OH-radical induced degradation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methyl

OH-radical induced degradation of2,4,5-trichlorophenoxyacetic acid

(2,4,5-T)and4-chloro-2-methylphenoxyacetic acid(MCPA):A pulse

radiolysis and gamma-radiolysis study

Robert Zona a,Sonja Solar b,n,Knud Sehested c

a HBLVA Rosensteingasse,Rosensteingasse79,A-1170Vienna,Austria

b University of Vienna,Faculty of Life Sciences,Department of Nutritional Sciences,Section Radiation Biology,Althanstrasse14,A-1090Vienna,Austria

c Risoe National Laboratories,DK-4000Roskilde,Denmark

a r t i c l e i n f o

Article history:

Received21September2011

Accepted21September2011

Available online29September2011

Keywords:

2,4,5-Trichlorophenoxyacetic acid(2,4,5-T)

4-Chloro-2-methylphenoxyacetic acid

(MCPA)

OH-radicals

Pulse radiolysis

Gamma-radiolysis

Degradation products

a b s t r a c t

The reactions of d OH,H d and e aqàwith2,4,5-trichlorophenoxyacetic acid(2,4,5-T)and4-chloro-2-

methylphenoxyacetic acid(MCPA)were studied by pulse radiolysis.The site of d OH-radicals addition to

the aromatic ring of2,4,5-T was found to be—C1:$18%,C2/C4/C5:total$28%and C3/C6:total$41%.

The overall rate constants with OH-radicals were k(d OHt2,4,5-T)?6.4(70.5)?109mol dmà3sà1and

k(d OHtMCPA)?8.5(70.8)?109mol dmà3sà1.The radiation induced decomposition of the pesti-

cides,chloride-and product formation(phenolic compounds,aliphatic acids)was studied by gamma

radiolysis as a function of dose.A mechanism for acetate formation is discussed.The presence of oxygen

during irradiation affected the decomposition rate only indiscernibly,however,chloride elimination,

ring fragmentation(formation of aliphatic acids),TOC-and toxicity reduction were strongly enhanced.

For complete removal of500m mol dmà3herbicides a dose of$4kGy was https://www.360docs.net/doc/b13533659.html,ing air

saturation during irradiation a reduction of37–40%of the TOC was observable at5kGy,detoxi?cation

(luminescence inhibition o20%)was achieved with10kGy.

&2011Elsevier Ltd.All rights reserved.

1.Introduction

In the last decades phenoxyacetic acid derivatives have been

one of the most commonly applied herbicides in agriculture,and

consequently,they can be present together with their degrada-

tion products in natural water resources.Contamination of waters

and soil can also occur from pesticide discharges from manufac-

turing plants,from storage sites and run offs from vegetation and

soil.For remediation of waters and waste advanced oxidation

processes(AOPs) e.g.ionizing radiation(Zona et al.,2002a;

Drzewicz et al.,2004;Peller and Kamat,2005;Bojanowska-

Czajka et al.,2006),UV-radiation/ozone/photocatalysis(M¨uller

et al.,1998;Benitez et al.,2004;Zertal et al.,2004)sonolysis

(Peller et al.,2001;Ueno et al.,2009),electrochemical degrada-

tion(Brillas et al.,2003a,2003b)as well as biodegradation

(Kuhlmann and Kaczmarzcyk,1995;Celis et al.,2008)have been

suggested.All AOPs are based on in-situ oxidation by d OH radicals

(e.g.Peller et al.,2004).Many microorganisms causing biodegra-

dation have been isolated(e.g.Kitagawa et al.,2002).In this

metabolic pathway hydroxylation has been found to be an

essential step.For2,4-D the following sequence of degradation

products was reported:2,4-dichlorophenol,3,5-dichlorocatechol,

2,4-dichloromuconate,2-chlorodienelactone and2-chloromaley-

latcetate.MCPA follows an analogous route(Smith and Beadle,

2008).In the present study water and ionizing radiation were

used as d OH radical source.

The objective of this report was to elucidate the mechanistic

details of d OH-radical attachment on the aromatic ring of2,4,5-T

and MCPA,and to analyze the consequently formed products.

Further,the effect of oxygen during irradiation on the degrada-

tion-and chloride elimination rate,on the aliphatic acid forma-

tion,as well as on the mineralization process was assessed.In

addition,the water quality after irradiation was determined by

toxicity measurements.

