Photodegradation of Imidacloprid Insecticide by Ag-Deposited

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Photodegradation of Imidacloprid insecticide by Ag deposited titanate nanotubes - A study of intermediates and their reaction pathways Inderpreet Singh Grover, Satnam Singh, and Bonamali Pal J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5041614 • Publication Date (Web): 02 Dec 2014 Downloaded from http://pubs.acs.org on December 10, 2014

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Journal of Agricultural and Food Chemistry

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Title: Photodegradation of Imidacloprid insecticide by Ag deposited titanate nanotubes - A

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study of intermediates and their reaction pathways

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Author’s name: Inderpreet Singh Grover, Satnam Singh and Bonamali Pal*

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Designation: First author is a Ph.D. student, second author is an Associate Professors and third

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author is a Professor.

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Address:

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School of Chemistry and Biochemistry,

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Thapar University,

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Patiala 147004,

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Punjab (India)

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Corresponding author:

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Bonamali Pal

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E-mail: [email protected]

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Tel: 91-175-2393128

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Fax: 91-175-2364498

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1 ACS Paragon Plus Environment


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Abstract

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The present work demonstrates the influence of Ag-loading (0.2-1.0 wt%) onto sodium titanate

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nanotubes (TNT) for complete photomineralization of neurotoxic imidacloprid (IMI) insecticide

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under UV-light illumination. It has been observed that degradation of IMI follows pseudo-first

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order-kinetics, where 0.5wt% Ag-loaded-TNT exhibited highest apparent-rate-constant

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(2.2 × 10-2 min-1) and corresponding least half-life (t1/2) of 31 min for IMI than with bare P25-

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TiO2 (3.4 × 10-3 min-1, t1/2 = 230 min). The mineralization of IMI intermediates to CO2 during its

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photooxidation has been described by the time course GC-MS and GC analysis and has been

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correlated with the kinetic analysis. The investigation for the role and quantitative estimation of

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the fate of heteroatoms (N, O and Cl) present in IMI revealed an increase in the amount of

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nitrate-, nitrite- and chloride-ions with time during its photooxidation. On the basis of these

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results a mechanistic pathway for photomineralization of IMI is being proposed.

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Key words: Photocatalytic activity of TNT, Photodegradation of Imidacloprid, Fate of

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heteroatoms, Inorganic ion formation, Mineralization, Intermediate’s identification

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Introduction:

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Highly porous monoclinic TNT possessing high surface area, interlayer water molecules, scroll

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structure, thin wall thickness for better charge transport and charge diffusion properties has been

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studied for the photocatalytic degradation

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exhibits higher photocatalytic activity (PCA)3-5,7,8 than commercially available most photoactive

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P25-TiO2 (P25). For instance, Zhu et al. 3 and Antoino et al. 4 showed its 5 and 7 times higher

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activity than P25 for degradation of sulforhodamine and methyl red dyes, respectively. Literature

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reports 1-6,10 also support that PCA of TNT is influenced by the Na-content in its crystal structure

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and calcination temperatures. For example, Morgado et al. 6 demonstrated that TNT having 2-5

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wt% of Na can show improved PCA for oxidation of rhodamine-B, while Qumar et al.1 showed

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its negligible and higher PCA at below and above 300 oC of calcination temperature,

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respectively. However, calcination of TNT at > 700-800 oC transforms it into orthorhombic

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nanorods with change in Ti-O bond length, having much less surface area and de-hydroxylated

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surface with a decent PCA7-9 to that of P25.

1-9

of many organic pollutants. Generally, TNT

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The PCA of TNT is well known to be improved11-14 by loading of metals (Au, Ag, Pt, Rh,

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Fe, Ru etc.) that act as electron sink and retards the recombination process of photoexcited

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charge carriers. For instance, Tsai et al.14 showed improvement in PCA of TNT for

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photooxidation of CO after incorporation of Au (0.39-2.53 wt%). Hence, bare/metal-loaded TNT

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despite having Na (generally decrease the activity of TiO2 catalyst) and the high temperature

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treatment has proven to show better activity than P25 for degradation of dyes. Therefore, it can

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be utilized for mineralization of other large heteroatom(s) containing molecules such as

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pesticides and has been recently studied by our group7,8.

