Generation of hydrogen peroxide and hydroxyl radical resulting from oxygen-dependent oxidation of l-ascorbic acid via copper redox-catalyzed reactions

Article abstract of DOI:10.1039/c6ra02843h

The generation of hydrogen peroxide (H₂O₂) and hydroxyl radical (HO^?) during the oxidation of l-ascorbic acid (l-AA) by oxygen with copper as a catalyst was investigated to set up the O₂/Cu/l-AA process with benzoic acid (BA) as a probe reagent. The high concentration of H₂O₂ that is generated undergoes an intramolecular two-electron transfer and is further activated by the intermediate cuprous copper [Cu(i)] to yield HO^? as a product, resulting in significant degradation of BA. Dehydroascorbic acid, 2,3-diketogulonic acid, and l-xylosone were the predominant detected products of the oxidation of l-AA. However, the generation of H₂O₂ and degradation of BA were regulated by variations in pH, which results from the contradiction between protonated l-AA that is difficult to chelate with Cu(ii) via electron transfer and hydrogen ions (H⁺), which are indispensable for the generation of H₂O₂. Furthermore, the concentration of H₂O₂ and degradation of BA increased with an increase in the dosage of l-AA. Trace amounts of Cu(ii) are effective for catalyzing the oxidation of l-AA, whereas the generation of H₂O₂ and degradation of BA increased with an increase in the dosage of Cu(ii). Owing to the formation of Cu(i) chloride complexes or Cu(ii) chloride complexes, the addition of chloride (Cl^-) could inhibit the generation of H₂O₂ and degradation of BA.

Full text of DOI:10.1039/c6ra02843h

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Generation of Hydrogen Peroxide and Hydroxyl Radical Resulting from
Oxygen-Dependent Oxidation of L-Ascorbic Acid via Copper-Redox Catalyzing
Reactions
Peng Zhou, Jing Zhang*, Yongli Zhang, Ya Liu, Juan Liang, Bei Liu, Wei Zhang
College of Architecture & Environment, Sichuan University
Chengdu 610065, P. R. China
Eꢀmail: zjing428@163.com
Abstract
The generation of hydrogen peroxide (H2O2)
and hydroxyl radical (HO•) during the
oxidation of Lꢀascorbic acid (LꢀAA) by oxygen
with copper as a catalyst was investigated to set
up the O2/Cu/LꢀAA process, with benzoic acid
(BA) as the probe reagent. The generated high
concentration of H2O2 undergoes an
intramolecular twoꢀelectron transfer and is
further activated by intermediate cuprous copper [Cu(I)] to induce the product of HO• resulting in
significant degradation of BA. Dehydroascorbic acid, 2,3ꢀdiketogulonic acid, and Lꢀxylosone
were the predominantly detected products of the oxidation of LꢀAA. However, the H2O2
generation and BA degradation were regulated with the variation of pH, which results from the
contradiction between protonated LꢀAA that is difficult to chelate with Cu(II) via electron transfer
and hydrion (H+) is indispensable for the generation of H2O2. Furthermore, the H2O2 concentration
and BA degradation increased with the increase of LꢀAA dosage. Trace amounts of Cu(II) is
effective to catalyze the oxidation of LꢀAA, while the H2O2 generation and BA degradation
increased with the increase of the Cu(II) dosage. Due to the formation of Cu(I)ꢀchloride
complexes or Cu(II)ꢀchloride complexes, the addition of chloride (Clꢀ) could restrict the
generation of H2O2 and BA degradation.
Key word: hydroxyl radical; hydrogen peroxide; copper; Lꢀascorbic acid; oxygen
1 Introduction

