Identification of photocatalytic degradation pathways of 2-Cl-s-triazine herbicides and detection of their decomposition intermediates

Chemosphere, Vol. 24, No. 7, pp. 891-910, 1992 Printed in Great Britain

0045-6535/92 $5.00 + 0.00 Pergamon Press plc

IDENTIFICATION OF PHOTOCATALYTIC DEGRADATION PATHWAYS OF 2-CI-$-TRIAZINE HERBICIDES AND DETECTION OF THEIR DECOMPOSITION INTERMEDIATES

E. Pelizzetti*, C. Minero, V. Carlin, M. Vincenti, E. Pramauro Dipartimento di Chimica Analitica, Universit~t degli Studi di Torino, Torino, Italy

M. Dolci Dipartimento di Chimica Agraria, Universit~ degli Studi di Torino, Torino, Italy

ABSTRACT The photocatalytic degradation of three 2-chloro-s-triazine herbicides (simazine, propazine, atrazine) over TiO 2 particles under simulated solar light has been investigated.

In the early steps, the degradation

progresses through competitive reactions such as oxidation of the side alkyl chain and substitution of chlorine with hydroxyl groups, the last process occurring for less than 10% of degradation. Further oxidative processes give rise to the formation of dealkylated products. Several intermediates have been detected by GC-MS and comprehensive degradation schemes are proposed. The three herbicides, hearing different alkyl groups, exhibit relevant differences in the relative role of the competitive degradation pathways.

KEY WORDS : s-triazine derivatives, degradation mechanism, photodegradation, GC-MS, atrazine, simazine, propazine.

891

892 INTRODUCTION The 2-chloro-s-triazines are amongst the most widely used herbicides. 1'2

Their degradation and

metabolic pathways have been largely investigated. Dehalogenated, dealkylated and deaminated derivatives of the s-triazine herbicides have been isolated in different sources. 3-11 Recently, herbicides. 12'13

fast and

efficient photocatalytic degradation

has been

reported

for some s-triazine

The catalytic process can be of interest in the fate of these compounds in the soil 14 and

aquatic environments and represent a viable route for the decontamination of polluted waters. 15'16 The present paper reports on the degradation pathways and the identification of some intermediates occurring in the photocatalytic degradation of the three widely used 2-chloro-s-triazines reported in Chart I. Particular efforts have been devoted to the study of early stages of the process, since the latter part of the degradation scheme has been already established for atrazine. 13 Emphasis is also given to the comparison of the mechanisms and decomposition rates for differently alkylated compounds.

Chart I. Structural formulas, trivial and chemical names of 2-chloro-s-triazines under study.

c4 Structural formula: R1

Symbol

R1

S

C~-Is

P

A

R2 CL=~'Is

Trivial name

Chemical name

Slmazlne

1,3,5-trlazlne-2,4-dlamlne-6-chloro-N,N'-diethyl

CH(CI-kj)z CH(CI-~j)2 Propazine

~

CH(CI-~2

R=

Atrazlne

1,3,5-trlazlne-2,4-dlamlne-6-chloro-N~l'-(1-methylethyl) 1,3,5-trlazlne-2,4-dlamlne-6-chloro-N-ethyI-N'-(1-methylethyl)

EXPERIMENTAL SECTION Materials and Reaeents.

Standard herbicide compounds (atrazine 99.5%, simazine 99.9%, propazine 99%) supplied by Istituto Superiore della Sanit~ (Rome), and cyanuric acid (Aldrich 98%) were used as received. Stock solutions

(1

gL -1) of triazine herbicides were prepared in ethyl acetate; reanalysis of stock solutions showed no effect of

893

the solvent on the stability of the considered herbicides. Cyanuric acid (compound X in Scheme V) is soluble enough in water for preparing stock solutions (1 gL -1) in bidistiiled water. Compounds III, V, VI, VIII, XVIII (see Schemes) were synthesized as described previously; 12 compounds I, XIII, XXI were obtained by hydrolysis of S, P, A respectively, according to the literature. 17 Acetonitrile and methanol (Lichrosolv Merck) and didodecyldimethylammonium bromide (Kodak) were used as received. Other reagents used were at least analytical grade. All degradation experiments were carried out by using TiO 2 Degussa P25 grade as photocatalyst.

