Diuron degradation in irradiated, heterogeneous iron/oxalate systems: The rate-determining step

Article abstract of DOI:10.1021/es001324q

The purpose of this study was to examine the various factors that control the kinetics of diuron degradation in irradiated, aerated suspensions containing goethite (α-FeOOH) and oxalate, in the following denoted as heterogeneous photo-Fenton systems. In t

Full text of DOI:10.1021/es001324q

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Environ. Sci. Technol. 2001, 35, 3314-3320
Diuron Degradation in Irradiated,
Heterogeneous Iron/Oxalate
Systems: The Rate-Determining
Step
FIGURE 1. Structures of diuron (3-(3,4-dichlorophenyl)-1,1-di-
methylurea) and the main degradation product found in this study,
3-(3,4-dichlorophenyl)-1-formyl-1-methylurea.
P A T R I C K M A Z E L L I E R † A N D
B A R B A R A S U L Z B E R G E R *
Swiss Federal Institute for Environmental Science and
Technology (EAWAG), Ueberlandstrasse 133,
CH-8600 Du¨ bendorf, Switzerland
In most surface waters, the major sources of HO• are
photochemical reactions of the water constituents NO3- and
colored dissolved organic matter (3, 4). On soil surfaces and
in iron-rich surface waters, photolysis of iron(III) complexes
with natural organic ligands (e.g., humic acids) acting as
electron donors and subsequent thermal reactions including
the Fenton reaction (oxidation of Fe(II) by H2O2) is likely to
be a significant additional or even the main source of HO•
(5, 6). In these heterogeneous environmental systems, a large
fraction of iron is present as particulate or colloidal iron(III)
(hydr)oxide (e.g., ref 7). Relative rates of HO• production and
consumption have been determined in irradiated suspen-
sions containing a well-defined iron(III) hydroxide phase
and humic acid as an electron donor (6). [We define photolysis
of Fe(III) complexes and subsequent thermal reactions
including the Fenton reaction as “photo-Fenton systems”.]
Oxidative transformation of atrazine through HO• attack
has been studied in detail in homogeneous photo-Fenton
systems containing oxalate as an electron donor (8). These
authors have shown that both oxalate concentration and pH
greatly affected the rate of atrazine degradation. For het-
erogeneous photo-Fenton systems, the question of whether
surface or solution reactions are the rate-determining step
requires resolution. This study investigates the effects of pH
and oxalate concentration on the kinetics of light-induced
diuron degradation in the presence of goethite. We have
chosen these heterogeneous photo-Fenton systems as model
systems for soil surfaces.
The purpose of this study was to examine the various
factors that control the kinetics of diuron degradation in
irradiated, aerated suspensions containing goethite (R-FeOOH)
and oxalate, in the following denoted as heterogeneous
photo-Fenton systems. In these systems, attack by hydroxyl
•
radicals (HO ) was the only pathway of diuron degradation.
Studies were conducted in systems containing initially
80 or 200 mg L-1 goethite (corresponding to 0.9 or 2.25 mM
total iron) and 20, 50, 75, 100, 200, and 400 µM oxalate at
3epH e6. Both oxalate concentration and pH greatly affected
the rate of light-induced diuron transformation. In the
presence of initial 200 µM oxalate, the rate of diuron
degradation was maximal at pH 4, coinciding with the maximal
extent of oxalate adsorption on the surface of goethite.
At pH 4, the rate of light-induced diuron degradation increased
with increasing oxalate concentration, reaching a plateau
at initial 200 µM oxalate, i.e., at the oxalate solution
concentration at which the extent of oxalate adsorption
on the surface of goethite reached a maximum. These
experimental results suggest that the rate of Fe(II)(aq)
formation through photochemical reductive dissolution of
goethite, with oxalate acting as electron donor, determines
the kinetics of diuron degradation in these heterogeneous
photo-Fenton systems.
Hydroxyl Radical Form ation in Heterogeneous Photo-
Fenton System s. Figure 2 shows the various surface and
solution reactions, the intermediates and products formed
in heterogeneous photo-Fenton systems in which iron(III)
oxalate surface and solution complexes are photolyzed.