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Copper (Cu) is an essential transition metal that is involved in a variety of physicochemical
reactions and physiological processes in natural aquatic systems and is vital to the viability of
almost all organisms,1,2 principally resulting from the redox transformations between cuprous
copper [Cu(I)] and cupric copper [Cu(II)].3 As such, numerous previous studies have focused on
oxygenꢀdependent oxidation of reducing agents catalyzed by redox cycling of copper, and
indicated that there are a number of potential important reactions and products.4,5 For example,
Cu(II) is capable of oxidizing 1,4ꢀhydroquinone, resulting in the formation of the semiquinone
anion radical, benzoquinone, and hydrogen peroxide (H2O2) through an oxidantꢀproducing
copperꢀredox cycle mechanism.6,7 The presence of a suitable ligand can allow Cu(II) to act as an
efficient aerobic redox catalyst for 1,4ꢀhydroquinone oxidation.8 Moreover, copper observably
catalyzes the oxidation of 2,3ꢀDihydroxybenzoic acid, which follows the generation of H2O2 and
hydroxyl radical (HO•).9 What the processes have in common is that Cu(I) is widely considered as
a vitally important intermediate.
Cu(I) is also considered to be an important scavenger of O2.10,11 Redox reactions of Cu(I)
could result in generation of reactive oxygen species, such as superoxide radical (O2•−) and H2O2,
that may subsequently induce a series of promoted reactions with other constituents in natural
waters.12,13 A significant production of HO•,14 could be generated from the further reaction
between Cu(I) with H2O2. Although Cu(I) is an excellent activator for H2O2 to induce the product
of HO•, Cu(I) was seldom investigated to activate H2O2 owing to its instability and difficulty in
dissolving.15 As commonly seen, the capacity of the reducing agent to reduce Cu(II) into Cu(I) is a
decisive prerequisite to induce chain reactions resulting in the generation of reactive species in the
copper catalyzed reducing agents oxidation processes.
Lꢀascorbic acid (LꢀAA, vitamin C), which has been proved to be an important antioxidant in
both plant and animal tissues by preventing oxidativeꢀinduced cellular damage, is a waterꢀsoluble
vitamin that is a necessary component for human health.16 It is able to reduce Cu(II) into Cu(I),
and further induce the generation of H2O2.17,18 Meanwhile, LꢀAA may overcome the drawback of
Cu(I) through reducing Cu(II) into Cu(I) to immediately activate intermediate H2O2 to product
HO•. However, the concomitant formation of H2O2 and HO•, and consumption of oxygen are not
thoroughly studied in water treatment. Especially, the generation and production of HO• received
little attention in the previous literatures, although HO• was recognized as an iconic intermediate

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in advanced oxidation processes (AOPs). Other than traditional AOPs, the generation of HO•
through oxidation of LꢀAA catalyzed by copper occurs without any dosage of conventional
oxidizers, such as H2O2, ozone, persulfate, and peroxymonosulfate, but it is induced from a most
frequent oxidizer O2, which was seldom reported in the previous literatures.
This study aims to investigate the generation of H2O2 and degradation of BA in the
O2/Cu/LꢀAA process, specifically focused on the mechanism of the generation of H2O2 and HO•
during the LꢀAA oxidation catalyzed by copper, while benzoic acid (BA) was selected as the
model compound to indirectly reveal the production of HO•.19 The products of LꢀAA in the
O2/Cu/LꢀAA process were examined with the gas chromatography/mass spectroscopy (GC/MS)
technique. Moreover, the effect of pH, LꢀAA dosage and Cu(II) dosage in the O2/Cu/LꢀAA process
were investigated. The chloride ion was introduced into the O2/Cu/LꢀAA process to investigate the
effect of copperꢀchloride complexes on the O2/Cu/LꢀAA process.
2 Materials and methods
2.1 Materials
Benzoic acid (BA, ≥ 99.5%) and copper sulfate pentahydrate (CuSO4·5H2O, ≥ 99.0%) are of
analytic purity and were supplied by SigmaꢀAldrich. Lꢀascorbic acid (LꢀAA, ≥ 99.7%), hydrogen
peroxide (H2O2, 30%), tertꢀbutyl alcohol (TBA, ≥ 99.5%), phosphoric acid, monosodium
phosphate (≥ 99.0%), sodium hydrogen phosphate (≥ 99.0%), sodium thiosulfate (≥ 99.0%),
sodium chloride (≥ 99.5%), and potassium titanium oxalate (≥ 98.5%) are of analytic purity and
were purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol (≥ 99.9%), phosphoric
acid, and dichloromethane (≥ 99.9%), which were purchased from SigmaꢀAldrich, were of HPLC
grade. Pure oxygen (O2, ≥ 99.2%) was stored in a special highꢀpressure gas cylinder.
2.2 Procedures
Most of the experiments were carried out at 25 ± 1 oC with a 500 mL beaker by heating in water
bath under constant feeding of pure O2 through an aerator in MilliꢀQ water (18.25 Mꢁ·cm). In
order to investigate the consumption rate of oxygen in the processes, part of experiments were
carried out in a 500 mL sealed florence flask under constant stirring with a PTFEꢀcoated magnetic
stirrer. Benzoic acid and Cu(II) (CuSO4) with desired concentrations were spiked in 500 mL
phosphoric acidꢀphosphate buffer. Each run was initiated by adding the desired dosage of fresh
LꢀAA. The pH changed less than ± 0.2 during the process. Samples were respectively withdrawn

