Environ. Sci. Technol. 2007, 41, 270-276
Oxidation of Aqueous EDTA and
Associated Organics and
Coprecipitation of Inorganics by
Ambient Iron-Mediated Aeration
J A M E S D . E N G L E H A R D T , * ,†
D A N I E L E . M E E R O F F , †,⊥
LUIS ECHEGOYEN,‡ YANG DENG,†
F R A N Ç I S C O M . R A Y M O , § A N D
T O M O Y U K I S H I B A T A †,#
Civil, Architectural, and Environmental Engineering,
University of Miami, PO Box 248294,
Coral Gables, Florida 33124-0630, Department of Chemistry,
Clemson University, 219 Hunter Laboratories,
Clemson, South Carolina 29634-0973, and Department of
Chemistry, University of Miami, 1301 Memorial Drive,
Coral Gables, Florida 33146-0431
Cationic metal and radionuclide contaminants can be
extracted from soils to groundwater with sequestering
agents such as EDTA. However, EDTA must then be removed
from the groundwater, by advanced oxidation or specialized
biological treatment. In this work, aqueous individual metalEDTA solutions were aerated with steel wool for 25 h, at
ambient pH, temperature, and pressure. Removal of
approximately 99% of EDTA (0.09-1.78 mM); glyoxylic
acid (0.153 mM); chelated Cd2+ (0.94 and 0.0952 mM), Pb2+
(0.0502 mM), and Hg2+ (0.0419 mM); and free chromate
and vanadate was shown. EDTA was oxidized to glyoxylic
acid and formaldehyde, and metals/metalloids were
coprecipitated together with iron oxyhydroxide floc. Free
arsenite and arsenate were each removed at 99.97%. Free
Sr2+, and chelated Ni2+ were removed at 92% and 63%,
respectively. Similar removals were obtained from mixtures,
including 99.996 ( 0.004% removal of total arsenic (95%
confidence). Traces of iminodiacetic acid, nitrilotriacetic
acid, and ethylenediaminetriacetic acid were detected after
25 h. Results are consistent with first-order, solutionphase oxidation of EDTA and glyoxylic acid by ferryl ion
and H2O2, respectively, with inhibition due to sludge
accumulation, and equilibrium metal coprecipitation. This
ambient process, to our knowledge previously unknown,
agrees with recently reported findings and shows promise
for remediation of metals, metalloids, and radionuclides
in wastewater, soil, and sediment.
Introduction
Recently metal pollution was shown to reduce bacterial
biodiversity in soil by more than 99.9%, underscoring the
* Corresponding author phone: 305-284-3391; fax: 305-284-3492;
e-mail: [email protected].
† Civil, Architectural, and Environmental Engineering, University
of Miami.
‡ Clemson University.
§ Department of Chemistry, University of Miami.
⊥ Current address: Department of Civil Engineering, Florida
Atlantic University.
# Current address: Department of Fisheries and Wildlife, Michigan
State University.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
eco-toxicity of metal contamination (1). However, technologies for cleanup of metals and radionuclides in soil, sediment,
and groundwater are less developed than those for organic
contamination, and cleanup is often complicated by the
presence of organic contaminants (e.g., tricholoroethylene).
Divalent and trivalent metal and radionuclide cations can
be extracted from soil with aqueous ethylenediaminetetraacetic acid (EDTA), either in-situ or ex-situ (2-6), partly
because the reported ordering of stability constants, Na+ <
Mg2+ < Ca2+ < Fe2+ < Al3+ < Zn2+ < Cd2+ < Pb2+ < Ni2+ <
Cu2+ < Hg2+ < Fe3+ (7-9), favors removal of metal contaminants over natural hardness ions. A principal limitation
is that, while EDTA is nontoxic, it is a metal-mobilizing
pollutant that would require removal from the groundwater.
Conventional removal would involve ex-situ advanced
oxidation or specialized biological treatment (10-13), and
regulatory permits would then be required for re-injection
of treated water to the aquifer.
