- Email: [email protected]
The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin
Accepted Manuscript The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin Geshan Zhang, Xuexiang He, Mallikarjuna N. Nadagouda, Kevin O'Shea, Dionysios D. Dionysiou PII:
S0043-1354(15)00027-5
DOI:
10.1016/j.watres.2015.01.011
Reference:
WR 11103
To appear in:
Water Research
Received Date: 10 October 2014 Revised Date:
5 January 2015
Accepted Date: 7 January 2015
Please cite this article as: Zhang, G., He, X., Nadagouda, M.N., O'Shea, K., Dionysiou, D.D., The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin, Water Research (2015), doi: 10.1016/j.watres.2015.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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The effect of basic pH and carbonate ion on the mechanism of
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photocatalytic destruction of cylindrospermopsin
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Geshan Zhanga, Xuexiang Hea, Mallikarjuna N. Nadagoudab, Kevin O'Sheac,
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Dionysios D. Dionysioua,*
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a
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OH 45221, United States.
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b
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United States.
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Environmental Engineering and Science Program, University of Cincinnati, Cincinnati,
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c
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33199, United States.
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US EPA, National Risk Management Research Laboratory, Cincinnati, OH 45268,
Department of Chemistry and Biochemistry, Florida International University, Miami, Fl
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*
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Tel: +1-513-556-0724; Fax: +1-513-556-4162
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Corresponding author Email: [email protected]
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Abstract
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This study investigated the mechanistic effects of basic pH and the presence of high
17
carbonate concentration on the TiO2 photocatalytic degradation of cyanobacterial toxin
18
cylindrospermopsin (CYN). A high-performance liquid chromatography combined with
19
quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LC/Q-TOF-
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ESI-MS) was employed for the identification of reaction byproducts. The reaction
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pathways were proposed based on the identified degradation byproducts and radical
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chemistry. In high pH system (pH = 10.5) similar reaction byproducts as those in neutral
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pH system were identified. However, high pH appeared to inhibit sulfate elimination with
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less sulfate elimination byproducts detected. In the presence of carbonate in the
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photocatalytic process, hydroxyl radical reaction would be largely inhibited since
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carbonate ion would react with hydroxyl radical to form carbonate radical. The second
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order rate constant of carbonate radical with CYN was estimated to be 1.4×108 M-1s-1,
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which is much smaller than that of hydroxyl radical. However, the more significant
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abundance of carbonate radical in the reaction solution strongly contributed to the
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transformation of CYN. Carbonate radical has higher reaction selectivity than hydroxyl
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radical and hence, played a different role in the photocatalytic reaction. It would promote
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the formation of byproduct m/z 420.12 which has not been identified in the other two
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systems. Besides, the presence of carbonate ion may hinder the removal of toxicity
34
originated from uracil moiety due to the low reaction activity of carbonate radical with
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uracil moiety in CYN molecule. This work would further support the application of
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photocatalytic technologies for CYN treatment and provide fundamental information for
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the complete assessment of CYN removal by using TiO2 photocatalysis process.
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Keywords: Basic pH; Byproduct; Carbonate radical; Cylindrospermopsin; Reaction
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pathway; Titanium dioxide photocatalytic degradation;
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List of Abbreviation
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4-CP
4-Cyanophenol
43
AOPs
Advanced oxidation processes
44
CYN
Cylindrospermopsin
45
CO3•-
Carbonate radicals
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ESI
Electrospray ionization
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HPLC
High performance liquid chromatography
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IEP
Isoelectric point
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kA/•B
Second order rate constant for the reaction of A with •B radical
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kobs(A)
Observed degradation rate constant of A
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m/z
Mass to charge ratio
52
MS
Mass spectrometry
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•
Hydroxyl radical
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P25_CO32-
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10.5)
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P25_HighpH
System with P25 TiO2 at high pH (pH = 10.5)
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P25_NA
System with P25 TiO2 at unadjusted pH (pH = 5.5)
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SI
Supporting information
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TOF
System with P25 TiO2 in the presence of Na2CO3 (1.5 mM, pH =
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Time-of-flight
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1. Introduction
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Cyanotoxins are a group of emerging contaminants that can be produced by certain
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cyanobacteria as secondary metabolites and released during their cell lysis (Carmichael
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1992, Wiegand and Pflugmacher 2005). One of the most problematic cyanotoxins,
65
cylindrospermopsin (CYN), has received great public attention due to its high toxicity,
66
prevalent distribution and widespread CYN-producing species (such as strains of
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Cylindrospermopsis
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Rhaphidiopsis curvata and Umezakia natans) in tropical, subtropical and temperate
69
areas (Fergusson and Saint 2003, Metcalf et al. 2004, Ohtani et al. 1992). The
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biosynthetic pathways of CYN in Cylindrospermopsis raciborskii have been proposed
71
based on the identified gene cluster for the biosynthesis of CYN (Mihali et al. 2008).
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CYN is a tricyclic guanidine alkaloid containing a hydroxymethyl uracil moiety with high
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water-solubility. Generally, this toxin is an inhibitor for the synthesis of protein and
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glutathione (Froscio et al. 2003, Lopez-Alonso et al. 2013, Young et al. 2008). It has a
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negative effect on many organs and has been reported as a potent hepatotoxin,
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cytotoxin and genetoxin (Bazin et al. 2010, Mazmouz et al. 2010, Ohtani et al. 1992).
77
Moreover, CYN has been proven to have carcinogenic potential at a concentration as
78
low as 1×10-7 µg/L (Maire et al. 2010). The cattle deaths and human poisonings in Palm
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Island of Australia in 1979 have been attributed to the CYN contamination in drinking
80
water (Griffiths and Saker 2003, Wiegand and Pflugmacher 2005). Besides, the natural
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photodegradation of CYN is very limited and highly dependent on UV-A radiation
82
(Wormer et al. 2010). Therefore, the effective treatment for this toxin is of extreme
83
significance for public health and environmental safety.
Aphanizomenon
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bergii,
ovalisporum,
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TiO2 photocatalysis has been applied in many areas including water treatment for
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emerging contaminants (Antoniou et al. 2009b, Miranda-Garcia et al. 2011, Pelaez et al.
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2009). It has been proven to be an effective and efficient approach for CYN removal
87
(Senogles et al. 2001, Zhang et al. 2014). Hydroxyl radical (•OH) is considered to be the
88
main reactive agent in the TiO2 photocatalysis process (Antoniou et al. 2009a, Kim and
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Choi 2002, Tachikawa et al. 2007). Our previous work has shown that hydroxylation and
90
ring opening on the hydroxymethyl uracil moiety and tricyclic guanidine moiety, along
91
with sulfate elimination, are the main reaction pathways for the TiO2 photocatalytic
92
degradation of CYN in water (Zhang et al. 2015). Since hydroxymethyl uracil moiety is
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believed to be responsible for the toxicity of CYN (Banker et al. 2001, Norris et al. 1999),
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the toxicity of water sample after TiO2 photocatalytic treatment can be reduced (Zhang
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et al. 2015).
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In the molecule of CYN, there is a negatively charged sulfate group and a positively
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charged guanidine group. The pKa of CYN was estimated to be around 8.8 (Onstad et al.
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2007). Therefore, the CYN molecule has both negatively and positively charged group
99
at pH < 8.8, while the CYN molecule is negatively charged at high pH condition (e.g., pH
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= 10.5). The isoelectric point (IEP) of TiO2 catalysts is around 7 (Gumy et al. 2006,
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Zhang et al. 2014), which means TiO2 would be positively charged at acidic pH and
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negatively charged at basic pH. In our previous study (Zhang et al. 2015), we have
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identified the reaction byproducts and pathways of photocatalytic degradation of CYN at
104
pH around 5.5 (without pH adjustment). Hence, it would be very scientifically meaningful
105
to explore the differences in the mechanism at a high pH. Besides, carbonates are
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widely present in various waters, and would therefore significantly affect the chemistry
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of the photocatalytic process (Dimitrijevic et al. 2011). When carbonate ions are in the
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system, they would result in not only higher pH but also other possible effects. Many
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studies have shown that •OH radical generated by photocatalytic activity can react with
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carbonate and bicarbonate ions to form carbonate radicals (CO3•-) with second-order
111
reaction rate constants of 3.9×108 and 8.5×106 M-1s-1, respectively (He et al. 2012,
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Pelaez et al. 2011). Moreover, the concentration of CO3•- in surface water under solar
113
light has been estimated to be two orders of magnitude higher than that of •OH radical
114
(Sulzberger et al. 1997). Thus, it is interesting to examine the effect of carbonate (or the
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generated carbonate radicals) on the photocatalytic degradation byproducts and
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pathways of CYN.
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In this study, we report for the first time the effects of initial high pH (pH = 10.5) and
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the presence of carbonate ion on the TiO2 photocatalytic degradation of CYN from the
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mechanism aspects, including the reaction byproducts and pathways. The reaction
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byproducts were detected and identified by using a high-performance liquid
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chromatography combined with quadrupole time-of-flight electrospray ionization tandem
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mass spectrometry (LC/Q-TOF-ESI-MS). The degradation pathways were proposed
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based on the identified byproducts and radical chemistry. This work can provide not
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only better understanding about the mechanism of CYN photocatalytic destruction but
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also further insights on the assessment of this technology for the treatment of CYN
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contaminated water.
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2. Materials and Methods
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2.1. Materials
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CYN (>95%) was purchased from GreenWater Laboratories. Aeroxide TiO2 P25 was
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obtained from Degussa AG. In the photocatalytic experiments, the water (HPLC grade)
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used was purchased from Tedia. Sodium carbonate (Na2CO3, 100%, Fisher) was used
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as the source of carbonate ions. For analysis, HPLC grade acetonitrile, deionized ultra-
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filtered water and glacial acetic acid (99.9%) were supplied by Fisher Scientific. Atrazine
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was purchased from Fluka (analytical standard) while 4-cyanophenol was purchased
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from Aldrich (95%). All reagents were used without further purification.
