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PHYSICAL/CHEMICAL INTERACTIONS ·OF WITH SOIL Jerome B. Weber * Abstract.

Reactions of herbicides in soils are dependent upon the physical/ chemical properties of the herbicides and of the soil colloids. Key properties of the soils that regulate reaction are expressed in the acronym "SCOOP" which includes "S" structure, "C" clay, "O" organic matter and "O" oxide contents, and "P" pH. Key properties of the herbicides which determine their reactivity with soil colloids are expressed in the acronym "SILVER" which includes "S" solubility, "I" ionizability, "L" longevity, "V" volatility,"E" extractability, and "R" reactivity. "SCOOP SILVER" then is a discussion of the interactions of basic, acidic, and nonionic herbicides with organic and inorganic constituents of the soil matrix.

Introduction

The reaction of a given herbicide with various soil colloids is dependent upon the composite physical and chemical properties of both the chemical and the soil (Weber 1972). The key properties of soils that regulate herbicide behavior are represented by the acronym "SCOOP" and include "S" structure (channels, pans, etc.), "C" clay (type and amount), "O" organic matter (type and amount), "O" oxide content, and "P" pH level. The key properties of the herbicides are represented by the acronym "SILVER" and include "S" solubility, "I" ionizability, "L" longevity, "V" volatility, "E" extractability, and "R" reactivity. Solubility refers to herbicide dissolution in water or an aqueous system. Ionizability refers to the type of functional groups present and whether the herbicide has basic, acidic or nonionizable properties. Longevity refers to the stability of the herbicide or how long it persists in the soil environment. Since we will be discussing herbicide/soil interactions primarily the longevity factor will not be discussed in this paper. Volatility refers to the tendency of a herbicide to evaporate from the soil and the vapor pressure of a given compound at ambient temperature is a relative index of the phenomenon. Extractability refers to the amount of a herbicide that can be removed from the soil with nonpolar organic solvents like octanol or hexane. It is also an indication of the lipophilicity of a given herbicide. Since extractability in nonpolar organic solvents is inversely related to aqueous solubility, and is an indication of the bioaccumulation or biomagnification potential of a herbicide, it will not be discussed in this paper. Reactivity refers to the presence of functional groups which are highly reactive with soil colloids. Groups such as P03 and As03 readily complex with clay minerals and hydrous oxides in soils and groups like No2 readily hydrogen bond to proteinaceous substances in soils. Interactions of the herbicides with soil colloids will be discussed following the classification scheme shown in the table.

*Professor, North Carolina State University, Box 7620, Raleigh, NC 27695- 7620.

96 Ionizability (the "I" in "SILVER") refers to the way a given herbicide ionizes in aqueous solution, if it does. It is of primary importance because positively charged (cationic) herbicides behave much differently than negatively charged (anionic) or uncharged (nonionic) herbicides. The most important property of the classification scheme in the Table is based on the ionizing characteristics of herbicides. The other chemical properties utilized in the classification scheme include "S" solubility, "R" reactivity and "V" volatility.

Strongly basic herbicides.

Strongly basic herbicides such as difenzoquat, , morfamquat, and (Table) ionize completely in aqueous solutions to yield cationic species as shown for paraquat in equation I (Weber 1972).

Paraquat dichloride~~~-)~ Paraquat2+ + 2 Cl- (I)

Table. Classification scheme for organic herbicides.

Category Species Connnon name

Strongly basic Cation Difenzoquat, diquat, morfamquat, paraquat

Weakly basic Cation/molecular Ametryn, amitrole, , , dipropetryn, fluridone, , , prometryn, propazine, , , terbutryn

Acidic Anion/molecular , , benazolin, ben­ sulide, , (free acid), bromacil, , buthidazole, , chlorimuron, chlorsulfuron, 2,4-D, dalapon, , diclofop (free acid), , , DNOC, DOWCO 290, DPX 6313, , fenac, fenoxaprop (free acid), flamprop (free acid), (free acid), , imazaquin, , ioxynil, isocil, (free acid),MCPA, MCPB, , mefluidide, naptalam, oryzalin, perfluridone, , quizalofop, , silvex, , 2,4,5-T, TCA, terbacil, , trisulfuron