A convenient method of removing EDTA and its metallic
complexes from water would facilitate remediation of soil
sediment and industrial wastewater. EDTA is subject to
Fenton oxidation, and in fact, the Fenton reaction can occur
at ambient pH when iron is complexed with EDTA or an
EDTA analog (14). Motekaitis et al. (15) subsequently reported
oxidation of EDTA without H2O2, by Fe(III) with and without
O2, at temperatures above 100 °C. Wubs and Beenackers (16)
further implied that H2O2 produced in the conversion of Fe(II)-EDTA to Fe(III)-EDTA by O2, at temperatures from 20 to
60 °C, could oxidize the organic compounds present. In
addition, Palmer and Boden (17) observed losses of aqueous
EDTA in steel process equipment at pH 7.0-10.5 at 70 °C
without added H2O2, and proposed a reductive mechanism.
Subsequently, we presented preliminary studies (18, 19)
showing efficient degradation of EDTA without H2O2 addition,
with iron under aerated conditions at ambient temperature.
Following the preliminary results just mentioned, expanded laboratory studies were conducted to confirm and
characterize the ability of ambient iron-mediated aeration
(IMA) to (a) decompose EDTA and associated organics, and
(b) simultaneously remove metal, metalloid, and radionuclide
cations and oxyanions by coprecipitation with the iron
oxyhydroxide floc formed. This work comprised the first phase
of development of a proposed soil/sediment/groundwater
remediation process. The purpose of this paper is to present
the results of those and follow-up studies (20), including the
oxidation of EDTA and glyoxylic acid; coprecipitation of Cd2+,
Ni2+, Pb2+, Hg2+, Sr2+, chromate, arsenite, arsenate, and
vanadate; and byproduct formation. Kinetic and scavenger
studies are presented to support mechanistic conclusions.
Experimental Section
All reagents were at least reagent grade and were used as
received, and solutions were prepared with Milli-Q water
(18.2 MΩ-cm), except as noted. All bottles, glassware, and
high-density polyethylene (HDPE) containers were acid
washed (20% aqueous v/v, HNO3, trace metal grade) prior
to use. Metal stock solutions, 0.1 M, were prepared in
deionized water from Na2HAsO4·7H2O, NaAsO2, Pb(NO3)2,
HgCl2, K2CrO4, NaO3V, Sr(NO3)2, and CdCl2. Cadmium salts
were dried at 110 °C for 2 h and stored in a desiccator. Nickel
was prepared from NiCl2‚6H2O as a 1000 mg/L stock solution.
Ethylenediaminetetraacetic acid was obtained as either the
fully protonated form (H4Y), or the disodium salt dihydrate.
A stock solution of 0.1 M H4Y was prepared as described in
Flaschka (21). From these solutions all calibration standards
and sample influent solutions were prepared daily. EDTA
10.1021/es061605j CCC: $37.00
 2007 American Chemical Society
Published on Web 11/18/2006
FIGURE 1. Schematic of the iron-mediated aeration test tube reactor.
standard solutions were prepared by the method of Bergers
and DeGroot (22).
Metal-EDTA sample solutions were prepared in simulated
natural water containing 0.5 mM CaCl2 and 1.63 mM NaHCO3
in Milli-Q water, adjusted to pH 7.5 with concentrated HCl.
Immediately after addition of cationic metal from stock
solution, EDTA was added from stock solution in 2:1 molar
ratio with the metal. Solutions containing anionic metal
oxides were prepared similarly, without EDTA. All sample
solutions were then equilibrated overnight with stirring in
the dark.
Oxidation/Coprecipitation Experiments. Typical experiments involved treatment of 23 mL samples, self-buffered at
pH 7-8 through continuous aeration over periods up to 25
h in the dark or under ambient light. Test tube reactors were
prepared as shown in Figure 1. Commercial steel wool (Brillo)
was prepared as the iron source by hexane rinse, complete
drying, 0.1 N HCl rinse, and final drying. The superficial
surface area based on fiber diameter measurements was
0.1608 ( 0.0002 m2/g. In each 30 mL test tube reactor 0.71.0 g steel wool was arranged uniformly between the outer
glass tube and the test tube wall. Samples were aerated or
deaerated with air or nitrogen gas, respectively, both prehumidified by bubbling through Milli-Q water. Gas was
delivered through the inner tube to the annular space
between the two glass tubes in the test tube center, causing
vertical circulation of sample upward through the annular
space and downward through the steel wool packing. Gas
flow was controlled at 5 ( 1 mL/s over the treatment period.
