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Abstract. PHYSICAL/CHEMICAL INTERACTIONS ·OF HERBICIDES

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Abstract. PHYSICAL/CHEMICAL INTERACTIONS ·OF HERBICIDES 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 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, 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- (I) Table. 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, ben­ sulide, 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 97 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). 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 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. 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
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