Mechanism of methylphosphonic acid photo-degradation based on phosphate oxygen isotopes and density functional theory

Article abstract of DOI:10.1039/c9ra05169d

Methylphosphonic acid (MPn) is an intermediate in the synthesis of the phosphorus-containing nerve agents, such as sarin and VX, and a biosynthesis product of marine microbes with ramifications to global climate change and eutrophication. Here, we applied the multi-labeled water isotope probing (MLWIP) approach to investigate the C-P bond cleavage mechanism of MPn under UV irradiation and density functional theory (DFT) to simulate the photo-oxidation reaction process involving reactive oxygen species (ROS). The results contrasted with those of the addition of the ROS-quenching compounds, 2-propanol and NaN₃. The degradation kinetics results indicated that the extent of MPn degradation was more under alkaline conditions and that the degradation process was more rapid at the initial stage of the reaction. The phosphate oxygen isotope data confirmed that one exogenous oxygen atom was incorporated into the product orthophosphate (PO₄) following the C-P bond cleavage, and the oxygen isotopic composition of this free PO₄ was found to vary with pH. The combined results of the ROS-quenching experiments and DFT indicate that the C-P bond was cleaved by OH^-/OH and not by other reactive oxygen species. Based on these results, we have established a mechanistic model for the photolysis of MPn, which provides new insights into the fate of MPn and other phosphonate/organophosphate compounds in the environment.

Full text of DOI:10.1039/c9ra05169d

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PAPER
Mechanism of methylphosphonic acid photo-
degradation based on phosphate oxygen isotopes
and density functional theory†
Cite this: RSC Adv., 2019, 9, 31325
Congcong Xia,‡c Huanhuan Geng,‡ Xiaobao Li, Yiyue Zhang, Fei Wang, *b
ab
b
a
b
bd
d
d
b
Xiaowen Tang,* R. E. Blake, Hui Li, Sae Jung Chang§ and Chan Yu
Methylphosphonic acid (MPn) is an intermediate in the synthesis of the phosphorus-containing nerve
agents, such as sarin and VX, and a biosynthesis product of marine microbes with ramifications to global
climate change and eutrophication. Here, we applied the multi-labeled water isotope probing (MLWIP)
approach to investigate the C–P bond cleavage mechanism of MPn under UV irradiation and density
functional theory (DFT) to simulate the photo-oxidation reaction process involving reactive oxygen
species (ROS). The results contrasted with those of the addition of the ROS-quenching compounds, 2-
3
propanol and NaN . The degradation kinetics results indicated that the extent of MPn degradation was
more under alkaline conditions and that the degradation process was more rapid at the initial stage of
the reaction. The phosphate oxygen isotope data confirmed that one exogenous oxygen atom was
4
incorporated into the product orthophosphate (PO ) following the C–P bond cleavage, and the oxygen
isotopic composition of this free PO
4
was found to vary with pH. The combined results of the ROS-
Received 8th July 2019
Accepted 6th September 2019
ꢀ
quenching experiments and DFT indicate that the C–P bond was cleaved by OH /cOH and not by other
reactive oxygen species. Based on these results, we have established a mechanistic model for the
photolysis of MPn, which provides new insights into the fate of MPn and other phosphonate/
organophosphate compounds in the environment.
DOI: 10.1039/c9ra05169d
rsc.li/rsc-advances
10,11
biogeochemical cycles of key elements—Fe, N and P,
and
1
. Introduction
12,13
thus indirectly affects the environment.
The net ux of methane is generally supersaturated at the
simple C–P bonded compounds. It is a precursor to more ocean surface. Photolysis of MPn maybe one of the pathways of
Methylphosphonic acid (MPn) is one of the most structurally
1–3
complex phosphonate molecules, such as glyphosate. MPn is methane production under aerobic environments, however, the
an intermediate in the synthesis of the chemical warfare agents detailed mechanism for the cleavage of the C–P bond still
4
–7
14
(
CWA), such as sarin and VX, and a biosynthesis product of remains elusive. The contribution of UV radiation to the MPn
most marine microorganisms, such as Nitrosopumilus mar- degradation in the uppermost euphotic zone of aquatic systems
8
9
itimus. MPn is very stable, and recent research shows that the could be highly signicant. In aquatic systems, UV photons
microbial metabolism of MPn has a signicant link to the usually interact with the reactant molecules to form radical
methane production in the open ocean and the large-scale species that degrade the dissolved phosphonate compounds by
15,16
stimulating an attack on the C–P bond.
