Ciprofloxacin degradation from aqueous solution by Fenton oxidation: Reaction kinetics and degradation mechanisms

Article abstract of DOI:10.1039/c3ra45709e

Pharmaceutical wastewater from a large number of manufacturing units is extremely contaminated by ciprofloxacin (CIP), an antibiotic drug. In this work, aqueous CIP solution was treated by Fenton oxidation (FO). The effects of typical process parameters on drug mineralization have been reported. The optimal Fe²⁺/H₂O₂ molar ratio of 0.125 and pH of 3.5 were determined with 15 mg L^-1 initial CIP at 25°C temperature. Maximum CIP, COD and TOC removal of 74.4, 47.1 and 37.9% were obtained under the optimal conditions. The mean oxidation number of carbon determined in terms of COD and TOC values was in accordance with that from the oxidation number of individual carbon atom. The concentration of hydroxyl radicals was measured using the N,N-dimethyl phenyl hydrazine method using dimethyl sulphoxide as a probe. Thirteen fragments appeared in the mass spectra and the proposed mechanism explored the routes of daughter ion formation. The cleavage of the piperazine ring was more effective in CIP oxidation due to high nucleophilic character of lone pair of electrons present on the nitrogen atom. A simple 2^nd order kinetic model was proposed for the oxidation of CIP and degradation products (DP_s) with respect to OH concentration. The rate constants of 3.13 × 10³, 4.89 × 10³ M^-1 s^-1 were estimated for CIP and DP_s. The initial concentration of OH was found to be 11.67 μM.

Full text of DOI:10.1039/c3ra45709e

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Ciprofloxacin degradation from aqueous solution
by Fenton oxidation: reaction kinetics and
degradation mechanisms†
Cite this: RSC Adv., 2014, 4, 6738
*
Ardhendu Sekhar Giri and Animes Kumar Golder
Pharmaceutical wastewater from a large number of manufacturing units is extremely contaminated by
ciprofloxacin (CIP), an antibiotic drug. In this work, aqueous CIP solution was treated by Fenton oxidation
(FO). The effects of typical process parameters on drug mineralization have been reported. The optimal
Fe2+/H2O2 molar ratio of 0.125 and pH of 3.5 were determined with 15 mg Lꢀ1 initial CIP at 25 ꢁC
temperature. Maximum CIP, COD and TOC removal of 74.4, 47.1 and 37.9% were obtained under the
optimal conditions. The mean oxidation number of carbon determined in terms of COD and TOC values
was in accordance with that from the oxidation number of individual carbon atom. The concentration of
hydroxyl radicals was measured using the N,N-dimethyl phenyl hydrazine method using dimethyl
sulphoxide as a probe. Thirteen fragments appeared in the mass spectra and the proposed mechanism
explored the routes of daughter ion formation. The cleavage of the piperazine ring was more effective in
CIP oxidation due to high nucleophilic character of lone pair of electrons present on the nitrogen atom.
A simple 2nd order kinetic model was proposed for the oxidation of CIP and degradation products (DPs)
with respect to OHc concentration. The rate constants of 3.13 ꢂ 103, 4.89 ꢂ 103 Mꢀ1 sꢀ1 were estimated
for CIP and DPs. The initial concentration of OHc was found to be 11.67 mM.
