Interaction of Organic Pesticides with Particulate ... - ACS Publications

4 Interaction of Organic Pesticides with

Downloaded by UNIV OF NEW SOUTH WALES on September 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1972 | doi: 10.1021/ba-1972-0111.ch004

Particulate Matter in Aquatic and Soil Systems J. B. WEBER Crop Science and Soil Science Departments, North Carolina State University, Raleigh, N. C. 27607

Organic pesticides occur in detectable amounts in many parts of the environment. An understanding of the fate and behavior of biologically-active substances in the total environment is a necessity for the chemicals to be used safely and for new products to be developed which will not produce adverse effects. The interactions of organic pesticides with particulate matter in surface waters and soil systems according to the chemical properties of the compounds and their reported behavior in these systems are discussed below. The particulate matter includes various clay minerals, soil organic matter, charcoal, and other specific adsorbents that were utilized to help identify adsorption mechanisms. Pesticides were classified according to ionizability, molecular size, functional groups present, water solubility, and vapor pressure. Adsorption included ion exchange, dipole interactions, and other physical adsorption mechanisms.

The fate and behavior of organic pesticides in the biosphere has concerned scientists almost from the time that the chemicals were discovered. In 1945 Carollo (1) was concerned about the effect on soldiers as a result of DDT in natural water supplies. He formulated biological methods to detect the pesticide. Forrest et al. (2) found in 1946 several toxic metabolites of DDT in soil systems. Four years later Chisholm et al. (3) and Fleming (4) found that DDT was persistent in soils and that it had accumulated in amounts of 137-194 pounds per acre under trees that had been sprayed to control Dutch Elm disease. Soon other investigators 55 In Fate of Organic Pesticides in the Aquatic Environment; Faust, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.


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found that D D T and other pesticides had accumulated i n soils in high amounts (5,6,7). In the early 1950's De W i t t (8) reported that feeding D D T to quail reduced the hatchability of their eggs and the viability of the chicks. About the same time Foster et al. (9) reported that D D T and several other pesticides had accumulated in the soils to the detriment of some crop plants. Several investigators implicated D D T and other pesticides with the death of fish and other aquatic life (10,11, 12) and many began finding deleterious effects on the ecosystem as a result of using pesticides. After Rachel Carson (13) described the adverse effects of pesticides in her book, "Silent Spring," the problem finally received much public attention. Since then many popular and scientific books and governmental reports have been published concerning pesticides in the biosphere (14, 15,16, 17, 18, 19, 20, 21, 22). The material which is discussed below concerns the interactions of a variety of pesticides with diverse particulate matter in aqueous and soil systems. Particulate Matter Natural surface waters range from clear spring water through yellow to black swamp water containing dissolved organic substances and reddish colored river and lake waters containing high amounts of iron. The p H of these waters ranges from as acidic as 3.0 to as alkaline as 9.6 (23, 24, 25, 26). The waters contain dissolved and suspended particles; the particles vary from individual ions, pairs of ions, or complexes made up of several ions in the dissolved state to colloidal aggregates from 0.0052.0/A in diameter and suspended solids which are larger than 2.0/x.. The colloidal and suspended particles are classified into the following size categories: clay = below 0.002 mm (2 micron), silt — 0.002-0.02 mm, fine sand = 0.02-0.20 mm, and coarse sand 0.2-2.0 mm. The particles are inorganic and organic substances originating from municipal, industrial, and agricultural wastes and soil erosion. Tons of particles result from decomposing of natural plants and animals living in waters. Suspended solids from municipal and manufacturing wastes amount to approximately 8 and 18 billion pounds per year, respectively (27). The municipal wastes are primarily organic substances and dissolved minerals. Manufacturing wastes come primarily from four major industry groups— paper, organic chemicals, petroleum, steel—and are composed of inorganic substances—e.g., metallic particles from grinding and milling processes—and organic substances—e.g., carbon particles as soot and ashes, cellulose fibers, oil, bits of plastics and rubber, and assorted debris. H o w ever, the greatest volume of wastes entering surface waters of the United States are suspended solids carried by soil erosion. These sedimentary materials accumulate to more than 700 times those derived from total

In Fate of Organic Pesticides in the Aquatic Environment; Faust, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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sewage discharges (27). They are washed into streams and rivers from croplands, unprotected forest soils, overgrazed pastures, strip mines, roads, or bulldozed urban areas. Soil erosion on agriculturally developed land is at least five to ten times that for natural cover, and exposure resulting from construction and land development increases the rate a hundred fold.