Homogeneous circulation was verified using bromocresol
purple dye. Samples were weighed before and after treatment,
and the difference in weight was used to correct all
concentrations for evaporative losses, as necessary.
Treated samples were prefiltered (Whatman no. 4) and
filtered through Millipore 0.45 µm nitrocellulose (HA) filter
membranes pretreated with 1:1 HNO3:H2O, rinsed with
Milli-Q water, and air-dried. Filtrates were then stored in
opaque HDPE bottles for solubles analysis. Samples to be
analyzed for dissolved metals were adjusted to pH 2.0 with
1:1 HNO3:H2O. Control samples were tested for metals and
EDTA before and after acidification and filtration, verifying
g95% recovery. All analytical results reported represent the
mean of three replicate samples, with error bars corresponding to (1 standard deviation, except as noted.
Analytical. Total chelatable EDTA in sample filtrates was
determined by reverse phase high performance liquid
chromotagraphy (HPLC) with ultraviolet light (UV) detection
(λ ) 254) by the method of DeJong et al. (23) and Bergers and
DeGroot (22). That is, EDTA in filtered samples was converted
to the Fe(III) complex with excess FeCl3 solution (g50:1 Fe3+:
EDTA molar ratio) and glacial acetic acid, and stirred to
equilibrate. Mobile phase (0.03 M acetate-acetic acid buffer,
pH 4.0 with counterion) was prepared as described by Bergers
and DeGroot (22). Analysis of Cd-EDTA, H4-EDTA, Fe(III)EDTA, and free Cd2+ and Fe3+ for absorption peaks near 254
nm by UV-vis spectrophotometry (HP8452A diode array,
Palo Alto, CA) confirmed no interfering absorption from
species other than Fe(III). Quantitative replacement of
contaminant metals by Fe(III) in the equilibrated samples
was verified versus replicate metal-EDTA solutions, at two
dilutions, heated at 80-90 °C in a water bath for 8 h. A quality
control (QC) standard was prepared daily from a current,
certified 0.1 M disodium EDTA reference standard (0.09951.005M), and used to validate the standard curve to within
( 0.1-3%.
Cadmium, nickel, mercury, lead, chromium, strontium,
and vanadium were quantified by inductively coupled
plasma-atomic emission spectroscopy (ICP/AES) using a JY
24 Sequential ICP (Jobin Yvon, Edison, NJ) following U.S.
Environmental Protection Agency (EPA) method 6010B, at
214.538, 221.647, 194.227, 220.353, 284.325, 421.522/346.446,
and 311.071 nm, respectively. Quality control samples of 100
mg/L were prepared from stock solutions of each metal and
verified against ICP analysis to within (5%. ICP calibration
standards for Cd, Ni, Pb, Cr, Sr, and V were prepared from
commercial stock solutions (1000 mg/L, 2% HNO3), diluted
with deionized water (Nanopure Infinity Ultrapure Water
System, Barnstead). ICP analyses were verified periodically
with 0.7 mg/L QC metal samples, adjusted to pH to 2.0 with
HNO3.
Total arsenic was determined according to EPA method
6020, by inductively coupled plasma-mass spectrometry
(ICP-MS) (HP4500 Plus with HP ChemStation Software
WinNT), to a detection limit of 0.2 µg/L. Calibration standards
and samples were prepared from 0.1 M Na2HAsO4‚7H2O stock
solution diluted with 2% HNO3 (trace metal grade), and 50
µg/L of an internal standard solution consisting of indium,
scandium, and yttrium. Selective ion monitoring of mass-75
was chosen for arsenic analysis. Interference correction for
the mass-75 isotope was automatically computed.
