tions ofmirex([M]b/ [DOC]) and X([X]b/ [DOC]) in the humic
matrix
FIGURE 3. Effect of added oxygen (oxygenated), nitrate (NO3-),
lindane (HCH), 2-chloroethanol (Cl-EtOH), methanol (MeOH), and
pentanol (PeOH) on the mirex (∼100 ng/L) apparent rate constant
(kM′) in Aldrich HA solution (30 mg/L DOC). Values of kM′ are relative
to argon-purged controls containing no additives. Each bar is the
average of replicates and error bars represent standard error.
and fd is the fraction of mirex that is truly dissolved. An
implicit assumption built into eqs 14 and 15 is that sorption
equilibrium is maintained during mirex phototransforma-
tion. Combining eqs 8, 9, and 13 with eq 15 and recognizing
that
d([M]b/ [DOC])
)
(
)
dt
rxn
-kx ([X]b/ [DOC])([M]b/ [DOC]) (10)
b
where [M]b is humic-bound mirex and [X]b is an interme-
diate species associated with the humic matrix, both
expressed as molar concentrations in the aqueous solution.
Note that eq 10 describes mirex loss from the humic matrix
due to reaction (rxn) only. Equation 10 simplifies to
1
fd )
(16)
1 + KDOC[DOC]
yields the overall rate constant as a function of [DOC]:
(d[M]b/ dt)rxn ) -kx ([X]b/ [DOC])[M]b
(11)
1
b
kM′ )
[(kxPx(λ)/ kr) +
1 + KDOC[DOC]
The expression for the steady-state value of [X]b is similar
to eq 5
(kx KDOC Px (λ)/ kr )] (17)
b
b
b
[Iλ(1 - 10-ꢀ [DOC]l) Φλ,x
]
λ
Px(λ) and Px (λ) are not simple functions of [DOC] except
when the product ꢀλ[DOC]l is e0.02 (28). When this is true,
b
b
[X]b )
(12)
kr
Px(λ) and Px (λ) are linear functions of [DOC] because the
b
b
rate oflight absorption, A (eq 7), reduces to a linear function
λ
where Φλ,x is the quantum efficiency for production of X
b
of [DOC] (31). Under these conditions, eq 17 predicts that
kM′ will be proportional to the product [DOC] × fd (eq 16)
as allother terms in eq 17 are constants. Equation 17 further
predicts that the proportionality will hold regardless of the
relative contributions of the dissolved and bound phases
to overall reaction rate constant.
in the humic matrix and kr applies to reactions removing
b
X from the humic matrix. Combining eqs 10 and 12 gives
an expression for the system-specific pseudo-first-order
rate constant for reaction of bound mirex with X inside the
humic matrix:
Equation 17 was fit to experimental data using KDOC
)
kx ′ ) kx (Px (λ)/ [DOC])/ kr
b
(13)
1.82 × 106 L/ kg and assuming that Px(λ) and Px (λ) were
b
b
b
b
linear functions of [DOC]. The resulting curve fit is shown
using a solid line in Figure 2a. This fit demonstrates that
eq 17 is a better predictor of mirex photochemical behavior
than eq 8 (R2 ) 0.97 for eq 17 vs R2 ) 0.7 for eq 8) and
indicates that the reaction of mirex in HA solution is more
accurately described as occurring in separate bound and
dissolved photochemical environments.
where Px (λ) is the wavelength-integrated photochemical
production rate of X in the humic matrix, similar to Px (λ)
in eqs 6 and 7.
If eq 8 applies to truly dissolved (not bound) mirex, and
eq 13 applies to bound mirex, then the overall loss of mirex
from the system can be expressed as
b
Scavenger Effects. Some compounds added to HA
solution in competition experiments decreased kM′ relative
to controls. These compounds included oxygen, nitrate,
lindane, and 2-chloroethanol but not methanol or pentanol
(Figure 3). The ability of some additives to decrease
apparent reaction rate constants is typical of HA-mediated
photoreactions. This is because HA-mediated photore-
d[M]T/ dt ) -kx′fd[M]T - kx ′fb[M]T
(14)
b
to give an overall rate constant of
kM′ ) kx′fd + kx ′fb
(15)
b
where [M]T is the total mirex concentration in the system
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VOL. 30, NO. 10, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 9 3 7

sistant with a homogeneus phase reaction between mirex
and the single reactive intermediate X.
