Influence of a reagents addition strategy on the
Fenton oxidation of rhodamine B: control of the
competitive reaction of $OH†
Wei Zhou,
a
Haiqian Zhao,
b
Jihui Gao,
*
a
Xiaoxiao Meng,
a
Shaohua Wu
a
and Yukun Qin
a
The Fenton system (Fe
2+
/H
2
O
2
) generates $OH with a high oxidation potential. However, as reactants
themselves, H
2
O
2
and Fe
2+
can act as $OH initiators as well as $OH scavengers, leading to the need
for a high dosage of reactants and increased costs. As a mixing-sensitive reaction, the $OH-related
reaction kinetics ($OH with Fe
2+
,H
2
O
2
, and RhB) was determined from the reaction rates (which were
a constant in this work) and stoichiometry, in which the latter could be regulated by an addition
strategy of Fenton reagents. This suggests that $OH competitive reactions could be controlled by
applying a macrolevel addition strategy. Herein, the effects of different addition approaches of Fe
2+
and H
2
O
2
on $OH competitive reactions were quantitatively and systematically studied by analyzing
the removal of the model pollutant RhB. We found that without stirring, and compared with a one-
time addition, once H
2
O
2
or Fe
2+
was added in a step-wise pattern (e.g., one drop by one drop, 2
times, or 4 times), a high concentration of H
2
O
2
or Fe
2+
existed in a localized place for a longer
period, resulting in a lower proportion of $OH reacting with RhB, which we ascribed to an enhanced
reaction between Fe
2+
,H
2
O
2
, and $OH. However, when H
2
O
2
and Fe
2+
were added from two close
points without stirring, a larger proportion of $OH was scavenged by H
2
O
2
and Fe
2+
; while under
stirring, even a one-time addition of H
2
O
2
or Fe
2+
could cause severe scavenging of $OH. The results
also revealed a linear relationship between the RhB removal percentage and wavelength blue-shifts.
This study showed that microlevel $OH competitive reactions could be controlled by applying
a macrolevel addition strategy of Fenton reagents without the addition of external chemicals. The
results suggest this methodology can also offer an approach to lower $OH invalid consumption by
regulating the addition strategy in bigger reactors.
Introduction
The Fenton system (Fe
2+
and H
2
O
2
), which is one of the
advanced oxidation processes (AOPs), generates $OH to initiate
free radical chain reactions and the decomposition of various
pollutants.
1–4
$OH is a reactive, nonselective radical, which has
a very high standard redox potential (1.9–2.7 V). However, $OH
has a short lifetime, approximately 1 s in the gas phase
5
or 10
9
to 10
6
s in the liquid phase.
6,7
Therefore, the enhanced
generation and effective utilization of $OH in the Fenton system
are key focuses of current research.
As previously reported, the enhanced generation of $OH in
the Fenton system has been well studied experimentally and
theoretically.
8–10
By the assistance of external energy (i.e.,
electro-Fenton, photo-Fenton, UV-Fenton, and US-Fenton) and
the introduction of specic additives (quinone, hydroxyl-
amine),
1,3,11
an improved regeneration of Fe
2+
was achieved,
which could then react with H
2
O
2
to generate more $OH.
However, the enhanced generation of $OH cannot guarantee the
effective utilization of the generated $OH. At the same time, the
above methods introduced additional chemicals into the
system, leading to more expensive running costs.
The reactivity of $OH is largely dependent on the concen-
tration and availability of hydroxyl radical scavenging
compounds.
12
Once formed, $OH reacts rapidly with organic
and inorganic substances in solution, with rate constants of 10
6
to 10
9
M
1
s
1
.
13
The key reactions in the Fenton system are
shown below.
14
Among the reactions involving $OH, reaction (3)
and (4) are the principle ones causing an invalid consumption
of $OH, that is, the desired radical chain propagation is termi-
nated by reaction (3), while weaker radicals (HO
2
$) are formed
by reaction (4). If the side reactions (3) and (4) could be
inhibited or weakened, there would be more $OH available to
react with the target pollutants, resulting in an improved overall
performance of the Fenton system.
a
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin
150001, P. R. China. E-mail: gaojh@hit.edu.cn
b
School of Civil Engineering & Architecture, Northeast Petroleum University, Daqing
163318, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra20242j
Cite this: RSC Adv. , 2016, 6, 108791
Received 10th August 2016
Accepted 11th October 2016
DOI: 10.1039/c6ra20242j
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2016 RSC Adv. , 2016, 6, 108791–108800 | 108791
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