A Green Method to Determine VUV (185?nm) Fluence Rate Based on Hydrogen Peroxide Production in Aqueous Solution

Article abstract of DOI:10.1111/php.12913

A mini-fluidic vacuum ultraviolet/ultraviolet (VUV/UV) photoreaction system (MVPS) was developed in our previous study. Based on the MVPS, a green method to determine VUV fluence rate has been developed using the production rate of H₂O₂

Full text of DOI:10.1111/php.12913

Article type : Research Note
A Green Method to Determine VUV (185 nm) Fluence Rate Based
on Hydrogen Peroxide Production in Aqueous Solution
Laxiang Yang1, Mengkai Li1, Wentao Li1, James R. Bolton2, Zhimin Qiang*1
1Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental
Sciences, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 18 Shuang-
qing Road, Beijing 100085, China
2Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G
1H9, Canada
*Corresponding author’s email address: qiangz@rcees.ac.cn (Z. Qiang).
ABSTRACT
A mini-fluidic vacuum ultraviolet/ultraviolet (VUV/UV) photoreaction system (MVPS) was
developed in our previous study. Based on the MVPS, a green method to determine VUV fluence rate
has been developed using the production rate of H2O2 when water is exposed to 185 nm VUV. The
H2O2 production followed pseudo-zero-order reaction kinetics well over the first 10 min of VUV/UV
exposure. This new method was well calibrated with a standard cis-cyclooctene cis-trans
photoisomerization actinometer as recommended by the International Union of Pure and Applied
Chemistry. The apparent quantum yield for H2O2 production by 185 nm VUV irradiation of water was
determined to be 0.024 ± 0.002. As the solution pH increased from 5.0 to 8.0, the H2O2 production
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rate decreased from 0.83 to 0.40 μM min‒1. Dissolved oxygen had a negligible influence on the H2O2
production. This study proposes a novel VUV fluence rate determination method with advantages of
non-toxicity, low detection limits, low costs and convenience, and it can be used as a good alternative
to traditional actinometers.
INTRODUCTION
As an advanced oxidation technology, the vacuum ultraviolet (VUV) process has been applied to
water treatment, production of ultrapure water and transformation of organic materials (1‒5). In the
process of VUV irradiation, determination of the VUV fluence rate is crucially important, because it
is an essential factor for reactor design, reaction rate determination, and operating cost calculation (6).
Several liquid-phase chemical actinometers for VUV fluence rate determination have been
reported so far (7‒10). Schuchmann and Sonntag (8) determined the VUV fluence rate by analyzing
the H2 production rate in the process of ethanol photolysis under 185 nm VUV irradiation (Farkas
actinometer). Schuchmann et al. (9) also used the photoisomerization reaction between cis-
cyclooctene and trans-cyclooctene to determine the VUV fluence rates at both 172 nm (Xe-excimer
lamp) and 185 nm (low-pressure mercury lamp) with different quantum yields. Both methods have
been recommended by the International Union of Pure and Applied Chemistry (IUPAC) to determine
the VUV fluence rates (7). Another chemical actinometer based on water photohomolysis in methanol
solution (0.2 M) was introduced by Heit et al. (10), and the quantum yield of 172 nm VUV photolysis
of water was determined to be 0.42. However, in all these methods, organic solvents must be used and
most of them are toxic. In addition, the analytical procedure of gas chromatograph (GC) is relatively
complicated and time-consuming. Therefore, there is a need to develop an accurate, green and rapid
method for VUV fluence rate determination.
As conventional VUV reactors have a relatively low accuracy for the determination of VUV
photochemical kinetic parameters, a mini-fluidic VUV/UV photoreaction system (MVPS) was
developed as described in our previous study (11). The MVPS provides a powerful tool for the
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determination of VUV photochemical kinetic parameters, such as VUV reaction rate constants and
quantum yields. However, a VUV fluence rate determination method for the MVPS has not yet been
developed, which limits the MVPS applications in photochemical studies. This study aimed to
examine the H2O2 production kinetics during 185 nm VUV photolysis of water using the MVPS, to
determine the apparent quantum yield for H2O2 production, and to evaluate its potential as a green,
effective and rapid method for VUV fluence rate determination.
MATERIALS AND METHODS
Chemicals. N,N-diethyl-p-phenylenediamine sulfate, peroxidase from horseradish and cis-cyclooctene
were purchased from J&K Scientific (Beijing, China). All other chemicals used were of at least
analytical grade. All aqueous solutions were prepared with ultrapure water produced by a Milli-Q
system (Advantage A10, Millipore, USA).
Experimental system. Irradiation experiments were conducted using the previously developed
MVPS (Fig. S1) (12). In brief, a low-pressure mercury lamp with a power of 8 W was installed along
the central axis of a cylindrical photoreactor. A synthetic quartz tube (the VUV/UV tube) and a Ti-
doped silica tube (the UV tube) were mounted parallel to the lamp at the same radial distance (about 5
mm) to the lamp surface in the photoreactor. The reaction solution could be exposed to either the
combined VUV/UV or the sole UV when flowing through the VUV/UV or UV tube, respectively, and
a nearly identical UV fluence rate could be achieved in the two tubes during the exposure time. The
temperature of the photoreaction system was stabilized at 25 ± 1 °C by recirculating water. The
reaction solution volume was 50 mL and the solution pH was stabilized at 6.0 with 1.0 mM phosphate
buffer. Water sample of 1.0 mL was taken for analysis at a time interval of 2 min during the
experiment.
Analytical methods. H2O2 concentration was analyzed by a photometric method (13) with a low
detection limit of 3 μg L‒1. VUV fluence rate was determined by a standard cis-cyclooctene cis-trans
photoisomerization actinometer as recommended by the IUPAC (7,9). The cis-cyclooctene and trans-
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cyclooctene concentrations were analyzed by GC (Shimadzu, 2010 Plus), equipped with a flame
ionization detector (FID). The separation column (DB-1701, 30 m × 0.25 mm, 0.25 μm) was from
Agilent Technologies. The injector temperature was 80 °C and the injection volume was 1.0 μL with a
split ratio of 5:1. The oven temperature was started at 50 °C, held for 5 min, and then ramped at 25 °C
min‒1 to 250 °C.
RESULTS AND DISCUSSION
H2O2 production
The bond dissociation enthalpy of water is 498 kJ mol‒1, so the threshold wavelength that is defined as
“the maximum wavelength for which the photon energy matches the bond energy of the bond that
breaks in the photolysis reaction” (14) is 240 nm. When water is exposed to 185 nm VUV, it is readily
photolyzed into hydroxyl radicals (HO•) and hydrogen atoms (H•) [Eq. (1)], which was confirmed by a
spin-trapping electron paramagnetic resonance (EPR) experiment (Fig. S2). The characteristic HO•
spin-trapping peaks were detected within 30 s after the solution was exposed to VUV/UV, while such
peaks were not observed when the solution was exposed to UV alone. Two hydroxyl radicals can
merge into one H2O2 molecule [Eq. (2)], and two other terminal reactions [Eqs. (3) and (4)] can also
occur at the same time. The H2O2 concentration increased with increasing reaction time until it
reached a maximum value after 2 h exposure (Fig. 1 inset). Then the H2O2 concentration decreased
slightly and leveled off at a quasi-stationary concentration of 41 μM after 5 h exposure. The quasi-
stationary state was reached when the production rate of H2O2 [Eq. (2)] was equal to its consumption
rate [Eqs. (5) and (6)] (15).
H2O VUV HO + H
HO + HO  H2O2
HO + H  H2O
= 0.33 (16)
(1)
k2 = 4 × 109 M‒1 s‒1 (16)
(2)
k3 = 1.5 × 1010 M‒1 s‒1 (17)
(3)
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H + H  H2
k4 = 1 × 1010 M‒1 s‒1 (16)
(4)
(5)
(6)
UV
H2O2  HO + HO
= 0.555 (18)
H2O2 + HO  HO2 + H2O
k6 = 3.3 × 107 M‒1 s‒1 (16)

