Carbamate insecticides: Removal from water by chlorination and ozonation

War. Res. Vol. 24, No. 1, pp. I I-21, 1990 Printed in Great Britain. All rights reserved

0043-1354/90 $3.00 + 0.00 Copyright 0 1990 Pergamon Press plc

CARBAMATE INSECTICIDES: BY CHLORINATION

REMOVAL FROM WATER AND OZONATION

YAEL(ZELICOVITZ) MASON‘v2,*,EHUD CHOSHEN’and CHAIMRAV-ACHA’ ‘Division of Environmental Sciences, School of Applied Sciencesand Technology, The Hebrew University of Jerusalem, Israel and 2Department of Environmental Science and Engineering, School of Public Health, University of California Los Angeles, Los Angeles, CA 90024, U.S.A.

(First received January

1989; accepted in revised form May

1989)

Abstract-A simple approach for removal of carbamates from drinking water by disinfection is presented. Four carbamates, aldicarb, methomyl, carbaryl and propoxur were reacted with excess of each of three disinfectants, Cl,, ClO, and 0,. Carbaryl and propoxur did not react with chlorine, none of the selected carbamates reacted with ClO, , and all reacted very rapidly with 0,. The reaction kinetics were determined for aldicarb and Cl, and for methomyl and Cl,. Product analysis for the reaction of aldicarb and Cl, was carried out using reverse-phase HPLC and GC-MS. The common degradation products, aldicarb-sulfoxide and aldicarb-sulfone were found together with other by-products. A mechanism is suggested based upon an electrophilic ionic attack by hypochlorous acid. A possible mechanism of electrophilic attack by ozone is also suggested. A preliminary bioassay using Daphnia magna, to compare the toxicity of aldicarb and chlorination by-products of aldicarb showed that the by-products were less toxic. Therefore, remova!/degradation of these carbamates can be achieved using Cl, and/or 0, but not ClOr. Key words-aldicarb, aldicarb-sulfoxide, chlorination, ozone, HPLC, bioassay

aldicarb-sulfone,

carbamates,

water, degradation,

kinetics,

(Christensen and Luginbyhl, 1975; Kuhr and Dorough, 1976; World Health Organization, 1976; Prabhaker and Fraumeni, 1978) and mutagens (Njagi and Gopalan, 1980). Carbamates such as aldicarb, its metabolites and methomyl are highly soluble in water (Guerrera, 1981; Lemley and Zhong, 1984; Rothschild et al., 1982; Miles and Delfino, 1984), and their stability under certain environmental conditions have made them a serious threat to drinking water (Rothschild et al., 1982; Zaki et al., 1982; Lemley and Zhong, 1984). Aldicarb is a systemic pesticide, and is applied directly to plants roots (Union Carbide, 1975) so that upon irrigation it can easily leach into groundwater (Jones, 1986). Such a possibility increases in the light of the persistence of carbamates in soil. The half-life of carbamates in the soil varies between 2 weeks to 3 months depending on temperature, pH, moisture, microbial population and inorganic matter (Rothschild et al., 1982; Jones, 1986). Low field capacity soils (porous soil), high water infiltrations from heavy rainfall, excessive rainy seasons, overirrigation, high-use rate and shallow water table are all major factors promoting leaching into groundwater (Rothschild et al., 1982; Zaki et al., 1982; Jones, 1986). Other potential sources of water contamination by carbamates include; run-off, aerial spraying, spray drift into drainage and irrigation canals, rivers and lakes (Wolfe et al., 1978; Ownan

INTRODUCTION

Carbamates are now of great significance in pest control and are increasingly used instead of organochlorine and organo-phosphorus pesticides (Kuhr and Dorough, 1976). Their advantage over other kinds of pesticides formerly used, stems from the fact that they are more biodegradable than the organochlorine compounds and are less toxic to mammals than the organo-phosphates (Schlagbauer and Schlagbauer, 1972; Kuhr and Dorough, 1976). The carbamates are substituted esters of carbamic acid (NH,COOH) with aliphatic or aromatic substituents on the oxygen and nitrogen atoms (Blaicher et al., 1980). Toxicity of carbamates is due to the inhibition of the enzyme acetylcholine esterase (Metcalf and Fokuto, 1965; Schlagbauer and Schlagbauer, 1972; Christensen and Luginbyhl, 1975; Kuhr and Dorough, 1976; Kolene and Soffer, 1981), but unlike the organophosphates, this inhibition is reversible and recovery from sub-lethal doses occurs very rapidly (Guerrera, 198 1; Zaki et al., 1982; Lemley and Zhong, 1984). However, some carbamates are highly toxic (the LC, for aldicarb in rats has been reported to be 0.93 mg/kg body weight) (Kuhr and Dorough, 1976; Watts, 1980; Guerrera, 1981) and some others are suspected carcinogens *Address all correspondence to: Dr Tony Mason, EAWAG, Ueberlandstrasse 133, CH-8600 Diibendorf, Switzerland. I1

YAEL (ZELICOVITZ)MASON PI ui.

12

and Belal, 1980). The half-life degradation rates in water range from days to several years (Lemley and Zhong, 1984; Jones, 1986). Toxic metabolites of aldicarb have leached into drinking water wells in New York, Wisconsin and Florida (Guerrera, 1981; Zaki et al., 1982; Rothschild et al., 1982; Miles and Delfino, 1984; Zhong and Lemley, 1984; Jones, 1986). The most severe contamination still appears to be in Long Island. Extensive groundwater sampling up until 1980 showed widespread contamination ranging from small amounts to as much as 500 ppb, whereas the recommended maximum contamination level is only 7ppb @g/l) (Guerrera, 1981; Zaki et al., 1982). This contamination may persist for decades. The discovery of drinking water contamination by carbamates, provides a case study to develop a detoxification methodology. Acid catalyzed hydrolysis and nucleophilic cleavage of carbamate are achieved in situ on ion-exchange beds charged with protons or hydroxyl ions (Lemley and Zhong, 1984; Lemley et al., 1984). Another method to remove these residues from water supplies suggested by Union Carbide was adsorption on activated carbon (Union Carbide, 1979). A sand/granular carbon filtration process gave results of an average removal efficiency of 72% for sand filtration and 91% removal for a combined sand and granular filtration treatment (Moore er al.. 1985).