Figure 1.

Electron micrograph of well-crystallized kaolinite clay particles (28)

Suspended materials in surface waters, stream bottom deposits, and soil systems are composed of sand, silt, and clay particles which are normally coated with thin films of organic and inorganic colloids, metallic oxides, and microbial growth. Sand particles are more or less rounded, irregularly angular, or even flat, depending upon the amount of abrasion and weathering that they have received. Silt particles are similar i n shape to sand particles, but they are much smaller. Clay particles, however, are flat, platelike, or mica-like. They result in the fine texture, high water-holding capacity, and stickiness of mineral soils. Figure 1 shows an electron micrograph of well-crystallized kaolinite clay particles. Not all clay minerals have the hexagonal flake-shape of kaolinite. Some are elongate and tubular in shape; others are lath-shaped crystals, and some appear to be fluffy amorphous aggregates (28). Several components make up the mixture of material referred to as soil clay. These include various hydrous silicates and oxides and the



F A T E

Ο

Oxygens

#

©Hydroxy*

Ο

Oxygens

®

ORGANIC

PESTICIDES

Aluminums

· Ο

Hydroxyts

O F

Silicons

φ

Aluminum, Iron, magnesium

Ο *nd φ Silicon, occasionally aluminum

R. E. G r i m , " C l a y M i n e r a l o g y , "

McGraw-HII

Figure 2. Diagrammatic sketch of structure of (top) 1:1 kaolinite and (bottom) 2:1 montmoriUonite clay minerals (28)


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important layer silicates. The layer silicates are formed structurally from two basic units:


(1) an infinite, 2-dimensional sheet of silica tetrahedra—all three basal oxygens are shared between adjacent tetrahedra (2) an infinite, 2-dimensional sheet of aluminum or magnesium octahedra—i.e., aluminum or magnesium i n octahedral coordination with six hydroxyls. A n example of the alternating tetrahedral and octahedral sheets is shown i n Figure 2 for a 1:1 clay (top) and for a 2:1 clay (bottom). T h e layer silicate structural types are formed by condensing tetrahedral and octahedral layers i n a ratio of 1:1 or 2:1—i.e., the apical oxygens of the silica sheet replace hydroxyls i n the octahedral sheet. According to structural considerations of ionic crystals, these oxygens are shared between a silica tetrahedron and two aluminum octahedra (or one tetrahedron and three magnesium octahedra). A tetrahedral sheet-octahedral sheet ratio of 1:1 characterizes the kaolinite minerals. Basic layer units are held together by hydrogen bonding between adjacent planes of oxygens and hydroxyls. The bonded layers do not readily separate, so the dimensions of the unit cells are relatively constant. Kaolinite minerals are common soil clays, particularly i n acid soils. In other layer silicates the tetrahedral sheet-octahedral sheet ratios are 2:1—i.e., an octahedral sheet sandwiched between two tetrahedral sheets (Figure 2, bottom). The 2:1 ratio characterizes the micas, vermiculites, and montmorillonites (smectites). During crystal growth of 2:1 clays individual unit layers are superimposed and held together by van der Waals forces between adjacent planes of oxygens. The relatively weakly bonded layers are readily separated by water molecules. Such clays therefore can swell upon wetting and shrink upon drying. Montmorillonite is probably the best known member of this mineral type and soils containing high amounts of it characteristically develop large cracks during drought periods. Usually during crystal growth, the tetrahedra are not solely occupied by silicon or by aluminum or magnesium. Aluminum may substitute for some of the tetrahedral silicon atoms, and iron, lithium, manganese, chromium, and other ions of suitable size may occupy a part of the octahedral sites. This isomorphous replacement of ions unbalances the overall charge of the crystalline lattice. A n excess negative charge develops which is balanced by cations that are retained on the external layer silicate surfaces. These cations—e.g., sodium, potassium, calcium, and others—are more or less exchangeable, depending on the nature of the replacing cations, the nature of the adsorbed cations, and the magnitude and distribution of the structural charge. They are held between unit layers and bond the layers together. The total number of exchangeable