2.Experimental

2.1.Chemicals,solutions and abbreviations

2,4,5-Trichlorophenoxyacetic acid(2,4,5-T),4-chloro-2-

methylphenoxyacetic acid(MCPA),2,4,5-trichlorophenol(2,4,5-

TCP),4-chloro-2-methylphenol(4-CMP),2,4-dichlorophenoxya-

cetic acid(2,4-D)and all chemicals used were of highest purity

Contents lists available at SciVerse ScienceDirect

journal homepage:https://www.360docs.net/doc/b13533659.html,/locate/radphyschem

Radiation Physics and Chemistry

0969-806X/$-see front matter&2011Elsevier Ltd.All rights reserved.

doi:10.1016/j.radphyschem.2011.09.012

n Corresponding author.Tel.:t431427754970;fax:t431427754964.

E-mail address:sonja.solar@univie.ac.at(S.Solar).

Radiation Physics and Chemistry81(2012)152–159

grade available (Aldrich,Merck).For sample preparation water was distilled and further puri?ed using Direct-Q3UV puri?cation system (Millipore,Bedford,MA,USA).The solutions (500m mol dm à3i.e.100ppm MCPA and 128ppm 2,4,5-T)were purged with N 2O (Stickoxydul,p.n.;Messer Austria,Vienna,Austria)for about 30min,or used directly (aerated solution,A),saturated with air (AS)and/or saturated with oxygen (OS)during irradiation.The pH ($9)was adjusted with NaOH.2.2.Irradiation facilities,water radiolysis

2.2.1.Pulse radiolysis

The 10MeV Linac at Ris?,(Haimson Research Corp.,HRC-712),which provided pulses of 0.2–1m s,was used (Sehested et al.,1975).The detection system consisted of a 450W xenon lamp,a quartz cell (light path 5.1cm),a Perkin-Elmer double quartz prism monochro-mator and a photomultiplier (IP28)equipped with a LeCroy digital oscilloscope,Model 9400,and an IBM PC/AT3computer on line.The irradiation doses were 4–10Gy per pulse.For determination of the

absorbed dose a hexacyanoferrate(II)dosimeter,G (d OH te aq à

)?6.1?10à7mol dm à3J à1,e 420?1000dm 3mol à1cm à1(Schuler et al.,1981)was applied.Some experiments were carried out with the ‘‘FEBETRON 708’’,a dual electron beam accelerator,delivering two equal intensities from synchronized 3ns electron pulses of 0.8MeV and designed for irradiation of a sample in a quartz cell from opposite sides (Quint and Getoff,2002).

The transients absorption spectra are presented in OD/cm normalized to a dose of 10Gy.The measuring points represent mean values of at least three measurements.The experimental uncertainty of the given rate constants and extinction coef?cients was about 710%.

2.2.2.Steady-state radiolysis

The irradiations were carried out with a Co-60-gamma source (‘‘Gammacell 220’’,Nordion International Inc.,Kanata,ON,Canada).In irradiation position the sample chamber of this gamma source is circular,surrounded by 16Co-pencils,therefore it provides a homogenous dose distribution.The dose rate was 76–87Gy min à1,which was determined by Fricke dosimetry using a radiation chemical yield (G -value)G (Fe 3t)?16.2?10à7mol dm à3J à1(Fricke and Hart,1966).

2.2.

3.Water radiolysis

The primary species formed by interaction of ionizing radia-tion with the water solvent are illustrated in gross reaction (1).The radiation chemical yields (G -values)of the primary radicals

are G (d OH)?G (e aq à

)?2.9?10à7mol dm à3J à1and G (H d )?0.6?10à7mol dm à3J à1.

H 2O àà!e à,g -rays

d H ,

e àaq ,d OH ,H 2,H 2O 2,H 3O t,OH

à

e1T

In N 2O saturated solutions e aq

à

are converted into d OH radicals,reaction (2).Thus,in the pH range 4–10,d OH radicals,G (d OH)?5.8?10à7mol dm à3J à1,and H d radicals,G (H d )?0.6?10à7mol dm à3J à1,are reacting with the solutes.At room tem-perature [N 2O]?2.8?10à2mol dm à3(Buxton et al.,1988).

e àaq tN 2O -d OH tOH à

tN 2,k 2?9:1?109dm 3mol

à1s à1

e2T

In the presence of oxygen d H and e aq

à

are scavenged forming formation of HO 2d and O 2d à

as shown in the following:

H d tO 2-HO d 2k 3a ?2?1010dm 3

mol

à1s

à1

eBuxton et al :,1988T

e3a T

e àaq tO 2-O d à2

k 3b ?2?1010dm 3

mol

à1s

à1

eBuxton et al :,1988Te3b T

HO d 2"H t

tO d à2pK ?4:8eGetoff and Prucha ,1983T

e3c T

Under these reaction conditions G (d OH)?2.9?10à7

mol dm à3J à1and G (O 2d à)?3.5?10

à7

mol dm à3J à1.In aerated solution [O 2]?0.25?10à3mol dm à3(8ppm)at room tempera-ture.Oxygen reacts very fast with carbon centered radicals by formation of peroxyl radicals,(R d tO 2)?ROO d ,with

k $1?109dm 3mol à1s à1

(Neta et al.,1990).2.3.HPLC analysis

Substrate degradation and aromatic product formation as a function of dose was determined by reverse-phase-chromatogra-phy (Hewlett-Packard series 1050and 1100,column:Spherisorb ODS 2RP-18(125?4mm I.D.,particle size:5m m)?tted with a guard column (4?4mm I.D.)of the same material;temperature:301C;injection volume:25m l;?ow rate:1ml/min;eluent:H 2O (0.1%w/v phosphoric acid)/CH 3OH ?60/40–50/50v/v.Detection:multiple wavelength UV-detector with diode array,detection wavelengths 210and 230nm and/or electrochemical detection,ECD (amperometric detection:glassy carbon working electrode,Ag/AgCl reference electrode and carbon auxiliary electrode;oxidation mode:t0.9V).

Quantitative analysis of chloride was carried out by ion chromatography (see later)and by HPLC,using an indirect absorption technique via phthalations (column:asahipak ODP-50,temperature:401C;injection volume:50m l,?ow rate:1.5ml/min,eluent:H 2O/CH 3CN/Hewlett Packard mobile phase,ratio 81/14/5,pH 8.6,according to the producer’s instructions).2.4.LC-ESI-MS analysis

For identi?cation of some phenolic compounds the mass spectra obtained by HPLC-ESI-MS analysis (M-1)mode were used.System:HPLC PE Series 200(column:Spherisorb ODS 2RP-18(5m m);temperature:301C;injection volume:50m l;?ow rate:1ml/min;eluent:H 2O (pH 3,adjusted with acetic acid/CH 3OH ?60/40v/v)and a mass spectrometer PE SCIEX API 150EX,operated in the negative ion mode by applying a voltage of 4kV to the capillary.The skimmer cone voltage was set at 25V.Ions were generated with electrospray ionization (ESI)using nitrogen as drying and nebulizing gas.Flow rate:300dm 3/h.Mass range 100.1–320.1m /z by 0.5amu.

2.5.SCIC (suppressed-conductivity ion chromatography)analysis Aliphatic acids and chloride were detected by high-performance ion chromatography using a Dionex Series 4500i ion Chromato-graph.Separation was performed on the IonPack AS11analytical column (4?250mm;Dionex)connected to the IonPack AG11guard column (5?50mm;Dionex);injection volume:20m l;?ow rate:2ml/min;gradient:sodium hydroxide from 0.75to 40mmol dm à

3.Detection:suppressed conductivity,ASRS-II,auto suppression recycle mode.

2.6.Toxicity measurements:

The acute toxicity of the aqueous pesticide solutions was determined using a luminescent bacteria test (LUMIStox measur-ing system,https://www.360docs.net/doc/b13533659.html,nge,DIN 38412L34,Dr.Bruno Lange Ges.m.b.H,Obergrafendorf,Austria).The bioassay was performed with the marine bacteria strain Vibrio ?scheri NRRL-B-11177provided with the test kit and following the manufacturer’s instructions (LUMIStox 300Manual).The salt content (NaCl)of the samples was 2%(w/v).Bovine catalase was added to each sample to

R.Zona et al./Radiation Physics and Chemistry 81(2012)152–159153

destroy residual H 2O 2,which is formed during irradiation.After cooling (151C)and incubating the test batches (30min),the bacterial light intensity was determined photometrically using a 2%NaCl solution as reference.From the change in the lumines-cence intensity the percentage inhibition after 30min incubation,I 30%,and the toxicological parameter EC 50(effective concentration required to reduce bacterial luminescence by 50%)were calcu-lated.An inhibition of r 20%can be regarded as non-toxic (Pudill,1992;https://www.360docs.net/doc/b13533659.html,nge,1998).