However, all the above mentioned methods have the disadvantages of high cost and complicated maintenance. A more simple approach would be to remove these contaminants by the routine process of disinfection. Therefore, the goal of this study was to compare reactivities of various disinfectants (Cl,, CIOz , 03) with representative carbamates from the thio oxime and aromatic carbamates (Fig. 1). This information can shed more light on the different routes by which the various disinfectants react with aquatic organic micropollutants in general. EXPERIMENTAL

Materials and methods

The inorganic reagents were of analytical grade (Merck, BDH or Frutarom-Israel). Carbamate pesticides and their derivatives methomyl, methomyl-oxime, carbaryl, 1-napthol and propoxur were kindly supplied as reference standards by EPA (Research Triangle Park, N.C.) and were of >99% purity. Aldicarb, aldicarb-sulfoxide, aldicarb-sulfone, aldicarb-oxime and aldicarb-sulfoxide-oxime were also of > 99% purity and supplied by Union Carbide Agricultural Products Company Inc. (Research Triangle Park, N.C.). Organic solvents were of the highest purity available (mostly > 99%, Frutarom-Israel) and redistilled before use. Water used in the experiments was purified by ionexchange, followed by passage through a Serdest SD 2000 water purifier equipped with a column of activated charcoal and a 0.2 p Millipore filter. The buffers used were phosphate buffer (0.025 M or 0.5 M) for pH 6.G8.5, and carbonate buffer (0.25 M) for pH 9-l 1. Stock solutions of chlorine in the form of hypochlorous acid were prepared by bubbling gaseous chlorine into

alkaline distilled water. These were stored in dark bottles at 4°C. Chlorine concentration was measured by the iodometric method (APHA, 1985). Stock solutions of ClO, were prepared from reagent grade sodium chlorite and acetic anhydride, according to the method of Masschelein (1967) which unlike other methods provided aqueous ClOr devoid of traces of chlorine (Masschelein, 1979). The CIOz solutions were stored in dark bottles at 4°C for several days. The ClO, concentration was measured simultaneously by two different methods: (1) the iodometric method (APHA. 1975) and (2) bv the U.V. absorbance at 360 nm (Stevenson et al.. 1981). . Ozone was generated by a Buchi-Fisher instrument model 501. For the reaction studies. distilled water was pre-ozonized in order to remove organic materials that may be oxidized by ozone. The reactions were then initiated by a second supply of ozone Ozone concentration was determined in aqueous solution by its uv. absorbance at 258 nm [f = 2900 Mm’ cm ’ (Hoigne and Bader, 1976)]. Reactions hetueert disin+ctartrs trnti curhamutes

Most of the studies with respect to kinetics, product analysis and bioassays were carried out with carbamate concentrations of 1 x IO-‘ M, disinfectant concentration of 1.22 x 10.‘~-2.84 x IO ’ M and 0.25 M of appropriate buffer, in distilled water Kinetic meu.~urement.\

The carbamate-disinfectant reactions were initiated by adding appropriate volumes of the carbamate stock solutions to 1 cm quartz cuvettes. containing the appropriate concentration of disinfectant (indicated in the results) in a buffered solution. The disappearance of the carbamate was followed spectrophotometrically (Varian 635) at 15°C by monitoring absorbance at the following wavelengths: carbaryl 220 nm, propoxur 275 nm, methomyl 234 nm and aldicarb 246 nm. Reaction mixtures were analyzed by:

(1) Reverse-phase HPLC, Tracer 985 LC pump, 980A programmer equipped with a 970A variable wavelength u.v.-vis detector. The column was a 5~ DuPont Zorbax C. (250 mm x 4.6 mm). 200 ~1100~ (and 2 ml sample loop for peak collection), mobile phase isocratic 50 or 20% acetonitrile in water, 1, = 2OOnm, flow rate: 1 ml/mm. (2) GC-MS under the following conditions; (a) MS-E1 was performed using a Finnigan MAT0054 instrument equipped with a SE-54 0.5 capillary column and a direct inlet probe. Some samples were analyzed using a Hewlett--Packard 5970 MSD linked to a 5890A gas chromatograph equipped with a DB5 column. The initial temperature was 35°C thereafter a 4’C:min rate change to 92°C for 15-25min, and then I O”C/min up to 280°C for 10 min. Injector port temperature was 250°C. Helium was the carrier gas. (b) MS-C1 was performed on a DuPont 21-49CB instrument equipped with a dual EIjCI ion source. ”

~

I.

.

_.

Procedures

The peak separation, collection and enrichment procedure used was as follows: a 0.03 M solution of aldicarb was allowed to react with 0.3 M Cl, (total volume 3 ml in distilled water) for 24 h at pH 4.0. 4°C in dark bottles. The mixtures were then analyzed by injecting a 1.5 ml sample into an HPLC. Each peak was collected separately and both these and the original reaction mixture were individually enriched by each of the following methods: (i) chloroform extraction followed by dehydration by molecular sieve 4A and evaporation with a dry stream of N, to 1 ml; (ii) the samples were passed through a C-18 Sep pak cartridge (Waters Associates). the cartridge was eluted with either

Carbamates degradation by water disinfection

CH, cH$-c-

A

I

c H, H 0

H

13

=q--C=-LN I s-cy

CH I-H

(a)

s

(4

H

Ii I

0

H

II

I

rC-N-cHs

(d)

ii

-N-@-C-N-CH

CH,S‘i-r CH, H

H (e)

Fig. 1. Chemical structure of the pesticides aldicarb (a), methomyl (b), carbaryl (c) and propoxur (d), and of the aldicarb oxidation products aldicarb-sulfone (e) and aldicarb-sulfoxide (f).