Received 10th October 2013
Accepted 2nd January 2014
DOI: 10.1039/c3ra45709e
www.rsc.org/advances
Proper removal of CIP from aqueous streams has an impor-
tant role in the prevention of diseases both in humans and
Introduction
animals. Fenton oxidation (FO) is known to be very effective
advanced oxidation processes (AOPs) for the treatment of phar-
maceutical wastewater.4 The primary reactions occurring in FO of
organics are shown in eqn (1)–(4). Rc indicates alkyl free radical.5
In India, annually around 0.33 million tons of pharmaceutical
waste is generated. For most of the cases, the contaminated
water is disposed of into the receiving stream without suitable
treatment or the water treatment facilities are not equipped to
treat/lter out pharmaceuticals.1 The main reason is very high
treatment cost. The concentration of pharmaceuticals in
disposed water from a production unit of 90 bulk drugs in
Patancheru, near Hyderabad, India, is reported to be the high-
est levels of pharmaceuticals in any effluent. The concentration
of ciprooxacin (CIP) was around 1000 times above the toxicity
level to some bacteria. In continuation of the earlier work,1 it is
reported that even a 0.2% (v/v) of this effluent could notably
reduce the growth rate of tadpoles and the underlying toxicity is
still unknown.2 Some pharmaceuticals can have biological
activity on animals and bacteria well below the concentration
that are usually used in safety tests.3 CIP is a synthetic antibiotic
drug of second-generation uoroquinolones class. It kills
bacteria by interfering with the enzymes that cause DNA to
rewind, which stops synthesis of DNA and protein.
Fe2+ + H2O2 / Fe3+ + OHꢀ + OHc
Fe2+ + OHc / Fe3+ + OHꢀ
OHc + RH / H2O + Rc
(1)
(2)
(3)
(4)
Rc + Fe3+ / R+ + Fe2+
Several in vitro experiments conrm that Fe2+ has the
capacity to reduce molecular oxygen to superoxide radical for
the production of the hydroxyl radical (OHc) (eqn (5)).6
Fe2+ + O2 / Fe3+ + O2cꢀ
(5)
The overall reaction of the combined steps eqn (1) and (5) is
called Haber–Weiss reaction.
O2cꢀ + H2O2 / O2 + OHc + OHꢀ
(6)
Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam,
781 039, India. E-mail: animes@iitg.ernet.in; Fax: +91 361-258-2292; Tel: +91 361-
258-2269
In general, OHc react with the organic pollutants (RH) by the
abstraction of H atom from C–H, N–H, or O–H bonds and by the
addition of C–C bonds to the conjugate aromatic rings. The
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45709e
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general rate equation of the key organic molecule can be written
as eqn (7).
X
X
dCRH
dt
ꢀ
¼
kOHc COHc CRH
þ
kOX COX CRH
i
(7)
i
i
i
i
i
OXi represent other oxidants than OHc such as ferryl [Fe(IV)
O]2+ or cOOH. Most of the studies assume that OHc formation
and disappearance rate are instantaneous.7 However, it is
prudent to consider [OHc] at higher pollutant concentration.8
Investigation on [OHc] estimation and its inclusion to the
kinetics of FO for the degradation of pharmaceutical
compounds still lack in literature.
Fig. 1 Chemical structure of CIP drug [1-cyclopropyl-6-fluoro-4-
oxo-7-(piperazin-1-yl)-quinoline-3-carboxylic acid].
Determination of OHc includes electron spin resonance
spectroscopy in which the electron paramagnetic resonance
spectrum of a spin adduct derivative is measured.9 This method
is less sensitive and difficult to employ readily to acquire
quantitative estimation of OHc as unstable OHc-adduct is
formed. OHc can be measured from the concentration of
hydroxylated products formed with aromatic compounds such
as phenol, benzoic acid and salicylic acid.10 But the problems
associated in OHc determination in AOPs include: (i) multiple
reactions, (ii) secondary generation of superoxide, (iii) limited
solubility adduct and, (iv) formation of iron (+2, +3)-salicylic
acid complex that hinders OHc formation in Fenton and Fen-
ton-like reactions.10
To overcome these limitations, dimethyl sulphoxide (DMSO)
as the chemical probe can be used for OHc determination. It is
based on the reaction between OHc with DMSO to produce
formaldehyde (HCHO). DMSO is highly water soluble and could
trap most hydroxyl radicals generated in AOP's. It does not form
complexes with iron or other metals ions in FO.11,12 Moreover,
limited insight has been provided to understand the degrada-
tion mechanism of CIP. Destruction of CIP from aqueous
solution is investigated using FO in this work. The inuence of
pH, reaction time, Fe2+ to H2O2 molar ratio on TOC, COD decay
and mean oxidation number of carbon (MONC) variation have
been studied. A mechanism of CIP oxidation is proposed and
supported by the results obtained in LC-MS spectra. An oxida-
tive kinetic equation involving OHc for [CIP] as well as for
degradation products [DPs] was developed. The concentration
of OHc was determined using DMSO as a chemical probe.