Byproduct Analysis. Analysis of EDTA oxidation products
was performed with a Hewlett-Packard (HP) 5890 GC with
HP 5970 Mass Selective MS Detector (Palo Alto, CA), HP
59970C MS ChemStation software, and capillary chromatography column (0.25 mm × 30 m) (Alltech Associates, Inc.,
Deerfield, IL) using Phase EC-1 with 0.25 µm film thickness.
Samples were derivatized as follows. Sample filtrate (5 mL)
was dried under argon at 100 °C. One mL of boron trifluoride
in methanol (10% w/v) was added, followed by heating to
100 °C for 40 min in a sealed ampule, and cooling. One mL
chloroform (nanograde) was added. The mixture was then
transferred to a test tube containing 3 mL of potassium
phosphate monobasic buffer to which 10 M NaOH had been
added to adjust the pH to 7.0. The test tube was shaken for
1 min and centrifuged. The chloroform extract was removed,
dried under argon, and reconstituted in 50 µL of chloroform
for analysis. Column temperature was ramped from 150 to
285 °C at 10 °C per minute. Injector and detector temperatures
were 285 °C. Samples were analyzed at a flow rate of 1 mL/
min using helium carrier gas at an inlet pressure of 5.0 psi.
Positive identification was made using mass spectral traces
and confirmed against analytical standards of pure compounds.
Glyoxylic acid was determined by UV-vis spectrophotometry by the method of Caprio and Insola (24). Formaldehyde was determined by the UV-vis spectrophotometric
method of Josimovic (25). Nitrate was determined qualitatively in the range of 14-52 mg/L NO3--N by the colorimetric
VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Removal of 1.78 mM EDTA by IMA (24 and 48 h) and by
aeration alone (48 h), and removal of 3.24 mM EDTA by aeration in
the presence of Fe(III) sludge (24 h). [Conditions: pH 7.6, ambient
temperature and light.]
FIGURE 2. Concentrations of EDTA, cadmium, and ferric oxyhydroxide floc (mM) versus time of dark iron-mediated aeration, and
iron-mediated nitrogen sparging under ambient light. Note: error
bars for aerated samples are hidden by data markers. [Conditions:
pH 7.6, ambient temperature.]
method of Cooper (26) and quantitatively by an adaptation
of the chromotropic acid test (27) with a spectrophotometer,
0.6 mg/L NO3- -N detection limit. Total ferric oxyhydroxide
precipitate was determined by flame atomic absorption
spectroscopy (Perkin-Elmer Analyst 800) after magnetic
removal of Fe(0) particles and sample homogenization.
Results
EDTA Oxidation. Results in Figure 2 show removal of EDTA
and Cd2+ versus time, in aerated/dark and deaerated/
ambient-light samples. Aerated samples acquired a pale
yellow color within 15-20 min, suggesting formation of the
Fe(III)-EDTA complex. After 30 min, an iron oxyhydroxide
layer developed on the steel wool. Thereafter, sludge accumulated continuously in the reactor, indicating minimal
Fe(0) passivation. Removal of 98.3% of initial EDTA and 97.4%
of Cd2+ were achieved after 25 h in aerated samples. Minimal
removal, e.g., by volatilization or photolysis, was observed
in the deaerated samples, indicating a non-photolytic,
ambient oxidation, though in subsequent tests the reaction
was accelerated with UV irradiation (20, 28). Iron corrosion
was modeled (correlation coefficient, r ) 0.9985, p ) 0.0001)
assuming mass transfer-inhibition with increasing FeIII(s)
sludge accumulation, as shown:
d[FeIII
(s)]
dt
a
) k1(Fe0surf - k2[FeIII
(s)] ),
(1)
a is an
in which Fe0surf indicates surface Fe(0), k2[FeIII
(s)]
0
empirical inhibition term, k1Fesurf ) 294, k1k2 ) 65.9, a )
0.165, and p is the probability of obtaining a correlation as
high as r by random chance, if the true correlation is zero.
