Organic Matter Reactions Involving Pesticides in Soil - ACS Publications

15 Organic Matter Reactions Involving Pesticides in Soil F. J. STEVENSON

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0029.ch015

Department of Agronomy, University of Illinois, Urbana, Ill. 61801

Adsorption by organic matter has been shown to be a key factor in the behavior of many pesticides i n s o i l . Numerous examples where bioactivity, persistence, biodegradability, leachability, and v o l a t i l i t y have been shown to bear a direct relationship to organic matter content can be found i n several reviews on the subject (1, 2, 3, 4, 5,). It has been well established, for example, that the rate at which any given adsorbable herbicide must be applied i n order to obtain adequate weed control can vary as much as 20-fold, depending upon the nature of the s o i l and the amount of organic matter it contains. Soils which are black i n color e.g., most Mollisols) have higher organic matter contents than those which are light colored (e.g., A l f i s o l s ) , and pesticide application rates must often be adjusted upward on the darker s o i l s i n order to achieve the desired result. Adsorption by organic matter depends to a considerable extent upon the physical and chemical properties of the pesticide -- each type has i t s own special features and must be considered separatel y . Information as to how pesticides react with s o i l organic matter may provide a more rational basis for their effective use, thereby reducing undesirable side effects due to carry-over, contamination of the environment, and, i n the case of herbicides, phytotoxicity to subsequent crops. The organic fraction of the s o i l also has the potential for promoting the nonbiological degradation of many pesticides (3, 6, 7), as well as for forming strong chemical linkages with residues arising from their p a r t i a l degradation by microorganisms (3). These aspects of pesticide-soil organic matter interactions deserve further study because such processes would play an important role i n detoxification and protection of the environment. Chemical binding of pesticide-derived residues would increase their persistence i n the s o i l but i n forms unharmful to the environment. Chemical Nature of Soil Organic Matter Soil organic matter chemistry is undoubtedly the least under180

In Bound and Conjugated Pesticide Residues; Kaufman, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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stood f i e l d o f s o i l s c i e n c e , and i n many ways the most p e r p l e x i n g * As Hayes (2) and Stevenson (3) have pointed out, mechanisms o f p e s t i c i d e - o r g a n i c matter i n t e r a c t i o n s w i l l remain obscure u n t i l more is known about the nature and chemical composition of the organic f r a c t i o n o f s o i l s . Organic matter e x i s t s i n many forms i n s o i l , i n c l u d i n g the unmodified remains o f p l a n t and animal t i s s u e s ( p l a n t d e t r i t u s , r o o t s , b a c t e r i a l and f u n g a l t i s s u e ) and secondary products o f m i c r o b i a l metabolism. The l a t t e r is commonly r e f e r r e d to as "humus" and is o f t e n used synonymously w i t h " s o i l organic matter . T h i s r e p o r t w i l l be devoted l a r g e l y to p e s t i c i d e r e a c t i o n s i n v o l v ing "humus" although it should be noted t h a t p l a n t residues and the m y c e l l i a l t i s s u e o f actinomycetes and f u n g i may a l s o be important i n p e s t i c i d e adsorption (5). Humus, o r s o i l organic matter, can f u r t h e r be c l a s s i f i e d i n to two main groups o f compounds, nonhumic substances and humic substances (8, 10, 11). The former i n c l u d e s substances belongi n g to the well-known c l a s s e s o f organic compounds, such as the carbohydrates, p r o t e i n s , f a t s , waxes, and r e s i n s . The l a t t e r group, the s o - c a l l e d humic and f u l v i c a c i d s , represents c h e m i c a l l y and b i o l o g i c a l l y modified substances which bear little if any resemblance to any o f the known organic compounds. The s o i l , being a graveyard f o r the bodies of micro- and macrofaunal organisms, would be expected to c o n t a i n most o f the biopolymer and b i o chemical compounds synthesized by l i v i n g organisms. As one might suspect, many of the biochemicals w i l l occur i n exceedingly s m a l l q u a n t i t i e s . N e v e r t h e l e s s , c e r t a i n t r a c e biochemicals have the p o t e n t i a l f o r forming conjugates w i t h p e s t i c i d e s , as w i l l be noted l a t e r . The humified m a t e r i a l , which represents the most r e a c t i v e component of humus, c o n s i s t s o f a s e r i e s of h i g h l y a c i d i c , y e l l o w to b l a c k - c o l o r e d , high-molecular-weight p o l y e l e c t r o l y t e s r e f e r r e d to by such names as humic a c i d , f u l v i c a c i d , e t c . The dynamic nature o f these substances is due t o t h e i r h i g h contents o f oxygen-containing f u n c t i o n a l groups, i n c l u d i n g COOH, p h e n o l i c - , a l i p h a t i c - , and enolic-OH, and C«0 s t r u c t u r e s o f v a r i o u s types. Amino, h e t e r o c y c l i c amino, imino, and s u l f h y d r y l groups may a l s o be present. The current view is that the v a r i o u s humic f r a c t i o n s represent a complex mixture of molecules which vary i n a systema t i c way w i t h regard to such p r o p e r t i e s as degree of p o l y m e r i z a t i o n , molecular weight, exchange a c i d i t y , and content o f oxygenc o n t a i n i n g groups (Figure 1 ) . I n c l a s s i c a l terminology, humic a c i d is d e f i n e d as the m a t e r i a l e x t r a c t e d from s o i l by a l k a l i n e s o l u t i o n s and which p r e c i p i t a t e s upon a c i d i f i c a t i o n : f u l v i c a c i d is the m a t e r i a l remaining i n s o l u t i o n . Humic a c i d can f u r t h e r be d i v i d e d i n t o brown humic a c i d (coagulated w i t h e l e c t r o l y t e under a l k a l i n e c o n d i t i o n s ) and gray humic a c i d (not coagulated w i t h e l e c t r o l y t e ) . In the o l d e r l i t e r a t u r e , c o n s i d e r a b l e importance was given to " c r e n i c " and "apocrenic" a c i d s , which are l i g h t y e l l o w f u l v i c a c i d - t y p e substances. Renewed i n t e r e s t has r e c e n t l y


1


1

BOUND AND CONJUGATED PESTICIDE RESIDUES

182


been shown to these rather low-molecular-weight substances because of their ubiquitous occurrence i n natural waters. The range of oxygen-containing functional groups i n humic and f u l v i c acids is given below. Total acidities of f u l v i c acids (usual range of 900 to 1,400 meq/100 g) are considerably higher than humic acids (usual range of 500 to 870 meq/100 j? 1. Both COOH and acidic OH groups (presumed to be phenolic OH) contribute to the acidic nature of these substances, with COOH being the most important.