The H2O2 production followed pseudo-zero-order reaction kinetics well over the first 10 min of
VUV irradiation (Fig. 1). The H2O2 production rate in that time period was determined to be 0.80 μM
min‒1 with a good correlation coefficient (r2 = 0.9992). When water was exposed to VUV/UV
irradiation up to 10 min, the produced H2O2 had a very low concentration (8.0 μM). As the H2O2
concentration increased, its UV photolysis rate also increased, but only 4.4% of H2O2 was photolyzed
at a concentration of 10 μM after 10 min UV exposure (Fig. S3). Therefore, the UV photolysis of
H2O2 [Eq. (5)] could be neglected at the beginning of the VUV/UV exposure (i.e., in the first 10 min).
The H2O2 consumption reaction [Eq. (6)] was also insignificant, because its rate constant is only 3.3 ×
107 M‒1 s‒1 (16), which is much smaller than that of Eq. (2). Without regard to its consumption at the
beginning of the VUV/UV exposure, the H2O2 production was dependent only on the concentration of
HO• which has a high diffusion-controlled second-order reaction rate constant [Eq. (2)] (16).
Therefore, Eq. (1) becomes the bottleneck reaction to the H2O2 production from VUV-generated HO•.
As the concentration of water is excessively high (55.6 M), the VUV photolysis of water follows
pseudo-zero-order reaction kinetics (19). Consequently, the H2O2 production also followed pseudo-
zero-order reaction kinetics corresponding to that of the VUV photolysis of water at the beginning of
the VUV/UV exposure.
Calibration of the new method
Since the H2O2 production rate [ rH O (μM min‒1)] followed pseudo-zero-order reaction kinetics, it
2
2
would be proportional to the absorbed VUV photon flux [q (einstein s–1)] at the beginning of the
VUV/UV exposure. Accordingly, q could be determined using Eq. (7):
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rH O
V
2
2
(7)
q 
Φ
where q can be converted to photon fluence rate (Eo,p) by a conversion coefficient (11,14) and is used
here for simplicity;
volume (50 mL).
Φ
is the apparent quantum yield for H2O2 production; and V is the reaction
To determine the apparent quantum yield, the H2O2 production rates were determined at different
VUV photon fluxes (Fig. 2). The horizontal coordinate represents the VUV photon fluxes that were
determined by the standard cis-cyclooctene cis-trans photoisomerization actinometer, and the vertical
coordinate represents the corresponding H2O2 production rates. As expected, a good linear
correlativity (r2 = 0.993) was achieved between the VUV photon flux and the H2O2 production rate.
The apparent quantum yield for H2O2 production was determined to be 0.024 ± 0.002 according to the
linear slope. Based on the apparent quantum yield for H2O2 production, Eq. (7) could be conveniently
used to determine the VUV photon flux.
Effects of pH and oxygen
Because of the pH variation of ultrapure water, phosphate buffer was used during the experiments.
The pH of the ultrapure water used in this study was determined to be about 6.5. After the ultrapure
water was buffered with phosphate (1.0 mM) and adjusted to the same pH, the H2O2 production rate
was almost the same as when no buffer was used (data not shown). It demonstrates that the phosphate
itself had little influence on the H2O2 production. The influence of solution pH on the H2O2
production is shown in Fig. 3a. When the solution pH increased from 5.0 to 8.0, the H2O2 production
rate decreased from 0.83 to 0.40 μM min‒1. This is probably because HO• has a higher oxidation
potential under acidic conditions (12), which favored the production of H2O2. Oxygen is ubiquitous in
water. The influence of dissolved oxygen (DO) on the H2O2 production rate was examined by purging
nitrogen or oxygen gas, separately. DO at concentrations ranging from 0.05 to 14.0 mg L‒1 had little
influence on the H2O2 production rate (Fig. 3b), which means that Eqs. (8) and (9) were insignificant
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for the H2O2 production at the beginning of the VUV/UV exposure. In fact, the rate constant of Eq.
(9) is only 2 × 106 M‒1 s‒1, which is much smaller than that of Eq. (2) (i.e., 4 × 109 M‒1 s‒1) (16).

