17th-IHSS Conference
September 1-5, 2014 Ioannina, Greece
The Dual Role of Humic Acid on Catalytic Decomposition of
Pentachlorophenol by the Iron-Fenton System
A. Smith (a) *, B. White(b), C. Rice(b)
(a) Address a
(b) Address b.
* Corresponding author e-mail: xxxxxx@yyyyyy
Keywords: up to 6 keywords
Abstract [UP TO 150 WORDS] The capacity of Fenton [Fe/H2O2] on the catalytic decomposition of
pentachlorophenol (PCP) in the presence and absence of humic acid (HA) was evaluated. Numerous studies have
shown that HA can act either as an enhancer or inhibitor of the Fenton reaction. Herein, by combining Electron
Paramagnetic Resonance (EPR) spectroscopic and catalytic data, it is shown that HA can act in a dual manner e.g.
either as an enhancer or as an inhibitor for PCP oxidation depending on the initial Fe/HA ratio. A quantitative limit is
established: [i] at low Fe/HA < 1.17 mmol/gr the presence of HA resulted to a decrease of PCP decomposition while
[ii] at high Fe/HA > 1.17 mmol/gr a significant enhancement was observed. EPR spectroscopy revealed that, HA
acted [i] as a chelator of Fe and [ii] as a redox agent..
Introduction [UP TO 350 WORDS]
Pentachlorophenol (PCP) is poorly biodegradable,
highly toxic therefore considerable research efforts
have been devoted to the development of abiotic
catalytic methods e.g. dechlorination by metal
catalysts and Advanced Oxidation Processes. The
Fenton system (FeII/ H2O2) offers tha basis for a ccost
–effective technology for the degradation of a broad
range of organic pollutants. Hydroxyl radical (HO˙) is
formed during the catalytic decomposition of H2O2 by
iron salts. HO˙ is a highly reactive, nonselective
radical (1). Since HO˙ is nonselective, in natural
aquatic samples it may react with dissolved natural
organic matter (NOM) i.e. beyond organic pollutants.
In this way NOM can be a significant sink of HO˙
resulting to a considerable decrease of the reaction’s
efficiency. Moreover, Fe-binding by compounds, such
as NOM, humic (HA) and fulvic acids (FA), can also
alter the rate constant for the reaction or the redox
cycle of iron and thereby change the formation rate of
HO˙ . It has been demonstrated that in a catalytic
Fenton system consisting of Fe, HA and H2O2, HO˙
radicals are produced (2). However, the role of HA on
the HO˙ radicals appears controversial e.g. HA has
been reported to either increase (3) or decrease (4) the
production of HO˙. To add to this complexity, recently
it has been reported that the initial catalytic conditions
may influence HO˙ production (5). The effect of HA
on the catalytic efficiency of the Fenton reaction still
remains controversial since the degradation of organic
pollutants has been reported to be either inhibited (6)
or enhanced (7) in the presence of humic materials.
All these cases are summarized in ref. 8. The aims of
the present work were (a) to examine the role of a well characterized HA- on the catalytic decomposition
of PCP by the Fenton system, (b) to frame quatitative
limits regarding the Fe/HA ratio for optimal yield/rate
of PCP decomposition, (c) to study the
physicochemical properties of Fe in the catalytic
Fenton/PCP system by EPR spectroscopy.
Experimental
The low-Fe HA used was a well characterised-HA
sample extracted from a mining site of Greece
according to the protocols of IHSS. All reactions were
conducted at room temperature and in dark to exclude
adverse photo-effects. The pH was adjusted to 3.5
using H2SO4 e.g. since production of HO˙ by Fenton
reaction declines at higher pH, in the presence or
absence of humic materials. A typical reaction mixture
contained 13ppm PCP, 0.87-32.5ppm FeSO4∙7H2O,
4.35-97ppm H2O2 and 20 ppm HA. The effect of
[H2O2], [Fe] and the presence of 20ppm HA was
studied (Table 1). Detailed control experiments were
performed
using
solutions
containing
[PCP+FeSO4∙7H2O]
or
[PCP+H2O2]
or
[PCP+FeSO4∙7H2O+HA] or [PCP+H2O2+HA]. All
these systems resulted to zero conversion of PCP.
Results and Discussion
Fenton Catalyst in the Absence of HA:[Figure 1A]
Fe-concentration appeared to have a decisive effect on
PCP degradation. For example, by increasing the
FeSO4∙7H2O content to 6.5ppm, the total of PCP was
removed within 120 h (compare Fig. 1A, B). Increase
of [H2O2] did not affect significantly the degradation
of PCP (Figure S1A). For example, increase of [H2O2]
by 2200% i.e. 22 times, (run 1 vs. 5) resulted to an
increase of PCP removal by 18%.