Total acidity

COOH

Acidic OH*

Weakly acidicplus alcoholic OH

C-0

normal range, meq/100 g Humic acids Fulvic acids

500-870 900-1,400

150-300 610-910

250-570 270-670

270-350 330-490

90-300 110-310

*Usually reported as "phenolic OH" For reasons outlined above, humic acid (the most extensively Investigated component of s o i l humus) cannot be regarded as a single chemical entity capable of being described by a single structural formula — no two molecules may have the precise chemical structure (6, j), 11). A "typical" molecule is believed to consist of micelles of polymeric nature, the basic structure of which is an aromatic ring of the d i - or trlhydroxyphenol type bridged by -0-> -NH-, N«, and -S- linkages and containing both free OH groups and quinone linkages. The dark color of humic acids, and their a b i l i t y to form adsorption complexes with a variety of inorganic and organic substances, is consistent with this concept. The "type structure" for humic acid shown i n Figure 2 meets many of the above requirements. While the humic acids i n any given s o i l w i l l vary widely i n composition, most molecules would be expected to contain the same basic units and the same types of reactive groups indicated by the model structure. A number of sites are i l l u s t r a t e d which can combine with herbicides, such as by electrostatic bonding (attraction of a positively charged organic cation to an ionized COOH or phenolic Oft group), H-bonding (note large numbers of COOH, OH, and C«0 groups), and ligand exchange (formation of a covalent bond with an attached metal ion)• Other type structures proposed for humic acids are given elsewhere (9, 10, 11). Humic substances also contain rather high concentrations of 8table free radicals, possibly of the hydroxyquinone type (12). These sites may be of considerable importance i n the binding of certain herbicides, particularly those capable of being ionized or protonated to the cation form. Several aspects of organic matter chemistry require further


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Humus (Decomposition products of organic residues)


(Known classes of organic compounds)

(Pigmented polymers)

Fulvic acid (Oden)

Humic acid (Berzelius)

Crenic acid ' Apocrenic acid (Berz alius) light yellow

Brown humic acids Gray humic acids (Springer)

| Yellow-brown

Dark brown

Gray-black

increase in degree of polymerisation 2,000?

-300,000?

increase in molecular weight

45%-----

increase in carbon content---

48%- -

decrease in oxygen content —

—•^30% —*»-500

1,400--- - - -decrease in exchange acidity -

Figure J. Classification and general chemical properties of humic substances (adapted from Ref. 10)

HC«0 I (Sugor) (HC-0H) 4

I

R-CH

I (Ptptide)

00

Figure 2. Type structure for humic acid


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elaboration regarding the fate of pesticides i n s o i l , including (i) organic matter-clay interactions, ( i i ) quantitative d i f f e r ences i n organic matter, and ( i l l ) potential chemical reactions between pesticides and organic substances i n s o i l . These items w i l l be discussed b r i e f l y i n the sections which follow. Chemical formulas of the pesticides mentioned i n this review are given i n Table 1.


Organic Matter Versus Clay as Adsorbent Clay and organic matter are the s o i l components most often implicated i n pesticide adsorption. However, individual effects are not as easily ascertained as sometimes assumed, for the reason that, i n most s o i l s , the organic matter is intimately bound to the clay, probably as a clay-metal-organic complex. Thus, two major types of adsorbing surfaces are normally available to the pesticide, namely, clay-humus and clay alone. Accordingly, clay and organic matter function more as a unit than as separate entities and the relative contribution of organic and inorganic surfaces to adsorption w i l l depend upon the extent to which the clay is coated with organic substances. As can be seen from the schematic diagram shown in Figure 3, the interaction of organic matter with clay s t i l l provides an organic surface for adsorption. Data published by Walker and Crawford (13) for adsorption of some s-triazines by 36 s o i l s having widely variable organic matter contents (Figure 4) suggests that, up to an organic matter content of about 6%, both mineral and organic surfaces are involved i n adsorption: at higher organic matter contents, adsorption w i l l occur mostly on organic surfaces. It should be noted, however, that the amount of organic matter required to coat the clay w i l l vary from one s o i l to another and w i l l depend on the kind and amount of clay that is present. For soils having similar clay and organic matter contents, the contribution of organic matter w i l l be highest when the predominant clay mineral is kaolinite and lowest when montmorillonite is the main clay mineral. Bailey et a l . (14) demonstrated that the adsorption capacity of clays for herbicides followed the order montmorillonite > i l l i t e > kaolinite. Comparative studies between known clay minerals and organic s o i l s suggest that most, but not all, pesticides have a greater a f f i n i t y for organic surfaces than for mineral surfaces. Scott and Weber (15) found that the phytotoxicities of 2,4-D, prometone, and CIPC to the test plant were reduced to a much greater extent by addition of an organic s o i l to the growth media than by addition of montmorillonite or kaolinite. Donerty and Warren's (16) results show that both fibrous peat and a well-decomposed muck were more adsorptive than bentonite for pyrazone, linuron, prometone, and simazine. Hance (17) concluded that diuron was a more effective competitor for water at organic matter surfaces than at mineral surfaces, and Deli and Warren (18) found that organic



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R_(

l f

H

(p«prtd«)

c«o NH

I

Figure 3. Clay-metal-organic matter complex

40 -

0

8

16

24

32

Kd Figure 4. Relationship between organic matter content and amount of atrazine adsorbed by 36 soils. Kd *= fimoles adsorbed per g/fimoles per ml equilibrium solution. From Ref. 3 as adapted from Ref. 13.


186


Table 1.

Chemical designations o f organics mentioned i n t e x t .

Common name

Chemical

formula

s-Triazines


Atrazine Simazine At ratone Ametryn Prometon Prometryn Propazine

2-chloro-4-ethylamino-6-isopropylamino-striazine 2-chloro-4,6-bis(ethylamino)-s-triazine 2-methoxy-4-ethylamino-6-isopropylamino-striazlne 2-methylthio-4-ethylamino-6-isopropylamino-striazine 2-methoxy-4,6-bis(isopropylamino)-s-triazine 2-methylthio-4,6-bis(isopropylamino)-striazine 2-chloro-4,6-bis(isopropylamino)-s-triazine

S u b s t i t u t e d ureas Diuron Monuron Fenuron Linuron Neburon

3-(3,4-dichlorophenyl)-1,1-dimethylurea 3-(p-chlorophenyl)-1,1-dimethylurea 3-phenyl-l,1-dimethylurea 3-(3,4-dichlorophenyl)-1-methoxy-l-methylurea l-butyl-3-(3,4-dichloropheny)-1-methylurea

