Nonbiological Degradation and Transformations of Organic Pesticides

35 Nonbiological Degradation and Transformations of Organic Pesticides in Aqueous Systems

Downloaded by UNIV OF CINCINNATI on November 12, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch035

SAMUEL D. FAUST Department of Environmental Sciences, Rutgers, The State University, New Brunswick, N.J. 08903

It may be stated categorically that all organic pesticides are thermodynamica11y unstable in natural aquatic environments. These so-called recalcitrant compounds have their carbon atoms in a reduced valence state. Consequently, there is an inherent tendency for the carbon to be oxidized to higher valence states in an aerobic environment. If conditions are favorable, CO (g)(+IVC)is the eventual product. Many terms are employed to describe the instability of organic pesticides in nonbiological systems: oxidation, transformation, degradation, etc. Definition and interpretation of these terms are, in mose cases, arbitrary for most investigators and authors. In this paper, oxidation and/or degradation refers to a change in the valence state of the organic C. Usually this change occurs whereby the carbon goes to a higher valence state which requires, of course, an electron acceptor. Transformation refers to any alteration in the configuration of an organic molecule. Examples of transformations would be: the hydrolysis of organic phosphorus, carbamates, and urea compounds, replacement of the = S by = 0 in organic phosphorus compounds, modification of the trichloroethane group in DDT, replacement of the Cl atom by OH in atrazine, etc. These two definitions are somewhat at variance with Kearney and Kaufman (1) who prefer the term "degradation" "to cover all transformations of organic herbicides without particularly trying to ascribe these to enzyme or particulate systems." 2

II.

Hydrolytic Transformations

Many organic pesticides undergo hydrolytic transformations in aquatic environments. Many of these compounds have been synthesized as an organic ester of some sort. This i s especially true for the pesticides that are phosphates, carbamates, ureas, and p h e n o x y a c e t a t e s · 572

In Marine Chemistry in the Coastal Environment; Church, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

35.

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Pesticides

A . Organophnaphat*a- The organophosphorus pesticide i s a tertiary phosphate or thiophosphate ester: S(0)

} Ρ - 0 - R» (S)

R - 0'


1

R i s usually a methyl or an ethyl group and R i s an organic moiety* Hydrolytic transformation of these compounds occur from rupture of either the P-O(S) bond or the (S)O-R bond. Very seldom i s the hydrolysis carried to rupture of the R-0 bond (tertiary hydrolysis). Once again the nature of the R group becomes important because the hydrolytic product may be environmentally hazardous. Whether the P-0(S)-R' bond i s ruptured depends upon the structure of the pesticide and the hydrolytic conditions. Various mechanistic studies i n 0*® r i c h water have shown that, in alkaline solutions, the P-O(S) bond i s broken and the (s)O-R i s usually replaced (2). In acidic hydrolysis, however, a rupture of the (S)O-R' bond apparently occurs as an i n i t i a l step. Secondary esters undergo additional hydrolysis under acid conditions to primary esters which usually does not oc cur under alkaline conditions. Generalized reactions may be written for these two conditions of hydrolysis: 1

1

0(S) II

(R0)

0(S) OH\

r 1

2

- Ρ τ 0-R (S)

H0 2

II

'

(R0)

2

- P-OH

1

0(S) II

(R0)

2

+

R'OH (S)

0(S) !