methanol or acetonitrile; (iii) the samples were extracted with methylene chloride and evaporated with a KD extractor or Buchi Rotavapur (Switzerland) at 38°C. The organic fraction was separated using anhydrous sodium sulfate and was gently evaporated to minimum liquid content using a dry stream of nitrogen. The sample was redissolved in either acetone or methanol; (iv) the reaction mixture was derivatized using trifluoroacetic anhydride (TFA, Aldrich). After evaporating with a gentle stream of N, to dryness, the sample was redissolved in 100~1 ethyl acetate and 100~1 TFA. This was then heated for 35min at 70°C and then evaporated using N, to dryness to remove TFA. The sample was then redissolved in ethylacetate or acetone prior to analysis. In order to determine the effect of pH on the chlorination reaction of aldicarb and on the reaction by-products, six 40ml unbuffered distilled water samples, each containing 0.02 M Cl, were prepared. Each sample was adjusted to the desired initial pH, allowed to stabilize and the reaction was started when 3.25 x lo-’ M aldicarb (final concentration) was added. Fifty ~1 of each sample were analyzed by reverse phase HPLC (mobile phase, 53% acetonitrile in water) in order to detect residual aldicarb. When 99% of the aldicarb had been degraded, half of each sample (20ml) was quenched with sodium thiosulfate. Both the quenched samples and the unquenched samples were analyzed by HPLC using 20% acetonitrile in water as the mobile phase for by-product detection. The reaction was followed over a period of 7 days. The pH was measured periodically during the experiment. Bioassay A rudimentary bioassay using Daphnia magna Straus (Crustacea: Cladocera) was carried out in order to estimate and compare the toxicity of aldicarkhlorine reaction products to that of the parent compound aldicarb. One hundred ml solution of chlorinated aldicarb was prepared by reacting 0.15 M chlorine with 0.03 M aldicarb for 24 h at pH 7.0 as previously described. The chlorine was then quenched by a molar equivalent of Na,S,O,. Several concentrations of 100. 300. 600. 800. 1000. 1200. 1300, 1400 pg/l of chlorinate> aldicarb ‘were’prepaied by addinp pre-calculated volumes of the reaction mixture to 100 ml tap water (that were previously aerated for 30 h to remove W.R. 14,1--8

chlorine residues). Ten D. magna were introduced to each solution (in five replicates). The number of dead D. magna in each solution was counted after 12 h and compared to two controls. One control consisted of 0.2 M Cl, to which 0.15 M Na,S,O, was added followed by 0.03 M aldicarb. The second control consisted of 0.2 M Cl, and 0.15 M Na,S,O, without aldicarb. RESULTS The Effect of Chlorine on Carbamates Kinetic studies When carbaryl or propoxur (1 x 10m5 M) were allowed to stand for 24 h with excess chlorine (up to 3 x 1O-4 M), at pH 7.0,2O”C, no reaction was found to occur as indicated by U.V. spectrophotometric and HPLC analyses. However, when aldicarb or methomyl (1 x lo-’ M) were reacted with excess chlorine (1.2 x 10-4-2.8 x 10w4M) in the pH range pH 6.0-8.5, pseudo-first reaction orders with respect to both aldicarb and methomyl were observed as indicated by the straight lines obtained when Al-A, - In ~ A,-A, was plotted against time (see Figs 2 and 3), where A, and A, are the values of absorbance at t = 0 and t = cc, respectively and A, is the absorbance at time t. It can also be seen that the reaction rate increases with increasing Cl, concentration. The observed reaction rate constant (kobs) for the reaction between both aldicarb and methomyl with Cl, was determined from the slopes in Figs 2 and 3 and also calculated using the expression: kbS = ln2/t,,, .

(1)

14

YAEL

(ZELICOVITZ)

MASON C, 01.

2

n

‘Table 3. Chlorme dlssoclatlon Ionic

species

found

$ I

PH PH

HOC1

OCI

;, ^e Q I

3.0 6.2 6.5 6.8 7.0 7.3 7.4 7.8 8.2 x3 X.6

97.51 96.45

0.00 3.55 6.84 12.77 18.84 3 I .65 36.82 59.52 78.62 82.24 00.23

TO 0

2

4

6

6 Time

10

12

tminl

Fig. 2. Removal of aldicarb from water by chlorination. Pseudo-first-order kinetics for aldicarb in reaction with various chlorine concentrations.

48

Q I

87.23 81.16 68.35 63. IX 40.5x 21.38 17 76 Y.77

Cl,(aq)+H,O=HOCl+H++Cl’

K,=4x

< I;

HOCl=

t ’

93.lb

CL 2.48 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

by pH changes. The chlorine dissociation in water is highly pH dependent (White, 1986) and can be described by equations (2) and (3) and by Table 3.

2 F

1 2 I 8

of

PO)

=?

=? ;j c

III water.

as a function

0

0

1

2

5

3

4

Time

lmml

6

7

6

Fig. 3. Removal of methomyl from water by chlorination. Pseudo-first-order kinetics for methomyl in the reaction with various chlorine concentrations.

Determination Cl,

Table

I.k,, ( x IO ’ s ’ ) asa function of pH in the

reaction between aldicarb

and Cl,

(W’M)

I .20 I .62 I .83 2.02 2.42 2.84

7.0

7.4

7.8

8.2

1.60 3.17 4.83 5.50 4.92

I .67 2.67 3.30 7.67

1.12 I.58 2.83 3.83 -

0.67 0.78 I .03 2.14 2.75

0.62 0.73 0.95 I .45 1.35

11.00

Table 2. k,,,(x 10mJs- ’ ) as a function of pH reaction between methomyl and Cl, PH

[Cl,1

(10-4M)

0.8 I 1.22

___._~.

-__

which resulted

in a non-linear curve. Therefore, (m) with respect to HOC1 was obtained from the gradient of the curve obtained by the reaction

7.4

2.06 5.18

1.22 3.10

I.17

0.83

5.76

4.67

2.33

0.50

5.50

2.67

0.67

6.33 -

4.33 -

1.62

8.10

1.83

14.00

2.02 2.22

36.00 41.30

243

--

2.84

56.20

9.45 16.00

-

order

plotting kobr against HOC1 on a logarithmic according to equation (6) (Figs 4 and 5).

log koha = m

log

[HOC11+ log K.