Dissolved iron(II) [Fe(II)(aq)], the superoxide/ hydroperoxyl
Introduction
•
radicals (O2•-/ HO2 ), and hydrogen peroxide (H2O2) are key
One of the unresolved environmental problems consists of
the pollution of soils and aquatic systems by chemicals used
in agriculture. One of the widely used herbicides is diuron
[3-(3,4-dichlorophenyl)-1,1-dimethylurea] since it acts on a
large variety of parasitic plants through inhibition of pho-
tosynthesis (1) (for the diuron structure, see Figure 1). Because
its herbicidal activity is required during seed germination,
diuron has to persist for 3-6 months; therefore, its degrada-
tion through chemical (hydrolysis) or biological processes is
very slow. As a result, diuron is found in surface waters and
groundwaters (1). On soil surfaces and in surface waters,
light-induced degradation may be an alternative pathway to
hydrolysis or microbial degradation. In laboratory studies,
it has been shown that diuron degradation occurs efficiently
by reaction with hydroxyl radicals (HO•) (2).
intermediates. In systems containing initially a solid iron-
(III) (hydr)oxide phase such as goethite (R-FeOOH), Fe(II)-
(aq) formation proceeds initially via photolysis of iron(III)
oxalate surface complexes (9, 10) (reaction sequence I in
Figure 2). Thereby, the first essential step consists of the
specific adsorption of oxalic acid on the surface of the iron-
(III) (hydr)oxide. Adsorption of oxalic acid on the surface of
a metal (hydr)oxide can be described with the surface
complex formation model (11-13). According to this model,
the pH dependence of adsorption of weak acids and anions
can be predicted with the aid of a surface ligand exchange
model incorporating the acid-base equilibria of both the
anion and the (hydr)oxide.
Photolysis of the Fe(III) surface complex is followed by
dissociation of the oxalate radical, C2O4•-, from the surface
and detachment of the reduced surface iron center from the
crystal lattice and transfer into solution. The oxalate radical
reacts with molecular oxygen yielding O2•-, where O2•- is in
* Corresponding author phone: +41-1-823 54 59; fax: +41-1-823
50 28; e-mail: sulzberger@eawag.ch.
† Present address: Laboratoire Chimie de l’Eau et de
l’Environnement, ESIP, F-86022 Poitiers, Cedex, France.
•
equilibrium with HO2• (pKa ) 4.8; 14). The product of HO2 /
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2001 Am erican Chem ical Society
Published on Web 07/10/2001

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ammonium acetate buffer (pH 4.5) were added. The absor-
bance was measured at the maximal absorption of the
iron(II)-ferrozine complex (λmax ) 562 nm, ꢀ562 ) 27 000 M-1
cm-1, as determined by a calibration curve). At the Fe(II)(aq)
and oxalate concentrations used in the experiments, oxalate
was not found to interfere with the Fe(II)(aq) measurements.
For the measurement of the concentration of total dissolved
iron, the procedure was the same as for the Fe(II)(aq)
measurement except that 100 µL of a reducing agent (104 g
of OHNH3Cl and 200 mL of 32% HCl in 500 mL of bidistilled
water) was used instead of H2SO4 in the first step. Also, the
sample/ reducing agent mixture was left to stand for several
minutes before addition of the ferrozine and buffer to allow
complete reduction of Fe(III)(aq) to take place.
Hydrogen peroxide was analyzed using the DPD method
developed by Bader and co-workers (22), modified to
minimize interference by Fe(II)(aq) and Fe(III)(aq). Bipyridine
was added to complex Fe(II)(aq); EDTA was added to complex
Fe(III)(aq). The modified procedure consisted in the following
steps: 0.4 mL of phosphate buffer solution (pH 6, 0.5 M
phosphate) and 0.1 mL of bipyridine solution (0.01 M) were
premixed in a 1-cm quartz cell; 2 mL of the sample, 0.02 mL
of EDTA solution (0.01 M NaEDTA), and 0.03 mL of DPD
(N,N-diethyl-p-phenylenediamine) reagent (1% in 0.1 M H2-
SO4) were then added, followed by 0.03 mL of POD
(horseradish peroxidase) reagent (about 0.8 g mL-1). The
absorbance at 551 nm was measured within 45 s.
FIGURE 2. Light-induced iron cycling, and surface (- -) and solution
(s) reactions in heterogeneous photo-Fenton systems.