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cations that a clay can retain is its cation exchange capacity ( C E C ) . Competitive ion studies indicate that the surface charge on the layer silicates is expressed as discrete adsorption sites rather than as a charge smear (29,30). Besides the C E C resulting from isomorphic substitution, electric charges develop at the edges of clay crystals from broken bonds. These charges are positive or negative, and they contribute to the exchange capacity of the clay mineral. Negative charges are believed to result from exposed hydroxyl groups which dissociate with p H changes and are termed pH-dependent charges as opposed to charges originating from isomorphic substitution which are considered to be independent of p H . In the 1:1 clays exchangeable cations are adsorbed only on the exterior surfaces, but in 2:1 clays the cations are also adsorbed on the interior surfaces between the tetrahedral sheets, along with the water molecules. The area between the silicate sheets is also known as the interlayer or basal spacing. For some of the 2:1 clays, potassium atoms are located between the neighboring tetrahedral sheets and because of the close geometrical spacings, potassium holds the sheets tightly together through multiple electrostatic bonds. Consequently these clays do not shrink or swell and are termed the nonexpanding 2:1 clays. E x panding 2:1 clays have a much greater specific surface area and C E C than the nonexpanding 2:1 clays. Some characteristics of selected clay minerals are given i n Table I. Table I.

Characteristics of Some Common Clay Minerals

Characteristics Type of layering Type of swelling C E C (meq/100 grams) Specific surface (mVgram) C axis basal spacing in ethylene glycol (A)

Montmorillonite

Illite

Kaolinite

80-120 700-750

2:1 limited expanding 120-200 500-700

2:1 nonexpanding 15-40 75-125

1:1 nonexpanding 2-10 25-50

17

14

10

7.2

2:1 expanding

Vermiculite

Vermiculite, which is formed in most soils from the weathering of micas, has a higher structural charge than the other clay minerals in Table I. Calcium and magnesium plus water of hydration generally occupy the interlayer region. These cations are readily exchangeable to other solution cations, and the limited-expanding interlayer region (4-5 A ) permits organic cations and polar organic molecules to enter with or instead of water, providing the organic molecules are not too large.



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Montmorillonite clay minerals are generally less well-crystallized and of much smaller particle-size than the others given i n Table I. Under natural conditions various cations occupy the exchange sites, including calcium, magnesium, sodium, and aluminum. The interlayer region is variable (10-20 A ) and readily accepts water and other polar molecules. Sodium-saturated montmorillonite exhibits unrestricted expansion in water, whereas the maximum interlayer spacing of the calcium-saturated mineral in water is 9.5-10.5 A . A l l of the clay minerals given in Table I may occur in the clay fractions of soils. They often occur in mixtures, but frequently one mineral predominates. Kaolinite and vermiculite generally characterize acid soils of the Southeastern United States. Illite and other clay-sized micas are common in the Northeast and the Midwest. Montmorillonite occurs commonly in the neutral and alkaline soils of the Midwest and Western states. However, any and all of the clays can and do occur throughout soils of the United States, depending on the natural parent material and the formation conditions. Other details concerning clay minerals are described in the works of G r i m (28), Marshall (31), and Bear (32). The exchangeable cations are most highly concentrated in the immediate vicinity of the clay surface and decrease with distance from the surface. The charged surface and the associated oppositely charged ions are collectively known as the double layer. The cation swarm from the surface out into the bulk of the medium, which is usually aqueous solution, is considered diffuse following a more or less exponential distribution. The adsorbed ions are hydrated somewhat with water molecules on the clay surfaces. These cations are replaced according to their valence and hydrated size. The water on the clay surface is less dense and more ordered than normal water and has an ice-like structure. A n example of the water net configuration at clay surfaces is shown i n Figure 3. The interactions of water at clay surfaces has recently been reviewed by L o w (33). H e reported that water adsorbed on clay surface is more ordered than that in bulk solution. Exchangeable ions on the clay surface affected the heats of wetting and adsorption of water. W h e n large or multivalent cations were present, there appeared to be little or no order i n the adsorbed water. The specific volume, viscosity, and freezing resistance of adsorbed water was directly related to its structural development. Consequently, the magnitudes of these properties decrease continuously with distance from the mineral surface. Nowhere was the structure of water so rigid that ions could not diffuse through it. L o w stated that although adsorbed water has a precise molecular arrangement and is ice-like, the structure is not that of ice (33). Studies by Mortland et al. (34) suggested that residual water on the interlayer surfaces of expanding clay minerals is dissociated more


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Silica laytf of cloy m intra I


Oriented water moleculet

Silica loytf of clay minorai

R.