2.7.TOC measurements

Organic carbon measurements were carried out using a multi N/C 3000from AnalytikJena AG.The TOC was calculated from carbon dioxide formed by catalytic oxidation (8501C,O 2,CeO 2)and determined by a non dispersive infra-red detector.

All measurements were done at least in triplicate,the given results represent mean values.

3.Results and discussion 3.1.Pulse radiolysis

Pulse radiolysis experiments (N 2O saturated solutions,pH 8.5–9.5)were carried out to determine the spectroscopic and kinetic characteristics of the initial transients formed by reaction of d OH with the herbicides.The only reaction of the electrophilic d OH radicals with phenoxyacetic acids is addition to the aromatic ring forming hydroxycyclohexadienyl radicals (Peller and Kamat,2005).OH-adduct formation on position C6is shown in the following reaction:

OCH 2COO Cl Cl

Cl

COO

Cl

Cl

H Cl

O

H H -°OH

.

e.g.

-e4T

The addition of d OH to C1of 2,4-D has been found to be a major pathway (Peller et al.,2003;Peller and Kamat,2005).These authors established that the ipso attack of d OH to the ether functionality of 2,4-D is followed by homolytic elimination of the ether side chain.For 2,4,5-T this is shown in the following

reaction:

OCH 2COO Cl Cl

Cl

Cl

Cl

Cl

H OCH 2

O H .

--

+OCH 2COO

.

-

e5a ;b T

The transient absorption spectra of the d OH addition products

on 2,4,5-T (e.g.reaction (5a))and MCPA are shown in Fig.1A and 1B.The absorption maxima are 320nm,e max ?4800(7250)dm 3mol à1cm à1(2,4,5-T)and 305nm,e max ?6200(7300)dm 3mol à1cm à1(MCPA).

The kinetics of d OH radical reactions were determined from the concentration dependence of the growth of the absorptions at 320and 305nm using three different substrate concentrations (5–10?10à5mol dm à3).The overall second-order rate constants were determined from plots of the pseudo ?rst order growth rates versus substrate concentration,k (d OH t2,4,5-T)?6.4(70.5)?109dm 3mol à1s à1and k (d OH tMCPA)?8.5(70.8)?109dm 3mol à1s à1.

To investigate if the presence of oxygen affects the 2,4,5-T-OH-adduct absorptions experiments in the presence of N 2O/O 2(4:1v/v)and with different oxygen concentrations (described in detail for 2,4-D,Zona et al.,2002b )were carried out.The yields of the OH-adducts did not change and no reaction of the OH-adducts with O 2was detectable.

The absorptions at 400–450nm (Fig.1A and B)are character-istic for phenoxyl radicals.They are originating from the ipso-chloro-OH-adducts (e.g.reaction (6)),which are known to elim-inate HCl spontaneously (Latif et al.,1978)and which were observed also in the case of 2,4-D (Zona et al.,2002b ).

OCH 2COO Cl Cl

Cl

OCH 2COO Cl

Cl

Cl

OH OCH 2COO Cl

Cl

O

-°OH

e.g.

-.

.

-+ HCl

e6T

The initial yields of the chlorophenoxyl radicals were deter-mined earlier by our group,using TMPD for their reduction and determining the yield of the TMPD d tradical cation (TMPD tR–Cl–PhO d tH t-TMPD d ttR–Cl–PhOH),which has an absorption maximum at l ?565nm (e max ?12500dm 3mol à1cm à1,Fujita and Steenken,1981).The chlorophenoxylradical-and phenoxyl radical yields,respectively,relative to G (d OH)were 28.2%for 2,4,5-T and 12.7%for MCPA (Zona et al.,2002b ).

nm

O D /c m

0.000

0.0050.010

0.0150.0200.025

0.030

nm

O D /c m

0.00

0.01

0.02

0.03

0.04

Fig.1.Absorption spectra resulting from the reaction of OH-radicals with 2,4,5-T (A)and MCPA (B).

R.Zona et al./Radiation Physics and Chemistry 81(2012)152–159

154

In addition to the reaction rate constants with d OH,the rate

constants of the herbicides with e à

aq (observing the decay of its absorption at 720nm in Ar saturated solutions)and with H-atoms (observing the absorbance growth at l max ,2,4,5-T:350nm;

MCPA:345nm;using pH 1.2(H tte aq à

-H d ,k ?2.2?1010mol dm à3s à1,Buxton et al.,1988),and 0.5mol dm à3t-butyl alcohol as d OH scavenger)were determined at three substrate concentrations (5–10?10–5mol dm à3).A compilation of deter-mined and reported rate constants for three phenoxyacetic acids is presented in Table 1.