The specific rate constant K can be calculated the intercept values from this plot.

plot

(6) from

1.ooo 7.4 7.6 8 2

1.66 ‘1.76 1 06

0.99 0.95 0 96

~~_

6.8

-

(5)

;I y” 0.100

6.2

-

(4)

kuhr= K [HOCllm

in the

___~ 7.8

(3)

where K, at 1YC = 10 -” and the reaction order with respect to HOC1 was obtained by plotting /cobsagainst HOC1 according to equation (5)

-_______

6.5

pK, = 1.5.

[HOCI] = C; “f’ ,F+;] *

PH [Cl,]

H’ +OCl

Among the chlorinated species, HOC1 is much more reactive than OCI- (c. IO4 times is typical), (Morris, 1978) therefore, the decrease in /cobscan be directly correlated to the decreasing HOC1 concentrations at alkaline pH values. Therefore, it is likely that HOC1 is the main attacking chlorinated agent. Its concentration can be calculated from equation (4).

of the reaction order (m) with respect to

Tables 1 and 2 show that for a fixed Cl, concentration, kobs decreases with increasing pH for both aldicarb and methomyl. Both base and acid catalyzed hydrolysis of aldicarb and methomyl are known to be very slow [35-58 weeks (Chapman and Cole, 1982)] in this pH range. This suggests that the concentration of the attacking chlorinated agent must be influenced

(2)

10 .J

8.3 0.33

-

~-

-~

2 00

IV.20

i.33

2.33

[HOC11

x 10-4

M

Fig. 4. kOb, as a function of HOC1 concentration in the reaction between aldicarb and Cl,. The reaction was. carried out

to

at various

pH

values ._

and.^the reaction .

order

i with

chlorine (WI)IS calculated trom the slopes of eat

respect

curve.

Carbamates degradation by water disinfection r!

15

5

8 a 1 .ooo B y”

I

4

a0 ;

3

A 0.5u . O.SM

?I 0.100

72 J ;;

1

E 0.010 0.2

0.4

0.6

0.5

CHOCII

1.0

x 10-d

’

1.1

-I

0 0

2

4 Time

M

Fig. 5. k,, as a function of HOC1 concentration in the reaction between methomyl and Cl,. The reaction was carried out at various pH values and the slope of each curve represents the reaction order with respect to chlorine.

The linear relationships obtained thus revealed the following information: (1) the reaction order with respect to HOC1 in the reaction with aldicarb is c. 1.8 in the pH range 6.5-7.8. At pH 8.2, the reaction order changes to first order. For methomyl, the reaction order with respect to HOC1 is consistently around 2.5 over the entire pH range examined. (2) The dependency of K, the specific rate constant, on pH is demonstrated in Fig. 6. For both pesticides, a decrease in rate was observed as the pH was increased from mildly acidic to pH 7.0. The reaction rate with aldicarb slows with increasing pH, whilst that with methomyl appears to be unaffected by alkaline pH values. (3) The reaction between methomyl and HOC1 is several orders of magnitude faster than the reaction between HOC1 and aldicarb. No effect of light on the reaction rate between chlorine and aldicarb was found.

10

6

12

(mini

Fig. 7. Effect of NaCl on the reaction rate (/cobs)for the reaction between aldicarb and chlorine. All reactions were carried out at pH 7.0. Table 4. kobr (x W4s-') for the reaction between aldicarb and Cl,at pH 7.0 in the presence of various concentrations of NaCl

[NsCtl

Slope

10-Z)

(x

(W

3.30 22.49 83.95 96.54

0 0.1 0.3 0.5

(x ,5%) 5.50 37.98 139.00 161.00

tration is insufficient to completely destroy all of the methomyl. Whilst a ratio of 2: 1 is sufficient to breakdown all of the methomyl, the addition of extra chlorine results in additional chlorine consumption probably due to reactions with by-products of the parent molecule. Aldicarb. Figure 9 shows that a stoichiometric ratio of 3: 1 is necessary to completely breakdown the

Eflect of salt on the reaction rate between carbamate and chlorine When aldicarb-chlorine (1 x lo-‘:0.7 x IO-‘M reactions were repeated in various concentrations of NaCl (0.1-0.5 M) at pH 7.0, the reaction was found to proceed faster with increasing salt concentration (Fig. 7). The values for k,, are shown at different NaCl concentrations in Table 4.

0 Initial

Stoichiometry Methomyl. The data shown in Fig. 8 show that at ratios of less than 2: 1 Cl,:methomyl, the Cl, concenAldlcorb Mathomyl

8.0

8.5

-’ 7.0

7.5

molar

4

concantrotions

rotio

Fig. 8. Determination of the stoichiometry for the reaction between methomyl and Cl,. A molar ratio of 2: 1 is sufficient to break down all of the methomyl.

? 0

_ .*.

3

2

: methomyl

0-c) l -- . -.zoco

0.0

1

Cl2

0.0

as

30

DH

Fig. 6. Effect of pH on the reaction rate constant between the carbamate insecticides methomyl and aldicarb with the disinfecting agent chlorine.

0

Initial

i Cl2

i

: aldlcorb

j molar

i

i

Concentrations

6 ratio

Fig. 9. Determination of the stoichiometric coefficients for the reaction between aldicarb and Cl,. When chlorine is present in a 3-fold molar excess, complete aldicarb removal is possible.