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2.2. TiO2 Photocatalytic experiments
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In order to obtain the detectable byproducts, a high initial concentration of 10 µM CYN
138
was applied in the photocatalytic experiments. P25 TiO2 nanoparticles were uniformly
139
dispersed in water by using ultrasonication (2510R-DH, Bransonic) for 15 min. The CYN
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stock solution and P25 TiO2 catalyst suspension (catalyst dose applied = 0.25 g/L) were
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spiked into a borosilicate glass Petri dish (Pyrex, 60 mm (ø) x15 mm (h)) reactor
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covered with a quartz cover. The reaction solution (10 mL) had an initial pH of 10.5
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adjusted by NaOH (Fisher). In the experiment with Na2CO3, the concentration of
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Na2CO3 was 1.5 mM with a pH of 10.5. The reactor was irradiated under two 15 W
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fluorescent lamps (Han et al. 2011, Zhang et al. 2014) (Cole-Parmer). The light intensity
146
was about 2.3 mW/cm2 measured with a radiant power meter (Newport Corp.).
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Continuous stirring and temperature control was applied during the reaction. 150 µL
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samples were taken after certain time intervals, diluted with 150 µL water and filtered
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with syringeless filters (0.2 µm, PTFE, Whatman).
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2.3. Analysis
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An Agilent 1290 infinity HPLC system (Agilent Technology) with a Zorbax Eclipse XDB-
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C18 Rapid Resolution column (2.1 × 50 mm, 3.5 µm, Agilent Technology) was
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employed for the separation of CYN and reaction byproducts. The mobile phases were
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H2O + 2% acetonitrile + 0.2% acetic acid (A) and acetonitrile + 2% H2O + 0.2% acetic
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acid (B). The programmed gradient elution was set as: 2% B for 1 min; linearly
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increasing to 95% B in the next 4 min; decreasing back to 2% B in the following 0.1 min;
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and holding 2% B for another 2 min for re-equilibrium. The follow rate was 0.2 mL/min;
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sample injection volume was 10 µL; and column temperature was 35 ºC. The HPLC
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was coupled with an Agilent 6540 UHD Accurate-Mass quadrupole time-of-flight tandem
160
mass spectrometer (Q-TOF-MS) with an electrospray ionization source (ESI). The
161
parameters set for ESI-MS and automatic MS/MS analysis were described in detail in
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our previous publication (Zhang et al. 2015). The quantification of CYN was confirmed
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through the HPLC analysis mentioned in our previous publication (Zhang et al. 2014).
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In order to elucidate the role of carbonate radical in the system with Na2CO3, atrazine
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and 4-cyanophenol (4-CP) were used for comparison with CYN. For atrazine and 4-CP
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quantification, an HPLC system (Agilent 1100 Series) with a photodiode-array detector
167
was used for their quantification. A C18 Discovery HS (Supelco) column (150 mm × 2.1
168
mm, 5 µm) was employed as a stationary phase. The mobile phase was acetonitrile and
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Milli-Q water with the ratio of 60:40 for atrazine and 50:50 for 4-CP. The flow rate was
170
0.4 mL/min for atrazine and 1 mL/min for 4-CP. The analysis was performed with a
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column temperature of 25 °C at the wavelength of 22 2 nm for atrazine and 246 nm for
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4-CP (Khan et al. 2013, Medendorp et al. 2006).
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3. Results and discussion
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3.1. Photocatalytic reaction byproducts and reaction pathways of CYN under
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different conditions
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CYN can be removed by photocatalytic degradation process (Fig. 1) at pH without
177
adjustment (pH = 5.5, P25_NA for short), high pH (pH = 10.5, P25_HighpH for short) or
178
in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32- for short). The pseudo first-
179
order rate constants were calculated as 0.168, 0.188 and 0.167 min-1 (± 0.005 min-1, R2
180
≥ 0.99), respectively. The major reaction byproducts generated in these three conditions
181
are summarized in Table 1. The structures of these byproducts were proposed
182
according to their possible formulae, retention times, mass spectra and radical
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chemistry (Antoniou et al. 2008a, b, Zhang et al. 2015). Agilent MassHunter workstation
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software (Qualitative Analysis B.04.00) was applied in the study, which can provide
185
users a list of possible formulae according to the mass to charge (m/z) ratio and isotope
186
distribution of targeting compound (for best matching formulae, Relative Mass
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Differences ⩽ 5 ppm). The MS spectra were detected under MS/MS mode. Moreover,
188
byproducts with higher polarity would elute earlier while those with lower polarity would
189
elute later under current analytical conditions. With respect to mass spectra, a
190
difference of 80 Da, 18 Da and 142 Da between two fragment ions indicates the
191
existence of sulfate group, hydroxyl group, and the whole hydroxymethyl uracil moiety,
192
respectively. The presence of fragment m/z 194.13 suggests the integrity of the tricyclic
193
guanidine moiety while the fragment m/z 192.11 implies the existence of a hydroxyl
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group (with the fragment of m/z 210.12) or another double bond on the tricyclic
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guanidine moiety (without m/z 210.12 fragment)
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(Silverstein 2005). More detailed
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description about the explanation of mass spectra was reported in our previous work
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(Zhang et al. 2015).
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3.1.1. Reaction byproducts and reaction pathways at high pH
199
TiO2 photocatalyst can generate hydroxyl radical, the main reactive agent (Antoniou et
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al. 2008b, Sun and Xu 2010, Xie and Lin 2007), in a wide pH range: preferring reaction
201
with HO- at high pH but preferring reaction with H2O in acidic pH ranges (Turchi and
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Ollis 1990). Therefore, the hydroxylation mechanism in P25_HighpH was expected to
203
be similar to that in P25_NA, which includes hydrogen abstraction and addition to the
204
C=C double bond (Antoniou et al. 2008b, Garrison 1987, Zhang et al. 2015). Based on
205
observation, most byproducts generated in the condition of P25_HighpH can be found
206
in system of P25_NA (Table 1). For example, the hydroxylation byproducts of CYN m/z
207
432.12 and 448.11 were observed in both systems. In our previous study, the
208
degradation byproduct m/z 448.11 had only one observed isomer (i.e., m/z 448.11(a) as
209
shown here) when no pH adjustment was applied (Zhang et al. 2015). In this study, two
210
more isomers of m/z 448.11 were observed in P25_HighpH (Fig. S1, Supporting
211
Information (SI)). The dehydration byproducts (e.g., m/z 414.11) were also observed.
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The sulfate elimination byproduct (i.e., m/z 334.15) and its main hydroxylation product
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m/z 350.16 cannot be found in P25_HighpH but only existed in the P25_NA system.
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From our observation, only very few sulfate elimination byproducts were detected in
215
P25_HighpH system, which can be attributed to the surface charge properties of the
216
catalyst. At high pH levels, the surface of TiO2 was negatively charged. Therefore, the
217
negative sulfate group would be more difficult to approach the TiO2 surface. Since the
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surface generated •OH would not diffuse far away from the TiO2 surface, it cannot easily
219
reach C12 to have sulfate elimination (Turchi and Ollis 1990, Zhang et al. 2015). The byproducts from the ring opening reactions on the hydroxymethyl uracil moiety,
221
such as m/z 392.12 and 375.10, were also detected under basic condition. Moreover,
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the geminal diol m/z 322.11 and its hydroxylation byproducts m/z 338.10 and 354.10 all
223
existed in both systems of P25_HighpH and P25_NA, while their sulfate elimination
224
products only existed in the condition of P25_NA, which further confirms the conclusion
225
about the unfavorable sulfate group elimination at high pH (e.g., the studied pH = 10.5).
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Other byproducts generated from the reaction on the tricyclic guanidine moiety,
227
including m/z 292.10 and 290.08 as well as their hydroxylation byproducts, were all
228
observed in the two systems. Moreover, one new byproduct m/z 338.14 was identified
229
in P25_HighpH system, which can be attributed to the reaction on tricyclic guanidine
230
moiety (Scheme 1). When both C14 and C15 have a hydroxyl group, the further
231
oxidation of this vicinal diol would result in the ring opening and form a carbonyl group
232
at C14 and a carboxyl group at C15 which can be easily released as CO2. (Antoniou et
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al. 2008b, Jiang et al. 2012). Similar to some phosphate esters, direct hydrolysis of the
234
sulfate ester can take place at high pH and generate the corresponding alcohol to form
235
byproduct m/z 338.14 (Samuni and Neta 1973). The mass spectrum of m/z 338.14 is
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shown in Fig. S2 (SI).
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3.1.2. Reaction byproducts and reaction pathways in the presence of Na2CO3
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Many studies have shown carbonate and bicarbonate ions play the role of •OH
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scavenger in the hydroxyl radical based advanced oxidation processes (AOPs)(Grebel
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et al. 2010, Haarstrick et al. 1996, Wong and Chu 2003). Moreover, the scavenging
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reactions would result in the generation of carbonate radicals (CO3•-) as shown in
242
Equation (1) and (2) (He et al. 2012, Pelaez et al. 2011). The carbonate radical has
243
attracted great scientific attention since it can be formed in the reaction of peroxynitrite
244
with CO2 in cellular environments (Squadrito and Pryor 2002). Furthermore, the
245
concentration of CO3•- in surface waters under solar irradiation has been reported to be
246
much higher than that of •OH (Canonica et al. 2005). The protonated form of this radical
247
is HCO3•, which has a pKa smaller than 0 (Augusto et al. 2002, Ferrer-Sueta et al. 2003).
248
•
OH + CO32- → OH- + CO3•-
k = 3.9×108 M-1s-1
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•
OH + HCO3- → H2O + CO3•-
k = 8.5×106 M-1s-1
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HCO3• ↔ H+ + CO3•-
pKa < 0
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(1)
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In current high pH system of P25_CO32-, the concentration of carbonate ion is around
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1 mM according to the calculation from the hydrolysis equilibrium of carbonate salt (pka1
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= 6.4, pka2 = 10.3). Since bicarbonate ion has a lower concentration (~0.5 mM) and
254
lower reaction rate with •OH radical, its impact would be negligible in the current system.