Photo-degradation
of the dissolved organic phosphorus (DOP) is an important
pathway for the generation of the biologically-available dis-
a
b
Jiangxi Transportation Institute China, 809 Jinsha Road, 330038 Nanchang, China
School of Energy & Environmental Engineering, University of Science and Technology
17,18
solved inorganic phosphate (DIP).
Therefore, the degrada-
tion of MPn is accompanied by the release of the dissolved
inorganic phosphate (PO or Pi). Under most environmental
conditions and in the absence of microbial/enzymatic activity,
Beijing, 30 Xueyuan Road, 100083 Beijing, China. E-mail: wangfei@ustb.edu.cn
c
4
School of Pharmaceutical Sciences, Sun Yet-sen University, 510006 Guangzhou,
China. E-mail: tangxw5@mail.sysu.edu.cn
d
Department of Geology and Geophysics, Yale University, P.O. Box 208109, New the oxygen in the P–O of the dissolved inorganic orthophos-
Haven, CT 06520-8109, USA
phate (DIP or PO
4
) is difficult to exchange with oxygen in the
†
1
‡
§
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra05169d
19
other substances, such as water. The ratio of the oxygen
18 16
18
isotopes, O/ O, in orthophosphate (d O ) has been widely
P
These authors contributed to the work equally.
applied to study the reaction pathways and P-cycling processes
over the past decade.
Present address: Division of Earth and Environmental Sciences, Korea Basic
20,21
Science Institute, 162, Yeongudanji-ro, Ochang-eup, Cheongwon-gu
Cheongju-si, Chungbuk 28119, Republic of Korea.
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1
8
The multi-labeled water isotope probing (MLWIP) approach, prepared in Milli-Q deionized water spiked with O-labeled
2
2
23,24
18
developed by Blake and widely employed by others,

was water with oxygen isotopic composition (d O ) of ꢀ6, 10.8
W
rst used to probe the C–P bond cleavage mechanism of and 32.4& (VSMOW). The pH values of the MPn solutions were
glyphosate and phosphonoacetic acid by Sandy et al. Previous adjusted to 3 and 8, respectively. The concentrations of MPn
results from our group indicate that O from an external source, were selected to release between 50 and 100 mmoles of Pi aer
25
2
possibly 2 sources (ambient water and O ), is incorporated into complete photolysis. About ve milliliters of each MPn solution
the orthophosphate and released along with the glyphosate C–P was treated with a 1200 W high-pressure mercury lamp in an
bond cleavage. However, different models have been put forth Ace glass 7900 UV photo-oxidation apparatus (in the Blake
ꢁ
to account for the source(s) of the external O incorporated into laboratory at Yale University) kept at #40 C. Each experiment
26,27
the released orthophosphate.
In order to verify which form was conducted in triplicate, and the degradation rates of MPn
of oxygen is involved in the C–P bond cleavage process, we were determined based on the concentration of the inorganic
studied the role of the ROS in the C–P bond cleavage phosphate released into the solution every 6 hours. The
mechanism.
phospho-molybdate blue assay was used to perform the color-
33
In surface waters, some organic contaminants undergo self- imetric phosphate analysis.
sensitization in sunlight and form an excited triplet state
The orthophosphate samples were processed using the
28
34
species that can perform direct photolysis. To investigate the following method described by Jaisi and Blake. The released
process of the organic contaminant degradation and test phosphate was rst precipitated as ammonium phosphomo-
whether ROS are produced during the degradation process, lybdate (APM) at an acidic pH, then dissolved and precipitated
1
NaN3 (an O2 and cOH scavenger) and 2-propanol (an cOH as magnesium ammonium phosphate (MAP) at an alkaline pH,
2
+
scavenger) are widely used as ROS quenchers to constrain and treated with a cation exchange resin to remove Mg and
29,30
photolysis.