Equation 13 describes the rate constant for mirex
reaction in the humic matrix in the absence of added
scavengers. If scavenger molecules must be in the humic
matrix to interfere in the reaction of bound mirex, then the
description for scavenging in the humic phase can be
assumed to be analogous to that for scavenging in micellar
systems (33, 34); that is, the number of reactive intermedi-
ates available to mirex in a humic molecule depends on the
number of scavenger molecules present in that humic
molecule. Under these conditions, the presence of an
added scavenger changes eq 13 to yield
kx ,s ′ ) (kx Px (λ)/ [DOC])[1/ (kr + nks )]
(20)
b
b
b
b
b
b
FIGURE 4. Comparison between the observed (9) and predicted
(dotted line, eq 19; solid line, eq 27) effect of 2-chloroethanol
concentration on the mirex (∼100 ng/L) apparent photoreaction rate
constant (kM′) in Aldrich HA solution (10 mg/L DOC).
where ks is the rate constant for the reaction of the added
b
scavenger with the reactive intermediate inside a humic
molecule and n is the number of scavenger molecules
present in that humic molecule. In considering the humic
phase as a whole, since n is an integer, there must be a
distribution of humic molecules with different values of n.
Expansion of eq 20 to include all values of n yields
actions commonly involve scavengable, short-lived excited
states or reactive species that occur at steady-state con-
centrations in HA solution under conditions of constant
irradiation. For example, Zepp et al. (25) used the cis-
trans isomerization of1,3-pentadiene as a probe for reactive
triplet states in HA solution. Under conditions of constant
irradiation, the isomerization followed first-order kinetics.
However, the addition of oxygen to HA solution reduced,
through competition, the number of reactive triplet states
available to 1,3-pentadiene. Isomerization of 1,3-penta-
diene consequently was described by different apparent
first-order rate constants in HA solutions containing
different oxygen concentrations. Scavenger-induced de-
creases in apparent reaction rate constant consequently
indicate that mirex is transformed in HA solution by a
second-order reaction with some intermediate X that is
photochemically generated by HAs, as assumed in the
derivations of eqs 8 and 17. Further, since mirex photo-
reaction follows apparent first-order kinetics both in the
presence and absence of scavengers, X must occur in HA
solution at steady-state concentrations.
∞
kx ,s ′ ) (kx Px (λ)/ [DOC])
[Prn/ (kr + nks )] (21)
b b
∑
b
b
b
b
n)0
where Prn is the probability offinding n scavenger molecules
in a humic molecule. Atik and Singer (33) and Atik and
Thomas (34) have shown that the distribution of scavengers
among micelles is described by a Poisson distribution:
Prn ) e-N Nn/ n!
(22)
where N in HA solution is the average number of humic-
associated scavenger molecules per humic molecule.
The value of N can be related to the bulk solution
concentration of scavenger through a partition coefficient,
Kp:
N ) KP[S]
(23)
where [S] is the truly dissolved (not bound) molar con-
centration of the scavenger. For scavenger molecules that
follow linear partitioning as described by KDOC, N can be
described by
The quantitative effects of adding scavengers to HA
solution support the hypothesis that dissolved and bound
mirex exist in different photochemical environments. If
mirexreacted in a homogeneous solution, eq 8 would apply.
For solutions with added scavenger, eq 8 becomes
KDOC
N ) 10-3fcMW
[S]
(24)
HA1 + KDOC[DOC]
kx,s′ ) kxPx(λ)/ (kr + ks[S]T)
(18)
where fc is the weight fraction of organic carbon in humic
molecules, MWHA is the gram molecular weight ofthe humic
material, and KDOC and [DOC] are expressed in units of
L/ kg and kg/ L, respectively. In this study, [DOC] was 10-5
Kg/ L, and the scavengers had KDOC < 104 L/ kg. Therefore,
where [S]T is the total concentration of added scavenger
and ks is the second-order rate constant for reaction of the
scavenger with the reactive intermediate, X. Equation 18
can be rearranged into a linear form
KDOC[DOC] for the scavengers was ,1, meaning that the
1/ kx,s′ ) (ks/ kxPx(λ))[S]T + (kr/ kxPx(λ))
(19)
fraction of scavenger dissolved (fd, eq 16) ≈ 1.
Equation 24 can be used to estimate values of N. For
example, KDOC for lindane and 2-chloroethanol are 1670
(35, 36) and 1.4, respectively (36, 37). Assuming fc ) 0.5
(38) and MWHA ) 105 g/ mol (39), N is 0-3 for lindane and
0-2 for 2-chloroethanolover the concentration ranges used.