255
Hydroxyl radical can efficiently react with organic compounds with very high second
256
order rate constants from around 109 to 1010 M-1s-1 (Antoniou et al. 2009a, Halliwell et al.
257
1987). The second order rate constant for the reaction of CYN with •OH radical has
258
been reported to be around 5×109 M-1s-1 (kCYN/•OH) (He et al. 2013, Onstad et al. 2007,
259
Song et al. 2012), which is one order higher than that of carbonate ion. However, the
260
concentration of carbonate ion is about 100 times higher than the concentration of CYN.
261
This means, in the P25_CO32- system, the carbonate ion would be more competitive to
262
react with •OH radical than CYN, generating carbonate radical. Therefore, we propose
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the •OH would mainly react with carbonate ion to generate carbonate radical and CO3•-
264
would be the predominant radical species for CYN degradation in P25_CO32- system. In order to confirm the hypothesis, a similar photocatalytic experiment was operated,
266
in which the CYN was replaced by atrazine with the same initial concentration (10 µM).
267
Carbonate radical has been reported to have oxidation capability for organic
268
compounds but its second order rate constants with organic compounds are in a much
269
wider range (from 102 to 109 M-1s-1) comparing to hydroxyl radical (Canonica et al. 2005,
270
Neta et al. 1988). Atrazine has a high second order rate constant with •OH radical
271
(kATZ/•OH, around 2.5×109 M-1s-1) but a relatively low rate constant with carbonate radical
272
(kATZ/CO3•-, around 4×106 M-1s-1) (Canonica et al. 2005, Meunier et al. 2006). Hence, if
273
•
274
degradation rate could be largely reduced; otherwise, the degradation rate would be
275
similar to the reaction rate in P25_HighpH system. The kinetic comparison of the two
276
systems with atrazine is shown in Fig. 2(a). The pseudo-first order rate constants were
277
calculated as 0.148 and 0.021 min-1 for P25_HighpH and P25_CO32- system,
278
respectively. The observed degradation rate constant of atrazine in P25_CO32- system
279
decreased seven times compared with that in P25_HighpH system. Therefore, we can
280
conclude •OH would mainly react with carbonate ion to form CO3•- in P25_CO32- system.
281
Through a competition study of atrazine and 4-cyanophenol (4-CP) in P25_HighpH
282
system (He et al. 2013), the second order rate constant for the reaction of 4-CP with
283
•
284
constant for the reaction of 4-CP with CO3•- (k4-cp/CO3•-) has been reported as 4×107 M-1s-
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OH would mainly react with carbonate ion to form CO3•- in P25_CO32- system, the
OH (k4-cp/•OH) can be determined as 2.4×109 M-1s-1 (Fig. 2(b)). The second order rate
(Canonica et al. 2005). Assuming the concentration of •OH and CO3•- in P25_CO32-
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system would remain the same with the same initial molar concentration of the target
287
compound, we can obtain the concentration of •OH and CO3•- in the system through the
288
following equations:
289
kobs(4-CP) = k4-cp/•OH[•OH] + k4-cp/CO3•-[ CO3•-]
(4)
290
kobs(ATZ) = kATZ/•OH[•OH] + kATZ/CO3•-[ CO3•-]
(5)
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Therefore, the concentration of hydroxyl radical and carbonate radical in P25_CO32-
292
were estimated as 1.15×10-13 M and 1.56×10-11 M, respectively. The concentration of
293
carbonate radical in P25_CO32- system was more than 100 times higher than that of
294
hydroxyl radical, which could be attributed to the longer lifetime of CO3•- compared with
295
•
296
decay rate (Neta et al. 1988). Considering the similar degradation rates of CYN in the
297
presence and absence of carbonate ion (Fig. 1), the contribution of carbonate radical
298
was expected to be significant. The second order rate constant for the reaction of CYN
299
with CO3•- can thus be estimated from equation (6) to be1.4×108 M-1s-1 using the
300
methodology used in this work. Hence, in the P25_CO32- system, about 80% of the CYN
301
degradation was contributed by carbonate radical reaction and the rest was due to
302
hydroxyl radical attack.
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kobs(CYN) = kCYN/•OH[•OH] + kCYN/CO3•-[ CO3•-]
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OH due to the relatively lower reaction rate of CO3•- with other species and lower self-
(6)
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The CO3•- has very high selectivity and could selectively attack the electron rich
305
groups, especially for N-containing organics and phenols (Canonica et al. 2005, Joffe et
306
al. 2003, Mazellier et al. 2002). This electrophilic property of CO3•- also supports the
307
relatively higher reaction rate of CO3•- radical with CYN (see the molecule structure of
308
CYN in Fig. 1). Research studies from the groups of prof. Geacintov, N.E. and prof.
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Shafirovich, V have investigated the mechanisms of guanine oxidation by carbonate
310
radical and found a unique end product, the diastereomeric spiroiminodihydantoin lesion,
311
which can be formed through a four electron oxidation of guanine (Crean et al. 2005,
312
Joffe et al. 2003). The carbonate radical was believed to mainly act as the electron
313
acceptor via electron transfer mechanisms, in which it would receive one electron from
314
the organic electron donor, such as amino groups (see Scheme 2a) (Elango et al. 1985,
315
Neta et al. 1988, Shafirovich et al. 2001). Besides, similar to hydroxyl radical, carbonate
316
radical can undergo hydrogen abstraction to generate the corresponding alcohol (see
317
Scheme 2b) (Crean et al. 2005, Mazellier et al. 2002, Medinas et al. 2007, Neta et al.
318
1988). In the hydrogen abstraction mechanism of carbonate radical, the attacked group
319
also needs to connect with a strong electron donor although the reaction rates would
320
generally be low (Elango et al. 1985, Neta et al. 1988). Therefore, in the presence of
321
carbonate radical, CYN can also have similar hydroxylation process on the tricyclic
322
guanidine moiety but with a different reaction mechanism (e.g., formation of m/z 432.12).
323
The byproducts of CYN detected were much less in P25_CO32- system (Table 1). The
324
reaction mechanism on the uracil moiety of CYN would be different in the presence of
325
carbonate radical (Crean et al. 2005, Elango et al. 1985). It has been reported that the
326
carbonate radical has very low reaction rate with uracil (<104 M-1s-1) (Chen and Hoffman
327
1973), while hydroxyl radical was reported to have a very high reaction rate with uracil
328
(5.7×109 M-1s-1) (Buxton et al. 1988). Therefore, the reaction on the uracil moiety of
329
CYN should more easily start with the reaction with hydroxyl radical (Scheme 3).
330
Hydroxyl radical can first have hydroxyl addition to C5=C6 double bond to form a carbon
331
centered radical at C6, which can further react with oxygen and release one HOO•
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radical to form byproduct m/z 432.12. This byproduct then can have a ring opening
333
reaction, similar to the reaction of byproduct m/z 392.12 formation in P25_HighpH
334
system (Jiang et al. 2012, Munk et al. 2008). The m/z 432.12 can generate byproducts
335
m/z 422.13 by losing C4, and would easily react with carbonate radical to produce m/z
336
420.12 as shown in Scheme 3. The mass spectrum of byproduct m/z 420.12 is shown in
337
Fig. S3 (SI) (the difference of 45 Da indicates the existing of carbonyl group). The
338
byproduct m/z 420.12 identified in P25_CO32- was not observed in the other two
339
systems of this study. Moreover, the uracil ring opening byproduct m/z 392.12, found in
340
other systems, was not detected in this system, which can further support the different
341
reaction mechanism with the presence of CO3•-. The m/z 420.12 can further react with
342
carbonate radical and release C5 to form a C6-centered radical, which eventually can
343
have ring reforming byproduct m/z 375.10 (Scheme 2a) (Crean et al. 2005, Elango et al.
344
1985, Zhang et al. 2015). The geminal diol byproduct m/z 322.11 can be generated
345
through bond splitting of C6-C7, further oxidation of m/z 420.12 or ring opening process
346
of m/z 375.10 (Jiang et al. 2012). After further oxidation, C7 can be released to form
347
byproduct m/z 292.10 or 290.08. Besides, it has been reported that uracil moiety is
348
mainly responsible for the toxicity of CYN (Banker et al. 2001). The presence of
349
carbonate ions may lower the toxicity reduction due to the low reaction rate of carbonate
350
radical with uracil moiety. Therefore, high carbonate concentration should be avoided
351
during the photocatalytic treatment for CYN considering the mechanism of degradation.
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352
With respect to sulfate elimination, similar as P25_HighpH system, only a few sulfate
353
group elimination byproducts were identified in P25_CO32- due to the high pH of the
354
system. The same byproduct m/z 338.14 identified in P25_HighpH was also observed in
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P25_CO32-. This byproduct can be formed through the hydroxylation by carbonate
356
radical at C14 and C15 and a ring opening process (similar as Scheme 1 mentioned in
357
3.1.1). Besides, the reaction byproduct m/z 212.10 was also identified in this system,
358
which can be obtained by cleavage of the hydroxymethyl uracil moiety of m/z 338.14.
359
4. Conclusions
360
The mechanistic effects of high pH and the presence of carbonate ion on the TiO2
361
photocatalytic degradation of CYN were investigated in this study. The reaction
362
pathways were proposed based on radical chemistry and the reaction byproducts were
363
identified using LC/Q-TOF-ESI-MS. While high pH or high carbonate concentration
364
would not largely affect the photocatalytic degradation kinetically under current
365
experimental conditions, high pH would inhibit sulfate elimination process, and
366
carbonate ions would scavenge the hydroxyl radical produced during the photocatalytic
367
reaction and generate carbonate radical which has higher reaction selectivity. Since
368
carbonate radical has very low reactivity with uracil moiety in CYN molecule, the
369
presence of high carbonate concentration may hinder the reduction of toxicity originated
370
from uracil moiety. Besides, the second order rate constant of carbonate radical with
371
CYN was estimated to be 1.4×108 M-1s-1. This work would further provide fundamental
372
information for the complete assessment of CYN removal by using photocatalytic
373
technologies.