Phenylcarbamate CIPC

isopropyl

m-chlorocarbanilate

Bipyridylium quaternary s a l t s 1

1

6,7-dihydrodipyrido(1,2-a:2 ,1 -c)pyrazidinium s a l t 1,1'-dimethyl-4,4 dipyridinium s a l t

Diquat

1

Paraquat


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Table 1 (Cont'd) Common name

Chemical formula


Others Amiben 2,4-D Picloram Dalapon Diphenamid Trifluralin DCPA DNPB Amitrole Pyrazone Lindane DDT

3-amino-2,5-dichlorobenzoic a c i d 2,4-dichlorophenoxyacetic a c i d 4-amino-3,5,6-trichloropicolinic acid 2,2-dichloropropionic a c i d N,N-dimethyl-2,2-diphenylacetamide ot, JC £ - t r i f l u r o - 2 , 6 - d i n i t r o - N - N - d i p r o p y l p-toluidine dimethyl-2, β, 5 , 6 - t e t r a c h l o r o t e r e p h t h a l a t e 4,6-dinitro-o-sec-butylphenol 3-araino-l,2,4-triazol 5-amino-4-chloro-2-phenyl-3-(2H)-pyridazone 1,2,3,4,5,6-hexachlorocyclohexane 1,1,l-trichloro-2,2-bis(p-chlorophenyl) ethane 9



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BOUND AND

CONJUGATED PESTICIDE

RESIDUES

matter was more e f f e c t i v e i n adsorbing diphenamid than c l a y ( b e n t o n i t e ) . In other s t u d i e s , Weber, P e r r y , and I b a r a k i (19) found t h a t , on a weight b a s i s , an organic s o i l was more e f f e c t i v e than m o n t m o r i l l o n i t e i n reducing the p h y t o t o x i c i t y of prometone to wheat. For the s - t r i a z i n e s , prometone and prometryne may p r e f e r m i n e r a l surfaces (20). Laboratory s t u d i e s have, i n g e n e r a l , corroborated f i e l d observations i n d i c a t i n g t h a t organic matter p l a y s a major r o l e i n the performance of s o i l - a p p l i e d p e s t i c i d e s . This work has gen e r a l l y i n v o l v e d m u l t i p l e c o r r e l a t i o n a n a l y s i s f o r p e s t i c i d e ad s o r p t i o n by a s e r i e s of s o i l s w i t h w i d e l y d i f f e r e n t p r o p e r t i e s , the u s u a l s o i l parameters being organic matter content, t e x t u r e ( c l a y c o n t e n t ) , c l a y m i n e r a l type, pH, and c a t i o n exchange capa c i t y . I n a t y p i c a l study, a given q u a n t i t y o f s o i l is added to a p e s t i c i d e s o l u t i o n of known c o n c e n t r a t i o n , the mixture is allowed to e q u i l i b r a t e , and the c o n c e n t r a t i o n of the p e s t i c i d e i n the s o l u t i o n phase is estimated. The amount of p e s t i c i d e adsorb ed is subsequently c a l c u l a t e d from the change i n c o n c e n t r a t i o n and is u s u a l l y expressed by such u n i t s as μ moles adsorbed per Kg of s o i l (x/m). By r e p e a t i n g the measurements a t s e v e r a l p e s t i c i d e c o n c e n t r a t i o n s , an adsorption isotherm can be obtained by p l o t t i n g the q u a n t i t y adsorbed (x/m) v s . the e q u i l i b r i u m concen t r a t i o n (C). In most i n s t a n c e s , a s t r a i g h t l i n e is obtained when the data are p l o t t e d as l o g x/m v s . l o g C, according to the F r e u n d l i c h a d s o r p t i o n equation, x/m

- KC

1 / n

where Κ and η are c o n s t a n t s . The constant Κ provides a measure of the extent of a d s o r p t i o n and has been used i n c o r r e l a t i o n s t u d i e s aimed a t determining the r e l a t i v e importance of the v a r i o u s s o i l parameters on a d s o r p t i o n . A l t e r n a t e l y , a d i s t r i b u t i o n c o e f f i c i e n t , Kd, can be obtained f o r a given s o l u t i o n c o n c e n t r a t i o n as the r a t i o of the amount of p e s t i c i d e adsorbed to the amount remaining i n s o l u t i o n K
t r i f l u r a l i n > 2,4-D > diphenamid > DCPA > DNPB > amiben > paraquat (no adsorption)· The most readily desorbed herbicide was 2,4-D; CIPC and DNPB showed little or no desorption. The p o s s i b i l i t y of using activated charcoal to detoxicate herbicide treated s o i l has been discussed by Ahrens (39) and Coffee and Warren (38). Special Role of Fulvic Acids Because of their low molecular weights and high a c i d i t i e s , fulvic acids are more soluble than humic acids, and they may have special functions with regard to herbicide transformations. F i r s t they may act as transporting agents for pesticides i n s o i l s and natural waters· Ogner and Schnitzer (40), and Schnitzer and Ogner (41), suggested that fulvic acids act as carriers of alkanes and other normally water-insoluble organic substances i n aquatic environments, and it is possible that these constituents also function as vehicles for the transport of pesticides. According to Ballard (42), the downward movement of the insecticide DDT i n the organic layers of forest soils is due to water-soluble, humicl i k e substances. Second, fulvic acids by virtue of their high a c i d i t i e s , may


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c a t a l y z e the chemical decomposition o f c e r t a i n p e s t i c i d e s . The suggestion has been made, f o r example, t h a t these c o n s t i t u e n t s might c a t a l y z e the h y d r o x y l a t i o n o f the c h l o r o - s - t r i a z i n e s (30). For a d d i t i o n a l i n f o r m a t i o n regarding f u l v i c a c i d s and t h e i r r e a c t i o n s , the reader is r e f e r r e d t o the recent book o f S c h n i t z e r and Khan (43).