In

order to avoid a possible interference from ions adsorbed on the photocatalyst in the analytical determinations, the TiO 2 powder was irradiated and washed several times with bidistilled water until no signal due to chloride, sulphate or sodium ions could be detected by ion chromatography (Cl- < 20 and Na + < 100 part per billion H20 in the presence of TiO 2 5 gL-I in H20). Irradiation ext~erim¢~. Experiments were carried out in Pyrex glass cells on 5 mL of an aqueous suspension containing the needed amount of semiconductor powder and substrate, by irradiating with a 1500 Watt Xenon lamp (CO.FO.MEGRA, Milan) equipped with a 340 cutoff filter and simulating AMI solar light. described elsewhere. 13

The cells were

The total photonic flux in both cells, as measured by ferrioxalate actinometry and

corrected for the fraction of light absorbed and the quantum yield of the actinometer 18 in the range 340-545 nm, was found 5.8x10 -5 einstein min -1 cm -2. Substrates were dissolved in water at ppm levels, under the solubility limit, 19 by putting the required volume of the stock solution in a volumetric flask, evaporating the solvent at room temperature under nitrogen stream and then by adding water till to the required volume.

The suspensions were stirred overnight.

Analytical determinations of substrate concentration in these solutions assured that quantitative transfer was achieved. All the irradiation experiments were carried out at 0.5 gL-1 of TiO 2.

Analytical determinations. The HPLC determinations were carried out with a Hitachi model L6200 pump with UV detection (model L4200), equipped with a bonded phase octadecylsilica column (LiChrospher C18 Merck, 125 mm long, 4 mm I.D., 5 /~m packing).

The primary degradation of triazine herbicides was followed by direct injection of the

filtered (0.45 /~m) irradiated sample; UV detection was performed at 240 nm; mobile phase was 65% methanol / 35% water at 1 mL rain -1 and retention times were: simazine (compound S in Chart I) 2.9 rain, propazine (compound P) 6.0 min and atrazine (compound A) 4.0 min. For the analysis of samples irradiated for short times (range of minutes, see figures 3,4,6,7 and 9), an extraction in ethyl acetate was carried out before GC-MS analysis. 1 /.~1of sample was injected splitlass (30 s) into a SE-52 capillary column (30 m, 0.32 mm). The other end of the column was inserted directly into the ion source via a heated transfer line. The oven temperature was programmed as follows: isothermal at 60"C for 1 rain; from 60"C to 300°C at 10*C/min; isothermal at 300°C for 10 min. Electron impact (EI) and chemical ionization (CI) mass spectra were obtained using a Finnigan-MAT 8400 double-focusing reverse geometry mass spectrometer, interfaced to a Varian 3400 gas chromatograph and controlled by a SuperIncos data system.

The EI spectra were recorded at a source temperature of 200°C,

setting the electron energy to 70 eV and the accelerating voltage to 3 kV. The magnet was scanned from 29 to 800 Da in 0.8 s (1.0 s/cycle) and the resolution was set to m / 6 m = 1000 (5% valley).

894

In CI experiments, isobutane was used as the reagent gas at a pressure of 0.3 mBar. The mass spectra of positive ions were recorded. The electron energy was set to 100 eV and the scanned mass range was 60-1000 IDa. The extraction in organic solvents of compounds formed after long irradiation times and therefore the gas chromatographic-mass spectrometric analysis, is prevented by their large hydrophilicity.

Ammeline

(compound VI), ammelide ( compound VIII) and (iso)cyanuric acid (compound X) (see Scheme V) were detected by ion interaction chromatography with the immobilized reagent technique (IIIR). The reverse phase column was loaded with didodecyldimethylammonium bromide (DDDMAB) by fluxing 30 mL of a lxl0 -2 M DDDMAB solution in acetonitrile/water 25%/75% at 1 mL min -1 and washing with 30 ml of water. A 5x10 -3 M phosphate buffer solution at pH 7.7 was used as eluant at 1 mL rain -1. The pH and the concentration of the buffer used as eluant were optimized to give reliable analytical conditions.

The complete elution of

bromide ion from the column required a conditioning time of about an hour and is evidenced at 220 nm by a baseline decrease followed by further stabilization. The retention times were 2.0, 2.9, 5.0 and 6.4 minutes for solvent front, ammeline (VI), ammelide (VIII) and cyanuric acid (X), respectively (UV detection at 220 nm). The formation of chloride ions was monitored by suppressed ion chromatography, using a Biotronik IC 5000 apparatus equipped with a BTIAN separation column (20 cm long, 4 mm I.D., Biotronik) and an alkaline buffer eluant containing K2CO 3 (3 raM) and KOH (1.5 mM) at the flow rate of 1.5 mL min -1.