3.2.Gamma radiolysis

3.2.1.N 2O saturated solutions

2,4,5-T.Primary d OH-attack.To elucidate the primary patterns of d OH addition to the aromatic ring of 2,4,5-T the phenol-type compounds were analyzed using HPLC.A typical HPLC chromato-gram is displayed in Fig.2.The attribution to phenolic derivatives is based on LC-ESI-MS measurements,(M-1)mode.Besides product 2,4,5-trichlorophenol,produced by reaction (5a,b),two compounds (peaks 1and 3)with m /z 235and a mass peak

splitting into three peaks (ratio 9:6:1)were observable.They correspond to dichloro-compounds (C 8H 6Cl 2O 4),and are two of the three expected isomers 2,4-dichloro-5-hydroxy-,2,5dichloro-4-hydroxy-and 4,5-dichloro-2-hydroxyphenoxyacetic acid,origi-nating from the phenoxyl radicals formed after addition of d OH to the ipso-chloro-positions of the ring, e.g.reaction (6).Two compounds (peaks 2and 4)exhibited an m /z 269and a splitting into 4peaks (33:32:11:1);they have three Cl-substituents (C 8H 7Cl 3O 4)and the non-ipso-OH-adducts as precursor,e.g.reac-tion (4).To estimate the quantity of the phenolic components the calibration curve of 2,4,5-T was used.Some quanti?cations were also based on amperometric detection,since the amperometric signal intensity dependence upon substrate concentration was similar for many chlorophenols (e.g.2,4,5-TCP,2,4-dichlorophe-nol,4-chlorophenol,Zona et al.,2002b ).

Since in the absence of an oxidant the OH-adducts of aromatic compounds disappear very fast by unimolecular and/or bimole-cular reactions,irradiations in the presence of K 3[Fe(CN)6]/N 2O (dose:200Gy)have been performed.Under these reaction con-ditions the non-ipso-OH adducts of benzene derivatives are oxidized very effectively to the corresponding phenols (Buxton et al.,1986;Raghavan and Steenken,1980).This method had also been used in case of 2,4-D (Zona et al.,2002b ).The initial yields of the phenolic compounds resulting from d OH-attack on the posi-tions C3and C6were determined in the absence and presence of

Fe(CN)6à

.The concentration of the phenols containing three Cl-substituents increased from 8(peak 2,Fig.2)and 15.5m mol dm à3(peak 4,Fig.2)in the absence of K 3[Fe(CN)6]to 12and 35m mol in the presence of K 3[Fe(CN)6].These concentrations correspond to a total non-ipso-2,4,5-T-OH-adduct yield of 41%relative to the d OH radicals.Based on the electrophilic properties of d OH,it can be assumed that the concentration ratio of about 1:3corresponds to that of the adducts C3:C6.The amount of the detectable two C 8H 6Cl 2O 4compounds was 21m mol corresponding to $19%of d

OH.Since under the used experimental conditions the expected third dichloro-compound was unascertainable,the yields of the precursors,the chlorophenoxyl radicals,determined in an earlier investigation (Zona et al.,2002b ),were taken as base.They were found to be 28%of d OH.The concentration of product 2.4.5-TCP,resulting from d OH-addition to C1,was determined to be $18%of d

OH,which was in agreement with the earlier results (Zona et al.,2002b ).

Summing up the following distribution of the d OH-radicals on the aromatic ring of 2,4,5-T could be assessed:C1:$18%,C2/C4/C5:total $28%and C3/C6:total $41%,which gives a material balance of $87%of d OH.These percentages have to be taken as minimum yields,since it can be assumed that neither the phenols resulting from the ipso-chloro-OH-adducts nor those from the non-ipso-OH-adducts could be determined quantitatively.In addition,product 2,4,5-TCP is involved in the radiolytic degrada-tion process already at low doses,see Fig.3B.However,the result is comparable to that obtained for 2,4-D.For this compound the material balance was 84%,and the distribution of OH-radical addition was C1:17%,ipso-chloro-positions C2/C4:20%and C3/C5/C6:47%(Zona et al.,2002b ).

2,4,5-T Degradation.The degradation of the substrate together with acetate-and chloride formation as a function of dose are presented in Fig.3A,the formation of 2,4,5-TCP and of the aliphatic acid products is shown in Fig.3B.