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Acknowledgements
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This study is funded by U.S. Department of Agriculture (58-3148-1-152). We are also
377
thankful to China Scholarship Council (CSC) Scholarships (2009617129) for the
378
financial support to G. Zhang through a Ph.D. scholarship.
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Disclaimer
381
The U.S. Environmental Protection Agency, through its Office of Research and
382
Development, funded and managed, or partially funded and collaborated in, the
383
research described herein. It has been subjected to the Agency’s peer and
384
administrative review and has been approved for external publication. Any opinions
385
expressed are those of the author(s) and do not necessarily reflect the views of the
386
Agency, therefore, no official endorsement should be inferred. Any mention of trade
387
names or commercial products does not constitute endorsement or recommendation for
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use.
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This material is available via the Internet at.
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Table 1- The main reaction byproducts detected using P25 TiO2 photocatalyst at unadjusted pH (pH = 5.5, P25_NA), high pH (pH = 10.5, P25_HighpH) or in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32-). m/z
Formula
P25_NAa
P25_HighpH
P25_CO32-
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464.11 C15H21N5O10S √ 448.11 C15H21N5O9S √ √ 432.12 C15H21N5O8S √ √ √ √ 420.12 C14H21N5O8S 414.11 C15H19N5O7S √ √ √ √ 392.12 C13H21N5O7S 375.10 C13H18N4O7S √ √ √ 370.09 C11H19N3O9S √ √ √ 354.10 C11H19N3O8S 350.14 C15H19N5O5 √ 338.14 C14H19N5O5 √ √ √ √ 338.10 C11H19N3O7S 336.09 C11H17N3O7S √ 334.15 C15H19N5O4 √ 322.11 C11H19N3O6S √ √ √ √ 322.07 C10H15N3O7S 320.09 C11H17N3O6S √ √ √ 308.09 C10H17N3O6S √ √ √ 306.08 C10H15N3O6S √ √ √ √ √ 292.10 C10H17N3O5S √ 292.06 C9H13N3O6S 290.08 C10H15N3O5S √ √ √ 272.12 C11H17N3O5 √ 256.13 C11H17N3O4 √ 256.09 C10H13N3O5 √ 240.13 C11H17N3O3 √ √ 212.10 C9H13N3O3 a Most of the results have been reported in our previous work (Zhang et al. 2015).
28
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100
O O S O H 9
12 13 20
H
7
10
8 18 NH
N 14 15
19
5 6
HN 1
17
NH
Uracil Moiety
C/C0
60
3 2
O
16
NH
O 4
SC
40
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H 11
80
P25_NA P25_HighpH P25_CO32-
HO
O
0 0
2
4
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20
6
8
10
12
14
Time (min)
Fig. 1 - The degradation of CYN using P25 TiO2 at unadjusted pH (pH = 5.5,
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P25_NA), high pH (pH = 10.5, P25_HighpH) or in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32-) for reaction byproducts identification. The inset is
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the structure of CYN.
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O
H
H
OH O
NH HN
N
O O S O O
NH O
H
NH
N O
C15 H21 N5 O9S m/ z = 448.11a
O O S O
OH
O
O HN
NH O
NH
NH
NH
O
C14H19 N5O5 m/ z = 338.14
OH
H
H
N
NH HN
SC
H
NH HN O
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H
N
O
HO
C15H21N5O7 S m/ z = 416.12
HO
H
HO
NH
OH
H
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O O S O
H2 N
O NH O
C14H19N5O8 S m/ z = 418.10
Scheme. 1 – Proposed pathway for the formation of byproduct m/z 338.14. (The byproduct with “[ ]” indicates it was not detected and was proposed based
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on current information. a The byproduct shown here is only one of the possible
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forms of m/z 448.11)
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a
1.0
0.6
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C/C0
0.8
0.4
ATZ in P25_HighpH ATZ in P25_CO32-
0.2
4-CP in P25_CO320
5
SC
0.0 10
15
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Time (min)
20
1.0
b
4-CP ATZ
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C/C 0
0.8
0.6
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0.4
0.2
AC C
0
2
4
6
8
10
12
14
Time (min)
Fig. 2 – (a) The degradation of atrazine or 4-CP by using P25 TiO2 in P25_HighpH or P25_CO32- systems and (b) The competition study of atrazine and 4-CP in P25_HighpH system.
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O R1 O
CH
R2
CO 32-
NR3 R4
R2
R1
C
(b)
O
CH
R1
C O
O
O
C R3
R2
H+
C R2
NR3 R4
C O
O NR3 R4
R1 H 2O C HCO 3-
R2
OH NR3 R4
O
O
R1
O
C NR3 R4
O
R1
C O
O
CH
R2
R1
HCO 3-
C
R2
R3
R2
O
C O
O R3
R1 H 2O C HCO 3-
R2
OH R3
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(a)
R1
C
Scheme. 2 - The mechanism of hydroxylation in the presence of carbonate
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radical.
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O O S O
OH OH
OH
H
H
N
NH HN
H
O
H
H
N
N
OH O
O
H
O
NH
O
NH
+
HCO 3-
O O S O
O
H
C O H
NH HN
N H
O
NH
NH
H
H
OH
OH
NH HN
N H
C15 H21 N5O7S m/ z = 416.12
NH O
NH
NH
C11 H19 N3 O6S m/z = 322.11
O
OH
O
NH
N H
O
OH
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H
O
O
O2
H
N H
O
NH HN
NH O
NH
O O S O O
OH
H
NH
O HCO3- + CO 2
C O
OH
O
H
O O S O
OH
H O NH HN
N H
isomer
O
NH
O H
H
N
NH HN
OH O
NH2 O H
C14H21N5O8 S m/ z = 420.12
NH
NH2 O
HOO
O O S O
O
OH O
HN
NH
NH2
O
O2
H
N
H
AC C
C15H21N5O8 S m/ z = 432.12
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H
OH
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O
OH
OH
NH HN
O O S O
HOO O O S O
H
NH
O
O O S O
OH
H
OH H
O
H N
C13 H18O7 N4S m/ z = 375.10
H 2O
O O S O
O O S O O
NH
NH
SC
O
RI PT
O O S O
NH 2
C O
O
H
N
NH
OH O
O
C14 H23 N5O8S m/ z = 422.13
OH H
O
HCO 3-
H
NH
HN
NH 2 O
Scheme. 3 – Proposed reaction on the hydroxymethyl uracil moiety in P25_CO32- system. (The intermediates with “[ ]” indicate they were not detected and were proposed based on current information.)
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Highlights:
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Basic pH and carbonate ion mechanistically impact on CYN photocatalytic destruction Basic pH can minimize sulfate elimination of TiO2 photocatalytic degradation of CYN Carbonate ion can scavenge produced hydroxyl radical to form carbonate radical 8 -1 -1 • The second order rate constant of CO3 with CYN was estimated to be 1.4×10 M s
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Supporting Information
1 2
The effect of basic pH and carbonate ion on the photocatalytic
4
destruction of cylindrospermopsin: the degradation byproducts and
5
pathways
6
Geshan Zhanga, Xuexiang Hea, Mallikarjuna N. Nadagoudab, Kevin O'Sheac,
7
Dionysios D. Dionysioua,*
SC
a
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati,
10
OH 45221, United States.
11
b
12
United States.
13
c
14
33199, United States.
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US EPA, National Risk Management Research Laboratory, Cincinnati, OH 45268,
Department of Chemistry and Biochemistry, Florida International University, Miami, Fl
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15
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8 9
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3
16
*
17
Tel: +1-513-556-0724; Fax: +1-513-556-4162
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Corresponding author Email: [email protected]
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Supporting Figure and Scheme Captions
19
Fig. S1 - MS spectra and proposed structures of reaction byproducts m/z 448.11(b) and
20
m/z 448.11(c). The red color indicates the group does not have a fixed position.
21
Fig. S2 - MS spectrum and proposed structure of reaction byproduct m/z 338.14.
22
Fig. S3 - MS spectrum and proposed structure of reaction byproduct m/z 420.12.
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23
SC
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350.14
100 O OH
O S O
OH
O
H
H
N
NH
368.16
O O
80
HN
NH
H
448.11
O
*
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C15H21N5O9S m/ z = 448.11 (b)
60
208.11
332.13
40
SC
Relative Abundance
NH
194.13
0 100
150
200
M AN U
20
250
300
350
400
450
m/z
26
368.16
100 HO
O
H
H
N
NH
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60
40
20
0 100
OH
HN
O
NH
O
H
HO
AC C
Relative Abundance
80
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O
O S O
NH
C15H21N 5O 9S m/ z =448.11 (c)
448.11
*
208.11
350.14
190.09
150
200
250
300
350
400
450
m/z
27 28
Fig. S1
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HO
OH
H
H
N
NH HN
60 NH
O
SC
O
O
NH
C 14 H 19N5 O5 m/z = 338.14
40
178.10
20
M AN U
Relative Abundance
80
RI PT
196.11
100
338.14
*
0 100 120 140 160 180 200 220 240 260 280 300 320 340
31
Fig. S2
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m/z
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295.14
100
O O S O
OH
H
RI PT
O
80
NH HN
N H
194.13
NH2
O
NH
C14H21N5O8 S m/ z = 420.12
60
420.12
SC
*
375.10
40
20
0 100
150
200
M AN U
Relative Abundance
OH H
O
250
300
350
400
Fig. S3
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m/z
5
S0043-1354(15)00027-5
DOI:
10.1016/j.watres.2015.01.011
Reference:
WR 11103
To appear in:
Water Research
Received Date: 10 October 2014 Revised Date:
5 January 2015
Accepted Date: 7 January 2015
Please cite this article as: Zhang, G., He, X., Nadagouda, M.N., O'Shea, K., Dionysiou, D.D., The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin, Water Research (2015), doi: 10.1016/j.watres.2015.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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The effect of basic pH and carbonate ion on the mechanism of
2
photocatalytic destruction of cylindrospermopsin
3
Geshan Zhanga, Xuexiang Hea, Mallikarjuna N. Nadagoudab, Kevin O'Sheac,
4
Dionysios D. Dionysioua,*
5 6
a
7
OH 45221, United States.
