Oxidative degradation of organic compounds using zero-valent iron in the presence of natural organic matter serving as an electron shuttle.
ABSTRACT This study aims to understand the oxidative degradation of organic compounds utilizing zerovalent iron (ZVI) which is further promoted by the presence of natural organic matters (NOMs) (as humic acid (HA) or fulvic acid (FA)) working as electron shuttle mediators. The main target substrate used was 4-chlorophenol. Both HA and FA can mediate the electron transfer from the ZVI surface to O2, while enhancing the production of Fe2+ and H2O2 that subsequently initiates the OH radical-mediated oxidation of organic compoundsthrough Fenton reaction. The electron transfer-mediating role of NOMs was supported by the observation that higher concentrations of H2O2 and ferrous ion were generated in the presence of NOM. The NOM-induced enhancement in oxidation was observed with NOM concentrations ranging 0.1-10 ppm. Since the reactive sites responsible for the electron transfer action are likely to be the quinone moieties of NOMs, benzoquinone that was tested as a proxy of NOM also enhanced the oxidative degradation of 4-chlorophenol in the ZVI suspension. The NOM-mediated oxidation reaction on ZVI was completely inhibited in the presence of methanol, an OH radical scavenger, and in the absence of dissolved oxygen.
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ABSTRACT: The distribution of reaction sites on iron particles exposed to water containing carbon tetrachloride has been examined by measuring the locations of reaction products. The uniformity or localization of reaction sites has implications for understanding and modeling the reduction of environmental contaminants by iron in groundwater systems. Granular iron surfaces similar to those being used for environmental remediation applications were studied using surface analysis techniques to develop an understanding of the physical and chemical structure of the surface and oxide films. Scanning Auger microscopy and imaging time-of-flight secondary ion mass spectroscopy revealed that granular iron exposed to carbon tetrachloride saturated water exhibits chloride-enriched regions with a high degree of localization. These results indicate that significant CCl4 reduction occurred at pits rather than on the passive oxide film on the metal.Langmuir. 08/2002; 18(20).
- Environmental Science & Technology - ENVIRON SCI TECHNOL. 01/1996; 30(2).
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Response to Comment on “Oxidative Degradation of Organic
Compounds Using Zero-Valent Iron in the Presence of
Natural Organic Matter Serving as an Electron Shuttle”
Seung-Hee Kang, and Wonyong Choi
Environ. Sci. Technol., 2009, 43 (10), 3966-3967• DOI: 10.1021/es900569n • Publication Date (Web): 10 April 2009
Downloaded from http://pubs.acs.org on May 13, 2009
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Response to Comment on “Oxidative
Degradation of Organic Compounds Using
Zero-Valent Iron in the Presence of Natural
Organic Matter Serving as an Electron
We reply to the comments from Noubactep (1) regarding
our recent paper on the oxidative reactions occurring on
zerovalent iron (ZVI). We have recently reported an acceler-
ated oxidative degradation of organic compounds [4-chlo-
ZVI in the presence of natural organic matters (NOMs) and
proposed their role as an electron shuttle that mediates the
mediated electron transfer on ZVI led to generating more
Fe2+and H2O2, which subsequently initiated the OH-radical
Noubactep raised four main issues on the above work and
the itemized replies follow.
contaminant removal is initiated by the direct electron
transfer from Fe0to substrates and added that “the premise
was already questioned and/or proven inconsistent” with
citing only his own papers (3, 4). This argument is hardly
acceptable since the role of the direct electron transfer in
ZVI-mediated reactions is well established and generally
accepted among the research community. Noubactep stressed
the importance of the sequestration of organic substrates
and maintained that the oxide film on Fe0should inhibit the
action of electron shuttles. However, it is obvious that the
simple sequestration of organic compounds cannot explain
the concurrent production of chlorides with the removal of
4-CP. More importantly, 4-CP and CA were not removed at
layers on Fe0were present in the as-received iron powder.
These results clearly rule out the possible role of adsorption
to remove 4-CP using wu ¨stite (FeO) and magnetite (Fe3O4)
instead of ZVI under the otherwise identical experimental
condition. The removal of 4-CP was negligible regardless of
the presence of NOMs. The adsorption/precipitation mech-
anism may contribute in some cases depending on the kind
of substrates and the experimental conditions but it should
be negligible in our study (2). Incidentally, it should be
not require the direct electron transfer from Fe0to organic
transfer takes place from Fe0to O2(not organic compound!)
and the Fenton-mediated oxidation of organic compounds
can occur in the solution bulk. Therefore, the adsorption on
the ZVI surface or the sequestration of organic compounds
within the oxide matrix is not even necessary for initiating
the oxidative degradation.