From the bending of the degradation curve at about 0.3kGy it is obvious that at higher radiation dose degradation products are already involved in the radiolytic process.This is also apparent from the yields of the primary product 2,4,5-TCP,which reaches its maximum concentration at 0.75kGy,and from the formation of secondary products (formate,glyoxylic acid,oxalic acid)at 40.5kGy.The degradation yield of 2,4,5-T at 200Gy was

Table 1

Reaction rate constants for MCPA,2,4-D and 2,4,5-T with the primary radicals of water radiolysis.

Substrate k (d OH tS),

dm 3mol à1s à1k (H d tS),

dm 3mol à1s à1k (e aq àtS),

dm 3mol à1s à1

MCPA (8.570.8)?109(2.170.2)?109(1.170.1)?1091.7?109a

2,4-D

1.6?109a

(1.470.1)?109

b

(2.570.3)?109

b

6,670.5)x109b (5.270.4)x109c 2,4,5-T

(6.470.5)x109

(1.470.1)?109(6.570.5)?109

a Mabury and Crosby (1996).

b Zona et al.(2002b).

c

Peller and Kamat (2005).

λ 210 nm 2,4,5-T

m A U

1 2 342,4,5-TCP

05

10t [min]

1520

Fig.2.HPLC-chromatogram (elution:H 2O (1%w/v H 3PO 4):MeOH ?47.5:52.5v/v)of an irradiated solution of 2,4,5-T (500m mol dm à3,pH 9,N 2O;dose:500Gy).Peak 1(3.0min)and peak 3(6.7min):C 8H 6Cl 2O 4,deprotonated molecule ion m /z 234.7.Peak 2(4.9min)and peak 4(15.3min):C 8H 7Cl 3O 4,deprotonated molecule ion m /z 268.7(OH-adducts).

R.Zona et al./Radiation Physics and Chemistry 81(2012)152–159155

G (-2,4,5-T)$5.6?10à7mol dm à3J à1,which corresponds approximately to the d OH radicals concentration.The chloride yields at 200Gy were with G (Cl à)$2.9?10à7mol dm à3J à1($50%of d OH)signi?cantly higher than that of the initial phenoxyl radical concentrations determined by pulse radiolysis (28%of d OH).This indicated that chloride is not only formed by the ?rst order decay of the ipso-chloro-adducts (e.g.reaction (6))but also by second order reactions of other chloro-hydroxycyclo-hexadienyl radicals.Similar results have been achieved for 2,4-D (Zona et al.,2002b ).From Fig.3A is evident that the formation rate of acetate follows that of a primary product.Its production from the C1–OH-adduct can be excluded due to the homolytic splitting of the ether side chain,reaction (5a,b),Peller and Kamat (2005).The acetate concentration at 200Gy (32m mol)correlates to that of the OH-adduct on C6(35m mol),which might indicate this transient as a precursor.In previous investigations concerning 2,4-D,where acetate as well was found as primary product,a mechanism for acetate production from the C6–OH-adduct was proposed and discussed (Zona et al.,2002a ).The analogous reaction is shown for 2,4,5-T in reaction (7).It can be assumed that this reaction also takes place in the presence of oxygen,since for the OH-adducts of 2,4,5-T no reaction with oxygen was observable,and therefore peroxyl radical formation is of minor importance.One possible decay pathway of the phenoxyl radicals formed in reaction (7)might be the formation of 3,4,6-trichlor-ocatechol,reaction (8).The formation of catechols is conceivable,since also during bacterial degradation of chlorophenoxy-herbi-cides a hydroxylation step under formation of chlorocatechols is of signi?cance (Benndorf and Babel,2002;Smith and Beadle,2008).

Cl

Cl

Cl

H O H O

C H

H COO Cl

Cl

Cl

O

O H CH 3COO

-

.

.

-

e7T

Cl

Cl

Cl

O

O

H .

+

RH

+

R

.

e8T

MCPA.The primary pathway of d OH attack on MCPA has not been determined quantitatively.The following products could be

detected by LC-ESI-MS:2-methyl-4-hydroxyphenoxyacetic acid,deprotonated molecule ion m/z 181.1,resulting from the ipso-chloro-d OH attack,and three OH-adducts,C 9H 9ClO 4,deproto-nated molecule ion m/z 215.1.The yields of the phenol 4-CMP (d OH attack on C1)and that of the phenoxyl radicals (d OH attack on C4)were determined earlier to be 20%and $13%relative to d

OH,respectively (Zona et al.,2002b ).The decrease,together with 4-CMP-and chloride formation,are presented in Fig.4.The yields at 200Gy relative to d OH concentration were degradation:$93%,formation of chloride:$17%,formation of 4-CMP:$22%.