EDTA oxidation was then modeled (r ) 0.99995, p )
0.0000005) as first order with respect to EDTA, decreasing
with accumulated ferric oxyhydroxide sludge:
d[EDTA]
b
) -k3[EDTA][FeIII
(s)] ,
dt
(2)
in which k3 ) 0.970 and b ) -0.784. Other orders did not fit
the data, nor did saturation relationships with respect to
EDTA and/or iron, indicating a solution-phase reaction.
Removal of metals by coagulation/flocculation generally
occurs within minutes, and, accordingly, the following
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
equilibrium model (r ) 1.0000, p ) 1.6 × 10-5) described
cadmium removal from 0.5 h onward, when [FeIII
(s)] g [EDTA]:
2+
2+
2+
c
[Cd(aq)
] ) [Cd(aq)
]0 - K[Cd(aq)
][FeIII
(s)]
(3)
Assuming essentially no Cd2+-EDTA (replaced by FeIIIEDTA), K ) 4.32 × 10-4 is the equilibrium constant for the
formation of cadmium-iron coprecipitate, and c ) 3.09.
To determine the contributions of solid-phase partitioning
and iron-catalyzed oxidation to the observed removal, a 1.78
mM EDTA solution was treated in the IMA reactor for an
initial 24-hour period. The reactor was then shaken and the
steel wool removed, leaving iron in the form of ferric sludge.
EDTA was added to bring the average concentration to 3.24
mM. These samples were then aerated for an additional 24
h. EDTA removal was measured after the initial 24 h
treatment, and again after the second 24 h treatment period.
Control samples were treated for 48 h (a) with steel wool and
aeration, and (b) with aeration only. As shown in Figure 3,
essentially complete EDTA removal was observed in the
samples treated with steel wool and aeration, whereas no
significant removal was observed in the sample aerated in
the presence of ferric sludge alone, or in the aerated control
lacking iron. Therefore, either the reaction was catalyzed by
Fe(0) or Fe(II), or the oxidation of Fe(0) or Fe(II) was required
such that iron participation was not strictly catalytic.
Furthermore, EDTA did not partition to the iron oxyhydroxide
precipitate, as seen by the lack of re-equilibration with the
sludge phase after 24 h in the samples brought to 3.24 mM,
in agreement with previous work (12).
Twenty 5-h, aerated samples of Figure 2 were tested for
reported byproducts of EDTA oxidation (11, 29-33). Results
of GC/MS analysis of a representative sample are shown in
Figure 4. Peaks were identified as iminodiacetic acid (IDA),
nitrilotriacetic acid (NTA), ethylenediaminetriacetic acid
(ED3A), and residual EDTA. If present, N-hydroxymethyliminodiacetic acid and, potentially, N-acetylethylendiamine, N,N′-methylacetylethylenediamine, N-methyltriacetylethylenediamine, and ethylenediamine diacetic acid would
have been expected to appear in the mass spectra, but were
not detected.
The concentration of glyoxylic acid as a function of time
in the Cd-EDTA replicate of Figure 4 is shown in Figure 5.
Surprisingly, the glyoxylic acid produced as a byproduct of
EDTA oxidation was also removed and presumably oxidized
to formaldehyde. Formaldehyde increased with treatment
time, particularly after 16 h, as shown for another replicate.
Formaldehyde measured 0.15 mM in the replicate of Figure
4 after 25 h treatment. (Although volatile, formaldehyde is
not stripped due to substantial hydration and a low Henry’s
Law constant.) Thus, for the sample of Figure 4 after 25 h of
FIGURE 4. GC/MS chromatogram of Cd2+-EDTA sample treated for 25 h. and having 99% EDTA removal. [Sample: one of four replicates
shown in Figure 1.]
FIGURE 5. Glyoxylic acid concentration, and formaldehyde absorbance, versus time in aerated and deaerated Cd2+-EDTA samples
of Figure 4.
treatment, 0.21 mM glyoxylic acid (Figure 5), 0.15 mM
formaldehyde, and 0.02 mM EDTA (Figure 2) remained.