The comment of Noubactep is rather surprising consid-
that ZVI-mediated reactions are largely based on the direct
electron transfer on Fe0is available in the literature (5, 6).
the surface is highly nonuniform (oxide films with lots of
Therefore, despite the presence of the passive oxide layer,
the electron transfer reactions occur mainly at pits (5). To
(POM) as an electron shuttle in the ZVI suspension for the
oxidative degradation of 4-CP (7). The removal of 4-CP was
accompanied by the rapid reduction of POM with the
appearance of blue color in the suspension. The blue
POM (POM + e-f POM-). This verified that the direct
electron transfer occurred from Fe0to POM although the
as-received iron powder (without acid washing) was used in
the study. This clearly indicates that the presence of oxide
films on Fe0does not prohibit the direct electron transfer.
suggested from our study was already known in the Becher
process (a mineral processing method that extracts metallic
iron from reduced ilmenite to produce synthetic rutile) that
utilized anthraquinone-based redox catalysts (8). I do not
feel this argument is well justified. Redox catalysts are
common in many chemical processes and the electron
shuttling properties of NOMs and quinone derivatives are
widely known. No research work is completely new and
and a ZVI-based water treatment method are drastically
different from each other and hardly comparable. The role
of the redox catalyst in Becher process is to accelerate the
iron dissolution from the reduced ilmenite whereas the role
of NOMs in ZVI process is to mediate the electron transfer
from Fe0to O2for the enhanced generation of OH radicals.
The two methods are totally different in their objectives and
very different in their chemical nature although both have
in common utilizing redox catalysts in the Fe0-involved
Third, it was pointed out that different amounts of Cl-
were generated according to the presence of fulvic acid (FA)
or humic acid (HA), while the kinetics of the 4-CP removal
be noted that only 4-CP case showed the difference. The
mismatch between the 4-CP removal and the chloride
generation was probably caused by the production of
and the production rate of Cl-do not have to be the same
when the intermediates are involved although we did not
carry out the analysis of the intermediates. It seems that the
chlorine-containing intermediates were generated in the
in 4-CP/HA/Fe0(see Figure 1a in ref 2).
Finally, it was mentioned that the pH variation was not
recorded during the reaction and the effects of HA and FA
might be related with their impact on the system pH. We
pH. In aqueous ZVI suspensions, the pH change is always
accompanied as a result of the iron dissolution (reactions 1
the following Fenton reaction is strongly favored at acidic
condition. Since NOMs have a pH-buffering capacity, their
Environ. Sci. Technol. 2009, 43, 3966–3967
39669ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 10, 200910.1021/es900569n CCC: $40.75
2009 American Chemical Society
Published on Web 04/10/2009
presence should retard the variation of pH. In our study, the
ZVI oxidation reactions were carried out in unbuffered
solutions since we did not want to add any chemical buffers
reactions. However, this pH effect does not seem to be
significant in the present case. The NOM-enhanced rate of
the oxidation in ZVI suspension was observed from the very
initial stage in which the pH variation from the initial pH is
insignificant (see Figure 1a in ref 2). The ZVI reactions were
such condition is unrealistic and this NOM-enhanced
oxidation effect “may not have a great practical value in the
real life systems” as we mentioned in the paper (2). It should
be noted that the main objective of the study was focused
on the fundamental understanding of the electron shuttling
role of NOMs in ZVI-oxidation process, not on exploiting
this phenomenon for practical water treatment.
compounds using zero-valent iron in the presence of natural
organic matter serving as an electron shuttle”. Environ. Sci.
Technol. 2009, 43, doi/10.1021/es900076m.
(2) Kang, S.-H.; Choi, W. Oxidative degradation of organic
compounds using zero-valent iron in the presence of natural
organic matter serving as an electron shuttle. Environ. Sci.
Technol. 2009, 43, 878–883.
removal in Fe0-H2O systems. Environ. Technol. 2008, 29, 909–
(4) Noubactep, C. Process of contaminant removal in “Fe0-H2O”
systems revisited: The importance of co-precipitation. Open
Environ. J. 2007, 1, 9–13.
(5) Gaspar, D. J.; Lea, A. S.; Engelhard, M. H.; Baer, D. R.; Mier,
R.; Tratnyek, P. G. Evidence of localization of reaction upon
reduction of carbon tetrachloride by granular iron. Langmuir
2002, 18, 7688–7693.
(6) Weber, E. J. Iron-mediated reductive transformations: Inves-
tigation of reaction mechanism. Environ. Sci. Technol. 1996,
2007, 41, 3335–3340.
(8) Bruckard, W. J.; Calle, C.; Fletcher, S.; Horne, M. D.; Sparrow,
G. J.; Urban, A. J. The application of anthraquinone redox
catalysts for accelerating the aeration step in the Becher
process. Hydrometallurgy 2004, 73, 111–121.
Seung-Hee Kang and Wonyong Choi*
School of Environmental Science and Engineering,
Pohang University of Science and Technology,
Pohang, 790-784, Korea
* Corresponding author e-mail: firstname.lastname@example.org; fax +82-
VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3967