3.2.2.Radiolysis in the presence of oxygen

During radiolysis in the presence of air,oxygen is continuously reduced according to reactions (3a)and (3b)and by reaction of the carbon centered radicals with O 2under peroxyl radical formation.A pronounced oxygen decrease during irradiation in aerated solution was found for 2,4-D,the oxygen uptake was linear up to a dose of 700Gy with $1.1ppm O 2/100Gy (Zona and Solar,2003).Degradation of 2,4,5-T and formation of products in aerated (A),air saturated (AS)and oxygen saturated (OS,[O 2]?1.25?10à3mol dm à3)solutions are presented in Fig.5A–D.The degra-dation rate is practically not affected by oxygen concentration (Fig.5A).This can be referred to the very slow reaction rate of oxygen with the primarily formed 2,4,5-T-OH-adducts.Elimina-tion of chloride (41kGy)and the formation of organic acids,however,increased signi?cantly.It is obvious that,in contrary to

kGy

μm o l

2000

400600

800kGy

μm o l

10

203040

50

60Fig.3.Degradation of 2,4,5-T (500m mol dm à3,N 2O,pH 9)and formation of chloride,acetate (A),2,4,5-TCP,formate,glyoxylic acid and oxalic acid (B)as a function of dose.

kGy

μm o l

0100200300400

500600Fig. 4.Degradation of MCPA (500m mol dm à3,N 2O,pH 9)and formation of chloride and 4-CMP (4-chloro-2-methylphenol)as a function of dose.

R.Zona et al./Radiation Physics and Chemistry 81(2012)152–159

156

the 2,4,5-T-OH-adducts,the transients produced by reaction of d

OH with the radiolytic products react ef?ciently with oxygen,implicating an enhanced formation of smaller molecules.This appeared for the intermediate product 2,4,5-TCP (Fig.5B),it decreased much faster when air or oxygen was supplied during irradiation.For MCPA,which has only one electron withdrawing Cl-substituent,in addition to product formation also the degrada-tion rate was also in?uenced by oxygen concentration to a certain extent (Fig.6).

Comparison of the degradation ef?ciency.The effect of oxygen concentration during irradiation on the degradation of three pesticides,MCPA,2,4-D and 2,4,5-T,is summarized in Table 2.The chloride elimination in A and OS solutions increased from 74%to 87%(MCPA),from 61%to 78%(2,4-DCP)and from 57%to

77%(2,4,5-T).Oxalic acid formation doubled for MCPA and more than trebled for 2,4-D and 2,4,5-T.Acetate intensi?es by 1.6for 2,4-D and 2,4,5-T.Acetate was not determined for MCPA in the present study,yet other authors reported the formation of about 350m mol dm à3by radiolysis of 500m mol dm à3MCPA in aerated solution at pH 7and 4kGy (Bojanowska-Czajka et al.,2006).3.3.TOC reduction

The impact of oxygen concentration on the mineralization process was determined at a dose of 5kGy,where the pesticides are already completely degraded.A comparison of the results obtained for 500m mol dm à3substrates is presented in Table 3.

kGy

μm o l

100200300400

500kGy

μm o l

020406080

100

120kGy

μm o l

02004006008001000

1200kGy

204060

80Fig.5.Degradation of 2,4,5-T (500m mol dm à3,pH 9)and formation of chloride (A),2,4,5-TCP,glyoxylic acid (B),acetate,formate (C)and oxalic acid (D)in aerated,air saturated and oxygenated solution as a function of dose.

kGy

μm o l

0100200300400

500600Fig.6.Degradation of MCPA (500m mol dm à3,pH 9)and formation of chloride in aerated,air saturated and oxygenated solution as a function of dose.

Table 2

Degradation and product formation at 4kGy using aerated,air saturated and oxygen saturated solution of MCPA,2,4-D and 2,4,5-T (500m mol dm à3,pH 9–9.5).Compound m mol dm à3(A)

m mol dm à3(AS)

m mol dm à3(OS)

MCPA 7004-CMP 20o 50Chloride 368400435Acetate a n.d.n.d.n.d.Oxalic acid 3856762,4-D b o 5o 5o 52,4-DCP b 2012o 5Chloride b 610710780Acetate b

270342421Oxalic acid b 2776992,4,5-T 0002,4,5-TCP 207o 5Chloride 85010151160Acetate 268340418Oxalic acid

33

68

102

a n.d.:not determined.

b

Zona et al.(2002a)and Zona and Solar (2003).