0 0.5

5

54 Time

91

145 Time

th)

cnl

1

40IJiG----

0

0.5

0 5

54 Time

91

0.5

145

54 Ttme

20

91

145

lh)

250 ui 5.70

“0 r

5

(h) m 6.17

1

1

m. 2w

15

x ;

15o

s 2

IO

L a

1~~

a

5

:: k

50

k! 0 O.!l

5

54 Time

01

0

145

0.S

3

IhI

54 Time

01

145

01

14s

a1

145

ih)

3oo Rt 0.5

no 250

D

0

c

.7

x 200

x

I.2 w ‘50

2

5

a Y

’ 6

4

% 3

lou

2

k 5

0.50

2 1

0

0 0.5

5

54 Time

91

145

0.5

5

54 Time

(hl

lh)

30 Rt 14.35

zoo

n

n

:

x

z x 150

20

;3

s 2 Y

d $j 100 10

:: E

0

0.5

01

54

Time

50

0 145

0.5

5

Ih)

$811

pH 6.6

m

pH 6.2

GL]

pH 7.3

x

0

PH 6.5

: [L a

m

pH 3.9

54 Time

(hl

54 Time

I h)

n

0 c

2

20

10

k! 0 0.5

5

91

145

Fig. 10. Reverse phase HPLC analysis of the products formed during the reaction between aldicarb and chlorine. The reaction was carried out in unbuffered aqueous solutions with various initial pH values. The appearance and disappearance of various peaks was followed during the course of the next 6 days. 16

Carbamates degradation by water disinfection

17

aldicarb molecule. Additional chlorine is consumed with higher ratios indicating consumption by reacting with the products of the aldicarhhlorination reaction.

PH *.ao-_o PH3.2.-0 $3%-t PH3.2G-0

Product Analysis

Reverse phase HPLC analysis of the reaction mixture, using a relatively high polar mobile phase (at 200 nm) suggests that at least seven by-products appeared within the first hour of reaction. These products were found in the quenched samples (0.5 h in Fig. 10). Four other major by-products were detected after a few hours of reaction in the unquenched samples. Differences in the by-product distribution and their temporal behavior were observed as a function of pH between acidic samples and more alkaline samples. Of the ten by-products only four were identified after injecting reference standards into the HPLC. Rt 4.0 was shown to be aldicarb-sulfoxide, Rt 3.58 is assumed to be aldicarb-sulfoxide oxime, Rt 6.17 was aldicarb-sulfone and Rt 18.17 is assumed to be aldicarb-oxime. The unidentified by-products will be referred to by using their HPLC retention time in 20% acetonitrile in water. Of the ten by-products found, two (Rt 5.07 and 16.8) were found only under basic conditions and one (Rt 5.70) only under acidic conditions. All the other products were found under both acidic and basic conditions, but their rates of accumulation and disappearance differed. Two products, (Rt 3.58 and 9.5) decomposed much faster (within a few hours) in acidic pH conditions as opposed to their behavior in basic conditions (within 7 days). Under alkaline conditions, aldicarb-sulfoxide (Rt 4.00) decomposed very fast, within 1 h giving aldicarb-sulfone (Rt 6.17). Aldicarb-sulfoxide shows a definite trend of decomposition under all pH conditions. Aldicarb-sulfone appears to be more stable at alkaline pH values where it initially decreases in concentration but later accumulates. Under acidic conditions, it shows a definite trend of decomposition with no accumulation. Aldicarb sulfoxide oxime (Rt 3.58) is produced at all pH values and increases in concentration after 7 days under basic pH conditions whilst it disappears completely after a few hours at acid pH values. Aldicarb-oxime was only found after a few hours of reaction. It was not detected after 1 day. After 5 h, a higher concentration was detected at acidic pH values than under alkaline pH conditions. After 6 days, most of the by-products had been destroyed at neutral to acid pH values, whilst at more alkaline pH, sometimes high concentrations persisted. Figure 11 shows the pH change during the reaction between aldicarb and chlorine. Each curve represents a different initial pH. When the initial pH was set at 8.2, the pH decreased very gradually and reached a stable value (3.1) after approx. 93 h. When pH 8.6 was used as the initial value the pH decreased extremely gradually and even after 145 h was still above 6.0. When the initial pH was set close to

-20

10

40

70 Time

100

130

160

lhl

Fig. 11 Change in pH during the reaction between aldicarb and chlorine. Each curve represents the change in pH during the reaction with different initial pH values. neutrality (7.3) the pH drops very rapidly and stabilizes after 5 h at pH 1.8. The acidic samples, (pH 6.5 and 3.2) both showed an extremely rapid decrease in pH within 1 min to pH 2.5 after which the pH finally stabilized at a value of 1.61.7. CC-MS

analysis

Analyzing each sample peak individually as well as sample mixtures (after enrichment with and without derivatization with trifluoroacetic anhydride) in GC-MS EI and CI was not fruitful and yielded only a few ionic fragments: the highest ionic fragment that could be observed was of m/z = 120 and its main fragments were m/z 12O(P + ); lOS(P-Me); 73(P-SMe) which suggests that it is CH,SC(CH3)2-CH,0H. Another fraction gave m/z = 115 which is possibly aldicarb nitrile [CH,SC(CH,),CN) and its fragments were m/z = 115 (M + ), lOO(M-CH3), 68 (M-CH,-S). Complimentary

studies

Complimentary studies of acid catalyzed hydrolysis of aldicarb (initial pH 2.2) showed that only a 2% reduction in aldicarb concentration was possible after 4 h giving aldicarb-oxime as a main product. Ninetyseven % of the formed aldicarb oxime decomposed further within 37 h. A study on the effect of Cl, at pH 7.0, 20°C on aldicarb-sulfoxide was performed and showed that within 32 min a 4% reduction in aldicarb-sulfoxide concentration occurred, after 44 h a 21% reduction and 39% was removed after 84 h. This study was repeated for aldicarb-sulfone with Cl, and no reduction in concentration could be detected after 45 h. Bioassay

A preliminary bioassay using D. magna was carried out in order to estimate and compare the toxicity of Cl, -aldicarb reaction products to aldicarb. Three aqueous stock solutions were prepared: (1) aldicarb (0.03 M) solution (2) a chlorine-aldicarb reaction mixture where 0.03 M aldicarb was reacted with 0.15 M C&(3) a control solution with no aldicarb in order to find out the natural death rate of D. magna. Several concentrations of each of the above stock solutions were prepared by adding precalculated vol-

YAEL (ZELICOVITZ) MASON er al.