8
b
9
United States.
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1
SC
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati,
10
c
11
33199, United States.
M AN U
US EPA, National Risk Management Research Laboratory, Cincinnati, OH 45268,
Department of Chemistry and Biochemistry, Florida International University, Miami, Fl
12 13
*
14
Tel: +1-513-556-0724; Fax: +1-513-556-4162
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Corresponding author Email: [email protected]
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Abstract
16
This study investigated the mechanistic effects of basic pH and the presence of high
17
carbonate concentration on the TiO2 photocatalytic degradation of cyanobacterial toxin
18
cylindrospermopsin (CYN). A high-performance liquid chromatography combined with
19
quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LC/Q-TOF-
20
ESI-MS) was employed for the identification of reaction byproducts. The reaction
21
pathways were proposed based on the identified degradation byproducts and radical
22
chemistry. In high pH system (pH = 10.5) similar reaction byproducts as those in neutral
23
pH system were identified. However, high pH appeared to inhibit sulfate elimination with
24
less sulfate elimination byproducts detected. In the presence of carbonate in the
25
photocatalytic process, hydroxyl radical reaction would be largely inhibited since
26
carbonate ion would react with hydroxyl radical to form carbonate radical. The second
27
order rate constant of carbonate radical with CYN was estimated to be 1.4×108 M-1s-1,
28
which is much smaller than that of hydroxyl radical. However, the more significant
29
abundance of carbonate radical in the reaction solution strongly contributed to the
30
transformation of CYN. Carbonate radical has higher reaction selectivity than hydroxyl
31
radical and hence, played a different role in the photocatalytic reaction. It would promote
32
the formation of byproduct m/z 420.12 which has not been identified in the other two
33
systems. Besides, the presence of carbonate ion may hinder the removal of toxicity
34
originated from uracil moiety due to the low reaction activity of carbonate radical with
35
uracil moiety in CYN molecule. This work would further support the application of
36
photocatalytic technologies for CYN treatment and provide fundamental information for
37
the complete assessment of CYN removal by using TiO2 photocatalysis process.
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Keywords: Basic pH; Byproduct; Carbonate radical; Cylindrospermopsin; Reaction
39
pathway; Titanium dioxide photocatalytic degradation;
40
List of Abbreviation
42
4-CP
4-Cyanophenol
43
AOPs
Advanced oxidation processes
44
CYN
Cylindrospermopsin
45
CO3•-
Carbonate radicals
46
ESI
Electrospray ionization
47
HPLC
High performance liquid chromatography
48
IEP
Isoelectric point
49
kA/•B
Second order rate constant for the reaction of A with •B radical
50
kobs(A)
Observed degradation rate constant of A
51
m/z
Mass to charge ratio
52
MS
Mass spectrometry
53
•
Hydroxyl radical
54
P25_CO32-
55
10.5)
56
P25_HighpH
System with P25 TiO2 at high pH (pH = 10.5)
57
P25_NA
System with P25 TiO2 at unadjusted pH (pH = 5.5)
58
SI
Supporting information
59
TOF
System with P25 TiO2 in the presence of Na2CO3 (1.5 mM, pH =
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Time-of-flight
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3
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1. Introduction
62
Cyanotoxins are a group of emerging contaminants that can be produced by certain
63
cyanobacteria as secondary metabolites and released during their cell lysis (Carmichael
64
1992, Wiegand and Pflugmacher 2005). One of the most problematic cyanotoxins,
65
cylindrospermopsin (CYN), has received great public attention due to its high toxicity,
66
prevalent distribution and widespread CYN-producing species (such as strains of
67
Cylindrospermopsis
68
Rhaphidiopsis curvata and Umezakia natans) in tropical, subtropical and temperate
69
areas (Fergusson and Saint 2003, Metcalf et al. 2004, Ohtani et al. 1992). The
70
biosynthetic pathways of CYN in Cylindrospermopsis raciborskii have been proposed
71
based on the identified gene cluster for the biosynthesis of CYN (Mihali et al. 2008).
72
CYN is a tricyclic guanidine alkaloid containing a hydroxymethyl uracil moiety with high
73
water-solubility. Generally, this toxin is an inhibitor for the synthesis of protein and
74
glutathione (Froscio et al. 2003, Lopez-Alonso et al. 2013, Young et al. 2008). It has a
75
negative effect on many organs and has been reported as a potent hepatotoxin,
76
cytotoxin and genetoxin (Bazin et al. 2010, Mazmouz et al. 2010, Ohtani et al. 1992).
77
Moreover, CYN has been proven to have carcinogenic potential at a concentration as
78
low as 1×10-7 µg/L (Maire et al. 2010). The cattle deaths and human poisonings in Palm
79
Island of Australia in 1979 have been attributed to the CYN contamination in drinking
80
water (Griffiths and Saker 2003, Wiegand and Pflugmacher 2005). Besides, the natural
81
photodegradation of CYN is very limited and highly dependent on UV-A radiation
82
(Wormer et al. 2010). Therefore, the effective treatment for this toxin is of extreme
83
significance for public health and environmental safety.
Aphanizomenon
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bergii,
ovalisporum,
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raciborskii, Anabaena
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4
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TiO2 photocatalysis has been applied in many areas including water treatment for
85
emerging contaminants (Antoniou et al. 2009b, Miranda-Garcia et al. 2011, Pelaez et al.
86
2009). It has been proven to be an effective and efficient approach for CYN removal
87
(Senogles et al. 2001, Zhang et al. 2014). Hydroxyl radical (•OH) is considered to be the
88
main reactive agent in the TiO2 photocatalysis process (Antoniou et al. 2009a, Kim and
89
Choi 2002, Tachikawa et al. 2007). Our previous work has shown that hydroxylation and
90
ring opening on the hydroxymethyl uracil moiety and tricyclic guanidine moiety, along
91
with sulfate elimination, are the main reaction pathways for the TiO2 photocatalytic
92
degradation of CYN in water (Zhang et al. 2015). Since hydroxymethyl uracil moiety is
93
believed to be responsible for the toxicity of CYN (Banker et al. 2001, Norris et al. 1999),
94
the toxicity of water sample after TiO2 photocatalytic treatment can be reduced (Zhang
95
et al. 2015).
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In the molecule of CYN, there is a negatively charged sulfate group and a positively
97
charged guanidine group. The pKa of CYN was estimated to be around 8.8 (Onstad et al.
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2007). Therefore, the CYN molecule has both negatively and positively charged group
99
at pH < 8.8, while the CYN molecule is negatively charged at high pH condition (e.g., pH
100
= 10.5). The isoelectric point (IEP) of TiO2 catalysts is around 7 (Gumy et al. 2006,
101
Zhang et al. 2014), which means TiO2 would be positively charged at acidic pH and
102
negatively charged at basic pH. In our previous study (Zhang et al. 2015), we have
103
identified the reaction byproducts and pathways of photocatalytic degradation of CYN at
104
pH around 5.5 (without pH adjustment). Hence, it would be very scientifically meaningful
105
to explore the differences in the mechanism at a high pH. Besides, carbonates are
106
widely present in various waters, and would therefore significantly affect the chemistry
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of the photocatalytic process (Dimitrijevic et al. 2011). When carbonate ions are in the
108
system, they would result in not only higher pH but also other possible effects. Many
109
studies have shown that •OH radical generated by photocatalytic activity can react with
110
carbonate and bicarbonate ions to form carbonate radicals (CO3•-) with second-order
111
reaction rate constants of 3.9×108 and 8.5×106 M-1s-1, respectively (He et al. 2012,
112
Pelaez et al. 2011). Moreover, the concentration of CO3•- in surface water under solar
113
light has been estimated to be two orders of magnitude higher than that of •OH radical
114
(Sulzberger et al. 1997). Thus, it is interesting to examine the effect of carbonate (or the
115
generated carbonate radicals) on the photocatalytic degradation byproducts and
116
pathways of CYN.
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In this study, we report for the first time the effects of initial high pH (pH = 10.5) and
118
the presence of carbonate ion on the TiO2 photocatalytic degradation of CYN from the
119
mechanism aspects, including the reaction byproducts and pathways. The reaction
120
byproducts were detected and identified by using a high-performance liquid
121
chromatography combined with quadrupole time-of-flight electrospray ionization tandem
122
mass spectrometry (LC/Q-TOF-ESI-MS). The degradation pathways were proposed
123
based on the identified byproducts and radical chemistry. This work can provide not
124
only better understanding about the mechanism of CYN photocatalytic destruction but
125
also further insights on the assessment of this technology for the treatment of CYN
126
contaminated water.
127
2. Materials and Methods
128
2.1. Materials
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CYN (>95%) was purchased from GreenWater Laboratories. Aeroxide TiO2 P25 was
130
obtained from Degussa AG. In the photocatalytic experiments, the water (HPLC grade)
131
used was purchased from Tedia. Sodium carbonate (Na2CO3, 100%, Fisher) was used
132
as the source of carbonate ions. For analysis, HPLC grade acetonitrile, deionized ultra-
133
filtered water and glacial acetic acid (99.9%) were supplied by Fisher Scientific. Atrazine
134
was purchased from Fluka (analytical standard) while 4-cyanophenol was purchased
135
from Aldrich (95%). All reagents were used without further purification.