R.Zona et al./Radiation Physics and Chemistry 81(2012)152–159157

Whereas in N 2O and aerated solutions the TOC reduction was only 10–15%,in AS and OS solutions it increased to 35–40%.It is obvious that the mineralization process is not enhanced by pure oxygen,air supply during irradiation is suf?cient.Economically,this is of fundamental importance.3.4.Toxicity determinations

The phenoxyacetic acid herbicides are moderately toxic com-pounds.Their EC 50values were found to be:MCPA:82mg dm à3(75.6mg dm à3,Bojanowska-Czajka et al.,2006),2,4-D:59mg dm à3(Zona and Solar,2003),2,4,5-T:23mg dm à3.How-ever,during irradiation several phenol-type compounds as well as aliphatic chlorine containing compounds are produced.Chloro-phenol intermediates are approximately ten times as toxic as the herbicide.The following EC 50values have been reported —4-CMP:1.55mg dm à3(Bojanowska-Czajka et al.,2006)and 2,4-DCP:2.4mg dm à3(Zona et al.,1999).The mineralization process passes very slowly i.e.high doses are needed for an ef?cient organic carbon decrease (e.g.:to attain a 85%TOC reduction in a 500m mol dm à32,4-D air saturated solution a dose of 20kGy was necessary,Zona and Solar,2003).The change of toxicity,%I 30(inhibition of luminescence after 30min incubation),as a func-tion of dose is presented in Fig.7for three herbicide solutions (AS)containing 50m mol substrate.The inhibition/dose plots are similar for all three compounds,having a maximum toxicity

around 200Gy,followed by a decrease,whereby the threshold level of o 20%inhibition is achieved at about 500Gy.The higher maximum values for 2,4-D and 2,4,5-T are re?ecting the different toxicity values of the herbicides and the greater toxicity of intermediates with a higher chlorine content.

Using 500m mol dm à32,4,5-T solutions (AS)the inhibition in non irradiated solution was 62%,between 1.5and 1.7kGy it increased to $90%and at 10kGy it was 22%.The curve shape is similar to that found for 500m mol 2,4-D in AS solution having a maximum inhibition of $80%and $20%at 10kGy (Zona and Solar,2003).For MCPA the maximum value was $70%and o 20%at 10kGy.The fact that $10kGy is suf?cient to guarantee detoxi?cation,although considerable amounts of organic-and chlorine containing organic compounds being present,elucidates the importance of toxicity measurements.

4.Conclusion

4.1.Primary pattern of OH-radical addition to the aromatic ring The addition to C1is $20%relative to d OH radical concentra-tion comparable for both herbicides.The ipso-chloro-positions are involved with about 10%each (MCPA:13%,2.4.5-T:total 28%).The addition of the electrophilic d OH radical to the non ipso-positions depends on the charge distribution on the aromatic ring.For 2,4,5-T the phenols (total 41%of d OH)formed on the activated C6and the non activated C3positions were in the ratio of $3:1.4.2.In?uence of oxygen concentration

Degradation of the herbicides is only insigni?cantly affected by the presence of oxygen,it is completed at $4kGy.Elimination of chloride,formation of phenolic intermediates and aliphatic acids is strongly https://www.360docs.net/doc/b13533659.html,ing 500m mol dm à3herbicides a TOC reduction of $40%is achieved with 5kGy,for a complete organic carbon removal doses 420kGy are required.However,detoxi?cation is obtained with 10kGy.This result demonstrates the particular importance of toxicity determinations to optimize reaction conditions for methods of water puri?cation.

Acknowledgment

The authors like to thank Univ.Prof.Dr.G.Reznicek (University of Vienna,Faculty of Life Sciences,Department of Pharmacognosy)for carrying out the LC-ESI-MS analysis.References

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In?uence of the presence of oxygen during irradiation on the total organic carbon (TOC)reduction at https://www.360docs.net/doc/b13533659.html,plete degradation of the substrates was achieved at $4kGy.Solutions:500m mol dm à3,pH 9.Substrate Irradiation condition TOC 5kGy /TOC 0kGy MCPA N 2O 0.87MCPA Air

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