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at set intervals and quenched by sodium thiosulfate (for BA) or chelated by potassium titanium
oxalate (for H2O2) before analysis. The quenching experiments employed tertꢀbutyl alcohol as
quencher, which was introduced in excess immediately after the addition of LꢀAA. The chloride
ion was introduced into the experiments to investigate the effect of chelated Cu(II) on the
O2/Cu/LꢀAA process.
2.3 Analysis
The concentration of BA was analyzed on HLPC (Waters, e2695), equipped with the
reverseꢀphase C18 column (4.6 × 150 mm). The binary phase consisted of (A) water with 0.1%
H3PO4 and (B) methanol, and the eluent was A and B (58:42, v/v) with a flow rate of 1.0 mL/min.
Detection was performed using a 2489 λ UV absorbance detector set at 227 nm for BA.
The products of LꢀAA oxidation were examined with the gas chromatography/mass
spectroscopy (GC/MS) technique, operating on a QP2010Plus GC/MS. Prior to GC/MS
determination, a 20 mL sample was extracted using 10 mL dichloromethane three times under
acidic (pH ≈ 2.0), neutral (pH ≈ 7.0), and alkaline (pH ≈ 12.0) conditions, respectively. The three
extracted layers were mixed, dehydrated and concentrated to 5 mL under nitrogen atmosphere.
The prepared dichloromethane solution was filtered by the 0.22 ꢂm polytetrafluoroethylene
membranes and stocked in amber bottles before analysis.
Furthermore, the pH was measured by a pH meter (PHBꢀ4). After chelated by potassium
titanium oxalate, the H2O2 concentrations were measured on UVꢀvis spectrometer (MAPADA,
UVꢀ1800) at 400 nm using a 1cm quartz cuvette, and the concentration of dissolved oxygen (DO)
was measured by a dissolved oxygen meter (JPBꢀ607A). Every experiment was carried out three
times and the standard deviation obtained was less than 2.0%.
3 Results and discussion
3.1 Generation of H2O2 and HO• in the O2/Cu/L-AA process.
The generation of H2O2 during the oxidation of 0.8 mM of LꢀAA catalyzed by copper was
investigated, with the results shown in Figure 1. Astoundingly, the highest concentration of H2O2
unexpectedly went up to 0.48 mM. Simultaneously, more than 40% of BA was degraded in the
O2/Cu/LꢀAA process. It is reasonable to presume that BA was degraded by HO• which could be
generated for the copper catalystꢀmediated decomposition of some reducing agents.6,18
To identify the contribution of HO• on the degradation of BA, TBA was introduced into the