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Imidacloprid, a neurotoxic insecticide having high water solubility (550 mg/l), half-life

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(t1/2) = 40-60 days is extensively used7 in Punjab (India). Under natural conditions its



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mineralization is incomplete and oxidize to other heteroatom(s) containing intermediates that are

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sometimes more toxic and persistent than IMI itself and are a cause of concern for the plausible

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threat for ecological system. As a result, studies have been carried out using P25 that showed 14-

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19% of mineralization along with presence of heteroatom containing persistent intermediates that

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may require longer light exposure for complete decomposition. Since, the presence of

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heteroatom(s) in the intermediate(s) can influence stability of a molecule by phenomenon of

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“resonance”,

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formation/transformation/stability of IMI’s intermediates and their final fate to the inorganic

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ions. Hence, present work considers the PCA of bare and Ag-loaded (0.2-1.0 wt%) TNT in

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relation to P25 for the photo mineralization of IMI to CO2, including quantitative estimation of

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the fate of its heteroatoms (N, O and Cl) to inorganic ions and identification of its photoproduced

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intermediates. A possible mechanistic path way for the mineralization and transformation of

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IMI’s intermediates into one-another, and a chemical mass balanced relation in comparison to its

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stoichiometrically balanced equation is being proposed in the present study.

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Experimental

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Materials

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Sodium Hydroxide (AR), Acetonitirile (HPLC), Methanol (GC) Ethyl aceteate (GC) Isopropyl

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alcohol (AR), Silver Chloride and Nitric acid were purchased from Loba Chemie. Commercially

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available P25-TiO2 (size = 30-50 nm, anatase phase = 70%, rutile phase= 30%surface area = 50

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m2g-1) and imidacloprid were obtained from Degussa Corporation, Germany and Ravi Organics

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Pvt. Ltd. (Mumbai), India, respectively. Standard CO2 gas (200 ppm) with N2 as a base was

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obtained from Centurion Scientific Pvt. Ltd, New Delhi, India. Sodium Nitrite, Sodium Nitrate

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and Sodium Chloride were used as standards of nitrite, nitrate and chloride ions, respectively and

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were obtained from Sigma Aldrich. All the chemicals were used without further purification.

so

it

is

worth

to

study

the

role


of

these

atoms

in

the

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Synthesis, characterization and photoactivity of bare and Ag-loaded TNT

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TNT was prepared through the hydrothermal route using P25-TiO2 as reported7,8 by our group.

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Surface modification of TNT by Ag loading (0.2-1.0 wt%) was carried out by irradiating (125 W

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Hg arc lamp) the de-aerated solution (50% v/v isopropyl alcohol) of TNT and AgNO3

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(equivalent to 0.2, 0.5 and 1.0 wt% of Ag) as reported7,8 recently by our group. Catalysts thus

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obtained are termed as 0.2-Ag-TNT, 0.5-Ag-TNT and 1.0-Ag-TNT.

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The photocatalytic activity of as-prepared catalysts was carried out in a test tube (rubber

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caped) by taking 5.0 mg of catalysts, 5 ml (50 ppm) aqueous solution of imidacloprid under UV-

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light irradiation for various time intervals. The reaction solution was analysed by HPLC

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(Agilent-LC1120) equipped with UV-detector (at λ = 270 nm ) having C-18 Column (BDS,

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Qualigens) of dimensions 250 mm × 4.6 mm and particle size of 5 µm using acetonitrile: water::

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80:20 as mobile phase (1 ml/min), at ambient temperature, using 20 µl injection volume. The

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CO2 evolution was determined by injecting 1 ml of gaseous mixture from the reaction vessel (gas

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tight test tube) into gas chromatograph (GC, NUCON-5765) using Propak-Q column with

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nitrogen as carrier gas (30 ml/min) and thermal conductivity detector. Column oven was

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maintained at 40 oC while injector and detector were at 70 and 80 oC, respectively. Identification

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of the intermediates formed during photooxidation of IMI was carried out using Gas

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Chromatograph (45X-GC)-Mass Spectrometer (MS-Scion-45P), by injecting 1 µl of extracted

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sample (Electronic Supplementary Information (ESI)-scheme1) on a HP-5MS column (15m ×

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0.25 mm × 0.25 µm), with Helium gas (1ml/min) as a carrier. The oven was programmed from

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60 oC (5 min hold) to 240 oC@6 oC, while transfer line and injector were kept isothermally at

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250 oC and 275 oC, respectively and the identified intermediates are termed as I-1 to I-8

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thereafter. The formation of inorganic anions (nitrate, nitrite and chloride) has been

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quantitatively estimated by injecting 100 µl of the sample into Ion chromatograph equipped with



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a Waters 501pump, a Waters 431 conductivity detector, and ion pack (50 mm × 4.6 mm,

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Metrhom) column using methanol: water :: 60:40 as mobile phase @ 0.6 ml min−1.