136
2.2. TiO2 Photocatalytic experiments
137
In order to obtain the detectable byproducts, a high initial concentration of 10 µM CYN
138
was applied in the photocatalytic experiments. P25 TiO2 nanoparticles were uniformly
139
dispersed in water by using ultrasonication (2510R-DH, Bransonic) for 15 min. The CYN
140
stock solution and P25 TiO2 catalyst suspension (catalyst dose applied = 0.25 g/L) were
141
spiked into a borosilicate glass Petri dish (Pyrex, 60 mm (ø) x15 mm (h)) reactor
142
covered with a quartz cover. The reaction solution (10 mL) had an initial pH of 10.5
143
adjusted by NaOH (Fisher). In the experiment with Na2CO3, the concentration of
144
Na2CO3 was 1.5 mM with a pH of 10.5. The reactor was irradiated under two 15 W
145
fluorescent lamps (Han et al. 2011, Zhang et al. 2014) (Cole-Parmer). The light intensity
146
was about 2.3 mW/cm2 measured with a radiant power meter (Newport Corp.).
147
Continuous stirring and temperature control was applied during the reaction. 150 µL
148
samples were taken after certain time intervals, diluted with 150 µL water and filtered
149
with syringeless filters (0.2 µm, PTFE, Whatman).
150
2.3. Analysis
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An Agilent 1290 infinity HPLC system (Agilent Technology) with a Zorbax Eclipse XDB-
152
C18 Rapid Resolution column (2.1 × 50 mm, 3.5 µm, Agilent Technology) was
153
employed for the separation of CYN and reaction byproducts. The mobile phases were
154
H2O + 2% acetonitrile + 0.2% acetic acid (A) and acetonitrile + 2% H2O + 0.2% acetic
155
acid (B). The programmed gradient elution was set as: 2% B for 1 min; linearly
156
increasing to 95% B in the next 4 min; decreasing back to 2% B in the following 0.1 min;
157
and holding 2% B for another 2 min for re-equilibrium. The follow rate was 0.2 mL/min;
158
sample injection volume was 10 µL; and column temperature was 35 ºC. The HPLC
159
was coupled with an Agilent 6540 UHD Accurate-Mass quadrupole time-of-flight tandem
160
mass spectrometer (Q-TOF-MS) with an electrospray ionization source (ESI). The
161
parameters set for ESI-MS and automatic MS/MS analysis were described in detail in
162
our previous publication (Zhang et al. 2015). The quantification of CYN was confirmed
163
through the HPLC analysis mentioned in our previous publication (Zhang et al. 2014).
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In order to elucidate the role of carbonate radical in the system with Na2CO3, atrazine
165
and 4-cyanophenol (4-CP) were used for comparison with CYN. For atrazine and 4-CP
166
quantification, an HPLC system (Agilent 1100 Series) with a photodiode-array detector
167
was used for their quantification. A C18 Discovery HS (Supelco) column (150 mm × 2.1
168
mm, 5 µm) was employed as a stationary phase. The mobile phase was acetonitrile and
169
Milli-Q water with the ratio of 60:40 for atrazine and 50:50 for 4-CP. The flow rate was
170
0.4 mL/min for atrazine and 1 mL/min for 4-CP. The analysis was performed with a
171
column temperature of 25 °C at the wavelength of 22 2 nm for atrazine and 246 nm for
172
4-CP (Khan et al. 2013, Medendorp et al. 2006).
173
3. Results and discussion
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3.1. Photocatalytic reaction byproducts and reaction pathways of CYN under
175
different conditions
176
CYN can be removed by photocatalytic degradation process (Fig. 1) at pH without
177
adjustment (pH = 5.5, P25_NA for short), high pH (pH = 10.5, P25_HighpH for short) or
178
in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32- for short). The pseudo first-
179
order rate constants were calculated as 0.168, 0.188 and 0.167 min-1 (± 0.005 min-1, R2
180
≥ 0.99), respectively. The major reaction byproducts generated in these three conditions
181
are summarized in Table 1. The structures of these byproducts were proposed
182
according to their possible formulae, retention times, mass spectra and radical
183
chemistry (Antoniou et al. 2008a, b, Zhang et al. 2015). Agilent MassHunter workstation
184
software (Qualitative Analysis B.04.00) was applied in the study, which can provide
185
users a list of possible formulae according to the mass to charge (m/z) ratio and isotope
186
distribution of targeting compound (for best matching formulae, Relative Mass
187
Differences ⩽ 5 ppm). The MS spectra were detected under MS/MS mode. Moreover,
188
byproducts with higher polarity would elute earlier while those with lower polarity would
189
elute later under current analytical conditions. With respect to mass spectra, a
190
difference of 80 Da, 18 Da and 142 Da between two fragment ions indicates the
191
existence of sulfate group, hydroxyl group, and the whole hydroxymethyl uracil moiety,
192
respectively. The presence of fragment m/z 194.13 suggests the integrity of the tricyclic
193
guanidine moiety while the fragment m/z 192.11 implies the existence of a hydroxyl
194
group (with the fragment of m/z 210.12) or another double bond on the tricyclic
195
guanidine moiety (without m/z 210.12 fragment)
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(Silverstein 2005). More detailed
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description about the explanation of mass spectra was reported in our previous work
197
(Zhang et al. 2015).
198
3.1.1. Reaction byproducts and reaction pathways at high pH
199
TiO2 photocatalyst can generate hydroxyl radical, the main reactive agent (Antoniou et
200
al. 2008b, Sun and Xu 2010, Xie and Lin 2007), in a wide pH range: preferring reaction
201
with HO- at high pH but preferring reaction with H2O in acidic pH ranges (Turchi and
202
Ollis 1990). Therefore, the hydroxylation mechanism in P25_HighpH was expected to
203
be similar to that in P25_NA, which includes hydrogen abstraction and addition to the
204
C=C double bond (Antoniou et al. 2008b, Garrison 1987, Zhang et al. 2015). Based on
205
observation, most byproducts generated in the condition of P25_HighpH can be found
206
in system of P25_NA (Table 1). For example, the hydroxylation byproducts of CYN m/z
207
432.12 and 448.11 were observed in both systems. In our previous study, the
208
degradation byproduct m/z 448.11 had only one observed isomer (i.e., m/z 448.11(a) as
209
shown here) when no pH adjustment was applied (Zhang et al. 2015). In this study, two
210
more isomers of m/z 448.11 were observed in P25_HighpH (Fig. S1, Supporting
211
Information (SI)). The dehydration byproducts (e.g., m/z 414.11) were also observed.
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The sulfate elimination byproduct (i.e., m/z 334.15) and its main hydroxylation product
213
m/z 350.16 cannot be found in P25_HighpH but only existed in the P25_NA system.
214
From our observation, only very few sulfate elimination byproducts were detected in
215
P25_HighpH system, which can be attributed to the surface charge properties of the
216
catalyst. At high pH levels, the surface of TiO2 was negatively charged. Therefore, the
217
negative sulfate group would be more difficult to approach the TiO2 surface. Since the
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surface generated •OH would not diffuse far away from the TiO2 surface, it cannot easily
219
reach C12 to have sulfate elimination (Turchi and Ollis 1990, Zhang et al. 2015). The byproducts from the ring opening reactions on the hydroxymethyl uracil moiety,
221
such as m/z 392.12 and 375.10, were also detected under basic condition. Moreover,
222
the geminal diol m/z 322.11 and its hydroxylation byproducts m/z 338.10 and 354.10 all
223
existed in both systems of P25_HighpH and P25_NA, while their sulfate elimination
224
products only existed in the condition of P25_NA, which further confirms the conclusion
225
about the unfavorable sulfate group elimination at high pH (e.g., the studied pH = 10.5).
226
Other byproducts generated from the reaction on the tricyclic guanidine moiety,
227
including m/z 292.10 and 290.08 as well as their hydroxylation byproducts, were all
228
observed in the two systems. Moreover, one new byproduct m/z 338.14 was identified
229
in P25_HighpH system, which can be attributed to the reaction on tricyclic guanidine
230
moiety (Scheme 1). When both C14 and C15 have a hydroxyl group, the further
231
oxidation of this vicinal diol would result in the ring opening and form a carbonyl group
232
at C14 and a carboxyl group at C15 which can be easily released as CO2. (Antoniou et
233
al. 2008b, Jiang et al. 2012). Similar to some phosphate esters, direct hydrolysis of the
234
sulfate ester can take place at high pH and generate the corresponding alcohol to form
235
byproduct m/z 338.14 (Samuni and Neta 1973). The mass spectrum of m/z 338.14 is
236
shown in Fig. S2 (SI).
237
3.1.2. Reaction byproducts and reaction pathways in the presence of Na2CO3
238
Many studies have shown carbonate and bicarbonate ions play the role of •OH
239
scavenger in the hydroxyl radical based advanced oxidation processes (AOPs)(Grebel
240
et al. 2010, Haarstrick et al. 1996, Wong and Chu 2003). Moreover, the scavenging
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reactions would result in the generation of carbonate radicals (CO3•-) as shown in
242
Equation (1) and (2) (He et al. 2012, Pelaez et al. 2011). The carbonate radical has
243
attracted great scientific attention since it can be formed in the reaction of peroxynitrite
244
with CO2 in cellular environments (Squadrito and Pryor 2002). Furthermore, the
245
concentration of CO3•- in surface waters under solar irradiation has been reported to be
246
much higher than that of •OH (Canonica et al. 2005). The protonated form of this radical
247
is HCO3•, which has a pKa smaller than 0 (Augusto et al. 2002, Ferrer-Sueta et al. 2003).
248
•
OH + CO32- → OH- + CO3•-
k = 3.9×108 M-1s-1
249
•
OH + HCO3- → H2O + CO3•-
k = 8.5×106 M-1s-1
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HCO3• ↔ H+ + CO3•-
pKa < 0
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In current high pH system of P25_CO32-, the concentration of carbonate ion is around
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1 mM according to the calculation from the hydrolysis equilibrium of carbonate salt (pka1
253
= 6.4, pka2 = 10.3). Since bicarbonate ion has a lower concentration (~0.5 mM) and
254
lower reaction rate with •OH radical, its impact would be negligible in the current system.