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O2/Cu/LꢀAA process, owing to its high reaction rate with HO• (k = 6.0 × 108 Mꢀ1sꢀ1).20 As shown
in Figure 1, in the absence and presence of TBA, the variations of H2O2 concentration over time
are similar. However, the addition of 25 mM TBA (2500 times of the initial BA concentration)
almost completely inhibited the BA degradation. Thus, it could be concluded that the primary
reactive oxidant was HO• in the O2/Cu/LꢀAA process.
3.2 Pathway of the Generation of H2O2 and HO•
LꢀAA is a binary acid with a bifunctional eneꢀdiol group built into a heterocyclic lactone ring, as
indicated by Formula 1 in Scheme 1. Due to resonance stabilization between the oxygens at the 1ꢀ
and 3ꢀpositions, the high acidity of the 3ꢀhydroxyl is easily ionized and the undissociated hydroxyl
group at 2ꢀposition of the monoanion may be hydrogen bonded to the adjoining negatively
charged oxygen at the 1ꢀpositions, as indicated by Formula 2. In addition, LꢀAA is a strong
twoꢀelectron reducing agent that is readily oxidized in oneꢀelectron steps by metal ions and metal
complexes in their higher valence states.18
By using copper as a catalyst, the oxidation of LꢀAA was involved in a classical chain
reactions, and an inner sphere mechanism is illustrated by Scheme 1.17,18 The first step of the
oxidation of LꢀAA is the interaction between Cu(II) and Formula 2 to form a monoprotonated
Cu(II) complex, indicated by Formula 3. The monoprotonated Cu(II) complex is an ephemeral
intermediate, and rapidly undergoes an intramolecular oneꢀelectron transfer to give an
unprotonated Cu(II) complex (Formula 4) or another monoprotonated Cu(II) complex (Formula 5).
Formula 5 in turn undergoes a second intramolecular electron transfer to produce dehydroascorbic
acid (DꢀAA, Formula 6), the final product of the oxidation of LꢀAA. Moreover, Cu(I), a strong
•−
reducing agent, and O2 are generated with the intramolecular electron transfer. Nevertheless,
under the attack from hydrion (H+), Formula 4 finally decompose into DꢀAA, Cu(II), and H2O2.
Accidentally, Cu(I) and H2O2 form a Fentonꢀlike system in the O2/Cu/LꢀAA process. Cu(I) is
actually a great activator for H2O2 to induce the generation of HO•, but limited by its instability
and difficulty in dissolving. However, as intermediates, Cu(I) and H2O2 could be in full use to
produce HO• in the O2/Cu/LꢀAA process, especially when the yield of H2O2 is high. The main
reactions considered in the O2/Cu/LꢀAA process is presented as follow Eqs. (1)ꢀ(5), due to low
rate constant, many of these possible reactions in this system were excluded from the main
mechanism.6,10,12 Furthermore, products Cu(I) and H2O2 contribute to enhance the generation of

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HO•, via Eq. (2) and Eq. (3) respectively, and it could infer that the consumption of H2O2 via Eq.
(4) and Eq. (5), which explained why the concentration of H2O2 increased firstly then decreased
latter in the O2/Cu/LꢀAA process over time as shown in Figure 1.
0.48 Mꢀ1sꢀ1
(1)
(2)
Cu(Ⅰ) + O2 →Cu(Ⅱ) + O2•−
Cu(Ⅱ) + O2•− → Cu(Ⅰ) + O2
6.6 × 108 Mꢀ1sꢀ1
+
2.0 × 109 Mꢀ1sꢀ1
4.7 × 103 Mꢀ1sꢀ1
(3)
(4)
Cu(Ⅰ) + O2•− 2H →Cu(Ⅱ) + H2O2
+
Cu(Ⅰ) + H2O2 2H →Cu(Ⅱ) + HO• + OH−
−2H+
Cu(Ⅱ)+ H2O2 →Cu(Ⅰ)+O2•−
460 Mꢀ1sꢀ1
(5)
3.3 Products of L-AA
In order to investigate the further oxidation of LꢀAA catalyzed by copper, the oxidation products
of LꢀAA were identified by GC/MS analysis. As shown in Figure 2, DꢀAA (peak area, 31.69%)
was the predominantly detected compound in the O2/Cu/LꢀAA process, which conforms to the
twoꢀelectron transfer oxidation of LꢀAA as seen in scheme 1. Moreover, 2,3ꢀdiketogulonic acid
(DKGA, peak area, 16.33%) and Lꢀxylosone (LX, peak area, 9.44%) were found to be the other
important constituents. In previous reports,21,22 LꢀAA is firstly oxidized to DꢀAA and then form
DKGA, after which the decarboxylation of DKGA forms LX. It could be inferred that the further
oxidation of LꢀAA by copper as a catalyst may follow the same pathway. Furthermore, several
obvious peaks eluted in GC/MS chromatogram could not be identified properly, due to lack of
authentic standards. However, the presence of several of unidentified compounds probably formed
through the compounds, such as LꢀAA, DꢀAA, DKGA, LX, and so on, attacked by HO• or other
reactive oxidants in the O2/Cu/LꢀAA process.
3.4 Effect of pH
To further investigate the mechanism of the O2/Cu/LꢀAA process, the effect of pH on the
generation of H2O2 and the degradation of BA was studied. As shown in Figure 3a, in the pH
range from 2.8 to 8.3, the generation of H2O2 was obvious, and the H2O2 concentrations increased
and then declined over time in all processes. In addition, the peak of H2O2 concentration appeared
later with the pH decreased, and the generation of H2O2 is strongly enhanced at pH 4.5. As shown
in Scheme 1, the generation of H2O2 with pH may be enhanced if the concentration of LꢀAA is
partly replaced by that of the monoanion (Formula 2). Firstly, the formation of the intermediate