- Ρ - Ο**

HJO 1

—4 H0 2

II

R0H +R" - Ν - R" + C 0 ( )

where the products are a hydroxy compound (phenol), an amine, and C0 (g). Aly and El-Dib (7) reported the hydrolytic sta b i l i t y of four carbamates: sevin, baygon , pyrolan, and d i me t i l a n over the pH value range of 2 to 10. First-order kinetics of hydrolysis was observed from which the h a l f - l i f e 2


2

g

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values were calculated (Table I I I ) . Pyrolan and dimetilan did not hydrolyze within this pH range and at 20°C. Baygon resisted hy drolysis at pH values 3 through 7 but did decay under alka line conditions. Sevin was the least stable of the four carba mates with hydrolysis occurring at pH values 7.0 and above with the rate increasing as the (OH") was increased. Casida et a l . (8) found four p-nitrophenyl N-alkyl carbamates to be quite un stable at a pH value of 7.8. These four compounds are not widely, i f at a l l , used as commercial products. Table I I I . HYDROLYTIC HALF-LIFE VALUES FOR SOME CARBAMATE PESTICIDES Compound 7 a

Sevin Baygon 1 2* 3 4 B

d

b

a. b.

a

10.5 days -

8 1.3 16.0 8.9 6.9 7.1 5.9

pH Value 9 days days min. min. min. min.

2.5 hrs. 1.6 days -

10 15 min. 4.2 hrs.

Ea Kcal/mole 16. 15.8

After Aly and El-Dlb (7) where T=20°C. After Casida et a l . (8) where T=22°C. and pH value 7.8. β

Colorimetrlc and radiometric analyses were used to study the persistence of carbaryl (sevin) i n estuarlne water and mud i n laboratory aquaria held at 8°C. and 20°C.(9). In systems where there was a sharp decrease i n the concentration of car baryl (8°C.), adsorption by the mud was the major reason for the decline. Approximately 90% of the carbaryl added to a control system without mud was present as unchanged insecticide or as 1-naphtholafter 38 days. Additional s t a b i l i t y experi ments performed i n the dark i n sea water (no mud) showed that, at low temperatures (3.5°C.) no hydrolysis of 10.0 mg/1 carbaryl could be detected after 4 days and after 8 days, only 9% of the compound was hydrolyzed (oH=7.8). The amount of carbaryl hydrolyzed i n 4 days at 17°C. was 44%, at 20°C. 55%, and at 28°C., 93%. These experiments were presumably per formed i n autoclaved sea water. In other systems, "complete conversion to non-detectable compounds occurred i n 4 days"at 19.5° and 28°C. This i s surprising. The authors do not at tempt to explain these results and the reader must assume that biological a c t i v i t y i s responsible for the rapid disappearance. The fate of 1-naphthoilJ-^C was examined i n a simulated estuarlne environment by Lamberton and Claeys (10). 1-Naphthol i s formed from the hydrolysis of carbaryl (sevin). Figure 1 shows the disappearance of 1-naphthol in light and dark systems f



Figure 1.

14

Effect of light and microorganisms on stability of 1-naphthol in seawater. After Ref. 10. A. Loss of 1-naphthol: Ο light, unsterile; • light, sterile. B. Loss of total C: · dark, unsterile; • dark, sterile.


3

ο*

§·

ο*

•i

ο

Η

G

>

pi

CO


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MARINE CHEMISTRY

and i n sterile and unsterile systems. In dark and s t e r i l e conditions, this compound i s relatively stable with approximately 75% remaining after 16 days. Light and the presence of microorganisms catalyze the disappearance of 1-naphbhol in sea water. This observation i s frequently the situation where synthetic organic compounds are concerned. Apparently, biological systems can supply the activation energy that i s necessary to degrade these compounds. Wauchope and Hague (11) reported upon the kinetics of hydrolysis of carbaryl (sevin) that was somewhat i n disagreement with Aly and El-Dib (7). For example, at pH value of 10.0 and 25°C., Wauchope and Hague reported a t ^ value of 20 minutes whereas Aly and El-Dib reported a value of 15 minutes at the same pH value and at 20°C. The temperature difference does not account for the disagreement which may be due to analytical techniques. The hydrolysis of zectran (4-dimethylamino-3.5xylylmethyl carbamate) i n alkaline water was reported by Hosier (12). The h a l f - l i f e value at a pH value of 9.5 was approximately two days whereas at pH 7.4, the h a l f - l i f e i s approximately 2 weeks at 12-13°C. No order of reaction rates was suggested. Also, Hosier reported a purple colored water solution of zectran under alkaline condtlons (pH9-9.5) when exposed to l i g h t . This was ascribed to the rapid formation of "xylenol" from hydrolysis which i s converted to i t s "xylenoxide" ionic form that i s , i n turn, "very sensitive to photooxidation." No proof was offered to substantiate this mechanism. However, Mathews and Faust (13) have made a similar observation. 0. Miscellaneous Compounds. An examination of the hydrolysis of three explosives i n sea water was reported by Hoffsommer and Rosen (14). These explosives were: TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trlazocyclohexane), and t e t r y l (N-methy1-N-nitro-2,4,6-triazocyclohexane)· Results at 25°Care summarized below: Compound TUT RDX Tetryl