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Results and discussion

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Morphological analysis of TNT (Fig. 1a-b) reveals the formation of open ended straw like

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nanotubes having length (L) = 80-115 nm and diameter (D) = 8-13 nm and has been reported by

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our group7,8. The Ag nanoparticles of narrow size (3-5 nm) distribution at the surface of many

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TNT particles can be distinctly noticed for 0.5-Ag-TNT sample.

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Time course PCA of bare and Ag-loaded TNT in relation to P25 is shown in Fig. 2. It is

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observed that TNT exhibited higher activity than P25 and is consistant with our7,8 and other

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reports3-5. However, Ag-loading increases the activity of TNT upto 0.5 wt% for the degradation

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of IMI and thereafter its activity decreases. Photooxidation of IMI was found to follow pseudo-

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first order kinetics (ESI-figureS1) with highest value of apparent rate constant (k) of 2.2×10-2

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min-1 for 0.5-Ag-TNT catalyst and the half-life of IMI found (ESI-table1) was ~32 min notably

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much lower in relation to TNT (~103 min) or P25 (~204 min) catalysts, under similar reaction

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conditions.

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Time course GC-MS (Fig. 3) analysis showed a gradual decrease in peak height for IMI

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along with appearance of some new peaks, each belonging to different intermediate. Some of

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these intermediates were identified by their mass spectra (Fig. 4a-b). There is a complete

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disappearance of peak corresponding to IMI (at retention time (Rt) = 11.2 min, Fig. 3) after 180

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min of its photooxidation in presence of 0.5-Ag-TNT catalyst, however its presence, was

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observed when studied with bare TNT under similar experimental conditions, confirming

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improved activity of the former catalyst.

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The quantitative estimation for CO2 formation (Fig. 5) during degradation of IMI by

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various catalysts was found to follow similar trends to its photooxidation. The highest amount of


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CO2 formation (ESI-table2) by 0.5-Ag-TNT catalyst with highest (2.4 × 10-3 min-1) pseudo-first

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order rate constant (ESI-table3) corresponds to ~70% of its actual mineralization despite its

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complete oxidation. This incomplete mineralization of IMI is because of the existence of some

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intermediates found (Fig. 3 and 4a-b) even after 180 min of its photooxidation.

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Investigation for the fate of heteroatoms showed that the amount of inorganic ions

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formed (ESI-figure S2) by 0.5-Ag-TNT are higher than that for P25 (Fig. 6). Moreover, the

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amount of inorganic ions increases with increase in time of irradiation, again confirming the

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mineralization IMI during its photodegradation.

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The chemical equation (eq. 1) obtained from these results is found to deviate from

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stoichiometrically balanced equation 2, evidencing incomplete mineralization of IMI.

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increase in amount of nitrate ion formation with irradiation time is probably due to

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mineralization of the imidazole or pyridine substituted ring(s). These results are in correlation

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with previous report27 for dissipation of cyrpoconazole, where the nitrogen atoms present in the

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triazole moiety were found to decompose to nitrate and ammonium ions.

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C9H10N5O2Cl + O2

ହ

0.5 wt% Ag-TNT

ଶ

hv (180 min)

_

_

The

_

2.88 CO2 + 0.04 NO2 + 0.62 NO3 + 0.6 Cl +_ Intermediates _

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C9H10N5O2Cl +

ଷଷ ଶ

_

O2

……….eq.

1

……….eq.

2

_

9CO2 +5 NO3 +Cl + 6H+ + 2H2O

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The PCA of TNT with Ag-depositions enhanced remarkably, certainly with an optimum

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amount (0.5 wt%) of the silver. With higher amount of Ag-photodeposition, the metal would

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rapidly become the recombination center for electron/hole (e-/h+) pairs and consequently

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diminishes the PCA. Moreover, Ag being plasmonic28 nanostructure supports the formation of

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resonant surface plasmons in response to a photon flux that in-turn could yield a high 7 ACS Paragon Plus Environment


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concentration of e-/h+ pairs in Ag-TNT samples. The formation of such plasmonic nanostructures

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also allow the selective formation of e-/h+ pairs at the interface of Ag-TNT which could readily

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separate these carriers from each other and easily migrate28-30 to the surface, where they can

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carry out PCA. As loaded Ag is well known28,31 to improve the adsorption capacity of TiO2 for

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molecular oxygen, therefore, it can scavenge more conduction band electrons by the formation of

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highly oxidative super oxide radicals. Whereas, h+ react with absorbed surface -OH group and/or

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H2O molecules to form hydroxyl radicals possessing high oxidizability and causing the improved

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PCA of Ag-loaded TNT nanoparticles.

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The formation of variety of intermediates during photooxidation of IMI, suggests various

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possible attack positions for hydroxyl radical leading to the formation of intermediates (Fig.

