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Chemosphere 87 (2012) 644–648
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Short Communication
Kinetics of the oxidation of sucralose and related carbohydrates by ferrate(VI)
Virender K. Sharma ⇑, Mary Sohn, George A.K. Anquandah, Nasri Nesnas
Department of Chemistry and Center of Ferrate Excellence, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA
a r t i c l e
i n f o
Article history:
Received 20 August 2011
Received in revised form 21 December 2011
Accepted 13 January 2012
Available online 15 February 2012
Keywords:
Ferrate
Rates
Saccharides
Carbohydrates
Radicals
Remediation
a b s t r a c t
The kinetics of the oxidation of sucralose, an emerging contaminant, and related monosaccharides and
disaccharides by ferrate(VI) (Fe(VI)) were studied as a function of pH (6.5–10.1) at 25 °C. Reducing sugars
(glucose, fructose, and maltose) reacted faster with Fe(VI) than did the non-reducing sugar sucrose or its
chlorinated derivative, sucralose. Second-order rate constants of the reactions of Fe(VI) with sucralose
and disaccharides decreased with an increase in pH. The pH dependence was modeled by considering
2
the reactivity of species of Fe(VI), (HFeO
4 and FeO4 ) with the studied substrates. Second-order rate constants for the reaction of Fe(VI) with monosaccharides displayed an unusual variation with pH and were
explained by considering the involvement of hydroxide in catalyzing the ring opening of the cyclic form
of the carbohydrate at increased pH. The rate constants for the reactions of carbohydrates with Fe(VI)
were compared with those for other oxidant species used in water treatment and were briefly discussed.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Sucralose (4-chloro-4-deoxy-a-D-galactopyranosyl-1,6-dichloro1,6-dideoxy-b-D-fructofuranoside) is a chlorinated derivative of
the disaccharide sucrose; in which three hydroxyl groups are replaced by chlorine atoms (Fig. 1). Sucralose is about 600 times
sweeter by weight than sucrose (Wiet and Miller, 1997). The
use of sucralose is widespread in over 80 countries and it is present in more than 4000 different products (Torres et al., 2011). For
example, sucralose is being used as a non-nutritive sweetener in
the sugar substitute known commercially as SPLENDA, which
contains 1% sucralose and 99% maltodextrin and dextrose in
the granulated product. The human consumption of sucralose
has been found safe (Grotz and Munro, 2009), however, it is also
considered an emerging contaminant due to its persistence in the
environment, with a half-life of up to several years (Lubick, 2008;
Richardson and Ternes, 2011). Recently, several studies have
determined sucralose concentrations up to 1 lg L1 in river,
coastal, and marine waters of North America and Europe (Loos
et al., 2009; Lubick, 2009; Mead et al., 2009). The input of sucralose to the environment is likely through municipal wastewater
treatment effluents (Torres et al., 2011). The present study thus
sought the removal of sucralose by ferrate(VI) (FeO2
4 , Fe(VI)) in
order to minimize release of this emerging contaminant to
aquatic ecosystems.
⇑ Corresponding author. Tel.: +1 321 674 7310; fax: +1 321 674 8951.
E-mail address: vsharma@fit.edu (V.K. Sharma).
0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2012.01.019
There have been several studies on the novel properties of Fe(VI)
as an oxidant, disinfectant, and coagulant (Eng et al., 2006; Jiang,
2007; Lee et al., 2009; Sharma et al., 2009; Sharma, 2010a; Sharma
et al., 2010), but no data has been published on rate constants for
the oxidation of sucralose or any carbohydrate by Fe(VI). In this paper, the kinetics of the reactions between Fe(VI) with sucralose and
carbohydrates were determined as a function of pH (6.5–10.1) at
25 °C. Structures of the selected carbohydrates are shown in
Fig. 1. The kinetic study of sucrose provided an interesting comparison with that of sucralose. Sucrose, also known as table sugar, is a
disaccharide of D-glucose and D-fructose. Therefore, the six-carbon
monosaccharides glucose and fructose were also chosen for this
study. D-Glucose, also called dextrose, was found to be the most
abundant sugar in groundwater (Spitzy, 1982). D-Fructose is an isomer of glucose (Fig. 1). Another carbohydrate of interest in this
study is the disaccharide maltose containing two D-glucose units.