Since N is predicted to be small, the probability that reaction
will continue unquenched in some humic molecules (n )
0) remains significant even in the presence of high
concentrations ofscavenger (e.g., 0.025 M 2-chloroethanol).
Figure 4 shows a sample plot of the fit of eq 19 (dotted line)
to experimental data assuming that kx,s′ ) kM ′, where kM
′
s
s
is the overall apparent first-order rate constant for mirex
photoreaction in HA solution containing an added scav-
enger. In these experiments, [DOC] was held constant so
that Px(λ) was constant. The experimental data were found
to be nonlinear by ANOVA (p < 0.05) using the method of
Peterson (32). The scavenger effects are therefore incon-
9
2 9 3 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996

Equation 24 implies that it may not be possible to quench
completely the reaction of bound molecules since solubility
(maximum [S]T) tends to decrease with increasing KDOC
.
Equations 21-23 combine to give a general equation
for humic-bound mirex reaction in the presence of added
scavenger:
e-K [S]KP[S]
kr + ks
P
kx Px (λ)
P
b
b
e-K [S]
kx ,s ′ )
+
+
b
b
(
kr
[DOC]
b
b
b
e-K [S](KP[S])2
e-K [S](KP[S])n
P
P
+ ... +
(25)
)
kr + 2ks
kr + nks
b
b
b
b
This equation indicates that when N is <0.1, >90% of the
humic molecules contain no added scavenger. It also
indicates that if ks is sufficiently large, reaction in a humic
b
molecule will be completely quenched by the presence of
one scavenger molecule.
Equations 18 and 25 were substituted into eq 15 to give
an equation that describes the overall rate constant for
mirex phototransformation as a function of concentration
of added scavenger. Preliminary curve fitting of experi-
mental data to that equation indicated that the terms for
bound mirex where Prn was >0 were insignificant, meaning
that a single scavenger molecule in the humic matrix was
sufficient to quench all reaction of mirex. Therefore, the
combined equation can be simplified to
FIGURE 5. Effect of 2-chloroethanol (a), nitrate (b), and lindane (c)
on the apparent rate constant for mirex photoreaction (kM ′) relative
s
to solutions containing no additives (kM′). Lines through data points
(9) are curve fits to eq 27.
TABLE 1
Comparison of Estimated and Calculated Values of
the Humic Acid-Water Partition Coefficient K
P
kx Px (λ)
kxPx(λ)
b
b
-K [S]fb
(26)
est K a (L/MHA)
SD
calcd K b
P
P
P
k
M ′ )
fd +
e
s
kr + ks[S]
kr [DOC]
b
lindane
2-chloroethanol
nitrate
2.3 × 104
5 × 103
8.4 × 104
NAc
65
6
1
70
where the total solution concentration of scavenger ([S]T)
in eq 18 has been changed to [S], the dissolved concentration
of scavenger, in order to restrict this description to the
dissolved phase.
When [S]T ) 0, eq 26 reduces to eq 17, the overall rate
constant for mirex in the absence of added scavengers.
Dividing eq 26 by eq 17 and combining with eqs 8 and 13
gives
6.3
a
Param eter estim ates and standard deviations are from curve fits
of eq 27 to individual scavenger data sets assum ing bound phase
b
reaction only. Calculated from KDOC using eqs 23 and 24 and assum ing
c
fc ) 0.5 and MWHA ) 105 g/m ol. KDOC unknown.
Parameter estimates for KP obtained from the curve fits
in Figure 5 are compared in Table 1 with values calculated
from KDOC and eqs 23 and 24. A calculated value of KP is
not included for nitrate because its KDOC is not known. The
values for KP in both cases reflect the expected relative
affinities of the scavengers for humic acids in water,
decreasing in the series lindane >2-chloroethanol>nitrate.
This supports the view that the ability of a scavenger to
interfere in bound phase reactions depends on the affinity
of the scavenger for the bound phase. Furthermore, the
good agreement between the calculated and curve fit
estimates of KP corroborates the overall accuracy of eq 21
in representing the effects of binding on HA-mediated
photoreactions. Consequently, as has been observed by
Blough (12), not only can bound phase molecules encounter
different activities of reactive intermediates, but they also
can encounter different activities of scavengers.