O2•- dismutation is H2O2, by the following reactions (reactions
II in Figure 2):
HPLC Analysis of Diuron and Diuron Degradation
Products. Separation of diuron and its metabolites was
performed with reversed-phase HPLC, using a C18 Spherisorb
column (5 µm) from PhaseSep (Waters), 250 × 4 mm, and
isocratic elution: methanol 70%-water 30%. Absorbance
was measured continuously in the range of 190-500 nm.
Diuron peaks were normally quantified at 250 nm.
+
+8 H2O2 + O2
(1)
H
/ 2H
•
•-
2HO2 / O2
+
+8 Fe(III)(aq) + H2O2 (2)
H
/ 2H
•
•-
Fe(II)(aq) + HO2 / O2
•
•-
Hydrogen peroxide formed from HO2 / O2 is another
significant oxidant of Fe(II)(aq) by the Fenton reaction
yielding HO• (reaction III in Figure 2):
Adsorption Experim ents. The adsorption isotherm of
oxalate was established at pH 4 in the dark by equilibrating
aliquots of 50 mL containing 200 mg L-1 goethite and various
oxalate concentrations in Erlenmeyer flasks at constant
temperature (25 ( 2 °C) for 30 min. NaClO4 was used as an
inert electrolyte (I ) 0.005 M). The suspensions were stirred
during the equilibration time. The suspensions were then
filtered through a 0.2-µm cellulose nitrate filter (Sartorius),
and the concentration of oxalate was determined in the
filtrates using capillary electrophoresis with a glass capillary
of 75 µm i.d. and 60 cm length, as described by Bu¨ rgisser and
Stone (23). The electrolyte consisted of 25 mM phosphate
buffer (pH 7) and 0.5 mM tetradecyltrimethylammonium
bromide. Samples were injected in the hydrostatic mode for
30 s. A constant run voltage of 25 kV was applied. Oxalate
was detected at 198 nm. The response was linear between
20 µM and 1 mM oxalate. The concentration of adsorbed
oxalate was then calculated as the difference between the
total oxalate concentration and the equilibrium oxalate
solution concentration.
The pH dependence of oxalate adsorption was established
in suspensions containing 200 mg L-1 goethite and 1 mM
oxalate. The procedure was the same as used for the oxalate
adsorption isotherm, except that oxalate was analyzed in the
filtrates by measuring the â-counts of 14C-labeled oxalate.
Irradiation Experim ents. All irradiation experiments were
carried out in a 350-mL Pyrex reactor, equipped with a water
jacket at constant temperature (25 ( 1 °C). The suspensions
were irradiated with a XENON-short arc lamp (XBO, Osram,
maximum power 1000 W) with PTI (Photon Technology
International) 02-A5000 arc lamp housing and PTI LPS 1000
power supply. The xenon lamp in combination with Pyrex
lenses for UV cutoff at approximately 300 nm was used for
sunlight simulation. Irradiation of suspensions was per-
formed at 500 W lamp power, corresponding to approximately
1.25 kW m-2 (50 cm2 irradiated area), which is about the
Fe(II)(aq) + H2O2 f Fe(III)(aq) + HO• + OH- (3)
The rate constants of Fe(II)(aq) oxidation by HO2 / O2•- and
•
by H2O2 depend critically on the Fe(II)(aq) speciation (15-
17). In the presence of excess oxalate, Fe(III)(aq) is present
as dissolved iron(III) oxalate complexes that undergo pho-
tolysis (18) (reactions IV in Figure 2) leading ultimately to
OH• production (reactions II and III in Figure 2).
Experimental Section
Materials. All reagents were reagent grade. pH measurements
were carried out with a combined glass electrode (Metrohm)
standardized with appropriate buffer solutions (Merck).
Diuron (technical grade) was obtained from Rhone-Poulenc.
Oxalate (K2C2O4, pro analysis, Merck) was used as purchased.
All solutions were prepared in Nanopure water (Q-H2O, grade
Barnstead). The diuron degradation product, 3-(3,4-dichlo-
rophenyl)-1-formyl-1-methylurea (for the structure of this
product, see Figure 1) was synthesized according to a
procedure described by Crosby and Tang (19). Goethite was
synthesized according to the procedure described by Atkinson
and co-workers (20). Infrared spectra and X-ray diffraction
patterns were identical with a pure goethite standard. The
specific surface area as determined by Brunauer-Emmett-
Teller (BET) measurements was 89 m 2 g-1. Stock suspensions
of goethite (1 g L-1) were prepared every 2 weeks.