255
Hydroxyl radical can efficiently react with organic compounds with very high second
256
order rate constants from around 109 to 1010 M-1s-1 (Antoniou et al. 2009a, Halliwell et al.
257
1987). The second order rate constant for the reaction of CYN with •OH radical has
258
been reported to be around 5×109 M-1s-1 (kCYN/•OH) (He et al. 2013, Onstad et al. 2007,
259
Song et al. 2012), which is one order higher than that of carbonate ion. However, the
260
concentration of carbonate ion is about 100 times higher than the concentration of CYN.
261
This means, in the P25_CO32- system, the carbonate ion would be more competitive to
262
react with •OH radical than CYN, generating carbonate radical. Therefore, we propose
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the •OH would mainly react with carbonate ion to generate carbonate radical and CO3•-
264
would be the predominant radical species for CYN degradation in P25_CO32- system. In order to confirm the hypothesis, a similar photocatalytic experiment was operated,
266
in which the CYN was replaced by atrazine with the same initial concentration (10 µM).
267
Carbonate radical has been reported to have oxidation capability for organic
268
compounds but its second order rate constants with organic compounds are in a much
269
wider range (from 102 to 109 M-1s-1) comparing to hydroxyl radical (Canonica et al. 2005,
270
Neta et al. 1988). Atrazine has a high second order rate constant with •OH radical
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(kATZ/•OH, around 2.5×109 M-1s-1) but a relatively low rate constant with carbonate radical
272
(kATZ/CO3•-, around 4×106 M-1s-1) (Canonica et al. 2005, Meunier et al. 2006). Hence, if
273
•
274
degradation rate could be largely reduced; otherwise, the degradation rate would be
275
similar to the reaction rate in P25_HighpH system. The kinetic comparison of the two
276
systems with atrazine is shown in Fig. 2(a). The pseudo-first order rate constants were
277
calculated as 0.148 and 0.021 min-1 for P25_HighpH and P25_CO32- system,
278
respectively. The observed degradation rate constant of atrazine in P25_CO32- system
279
decreased seven times compared with that in P25_HighpH system. Therefore, we can
280
conclude •OH would mainly react with carbonate ion to form CO3•- in P25_CO32- system.
281
Through a competition study of atrazine and 4-cyanophenol (4-CP) in P25_HighpH
282
system (He et al. 2013), the second order rate constant for the reaction of 4-CP with
283
•
284
constant for the reaction of 4-CP with CO3•- (k4-cp/CO3•-) has been reported as 4×107 M-1s-
285
1
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OH would mainly react with carbonate ion to form CO3•- in P25_CO32- system, the
OH (k4-cp/•OH) can be determined as 2.4×109 M-1s-1 (Fig. 2(b)). The second order rate
(Canonica et al. 2005). Assuming the concentration of •OH and CO3•- in P25_CO32-
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system would remain the same with the same initial molar concentration of the target
287
compound, we can obtain the concentration of •OH and CO3•- in the system through the
288
following equations:
289
kobs(4-CP) = k4-cp/•OH[•OH] + k4-cp/CO3•-[ CO3•-]
(4)
290
kobs(ATZ) = kATZ/•OH[•OH] + kATZ/CO3•-[ CO3•-]
(5)
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Therefore, the concentration of hydroxyl radical and carbonate radical in P25_CO32-
292
were estimated as 1.15×10-13 M and 1.56×10-11 M, respectively. The concentration of
293
carbonate radical in P25_CO32- system was more than 100 times higher than that of
294
hydroxyl radical, which could be attributed to the longer lifetime of CO3•- compared with
295
•
296
decay rate (Neta et al. 1988). Considering the similar degradation rates of CYN in the
297
presence and absence of carbonate ion (Fig. 1), the contribution of carbonate radical
298
was expected to be significant. The second order rate constant for the reaction of CYN
299
with CO3•- can thus be estimated from equation (6) to be1.4×108 M-1s-1 using the
300
methodology used in this work. Hence, in the P25_CO32- system, about 80% of the CYN
301
degradation was contributed by carbonate radical reaction and the rest was due to
302
hydroxyl radical attack.
303
kobs(CYN) = kCYN/•OH[•OH] + kCYN/CO3•-[ CO3•-]
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OH due to the relatively lower reaction rate of CO3•- with other species and lower self-
(6)
304
The CO3•- has very high selectivity and could selectively attack the electron rich
305
groups, especially for N-containing organics and phenols (Canonica et al. 2005, Joffe et
306
al. 2003, Mazellier et al. 2002). This electrophilic property of CO3•- also supports the
307
relatively higher reaction rate of CO3•- radical with CYN (see the molecule structure of
308
CYN in Fig. 1). Research studies from the groups of prof. Geacintov, N.E. and prof.
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Shafirovich, V have investigated the mechanisms of guanine oxidation by carbonate
310
radical and found a unique end product, the diastereomeric spiroiminodihydantoin lesion,
311
which can be formed through a four electron oxidation of guanine (Crean et al. 2005,
312
Joffe et al. 2003). The carbonate radical was believed to mainly act as the electron
313
acceptor via electron transfer mechanisms, in which it would receive one electron from
314
the organic electron donor, such as amino groups (see Scheme 2a) (Elango et al. 1985,
315
Neta et al. 1988, Shafirovich et al. 2001). Besides, similar to hydroxyl radical, carbonate
316
radical can undergo hydrogen abstraction to generate the corresponding alcohol (see
317
Scheme 2b) (Crean et al. 2005, Mazellier et al. 2002, Medinas et al. 2007, Neta et al.
318
1988). In the hydrogen abstraction mechanism of carbonate radical, the attacked group
319
also needs to connect with a strong electron donor although the reaction rates would
320
generally be low (Elango et al. 1985, Neta et al. 1988). Therefore, in the presence of
321
carbonate radical, CYN can also have similar hydroxylation process on the tricyclic
322
guanidine moiety but with a different reaction mechanism (e.g., formation of m/z 432.12).
323
The byproducts of CYN detected were much less in P25_CO32- system (Table 1). The
324
reaction mechanism on the uracil moiety of CYN would be different in the presence of
325
carbonate radical (Crean et al. 2005, Elango et al. 1985). It has been reported that the
326
carbonate radical has very low reaction rate with uracil (<104 M-1s-1) (Chen and Hoffman
327
1973), while hydroxyl radical was reported to have a very high reaction rate with uracil
328
(5.7×109 M-1s-1) (Buxton et al. 1988). Therefore, the reaction on the uracil moiety of
329
CYN should more easily start with the reaction with hydroxyl radical (Scheme 3).
330
Hydroxyl radical can first have hydroxyl addition to C5=C6 double bond to form a carbon
331
centered radical at C6, which can further react with oxygen and release one HOO•
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radical to form byproduct m/z 432.12. This byproduct then can have a ring opening
333
reaction, similar to the reaction of byproduct m/z 392.12 formation in P25_HighpH
334
system (Jiang et al. 2012, Munk et al. 2008). The m/z 432.12 can generate byproducts
335
m/z 422.13 by losing C4, and would easily react with carbonate radical to produce m/z
336
420.12 as shown in Scheme 3. The mass spectrum of byproduct m/z 420.12 is shown in
337
Fig. S3 (SI) (the difference of 45 Da indicates the existing of carbonyl group). The
338
byproduct m/z 420.12 identified in P25_CO32- was not observed in the other two
339
systems of this study. Moreover, the uracil ring opening byproduct m/z 392.12, found in
340
other systems, was not detected in this system, which can further support the different
341
reaction mechanism with the presence of CO3•-. The m/z 420.12 can further react with
342
carbonate radical and release C5 to form a C6-centered radical, which eventually can
343
have ring reforming byproduct m/z 375.10 (Scheme 2a) (Crean et al. 2005, Elango et al.
344
1985, Zhang et al. 2015). The geminal diol byproduct m/z 322.11 can be generated
345
through bond splitting of C6-C7, further oxidation of m/z 420.12 or ring opening process
346
of m/z 375.10 (Jiang et al. 2012). After further oxidation, C7 can be released to form
347
byproduct m/z 292.10 or 290.08. Besides, it has been reported that uracil moiety is
348
mainly responsible for the toxicity of CYN (Banker et al. 2001). The presence of
349
carbonate ions may lower the toxicity reduction due to the low reaction rate of carbonate
350
radical with uracil moiety. Therefore, high carbonate concentration should be avoided
351
during the photocatalytic treatment for CYN considering the mechanism of degradation.
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352
With respect to sulfate elimination, similar as P25_HighpH system, only a few sulfate
353
group elimination byproducts were identified in P25_CO32- due to the high pH of the
354
system. The same byproduct m/z 338.14 identified in P25_HighpH was also observed in
16
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P25_CO32-. This byproduct can be formed through the hydroxylation by carbonate
356
radical at C14 and C15 and a ring opening process (similar as Scheme 1 mentioned in
357
3.1.1). Besides, the reaction byproduct m/z 212.10 was also identified in this system,
358
which can be obtained by cleavage of the hydroxymethyl uracil moiety of m/z 338.14.
359
4. Conclusions
360
The mechanistic effects of high pH and the presence of carbonate ion on the TiO2
361
photocatalytic degradation of CYN were investigated in this study. The reaction
362
pathways were proposed based on radical chemistry and the reaction byproducts were
363
identified using LC/Q-TOF-ESI-MS. While high pH or high carbonate concentration
364
would not largely affect the photocatalytic degradation kinetically under current
365
experimental conditions, high pH would inhibit sulfate elimination process, and
366
carbonate ions would scavenge the hydroxyl radical produced during the photocatalytic
367
reaction and generate carbonate radical which has higher reaction selectivity. Since
368
carbonate radical has very low reactivity with uracil moiety in CYN molecule, the
369
presence of high carbonate concentration may hinder the reduction of toxicity originated
370
from uracil moiety. Besides, the second order rate constant of carbonate radical with
371
CYN was estimated to be 1.4×108 M-1s-1. This work would further provide fundamental
372
information for the complete assessment of CYN removal by using photocatalytic
373
technologies.