Carbohydrate-type structures are also found in humic substances
(Canellas et al., 2010) and the results of the present study may thus
have implications in reducing levels of humic substances by reaction with Fe(VI).
The objectives of the study were: (i) to determine rate laws and
rate constants for the oxidation of sucralose and various related
carbohydrates by Fe(VI); (ii) to compare the rate constants of the
carbohydrates with Fe(VI) to each other to see if differences in rate
constants can be related to structural differences. This should
allow for better predictions to be made regarding the reactivity
of Fe(VI) with a greater variety of carbohydrates and (iii) to understand the pH dependence of the oxidation of carbohydrates by
Fe(VI) in order to expand the relevance of the results to a greater
variety of environmental conditions.
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V.K. Sharma et al. / Chemosphere 87 (2012) 644–648
OH
HO
O
HO
HO
1
OH
H
HO
OH
HO
O
1
Fructose
OH
4
O
OH
HO
OH
OH
Glucose
H
O
HO
2
H
1
OH
HO HO
O
HO
O
HO
1
O
HO
2
OH O
H
HO
OH
OH
HO HO
Maltose
Sucrose
H
HO
O
Cl
Cl
1
O
HO
2
OH O
HO
Cl
HO
Sucralose
Fig. 1. Structures of studied carbohydrates and sucralose.
2. Experimental approach
All glassware used in this study was soaked overnight in 50%
nitric acid and rinsed thoroughly with distilled water before use.
All solutions were prepared in water which had been distilled and
then further purified through a Milli-Q system (18.2 MO cm). A
solution of 200 lM Fe(VI) was prepared by dissolving solid potassium ferrate(VI) (K2FeO4) (purity > 98%) into a 5 mM Na2HPO4/
1 mM borate ((Na2B4O710H2O) buffer solution at pH 9.0 (Luo
et al., 2011). Substrate solutions were prepared in 0.01 M phosphate
buffer in a concentration range of 0.05–0.25 M. This concentration
range was selected based on the desire to maintain substrate concentrations at least ten times greater than ferrate concentrations
for pseudo-order conditions to perform kinetics measurements.
The pH of the substrate solution was adjusted by adding either concentrated phosphoric acid or sodium hydroxide solution.
All kinetic studies were performed on a stopped-flow spectrophotometer (SX-18 MV, Applied Photophysics, UK) with a photomultiplier detector. Time spectra were collected in the wavelength
range from 350 to 750 nm. Kinetic traces were collected at a wavelength of 510 nm to determine the pseudo-first-order rate constants
using solutions in which the substrate was in excess. Stopped flow
experimental data was analyzed using the nonlinear least-square
algorithm of the SX-18MV global software (Applied Photophysics,
UK). Rate constants reported represent the average of six replicate
runs. The spontaneous self-decay rate of Fe(VI) was measured under
conditions identical to the experimental oxidation conditions to
allow for a correction in the rate constant. Routine spectral
measurements were performed on an Agilent Model 8453 UV–Vis
spectrophotometer.
3. Results and discussion
Initially, the reactivity of Fe(VI) with monosaccharides was
studied at pH 9.0 and 25 °C. The spectral measurements for
monitoring the reactions are shown in Fig. SM-1 and indicate the
decay of Fe(VI) without any suggestion of the formation of any
intermediate(s) such as Fe(VI)-carbohydrate complexes. The complexation of carbohydrates with high-valent chromium species
has been observed (Roldán et al., 2002; Codd et al., 2003; García
et al., 2006), however it appears that the oxidation of monosaccharides by Fe(VI) occurred without precursor complexation for the
time scale of the studied reactions. Similar results were observed
for the reactions of Fe(VI) with disaccharides and with sucralose
(Fig. SM-2).
The rates of the reactions were followed at 510 nm at different
concentrations of substrates (S) under pseudo-first order conditions at pH 9.0. The insets of Figs. SM-1 and SM-2 show the
decrease in absorbance of Fe(VI) with time, which fit nicely to
single exponential decays. This suggests that the reactions are
first-order with respect to the concentration of Fe(VI). The
pseudo-first-order rate constants (k0 ) were obtained at different
concentrations of monosaccharides, disaccharides, and sucralose.