Assuming production of four gyloxylic acid/formaldehyde
molecules per EDTA molecule, glyoxylic acid, formaldehyde,
and EDTA accounted for 3, 2, and 2% of the original EDTA
mass, respectively. If significant ED3A or other large fragments
remained after 25 h, the percentages represented by glyoxylic
acid and formaldehyde would be higher, though this appears
unlikely based on mass spectra. Nitrate was not detectable
at the 1.0 mM level in a Cd-EDTA sample treated for 301
min. Unaccounted mass was presumably converted to low
molecular weight compounds including simple organic acids
and CO2, as characteristic of EDTA oxidation and reported
for this system (34).
The oxidation of glyoxylic acid by IMA in the absence of
EDTA, with and without the hydroxyl radical scavenger
p-chlorobenzoic acid (pCBA), and with simple aeration as a
control, is shown in Figure 6. As shown, 99.9% removal of
glyoxylic acid was achieved with IMA in 27 h, with or without
pCBA, effectively ruling out hydroxyl radical as the oxidant
in this case. Iron corrosion was modeled closely with eq 1
(r ) 1.000, p ) 2.5 × 10-7) for the EDTA system, with k1Fe0surf
) 384, k1k2 ) 2.64, and a ) 0.636. That is, iron corrosion was
more rapid and more time-dependent than in the EDTA
FIGURE 6. Glyoxylic acid (GA) and ferric oxyhydroxide sludge
concentration versus IMA treatment time, with and without 0.256
mM pCBA as a hydroxyl radical scavenger [pH 7.5; initial
concentration 0.153 mM].
system. Glyoxylic acid oxidation closely followed the analog
to eq 2 (r ) 0.9994, p ) 1.6 × 10-5). However, in contrast with
EDTA, oxidation was largely independent of iron sludge
accumulation with b ) 0.0831 and k3 ) 0.557.
Removal of Chelated Metal Cations, Free Strontium,
and Metal/Metalloid Oxyanions. The rate of removal of
metals, simultaneous with the oxidation of EDTA and
glyoxylic acid, was studied in parallel tests. Removal from
individual, ca. 10 mg/L solutions of EDTA-chelated Hg2+,
Ni2+, Pb2+, and Cd2 was tested (Figure 7 and Table 1). Also,
removal of 9.3 mg/L unchelated Sr2+ (mobile in groundwater)
and 9-20 mg/L arsenate, arsenite, vanadate, and chromate
from individual solutions was tested. Iron oxyhydroxide layers
developed within approximately 45 min in all samples except
for strontium, which required 5-7 h. As observed in cadmium
tests, EDTA removal was greater than 98%, with most
concentrations at or near the 0.5 mg/L detection limit, after
25 h of treatment. Removal of Cd2+, Hg2+, and Pb2+ was also
greater than 98%. Nickel removal was variable and averaged
63%. Average Sr2+ removal was 92.1 ( 3.6%. Greater than
99% of chromate and vanadate, and 99.97% of total arsenic,
were removed. Dispersed black particles and blackening of
the steel wool surface in samples containing arsenic were
observed, though this occurrence was unexplained. Similar
removals were observed from mixtures of cations and
mixtures of oxyanions (Table 1).
VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
273
FIGURE 7. Initial and final concentrations of (a) EDTA and (b) cations treated by IMA individually for 25 h. Triplicate samples except
cadmium (duplicate). [Conditions: ca. 10 mg/L metal, 2.0 EDTA:metal molar ratio, pH 7.6, ambient temperature and pressure.]