Measurem ent of Dissolved Iron and Hydrogen Peroxide.
The concentration of Fe(II) was measured colorimetrically
using the ferrozine method (21) with a modified procedure.
A total of 2 mL of the sample, after filtration through a 0.2-
µm cellulose nitrate filter (Sartorius), was added to 100 µL
of H2SO4 (3.6 M) in a 1-cm quartz cell. After the solution was
mixed, 200 µL of ferrozine solution (5 mM) and 100 µL of
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contamination. In the absence of oxalate, no diuron deg-
radation was observed after 10 h of irradiation (data not
shown) indicating that the presence of oxalate is a prerequisite
for HO• formation through reactions I-III (see Figure 2).
Adsorption of diuron on the surface of goethite was negligible
in the pH range used in this study. Hence, diuron degradation
does not occur on the surface of goethite, e.g., through surface
ligand-to-metal charge-transfer reactions, but in solution by
reaction with HO•. Irradiation of an aqueous solution of
diuron (100 µM) at λ > 300 nm in the absence of oxalate and
goethite did not result in diuron degradation within the time
scale typical for our experiments in this study. This is not
surprising since light absorption by diuron is insignificant
above 300 nm. In the absence of light, neither diuron was
degraded nor dissolved iron was formed during 24 h in a
broad pH range (3 < pH < 7) and at various oxalate
concentrations (data not shown).
Degradation Products of Diuron in these Heterogeneous
Photo-Fenton System s. We have determined only one diuron
degradation product by HPLC analysis with UV/ VIS detection.
This product was identified as 3-(3,4-dichlorophenyl)-1-
formyl-1-methylurea (for the structure, see Figure 1) by
comparison with an authentic sample. It represented 40-
50% of the initial diuron concentration after 15 min of
irradiation of a goethite-oxalate-diuron suspension under
the experimental conditions shown in Figure 3. This product
did not adsorb on the surface of goethite under our
experimental conditions, i.e., no loss was observed in a
suspension containing 80 mg L-1 goethite, 200 µM oxalate,
and 10 µM 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea at
pH 4, even after 24 h of stirring in the dark.
The formation of the N-formyl derivative involves an
abstraction of a hydrogen atom from the methyl group
through HO• attack. The alkyl radical (R•) reacts with oxygen
to form a peroxyl radical (ROO•), which leads to an alcoxyl
radical (RO•) via formation and subsequent decomposition
of the unstable intermediate ROOOOR (25). The classical
â-scission of RO• yields the N-formyl derivative (26).
It is worth noting that in a previous study on light-induced
diuron degradation in homogeneous systems, where HO•
was formed through photolysis of dissolved iron(III) hydroxo
complexes [mainly Fe(OH)2+] (2), the N-formyl derivative
was also the major diuron degradation product. However, at
least five additional products were identified that cor-
responded to attack of HO• at the methyl groups or at the
aromatic ring. We did not find these additional products in
this study. This discrepancy between this and the previous
study (2) could be due to fast degradation of these additional
products by HO• and/ or to their adsorption on the goethite
surface, preventing them from being detected by HPLC.
Kinetics of Light-Induced Diuron Degradation in these
Heterogeneous Photo-Fenton System s. As Figure 3 shows,
the rate of diuron degradation was constant after an induction
period. An induction period has also been observed in the
degradation of atrazine by HO• in homogeneous photo-
Fenton systems (8). This phenomenon occurs because
Fe(II)(aq) and H2O2 have first to produce HO• that degrade
a considered pollutant. However, in the study by Balmer and
Sulzberger (8), atrazine degradation obeyed first-order kinet-
ics after the induction period, whereas in this study, a constant
rate of diuron degradation was observed after an induction
period (see Figure 3). To rationalize a constant rate of diuron
degradation, sources and sinks of HO• have to be considered.