As for the environmental chemical effect on other contaminants before the nal precipitation as silver
degradation, pH is one of the most important factors inu- phosphate (Ag
3
PO
4
), the compound used for the phosphate
ꢁ
encing photo-oxidation due to the different proportion of ROS oxygen isotope analysis. The Ag
3
PO
4
was roasted at ꢂ550 C for
31,32
produced at different pH.
Therefore, different pH conditions 3 min under vacuum for further purication and the removal of
can help predict the reaction path of MPn in models of its water and any possible remaining organic matter. Silver
transport and degradation in natural environment, and also aid capsules, containing 250–350 mg of the roasted Ag PO , were
3
4
in the understanding of the C–P bond cleavage mechanisms.
prepared in triplicate for the oxygen isotope analysis and
In this study, we present the results of the experiments examined on a Thermo-Chemolysis/Elemental Analyzer (TC/EA)
1
8
performed on the degradation of MPn using the O-labeled coupled to a Delta V continuous ow isotope ratio mass spec-
1
8
water to probe the mechanism and kinetics of the C–P bond trometer (IRMS, Finnigan-MAT, Bremen, Germany). The d O
cleavage and track the oxygen isotope composition of the values of the silver phosphate samples were calibrated against
P
1
8
inorganic product PO
4 P
or Pi (d O ). We also studied the role of two laboratory silver phosphate standards, YR1a and YR-32,
1
8
the reactive oxygen species (ROS) and pH in the MPn degrada- with the d OPO4 values of ꢀ5.49& and 33.63&, respectively.
1
8
tion in aquatic systems. DFT was used to verify the role of ROS The analytical precision of the silver phosphate d O values
P
35
in the MPn photo-degradation as it may not be possible to based on replicate analyses was better than ꢃ0.3& (1SD).
isolate and explicitly determine the O isotopic composition of Isotope analysis was performed at the Yale Institute for
ROS the way it is typically done to assess the contribution of the Biospheric Studies (YIBS)–Yale Analytical and Stable Isotope
26,27
potential O sources, involved in the C–P cleavage.
Also, since Center (YASIC).
the MPn structure is very simple, the degradation products are
detected by nuclear magnetic resonance (NMR). Hence, based
on the combined results of the abovementioned studies, we
posit plausible MPn photo-degradation pathways.
2
.3 Methylphosphonic acid degradation experiments w/ROS
A series of experiments were carried out to investigate the
inuence of ROS under UV irradiation. Twenty mL aliquots of
0.1 mM MPn were prepared and transferred to separate 35 mL
2
. Materials and methods
sterilized quartz tubes to which 20 mM 2-propanol (an cOH
1
scavenger) and 20 mM NaN
added to quantify the contribution of cOH and
3 2
(an O and cOH scavenger) were
2.1 Reagents
1
2
O to the
MPn was purchased from Sigma-Aldrich, and NaN
3
and 2-
photolysis, respectively. The samples were treated with a photo-
chemical reaction instrument (BL-GHX-V; Shanghai, China)
propanol were purchased from Sinopharm Chemical Reagent
Co., China. Deuterium oxide for the NMR analysis was obtained
from Sahn Chemical Technology. All other materials were
ꢁ
under a 1000 W mercury lamp with a temperature of #25 C
controlled by a cooling water circulation system at the Univer-
sity of Science and Technology Beijing.
$
99% pure and obtained from commercial sources.