RESULTS

StoichiQmetrv. In the previous report 12, the final products originating from the photocatalytic degradation of s-triazine derivatives have already been shown. At long irradiation times, the process leads to the formation of cyanuric acid (X), as the major final product, and stoichiometric chloride for 2-chloro-s-triazines together with nitrate corresponding to the two amino groups in 4 and 6 positions of the triazine ring.

Intermediate identifications. Compounds I, III, V, VI, VIII, X and XVIII have been identified as intermediates through HPLC analysis of the slurry at different irradiation times by comparison with the retention times of the pure products under the same conditions. Other sample constituents, arising from photodecomposition of the various starting materials, were identified from their EI mass spectra by (i) comparison with pure standards, when available; (ii) comparison with published spectra either from databases or research papers and (iii) careful interpretation based on the fragment ions formed in the EI induced decomposition. Further information can be obtained by the CI mass spectra 20,21

In these experiments, the base peak

corresponded to the protonated molecular ion for all the substances investigated, allowing an easy determination of the molecular weight both for major constituents and trace compound. This was particularly useful for molecules such as XIV and XV where extensive fragmentation prevents the detection of a significant abundance of molecular ion in the EI mass spectrum. In

Figures

1 and

2

the

EI

mass

spectra

are

depicted

for

starting

materials

and

various

photodecomposition products identified in the reaction mixtures. These spectra show that the fragmentation of most molecular ions mainly occurS by competitive ruptures on the substituents of the triazine ring.

I

'

68

'

9i104 122132 138 158

N

1O0

I

N.-~-.N

Cl

'

'

'

'

'

158

150

I

'

'

'

200

m/z

....

215

m/z

215

] IL '8,7200

145 172

'

200

,~[ ,.,,, I!, ,,..,d., L.! ,.. !,. J, ]J.,..IL :glLj,.k !, 50 100 150 200

I

c..c,.., \,J-,,coc,.

'

,. [ .,,,iJ,., IL, I, ~,,.. ].. l,,, d,~7

50

i3

58

H3C/

N'~N II HCHN~ . ~ NHCH2CH3

H,C\ L

OI

250

C

250

A

0

50-

100 -

50-

100 -

.l.

50

I

? '

Cl

'

'

I

,

,

50

I

'

'

'

'

'

'

schemes II and IV, respectively).

I

150

'

'

100

I

'

'

[li5

138 158

'

'

','

201 ,

200

,

200

1~6 IL

'

'

.~oI

'

,

m/z

,

m/z

'

,

I

250

250

I

D

'

B

c o m p o u n d s II and XI in

150

I

'

173

'

138 158 /

la2/14s I d,

, ,Ih, I,,L........,,,,,,hIJ I

3,0 jil ~!,.,i,,. 91104

68

'

,,c.o

100

186 173

....Lt,LJ..,,.,,,J,J,I,,,L,Ihl

N-'~L-N

'

,. IJ

/

68 85 96

c..c.... \.J-

,o

44

CI

Figure 1. Electron impact mass spectra of atrazine, simazine and of two degradation products of simazine ( see

0

50

100

50

100

O0

896

× X

~..O

>

121

m o

_

N

~ ~-~~

>N

to

E O "O

cO

E

O O tXl

o

0 Q.

E

cO t'M

8

~J

O LID

O -

I-

lid

o ~

O

r~

o-to \0 / -r Z

0 ~

--: -

Z -r /(.)~ -r i

,

,

,

,

O -O

o- r

o

-r Z

"~-~ 0

.E

i

~

0 0

o Z

cO_ ~.).

O LO

o "tO

g

~

1i

,

,

,

,

t

O O

;

,

,

,

i

,

,

,

,

o

0

O If)

o

O r

o

U'~

O t/3 Od

o

.ca:

i

_ _

00 c,J~ t~M

E

E

o

..E N

O O

o° _ _

04

O c~

O

~oZ_

o: o

E

olo

O "-(~/ l-O I

0 .~ ~------

0 0

Z

0 0

z%

0 0

I Z

O O

E

O

(-i

o~

~O Z I-

co

cO

Z 1-

O -~i

O O

,

,

,

,

,

"I,

,

,

,

i

O LO

,

,

,

,

,

,

,

,

i , , , , i , , , , i . . , , ,

,

O

o o

o

''

o

897

AU mass spectra display several common ions (m/z= 104, 110, 130, 145, etc.), which correspond to fragments retaining

the

triazine

moiety.