In these systems, HO• is formed through the Fenton
reaction with a formation rate (rf):
FIGURE 3. Concentration versus time courses of diuron (]), Fe(II)(aq)
(b), Fe(III)(aq) (O), and H O (1) in an irradiated, aerated suspension
2
2
containing initially 80 mg L-1 goethite, 200 µM oxalate, and 10 µM
diuron at pH 4. The figure shows also the effect of excess 2-propanol
on diuron degradation ([).
sunlight intensity at mid-latitude summer noon, as estimated
by ferrioxalate actinometry (8). The test volume was 250 mL
in all irradiation experiments, and the optical depth was 3.8
cm. The suspensions were stirred before and throughout the
experiments and left open to the ambient air. NaClO4 was
used as an inert electrolyte (I )0.005 M). The pH was adjusted
by addition of HClO4 or NaOH and kept constant by automatic
titration of HClO4 during the experiments if necessary. Diuron
from an aqueous solution was added after an equilibration
time of the goethite/ oxalate suspension of at least 45 min,
followed by another 10 min of equilibration before starting
irradiation. The initial diuron concentration was 10 µM in all
irradiation experiments. The dark control experiments were
performed as described for the irradiation experiments,
except that the suspensions were kept in the dark.
Results and Discussion
Pathways of Light-Induced Diuron Degradation in These
Heterogeneous Photo-Fenton System s. Irradiation (with λ
> 300 nm) of an aerated suspension containing initially 80
mg L-1 goethite (corresponding to 0.9 mM total iron), 200
µM oxalate, and 10 µM diuron at pH 4.0 resulted in rapid
disappearance of diuron, accompanied by the formation of
dissolved iron [both Fe(II)(aq) and Fe(III)(aq)] and H2O2
(Figure 3). The Fe(II)(aq) concentration reached a maximum
after about 10 min of irradiation, followed by its decrease
due to Fe(II)(aq) reoxidation, while Fe(III)(aq) and H2O2
accumulated in the systems on the time scale of the
experiment. In the presence of excess 2-propanol (0.28 M),
no degradation of diuron was observed at otherwise the same
conditions (Figure 3), whereas the kinetics of formation of
dissolved iron was not affected (data not shown). 2-Propanol
is known to react efficiently with HO• with a second-order
rate constant, k ) 3 × 109 M-1 s-1 (24). This provides evidence
that attack by HO• is the only pathway of diuron degradation
in these heterogeneous photo-Fenton systems.
In a deaerated, irradiated suspension containing initially
80 mg L-1 goethite, 200 µM oxalate, and 10 µM diuron at pH
4.0, the rate of diuron degradation was smaller by a factor
of 6 than in the aerated suspension under otherwise the same
conditions (data not shown). A much smaller rate of diuron
disappearance is expected in deaerated suspensions since
•
molecular oxygen is needed to produce HO2 / O2•- by reaction
rf ) kF[H2O2][Fe(II)(aq)]
(4)
of O2 with C2O4•- (reaction II in Figure 2). The fact that the
rate of diuron disappearance did not drop to zero in
irradiated, deaerated suspensions is most likely due to oxygen
where kF is the second-order rate constant of the Fenton
reaction. Possible sinks of HO• are as follows: (i) reaction
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FIGURE 5. pH dependence of oxalate adsorption on the surface of
goethite, established in suspensions containing 200 mg L-1 goethite
and 1 mM oxalate. As expected, the extent of adsorption of oxalic
FIGURE 4. pH dependence of diuron degradation in irradiated,
aerated suspensions containing initially 200 mg L-1 goethite, 200
µM oxalate, and 10 µM diuron. The inset shows the rate of diuron
degradation, i.e., the slope of the linear parts of the diuron
concentration versus time curves, as a function of pH.
acid on the surface of goethite was maximal near the second pK
a
value of oxalate, which is pK ) 3.8 ( 0.2 (28, 29).
a
oxalate, and 10 µM diuron. At all pH values, the rate of diuron
degradation was constant after an induction period and was
strongly pH-dependent. The inset in Figure 4 shows the rate
of diuron degradation, i.e., slopes of the linear portion of the
diuron concentration versus time curves, as a function of
pH. The rate was maximal at pH 4 and decreased strongly
above and slightly below this pH value. The shape of the rate
versus pH curve shown in the inset in Figure 4 strongly
resembles the pH dependence of the extent of oxalate
adsorption on the surface of goethite in suspensions con-
taining 200 mg L-1 goethite and no diuron (Figure 5). This
supports the contention that the pH dependence of oxalate
adsorption determines to a large extent the pH dependence
of diuron degradation in these heterogeneous photo-Fenton
systems.