2.2 Methylphosphonic acid degradation experiments
2.4 DFT calculations
A series of MPn degradation experiments were carried out to The DFT calculations were used to verify the contribution of
investigate the effect of pH and water O isotopic composition on ROS and the theoretical reaction mechanism of MPn solution
the degradation kinetics. Aqueous solutions of MPn were under UV conditions. All calculations were performed using the
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8
Gaussian 09 package. The 3D-structures of the optimized the degradation rate of the different O-labeled solutions.
species were generated using GaussView 5.0. Geometry opti- Fig. 1b shows the photolysis of MPn at pH 3 and pH 8; the
ꢀ1
mizations and the vibrational frequencies of all the stationary corresponding photolysis rate constants were 0.037 h and
ꢀ1
points were performed at the M06-2X/6-311G+(d,p) level. 0.054 h , respectively. Pajares et al. found that under the
Simulation of the solvent effect was performed using the SMD alkaline conditions, phenol exists in a more active anion state,
37
model with water as the solvent. With geometry optimization at and its degradation rate is faster. Previous studies indicated
ꢁ
each point, the relaxed potential energy surface (PES) scan was that 2-chloroethylphosphonic acid hydrolyzes at 35–55 C,
performed to ascertain the existence of the transition state in appreciably only at alkaline pH (6–9) based on the different
38
each step. A lower level, M06-2X/6-31G(d), was adopted for the ionic species present at different pH. Nevertheless, the control
PES scan. Intrinsic reaction coordinate (IRC) calculations were experiments revealed that without UV exposure, MPn hydrolysis
ꢁ
performed to assure that the transition states connected the is almost nonexistent in solution at about 40 C. On the other
reactants and the products for each elementary reaction. The hand, the cleavage of the C–P bond in alkylphosphonates is
activation free energy barrier was dened as the free energy assumed to be a redox-dependent free-radical process, which is
difference between the transition state and the lowest-energy probably dependent on the aeration and the pH of the
39,40
stationary point in the reaction pathway.
medium.
Therefore, we posit that MPn is more likely to
undergo redox reactions under alkaline conditions.
2.5 NMR analysis of complexation in the solution
3
.2 Stable oxygen isotope analysis based on the amount of
Spectroscopic studies were carried out in order to investigate
the structure of the product of MPn photo-oxidation. This
PO released from MPn as a result of UV photo-oxidation
4
1
8
included the analysis of the proton NMR spectrum obtained. The UVR degradation data of MPn in different O-labeled water
MPn (1.87 mM) was directly dissolved in 10 mL D O and solutions based on the phosphate concentration measure-
2
exposed to UV irradiation. The photo-degradation conditions ments, expressed as the percentage of the total available P, are
were consistent with those of the ROS-quenching reactions. shown in Table 1. As shown in the table, at least 90% of the MPn
1
3
1
Carbon ( C) NMR and proton ( H) NMR spectra were recorded hydrolysis was achieved in 68 h of the UVR treatment. The
in D O at room temperature on a Bruker-500 MHz nuclear phosphorus atom in MPn is bonded to three oxygen atoms.
magnetic resonance (NMR) spectrometer (Bruker).
2
Thus, the cleavage of the C–P bond results in the incorporation
of an external oxygen atom from an ambient source to generate
phosphate. In water, O from water have the greatest chance of
3
. Results and discussion
binding to released PO4 when the C–P bond breaks. So the
3.1 MPn degradation kinetics
18
releasing phosphate oxygen isotope values (d O
P
) have a posi-
). The slope of
was used to express the amount of O from water
degradation is uncommon. Phosphate (Pi) release in different incorporated into the released PO . If one oxygen atom (1 out of
O-labeled water samples based on pH during UVR photo- 4) from water is incorporated into the released PO , the slope of
oxidation of MPn is shown in Fig. 1a. Degradation kinetics of the linear correlation curve of d O vs. d O should be ca.
1
8
18
Microbial metabolism was reported to be an important pathway tive correlation with O-labeled water (d O
W
1
8
18
P W
for MPn degradation in the environment, while chemical d O vs. d O
36
4
1
8
4
1
8
18
P
W
25
MPn (Fig. 1a) followed pseudo-rst order kinetics with a rapid 0.25. The slopes obtained for MPn indicated a 13% (acidic)
initial rate followed by a gradual slower rate before approaching and 25% (alkaline) ambient water oxygen incorporation into the
41
maximum degradation. There was no signicant difference in released phosphate (Fig. 2). Yu found that only 7% of the
Fig. 1 (a) UVR degradation of MPn in different labeled water samples based on the released inorganic phosphate (Pi) concentration; (b)
photolysis kinetics of MPn irradiation by a UV lamp at pH 3 and pH 8.