P o t e n t i a l Chemical Reactions I n v o l v i n g P e s t i c i d e s and Organic Substances i n S o i l There seems little doubt but t h a t the organic f r a c t i o n o f the s o i l has the p o t e n t i a l f o r promoting the n o n b i o l o g l c a l degradation of many p e s t i c i d e s . Organic compounds c o n t a i n i n g n u c l e o p h i l i c r e a c t i v e groups o f the types b e l i e v e d t o occur i n humic and f u l v i c a c i d s (e.g., COOH, p h e n o l i c - , e n o l i c - , h e t e r o c y c l i c - , and a l i phatic-OH, amino, h e t e r o c y c l i c amino, imino, semiquinones, and o t h e r s ) are known t o produce chemical changes i n a wide v a r i e t y of p e s t i c i d e s (7, 44). Of a d d i t i o n a l i n t e r e s t is that humic sub stances are r a t h e r s t r o n g reducing agents and have the c a p a b i l i t y of b r i n g i n g about a v a r i e t y o f r e d u c t i o n s and a s s o c i a t e d r e a c t i o n s , as discussed by Crosby ( 7 ) . The occurrence o f s t a b l e f r e e r a d i c a l s i n humic and f u l v i c a c i d s f u r t h e r i m p l i c a t e s organic matter i n chemical transformations o f p e s t i c i d e s . F o r example, t h e h e t e r o c y c l i c r i n g o f amitrοle is known t o be h i g h l y s u s c e p t i b l e to a t t a c k by f r e e r a d i c a l s (45, 46). B a s i c amino a c i d s and s i m i l a r compounds have the p o t e n t i a l f o r c a t a l y z i n g the h y d r o l y s i s o f organophosphorus e s t e r s (47), as w e l l as the d e h y d r o c h l o r i n a t l o n o f DDT and l i n d a n e (48)· Miskus et a l . (49) demonstrated that c e r t a i n c h l o r o p h y l l degradation products (reduced porphyrins) can convert DDT t o DM). Substances i n s o i l organic matter which c o n t a i n hydroxy1 and amino groups, such as humic and f u l v i c a c i d s , are p o t e n t i a l l y capable o f being a l k y l a t e d by the a c t i o n o f c h l o r i n a t e d a l i p h a t i c a c i d s (e.g., c h l o r o a c e t i c , d i c h l o r o p r o p i o n i c ) , as shown below (50). R-NH R-OH

2

+ +

Cl-CH -(CH2) -C00H -> R - N H - ( C H ) 2

n

2

n + 1

Cl-CH -(CH ) -C00H -> R - 0 - ( C H ) i " 2

2

n

2

-

C 0 0 H

C O O H

n +

S p e c i f i c examples o f n o n b i o l o g l c a l transformations brought about by the o r g a n i c f r a c t i o n of the s o i l i n c l u d e s h y d r o x y l a t i o n of t h e c h l o r o - s - t r i a z i n e s (51-56) and decomposition o f a m i t r o l e (45>46). w i t h regard t o the former, Armstrong and Chesters (51) concluded t h a t h y d r o l y s i s o f a t r a z l n e r e s u l t e d from the sequence of events shown i n F i g u r e 5. Adsorption was b e l i e v e d t o take p l a c e between a r i n g n i t r o g e n atom and a protonated COOH group o f t h e organic matter. Hydrogen bonding o f the r i n g n i t r o g e n was b e l i e v ed t o cause the withdrawal o f e l e c t r o n s from the e l e c t r o n d e f i c i e n t carbon atom bonded t o the c h l o r i d e ; thereby enabling water to r e p l a c e the c h l o r i d e atom. Nearpass (55)found t h a t propazine h y d r o l y s i s was enhanced i n the presence o f organic matter i r r e s p e c t i v e o f the pH o f the system and was r e l a t e d i n some way t o


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193


a d s o r p t i o n . I n other work, Hance (53) was unable to e s t a b l i s h a r e l a t i o n s h i p between r a t e o f a t r a z l n e decomposition and extent o f adsorption. The review of Crosby (7) should be consulted f o r other examples o f n o n b i o l o g l c a l degradation of p e s t i c i d e s by r e a c t i o n w i t h o r g a n i c substances.


Chemical B i n d i n g o f P e s t i c i d e s and T h e i r Decomposition Products S u b s t a n t i a l evidence e x i s t s t o i n d i c a t e t h a t p e s t i c i d e d e r i v e d r e s i d u e s can form s t a b l e chemical l i n k a g e s w i t h o r g a n i c substances and t h a t such b i n d i n g g r e a t l y i n c r e a s e s the p e r s i s t ence o f the p e s t i c i d e r e s i d u e i n the s o i l (57-61). Two main mechanisms can be e n v i s i o n e d : ( i ) d i r e c t chemical attachment o f the r e s i d u e s t o r e a c t i v e s i t e s on c o l l o i d a l o r g a n i c s u r f a c e s and ( i i ) i n c o r p o r a t i o n i n t o the s t r u c t u r e s of newly formed humic and f u l v i c a c i d s d u r i n g the h u m i f i c a t i o n process ( 3 ) . A key to the f a t e o f p e s t i c i d e s and t h e i r i n t e r m e d i a t e decomposition products may be provided by c o n s i d e r a t i o n o f the p r o cess whereby humic and f u l v i c a c i d s are formed. The l i g n i n - p r o t e i n theory i n i t s o r i g i n a l form is now b e l i e v e d by many i n v e s t i gators to be o b s o l e t e , and the modern view is t h a t humic substance are formed by a m u l t i p l e stage process which i n c l u d e s : (1) decomp o s i t i o n o f all p l a n t components, i n c l u d i n g l i g n i n , i n t o s i m p l e r monomers, (11) metabolism of the monomers w i t h an accompanying i n c r e a s e i n the s o i l biomass, ( i l l ) repeated c y c l i n g of the b i o mass carbon w i t h s y n t h e s i s of new c e l l s , and (jLv) concurrent p o l y m e r i z a t i o n o f r e a c t i v e monomers i n t o high-molecular-weight polymers (8-11). The g e n e r a l consensus is t h a t polyphenols (quinones) s y n t h e s i z e d by microorganisms, together w i t h those l i b e r a t e d from l i g n i n , polymerize alone o r i n the presence o f amino compounds (amino a c i d s , e t c . ) t o form brown c o l o r e d polymers. An a l t e r n a t e pathway is by condensation of amino a c i d s and r e l a t e d substances w i t h reducing sugars, a c c o r d i n g t o the M a i l l a r d r e a c t i o n s . The r e a c t i o n between polyphenols and amino compounds i n v o l v e s simultaneous o x i d a t i o n o f the polyphenol t o the quinone form, such as by polyphenol oxidase enzymes. The a d d i t i o n product r e a d i l y polymerizes t o form brown nitrogenous polymers according to the general sequence shown i n F i g u r e 6. I n the case o f the M a i l l a r d r e a c t i o n , the i n i t i a l step i n v o l v e s a d d i t i o n o f the amine to the C«0 group o f the sugar, w i t h the formation o f an aldosylamine ( F i g u r e 7 ) . This is f o l l o w e d by the Amador1 rearrangement t o form the N - s u b s t i t u t e d keto d e r i v a t i v e , which subsequently undergoes dehydration and fragmentation to y i e l d a v a r i e t y o f unsaturated intermediates (62, 63). I n the f i n a l stages o f browning, the intermediates polymerize i n t o brown polymers and copolymers. The r a t e o f the r e a c t i o n i n c r e a s e s w i t h temperature, pH, and the b a s i c i t y o f the amine. Under l a b o r a t o r y c o n d i t i o n s , brown polymers can r e a d i l y be s y n t h e s i z e d from amino acid-sugar mixtures w i t h i n hours i n aqueous s o l u t i o n a t 50 C. c