with diuron, (ii) reaction with diuron degradation products,
and (iii) reaction with oxalate. Although the initial oxalate
concentration was 20 times higher than that of diuron in the
experimental system shown in Figure 3, oxalate is neverthe-
less unlikely to be an important sink of HO• as compared to
diuron and its degradation products. This is to be expected
in view of the relatively low second-order rate constants of
oxalate reaction with HO• which are 7.7 × 106 M-1 s-1 for
2-
-
C2O4 and 4.7 × 107 M-1 s-1 for HC2O4 (16). However,
reactions of diuron degradation products with HO• are likely
to be diffusion-controlled. The rate of HO• consumption can
then be expressed as
rc ) kHO ,D[HO•][D] +
kHO ,DP [HO•][DPi]
(5)
•
•
∑
i
However, the pH dependence of solution reactions also
has to be considered: The quantum yield of photolysis of
dissolved ferric iron complexes depends on the Fe(III)(aq)
speciation, and the rate constant of the Fenton reaction
depends on the Fe(II)(aq) speciation. According to speciation
where the second term in eq 5 is the rate of HO• consumption
through reaction with diuron degradation products, DP.
The rate of diuron degradation through reaction with
HO• is
calculations (not shown), Fe(III)(aq) is present as FeIII(C2O4)2
-
and FeIII(C2O4)33- for 200 µM oxalate concentration over the
whole pH range shown in Figure 4. Both of these iron(III)
oxalate complexes are efficiently photolyzed (30). Regarding
Fe(II)(aq), [FeII(C2O4)] increases with increasing pH according
to speciation calculations (not shown). FeII(C2O4) is oxidized
by H2O2 at a rate constant of kF ) 3.1 × 104 M-1 s-1 (16),
whereas kF involving Fe2+ is orders of magnitude smaller: kF
) 53-76 M-1 s-1 (15-17). Therefore, the Fenton reaction,
resulting in HO• generation, is faster at higher pH. On the
basis of these arguments, the sharp drop in the rate of light-
induced diuron degradation above pH 4 (see inset in Figure
4) cannot solely be due to dissolved iron speciation. Rather,
it reflects mainly the pH dependence of oxalate adsorption
on the surface of goethite as well as the pH dependence of
the efficiency of detachment of reduced surface iron species
from the crystal lattice of goethite (29) (for the latter, see
discussion below). The decrease of the rate of diuron
degradation below pH 4 could reflect both the pH depen-
dence of oxalate adsorption to the surface of goethite and
the pH dependence of Fe(II)(aq) speciation and thus of the
Fenton reaction rate.
[D]
dt
-
(
) kHO ,D[HO•][D]
(6)
•
)
•
where kHO ,D is the second-order rate constant of reaction of
diuron with HO•, which is 4.6 × 109 M-1 s-1 (27). A constant
rate of diuron degradation, after an induction period (see
Figure 3), suggests that the HO• concentration increases
according to the decrease of [D], without reaching a steady-
state within the time scale of the experiment shown in Figure
3. This is plausible considering that the HO• formation rate
(rf) increases with irradiation time due to the increase of
[Fe(II)(aq)]‚[H2O2] (see Figure 3). The rate of HO• consumption
(rc), on the other hand, is unlikely to increase with irradiation
time. At the maximum, rc would remain constant if all the
diuron degradation products would react with HO• with the
same second-order rate constant as diuron and if no
mineralization would take place. A suitable tool to obtain
more quantitative information on the mechanisms control-
ling the kinetics of light-induced diuron degradation in these
heterogeneous photo-Fenton systems is mathematical kinetic
modeling. This was, however, beyond the realm of this study
and will be published later.
Effect of Oxalate Concentration on Light-Induced Diu-
ron Degradation. Figure 6a shows diuron degradation at
various initial oxalate concentrations in irradiated, aerated
suspensions containing initially 80 mg L-1 goethite and 10
µM diuron at pH 4. Again, at all oxalate concentrations, the
Effect of pH on Light-Induced Diuron Degradation.
Figure 4 shows diuron degradation at various pH values in
irradiated, aerated suspensions containing initially 200 mg
L-1 goethite (corresponding to 2.25 mM total iron), 200 µM
9
VOL. 35, NO. 16, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 3 1 7