Experimental
Fenton experiment. One litre capacity cylindrical (B 105 mm)
borosilicate vessel was used as the batch reactor system. The
experiment was performed at room temperature (23–25 ꢁC) with
400 mL drug solution. The initial concentration of drug was
chosen based on the maximum reported concentration in liter-
ature.13 pH of the solution was adjusted using 0.05 N H2SO4 prior
to addition of Fe2+ catalyst. Solution pH was measured using a
precision pH meter of M/s Eutech instruments (Model: pH/ion
510, Oaklon, Japan). Predetermined amount of Fe(NH4)2
(SO4)2$6H2O was then added and mixed for 5 min at 260 rpm for
better homogeneity.14 H2O2 was then added. The agitation was
continued at the same speed using a magnetic stirrer (stirrer bar:
length 40 mm, B 0.8 mm) of M/s Tarson, Kolkata (Model: Spinot
6020). Samples were taken out at different time intervals and 0.1
N NaOH was immediately to stop the reaction at 10 : 1 (v/v).
Addition of NaOH increased the solution pH around 12.7. Sludge
formed was separated by centrifugation at 1600 rpm for 30 min.
The clear supernatant was analyzed for pH, drug concentration,
COD and TOC. COD analysis was carried out heating the solution
at 70 ꢁC preceded by sludge separation.
Derivatization procedure. Fenton experiments were per-
formed in duplicate under similar conditions and the second
test was used for determination of OHc concentration. Sample
was added into glass vial with previously added DMSO reagent
(250 mM) at 1 : 0.4 (v/v). It was then mixed with 5 mL 2,4-di-
nitrophenylhydrazine (DNPH)–phosphate buffer reagent. It was
prepared by mixing phosphate buffer of 2.5 mL at pH 4 with
0.2 mL DNPH (6 mM in ethanol) and diluted to 5 mL with DI
water. Hydrazone colored derivative formed by the reaction
between HCHO and DNPH is shown in Fig. S1 of the ESI.† The
reaction mixture was analyzed by LC-UV at xed wavelength of
365 nm at room temperature. An eluent phase of 40 : 60 (v/v)
water–methanol at 0.5 mL minꢀ1 was employed. The retention
time of the HCHO–DNPH colored derivative under these
conditions was found as 7.8 min. The reactions involved for the
formation of HCHO using DMSO and OHc are shown through
eqn (8)–(11). The amount of HCHO is formed at the stoichio-
metric ratio of 1 : 2.17 with respect to OHc.11
Material and methods
Reagents
HPLC grade CIP (purity >98%, w/w) and 2,4-di-nitrophenyl
hydrazine (DNPH) (purity >98%, w/w) were procured from M/s
Sigma Aldrich Chemical Ltd. (USA). The chemical structure of
CIP is illustrated in Fig. 1. HPLC grade of methanol (purity 98%,
v/v), ferrous ammonium sulphate hexahydrate (99% purity,
w/w) and sulfuric acid (98% purity, w/w), H2O2 (50% purity, v/v),
Ag2SO4 (purity >98%, w/w), K2Cr2O7 (purity >98%, w/w), di-
potassium hydrogen orthophosphate (98–100% purity, w/w)
and DMSO (purity >96–99%, w/w), were obtained from M/s
Merck Specialties Pvt. Ltd. (India). Milli-Q water (Model: Elix-3,
USA) was used in preparation of reagents and drug solutions.
2(CH3)2SO + 2OHc / 2CH3SO2H + 2CH3c
2CHc3 + 2H–R / 2CH4 + 2Rc
(8)
(9)
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Optimal Fe2+/H2O2 molar ratio was found out by varying it in
the range from 0.05 to 0.18 and H2O2 dose of 10 mM was used in
all experiments. The change in [OHc] can be divided into two
phases. At the initial phase, OHc radicals production is faster. It
resulted in higher CIP degradation (Fig. S2 of the ESI†). In the
second phase, [OHc] decreased even though CIP oxidation
mostly occurred within the 1st stage.15 It was primarily due to
progressive depletion of H2O2.