Obviously,

these peaks

provide

no

information on

the

photochemi~lly modified substituents of the ring. In contrast, other spectral features (fragment ions and neutral losses) point out that some specific groups are included in the molecular structure.

For example, the peak at m/zffi 58 Da is observed only when an

isopropylamino- group is linked to the triazine. Note that the isobaric acetylamino- group does not produce the ion m/zffi 58 (Fig. IC). Likewise, the presence of an ethylamino- group in the structure is often evidenced by a rather abundant ion at m/zffi 44 Da (Fig. IB, IC, ID and Fig. 9 in Ref.13). The mass spectra of triazines substituted with a formylamino- group show an intense [M-28] + peak, corresponding to the facile loss of a CO molecule with rearrangement of a hydrogen atom; consequently, the molecular ion signal is rather weak (Fig. ID, 2D). The cleavage of the amide bond in acetylamino-substituted rings is less favorable. Thus, the molecular ion is abundant and alternative fragmentations compete with the expected losses of the acetyl radical ([M-43] +) and ketene ([M-42]+).

The latter peaks originate from either

the homolytic cleavage of the amide bond or a four- or six-membered rearrangement (Fig. IC and Fig. 7 in Ref. 13). It is noteworthy that the two processes occur to a similar extent in both examples considered.

Scheme I. Proposed reaction pathways of isopropyl degradation.

,,01% OH /OH3 02. "-NH-CH,,cI_I 3 " -NH-CLCI_I3

~ -NH-?/'OH3CI-~

OH~

-NH-OH:;: H.zO1 -NH-CH#I'I'zOH 0~2

01"t3

02

O'-OH

/CH2-O-OH l, -NH-OH\oHa / ehydration ,,CliO02

-NH-OH,,c~

-NH-CO-CHa .COOH

~-NH-CH\cHa ~I-C02 -NH-Ct-~-C~

-NH-OHO' ~

As shown in Scheme I, a possible preliminary step of the photocatalyzed decomposition of an isopropyl substituent consists in the oxidation of one methyl group to the corresponding alcohol or aldehyde (see compounds XIV and XV, respectively). The resulting products fragment extensively upon EI excitation by loss of the hydroxymethyl- or formyl- radical, respectively. Cleavage of the C-C bond leads to an extremely

898

stable ion, in which the charge is delocalized on the adjacent nitrogen and the conjugated aromatic ring (Fig. 2B, 2C).

Once it is ionized, XV promptly releases the formyl group, giving the fragment ion m/z 214 Da

(Fig. 2C). The same fragment is also produced from compound XIV by the loss of a -CH2OH radical, while the molecular ion can be barely detected (Fig. 2B). In this spectrum, the occurrence of the [M-I8] + ion (m/z 227), corresponding to the neutral loss of a water molecule, is a further indication that a hydroxyl group is present in the structure. The

products

found

in

samples

and

their

relative abundance

are

fully

consistent

with

the

photodegradation mechanisms proposed, viz. no product expected based on the photodecomposition pathway is actually missing, and the product distribution observed clearly correspond to the relative favorableness of competitive degradations. For example, in the decomposition of atrazine the identified products originate from either the characteristic oxidation of the ethyl or the isopropyl group. The same oxidation processes have been previously recognized for the symmetrical structures of simazine and propazine.

The abundance of the

corresponding products suggests that the isopropyl group is more readily oxidized than the ethyl. In fact, III is much more abundant than XVIII, although XVII and X X are produced in larger amounts than XXII and XXIII. Deeradation kinetics.

The evolution of the concentration of initial compounds, identified intermediates and the final product (cyanuric acid) was followed as a function of the irradiation time. The results are displayed in Figures 3-10.

15

25

A

B

O O X

(~\\comp.(lll) I \ i "e -V1 I

20 o

LO

O

10

rN

o~

X

E

I / I / I /

CO

(N eU

15

E

Ii1

5

E

\

I [~]

cO £3.

\

10

. I

GO

\

1/'

5

"'~, ........

I

0

I

I

I

i

10 20 30 40 50 60

Irradiation time (min)

Figure 3.

\ .......

O O

0

\\ \\

,~...comp.(I)

0

I

0

I

I

I

I

10 20 30 40 50 60

Irradiation time (min)

Simazine photodegradation in presence of TiO 2 0.5 gL -1 (A) and intermediate

compound formation at short irradiation time (B).