Complexing-type , DSMA, , fosamine, , glyphosine, MAA, MSMA

97 Table. Classification scheme for organic herbicides (continued)

Category Species Common name

Nonionic Moleculer Anilide: , , , diethatyl-ethyl, , ,

Amide: Diphenamide, napropamide, pronamide

Phenylurea: , diuron, fenuron, fluometuron, , monuron, siduron

Carbamate: Barban, chlorpropham, desmedipham, karbutilate, pherunedipham, propham, terbutol

Thiocarbamate: Butylate, CDEC, cycloate, diallate, EPTC, molinate, pebulate, thiobencarb, triallate, vernolate

Phenoxybenzene: Fluorodifen, nitrofen, oxyf luorf en

Dinitroaniline: Benefin, dinitramine, ethalfluralin, fluchloralin, isopropalin, , prodiamine, prof luralin,

Misc. (Low solubility): DCPA, , oxadiazon

Misc. (Moderate solubility): Cirunethylin, dichlobenil, norflurazon

Misc. (High solubility): Dimethazone, ethofumesate, , isouron, norea, pyrazon

These organic cations readily replace inorganic cations on the exchange complex of soil colloids where they are held by strong ionic forces as shown for paraquat by organic matter in Figure 1. (Weber et al. 1965).

98 PARAQUAT

OM

Figure 1. Paraquat, a strong base herbicide ionically bound to soil organic matter (OM).

Cationic herbicides are ionically bound to both organic colloids (the "O" in "SCOOP") (Best et al. 1972) and to clay minerals (Figure 2) (the "C" in SCOOP") (Weber and Weed 1968) and their biological availability to plants and microorganisms is regulated by the geometry of binding (Scott and Weber 1967, Summers 1980, Weber and Scott 1966, Weber and Weed 1974). Cationic herbicides bound to the exterior of nonexpanding clays are biologically available while those bound on the interior surfaces of expanding clays are only very slowly available or not available at all.

PARAQUAT

MONTMORILLONI TE CLAY

Figure 2. Paraquat ionically bound on the interlayer spaces of smectite clay minerals.

99 Weakly basic herbicides.

Weakly basic herbicides such as ametryn, amitrole, atrazine, cyanazine, dipropetryn, fluridone, metribuzin, prometon, prometryn, propazine, simazine, tebuthiuron, and terbutryn (Table), ionize in aqueous solution according to the equilibrium shown in equation II (Weber 1972). B + ~ ---•'•... HB+ (II) where: B = molecular species of weakly basic herbicide ~ = hydrogen ion HB+ = cationic species of weakly basic herbicide

The equilibrium equation is pH dependent (the P in "SCOOP"). Thus, the lower the pH, i.e. the higher the hydrogen concentration, the more the reaction is driven to the right and the greater the proportion of cationic to molecular species present at any given time. Greater adsorption by organic (the "O" in "SCOOP") soil colloids and by clay minerals (the "C" in "SCOOP") occurs at low pH since cations are more readily adsorbed by soil colloids than are molecular species. (Figure 3).

--o_--o_- ~-H• K•

Figure 3. Prometryn, a weakly basic herbicide, ionically bound to clay and organic matter in soil.

Leachability of basic herbicides is also less under acid conditions than it is under neutral or alkaline conditions since cationic species are bound more strongly and in greater amounts than molecular species. Although the basic herbicides as a group tend to be very low in volatility (the "V" in "SILVER"), they are more readily vaporized when in the molecular form at neutral or high pH conditions than when in the cationic form under acidic conditions. Adsorption of basic herbicides is greater for relatively strong bases like amitrole and prometryn than it is for weaker bases like atrazine

100 and cyanazine (Weber 1966, Weber et al. 1969). Bioavailability of weakly basic herbicides is also affected by soil pH (Best et al. 1975, Lowder and Weber 1982, Weber 1970). Greater adsorption at lower pH levels causes lower bioavailability and thus poorer weed control performance than is the case at high pH levels. In addition, pH affects the longevity of a herbicide in the soil. Some chemicals like atrazine persist longer at high pH than at low pH and others like prometryn do the opposite.