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18
Table 1 Concentration of released Pi at different pH values after the UVR treatment and measured Pi–d
O
P
in different labeled water solutions
Released Pi
(mmoles)
Percent released
based on [Pi] (%)
18
18
Phosphonate
MPn
Water d
O
W
(&, VSMOW)
Pi-d
O
P
, & (VSMOW)
SD
n
ꢀ6 (pH 3)
73.75
76.03
75.59
79.91
79.66
88.65
91.28
91.54
90.63
95.30
96.27
98.83
5.123
7.093
10.076
3.521
6.459
13.179
0.302
0.097
0.149
0.113
0.126
0.068
3
3
4
4
4
4
1
3
0.8 (pH 3)
2.4 (pH 3)
ꢀ
6 (pH 8)
0.8 (pH 8)
2.4 (pH 8)
1
3
combines with the excited organic compounds to form peroxy
25
radicals, which are subsequently hydrolyzed. To qualitatively
investigate the role of ROS in the MPn photodegradation,
solutions containing 0.1 mM MPn were irradiated in the pres-
ence of 2-propanol and NaN
various radical species was calculated using eqn (1) and (2).
3
, respectively. The contribution of
44
While k represents the rate constant of the original reaction
without the addition of the radical quenchers, kisopropanol and
k
NaN3 represent the rate constants in the presence of the
scavengers.
R
cOH ¼ kcOH/k z (k ꢀ kisopropanol)/k
(1)
(2)
.
ꢀ
ꢁꢂ
1
1
R
¼ k
k z kisopropanol ꢀ kNaN3
k
O2
O2
1
8
18
Fig. 2 Plots of d
P W
O vs. d O of phosphates obtained from UVR
As shown in Fig. 3 and Table 2, the photolysis reaction
conforms to the rst order kinetics under acidic and alkaline
conditions. The photolysis rate constant without additives and
photo-oxidative hydrolysis of MPn at different pH values and the
comparison to the glyphosate data from Sandy' paper.
ꢀ2
ꢀ1
ꢀ2 ꢀ1
MPn alone was 0.98 ꢄ 10
h
at pH 3 and 1.77 ꢄ 10
h
at
ambient water oxygen was incorporated into the released PO
4
pH 8. All samples containing radical scavengers showed
for MPn under the 400 W and 1200 W mercury lamp UV photo-
oxidation aer 2162 h. This observation indicates that different
reaction conditions affect the contribution rate of each
substance involved in the reaction. Both Sandy and Jaisi
studied the mechanism of the glyphosate degradation under
a decrease in the photolysis rate. The value for kisopropanol and
ꢀ2 ꢀ1
ꢀ2 ꢀ1
k
NaN3 was 0.07 ꢄ 10
h
and 0.31 ꢄ 10
h
at pH 3, and 0.06
ꢀ2 ꢀ1
ꢀ2 ꢀ1
ꢄ 10
h
and 0.6 ꢄ 10
h
at pH 8, suggesting that the
25
26
reactive oxygen species play an important role in the reaction. It
was calculated that the contribution of the cOH radical is
1
8
18
P W
ultraviolet radiation; the slope of d O vs. d O was 0.175 at
92.86% at pH 3 and 96.61% at pH 8. Because kisopropanol < kNaN3,
pH 2.5 and 0.21 at pH 7.26, respectively. These results are
similar to those of the present experiments, and it was
concluded that more O from ambient water is incorporated into
we hypothesized that, on the one hand, the singlet oxygen
consumes sodium azide and alleviates the quenching effect of
sodium azide on the hydroxyl group, and on the other hand,
methylphosphonic acid is unreactive under these reaction
conditions. Also, the degradation of MPn is faster under alka-
line conditions than under acidic conditions. This may be
because hydroxyl radicals are more abundant under alkaline
conditions relative to acidic conditions. In summary, hydroxyl
groups play a major role in the C–P bond cleavage process. As
Jaisi observed, the hydroxyl ion and the cOH radical have similar
barriers opposing the C–P bond cleavage, indicating that
the product PO at higher pH. The lower the slope, the fewer O
4
atoms come from water, and we assume that the other source of
O is from O2. The dissolved oxygen is usually converted to the
radical O species and decomposes organic matter in the aqueous
solution. Thus, at low pH, there are more radical O species from
the exogenous oxygen present in the solution. Based on the acid–
base equilibrium relationship, there are few hydroxyl groups
present at pH 3, and the dissolved oxygen is one of the main
sources of hydroxyl groups under the photolysis conditions.