September 1-5, 2014 Ioannina, Greece
80
0.87 ppm
% [PCP] Remained
40
20
0
dX''/dH (au)
[FeSO47H2O]
60
run 16
(e)
run 19
0
runs 11-15
1500
3000
[HA] ppm
4500
(d)
Signal Intensity
A
A
runs 4, 6-10
100
g=4.3 EPR Signal Intensity
17th-IHSS Conference
(c)
32.5 ppm
runs 14, 16-20
100
B
(b)
(a)
[FeSO47H2O]
0.87 ppm
80
400
60
800
1200 1600 2000 2400
Magnetic Field (G)
40
40
B
runs 11-15
20
32.5 ppm
0
72
144
216
time (h)
Figure 1. Effect of [FeSO4∙7H2O] on the removal of PCP by
Fenton reaction (A) in the absence and (B) in the presence of
20ppm HA. Catalytic conditions: 13 ppm PCP, 52 ppm H2O2,
0.87-32.5 ppm FeSO4∙7H2O. Run 4 and 14 (), run 6 and 16
(), run 7 and 17 (), run 8 and 18 (), run 9 and 19 (),
run 10 and 20 ().
% [Fe-HA ]
30
0
run 16
20
RFe/HA=1.17
10
run 19
0
3.6
HA-Modified Fenton Catalyst: [Figure 1B] FeII
concentration had a prominent effect i.e. depending on
the initial [FeII]:HA ratio. Noticeably, in the presence
of HA, increase of H2O2 concentration had a beneficial
effect which was higher than the effect of H2O2 in the
absence of HA: [i] under the conditions of these
reactions, HA inhibited PCP decomposition; [ii] in the
presence of HA, more [H2O2] oxidant was required to
achieve the same PCP decomposition as in the absence
of HA [iii] further increase of [H2O2] resulted to a
linear increase of the total PCP conversion at 9 days in
the presence or absence of HA.
Overall, under the conditions of our experiments:
[i] HA appears to play a dual Janus-like role e.g. HA
can act either as an inhibitor or as an enhancer of the
Fenton reaction, on both PCP decomposition yield and
reaction rates; [ii] the function of HA is gated by the
ratio Fe/HA (RFe/HA). When
Fe(mol)
R Fe/HA 
 1.17
HA(mg)
both PCP decomposition yield and reaction rates were
increased otherwise they remained constant or
decreased vs. the unmodified Fenton.
EPR Spectroscopy [Figure 2A] shows the percent
of monomeric FeIII-HA and the reduced FeII (due to
the presence of HA) formed in each sample used for
the Fenton catalysis of PCP, i.e. the percent of Fe (FeIII
and FeII) interacting with HA (Fe-HA). It is seen that
increase of the HA concentration keeping the Fecontent constant (i.e. decrease of RFe/HA ratio) resulted
to an increase of the [Fe-HA] species. The downarrow in Figure 4B mark the RFe/HA=1.17, while the
horizontal arrows mark the RFe/HA regimes where
oxidation of PCP is increased/decreased in the
presence of HA (Figure 2B).
Increase Decrease
of PCP of PCP
oxidation oxidation
Low RFe/HA
High RFe/HA
3.0
2.4
1.8
1.2
0.6
Fe/HA (mol/mg)
Figure 2. A) EPR spectra of 0.4 mM Fe2(SO4)3∙xH2O
incubated in the absence of HA, spectrum (a) and (b)
(spectrum (b) has 30 % v/v glycerol) and in the presence of
(c) 230, (d) 1710 and (e) 5110 ppm HA (i.e. corresponding to
RFe/HA = 0.16, 0.47 and 3.5 μmol/mg respectively) for 2 h at
pH 3.5. Inset plot: EPR FeIII signal intensity as a function of
[HA] incubated for t 30 min () and 2 h (), the lines
correspond to a linear regression of the data. B) Percent of
the [Fe-HA] after incubated for 2 h at pH 3.5. Each case is
labelled with the corresponding Fenton reaction with the
same ratio RFe/HA
REFERENCES [UP to 8 references]
(1) Rice, N.; Buxton, G.V; Nixon, A.B. J. Phys. Chem.
Ref. Data 1988, 17, 513-886.
(2) Paciolla, M.D. and White A.B. Environ. Sci.
Technol. 1999, 33, 1814-1818.
(3) Huling, S.G.; Water Res. 2001, 35, 1687-1694
(4) Lindsey, M.; Tarr, M.A.. Chemosphere 2000, 41,
409-417.
(5) Ciotti, C.; Baciocchi, R.; Tuhkanen, T J. Hazard.
Mater. 2009, 161, 402-408.
(6) Lindsey, M.; Tarr, M.A. Environ. Sci. Technol.
2000, 34, 444-449.
(7) Fukushima, M.; Tatsumi, K. Environ. Sci. Technol.
2001, 35, 1771-1778.
(8) Georgi, A. Appl. Catal. B: Environ. 2007, 72, 26-36.
Acknowledgments:
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