Download Abstract. PHYSICAL/CHEMICAL INTERACTIONS ·OF

PHYSICAL/CHEMICAL INTERACTIONS ·OF HERBICIDES 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 herbicide 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 No 2 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 276957620.
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, diquat, morfamquat, and
paraquat (Table) ionize completely in aqueous solutions to yield cationic
species as shown for paraquat in equation I (Weber 1972).
Paraquat dichloride~~~-)~ Paraquat2+ + 2 Cl-
Table.
(I)
Classification scheme for organic herbicides.
Category
Species
Connnon name
Strongly basic
Cation
Difenzoquat, diquat, morfamquat,
paraquat
Weakly basic
Cation/molecular
Ametryn, amitrole, atrazine, cyanazine,
dipropetryn, fluridone, metribuzin,
prometon, prometryn, propazine,
simazine, tebuthiuron, terbutryn
Acidic
Anion/molecular
Acifluorfen, asulam, benazolin, bensulide, bentazon, bifenox (free acid),
bromacil, bromoxynil, buthidazole,
chloramben, chlorimuron, chlorsulfuron,
2,4-D, dalapon, dicamba, diclofop (free
acid), dichlorprop, dinoseb, DNOC, DOWCO
290, DPX 6313, endothall, fenac,
fenoxaprop (free acid), flamprop (free
acid), fluazifop (free acid), fomesafen,
imazaquin, imazapyr, ioxynil, isocil,
lactofen (free acid),MCPA, MCPB,
mecoprop, mefluidide, naptalam,
oryzalin, perfluridone, picloram,
quizalofop, sethoxydim, silvex,
sulfometuron methyl, 2,4,5-T, TCA,
terbacil, triclopyr, trisulfuron
Complexing-type
Cacodylic acid, DSMA, ethephon,
fosamine, glyphosate, glyphosine, MAA,
MSMA
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Table.
Classification scheme for organic herbicides (continued)
Category
Species
Common name
Nonionic
Moleculer
Anilide: Acetochlor, alachlor,
butachlor, diethatyl-ethyl, metolachlor,
propachlor, propanil
Amide: Diphenamide, napropamide,
pronamide
Phenylurea: Chloroxuron, diuron,
fenuron, fluometuron, linuron, 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, pendimethalin, prodiamine,
prof luralin, trifluralin
Misc. (Low solubility): DCPA, methazole,
oxadiazon
Misc. (Moderate solubility):
Cirunethylin, dichlobenil, norflurazon
Misc. (High solubility): Dimethazone,
ethofumesate, hexazinone, 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).
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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 surf aces 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.
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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
where:
B
+
~ ---•'• HB+
...
(II)
= molecular
species of weakly basic herbicide
ion
HB+ = cationic species of weakly basic herbicide
~
= hydrogen
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•
Figure 3.
K•
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
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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
where:
(III)
HA = molecular species
a+ • hydrogen ion
of weakly acidic herbicides
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
Cl
O-CH 2-c
2,4-D
~p
.
.·.
-H C
2
Figure 4.
/OH
OM
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).
Cl
-Q-
~o
O-CH2C'o.-
CI
2,4-D
. _ +,..OH
Fe
\
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 arsenic 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~
~
GLYPHOSATE
0-
OXIDE
OH
,j,·3
I
----o---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 surf aces 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
C-N-011
0:
..
.
OM
I
H:
..
.
Ho
Cl
I II
CH
N-C-N/
3
'CH3
DIURON
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
Cl
0
Cl
Figure 9.
~o
N
-0- 'o
NITROFEN
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.
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107
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