899

15

LO

0

p.(lll)

10

X -C] ¢-

o

Q.

E

5

-E ]

0

0

0

)

i

0

10

20

30

Irradiation time (min) Figure 4.

Photodegradation of 1,3,5-triazine-2,4-diamine-6-chloro-N-ethyl ( compound III) and

CI- evolution in presence of TiO 2 0.5 gL-1.

DISCUSSION

Primary events in Dhotocatalvtic process over TiO 2.

The photocatalyzed decomposition of organic molecules on irradiated TiO 2 in aqueous aerated solutions occurs through a sequence of reactions involving different reacting species. 15'19'22'23 Apart from direct reactions of the various organics with the photogenerated charge carriers (holes and electrons), strong evidence is reported in the literature for reactions involving hydroxyl and other oxygencontaining radicals. The latter reactions could in principle occur either at the surface of the catalyst or in the homogeneous aqueous phase. 24 The availability of readily reducible and oxidizable species at the surface of the catalyst inhibits the recombination of the photogenerated charge carriers that otherwise would take place. The organic radicals so formed give rise to further intermediates progressively degraded, until stable compounds are formed.

It has

been observed that many organic compounds are completely mineralized to CO 2 and, in case, anions (e.g. chloride, sulphate, etc.). 15,19,22

This process requires the consumption of oxygen or of oxygen releasing

compounds.25,26 For the substances presently investigated, only a partial conversion to CO 2 has been observed, while cyanuric acid is the stable final product most abundantly obtained. 12

900

CI

OH,CHiN~ N ~L NHCH2CHs CI

OH

N~N

N~'~N (I)

;bsl

(Xl)

OH

N~''N

"~2

asl

~1

b/

(IV) lbsa OH

Cl

aa

CH,CH2HN ~.~N./~1" NHCOCH,

c,

bso~

(it)

N~J'-..N CH,OHxk-IN~'-~N~" NH,

OH,CHiN-J'-~.~N..~L-NH,

(Vl) ;c'

I

CH,CHz~rtN-J'~N~L NHCHO

CHsCH~N-J~.~N~j" NHCH,CHs

N~N

ds2

(ill)

CI

N~N J I

~L~N/J~ NHOOOH3

OHOHN

(xll)

C~ (V)

Scheme II. Degradation pathways of simazine (first part). Beside each structural formula the roman reference number used in the text is reported. Simbols close to arrows indicate the kind of reaction (a: dehalogenation, b: dealkylation, c: deamination, d: alkyl chain oxidation); the first subscript refers to the substrate to which reaction pertains ( A: atrazine, P: propazine, S: simazine) and the second one enumerates progressively the pathways. Last part of the scheme is depicted in Scheme V.

901

Simazine Degradation. The major photocatalytic degradation paths of simazine are depicted in Scheme II and V, as deduced from chromatographic analysis and mass spectrometric data. As mentioned in the previous section,

N-

ethylammelide (originating from IV) and 2-chloro-4-hydroxy-6-ethylamino-l,3,5-triazine (originating from III) are present only in traces and not shown in the scheme.

These compounds together with other

dechlorinated derivatives were also found in the treatment of atrazine with UV/ozonation. 27,28 From the comparison of Figure 3A and 3B, it can be deduced that path as0 accounts for ca. 10% of simazine disappearance (after 15 minutes ca. 75% of simazine is degraded, but compound I and chloride ions reach the stoichiometric molar ratio of ca. 0.06 and 0.10, respectively). Figure 3B shows also the concentration variation as a function of time for compound III, which is the most relevant intermediate after 5 rain of irradiation, but decreases rapidly at longer irradiation times.

Compound III can originate from deacetylation

of compound II and through the formation of XI and subsequent oxidation/decarboxylation.

Compound III

disappears even faster than simazine as shown in Fig. 4, but in this case dehalogenation route seems to play a more important role (note that after 15 minutes, when the concentration of III is reduced at ca. 5% of the initial one, chloride ions are ca. 30% of stoichiometric).

~t

o

75

~t

viii

A

o

6O

40

I~

~

~ ~,~

..A.. /

~-

~ot :/v

/ x

I

B ,

v ~,, ',,, /,~/ ,,"'.....~--.....