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monoprotonated (Formula 3), which is the reactive species in O2/Cu/LꢀAA process, is scarcely
possible when the 3ꢀhydroxyl is not ionized. However, without the attack by H+, it is difficult for
Formula 3 and Formula 4 to form DꢀAA and H2O2 through electron transfer. It should be noted
that about 68% of LꢀAA here existed in the form of Formula 2 with pKa1 = 4.10.23 Thus, the
generation of H2O2 was regulated by H+ with the pH decrease, although the decrease of pH could
enhance the formation of Formula 3.
As shown in Figure 3b, increase of BA degradation was observed with the increase of pH
range from 2.8 to 4.5, then the increase of pH resulted in a decrease of BA removal. Obviously,
the degradation of BA with the variation of pH greatly conforms to the generation of H2O2,
resulting from that the elevated levels of H2O2 could accelerate the generation of HO• via Eq. (4)
to degrade BA. The generation of O2 and Cu(I) were regulated by H+, which is similar to the
•−
effect of H+ on H2O2 generation; it was another way for pH to impact on BA degradation.
Moreover, the degradation of LꢀAA by HO• is not ignorable, and the rate constant between LꢀAA
and HO• increased with the increase of pH.20 Therefore, more LꢀAA would exist with the form of
Formula 2 and less H+ was involved in the generation of H2O2 and Cu(I) with the increase of pH,
which could be the major causes to the variation of the BA degradation efficiency with the
increase of pH in the range of 2.8 to 8.3.
3.5 Effect of L-AA concentration.
To further investigate the role of LꢀAA, the effect of LꢀAA concentration on H2O2 generation and
BA degradation in the O2/Cu/LꢀAA process was studied. As shown in Figure 4, with the increase
of pH in the range of 0.1 to 3.0 mM, the generation of H2O2 was strongly enhanced, while,
resulting in the increase of the degradation of BA. Moreover, the degradation of BA was inhibited
in the initial phase of the O2/Cu/LꢀAA process, especially when the LꢀAA dosage is high. It should
be noted that the degradation of BA was regulated by LꢀAA. Although the increase of LꢀAA
concentration could accelerate the generation of H2O2 and Cu(I) to induce the generation of HO•,
large amount of HO• could be quenched by excess LꢀAA ( k = 1.1 × 1010 Mꢀ1sꢀ1)20 in the initial
phase. However, due to the continual feeding of the oxygen into the solution with the oxidation of
LꢀAA, BA could be continually degraded until the LꢀAA was completely oxidized to terminate the
process. Hence, in improving the degradation efficiency of probe compounds at the highest extent
and reducing the cost, a proper dosage of LꢀAA should be selected.