2CHc3 + 2O2 / 2CH3OOc
2CH3OOc / HCHO + CH3OH + O2
(10)
(11)
Analytical methods
High performance liquid chromatography (HPLC). A 20 mL
sample was injected directly into a C18 HPLC column (150 mm
length, B 3.5 mm) for the determination of concentration of
CIP. HPLC (Model: 26462) of M/s Shimadzu (Japan) equipped
with UV-visible detector was employed for the chromatographic
measurement. Acetonitrile (98% purity, v/v), water and tri-
ethylamine (98% purity, v/v) at the ow rate of 1 mL minꢀ1
(20 : 80 : 0.1 v/v/v) was used as the mobile phase. pH of the
mobile phase was adjusted to 3.0 using 5% (v/v) o-phosphoric
acid. The suspended particle appeared was removed by ltra-
tion using 0.45 mm cellulose acetate lter. The scanning was
performed at xed wavelength of 280 nm.
The results at the end of the run are shown in Fig. 2. CIP
removal was increased with increase in Fe2+/H2O2 molar ratio
and maximum drug removal of 74.4% was obtained at 0.125.
CIP removal decreased with further rise of Fe2+/H2O2 ratio.
Generally, the rate of OHc formation increases with rise of Fe2+
/
H2O2. However, excess H2O2 with respect to Fe2+ can directly
oxidize Fe2+ to Fe3+. It reduces the production rate of OHc and
drug removal fell down.16,17 CIP removal per unit Fe2+/H2O2
molar ratio dropped almost linearly (Fig. 2). It suggests that FO
was more procient in consumption of Fe2+ and H2O2 for drug
degradation at lower dose. At Fe2+/H2O2 >0.125, CIP removal
dropped a bit. It implies that the optimal Fe2+/H2O2 ratio was
around 0.125 in terms of percentage CIP removal.
Liquid chromatography-mass spectroscopy (LC-MS). HPLC
method outlined above was adopted for subsequent sample
introduction to MS detector. The chromatographic separation
was performed on a YMC Hydrosphere C18 150 mm ꢂ 4.6 mm
(5 mm particle size) reverse phase analytical column (Wilming-
ton, NC, US) following a YMC Hydrosphere 10 mm ꢂ 4 mm (5
mm particle size) guard column. The mobile phase ow rate was
0.8 mL minꢀ1. H2O and acetonitrile with 0.1% (v/v) formic acid
was employed as the mobile phase. A linear gradient of 95 to
50% water for 15 min was used. A 10 mL of both sample and
calibration solution were injected into the column using an AS
3000 auto injector (Thermo Finnigan, USA). Atmospheric pres-
sure chemical ionization method in positive ion mode over the
mass range of 100 to 500 amu was adopted. N2 at a ow rate of
400 L hꢀ1 for drying and 150 L hꢀ1 for sheathing were purged.
Optimal pH
pH of the solution has notable inuence on FO. It controls the
production rate of hydroxyl radical and the nature of iron
species. pH was varied in the range from 2 to 4.5 (Fig. S3 of the
ESI†). The dynamics of drug removal followed similar trend at
different pH. Removal efficiency gradually increased with rise of
pH and it reached a maximum value of 74.4% at pH 3.5 (Fig. 3).
Further pH elevation decreased drug removal. OHc could be
consumed by the scavenging effects of H+ (eqn (13)) at pH < 3.5
which would limit the degradation rate.18 It might also be
possible that H2O2 was stable by acquiring a proton to form an
oxonium ion (eqn (14)) at lower pH.19
ꢁ
The source temperature at 120 C and a cone voltage of 25 V
were maintained to determine the m/z ratio of the parent ion
and isotope. A cone voltage of 75 V was used for the fragmen-
tation of the daughter ions. The electrospray source voltage was
OHc + H+ + eꢀ / H2O
(13)
ꢁ
held at 3.5 kV and the temperature was maintained at 200 C.