194

BOUND

A N DCONJUGATED

PESTICIDE

RESIDUES

8Φ Η. δθ CI

I CI

N^S SORPTION N^N5-HO-?-SOM Hx L ï M +SOM-COOH-^zz=^ „ f V R ^ ' N T ^ R DESORPTON R . ^ % M

N

CHLORO - 1 - TRIAZINE

A

CHLORO - £ - TRIAZINE (SORBED)


HYDROLYSIS OH

OH

N ^ N

DESORPTION

N^\l

HO-C- SOM + HCI

HYDROXY-S-TRIAZINE

HYDROXY - £ - T R I A Z I N E ( SORBED)

+ SOM-COOH SOM-COOH = CARBOXYL

FUNCTIONAL

GROUP ON SOIL ORGANIC MATTER. Pesticides in Soil and Water

Figure 5. Proposed model for the sorption-catalyzed hydrolysis of chloro-striazines by soil organic matter (6)

NH -CH -COOH

Q

2

H

2

ρ

NH -CH -COOH 2

2

/OH

N-CH -COOF 2

4H +4e +

Condensation of intermediates

NH CH COOH 2

OH

OH NH

B r o w n nitrogenous

CH-COOH

II 2

polymers CHO-COOH NH

NH

I

I CH -COOH 2

CH -COOH 2

Figure 6. General scheme for the formation of brown nitrogenous polymers by con densation of polyphenols and amino acids as exemplified by the reaction between catechol and glycine



15.

STEVENSON


195

Condensation r e a c t i o n s between p e s t i c i d e s and t h e i r degradat i o n products with organic substances i n s o i l would be enhanced by such processes as f r e e z i n g and thawing, wetting and d r y i n g , and the i n t e r m i x i n g of r e a c t a n t s w i t h m i n e r a l matter having c a t a l y t i c properties. I t is r a t h e r evident that r e a c t i o n s s i m i l a r to these shown i n F i g u r e s 6 and 7 could be i n v o l v e d i n p e s t i c i d e transformations i n s o i l . Many weakly b a s i c compounds, i n c l u d i n g amino a c i d s , p y r r o l s , amides, amines, and imines, are known to have the a b i l i t y to combine c h e m i c a l l y with an array of c a r b o n y l - c o n t a i n i n g substances, i n c l u d i n g reducing sugars, reductones, the common aldehydes and ketones, and f u r f u r a l . Many of the common p e s t i c i d e s f a l l i n t o one of these c a t e g o r i e s . Those p e s t i c i d e s which are b a s i c i n character, have the p o t e n t i a l f o r forming a chemical l i n k a g e with C*0 c o n s t i t u e n t s of s o i l organic matter; those c o n t a i n i n g a C=0 group are t h e o r e t i c a l l y capable of r e a c t i n g with amino c o n s t i t uents. Condensation and conjugate r e a c t i o n s of p e s t i c i d e s with metabolic products have been p o s t u l a t e d to c o n s t i t u t e a form of p e s t i c i d e transformations by microorganisms and high p l a n t s . Another f a c t o r to consider is that the p a r t i a l degradation of many p e s t i c i d e s by microorganisms leads to the formation of c h e m i c a l l y r e a c t i v e intermediates which can combine with aminoor C=0 c o n t a i n i n g compounds, as i l l u s t r a t e d i n Figure 8. Thus, l o s s of the s i d e chain from the phenoxyalkanoic a c i d s by enzymatic a c t i o n leads to the formation of p h e n o l i c c o n s t i t u e n t s which can e i t h e r be o x i d i z e d f u r t h e r v i a the enzymatic route or undergo condensation (probably as quinones) w i t h amino compounds to form "humic-like" substances. On the other hand, amines (or c h l o r o amines) produced by b i o l o g i c a l decomposition of such h e r b i c i d e s as the a c y l a n i l i d e s , phenylcarbamates and phenylureas may r e a c t with C=0 c o n s t i t u e n t s o c c u r r i n g n a t u r a l l y i n s o i l . Entry i n t o the carbon c y c l e by t h i s mechanism may c o n s t i t u t e a form of n a t u r a l detoxification. Thus, it must be concluded that some p e s t i c i d e s or t h e i r decomposition products can become p a r t of the p o o l of precursor molecules f o r humus s y n t h e s i s , and, i n so doing, l o s e t h e i r identity. Bartha (57), Bartha and Pramer (58), and Chisaka and Kearney (59) concluded that the bulk of c h l o r o a n i l i n e s l i b e r a t e d by part i a l degradation of the phenylamide h e r b i c i d e s ( a c y l a n i l i d e s , phenylcarbamates, and phenylureas) becomes immobilized i n s o i l by chemical bonding to organic matter. The chemically bound residues could not be recovered by e x t r a c t i o n with organic s o l vents or i n o r g a n i c s a l t s ; p a r t i a l r e l e a s e was p o s s i b l e by a c i d or base h y d r o l y s i s (60). According to Hsu and Bartha (60), b i n d i n g occurs when the amino group of the a n i l i n e s react with C=0 and COOH groups a p p r o p r i a t e l y p o s i t i o n e d on the humic a c i d core with formation of a h e t e r o c y c l i c r i n g . The soil-bound c h l o r o a n i l i n e r e s i d u e s r e s i s t a t t a c k by microorganisms (57, 61).


196


ALDOSE SUGAR

AMINO COMPOUND

N-SUBSTITUTED GLYCOSYLAMINE

AMADORI REARRANGEMENT

1-AMINO-1 - DEOXY - 2 - KETOSE (1,2-ENOL FORM)

DEHYDRATION


• oc-AMINO ACID STRECKER DEGRADATION ALDEHYDE

REDUCTONES DEHYDROREDUCTONES FURFURALS

AMINO COMP'D

AMINO COMP'D

J

FRAGMENTATION

11

FISSION PRODUCTS (ACETOL, DIACETYL, PYRUVALDEHYDE, ETC.)

AMINO COMP'D

ι

|BROWh^JJTROGENOU^

Figure 7. Formation of brown nitrogenous polymers according to the MaiUard reaction

PHENYLCARBAMATES PHENYL UREAS

PHENOXYALKANOIC ACIDS

SIDE CHAIN

SIDE CHAIN

PHENOLIC

AMINES

CONSTITUENTS

ENZYMATIC

ENZYMATIC *OH OH AMINES FROM O.M.

CHEMICAL

•C=0's FROM O.M.

HUMIC-LIKE SUBSTANCES Environmental Quality

Figure 8.