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3.6 Effect of Cu(II)
The process which uses copper as a catalyst described above, accomplishing twoꢀelectron
oxidation of LꢀAA, may be employed in catalytic process in which the Cu(II) is only a minor
constituent.17 As shown in Figure 5a, the generation of H2O2 was significant when the dosage of
Cu(II) is only 2ꢂM, and the H2O2 concentration decreased with the increase of Cu(II) dosage in
the O2/Cu/LꢀAA process. The higher dosage of Cu(II) could accelerate the oxidation of LꢀAA to
produce H2O2, thus, the generation of H2O2 was accelerated in the initial phase with the increase
of Cu(II) dosage as shown in Figure 5a. However, the increase of the Cu(II) dosage could also
enhance the generation of Cu(I), which could accelerate the decomposition of H2O2 to product
HO• via Eq. (4). Precisely because of the enhanced HO• generation, the degradation of BA was
increased with the increase of the Cu(II) dosage, as shown in Figure 5b.
In natural waters, the variety of potential copperꢀbinding ligands is large, and the
determination of the actual Cu(I) and Cu(II) species present is difficult and the elucidation of
constant rates for reaction of these species is essentially impossible. However, the effect of ligand
of copper species on the O2/Cu/LꢀAA process is worth investigating. As shown in Figure 6a, the
generation of H2O2 was enhanced with the dosage of Clꢀ in the range of 1 to 10 mM. The addition
of Clꢀ at these dosages could almost not form Cu(II)ꢀchloride complexes with Cu(II) resulting
from the relatively low affinity of Clꢀ for Cu(II).8 In addition, these dosages of Clꢀ had minimal
effect on the oxygen consumption rates in the O2/Cu/LꢀAA process as shown in Tab. 1. Therefore,
it could be concluded that the addition of Clꢀ in the range of 1 to 10 mM has less impact on the
consumption of oxygen and the generation of H2O2 through electron transfer catalyzed by copper.
ꢀ
ꢀ
However, Cu(I)ꢀchloride complexes (CuCl, CuCl2 , and CuCl3 ) are expected as the predominant
Cu(I) species due to the higher dosages of Clꢀ in this work and the large stability constants of these
complexes.12 The constant rates of the reaction between H2O2 with Cu(I) are inhibited when the
ꢀ
ꢀ
Cu(I) species are in the form CuCl, CuCl2 , and CuCl3 , which could inhibit the generation of HO•.
Although HO• can reacts rapidly with Clꢀ, the reaction forms ClOHꢀ• reversibly, and the formation
of Cl• generally is only significant at low pH as Eq. (6) and Eq. (7).24 It conforms that the addition
of Clꢀ obviously inhibited the degradation of BA as shown in Figure 6b.
kfor = 4.3×109 Mꢀ1sꢀ1 krev = 6.1 × 109 sꢀ1
(6)
•−
HO • + Cl − ↔ ClOH

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kfor = 2.1 × 1010 Mꢀ1sꢀ1
(7)
ClOH •− + H + ↔ Cl• + H2O
Furthermore, when the dosage of Clꢀ ranges from 30 to 500 mM, the Cu(II) species mainly
exist in the form of Cu(II)ꢀchloride complexes (CuCl+ and CuCl2)8 and the oxygen consumption
rate was significantly decreased in the O2/Cu/LꢀAA process as shown in Tab. 1. It could be
concluded that the high addition of Clꢀ may inhibit the electron transfer of oxidation of Formula 2
catalyzed by Cu(II) to form Formula 3 resulting in the generation of H2O2, which conforms to the
generation of H2O2 as shown in Figure 6a. It is clear from these results that at proper concentration,
Clꢀ is an effective coordinating agent for copper redox catalysis for generation of H2O2, but not for
HO•.
4. Conclusions
The study introduced an ignored phenomenon that the strong generation of H2O2 and HO• during
the oxygenꢀdependent oxidation of LꢀAA catalyzed by Cu(II)/Cu(I) redox couple. Undergoing an
intramolecular twoꢀelectron transfer, high concentration of H2O2 was generated and further
activated by intermediate Cu(I) to induce the product of HO• resulting in significant degradation
of BA in the O2/Cu/LꢀAA process. Dehydroascorbic acid (DꢀAA), 2,3ꢀdiketogulonic acid (DKGA),
and Lꢀxylosone (LX) were the predominantly detected products of the oxidation of LꢀAA based on
GC/MS technique.
Moreover, pH and the form of Cu(II) species and Cu(I) species are important influencing
factors of the H2O2 generation and BA degradation via regulating the election transfer process.
Trace amounts of Cu(II) is effective to catalyze the oxidation of LꢀAA. Meanwhile the H2O2
generation and BA degradation increased with the increased dosage of Cu(II) and LꢀAA. The
presence of chloride with low dosage could enhance the concentration of H2O2 through forming
Cu(I)ꢀchloride complexes to inhibit the activation of H2O2 by Cu(I) species which decreases the
BA degradation efficiency, and the high dosage of chloride could simultaneously decrease the
concentration of H2O2 and the degradation efficiency of BA through forming Cu(II)ꢀchloride
complexes to inhibit electron transfer.
In the O2/Cu/LꢀAA process, LꢀAA is the driving force resulting from that LꢀAA can reducing
Cu(II) into Cu(I) which is the key intermediate to induce the chain reactions to produce reactive
oxygen species. Based on the role of LꢀAA in the O2/Cu/LꢀAA process, it could be inferred that