IX

umes to 100ml tap water into which 10 D. magna were immediately introduced. The average natural death rate after 12 h was 2.5 D. magna. In order to find out the lethal concentration (LC,,) a number of 6.25 D. magna out of the observed have been should original 10, [(6.25 - 2.5)/(10-2.5) = 3.75/7.5 = 50%]. The LC,, after 12 h for aldicarb was found to be 300 ~g/l (1.6 x 10e6 M) whereas the LC, after 12 h for the reaction products was found to be 1400 pg/l (7.4 x 1O-6 M). These results suggest that the toxicity of the chlorine-aldicarb reaction products is almost 5 times less than the toxicity of the parent compound aldicarb. Therefore, it is possible that the chlorination of aldicarb is a detoxification process. The effect of C/U, and 0,

When 1 mg/l of each of the selected carbamates was allowed to stand with up to 6mg/l of CIOZ at room temperature (pH 7.0), no reaction could be observed after 24 h as indicated by both spectrophotometer and HPLC analysis. When 1 mgjl of each of the selected carbamates was reacted with ozone, the reaction was too rapid to be followed spectrophotometrically thereby preventing kinetic studies. It was however observed that the oxime carbamates (aldicarb and methomyl) reacted even faster (seconds) than the aromatic carbamates. A partial product analysis was undertaken in which aldicarb-sulfoxide was identified as one of the products. DISCUSSION

In this work, the chemistry of disinfection of two groups of carbamate insecticides (aromatic and aliphatic thio oxime carbamates) using three different disinfecting agents was compared and studied. Base calalyzed hydrolysis

Carbaryl and propoxur, the representatives of aromatic carbamate insecticides are known to undergo a relatively fast hydrolysis [I,,~ of carbaryl in sterile water at pH 7.0 and at pH 8.0 is 2.0 and 0.07 weeks, respectively. (Chapman and Cole, 1982), t,:, of propoxur at pH 7.0 and at pH 8.0 is 20 and 2.3 weeks, respectively (Osman and El-Dib, 1972)]. This indicates that their chemical structure includes good leaving groups (propoxur, a phenoxy group and carbaryl, a napthoxy group). Aldicarb and methomyl are aliphatic oxime carbamates containing a thio group which indicates their ability to undergo facile oxidation but their slow base and acid-catalyzed hydrolysis [t, z of aldicarb at pH 7.0 is 35 weeks and at pH 8.0 is 38 weeks, and for methomyl, r,,* at pH 7.0 is 38 weeks and at pH 8.0 is 20 weeks (Chapman and Cole, 1982)] indicates their lack of a good leaving group. The oxime carbamates, aldicarb and methomyl are much more soluble in water and toxic to mammals

compared to carbaryl and propoxur, and their slow biodegradation and chemical hydrolysis results in their long term persistence in the environment. Therefore, several disinfecting agents were tested for their effectiveness in removal of oxime carbamates from drinking water. EJkct of chlorine on carbamates

The results show that both aldicarb and methomyl react in the presence of water with Cl, in a manner suggesting an electrophilic reaction. This is supported by the fact that kobsdecreases to 50% from its initial value (Tables 1 and 2) at pH 7.4 a value very close to the pK, of HOCl. Salt is known to effect ionic reactions by stabilization of the charged intermediates. The results of the effect of salt (NaCl) on the reactions here therefore indicate that it is not a radical reaction that is occurring. This is further supported by the observation that light had no effect on the reaction rate. Enhancement of the electrophilic nature of attack by HOC1 in similar reactions is well documented with many examples of higher order reaction kinetics (Dewer and Fahey, 1963). In this study, the reaction order with respect to Cl, was shown to be 1.8 with aldicarb and 2.5 with methomyl. Whilst both aldicarb and methomyl were found to react with Cl, there were several differences between them in their reaction characteristics. The reaction orders with respect to Cl, and the stoichiometric ratios differed. Also. a much faster reaction was observed between methomyl and Cl,(K = lO’M_‘s ‘) than with aldicarb (K = 104M-‘s-‘). These results strongly indicate that differences in the reaction mechanism must exist. Moreover, the reaction order (and therefore the mechanism) for Cl, in the reaction with aldicarb was strongly pH-dependent whilst it was unaffected by pH in the reaction with methomyl. Methomyl is a smaller molecule than aldicarb. The slower reaction rate with aldicarb and the effect of pH on the reaction order could thus be due to a number of different factors: (a) competing reactions could limit the availability of HOC1 at the most reactive site on the aldicarb molecule; (b) the higher stoichiometric ratio for aldicarb and Cl, also suggests that more HOC1 is required to break down 1 mole of aldicarb than 1 mole of methomyl and therefore, a lower free HOC1 concentration would be available than in the reaction with methomyl; and (c) it is possible that OCl . which is the most abundant chlorinated species under alkaline pH conditions, can successfully react with methomyl but does not react with aldicarb. Aldicarb by-product analysis shows that HOC1 attacks at the thio group giving aldicarb-sulfoxide and aldicarb-sulfone. It is clearly shown that other by-products are being formed, probably when other sites on the aldicarb molecule are being attacked. Following the by-products behavior and their distribution at different initial pHs reinforces the assump-