TABLE 1. Removal of Chelated Cations, Strontium, and Anions,
Individually and from Mixtures after 25 h Treatment Timea
inorganic solute
C0 (µM)
inorganic
solute
individual
cations
Pb2+
Hg2+
Ni2+
Cd2+
Sr2+
meanb
50.2
41.9
170
95.2
EDTA
C25/C0
0.0212
<0.018
0.377
<0.007
(C25/C0)
std
dev.c
C0
(µM)
0.0079
93.5
(4/4 < 1.7) 92.8
0.245
294
(2/2 < 0.9) 179
0
meanb
<0.018 (4/4 < 1.7)
<0.018 (4/4 < 1.7)
0.0251 0.0184
0.0095 0.0010
0
(4/4 < 1.7)
individual
anions
chromate 354
vanadate 186
arsenate 108
arsenite 115
<0.0073 (2/3 < 1.9) 0
<0.0003 (4/4 < 2.0) 0
0.00034 0.00050 0
0.00034 0.00050) 0
cationic
mixture
Cd2+
Pb2+
Hg2+
Ni2+
0.0200 0.0099
626
0.0093
<0.0107 (3/4 < 0.5) (same) (same)
<0.014 (4/4 < 0.5) (same) (same)
0.366
0.231
(same) (same)
90.0
47.9
35.4
170
anionic
mixture
chromate 391
vanadate 190
arsenite 239
<0.005 (4/4 < 2)
<0.01 (4/4 < 2)
0.00004 0.00002
std
dev.c
k2,k-2
[FeII(edta)(H2O)]2- + O2 798 [FeII(edta)(O2)]2- + H2O
(4)
k3
0.0059
(same)
(same)
(same)
0
(same)
(same)
Conditions: ambient temperature and light, self-buffered pH 7-8.
b Nondetects assumed equal to the detection limit. c In parentheses:
fraction of nondetects < detection limit (µM).
Discussion
To our knowledge, the ambient, iron-mediated oxidation of
EDTA and glyoxylic acid by dioxygen was unknown prior to
the work presented here and previously (18, 20), though it
is consistent with recent studies. In particular, Noradoun et
al. (34, 35) reported the oxidation of 4-chlorophenol in ironmediated, aerated aqueous EDTA solution. In addition,
observed removal of arsenic from water is consistent with
that reported by Hug and Leupin (36) in aerated Fe(II)
solution. Joo and co-workers (37, 38) showed traces of H2O2,
and oxidation of <0.1% benzoic acid, in aerated ambient
9
To generalize oxidation by IMA to other organics, the
mechanism by which the O-O bond of dioxygen is destabilized and cleaved is of interest. In the case of EDTA, the
first-order solution-phase oxidation indicated in eq 2 is
consistent with the mechanism proposed by Seibig and van
Eldik (29) for the oxidation of Fe(II)-EDTA with O2 to Fe(III)-EDTA, in which eq 5 is the rate-limiting step:
2[FeII(edta)(O2)]2- 98 [FeIII(edta)(O2 )]
a
274
suspensions of zerovalent iron nanoparticles without EDTA.
Subsequently, Feitz et al. showed >50% oxidation and/or
coagulation of the carbothiolate herbicide, molinate, by IMA
(39).
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
(5)
k4
2[FeII(edta)(H2O)]2- + [FeIII(edta)(O98
2 )]
III
4[(edta)FeIII(O2+ H2O (6)
2 )Fe (edta)]
k5 H+, fast
V
2[FeIII(edta)H2O]- + H2O2
(7)
To oxidize the EDTA molecule (not subject to H2O2 attack
(40)), H2O2 may react with Fe(II) to generate hydroxyl radical
in a Fenton-like step (39), as suggested by Walling for lowpH Fenton degradation of EDTA (41). However, Seibig and
Van Eldik (29) showed electron paramagnetic resonance
(EPR) results ruling out hydroxyl radical formation in the
Fe(II)-EDTA-O2 system (pH 2.2), and the results of Figure
6 further rule out hydroxyl radical in the neutral glyoxylic
acid-Fe(II)-O2 system. Also, in Fenton oxidation of EDTA
at circum-neutral pH, reaction products and EPR spectra
indicate a ferryl-like oxidant and not OH‚ radical (42, 43).
Walling further showed evidence against singlet oxygen (44).
In addition, Hug and Leupin (36) suggested an alternate
oxidant such as ferryl ion in the iron-mediated, aerated
oxidation of arsenic (III) to arsenic (V) at ambient pH, based
on lack of the 2-propanol inhibition seen at low pH. Also,
Buda et al. suggested ferryl ion, [FeIVO]2+, as the principal
oxidant in Fenton systems, based on density functional
computations for a system containing unchelated iron (45).