TOC and COD analysis. Total organic carbon (TOC) analyzer
of M/s O.I. Analytical (Model: 1030C Aurora, USA) was employed
for determination of TOC before and aer experiments. Non-
dispersive infrared method was adopted for the detection.
Chemical oxygen demand (COD) was determined by scanning
the sample at 360 nm wavelength using HACH DRB 200 digester
following the protocol recommended by the manufacturer.
Results and discussion
Optimal Fe2+/H2O2 molar ratio
The dose of Fe2+ and H2O2 determine the operational cost as
well as the efficiency of degradation of contaminant in FO. The
concentration of Fe2+ drops quickly with the progress of FO (eqn
(1)). It is consequently balanced by the formation of Fe2+
through reduction of Fe3+ (eqn (12)) and Fe2+ concentration
reaches at the steady state.
Fig. 2 Effect of Fe2+/H2O2 molar ratio on CIP degradation. Experi-
mental conditions: [CIP]0 ¼ 15 mg Lꢀ1, reaction time ¼ 45 min, pH ¼
Fe3+ + H2O2 / Fe2+ + OOHc + H+
(12) 3.5 and temperature ¼ 25 ꢁC.
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Fig. 4 Removal of CIP, COD and TOC along with the variation
hydroxyl radical concentration with the progress reaction time.
Experimental conditions: [CIP]0 ¼ 15 mg Lꢀ1, Fe2+/H2O2 molar ratio ¼
0.125, pH ¼ 3.5 and temperature ¼ 25 ꢁC.
Fig. 3 CIP degradation as a function of solution pH. Experimental
conditions: [CIP]0 ¼ 15 mg Lꢀ1, Fe2+/H2O2 molar ratio ¼ 0.125, reac-
tion time ¼ 45 min and temperature ¼ 25 ꢁC.
+
H2O2 + H+ / H3O2
(14)
water and dissolved in conc. H2SO4 solution. Amount of CIP
adsorbed on sludge was determined aer 45 min of FO under
the optimal condition (Fe2+/H2O2 molar ratio 0.125 and pH 3.5).
About 3, 4 and 6% CIP, COD and TOC reduction was observed
due to sludge appearance. It implies that CIP adsorption on iron
sludge would not affect the kinetics of CIP oxidation. Stable CIP
(or intermediates)–iron complex was formed which was resis-
tant to FO. It is in line with the steady appearance of iron–
organic sludge.
The concentration iron used in Fenton experiment was lower
than the theoretical solubility of Fe(II) and Fe(III).20,21 [Fe2+] is
more than 99.9% at pH < 4.22 However, brownish appearance of
the solution was visually noted with addition of CIP into Fenton
reagent and it transformed into steady particle form in about
10 min. It implies that probably iron-organo complexes of lower
solubility were formed. It reduced the total available iron,
primarily Fe2+, and CIP in solution for FO. Sludge may be con-
verted to more stable solid modications such as a-FeOOH(s)
and a-Fe2O3(s) with progress of the reaction. Indeed, highly Mean oxidation number of carbon
heterogeneous nature of the iron sludge is formed in FO.23,24
The results obtained at different pH indicate that the optimal
The mean oxidation number of carbon (MONC) reveals the
experimental errors incurred in COD and TOC measurements.
Chemically, the MONC must lie in the range from ꢀ4 to 4 with
respect to the simplest organic compound, CH4. The MONC for
a single organic molecule containing ‘n’ number of carbon
atoms can be represented as in eqn (15). ONi indicates the
oxidation number of ith carbon atom (eqn (15)).26 The oxidation
number of the individual carbon atom is provided in Fig. 1 in
round parenthesis for the CIP molecule. An oxidation number
of 0, +1, +0.5, +2 were considered for the C–C s- and p-bonds;
C]C s- and p-bonds and; C]O s- and p-bonds, respectively.27
It gave net ‘zero’ oxidation number of carbons in both pipera-
zine ring and cyclopropyl group.
pH of CIP removal was around 3.5.