Time-days

Hydrolysis-%

108 112 101

0 11·6 88.

It appears that disposal of these compounds (tetryl excepted) at sea i s not feasible. In an attempt to simulate natural conditions, Bailey, et a l . (15) studied the hydrolysis of the propylene glycol butyl ether ester of 2-(2,4,5)TP (silvex) i n three pond waters. There was an apparent rapid decay of this ester as seen by the tj^ values :


35.

Organic

FAUST

579

Pesticides

PGBE of 2-(2,4,5) TPs

values i n pond waters: Pond A 5 hrs. Pond B 7 hrs. Pond C 8 hrs.

three natural 1 y-occurring (pH*6.25) (pH»6.09) (pH«6.07)


D

» Comment. Hydrolysis i s , indeed, an important variable af fecting the fate of the appropriate organic pesticide i n aquatic environments. The rate at which hydrolysis occurs i s , of course, unique to the individual compound and i s dependent, also, upon such environmental factors as (H3O) and tempera ture (6). Hydrolytic s t a b i l i t y should be viewed with con cern afeout the length of the time period required for com plete hydrolysis and about the products that may or may not be more toxic than the parent molecule* III.

Chemical Systems

A. Thermodynamic S t a b i l i t y . A natural water i n e q u i l i brium with the atmosphere should be saturated with dissolved oxy gen (neglecting for the moment aerobic biological transforma tions of organic matter). In this situation, this water has a well-defined redox potential of approximately + 800 mv (PO2 0 . 2 1 atm, pH - 7 . 0 , 25°G). A simple equilibrium calcu lation shows that, at this E value, a l l organic carbon should be present as C(+IV) or as G 0 , HCO3, or C 0 ~ , Sand Ν should occur i n the form of S 0 ^ 2 - and NO3, respectively. That or ganic compounds are unstable i n aquatic environments may be shown from a simple thermodynamic model: s

n

2

CH 0 + H 0 2

3

=

2

C 0 ( ) + 4H+ + 4e 2

°2(g) + 4H+ + 4 e -

2H 0

CH 0 + 0

C0

2

2

(

g

)

2

g

2

*

2 ( g )

+

H0 2

The free energy change of this reaction i s - 117.99 Kcal mole" which gives a E g value of 1.28 volts. Under the Euro pean sign convention, a negative ΔG° reaction value and a positive Eg values denotes that the l e f t to right reaction i s feasible. A similar model may be calculated for the organic herbicide 2,4-D i n an oxygenated environment^ CgH,03 C l + 2(g) 2 ( f i ) 4H++4C1 + 4 H 0 . The free energy change for this reaction (25°0) i s -1600.78 Kcal mole" which i n d i cates that i t i s feasible for the carbon i n 2,4-D to be o x i dized to C0 (g) with 0 (g)ae an electron acceptor. This model does not, however, indicate the reaction knetics or immea surably, the reaction w i l l even occur. Only laboratory ex perimentation w i l l answer these two points. More and de t a i l e d information on the thermodynamic s t a b i l i t y of organic pesticides i n aquatic systems i s given by Gomaa and Faust (16). 1