Acidic herbicides.

Acidic herbicides such as acifluorfen (Table) ionize in aqueous solution according to the equilibrium shown in equation III (Weber 1980a).

HA (III) where: HA = molecular species of weakly acidic herbicides a+ • hydrogen ion A- • anionic species of weakly acidic herbicides

As was the case for basic herbicides, the equilibrium is pH dependent (the "P "in "SCOOP"). High soil acidity drives the equation to the left and high alkalinity drives the equation to the right. Thus, under acid conditions more of the acid herbicides are in the undissociated state (molecular form) and are more readily bound particularly by organic (the "O" in "SCOOP") soil colloids than they are when in the anionic form at high pH levels (Weber 1980b).

In the molecular form they are probably bound by charge transfer bonds (n) and/or hydrogen bonds, as shown in Figure 4 for 2,4-D.

Cl /OH 2,4-D Cl O-CH2-c ~p.. .·. -H C 2 OM

Figure 4. Retention of undissociated 2,4-D molecule by soil organic matter by way of bonds and/or hydrogen bonding.

101 At high pH levels in clay soils, anionic species of acid herbicides predominate and they are repelled from the negatively charged surfaces. In certain high oxide (the 11 0 11 in "SCOOP") content tropical and subtropical soils, the anionic species of acid herbicides may be bound by way of anion exchange reactions with the positively charged hydrous oxides as shown in Figure 5.(Strek 1984).

~o Cl -Q- O-CH2C'o.- CI .._ +,..OH Fe \ 2,4-D OH

HYDROUS OXIDE

Figure 5. Retention of 2,4-D anion by hydrous iron oxide in soils.

Acid herbicides are also more leachable under high pH conditions in most soils (those low in hydrous oxide content) because the anionic species are predominate and are repelled by negatively charged soil colloids. Bioavailability of acid herbicides is normally higher under acid conditions where the molecular form predominates, in soils containing low amounts of soil organic matter (the "O" in "SCOOP") (Shea et al. 1983). However, adsorption of acid herbicides does occur on soil organic matter and is greater under acid soil conditions (Weber 1980b). Thus, in soils containing significant amounts of organic matter bioavailability may be lower at low pH levels than at high pH levels (Shea et al. 1983).

Complexing-tyPe herbicides.

Complexing-type herbicides such as the containing compound cacodylic acid and the phosphorus containing compound glyphosate dissociate as acids in aqueous solution. However, these acidic herbicides contain arsenic and phosphorus groups which readily complex with clay mineral (the "C" in "SCOOP") and with hydrous oxides (the "O" in "SCOOP") in soils (Woolson 1975). They are bound through complexes shown in Figure 6.

102 0 0 OH HO-~-CH 2-N-CH 2-r-O-····~F~ OXIDE ~ 0 - OH GLYPHOSATE ,j,·3 ----o----I CLAY

Figure 6. Complexing-type herbicides such as glyphosate are bound to hydrous oxides and clay minerals in soils.

Because of the high reactivity with soil colloids (the "R" in "SILVER"), these herbicides are nearly immobile in soils. Bound herbicides such as the complexing-type compounds are degraded in soils but availability to weeds is very slow and ineffectual.

Nonionic herbicides.

Nonionic herbicides (Table) exist in the soil solution only in the molecular form and their reactivity with soils is dependent upon their water solubility (the "S" in "SILVER"). the types of reactive functional groups present (the "R" in "SILVER"), and their volatility (the "V" in "SILVER"). Bioavailability and bioaccumulation of herbicides has also been correlated with their extractability (the "E" in "SILVER") into organic solvents such as octanol but this aspect will not be addressed here.

Anilides.