020

,~,

~

:

-"... D

A

I 0

5

12

16

Irradiation time (hours)

0

5

x

'

14

16

Irradiation time (hours)

Figure 5. Intermediate evolution in the photodegradation of simazine depicted in Fig.3 (A) and of compound III depicted in Fig.4 (B); ammeline (compound VI), ammelide (compound VIII), 1,3,5triazine-2,4-diamine-6-chloro (compound V) and cyanuric acid (compound X).

902

Then, the decomposition at the early stages apparently proceeds through a competition between the chlorine substitution (at position 6) by a hydroxyl group (a wall-known process in the photocatalytic transformation of other haloaromatic compounds) 15,16,22 and the oxidation of the ethylamino groups at the 2 and 4 positions of the 1,3,5-triazine ring. The mechanism of the latter process, on the basLs of the product identified (II and, in a lesser extent, XI), can be attributed to the reaction of a hydroxyl radical on the methylene carbon of the ethyl group. The free radical formed thereafter can react with molecular oxygen, as follows: 0-0 •

o2

"OH -NH-CH 2-cH 3

> -NH-CH-CH 3

I > -NH-CH-CH 3

O-OH

I > -NH-CH-CH 3

the hydroperoxide can either dehydrate or rearrange, yielding II and XI, respectively. Further oxidation steps can cause carboxylic group formation.

The subsequent photocatalytic decarboxylation (also a well-known

process) leads to dealkylated compounds. 19'23'29'30 The onset and decrease of intermediates V, VI (ammeline), VIII (ammelide) and the final product X (cyanuric acid) is depicted in Fig. 5A and 5B for the degradation of simazine and compound III, respectively. As expected, the general trend is similar for simazine and III, being compound V the intermediate appearing first and ammelide the intermediate that reaches the highest concentration.

Interestingly, compound XII

(although detected in low concentration) suggests that V can originate also through the sequence ds0 + ds2 + dss.

Since it has been shown that path aa (giving ammeline from V) is negligible (essentially only traces of

ammelide are observed in the photocatalytic degradation of V and the formation of cyanuric acid goes mainly through path c + c"' + aa'), 13 ammeline formation in simazine degradation (up to the maximum value of ca. 15%) has to occur mainly through the sequence as0 + bSl + bs3 from simazine and aSl + bs3 from III. The final slow formation of cyanuric acid from ammelide and V has been reported. 13 Several intermediates present in Scheme II have been identified in previous studies on s-triazine herbicides transformations. The main differences arise from the relative importance of the different pathways. While soil transformation occurs mainly through substitution of chlorine with a hydroxyl group, 3 microbial degradation takes place essentially through dealkylation paths. 7 Metabolic degradation observed in chicken goes in both directions.5 The action of chemical oxidizing agents such as ozone27'28 and Fenton's reagent 31 (ferrous sulfate hydrogen peroxide) similarly lead to extensive dealkylation together with partial hydroxy substitution of the chlorine atom.

ProDazine Deeradalion. Scheme III represents the degradation routes for photocatalytic degradation of propazine. Although the scheme is similar to that for simazine (propyl instead of ethyl substituents at the amino groups), some qualitative and quantitative differences can be observed in the plots of intermediate concentrations versus time. Compounds XIV and XV (Scheme III), that are amongst the possible compounds formed through oxidation of the propyl group, can be detected after short irradiation times (5 and 10 minutes). same irradiation period compound XVIII is the major intermediate detected.

After the

H=C~

OH

lbp1

N

\CH~

$c'

NH=

,t aa

....

x

I\

H

~

. \

_ s

,,Cl.~

(v) ~c

\

bp2

'-

dA3

\"

d. v \ap2

H~I'~N.~"NH, =

-(xx)

H,c' ,"

N/~"N

dp1 ~ ~ I 'i, o ~

H3C,, 'J'

.~. (P)

\

\

"CHI

"J"-N

(XV,=A)

Legend as in Scheme II.

(xvltt)

~HN ~'~NH, (XVll)

b

~c

a.,,.2c',

~c,'_ ?

" "~N

H3C', ~,~ I[

d,,c,, Ix,vl

N,

N,,,'~',,.,N

o,

,,~L"N /Clio HCHN-"~..~NHCH H,c' ", "c~ dp4 ~ (XV)

H,,C/

Degradation pathways of propazine (first part). Atrazine is found as byproduct (compound XVI).

H~ ~

N~',,,N

OH

lbe3

N

N-'2"L.N H3C~ I HCHN'J~.~. ~ NH,

Scheme III.