A plot of k0 versus concentration of substrate ([S]) consistently
showed linear relationships (Fig. 2). A log–log plot of the data from
Fig. 2 was constructed to obtain order with respect to the concentration of substrate (n) (Fig. SM-3). Slopes of the plot were nearly
unity (Table 1); indicating the reaction is also first-order with
respect to the concentration of substrate. The rate law is represented by the following equation:
dð½FeðVIÞÞ=dt ¼ k½FeðVIÞ½S
ð1Þ
where k is the second-order rate constant for the reaction of Fe(VI)
with substrate. The values of k obtained at pH 9.0 are given in Table
1. The values of k did not span a large range varying from 1.0 101
to 2.5 101 M1 s1. The slowest rate constants in this study were
those for sucrose and sucralose (Table 1). Maltose had the highest
rate constant (Table 1). The order of reactivity for monosaccharides
was fructose > glucose while maltose > sucralose > sucrose was the
order of reactivity for disaccharides.
Of the carbohydrates studied here, only sucrose is classified as a
non-reducing sugar. Carbohydrates which can be oxidized by mild
oxidizing agents such as Fe(III) or Cu(II) are referred to as reducing
sugars (Nelson and Cox, 2005). The oxidation of sugars with Fe(III)
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V.K. Sharma et al. / Chemosphere 87 (2012) 644–648
HFeO4 þ S ! FeðIIIÞ þ productðsÞ
2.5
k', 10-2 s-1
FeO2
4 þ S ! FeðIIIÞ þ productðsÞ
Maltose
2.0
ð3Þ
(H3 FeOþ
4;
Fructose
1.5
ð2Þ
The more highly protonated species of Fe(VI),
and
H2 FeO4 ) were not considered because their pKa values are much
lower than the pH of this study and contributions of these species
would be negligible. The rate constant can thus be expressed as
k½FeðVIÞ ¼ k2 ½HFeO4 þ k3 ½FeO2
4 ð4Þ
where k2 and k3 are the species-specific rate constants for reactions
(2) and (3), respectively. The expression for k can be simplified to
Eq. (5) using an equilibrium expression for HFeO
4 species
1.0
Glucose
k ¼ k2 f½Hþ =ðKa3 þ ½Hþ Þg þ k3 fK a3 =ðK a3 þ ½Hþ Þg
0.5
Sucralose
Sucrose
0.0
0
20
40
[Substrate],
60
10-3
80
100
M
Fig. 2. A plot of pseudo-first-order rate constants, k0 (s1) versus concentration of
various carbohydrates and sucralose at pH 9.0 and 25 °C.
or Cu(II) occurs at the anomeric carbon and only the linear form of
the carbohydrate is reactive, not the cyclic form which is in equilibrium with the linear form. When two monosaccharides combine
to form a disaccharide, a glycosidic bond forms as the hydroxyl
group of one molecule reacts with the anomeric carbon of the
other molecule (see Fig. 1). Sucrose, a disaccharide formed by the
bonding of glucose and fructose has no free anomeric carbons since
both are involved in the glycosidic bond. Thus sucrose is classified
as a non-reducing sugar, e.g. it is not oxidized by either Fe(III) or
Cu(II). Sucralose, its chlorinated derivative would also be expected
to show similar behavior. All of the other substrates used in this
study (glucose, fructose, and maltose) are classified as reducing
sugars and are relatively easier to oxidize by Fe(VI) than are
sucrose and sucralose (Table 1).
The k values for the reaction of Fe(VI) with substrates were then
determined at different pHs (Fig. 3). Generally, the k values
increased with decrease in pH (Fig. 3a and b), similar to results from
most studies with Fe(VI) (Sharma et al., 2011; Sharma, 2011).