The reaction of mirex in the bound phase might
appropriately be called an intrahumic reaction. The
description in eqs 20-23 for scavenger effects in the bound
phase is based on a kinetic model developed by Atik and
Singer (33) and Atik and Thomas (34) to predict the ability
of scavengers to quench intramicellar reactions. A key
feature distinguishing this model from that used for
homogeneous solutions is the dependence of intramicellar
quenching on the number of scavenger molecules in
micelles, not the bulk solution scavenger concentration.
The associated rate constant for intramicellar quenching
kx′/ kx ′
b
fd + e-K [S]fb
P
kM
′
1 + (ks/ kr)[S]
s
)
(27)
kM′
[(kx′/ kx ′)fd] + fb
b
Using measured values of fd (0.05) and fb (0.95), eq 27 was
applied simultaneously to observed effects of three scav-
engers to determine the relative reactivities of mirex in the
dissolved and bound phases (i.e., kx′/ kx ). Figure 5 il-
b′
lustrates the resulting curve fits using 2-chloroethanol,
nitrate, and lindane as scavengers. While values of the
parameters KP and ks/ kr were allowed to vary for each
scavenger, the parameter kx′/ kx ′ was held to be the same
b
for the three scavengers because experiments were per-
formed in HA solution having a constant [DOC] of 10 mg/
L. The curve fitting results indicate that reactivity of mirex
in HA solution is restricted to the bound phase (e.g., kx′/ kx ′
b
≈ 0). An additional curve fit of eq 27 (solid line) to the
2-chloroethanol data appears in Figure 4. This curve fit
was performed using a rearranged form of the equation
assuming kx′/ kx ′ ) 0. In conjunction with Figure 5a, this
b
fit demonstrates that eq 27 better describes the data than
eq 19. Therefore, the scavenger effects support the reaction
of mirex wholly within the bound phase over reaction of
mirex in homogeneous solution.
9
VOL. 30, NO. 10, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 9 3 9

is specific for reaction in a micelle containing one scavenger
molecule and has units of time-1. The model consequently
operates on a molecular level, using the Poisson distribution
to sum the molecular level kinetics into an overall rate
constant. The successful application of this model in
describing the photoreaction of mirex in the bound phase
of HA solution suggests that this reaction, like intramicellar
reactions, occurs within discrete microenvironments. The
aqueous matrix of individual HA molecules may constitute
such a microenvironment, as suggested in recent work by
Engebretson and von Wandruszka (40, 41) on the behavior
of fluorescence molecules bound to HAs. Therefore, the
appellation of “intrahumic” may be appropriate for bound
phase reactions.
19 are not in themselves sufficient to invoke intrahumic
reaction. The same deviation potentially could arise in a
purely homogeneous phase reaction if, for example, that
reaction was mediated by multiple reactive intermediates
having different rates of reaction with a given scavenger.
However, the homogeneous solution model failed to
describe the reaction of mirex in the absence of scavengers,
making necessary an alternate description for that reaction.
Taken together, the HA concentration effects and scavenger
effects support intrahumic reaction but not homogeneous
phase reaction.
The different dependencies that intra- and extrahumic
reactions are predicted to have on scavenger concentrations
ultimately are derived from scavenger effects on the steady-
state activities of reactive intermediates in the two phases.
As a consequence, these different dependencies may mean
that bound phase molecules encounter activities of HA-
generated reactive intermediates that are much higher than
currently thought. Intrahumic reactions nevertheless
complicate using model compounds and solutions to make
predictions about pollutant behavior in the environment.
Such predictions require measurements of reactive inter-
mediate quantum yields, production rates, or steady-state
concentrations. To measure the production rates of
quantum yields, probe molecules must successfully out-
compete all other loss processes. The bulk solution probe
concentration necessary to accomplish this within the
humic matrix may not be possible without changing the
solvent properties of water. An additional complication is
that numerous studies have demonstrated that the relative
magnitudes of different rate constants in homogeneous
solution do not necessarily agree with the relative mag-
nitudes of rate constants for the same reactions in micelles
(42). This means that it may not be possible to use the
intrahumic reaction rates of model compounds in con-
junction with homogeneous phase reaction rate constants
to draw conclusions about other intrahumic reaction rates.