Anilides such as acetochlor et al. (Table) have water solubilities which range from 23 to 580 ppm (low to moderate) and vapor pressures which range from 1 x 10-6 to 2.4 x 10-4mm of Hg at 20 to 30 C (low to moderate). In soils, the acetanilides are bound to organic matter (Kozak et al. 1983, Weber and Peter 1982, Peter and Weber 1985a) and to certain clay minerals (Weber and Peter 1982). Bonding to organic colloids (humic matter) is probably through charge transfer bonds (~) between electron deficient aromatic rings of the herbicide molecules and electron rich aromatic rings of the soil humic matter in a manner analagous to that shown for undissociated 2,4-D in Figure 4. Bonding to clay minerals is probably by way of bridges between Ca on the clay surfaces and 0 of the carbonyl group of the herbicide molecules (Weber and Peter 1982) as shown for alachlor in Figure 7.

103 Leachability of the chemicals in soils ranges from low to moderate (Peter and Weber 1985b). Bioavailability of anilide herbicides is highly correlated with the organic matter and clay contents of soils (Weber and Peter 1982, Peter and Weber 1985b).

. A LACH LOR . Ca I ~~~-o~~~~--- c LAY

Figure 7. Physical bonding between an alachlor molecule and a clay colloid by way of a calcium bridge.

Amides.

Amide herbicides such as diphenamide et al. (Table) have solubilities which range from 15 ppm to 260 ppm (low to moderate) and vapor pressures which range from 1 x 10-8 to 8.5 x 10-7 nan of Hg at 20 to 30 C (extremely low to very low). Adsorption and biological inactivation of amide herbicides in soils has been most highly correlated with soil organic matter content and binding probably occurs through bonds similar to those postulated for the anilide herbicides (Weber 1972, Weed and Weber 1974).

Phenylureas.

The phenylurea herbicides such as chloroxuron et al. (Table) range in water solubility from 2.7 to 3375 ppm (very low to highly soluble) and this adsorption in soils has been shown to be inversely related to their water solubility (Carringer et al. 1975). The chemicals have very low to extremely low vapor pressures and are considered to be relatively nonvolatile. Adsorption, mobility and biological inactivation of phenylureas in soils have been highly correlated with soil organic matter content (Carringer et al. 1975, Harrison et al. 1976, Weber 1972)) and to a lesser extent with clay mineral content (Weber 1972). Bonding to soil colloids probably occurs through charge transfer bonding ( ) between aromatic herbicide molecules and aromatic structures of soil humic matter and/or by way of hydrogen bonding between the amino and carbonyl groups of the herbicide molecules and carbonyl and amino groups, respectively of soil humic matter, as shown for diuron in Figure 8. Adsorption by soils ranges from low to high and leachability generally ranges from low to high.

104 OM C-N-0-11 I 0 H :. : . H. o . I II CH DIURON Cl N-C-N/ 3 'CH3 Cl

Figure 8. Retention of diuron molecule to soil organic matter through charge transfer bond (?t) and/or hydrogen bonds.

Carbamate.

Carbamate herbicides such as barban et al.(Table) range in water solubilities from 6 to 325 ppm (very low to moderate) and in vapor pressure from 1 x 10-6 to 1 x 10-S mm of Hg at 20 to 30 C (very low to low). Adsorption in soils ranges from moderate to very high (Weber 1972). Adsorption, mobility, and biological inactivation have been most highly correlated with soil organic matter content (Scott and Weber 1967, Weber and Weed 1974). Bonding probably occurs primarily through bonding between aromatic rings of herbicide and humic matter and by way of hydrogen bonding between amino and carbonyl groups of the herbicide molecules and carbonyl and amino groups, respectively, of soil organic matter in a manner analogous to that shown for diuron in Figure 8.

Thiocarbamate.