(Vl)

(XlX)

(XIII)

1.130/

I~C., ~.~ I I /CH3 HCHN"~. ~ NHCH

J"'N

OH

9O4

Other interesting compounds detected at non-negligible concentration arc XVII and XX, present also in the atrazine degradation (see Scheme IV). degradation side-product, present in traces.

It is worth noting that also atrazine is likely a propazine Different pathways (some of which not depicted in the scheme)

can be responsible of the presence of these derivatives.

From the products identified above, the mechanism

depicted in Scheme III can be hypothesized; the reaction sequence proposes an initial attack of hydroxyl radicals (or other hydrogen atom abstracting species) to the side chains to form primary radicals, which further react with 0 2 and H20. Dehalogenation paths in Scheme III (ap0 and apl ) are less relevant than in the simazine degradation. In fact, inspection of Fig.6A and 6B shows that after 15 minutes more than 90% of propazine is degraded, but only 5-6% of chloride is formed.

Likewise, Fig.7 reveals that after 15 minutes more than 95% of compound

XVIII is disappeared and ca. 12% of chloride ion is formed. Thus, the rapid formation and disappearance of compound XVIII, together with the slow formation of CI-, support this proposal. Moreover, compound XVIII disappears slightly faster than its ethyl homologous compound III does, as shown by the lowest maximum in Fig.6B (ca. 10%) and Fig.7 in comparison with Fig.3 and Fig.4, confirming that the propyl group is more easily oxidized than the ethyl group.

12

A

O O

20

X

CF a j ~ [ ] o

Q.) oN

O

'1=-"

J

15

c0mp.(XVlll)

s s s J

X r-'-i

O

r" N Q. O

13_

10 !

I,,--I

"0 c"-I 0

4

El_

I,--,..I

[

5

,t" ,t

,

I

I

E o 0

0

I

0

5

I

I

I

I

10 15 20 25 30

Irradiation time (min) Figure 6.

0

I

0

I

!

10 20 30 40 50 60

Irradiation time (min)

Propazine photodegradation in presence of TiO 2 0.5 gL -1 (A), CI- and intermediate

compound XVIII evolution (B).

905

15 2~ O x

10

"o c 0 ~L

mp.(XVlll)

O o

k~.Ey

E

0

o r .... r.

10

20

30

Irradiation time (min)

Figure 7. Photodegradation of 1,3,5-triazine-2,4-diamine-6-chloro-N-(l-methylethyl) (compound XVIII) and C1- evolution in the presence of TiO 2 0.5 gL-1.

o o

60

rr

A

"&

,-x

50

._ N

40

0 x

,f,.. V

Pf

B 60

%.

z%..

_~ 50

O

.a_,

o='E

0 ,o

>

30

20

X£~_ 40 E

X'.. i

/

";. .,

O

"

o ao

ti ~,z~ "e~'....'-,, viii

0

-j

"',

10 ~S,I,I~

0~

F

/

v~"\ ,

~ .. ..12

,

,

5

10

..-43-7"

oE 1° 1 J~"~-v-'~.,

.o

"@ 15

Irradiation time (hours)

"

'1

0 IP-,-e--~..e-~ 0

2

5

14 16

Irradiation time (hours)

Figure 8. Time evolution of ammeline (compound VI), ammelide (compound VIII), 1,3,5-triazine2,4-diamine-6-chloro (compound V) and cyanuric acid (compound X) in the photodegradation (A) of propazine depicted in Fig.6 (A) and (B) of compound XVIII depicted in Fig.7 .

(IV)

~

~

N"~""N

.._

~lr

~

~ "

ap1

(vl)

aa

~

//bs 4

(''')

Hi~l"L'~N~ NHCi'IICH: CI j l ~ N/~L"N

,~c

lUA4 I'

,N~ "N

H~~N~ NH, (v)

bp2~

~xv,,,)

d A1

(XXll)

OHHC~HN~,~-.NHCH.CH:

(xx)

I

HHCHN-~N,/~NHCH:CH.