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4.7a). The primary attack sites by hydroxyl radical during photooxidation of IMI yielded I-2 and

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I-4 by cleavage of the N-N bond between N1and N2 positions and C-N bond at C-1 position. The

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intermediate I-2 thus formed could be degraded to I-6 via attack of hydroxyl radical on the C-N

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bond of –CH2–N– moiety. On the other hand, intermediate I-4 (1o alcohol) is being oxidized to

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corresponding acid (I-5). The formation of carboxylated compounds has been observed during

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the photocatalytic degradation pesticides such as for cyproconazole33 (fungicide) that has triazole

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and chlorobenzene moieties in its structure. Alternatively, hydrolysis which is often reported34-36

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during photocatalytic degradation of pesticides could result (Scheme-1a) in the formation of

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intermediate I-8.The successive oxidation of I-8 on methyl group at C-2 position of imidazole

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ring could lead to formation of I-3. The formation of intermediate I-6 is proposed to be via

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fragmentation of I-3 and is reported to be an intermediate product26 of IMI photooxidation.

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Interestingly, I-7 formed (Scheme-1b) by removal of Cl and NO2 radicals from IMI has been

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identified in the present study using Ag-loaded TNT samples. This could be attributed to the fact

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that (i) I-7 intermediate formed remained below the detection limit of GC-MS and (ii) that it may

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have formed/consumed/transformed into some other product during the photooxidation. The 8 ACS Paragon Plus Environment

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formation of I-7 via., proposed radical mechanism is also suggested by Donald et al.37 during the

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gas phase fragmentation of IMI. Since, there is evolution of CO2 and formation of inorganic ions

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during the photooxidation of IMI, therefore it is possible that intermediates might further oxidize

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to smaller compound(s) and eventually got mineralized. Similar to identified intermediates,

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previous studies also identified these intermediates15,18,26 during the photooxidation of IMI using

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P25. However, the extent of IMI mineralization was 14-29%32and is 1.7 times less than the

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present study, revealing the improved efficiency of as-prepared catalysts than P25.

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The proposed mechanism is further supported by kinetic analysis of the intermediates

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(Fig. 8a-c) and by relating the ratio of peak area at any time (Area(t)) to the maximum peak area

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(Area(max)), due to unavailability of authentic intermediates. Intermediates I-2, I-4 and I-8 appear

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promptly and concentration becomes maximum after 30 min before being degraded. Whereas,

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their corresponding transformed intermediates viz., I-1, I-5; I-3 and I-6 were found to be at their

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maximum level after 15-30 min of photooxidation of I-2, I-4 and I-8 intermediates, respectively.

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Afterwards, intermediates starts disappearing and within 180 min of photooxidation are almost

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leveled off, except for I-2 and I-5. The existence of heteroatom containing intermediates (I-2 and

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I-5) even after complete degradation of IMI, signifies their stability in the reaction medium and

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is believed to be a cause for the incomplete mineralization for IMI. The results of the present are

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also supported by the results of Lhomme et al.33 and Lambropoulou et al.38 suggesting

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phoromineralization of heteroatoms (present in their moieties) to inorganic ions and leveling off

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the intermediates with time during photocatalytic degradation of cyproconazole and Fenhexamid,

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respectively, in presence of TiO2.

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Thus, Ag-photodeposited TNT is an efficient catalyst for the decomposition of IMI to

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CO2 and inorganic ions, and it notably reduces half-life of IMI to that found by most photoactive

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P25-TiO2. Although, structural parameters of TNT were found to play a vital role for the

212

improved decay constants, yet detrimental influence of Na-atoms was not observed in the present 9 ACS Paragon Plus Environment


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study. The formation of a number of intermediates identified revealed the complexity of the

214

photocatalytic process, suggesting the existence of various oxidation routes and non-specific

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attack of hydroxyl radicals that resulted in multi-step and interconnected pathways for

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decomposition of IMI. The fate of heteroatoms suggests the partial mineralization to inorganic

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ions and is manifested by the presence of heteroatom containing persistent intermediates even

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after the complete degradation of IMI. Abbreviations used:

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e-

electron

h+

hole

TNT

Sodium titanate nanotubes

0.2-Ag-TNT

0.2 wt% Ag deposited sodium titanate nanotubes

0.5-Ag-TNT


1.0-Ag-TNT


P25

Commercially available P25-TiO2

MCPS

Mega Counts Per Second

IC

Ion Chromatograph

GC

Gas Chromatograph

HPLC

High Performance Liquid Chromatograph

GC-MS

Gas Chromatograph coupled with Mass Spectrometer

TEM

Transmission Electron Microscope

I-1 to I-8

Identified intermediates

IMI

Imidacloprid

t1/2

Half-life time

AR

Analytical Reagent Grade


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Acknowledgements

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The authors acknowledge the University Grants Commission and Department of Science and

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Technology, New Delhi, Government of India for providing financial support. Degussa

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Corporation, Germany is gratefully acknowledged for the gift sample of TiO2.