Thiocarbamate herbicides such as butylate et al. (Table) range from 4 to 800 ppm in water solubility (very low to moderate) and with the exception of thiobencarb have vapor pressures which range from 1.2 x 10-4 to 3.5 x 10-2 mm of Hg at 20 to 30 C (high to very high). Because of the high vapor pressures these chemicals must be incorporated into the soil to be effective (Gray and Weierich 1965). They are bound in moderate amounts by soil organic matter probably through weak physical adsorption forces and are lost through volatilization particularly at high soil moisture contents and high temperatures (Gray and Weierich 1965). The thiocarbamates exhibit moderate mobility in soils and their mobility is directly related to the water solubility of the chemicals (Gray and Weierich 1968). Binding of the thiocarbamates to soil organic matter probably occurs by way of hydrogen bonding between the carbonyl group of the herbicide molecule and active

105 hydrogen atoms in soil humic matter. Additional bonding may occur by way of weak van der Waals and London forces between alkyl groups of the herbicide molecules and the soil collodial surfaces. Thiobencarb has very low solubility and very low vapor pressure and, thus, is very i111Dobile and nonvolatile in soils.

Phenoxybenzene.

Phenoxybenzene herbicides such as fluorodifen (Table 1), have water solubilities ranging from 0.1 to 3 ppm (very low to extremely low). They are strongly adsorbed in soils and are very immobile (Fadayomi and Warren 1977). Since their solubilities are very low the compounds may exist as lipophillic micelles in the soil matrix. Herbicides which are present in the solution phase are probably removed from solution and bound to soil colloids through charge transfer (n) bonds or hydrogen bonds, as shown for nitrofen in Figure 9. The phenoxybenzene herbicides have very low vapor pressures and are generally considered to be relatively nonvolatile.

CH 2-CH2-~-CH~ OM H

~o NITROFEN Cl 0 N -0- 'o Cl

Figure 9. Adsorption of nitrofen to soil organic matter through charge transfer (n) bonds and/or by way of hydrogen bonds.

Dinitroaniline.

Dinitroaniline herbicides such as benefin et al. (Table) are among the most water insoluble of the herbicide families with solubilities generally less than 1 ppm (extremely low) (Weber 1978). The presence of the nitro groups greatly decreases water solubility of the compounds because nitro groups readily hydrogen bond to alkyl groups of neighboring molecules creating lipophilic micelles which resist solvation into the water structure. The compounds are i111Dobile in soils because the herbicide molecules exist as insoluble micelles and/or because nitro groups on the molecules are readily hydrogen bonded to proteinaceous sites on soil organic matter in a manner analogous to that shown for nitrofen in Figure 9. Adsorption and inactivation of dinitroaniline herbicides in soils has been most highly associated with soil organic matter content (Carringer et al. 1975, Peter and

106 Weber 1985b). Because of the deficiency of electrons in the aromatic rings of dinitroaniline molecules, the herbicides may also be bound to aromatic rings in soil organic matter by way of charge transfer complexes (TI) as shown for nitrofen in Figure 9. Most of the dinitroaniline herbicides (exceptions are dinitramine and prodiamine) have moderate to high vapor pressures and must be incorporated into the soil to prevent loss through volatilization.

Miscellaneous nonionic herbicides.

The remaining miscellaneous nonionic herbicides in the Table are divided into three groups according to their relative aqueous solubilities. Their adsorption to soils is regulated primarily by water solubility since none of the chemicals is ionizable or is of the complexing-type. Herbicides of lowest solubility are adsorbed to soil colloids, particularly organic colloids, in greater amounts than those of moderate or high solubility, respectively. Bonding to soil particles is primarily through charge transfer bonding (n), hydrogen bonding, or other weak physical forces analogous to that shown for diuron in Figure 8. Leachability of the herbicides through soils is inversely related to their adsorption by soils and thus the most soluble herbicides would be expected to be the most leachable and the least soluble the most mobile. With the exception of dichlobenil, which is moderately volatile, all of the miscellaneous nonionic herbicides have relatively low vapor pressures and thus their losses from the soil through volatilization would be expected to be low. Bioavailability of the miscellaneous nonionic herbicides would be directly related to their adsorption by soils, i.e., those bound in the greatest amounts would be the least available and vice versa. Inactivation of these herbicides by soils would increase as the organic matter and clay contents of the soils increase.

LITERATURE CITED

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3. Garringer, R. D., J.B. Weber, and T. J. Monaco. 1975. Adsorption­ desorption of selected pesticides by organic matter and montmorillonite. J. Agric. Food Chem. 23:568-572.

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