Upon formation of reactive oxygen species, e.g., ferryl,
accompanying organics would be subject to oxidation. In
particular, aliphatic tertiary amines undergo R-cleavage upon
oxidation in the presence of water (46, 47). Presumably, the
iron-mediated generation of strong oxidants causes the
oxidation of EDTA to the corresponding radical cation. This
intermediate can then lose carboxylate arms in the form of
glyoxylic acid after hydrolytic R-cleavage.
In the case of glyoxylic acid, a 2-electron-transfer mechanism for the reduction of oxygen to H2O2 in neutral-basic
solution is proposed as follows:
Fe(OH)2(aq) + O2 f Fe(OH)2+ + O2-·
(8)
Fe(OH)2(aq) + O2-· + 2H2O f Fe(OH)2+ + H2O2 + 2OH(9)
Equations 8 and 9 are based on arguments of Wehrli (48)
that Fe(OH)2 is oxygenated more rapidly than Fe2+ and FeOH+
to produce O2-· in the rate-limiting step, though Fe2+ is
thermodynamically favored. While H2O2 formation by this
route may be less productive than that of eqs 4-7, H2O2
alone can oxidize glyoxylic acid at ambient pH (49). In
comparison with the suggested mechanism of EDTA oxidation, this route represents fewer steps requiring mass transfer,
and may therefore explain the lack of Fe(s)III dependence in
Figure 6.
Several factors may contribute to the overall efficacy of
the IMA process presented here. First, dioxygen has generally
been found to accelerate the Fenton reaction, while acting
as a supplemental oxidant, in acidic and basic solution (50,
51). Also, the continuous supply of Fe(II) obviates the low
pH requirement for maintaining iron solubility. The use of
Fe(0) is in turn facilitated by the addition of O2 rather than
H2O2, the latter of which accelerates Fe(0) surface passivation
at neutral pH. Also, two limitations to high-pH Fenton
systemssthe increase in OH‚ scavenging by bicarbonate,
and reduction in the oxidation potential of OH‚ with increased
pH (52)smay not affect oxidation by ferryl or H2O2. Finally,
chloride in the test water could have acted to reduce Fe(0)
surface passivation (53); however, Fe(0) passivation may not
be important in the absence of H2O2 addition.
Results indicate that the removal of chelated Cd2+, Pb2+,
and Hg2+ is limited by the rate of EDTA oxidation. The variably
low nickel removal has been confirmed in subsequent tests
of the same type (data not shown). This variability is not
explained by the high solubility of Ni(OH)2, because equilibrium modeling of the aerated CaCl2-NaHCO3-NiCl2 system
tested indicates >99% nickel carbonate precipitation, even
assuming significant nickel impurity in the steel wool. Further,
efficient Ni-EDTA oxidation is confirmed in Figure 7, and is
attributed either to auto-oxidation of Ni-EDTA in the presence
of dioxygen (54), or to replacement of Ni(II) by Fe(III) within
10 min under conditions of these tests (20). However,
ammonia, an EDTA oxidative byproduct, forms stable
complexes with NiII in solution (55) {e.g., diaquotetrammine
nickel (II) nitrate ([Ni(NH3)4(H2O)2](NO3)2) and hexamminenickel (II) chloride ([Ni(NH3)6]Cl2)}. Competing with
ammonia complexation would be stripping of free ammonia,
and this dynamic interaction under conditions of variable
mass transfer due to uneven sludge accumulation may
explain the variable nickel removal in our reactors.
Results of this study indicate that the ambient IMA process
is efficient for oxidizing EDTA and associated organics, and
coprecipitating many common metals, radionuclides, and
arsenic from aqueous EDTA solution. Therefore, the process
may be useful for remediating contaminated soil, sediment,
and industrial wastewater.
Acknowledgments
This work was supported by the National Energy Technology
Laboratory, U.S. Department of Energy grant DE-AC2601NT41302. Curt Woolever is thanked for chemical analysis
of metals and EDTA oxidation byproducts. Yong Cai and
Bernine Khan are thanked for conducting the arsenic
analyses.
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Received for review July 6, 2006. Revised manuscript received
September 6, 2006. Accepted September 27, 2006.
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