42
43
26
a strong competitive relationship affects their respective rates.
1
8
18
Therefore, the slope of d O vs. d O is lower due to the higher
amount of exogenous oxygen incorporated into the released
product PO at pH 3.
P
W
It is known that molecular oxygen may act as an electron
acceptor in the photosensitization process and produce super-
4
45
oxide anions or hydroxyl radicals. It was not known whether
the superoxide anions are involved in the MPn photolysis
reactions. On the other hand, there could be a reverse
phenomenon: kisopropanol < kNaN3. So, we used the DFT method to
3
.3 Participation of ROS
Ultraviolet radiation is known to stimulate the formation of free
radicals. Under the UVR conditions, atmospheric oxygen
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Fig. 3 Comparison of photolysis of MPn in the absence and the presence of 20 mM isopropanol and 20 mM NaN at pH 3 and pH 8.
calculate the reaction barrier and verify the results of the ROS- 3.4 Mechanism of C–P bond cleavage
quenching experiments.
Since the structure of MPn is relatively simple, we detected the
The reaction occurs according to the nucleophilic substitution
mechanism, and the nucleophile ROS can attack either the
methyl group or the phosphoryl group of the methylphosphonic
acid molecule. However, the calculated free energy barriers
1
13
change of groups in the reaction by H NMR and C NMR
spectroscopy, and then deduced the degradation products.
Sample solutions (0.5 mL aliquots) were used for the NMR
analysis at different times during the reaction and the results
1
ꢀ
involved O
2
and O
2
c radical triggered cleavage of the C–P bond
1
are shown in Fig. 5. The H NMR results showed a new single
in MPn are extremely high, which can be considered in the
ordinary conditions is difficult to happen as listed in Tables S1
and S2.† In comparison, the cOH radical is more likely to trigger
the cleavage of the C–P bond of methylphosphonic acid. As
shown in Fig. 4 and Table S3,† the C–P bond can be broken by the
cOH radical either from the phosphoryl side under acidic
conditions or the methyl side under alkaline conditions. The
former refers to an asynchronous process with a free energy
peak at 8.14 ppm on the fourth day of reaction, and two new
peaks at 81.76 ppm and 165.65 ppm were found in the C NMR
spectrum. As mentioned above, we speculate that the organic
degradation products of MPn are methanol and formic acid,
13
which are different from the microbial degradation pathway
10,36
product, methane.
The generation of ROS and degradation
46–48
of MPn are represented by formulas (3)–(9).
Under ultraviolet radiation, MPn absorbs energy and tran-
sitions into a single-line excited state. Then, the energy, trans-
ferred to the dissolved oxygen in solution, generates singlet
oxygen, or electrons are transferred to produce superoxide
anion radicals. The superoxide anion radicals combine with the
hydrogen ion to form hydrogen peroxide, and nally hydroxyl,
which then reacts with MPn, producing methanol and phos-
phoric acid. Methanol is further oxidized to formic acid and
ultimately generates stable substances.