Comparative CIP, COD and TOC removal at optimal condition
CIP removal showed two distinct rate periods (Fig. 4) i.e. initial
faster drug removal followed by an almost constant rate even
though there was considerable amount of unreacted CIP.
Percentage COD and TOC removal increased gradually with the
reaction time. On the other hand, the change in CIP decom-
position was <5.8% aer 5 min of FO (Fig. 4). This is in accor-
dance to the change in OHc concentration (Fig. 4). It dropped
quickly within rst 5 min of FO and then it decreased gradually.
The rate of formation of OHc decreased as a net result of Fe2+
catalytic effect reduction, OHc scavenging by H+ (eqn (13)) and
H2O2 auto decomposition (eqn (14)).25 The high initial rate of
OHc formation was resulted in higher initial rate of CIP (till
5 min), COD and TOC (till 15 min) removal. Maximum COD and
TOC removal were found as 47.1 and 37.9% in 15 min against
the drug removal of 74.4%. Aer that, there was no notable
effect of treatment time on CIP, COD and TOC removal.
n
X
ONi
i¼1
MONC ¼
(15)
n
COD and TOC values also can be combined together to
compute the MONC irrespective to the number of organic
molecule present in the sample (eqn (16)).28
COD
MONC ¼ 4 ꢀ 1:5
(16)
Sludge formed during FO was separated by ltration using
0.45 mm cellulose ester lter. It was then washed in distilled
TOC
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COD and TOC are expressed in mg Lꢀ1. Iron sludge formed symbol ‘D’ following the integer to count the number of frag-
during FO was removed preceded by Fe2+ oxidation at high pH ments. D1 with m/z 305 has the same specic mass of 7-[(2-
(ref. 29) and residual OHc and H2O2 were destroyed by heating amino-ethyl)amino]-6-uoroquinoline formed by the loss of the
the samples to avoid the interference with inorganic COD. The piperazine ring (Fig. 7a–c) under acidic conditions. It appeared
experimentally determined COD value was therefore readily with the cleavage of piperazine ring via an unstable interme-
employed for the estimation of MONC. The results are shown in diate product as shown in Fig. 7a. The nitrogen atom of piper-
Fig. 5. The initial MONC values estimated using eqn (15) and azine ring with a lone pair electron, abstracted the proton from
(16), were of 1.17 and 1.28. It implies that COD and TOC were the solvent at acidic pH and the C–C bond was broken.
determined with reasonable accuracy. The MONC increase was
The fragmentations with m/z of 186, 198, 239, 261, 304, 354
faster at <5 min and then it increased gradually. The MONC and 386 at 5.76 min of retention are presented in Fig. 6b. The
raised to 1.68 in 45 min of FO. The result can be corroborated by daughter ions with m/z ratio of 304 (D2) were formed due to
the refractory nature of the quinolone moiety. It contains 9 piperazine ring breaking along with deuorination followed by
carbon atoms out of 17 in CIP molecule.
hydroxylation of D1 (Fig. 7a). D3 (m/z of 239) appeared with the
loss of uorine atom at position 6 and expulsion of ethyl-
enediamine molecule (Fig. 7a).
Proposed mechanisms of CIP degradation
Decarboxylation of CIP led to formation of D4 having the
same m/z of D2. MS spectra of the compound with m/z ¼ 301
(D5) theoretically represents a fragment of CIP which might be
formed by the loss of uorine atom (Fig. 7b). The steric effect
between carbonyl and carboxylic (–COOH) groups and the
presence of uoride group could trigger such decarboxylation.
The peaks with m/z of 205, 239, 279 and 301 in the mass
spectra corresponding to retention time of 6.50 min are shown
in Fig. 6c. –OH group was expelled out from the quinolone
moiety in presence of electron withdrawing N atom and carbonyl
group. Therefore, D1 was further broken into D6 (m/z of 287) by
dehydroxylation. Piperazine ring suffers an angle stress in
presence of high electronegative uorine atom. It led to forma-
tion of the three-membered cyclic amine ring (D7) (Fig. 7c).