Chemical reactions involving intermediate products of herbicide decompo sition and constituents of soil organic matter (3)


15.

STEVENSON


197


Fate of Organics In Sediments The r o l e o f organic matter i n chemical transformations deserves s e r i o u s a t t e n t i o n i n determining the long-time f a t e o f p e r s i s t e n t p e s t i c i d e s i n the environment. I n d i r e c t i n f o r m a t i o n on t h i s s u b j e c t is provided by the many biogeochemical s t u d i e s d e a l i n g w i t h the f a t e of n a t u r a l l y o c c u r r i n g organics i n sediments and sedimentary rocks (64). Since t h i s subject is beyond the scope o f the present review, the d i s c u s s i o n which f o l l o w s w i l l be confined to a c o n s i d e r a t i o n o f the diagenesis o f amino a c i d s i n sediments, as o u t l i n e d elsewhere (65)· The net e f f e c t of n o n b i o l o g l c a l r e a c t i o n s i n v o l v i n g amino a c i d s (see Figures 6 and 7) is i n c o r p o r a t i o n of n i t r o g e n i n t o the s t r u c t u r e s of humic and f u l v i c a c i d s . Thus, whereas amino a c i d s c o n s t i t u t e 80% or more o f the organic n i t r o g e n of m i c r o b i a l t i s sue (biomass), they account f o r only about o n e - t h i r d of the n i t r o g e n i n s o i l and marine humus. F o l l o w i n g b u r i a l i n sediments, f u r t h e r changes occur, w i t h t r a n s f e r o f amino acid-N i n t o the humified remains. The s i g n i f i c a n c e of chemical transformations o f amino a c i d s f o l l o w i n g b u r i a l can be i l l u s t r a t e d by c o n s i d e r a t i o n o f data obtained f o r the forms of n i t r o g e n i n sediments o f the Argentine B a s i n (66) and the Experimental Mohole (67). I n the case o f the Argentine Basin sediments (estimated maximum age o f 125,000 y e a r s ) , data were reported (65) f o r t o t a l amino a c i d s and organic carbon i n 72 i n d i v i d u a l samples from two cores (V-15-141 and V-15-142). As i n d i c a t e d i n F i g u r e 9, there was a p r o g r e s s i v e decrease i n the percentage o f the organic carbon as amino a c i d s w i t h i n c r e a s i n g age. A s i m i l a r r e s u l t was obtained f o r the somewhat o l d e r Experimental Mohole sediments, f o r which q u a n t i t a t i v e data were a v a i l able f o r e i g h t samples ranging i n age from 3 t o 14 m i l l i o n y e a r s . These data are shown i n F i g u r e 10. For both b a s i n sediments, the disappearance o f amino a c i d s from the sedimentary organic matter, as estimated from composit i o n a l s t u d i e s , was c o n s i d e r a b l y greater than would be a n t i c i p a t e d from k i n e t i c s t u d i e s o f amino a c i d s i n aqueous s o l u t i o n . This o b s e r v a t i o n lends support t o the c o n c l u s i o n t h a t , during d i a g e n e s i s , amino a c i d s are transformed to other products as a consequence o f chemical r e a c t i o n s w i t h other o r g a n i c s , presumably by r e a c t i o n s of the type shown i n Figures 6 and 7. D i a g e n e t i c changes i n the amino a c i d s o f sediments have a l s o been observed f o r Saanich I n l e t , B r i t i s h Columbia, where it was found t h a t l o s s e s o f amino a c i d s w i t h depth exceeded that f o r organic carbon (68), Abelson (69) e a r l i e r p o s t u l a t e d t h a t nonb i o l o g l c a l processes i n v o l v i n g complex heteropolymers were r e s p o n s i b l e f o r the disappearance o f amino a c i d s from sediments. Coupling of amino a c i d s w i t h porphyrins may be o f geochemical s i g n i f i c a n c e (70). The h i g h s t a b i l i t y o f amino a c i d s i n petroleum b r i n e s has been a t t r i b u t e d t o t h e i r l i n k a g e w i t h phenol- and quinone-containing substances (71).


BOUND AND CONJUGATED PESTICIDE RESEDUES


198

4Q000

120,000

80,000

YEARS BEFORE PRESENT Advances in Organic Geochemistry

Figure 9. Relationship between estimated age of Argentine Basin sediments and the percentage of organic carbon which occurred as amino acids. Regression equation: Y =- 22.92 - 0.05 x(r = 0.47, significant at ρ — 0.01) (65).

30h

z ό Ο • EM 8-11 EM 8-9 \

S»' Figure 10. Relationship between age of sedimentary material in Experimental Mohole sediments and the percentage of organic nitrogen which occurred as amino acids. Value shown in brackets in the upper left hand corner represents an average for the younger Argentine Basin sediments. Regression equation: Τ «= 32.03 — 0.001 χ (τ = 0.95, significant at ρ — 0.01 ).

EM 8-13 E M 8-141 • \ EM 8-12 V

10 4

8

MILLIONS OF YEARS


12

15.

STEVENSON

199


U s i n g the analogy given above, one would expect t h a t n a t u r a l organics would e x e r t an a p p r e c i a b l e k i n e t i c i n f l u e n c e on the disappearance o f c e r t a i n p e r s i s t a n t p e s t i c i d e s (or t h e i r i n t e r mediate decomposition p r o d u c t s ) . Rate o f l o s s would, o f course, depend upon the nature o f the compound and environmental c o n d i t i o n s e x i s t i n g i n the sediment, such as pH and temperature.


A d s o r p t i o n Mechanisms Bonding mechanisms f o r the r e t e n t i o n o f p e s t i c i d e s by o r g a n i c substances i n s o i l i n c l u d e Ion exchange, p r o t o n a t i o n , H-bonding, van der Waal's f o r c e s , and c o o r d i n a t i o n through an attached metal i o n ( l i g a n d exchange). In a d d i t i o n , nonpolar molecules may be p a r t i t i o n e d onto hydrophobic s u r f a c e s through "hydrophobic bondi n g ." For some p e s t i c i d e s , a d s o r p t i o n is a p p a r e n t l y not completely r e v e r s i b l e (18, 20, 32, 3£, 72), a f a c t o r which is o f c o n s i d e r a b l e importance i n determining the environmental impact o f p e s t i c i d e s i n s o i l and water. Ion Exchange and P r o t o n a t i o n . A d s o r p t i o n through i o n exchange is r e s t r i c t e d t o those p e s t i c i d e s which e i t h e r e x i s t as c a t i o n s (diquat and paraquat) o r which become p o s i t i v e l y charged through p r o t o n a t i o n ( s - t r i a z i n e s ; amitrole). Diquat and paraquat, b e i n g d i v a l e n t c a t i o n s , have the potent i a l f o r r e a c t i n g w i t h more than one n e g a t i v e l y charged s i t e on s o i l humic c o l l o i d s , such as through two COO i o n s ( i l l u s t r a t e d below f o r d i q u a t ) , a COO" i o n p l u s a phenolate i o n combination, o r a COO" i o n (or phenolate i o n ) p l u s a f r e e r a d i c a l s i t e .