However, the reactions of glucose and fructose with Fe(VI) showed
unusual pH dependence behavior in which rates slightly increased
with increasing pH (pH P 8.0; Fig 3a). The variation in the k values
with pH in Fig. 3 can be explained by considering reactions between
+
acid–base species of Fe(VI) (H3 FeOþ
4 H + H2FeO4, pKa1 = 1.9;
+
H2FeO4 H + HFeO4 , pKa2 = 3.5; HFeO4 H+ + FeO2
4 , pKa3 =
7.23 (Sharma et al., 2001)) and substrates. In the pH range studied,
substrates do not exhibit more than one pH-dependent species,
hence the pH dependence of k for the reaction of Fe(VI) was
modeled using only two reactions (Eqs. (2) and (3)).
ð5Þ
Values of k2 and k3 were determined using Eq. (5) by fitting the
experimentally determined values of k at different pH. Estimated
values of species-specific rate constants are given in Table 1 for
disaccharides, which fit reasonably well with the experimental
results (solid lines, Fig 3b). The rate constants for the reaction of
2
HFeO
4 were greater for disaccharides than for FeO4 , which
confirms results from earlier studies which indicate that monoprotonated species of Fe(VI) react faster with substrates than do
unprotonated species of Fe(VI) (Sharma, 2010b). Comparatively,
values of k for oxidation of monosaccharides at different pH could
not be fitted using Eq. (5) (dashed lines, Fig. 3a). It seems that other
factors besides the change in the fraction of species with pH are
involved in the reactivity of Fe(VI) with monosaccharides. Base
catalysis of monosaccharides may be one of the factors involved
where the slight increase in the reaction rate at pH > 8 was
observed (Fig. 3a). The tendency of the conversion of fructose to
glucose in alkaline medium increases with increase in pH because
such transformation is base catalyzed. Significantly, this conversion
involves opening of the cyclic ring structure before fructose can be
transformed to glucose. As the pH increased beyond 8.0, higher
concentrations of open structures of monosaccharides are expected
than those of closed (or ring) structures. Open structures of carbohydrates are more susceptible to oxidation than the ring structures.
Proportions of open structures (i.e. active reductant) would
increase with increase in pH and may increase the reaction rate at
higher pH as observed in the present study. Increase in the formation of the active reductant form of glucose with increase in pH has
also been experimentally determined (Roepke and Ort, 1931).
The disaccharide, sucrose and its chlorinated derivative,
sucralose behave differentially fron monosaccharides because they
are non-reducing agents due to the link between C-1 and C-2
(1 ? 2) anomeric carbons (Fig. SM-1). Rates are not expected to
be influenced by an increase in the hydroxide ion concentration.
Therefore their rates of oxidation are solely dependent on the
predominant Fe(VI) species present at that pH. Although maltose
is a reducing sugar, it behaved more like the other disaccharides,
and explanation of such observed rates will stimulate further studies in this area through the investigation of oxidation rates of a
multitude of other disaccharides.
Table 1
n and k (M1 s1) for oxidation of carbohydrates and sucralose by Fe(VI) at 25 °C.
Substrate (S)
Glucose (G)
Fructose (F)
Maltose (G-G)
Sucrose (G–F)
Sucralose
n
1.02 ± 0.07
0.97 ± 0.04
0.93 ± 0.07
1.19 ± 0.07
1.20 ± 0.11
k, 101 M1 s1
pH 9.0
k, 101 M1 s1
HFeO
4 þS
FeO2
4 þS
k, 101 M1 s1
1.6 ± 0.1
2.1 ± 0.2
2.5 ± 0.3
1.0 ± 0.1
1.3 ± 0.1
–
–
(6.8 ± 0.3) 101
(5.3 ± 0.4) 101
(5.7 ± 0.3) 101
–
–
(2.2 ± 0.2) 101
(0.7 ± 0.1) 101
(1.2 ± 0.1) 101
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V.K. Sharma et al. / Chemosphere 87 (2012) 644–648
8.0
Table 2
Rate constants of carbohydrates with different oxidants at 25 °C.