H + O2  HO2
k8 = 1 × 1010 M‒1 s‒1 (16)
k9 = 2 × 106 M‒1 s‒1 (16)
(8)

2HO2  H2O2 + O2
(9)
CONCLUSIONS
A novel VUV fluence rate determination method using the MVPS has been developed based on the
H2O2 production by VUV photolysis of water. The method was well calibrated with a standard
actinometer and the apparent quantum yield for H2O2 production was determined to be 0.024 ± 0.002.
The H2O2 production rate decreased with increasing solution pH, but was almost unaffected by the
DO concentration. This novel method has advantages of non-toxicity, low detection limits, low costs
and convenience, and thus can be used as a good alternative to traditional actinometers that commonly
use organic solvents.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support from the National Natural Science
Foundation of China (51525806, 21590814, 51611540344), Ministry of Science and Technology of
China (2016YFC0400802), and National Geographic Air and Water Conservation Fund of US
(GEFC23-15).
SUPPORTING INFORMATION
Additional Supporting Information is available in the online version of this article:
Figure S1. Schematic diagram of the mini-fluidic VUV/UV photoreaction system (MVPS).
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Figure S2. Electron paramagnetic resonance (EPR) spectrum of HO• formed during the VUV/UV
exposure, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 0.1 M) used as a spin trapping agent.
Figure S3. UV photolysis of H2O2 at different initial csoncentrations.
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FIGURE CAPTIONS
Figure 1. H2O2 production as a function of VUV irradiation time (pH = 6.0).
Figure 2. H2O2 production rate as a function of VUV photon flux.
Figure 3. Effects of pH (a) and DO (b) on H2O2 production rate.
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