Reported elsewhere (21, 43) are complete results of work
investigating the identity of the transient reactant respon-
sible for the transformation of mirex in HA solution. These
results suggest that the hydrated electron is the reactive
intermediate responsible for the transformation of mirex
in HA solution. Photoproduction of hydrated electron from
humic materials and natural organic matter has been
demonstrated previously (16), and the ability of hydrated
electron to initiate dechlorination reactions is well known
(cf. ref 44). However, because the hydrated electron reacts
with oxygen at diffusion-controlled rates, steady-state
hydrated electron concentrations in sunlit natural waters
generally are believed to be too low (∼10-17 M; 16) to
contribute significantly to the transformation of most
aquatic pollutants. Zepp et al. (16) excluded from this
conclusion pollutants in the bound phase. Supporting this
conclusion, the reaction of mirex in sunlit, air-equilibrated
HA solution, or natural water has a demonstrated t1/ 2 ) 7
days (14, 15).
Implications
The success of eqs 17 and 27 in describing mirex photo-
chemical behavior corroborates their use in predicting the
effect of binding on humic-mediated photoreaction. Based
on these equations, the description for the second-order
reaction of any compound C with any single HA-photo-
generated reactive intermediate X is provided by
d[C]
-
)
dt
kx Px (λ)
∞
kxPx(λ)
Prn
b
b
[C]T fd
+ fb
(28)
∑
(
)
n)0 k + nks
kr + ks[S]
[DOC]
rb
b
Equation 28 represents a simplified model applicable only
to reactions involving a single reactive intermediate and
only when certain assumptions hold true. The first of these
assumptions is that sorption equilibrium is maintained
during photoreaction and that the bound and dissolved
fractions of a compound in HA solution are therefore
constant. A second, less apparent assumption is that the
partitioning ofscavenger molecules to HAs can be described
using an average molecular weight for HAs (eq 24). Arelated
assumption is that this average molecular weight does not
change with HA concentration (e.g., due to aggregation).
It is also assumed that scavenger molecules in one phase
of HA solution do not affect the steady-state concentration
of reactive intermediates in the other phase. Equation 28
nevertheless can be expanded to describe more than one
reactive intermediate, to include descriptions of how fb and
fd vary with time, and to provide more complex descriptions
of HA molecular weight distributions and scavenger effects.
In spite of the specific limitations of the model, the
description of HA-mediated photoreaction as a homoge-
neous phase phenomenon (eq 8) is demonstrated to be a
special case of eq 28, one that is applicable only when fd
≈ 1. Additionally, provided that the assumptions behind
the model are true, eq 28 permits the reactivities of the
bound and dissolved phases of HA solution to be assessed
by looking at scavengers effects. This is possible because
the effects of scavengers in the two phases depend on
different distributions. Scavengers molecules are distrib-
uted according to a normal distribution in the dissolved
phase and according to a Poisson distribution in the bound
phase. The result is that bound and dissolved phase
reaction rates are predicted to have detectably different
dependencies on scavenger concentration. It should be
noted, however, that scavenger effects observed to deviate
from the homogeneous solution description in eqs 18 and
Regardless of the identity of the reactive intermediate
responsible for mirex transformation, a mirex half-life on
the order of 7 days means that intrahumic reaction can
contribute significantly to the environmental fate of pol-
lutants. In Lake Ontario, where mirex is a pollutant,
phototransformation of mirex has been identified as the
source of photomirex to the lake (15). Concentration ratios
of photomirex to mirex in lake sediments are on the order
of 0.1 (15, 45), while ratios in biota from the lake (e.g., fish
9
2 9 4 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996

(23) Yin, C.; Hassett, J. P. Chemosphere 1989, 19, 1289.
(24) Zepp, R. G.; Baughman, G. L.; Schlotzhauer, P. F. Chemosphere
1981, 10, 109.
(25) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol.
1985, 19, 74.
and benthic invertebrates) are as high as 0.93 (45). These
ratios reflect the efficiency of natural dissolved organic
matter in transforming mirex in this system. As a conse-
quence, these ratios also exemplifythe potentialsignificance
of intrahumic reactions in the aquatic environment.
(26) Zepp. R. G.; Wolfe, N. L.; Gordon, J. A.; Fincher, R. C. J. Agric.
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(27) Skurlatov, Y. I.; Zepp, R. G.; Baughman, G. L. J. Agric. Food Chem.
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(28) Leifer, A. The Kinetics of Environmental Aquatic Photochemistry;
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Acknowledgments
M.V.R. is grateful to CNPq (Conselho Nacional de Pesquisa,
Brasilia, Brazil) for financial support.
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Received for review December 4, 1995. Revised manuscript
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ES950906I
X Abstract published in Advance ACS Abstracts, August 1, 1996.
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