(a)
Glucose
Fructose
k, 10-1 M-1s-1
6.0
Oxidant
4.0
a
b
c
(b)
Maltose
Sucralose
Sucrose
k, 10-1 M-1s-1
6.0
4.0
k, M1 s1
pH
k, M1 s1
Glucose
Fe(VI)
ClO2
O3
OH
CO
3
2.0
pH
7.0
–
6.0
7.5
9.3
2.0 101
<1.0 102a
4.8 100b
1.5 109b
4.3 105c
7.0
–
6.0
6.5
–
5.0 101
–
6.9 100b
2.3 109b
–
pH
k, M1 s1
Maltose
Sucrose
7.0
–
5.5
7.0
8.3
3.7 101
–
2.3 100b
2.3 109b
2.4 104c
Hoigne and Bader, 1994.
National Institute of Science and Technology, 2002.
Stenman et al., 2003.
the anomeric OH group (i.e. the hemiacetal or the aldehyde in
the open form). However, in the case of the non-reducing sucrose
and sucralose, it is likely that oxidation occurs at the hydroxymethyl groups as described in the oxidation of alkyl glycosidic type
carbohydrates by Fe(VI) (BeMiller et al., 1972).
4. Conclusions
2.0
0.0
6.0
7.0
8.0
9.0
10.0
pH
Fig. 3. Dependence of second-order rate constants (k, M1 s1) on pH for the
oxidation of carbohydrates and sucralose by Fe(VI) at 25 °C.
Reactivity of carbohydrates with Fe(VI) and other oxidants are
compared (Table 2). Fe(VI), ClO2, and O3 are much less reactive
than the radical species, OH and CO
3 . ClO2 had no reactivity while
both Fe(VI) and O3 reacted sluggishly with carbohydrates. CO
3 has
been reported to be less reactive than OH (Gilbert et al., 1999;
Stenman et al., 2003). The reaction of OH with carbohydrates
was not selective and oxidation occurred through abstraction of
carbon bound hydrogen atoms to form different radicals (Diehl
et al., 1978; Gilbert et al., 1982). Comparatively, oxidation of carbo
hydrates by CO
3 , and SO4 were more selective than was OH radical (Gilbert et al., 1999; Gierer et al., 2001; Reitberger et al., 2001;
Stenman et al., 2003). The mechanism of oxidation of carbohydrates by CO
involved hydrogen abstraction and/or electron
3
transfer steps (Stenman et al., 2003). In the case of oxidation of carbohydrates by SO
4 , the preference for hydrogen abstraction was
from the C-H a to a ring oxygen atom in five membered ring carbohydrates (e.g. sucrose). The reactivity of SO
4 with a-D-glucose
and b-D-glucose also differed. The reaction with a-D-glucose was
selective towards the C-2, C-5, and C-6 positions while a hydrogen
atom was abstracted from C1 from b-D-glucose by SO
4 (Gilbert
et al., 1999). In the case of Fe(VI), oxidation of tested carbohydrates
with Fe(VI) showed the formation of aldehydes via attack on the
primary hydroxymethyl group (BeMiller et al., 1972). The tested
carbohydrates in this study were essentially the alkyl glycosidic
type, e.g. D-methyl hexopyranoside, which are non-reducing, i.e.
they lack the anomeric OH. Another study of oxidation of aliphatic
alcohols by Fe(VI) include similar examples in which the corresponding aldehydes were formed (Bartzatt et al., 1985). In the
present study, however, in the case of glucose, fructose, and maltose, the sugar substrate was present in excess under studied conditions of determining rate constants, which should result in the
oxidation of the most oxidant susceptible functionality; namely,
The kinetics of the oxidation of selected carbohydrates and
sucralose by Fe(VI) were shown to be first-order with respect to
each reactant and the observed second-order rate constants, k,
increased with a decrease in pH for disaccharides. However, the
pH dependence for monosaccharides was different in which initial
rate constant decreases were followed by increases at pH P 8.0,
which were attributed to the hydroxide catalyzed ring opening/
isomerization of fructose and glucose. Comparison of the reactivity
of Fe(VI) with other oxidants suggests that free radical species such
as OH, have much higher reactivity than Fe(VI) towards carbohydrates. The reactivity of Fe(VI) with sucralose and carbohydrates
may be enhanced by an Fe(VI)–TiO2–UV system, which would
likely generate the reactive species OH and Fe(V) to oxidize
substrates more efficiently. An ozone–TiO2–UV system may also
be appropriate in degrading carbohydrates in water.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.chemosphere.2012.01.019.
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