Diquat

On the b a s i s o f i n f r a r e d s t u d i e s , Khan (T3 74) suggested t h a t b i p y r i d y l i u m h e r b i c i d e s form c h a r g e - t r a n s f e r complexes w i t h humic substances. T h i s could not be confirmed by Burns e t a l , (75) who s u b j e c t e d some paraquat-humic a c i d complexes t o u l t r a violet analysis. Paraquat has been found to be complexed i n g r e a t e r amounts by humic and f u l v i c a c i d s (73), and by an organo-clay complex ( 7 4 ) , than d i q u a t . A h i s t o s o l and i t s humic and f u l v i c f r a c t i o n s has 9

f




200

a l s o been observed t o show s e l e c t i v e preference f o r paraquat (76). F a c t o r s which i n f l u e n c e the a v a i l a b i l i t y o f exchange s i t e s f o r a d s o r p t i o n i n c l u d e the presence of competing metal c a t i o n s and pH. S o i l pH has a d i r e c t b e a r i n g on the r e l a t i v e importance of o r g a n i c matter and c l a y i n r e t a i n i n g o r g a n i c c a t i o n s . U n l i k e c l a y , o r g a n i c c o l l o i d s have a s t r o n g l y pH-dependent charge. Therefore, the r e l a t i v e c o n t r i b u t i o n o f o r g a n i c matter t o c a t i o n exchange c a p a c i t y , and subsequently r e t e n t i o n o f c a t i o n s , w i l l be h i g h e r i n n e u t r a l and s l i g h t l y a l k a l i n e s o i l s than i n a c i d i c ones. For each u n i t change i n pH, the change i n c a t i o n exchange c a p a c i t y f o r o r g a n i c matter is s e v e r a l times greater than f o r c l a y . Less b a s i c compounds, such as the s - t r i a z i n e s , may become c a t i o n i c through p r o t o n a t i o n . Whether o r not p r o t o n a t i o n occurs w i l l depend upon: (1) the nature o f the compound i n q u e s t i o n , as r e f l e c t e d by i t s p l ^ , and ( i i ) the proton-supplying power o f t h e humic c o l l o i d s . Reactions l e a d i n g t o a d s o r p t i o n o f the s - t r i a z i n e s , as p o s t u l a t e d by Weber e t a l . ( 7 7 ) , are shown by the f o l l o w i n g equations : Τ + H 0 2

^

RCOOH + H 0

RCOOH + Τ

+

+

+

OH"

R-COO" +

2

RC00~ + HT

HT

^

[1]

H 0

+

3

[2]

R-COO-HT

[3]

= ^ R-COO-HT

[4] +

where R is the o r g a n i c c o l l o i d , Τ the s - t r i a z i n e molecule, HT the protonated molecule, and the hydronium i o n . Equation [1] represents pH-dependent a d s o r p t i o n through p r o t o n a t i o n i n the s o i l s o l u t i o n w h i l e equation [2] represents i o n i z a t i o n o f the c o l l o i d COOH group. I o n i c a d s o r p t i o n o f t h e c a t i o n i c s - t r i a z i n e molecule, formed by r e a c t i o n [ 1 ] , is shown by equation [ 3 ] . A d s o r p t i o n through d i r e c t p r o t o n a t i o n on the s u r face o f the o r g a n i c c o l l o i d is shown by r e a c t i o n [ 4 ] . A m i t r o l e is another example o f a weak base that can be adsorbed t o s o i l o r g a n i c matter through i o n exchange (78). S o i l pH has a profound e f f e c t on a d s o r p t i o n o f s - t r i a z i n e s and other weakly b a s i c p e s t i c i d e s by o r g a n i c matter. S o i l reac t i o n governs not o n l y the i o n i z a t i o n o f a c i d i c groups on the o r g a n i c c o l l o i d s but the r e l a t i v e q u a n t i t y o f the p e s t i c i d e which occurs i n c a t i o n i c form, i n accordance w i t h equation [ 1 ] . The p K o f a c i d i c groups i n humic a c i d s (COOH p l u s p h e n o l i c - and/or enolic-OH) is o f the order o f 4.8 t o 5.2. Thus, it would appear t h a t i o n exchange would not be an important mechanism f o r adsorp t i o n o f weakly b a s i c p e s t i c i d e s w i t h p K s much lower than 3.0. I t should be p o i n t e d out however, t h a t the pH a t the s u r f a c e o f s o i l organic c o l l o i d s may be as much as two pH u n i t s lower than that o f the l i q u i d environment. The a d s o r p t i o n c a p a c i t i e s o f s o i l o r g a n i c matter p r e p a r a t i o n s f o r the s - t r i a z i n e s has been a

f

a


15.

STEVENSON

201


found t o f o l l o w t h e order expected on the b a s i s o f p K values f o r the h e r b i c i d e s , w i t h maximum a d s o r p t i o n o c c u r r i n g a t pH values near t h e p K o f t h e r e s p e c t i v e compound (78). Ion exchange is but one o f s e v e r a l mechanisms f o r a d s o r p t i o n of t h e s - t r i a z i n e s t o organic c o l l o i d s , as i l l u s t r a t e d below: a