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319

Figure Captions:

320

Fig. 1 (a and b) TEM images for 0.5 wt% Ag photodeposited sodium titanate nanotubes; scale

321

bar = 100 nm.

322

Fig. 2 Time course study and pseudo-first order decay constants for photocatalytic degradation

323

of imidacloprid using various catalysts. Error bars represents standard deviation from three

324

replicate experiments.

325

Fig. 3 Gas Chromatographs for identified intermediates (I-1 to I-8) obtained after photocatalytic

326

degradation of imidacloprid using 0.5 wt% Ag-photodeposited sodium titanate nanotubes after

327

(a) 30 min, (b) 120 min, (c) 180 min, in comparison to (d) bare sodium titanate nanotubes after

328

180 min; inset: corresponding enlarged view.

329

Fig. 4a Mass spectrum for identified intermediates (I-1 to I-4); inset: retention time (Rt) as per

330

gas chromatographs, molecular structure and mass/charge ratio (m/z).

331

Fig. 4b Mass spectrum for identified intermediates (I-5 to I-8); inset: retention time (Rt) as per

332

gas chromatographs, molecular structure and mass/charge ratio (m/z).

333

Fig. 5 Time course of CO2 formation during the photooxidation of imidacloprid by different

334

catalysts. Error bars represents standard deviation from three replicate experiments.

335

Fig. 6 Evolution of inorganic ions during photooxidation of imidacloprid by 0.5 wt% Ag

336

photodeposited TNT and bare P25.

337

Fig. 7 (a) Various attack positions by hydroxyl radicals at imidacloprid and (b) probable pathway

338

proposed for the degradation of imidacloprid.

339

Scheme 1 Mechanism for formation of intermediates (a) I-8 and (b) I-7 after photooxodation of

340

imidacloprid.



341

Fig. 8 (a-c) Time evolution for the intermediates formed during the photooxidation of

342

imidacloprid.

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359


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Figures

360 361 362

Fig. 1

363 364 365

Fig. 2

Residual amount (ppm)

50 40

P25 ( 3.4×1 -3 0

30 TNT

20 10

(6.7×

10 -3

(1.4×1 -2 0

1.0-Ag-TNT (9.2×10-3 min-1)

0 0 366

0.2Ag-T NT

0.5-Ag-TNT (2.2×10-2 min-1)

30

60

90

120

min -1 )

min -1

)

min -1 )

150

180

Time (min)

367 368 369 370 371 17 ACS Paragon Plus Environment


372

Fig. 3

373 374 375 376 377 378 379 380 381 18 ACS Paragon Plus Environment

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382


Fig. 4a

383 384 385 19 ACS Paragon Plus Environment


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Fig. 4b

387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411


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412


Fig. 5

413

35 -1

Rate Constant (m in ) -3 2.4 × 10 -3 1.6 × 10 -3 1.0 × 10 -4 7.4 × 10 -4 3.7 × 10

Amount (ppm)

30 25 20 15

0 .5

-T -A g

NT

g -T 0 .2 A g -T 1.0 A

NT

NT

TNT

10 P25-TiO 2

5 0 0

60

90

120 150 180 210

Time (min)

414 415

30

Fig. 6

416 417 418 419 420 21 ACS Paragon Plus Environment


421

Fig. 7

422 423

424 425 426 427 428 429 430


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431


Scheme-1

432 433

Fig. 8

435

1.0

437 438 439 440 441

Area(t)/Area(max)

436

1.0

(a)

0.8 0.6 0.4 I-2

0.2

I-5

0.8 0.6 0.4 0.2

I-1

0.0 0

30

60

90

120

150

0.0

180

0

30

1.0

(c)

446

Area(t)/Area(max)

443

445

0.8 0.6

I-6

I-8

0.4

I-3

0.2

447 0.0

448

60

90

120

Time (min)

Time (min)

442

444

(b)

I-4

Area(t)/Area(max)

434

0

30

60

90

120

150

Time (min)

449 450 451 452 23 ACS Paragon Plus Environment

180

150

180


453 454

Graphic for Table of Content

455 456 457


Page 24 of 24

Photodegradation of Imidacloprid Insecticide by Ag-Deposited

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