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other reducing agents which can reduce Cu(II) into Cu(I) may act similar to LꢀAA in the
O2/Cu/LꢀAA process. Nevertheless, it should be noted the O2/Cu/LꢀAA process is far from
practical. Our work just introduced an interesting phenomenon and proposed a preliminary
interpretation that the oxidation of LꢀAA by copper catalyzed could induce the production of H2O2
and HO•.
Acknowledgements
Appreciation and acknowledgment are given to the National Natural Science Foundation of
China (No. 51508353), the National Natural Science Foundation of China (No. 51408349), the
National Natural Science Foundation of China (No. 51008052) and the Program for New Century
Excellent Talents in University (NCETꢀ11ꢀ0082).

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RSC Advances
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DOI: 10.1039/C6RA02843H
Table 1. Effect of chloridion on the initial oxygen consumption rate in the O2/Cu/L-AA process.
[CuSO4], µM
[Cl-], mM
0
Initial rate, µM/min
10
10
10
10
10
10
10
10
126.6±0.4
127.2±0.8
126.8±0.3
131.3±1.2
128.8±1.3
86.3±0.5
19.1±0.2
7.5±0.1
1
3
5
10
30
100
500
Conditions: [L-AA]0 = 0.5 mM, [Cu(II)]0 = 10 µM, [BA]0 = 10 µM, [Cl-]0 = 0, 1, 3, 5, 10, 30, 100,
and 500 mM, [O2]0 = 7.24 ± 0.02 mg/L, pH = 7 ± 0.2, 25 oC. The experiments were carried out at
a 500 mL sealed florence flask.

Page 13 of 19
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Scheme 1. Generation of hydrogen peroxide and hydroxyl radical resulting from
oxygen-dependent oxidation of L-ascorbic acid via copper-redox catalyzing Reactions.

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Figure 1. Effect of TBA on H2O2 generation and BA degradation in the O2/Cu/L-AA process.
Conditions: [L-AA]0 = 0.8 mM, [Cu(II)]0 = 10 µM, [BA]0 = 10 µM, [TBA]0 = 25 mM, O2 flow
rate = 0.4 L/min, pH = 7 ± 0.2, 25 oC.

Page 15 of 19
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Figure 2. The GC/MS chromatogram of the products of the oxidation of L-AA in the O2/Cu/L-AA
process.
Conditions: [L-AA]0 = 1.0 mM, [Cu(II)]0 = 10 µM, O2 flow rate = 0.5 L/min, pH = 7 ± 0.2, 25 oC,
reaction time = 150 min.

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Figure 3. Effect of pH on H2O2 generation (a) and BA degradation (b) in the O2/Cu/L-AA process.
Conditions: [L-AA]0 = 1 mM, [Cu(II)]0 = 10 µM, [BA]0 = 10 µM, O2 flow rate = 0.4 L/min, pH =
2.8, 4.5, 5.9, 7.0, and 8.3, 25 oC.

Page 17 of 19
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Figure 4. Effect of L-AA concentration on H2O2 generation (a) and BA degradation (b) in the
O2/Cu/L-AA process.
Conditions: [L-AA]0 = 0.1, 0.3, 1.0, 2.0, and 3.0 mM, [Cu(II)]0 = 10 µM, [BA]0 = 10 µM, O2 flow
rate = 0.4 L/min, pH = 7 ± 0.2, 25 oC.

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Figure 5. Effect of Cu(II) dosage on H2O2 generation (a) and BA degradation (b) in the
O2/Cu/L-AA process.
Conditions: [L-AA]0 = 1.0 mM, [Cu(II)]0 = 2, 5, 10, 30, 50, and 100 µM, [BA]0 = 10 µM, O2 flow
rate = 0.4 L/min, pH = 7 ± 0.2, 25 oC.

Page 19 of 19
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Figure 6. Effect of chloridion on H2O2 generation (a) and BA degradation (b) in the O2/Cu/L-AA
process.
Conditions: [L-AA]0 = 0.5 mM, [Cu(II)]0 = 10 µM, [BA]0 = 10 µM, [Cl-]0 = 0, 1, 3, 5, 10, 30, 100,
and 500 mM, O2 flow rate = 0.5 L/min, pH = 7 ± 0.2, 25 oC.