Amine group makes the quinolone moiety as an electron rich
compound and it is easily targeted by OHc because of electro-
phile nature of the free radical. As a result, D8 was formed by
hydroxylation. D9 (m/z 198) and D10 (m/z 217) were yielded due to
cleavage of N–C bond of cyclic amine ring (Fig. 7c). D1 broken
into D11 (m/z 242) on deuorination and partial piperazine
moiety oxidation because of steric hindrance (Fig. 7c). A similar
explanation for D12 is also applicable. The possible route for the
formation of D13 with m/z 279 is shown in Fig. 7c. The pathways
for the formation of daughter ions are summarized in Table S1
(ESI†). Lower TOC removal is in accordance with the proposed
mechanism of CIP degradation as carbon atoms mainly
remained to the quinolone moiety. The variation of MONC also
gives the hint for the successive oxidation of the CIP molecule.
The MONC for the individual daughter ions were varied in the
range of 0.8 (D11) to 1.72 (D8) at 10 min of FO with 28%
unreacted CIP against the same of 1.18 for CIP.
N4 in piperazine ring of uoroquinolone compounds is typically
the specic site of OHc attack. N1 is likely less reactive than N4
because of its weaker basicity.30 Two strong electron-withdrawing
nitrogen substituents i.e. uorine and –COOH groups are therein
the aromatic ring (Fig. 1). On the other way it indicates that
uoroquinolone ring is less electron-donating because of highly
electron-withdrawing uorine substituent.22 In general, the loss
of carboxyl moiety ([M + H-44]+) and uoride ([M + H-22]+), are
the typical fragmentation pathways of uoroquinolone.31 The
appearance of such fragmentations in the MSn spectra gives
some hints for the subtraction of different moieties based on
their mass losses such as 44 for carboxyl group.
The mass spectra are depicted in Fig. 6. The formation of
daughter ions could follow two pathways for the degradation of
CIP i.e. piperazine moiety degradation and decarboxylation. The
possible routes of oxidation of CIP molecule are illustrated in
Fig. 7. The peaks in the mass spectra with m/z of 305, 186, 198,
217, 242 and 261 were for the retention time of 5.36 min
(Fig. 6a). All the degradation products are denoted with the
Kinetic model for the oxidation of CIP and degradation
products
CIP molecules are targeted by OHc immediately upon its
generation (eqn (1)) and a number of products are formed. The
solution in the batch reactor was completely mixed and the
reaction temperature was controlled in a narrow range (23 to
ꢁ
25 C). The pH variation was insignicant during degradation
Fig. 5 Variation of MONC with progress of reaction: experimental
conditions: [CIP]0 ¼ 15 mg Lꢀ1, Fe2+/H2O2 molar ratio ¼ 0.125, pH ¼
3.5 and temperature ¼ 25 ꢁC.
experiment. It was assumed that active OHc was the main
oxidant to cleavage the organic substances in the solution.
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Fig. 6 (a) Mass spectra recorded at 10 min of FO for daughter ion 1 (D1) with m/z ¼ 305. (b) Mass spectra recorded at 10 min of FO for daughter
ion 2 (D2) with m/z ¼ 304. (c) Mass spectra recorded at 10 min of FO for daughter ion 3 (D3) with m/z ¼ 301. Experimental conditions: [CIP]0 ¼ 15
mg Lꢀ1, Fe2+/H2O2 molar ratio ¼ 0.125, pH ¼ 3.5 and temperature ¼ 25 ꢁC.
OHc radical was utilized for the oxidation of CIP (eqn (17)) structure of intermediates (13 nos) as in the mechanisms
and degradation products (DPs). The initial CIP concentration (Fig. 7a–c). Hence, the concentration of DPs was calculated from
was quite high and OHc showed exponential decay with oxida- their TOC values. Therefore, a better estimation of initial OHc
tion time (Fig. 4) at the optimal Fe2+/H2O2 and pH. It implies radical concentration ðCO0 Hc Þ and the second order rate constant
that the rate of CIP oxidation was dependent on [OHc]. So the of CIP oxidation can be done. However, the uncertainly may
rate equation is expressed in terms of concentration of CIP (eqn arise from the calculation of average molecular weight of DPs.