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Acknowledgements
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This study is funded by U.S. Department of Agriculture (58-3148-1-152). We are also
377
thankful to China Scholarship Council (CSC) Scholarships (2009617129) for the
378
financial support to G. Zhang through a Ph.D. scholarship.
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Disclaimer
381
The U.S. Environmental Protection Agency, through its Office of Research and
382
Development, funded and managed, or partially funded and collaborated in, the
383
research described herein. It has been subjected to the Agency’s peer and
384
administrative review and has been approved for external publication. Any opinions
385
expressed are those of the author(s) and do not necessarily reflect the views of the
386
Agency, therefore, no official endorsement should be inferred. Any mention of trade
387
names or commercial products does not constitute endorsement or recommendation for
388
use.
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Supporting information
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This material is available via the Internet at.
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nanoparticles
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photocatalytic
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light-activated
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596 597 598
Table 1- The main reaction byproducts detected using P25 TiO2 photocatalyst at unadjusted pH (pH = 5.5, P25_NA), high pH (pH = 10.5, P25_HighpH) or in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32-). m/z
Formula
P25_NAa
P25_HighpH
P25_CO32-
599
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464.11 C15H21N5O10S √ 448.11 C15H21N5O9S √ √ 432.12 C15H21N5O8S √ √ √ √ 420.12 C14H21N5O8S 414.11 C15H19N5O7S √ √ √ √ 392.12 C13H21N5O7S 375.10 C13H18N4O7S √ √ √ 370.09 C11H19N3O9S √ √ √ 354.10 C11H19N3O8S 350.14 C15H19N5O5 √ 338.14 C14H19N5O5 √ √ √ √ 338.10 C11H19N3O7S 336.09 C11H17N3O7S √ 334.15 C15H19N5O4 √ 322.11 C11H19N3O6S √ √ √ √ 322.07 C10H15N3O7S 320.09 C11H17N3O6S √ √ √ 308.09 C10H17N3O6S √ √ √ 306.08 C10H15N3O6S √ √ √ √ √ 292.10 C10H17N3O5S √ 292.06 C9H13N3O6S 290.08 C10H15N3O5S √ √ √ 272.12 C11H17N3O5 √ 256.13 C11H17N3O4 √ 256.09 C10H13N3O5 √ 240.13 C11H17N3O3 √ √ 212.10 C9H13N3O3 a Most of the results have been reported in our previous work (Zhang et al. 2015).
28
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100
O O S O H 9
12 13 20
H
7
10
8 18 NH
N 14 15
19
5 6
HN 1
17
NH
Uracil Moiety
C/C0
60
3 2
O
16
NH
O 4
SC
40
RI PT
H 11
80
P25_NA P25_HighpH P25_CO32-
HO
O
0 0
2
4
M AN U
20
6
8
10
12
14
Time (min)
Fig. 1 - The degradation of CYN using P25 TiO2 at unadjusted pH (pH = 5.5,
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P25_NA), high pH (pH = 10.5, P25_HighpH) or in the presence of Na2CO3 (1.5 mM, pH = 10.5, P25_CO32-) for reaction byproducts identification. The inset is
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the structure of CYN.
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O
H
H
OH O
NH HN
N
O O S O O
NH O
H
NH
N O
C15 H21 N5 O9S m/ z = 448.11a
O O S O
OH
O
O HN
NH O
NH
NH
NH
O
C14H19 N5O5 m/ z = 338.14
OH
H
H
N
NH HN
SC
H
NH HN O
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H
N
O
HO
C15H21N5O7 S m/ z = 416.12
HO
H
HO
NH
OH
H
RI PT
O O S O
H2 N
O NH O
C14H19N5O8 S m/ z = 418.10
Scheme. 1 – Proposed pathway for the formation of byproduct m/z 338.14. (The byproduct with “[ ]” indicates it was not detected and was proposed based
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on current information. a The byproduct shown here is only one of the possible
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forms of m/z 448.11)
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a
1.0
0.6
RI PT
C/C0
0.8
0.4
ATZ in P25_HighpH ATZ in P25_CO32-
0.2
4-CP in P25_CO320
5
SC
0.0 10
15
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Time (min)
20
1.0
b
4-CP ATZ
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C/C 0
0.8
0.6
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0.4
0.2
AC C
0
2
4
6
8
10
12
14
Time (min)
Fig. 2 – (a) The degradation of atrazine or 4-CP by using P25 TiO2 in P25_HighpH or P25_CO32- systems and (b) The competition study of atrazine and 4-CP in P25_HighpH system.
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O R1 O
CH
R2
CO 32-
NR3 R4
R2
R1
C
(b)
O
CH
R1
C O
O
O
C R3
R2
H+
C R2
NR3 R4
C O
O NR3 R4
R1 H 2O C HCO 3-
R2
OH NR3 R4
O
O
R1
O
C NR3 R4
O
R1
C O
O
CH
R2
R1
HCO 3-
C
R2
R3
R2
O
C O
O R3
R1 H 2O C HCO 3-
R2
OH R3
RI PT
(a)
R1
C
Scheme. 2 - The mechanism of hydroxylation in the presence of carbonate
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radical.
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O O S O
OH OH
OH
H
H
N
NH HN
H
O
H
H
N
N
OH O
O
H
O
NH
O
NH
+
HCO 3-
O O S O
O
H
C O H
NH HN
N H
O
NH
NH
H
H
OH
OH
NH HN
N H
C15 H21 N5O7S m/ z = 416.12
NH O
NH
NH
C11 H19 N3 O6S m/z = 322.11
O
OH
O
NH
N H
O
OH
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H
O
O
O2
H
N H
O
NH HN
NH O
NH
O O S O O
OH
H
NH
O HCO3- + CO 2
C O
OH
O
H
O O S O
OH
H O NH HN
N H
isomer
O
NH
O H
H
N
NH HN
OH O
NH2 O H
C14H21N5O8 S m/ z = 420.12
NH
NH2 O
HOO
O O S O
O
OH O
HN
NH
NH2
O
O2
H
N
H
AC C
C15H21N5O8 S m/ z = 432.12
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H
OH
EP
O
OH
OH
NH HN
O O S O
HOO O O S O
H
NH
O
O O S O
OH
H
OH H
O
H N
C13 H18O7 N4S m/ z = 375.10
H 2O
O O S O
O O S O O
NH
NH
SC
O
RI PT
O O S O
NH 2
C O
O
H
N
NH
OH O
O
C14 H23 N5O8S m/ z = 422.13
OH H
O
HCO 3-
H
NH
HN
NH 2 O
Scheme. 3 – Proposed reaction on the hydroxymethyl uracil moiety in P25_CO32- system. (The intermediates with “[ ]” indicate they were not detected and were proposed based on current information.)
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Highlights:
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Basic pH and carbonate ion mechanistically impact on CYN photocatalytic destruction Basic pH can minimize sulfate elimination of TiO2 photocatalytic degradation of CYN Carbonate ion can scavenge produced hydroxyl radical to form carbonate radical 8 -1 -1 • The second order rate constant of CO3 with CYN was estimated to be 1.4×10 M s
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Supporting Information
1 2
The effect of basic pH and carbonate ion on the photocatalytic
4
destruction of cylindrospermopsin: the degradation byproducts and
5
pathways
6
Geshan Zhanga, Xuexiang Hea, Mallikarjuna N. Nadagoudab, Kevin O'Sheac,
7
Dionysios D. Dionysioua,*
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a
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati,
10
OH 45221, United States.
11
b
12
United States.
13
c
14
33199, United States.
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US EPA, National Risk Management Research Laboratory, Cincinnati, OH 45268,
Department of Chemistry and Biochemistry, Florida International University, Miami, Fl
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*
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Tel: +1-513-556-0724; Fax: +1-513-556-4162
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Corresponding author Email: [email protected]
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Supporting Figure and Scheme Captions
19
Fig. S1 - MS spectra and proposed structures of reaction byproducts m/z 448.11(b) and
20
m/z 448.11(c). The red color indicates the group does not have a fixed position.
21
Fig. S2 - MS spectrum and proposed structure of reaction byproduct m/z 338.14.
22
Fig. S3 - MS spectrum and proposed structure of reaction byproduct m/z 420.12.
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350.14
100 O OH
O S O
OH
O
H
H
N
NH
368.16
O O
80
HN
NH
H
448.11
O
*
RI PT
C15H21N5O9S m/ z = 448.11 (b)
60
208.11
332.13
40
SC
Relative Abundance
NH
194.13
0 100
150
200
M AN U
20
250
300
350
400
450
m/z
26
368.16
100 HO
O
H
H
N
NH
EP
60
40
20
0 100
OH
HN
O
NH
O
H
HO
AC C
Relative Abundance
80
TE D
O
O S O
NH
C15H21N 5O 9S m/ z =448.11 (c)
448.11
*
208.11
350.14
190.09
150
200
250
300
350
400
450
m/z
27 28
Fig. S1
3
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HO
OH
H
H
N
NH HN
60 NH
O
SC
O
O
NH
C 14 H 19N5 O5 m/z = 338.14
40
178.10
20
M AN U
Relative Abundance
80
RI PT
196.11
100
338.14
*
0 100 120 140 160 180 200 220 240 260 280 300 320 340
31
Fig. S2
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m/z
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295.14
100
O O S O
OH
H
RI PT
O
80
NH HN
N H
194.13
NH2
O
NH
C14H21N5O8 S m/ z = 420.12
60
420.12
SC
*
375.10
40
20
0 100
150
200
M AN U
Relative Abundance
OH H
O
250
300
350
400
Fig. S3
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m/z
5