Scheme IV. Degradation pathways of atrazine (first part). Legend as in Scheme II.

c'[

as'

~

N

HCI-IN~./~'NI-12

H,C ~

N/~"N

I~(~HN"L~N"~'NH'

/b

H~C' - -

x~

~d A3

^ 2 ~ Cl /UA3

(xv")b\

N? ",N ~

\b^,

\

H,c' N

N.~N ~ A 2

//d

.~,

cl

HOH2C\ -j- j j

dAo

~ N N H:C\ ~ I H:C\ I HCHN.-L~.,...~k.NHCOCH: HCHN--~~k. NHCHO

/ (x,x)~

/

NHCH2CI'I3

OH ~

i~'~.~

/

~

\

/

/

/

(XXl)/

H:cH?HN"~N/~N "HCH~X~H:

I-I:C\ ~ II

bAo/

/

aA

,-~'-, ~ ' ~ , N ~I.I:C./ ~ NN, = , . c , . ~

OH

,Q (A)

907

l

from previous schemes I

(Vl)

OH

N.J"-.N

aa

I

j

C

(Vlll)

OH

aa'

IC"

(Vll)

N~"-N

IC'" ~,

Scheme V.

C

c,

N-"~N HO-~.~N/~N "H,

(X) N..~ N

Cl N.J.~,N (V)

aa"

N I N (IX) .O\NJ-O=

Degradation pathways for simazine, propazine and atrazine (last part). Legend as in

Scheme II.

These considerations suggest that the degradation paths occurring through compounds XVIII and X X are predominant.

Accordingly, compound V is the main intermediate detected in the degradation of both

propazine (Fig. 8A) and compound XVIII (Fig. 8B) and ammeline has been produced in lower amount in comparison with the case of simazine.

Ammeline originates from propazine through routes ap0 + bp1 + bp3

and from XVIII through apl + bp3. Atrazine deeradalion.

The degradation of atrazine (the asymmetrically substituted compound) can be rationalized in the light of the previously cited findings. In Scheme IV, the main degradation pathways are depicted.

While compounds

XXI, XXII and XXIII are peculiar of the atrazine photodegradation, compounds XVII and X X were detected also in the propazine decomposition. Anyhow, the chemistry of the atrazine degradation processes is the same discussed for simazine (on the ethyl substituent) and propazine (on the isopropyl substituent).

908

25

10

A

O O x

B

cl[//

20

/ cD

tt)

O x

N

,e'-,e

15

E U

t'--I

E O O.

<

E

/

jz , d

<

cD

/

I

10

2f'-x3~... ',, 5

XVlll

'~...,." ,

O o

0

5

10

0

15

20

30

Irradiation time (min)

Irradiation time (min)

Figure 9.

10

Atrazine photodegradation in the presence of TiO 2 0.5 gL =1 (A) and evolution of

intermediates (B): compound III, compound XVIII, compound I and CI-.

o 100 O 'I"-X ~

~

.___

X<~ >

5o 4o

/

N

~

3o

~E O

2o

E

lO

<

O o

0 .--ca , 0

, 2

,'-~-, 4 1570

Irradiation time (hours)

Figure 10. Ammeline (compound VI), ammelide (compound VIII), 1,3,5-triazine-2,4-diamine-6chloro (compound V) and cyanuric acid (compound X) evolution in the photodegradation of atrazine depicted in Fig.9a.

9O9

Dehalogenation route aA can account for ca. 8%, as from comparison of data in Fig 9B and 10. In the meanwhile, compound III reaches higher concentrations than XVIII, as expected if the attack to the isopropyi group is favored. The behavior of intermediates reported in Fig. 10 shows that several paths contribute comparably to the progress of the degradation: thus, V originates through paths bA2 (or hA3) + bp2 and hA4 + bs4; ammeline through paths involving dehalogenation and further dealkylation of intermediates III, XVIII, IV and XIX.

CONCLUSIONS From the present study it can be concluded that the photoeatalytic degradation of 2-ehloro-s-triazine pesticides occurs through dehalogenation routes only in a very limited extent.

Alkyl chain oxidation and

subsequent dealkylation represent the main degradation path, followed by the formation of hydroxy derivatives, up to the final product, namely cyanuric acid. The choice of proper degradation conditions and of the analytical procedures allows a detailed identification of the reaction intermediates. The same compounds found in this in vitro study are likely to be present also in other transformation routes taking place in the environment or in metabolic processes. It is noteworthy that upon the change of the alkyl substituent in the side amino groups, viz. from ethylethyl (simazine), to propyl-propyl (propazine) and finally to the asymmetric derivative bearing an ethyl and a propyl (atrazine), relevant variations are induced in the relative role of different pathways depicted in the degradation schemes.

ACKNOWLEDGEMENTS This research received generous support from Eniricerche and from EEC (Contract EV4V-0068-C(CD)).

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