(18)) and DPs (eqn (19)). k1 and k2 (Mꢀ1 sꢀ1) are the second order
rate constants. CIP, DPs and OHc in square parenthesis indicate constants and CO0 Hc . The best t graphics are shown in Fig. S4–
The least square technique was used to estimate the kinetic
their concentration.
S6 (ESI†). The results imply that both CIP and DPs fairly fol-
lowed the 2nd order kinetics. CO0 Hc , k1 and k2 were found as 0.051
mM, 3.13 ꢂ 103 and 4.89 ꢂ 103 Mꢀ1 sꢀ1, respectively. Higher k2
value implies that the overall reactivity of DPs towards OHc was
more than CIP. CO0 Hc exhibited good accordance to the earlier
reports. Hayashi et al.32 showed that [OHc] in FO typically varies
in the range from 1 to 0.01 mM. The average OHc generation and
CIP degradation rates were found as 3.83 ꢂ 10ꢀ8 and 2.97 ꢂ
10ꢀ8 M sꢀ1, respectively. Maezono et al.33 reported a close value
of the average OHc formation rate as 9.3 ꢂ 10ꢀ10 M sꢀ1 for the
oxidation of Orange II with 60 mg Lꢀ1 at pH 3 and Fe2+/H2O2
molar ratio of 0.01 in photo-assisted Fenton process.
CIP + OHc / CIPoxidised
(17)
(18)
ꢀ
ꢁ
ꢁ
d½CIPꢃ
dt
ꢀ
ꢀ
¼ k1½CIPꢃ OHc
ꢀ
d½DPsꢃ
dt
¼ k2½DPsꢃ OHc
(19)
eqn (18) and (19) can be combined as in eqn (20):
ꢂ
ꢃ
d½OHcꢃ
dt
d½CIPꢃ
d½DPsꢃ
dt
ꢀ
¼ ꢀ
þ
dt
ꢀ
ꢁ
ꢀ
ꢁ
¼ k1½CIPꢃ OHc þ k2½DPsꢃ OHc
(20)
Conclusions
The TOC values of unreacted CIP and DPs were experimen-
tally found out at different time intervals. The contribution of
TOC due to unreacted CIP was determined at the same
concentration of CIP. TOC of DPs was calculated by subtracting
the share for unreacted CIP. An atomic number average mole-
cule (molecular weight 218 g molꢀ1) was proposed from the
The following conclusions are drawn from this investigation:
ꢄ The maximum CIP removal of 74.4% was found with [CIP]0
¼ 15 mg Lꢀ1 at optimal pH ¼ 3.5 and Fe2+/H2O2 ¼ 0.125. COD
and TOC removal were of 47.1 and 37.9%. A large portion of
intermediates were resistant to FO.
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ꢄ The residual hydroxyl radical concentration of 0.792 mM
was determined using DMSO probe aer 30 min of FO even with
25.7% unreacted CIP.
ꢄ The mean oxidation number of carbon increased from 1.28
to 1.68 at the optimal conditions.
ꢄ Four primary daughter ions were originated upon degra-
dation of CIP during FO. D1 (m/z 305) and D2 (m/z 304) were
produced by piperazine moiety degradation. Whereas, D3
(m/z 304) along with D4 (m/z 301) were formed by decarboxyl-
ation. The daughter ion with m/z ratio of 305 was further broken
into smaller fragments.
ꢄ A 2nd order kinetic model for the cleavage of both CIP and
degradation products (DPs) exhibited excellent agreement to
the experimental data. The rate constant of CIP and DPs
oxidation were obtained as 3.13 ꢂ 103 and 4.89 ꢂ 103 Mꢀ1 sꢀ1
.
The model gave an initial hydroxyl radical concentration of
11.67 mM.
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