a

CI

s>

s-Triazine

On the b a s i s o f an i n f r a r e d study o f some s - t r i a z i n e - h u m i c a c i d complexes, S u l l i v a n and Felbeck (79) concluded t h a t one secondary amino group was bound t o e i t h e r a C«0 o r quinone group of the humic a c i d through a hydrogen bond whereas the other secondary amino group became protonated and was bound by i o n exchange t o a COO" group. I t should be noted t h a t the mechanism described above is somewhat d i f f e r e n t from that shown e a r l i e r i n Figure 5. Walker and Crawford (13) suggested that a d s o r p t i o n o f the s - t r i a z i n e s by organic matter could best be regarded as p a r t i t i o n i n g out o f s o l u t i o n onto hydrophobic surfaces (discussed l a t e r ) . For a n i o n i c p e s t i c i d e s , such as the phenoxyalkanoic a c i d s , r e p u l s i o n by the predominantly n e g a t i v e l y charged s u r f a c e o f organic matter may occur. P o s i t i v e a d s o r p t i o n o f a n i o n i c h e r b i c i d e s a t pH values below t h e i r pKa can be a t t r i b u t e d t o a d s o r p t i o n of t h e unionized form o f t h e h e r b i c i d e t o organic s u r f a c e s , such as by Η-bonding between t h e COOH group and C«0 o r NH2 groups o f organic matter. Η-Bonding, van der Waals Forces, and C o o r d i n a t i o n . Adsorption mechanisms f o r r e t e n t i o n o f n o n i o n i c p o l a r p e s t i c i d e s , such as the phenylcarbamates and s u b s t i t u t e d ureas, a r e i l l u s t r a t e d i n F i g u r e 11. The great importance o f Η-bonding i n r e t e n t i o n is suggested, w i t h m u l t i p l e s i t e s being a v a i l a b l e on both p e s t i c i d e and organic matter s u r f a c e . Other a d s o r p t i o n mechanisms i n c l u d e van der Waals f o r c e s ( p h y s i c a l a d s o r p t i o n ) , l i g a n d exchange ( - M 0»C), and, f o r p e s t i c i d e s c o n t a i n i n g an i o n i z a b l e COOH group, a s a l t l i n k a g e through a d i v a l e n t c a t i o n on the organic exchange s i t e . For c h l o r i n a t e d phenoxyalkanoic a c i d s , such as 2,4-D, p h y s i c a l adsorption t o aromatic c o n s t i t u e n t s



202

of o r g a n i c matter may be i n v o l v e d ; Η-bonding w i l l be l i m i t e d t o a c i d c o n d i t i o n s where COOH groups are u n i o n i z e d . Considerable v a r i a t i o n can be expected i n the a d s o r p t i o n c a p a c i t y o f o r g a n i c matter f o r n o n i o n l c p o l a r h e r b i c i d e s , depend i n g upon s t e r i c e f f e c t s and the number and kinds of e l e c t r o n e g a t i v e atoms i n the molecule.


Hydrophobic Bonding P a r t i t i o n i n g on hydrophobic s u r f a c e s has been proposed as a mechanism f o r r e t e n t i o n of nonpolar o r g a n i c p e s t i c i d e s by s o i l o r g a n i c matter. A c t i v e s u r f a c e s i n c l u d e the f a t s , waxes, and r e s i n s , as w e l l as p o s s i b l e a l i p h a t i c s i d e chains on humic and f u l v i c a c i d s . Weber and Weed (80) p o i n t e d out that "humus," by v i r t u e o f i t s aromatic framework and presence o f p o l a r groups, may c o n t a i n both hydrophobic and h y d r o p h y l i c a d s o r p t i o n s i t e s . P i e r c e e t a l . (81, 82) suggested that c h l o r i n a t e d hydrocar bons (such as DDT) would have g r e a t e r a f f i n i t y f o r hydrophobic s i t e s on o r g a n i c substances than f o r c l a y and that scavenging by o r g a n i c p a r t i c u l a t e s provided a means whereby these p e r s i s t e n t p o l l u t a n t s are t r a n s p o r t e d through the water column and concen t r a t e d i n bottom sediments. For DDT, it would be noted that c h l o r i n e atoms on the e t h y l group may impart a s l i g h t n e g a t i v e charge to the molecule (83); consequently, p a r t o f the a d s o r p t i o n a t t r i b u t e d to hydrophobic bonding may be due to a t t r a c t i o n to p o s i t i v e l y charged s i t e s such as t o an amino group. Considerable emphasis has been given to hydrophobic bonding as a mechanism f o r a d s o r p t i o n o f the s - t r i a z i n e s (13) and the phenylureas (17) by s o i l o r g a n i c matter. These claims r e q u i r e c o n f i r m a t i o n ; f o r these p e s t i c i d e s , the b u l k of the evidence favors the i d e a that s p e c i f i c a d s o r p t i o n s i t e s ( f u n c t i o n a l groups) are i n v o l v e d (see e a r l i e r d i s c u s s i o n ) . R e l a t i v e A f f i n i t i e s o f P e s t i c i d e s f o r Organic Matter The d e l i b e r a t i o n s o f the previous s e c t i o n serve to empha s i z e t h a t the v a r i o u s p e s t i c i d e s d i f f e r g r e a t l y i n t h e i r r e l a t i v e a f f i n i t i e s f o r s o i l o r g a n i c c o l l o i d s . The approximate order f o r some common h e r b i c i d e s are given i n F i g u r e 12. Thus, the c a t i o n i c h e r b i c i d e s (diquat and paraquat) would be expected to be the most s t r o n g l y bound, followed by those weakly b a s i c types capable of b e i n g protonated under moderately a c i d i c c o n d i t i o n s . For the s - t r i a z l n e s , d i f f e r e n c e s i n a d s o r b a b i l i t y can be account ed f o r by v a r i a t i o n s i n pKg, w i t h the more b a s i c compounds (high pKa) b e i n g adsorbed the s t r o n g e s t . H e r b i c i d e s i n the next order of a d s o r p t i o n are those having v e r y low pK values but which con t a i n one or more p o l a r groups s u i t a b l e f o r H-bonding. A n i o n i c p e s t i c i d e s may o r may not be adsorbed, depending upon s o i l pH. a


15.

203


STEVENSON

PHENYLCARBAMATES SUBST UREAS 0

/

, ,

R,

0

VN-C-O-CH \=J Η V R

f

R,

2

+

VAN DER WAALS

R, II 1 R - N - C ,*C-N-R Η Ν Η

2

PHENOXYALKANOIC ACIDS

s-TRIAZINES

CI ^ ~ ^ - Q - C H - C O O H 2

3

CI

+

+

+

+

-

+ (R,'OH)

-

-

-

-

+

+

-

+

+

+

-

-

—

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—

+ (pH>7.0)

H-BONDING N

/

H

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- O H — · ••••0= R -C-O — · \ '

-

HAJ

HOHN = HA

0

£ C = 0- ••·

H

HN-

(pH< K ) P

0

LIGAND EXCHANGE Z

M *-(HÂ]

^c=o

SALT LINKAGE -C-0-M-O-C-fHÂ] λ

ο

d

Environmental Quality

Figure 11.

Typical bonding mechanisms for adsorption of some common herbicides by soil organic matter (3)

ION

DIQUAT

EXCHANGE

A N D PARAQUAT PROMETONE ATR ATONE PROMETRYNE AMETRYNE

ATRAZINE SIMAZINE

PHENYLCARBAMATES SUBSTITUTED UREAS OTHERS

Environmental Quality

Figure 12. Relative affinities of herbicides for soil organic matter surfaces (3)


204


Literature Cited 1. 2. 3. 4.


5.

6.

7.

8. 9. 10. 11.

12.

13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

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83.



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