Carbamates degradation by water disinfection

tion that the mechanism of reaction between aldicarb and Cl2 is different from that of Cl, with methomyl and is determined by the initial pH, which determines not only the HOC1 concentration, but also the extent of the base-catalyzed hydrolysis reaction. Two byproducts are formed only when the initial pH is basic and one is formed only at an initial acidic pH. The other seven detected by-products appear at all pH values but at basic pH values there is a fluctuating trend during the initial phase of the reaction followed by a build-up of the by-product towards the end of the reaction. This suggests that the reactions are reversible at basic pH values and irreversible at acidic pH values, whilst at initial acidic pH values the trend is of definite decomposition with time and no buildup. Following pH change during the reaction between aldicarb and Cl, suggests that at initial acidic conditions, an electrophilic attack by HOC1 results in oxidation of the thio group followed by acidcatalyzed hydrolysis (the pH decreases because of the formation of HCl) giving secondary products such as aldicarb-sulfoxide-oxime and aldicarb-oxime. Further oxidation results in the formation of aldicarb-sufone from aldicarb-sulfoxide which is resistant to reaction with chlorine. At initial alkaline pH values, there is initial HOC1 so less HCl is formed and also acid-catalyzed hydrolysis competes with base-catalyzed hydrolysis resulting in a slower process since acidic conditions are achieved only after 93 h, as opposed to 5 h for initial acidic pH values. The finding that the thio group is oxidized by Cl, to give aldicarb-sulfoxide and aldicarb-sulfone is also supported by the findings of Reamonn and O’Sullivan (1976) who found that the electrophilic attack by HOC1 on sulfur containing compounds resulted in the sulfoxide and that this is followed by further reaction with the hypochlorous ion to give sulfoxide-epoxides (various isomers). This process is highly pH-dependent yielding different ratios at different pH values (pH 4,6, 13) (Reamonn and O’Sullivan; 1976). That aldicarb-sulfoxide can be formed as a by product in these reactions is further supported by the work of Grossert et al. (1977) where they discuss the potential in such reactions for the formation of oxidation products such as sulfones. Other possible by-products that correspond to evidence already in the literature include chlorinated sulfoxides (Bennett et al., 1963; Drabowicz et al., 1988). A possibility of chlorine attack on the C--N double bond exists but only identification of the unknown products can support this assumption. At pH values close to neutrality, it appears that aldicarb and methomyl are more resistant to attack by HOC1 (Fig. 7). This result is in accordance to the results of Given and Dierberg (1985) who showed that chemical hydrolysis is less effective at pH 7.0 due to competing processes such as alkaline hydrolysis and acid-catalyzed hydrolysis and, in this case, electrophilic attack of HOC1 competes as well. The variety of by-products can also be explained by the

19

fact that minor by-products are results of the reaction of chlorine with the anti-isomer of aldicarb as well as of the syn-isomer as reported previously by Bank and Tyrell (1984). The effect of pH in changing the reaction mechanism between alkaline and acid pH values has also been reported by Bank and Tyrell (1984) suggesting that at pH values below 5.0 an unusual acid-catalyzed hydrolysis reaction occurs. Results from the preliminary bioassay performed on D. magna supports the finding that by-products are not as toxic as the parent compound. In order to suggest a complete mechanism for the reaction of Cl, and oxime carbamates, different tools of analysis are needed such as FT NMR or LC-MS since the reaction by-products seem to be relatively polar non-volatile and thermally unstable which explains the unfruitful results of the GC-MSD (CI and EI) analyses. The two aromatic carbamate insecticides selected, carbaryl and propoxur, did not react with Cl2 under conditions used in these experiments where the Cl,:carbaryl ratio was 28: 1, molar concentration ratios which were selected to simulate realistic chlorine concentrations in treated drinking water. Previous work by Poncin et al. (1980) showed that carbaryl does indeed react with chlorine, however they used considerable excess of Cl* (50,000: 1). Poncin et al. (1980) also observed an increase in the reaction rates with increasing pH in the reaction with carbaryl, a result which contradicts the findings here for the behavior of the aliphatic oxime carbamates. This therefore suggests that a different reaction mechanism altogether must exist when using very large concentrations of Cl?. No reaction was observed when chlorine dioxide was used as the disinfecting agent with any of the four selected carbamates using 14-l 8 : 1 (CIOz :carbamate on a molar ratio basis) at pH 7.0 within 24 h. This may be explained by the fact that C102 is a very selective oxidant attacking as an electron transfer reaction and being an electron acceptor, it will only react if a stable cation is formed. Obviously, no stable cationic radical can be formed in this case since C-N-O and H-C-N are very unstable ions. The reaction of ozone with the thio-carbamates giving aldicarb-sulfoxide can be explained as an electrophilic attack by ozone on the sulfur atom giving an adduct R, S-O-O-O-, which decomposes to molecular oxygen and the sulfoxide (Bailey, 1982): R, &-O-O..

2

R, S&j:

Attack on the carbon-nitrogen double bond (Bailey, 1982), and/or hydrolysis of the ester linkage (Prengle and Mauk, 1978) are also possibilities but no evidence for these modes of attack were found in this study. 03 oxidation of aromatic carbamates such as propoxur results in their complete destruction by hydrolysis and ring hydroxylation (Prengle and Mauk, 1978).

20

YAEL (ZELICOVITZ) MASON et td.

In conclusion, the use of chlorination for the removal of oxime-carbamates is feasible. One of the problems of using chlorine to remove oxime carbamates is that when the pH is neutral to alkali, the reaction with aldicarb results in the formation of even more persistent by-products such as aldicarbsulfone. Similar effects on other oxime carbamates must thus also be considered possible. A solution to this problem might be chlorination at neutral to low pH values which appears to be more effective in the removal of all by-products. It should be borne in mind that the chlorine dose might need to be increased in the treatment of groundwaters containing persistent oxime-carbamate residues. In groundwater treatment, only a very small amount of chlorine is currently used and this is consumed mainly in reactions with inorganic compounds which are often present at high concentrations, such that the more unreactive organic compounds might still be found following treatment. Ozonation also appears to be a successful process for the removal of the parent compounds from water and appears to be less selective than chlorine in destroying both aromatic and oxime carbamates. The use of chlorine dioxide is not recommended in this case. Acknowledgemenls-We are very grateful to W. Glaze for his valuable comments on this work and to M. Hoffman for his comments. REFERENCES

APHA (1985) Standard Methods for Examination of Water and Wastewaters, 16th edition. American Public Health Association, Washington, D.C. Bailey P. S. (1982) Ozonafion in Organic Chemisrry, Vol. I. Academic Press, New York. Bank S. and Tyrell R. J. (1984) Kinetics and mechanism of alkaline and acidic hydrolysis of aldicarb. J. agric. Fd Chem. 32, 1223-1239. Bennett L., Goheen D. W. and Macgregor W. S. (1963) Aqueous chlorination of dimethyl sulfides. J. org. Chem. 28, 2485-2486.

Blaicher G., Pfannhauser W. and Woidich H. (1980) Problems encountered with the routine application of HPLC to the analysis of carbamate pesticides. Chromat0graph.y 13, 438-446.