2

1 5 0

β

1 6

c o

+

2

1

2

2


MARINE

580

CHEMISTRY

Β. A r t i f i c i a l Systems. There are numerous electron ac ceptors available for oxidation of organic pesticides i n ar t i f i c i a l l y created environments. However, where the natural environment i s concerned, the oxidative reactions are extremely slow and are incomplete, i n the sense, that toxic r e a c t i o n products may be formed. I t i s extremely deslreable to effect "complete" degradation to CQ , H^O, etc. Historically, KMnO^, C10 , C I , and 0^ have been em ployed for the oxidation of organic compounds at water treat ment plants. Consequently, these oxidants have been investi gated for their capacity to degrade organic pesticides (16, 17, 18, 19» 20). Several basic concepts have evolved from these five studies for the oxidation of organic compounds. 2


2

2

1. Stoichiometry: Oxidative processes at water treat ment plants, for example, are often ineffective because inade quate stoichiometries, i.e., chemical dosages are employed. Gomma and Faust (16) proposed the exact stoichiometric r e l a tionship for several organic pesticides and inorganic oxidants. For example, parathlon and KMnO^ gave: C H O NSM0ifaO^+l4H =20îftiO H0CO +SO^+PO^14H O • MO3 +

10

l4

5

2

2

2

3C H O NSP+50MnO4=50^iO +15C o|" -f ^3+3SO|-+3K) | +20H 0^2 0H ?

3L0

14

5

2

-

2

2

These two reaction models suggest rather complex molar ratios of oxidant to compound. The f i r s t reaction represents acidic conditions i n which the ftinO^/parathion molar ratio i s 20:1. In the second reaction, the molar ratio under alkaline conditions is 16.67:1. In order to write these two reactions and others (16), several assumptions were made for the products: a. b. c. d.

H1O4" goes to Mn0 / \ and not to Mn organic C goes to u L i n acid conditions (C+IV) organic C goes to C u,2~ i n alkaline conditions (C+III) _ nitro group goes to N0g~, Ρ group goes to 4 " » and S group goes to SO^ ". 2+

2

2

P0

3

2

That there i s some credibility to the proposed reactions i s seen in Table IV where the moles of KfâiO^ consumed per mole of compound i s i n good agreement with the calculated values. 2. Reaction Kinetics. Well-designed kinetic experiments yield data from chemical oxidation systems that w i l l : a.

confirm thermodynamic reactions models with respect to feasibility and stoichiometry

b.

determine the order of the disappearance reaction


35.

Organic

FAUST

Pesticides

581

c. determine the rate and reaction times required to process design


Table IV.

POTASSIUM PERMANGANATE CONSUMED BY THE OXIDATION OF PARATHION, PARAOXON, AND p-NITROPHENOL (16)*

Compound

£2

Parathion Paraoxon p-Nitrophenol Parathion Paraoxon p-Nitrophenol

3.1 3.1 3.1 9.0 9.0 9.0

(a)

Mole MnO^/Mole Comp.

Contact Time (Hours)

Ratio

Experimental Calc.

120 120 96 48 48 96

0.386 0.364 0.404 0.392 0.403 0.367

19.48 16.69 9.10 17.08 14.17 7.62

20.00 17.33 9.33 16.66 14.00 7.33

Temperature * 20° + 0.2°C.

Determination of the order of the reactions may be accomplished by f i t t i n g the data into several rate equations. These r e actions are, of course, extremely complex and an exact order i s not expected over the entire course of events. Intermediate oxidation products w i l l compete with the parent molecule for the oxidant. Nontheless, Gomma and Faust (21) found that an integrated form of a second-order expression provided reasonably constant rate constants for the KMnO* oxidation of two d i pyridylium quarternary herbicides - Diquat and Paraquat. This expression was: 2

aC

·

3

0

l

b

°ox "

Nonbiological Degradation and Transformations of Organic Pesticides

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