ꢀ1
barrier of 16.8 kcal mol for the rst half and a barrier-free step
for the second half; the process is exothermic with a free energy
ꢀ
1
change of ꢀ18.9 kcal mol . The latter is a synchronous process
with a free energy barrier of 12.5 kcal mol; it is exothermic with
ꢀ
1
a free energy change of ꢀ18.4 kcal mol . The lower the potential
energy barrier, the more spontaneous will be the reaction. Our
calculations suggest that the alkaline conditions are more
favorable for the cleavage of the C–P bond in methylphosphonic
acid than the acidic conditions, which is consistent with the
results of the ROS-quenching experiments.
The proposed mechanism for the UVR photolysis of the C–P
bond in MPn is presented in Fig. 6. It can be proposed that
Table 2 The contribution of various radical species to the degradation of MPn and the rate constants of the degradation after adding various
scavengers
MPn
pH 3
pH 8
Rate constant
Rate constant
ꢀ
1
ꢀ1
Scavengers
Inhibition rate (%)
(h
)
Inhibition rate (%)
(h
)
ꢀ2
ꢀ2
ꢀ2
ꢀ2
ꢀ2
ꢀ2
Blank
Isopropanol
—
92.86
68.37
0.98 ꢄ 10
0.07 ꢄ 10
0.31 ꢄ 10
—
96.61
66.1
1.77 ꢄ 10
0.06 ꢄ 10
0.6 ꢄ 10
NaN
3
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Fig. 4 The free energy profile of the cOH radical triggered cleavage of the C–P bond in MPn. The pathway colored in black represents the
nucleophilic attack on the phosphoryl side of MPn under acidic conditions, and the pathway colored in gray represents the nucleophilic attack on
ꢀ1
the methyl side of MPn under alkaline conditions. The free energy is measured in kcal mol and the distance is measured in A˚ . The structures are
presented in a ball-and-stick model with carbon in gray, oxygen in red, phosphorus in orange and hydrogen in white.
a
series of reactions between phosphonate and other subsequently recombine or hydrolyze to form radicals that react
substances in the solution were induced by ultraviolet photons, with oxygen to produce peroxy radicals. Upon further photol-
and a nucleophilic substitution occurred with the generated ysis, the peroxy radicals are converted to hydrogen peroxide,
49
radical O species and phosphonate. The nucleophilic attack and nally decomposed to hydroxyl radicals that attack the
43,50
could be inuenced by the nature of the group adjacent to the methyl group.
The reaction between water and methyl
C–P bond in phosphonate. The hydrolysis mechanism and the phosphonate cationic states also produces hydroxyl radicals,
water incorporation pattern/slope for phosphate released from and we hypothesized that as the pH increases, the proportion of
the phosphonate studied here differ markedly from those re- O from water incorporated into the released PO increases.
4
ported for the phosphomonoesters hydrolyzed by UVR in
hv
MPn !MPn*
27
(3)
similar labeled water in the previous experiments. We posit
that C–P bond cleavage during UV photo-oxidation results from
the excitation of the phosphonate molecules into more reactive
species, which transfer electrons to acceptors present in the
reaction medium to form free radicals. The free radicals
1
MPn* + O / MPn + O
(4)
(5)
2
2
+
ꢀ
MPn* + O
2
/ MPnc + O
2
c
13
1
Fig. 5
C NMR spectra (A–C) and H NMR spectra (D–F) obtained at the start of the reaction (A and D), the first day (B and E) and the fourth day
(
C and F).
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experiment, the source and reaction amount of oxygen that
incorporated into the orthophosphate can be derived.
Conflicts of interest
The authors declare no conict of interest.
Acknowledgements
The labelled-water experiments and the stable oxygen isotope
analysis were carried out at Yale University and supported in
part by the grants, awarded to R. E. Blake from the US National
Science Foundation (NSF-EAR-0746241, NSF-OCE-0928247 and
NSF-OCE-1129499) and NASA (Award No. NNX13AJ36G). This
study was also supported in part by the grants from the National
Natural Science Foundation of China (41822706, 41473096,
Fig. 6 Proposed mechanism for MPn photo-degradation and ROS
formation.
41703109 and 21903089) and Beijing Natural Science Founda-
tion (8182034).
ꢀ
+
O
2
c
+ H / HO
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HO c + O c + H / H O + O
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