Chapman R. A. and Cole C. M. (1982) Observations on the influence of water and soil pH on the persistence of insecticides. J. enoir. Sci. Hith B17, 487-504. Christensen H. E. and Luginbyhl T. T. (1975) Suspected carcinogens-a subfile of the NIOSH toxic substance list. U.S. Department of Health, Education and Welfare. Rockville, Md. Dewer M. J. S. and Fahey R. C. (1963) Electrophilic addition to olefins. J. Am.. Chem. Sot. 85, 22452iS2. Drabowicz J. Kiexbasinski P. and Mikoxaiczvk M. (1988) Synthesis of sulfoxides. In The Chemistry o/ Suljones and Sulfoxides (Edited by Patai S., Rappoport 2. and Stirling C.). Wiley, Chichester. Given C. J. and Die&erg F. E. (1985) Effect of pH on the rate of aldicarb hydrolysis. Bull. envir. conram. Toxic. 34, 627-633. Grossert J. S., Hardstaff W. R. and Langler R. F. (1977) Intermediate stages of sulfohaloform reaction. Preparation of a-halosulfoxides and sulfinyl chlorides. Oxygen transfer reactions. Can J. Chem. 5, 421426.

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Lemley A. T.. Zhong W. Z., Janauer J. E. and Rossi R. (1984) Investigation of degradation rates of carbamate pesticides: exploring a new detoxification method. Am. Chem. Sac. Symp. Ser. 259, 245-259.

Masschelein W. J. (1967) Preparation of pure CIO,. Reactions of chlorine or sodium hypochlorous solution with sodium chlorite in the presence of acetic anhydride. Indusr. Engng Chem. Prod. Res. Dev. 6, 137-142. Masschelein W. J. (1979) Chlorine Dioxide Chemistry and Environmenral Impact qf Oxychlorine Compounds. Ann Arbor Science, Ann Arbor, Mich. Metcalf R. L. and Fokuto T. R. (1965) Effects of chemical structure on intoxication and detoxication of phenyl-Nmethyl carbamates in insects. J. agric. Fd Chem. 13, 220-23 1. Miles C. J. and Delfino J. J. (1984) Determination of aldicarb and its derivatives in ground water by HPLC with U.V. detection. J. Chromat. 299, 275-280. Moore J. C.. Hansen D. J. Garnas R. L. and Goodman L. R. (1985) A sand/granular carbon filtration treatment system for removing aqueous pesticide residues from a marine toxicology laboratory effluent. War. Res. 19, 1601 -1604. Morris J. C. (1978) The chemistry of aqueous chlorine in relation to water chlorination. In Water Chlorination: Environmental Impact and Health Effects (Edited by Jolley R.), Vol. I. pp. 21. 35. Ann Arbor Science, Ann Arbor, Mich. Njagi G. D. E. and Gopalan H. N. B. (1980) Mutagenicity testing of some selected food preservatives, herbicides, insecticides. II: Ames test. Bangladesh J. Bof. 9, 141-149. Osman M. A. and El-Dib M. A. (1972) Studies of the persistence of some carbamate insecticides in the aquatic environment. In Fate of Organic Pesticides in the Aquaric Encironmenr (Edited by Gould R. F.), pp. 210-243. Advances in Chemistry Series 1I I, American Chemical Society. Osman M. A. and Bela1 M. H. (1980) Persistence ofcarbaryl in canal water. J. envir. Sci. Hlth B15, 307-311. Poncin J., Plusquellec D. and Martin G. (1980) Oxydation de carbamates en milieu aqueux. Example de la chloration du Carbaryl. Jour. Fran&s d’Hydrol: 11, 101-121. Prabhaker J. M. and Fraumeni J. F. (1978) Possible relationship of insecticide exposure to kmbrional cell carcinomas. J. Am. med. Ass. 240, 288. Prengle H. W. Jr and Mauk C. E. (1978) Ozone/UV oxidation of pesticides in aqueous solution. In Ozone/Chlorine Dioxide Oxidation Products of Organic Marerial. Proc. Conf. Ozone Press, pp. 302-320.

Rtamonn L. S. S. and O’Sullivan I. (1976) Selective oxidation of 2-Benzylidene-2,3-dihydro-S-methylbenzo[b]-

Carbamates degradation by water disinfection thiophen-3-one by sodium hypochlorous-hypochlorous acid. J. Chem. Sot. Chem. Commun. 1012-1013. Rothschild E. R., Manser R. J. and Anderson M. P. (1982) Investigation of aldicarb in groundwater in selected areas of the central sand plain of Wisconsin. Grnd War. 20, 437-445. Schlagbauer B. G. L. and Schlagbauer A. W. J. (1972) The metabolism of carbamateea literature analysis. In Residue Reuiews (Edited by Gunther F. A.). Vol. 42, pp. I-90. Springer, Berlin. Stevenson R. G.. Dailev L. L. and Ratiaan B. J. (1981) Chemistry in Water keuse (Edited by-cooper W. J.), Vol. I, Chap. 21. Ann Arbor Science, Ann Abor, Mich. Union Carbide (1975) Temik-Aldicarb pesticide. Technical information. Union Carbide Company, Salinas, Calif. Union Carbide (1979) Temik-Aldicarb pesticide removal of residues from water. Union Carbide Agricultural Company (internal publication). Watts R. R. (1980) Analytical reference standards and

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supplemental data for pesticides and other organic compounds. EPA600/2-8 l-01 1. White G. C. (1986) The Handbook of Chlorination, 2nd edition. Von Nostrand Reinhold, New York. Wolfe N. L., Zepp R. G. and Paris D. E. (1978) Carbaryl, propham, and chloropham: a comparison of the rates of hydrolysis and photolysis with the rate of biolysis. War. Rex 12, 565-571. World Health Organization (1976) Evaluation of carcinogenic risk of chemicals on man. International Agency for Research on Cancer, IARC Monographs 12, WHO, Lyon. Zaki M. H., Moran D. and Harris D. (1982) Pesticides in groundwater. The aldicarb story in Suffolk County, New York. Am. J. Publ. Hlth 72, 1391-1395. Zhong W. Z. and Lemley A. T. (1984) Quantitative determination of ppb levels of carbamate pesticides in water by gas chromatography. J. Chromat. 299, 269-274.