Figure 3.

E. G r i m ,

"Clay Mineralogy,"

McGraw-Hill

Configuration of water net at clay surfaces (28)

than free water. The acidity at clay surfaces in aqueous systems is greater than that in the suspension. Studies by Harter and Ahlrichs (35) showed that when the suspension p H was 6, 5, 4, and 3 organic acids adsorbed to clay surfaces dissociated as if they were in p H environments of 4.5, 4.0, 3.2, and 2.5, respectively. Therefore water on the surface of clay minerals is more acidic than i n the bulk solution. Besides the mineral particulate matter in surface waters and soil systems, there are organic substances occurring as dissolved solids, undecayed plant and animal tissues, and the myriads of humic substances present in soil organic matter. This humic material is called humus and is generally considered to consist of two main groups (36). Group one (decomposing plant and animal components and products of resynthesis in bacterial cells) consists of various nitrogenous and non-nitrogenous organic compounds belonging to well-known organic chemical groups— e.g., enzymes, proteins, carbohydrates, organic acids and sugars, fats, waxes, resins, lignin, pigments, vitamins, antibiotics, and hormones. These compounds make up approximately 10-15% of the total organic matter in soils. Organic nitrogen compounds which make up 20-50% of the total nitrogen in most surface soils are in bound amino acids and sugars. Less than 1% of the organic nitrogen in soils occurs as purine and pyrimidine bases. More than 30 amino acids have been detected i n soil, including



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basic amino acids—e.g., arginine, histidine, methylhistidine, lysine, and ornithine—acidic amino acids—e.g., aspartic, glutamic, and cysteic—and neutral amino acids—e.g., glycine, serine, valine, alanine, leucine, isoleucine, tyrosine, threonine, proline, methionine, and cystine. Amino sugars, such as glucosamine and galactosamine, occur i n soils i n significant amounts. Several purine and pyrimidine derivatives, including adenine, guanine, xanthine, hypoxanthine, cytosine, uracil, and thymine, have been found in soils or soil preparations. Other organic nitrogen compounds have been detected i n soils or soil products. These include t r i methylamine, ethanolamine, choline, histamine, creatinine, allantoin, cyanuric acid, a-picoline-y-carboxylic acid, urea, asparagine, and glutamine. Also, enzymes—e.g., amylases, catalases, glycosidases, invertase, nuclease, proteases, phosphatases, and urease—occur i n soils i n small amounts. Organic phosphorus compounds, primarily inositolhexaphosphates (probably more than 5 0 % of all organic phosphates), occur i n soils. The parent cyclic polyol, inositol, exists i n numerous stereoisomeric configurations, of which myo-, scyllo-, neo-, and dZ-inositol have been isolated from soils as phosphate esters. The hexaphosphate of myoinositol (myoI H P ) , phytic acid, occurs in plant tissues. It often occurs as phytin, the calcium magnesium salt. Esters of myo-IHP are readily adsorbed in acidic soil solution by clay minerals and finely divided hydrated oxides of iron and aluminum. Organic sulfur compounds present in soils probably occur primarily as amino acids—e.g., cysteine, cystine, and methionine. Approximately 5-15% of the soil organic matter is carbohydrate. Carbohydrates which have been isolated from soils includes monosaccharides—e.g., hexoses (glucose, galactose, mannose, and fructose) and pentoses (arabinose, xylose, ribose, fucose, rhamnose), disaccharides (sucrose, cellobiose, and gentiobiose), oligosaccharides (cellotriose); polysaccharides (cellulose, hemicellulose), amino sugars (glucosamine, galactosamine, and N-acetyl-D-glucosamine ), sugar alcohols (mannitol, inositol); sugar acids (galacturonic, glucuronic), and methylated sugars (2-O-methyl-D-xylose, 2-O-methyl-D-arabinose, 2-O-methylrhamnose, and 4-O-methylgalactose). Free carbohydrates constitute less than 1% of the total sugars and are present chiefly as monosaccharides. Approximately 10-20% of all carbohydrates have been extracted as polysaccharides, primarily cellulose, and the nature of the remaining 70-80% of soil carbohydrates is still obscure. The nature, origin, and properties materials are not yet fully understood mately 85-90% of the soil humus. The made of various humic acids and small

of the second group of humic (36). They comprise approxiactive portion of soil humus is amounts of stable free radicals.


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The humic acids vary i n water solubilities and i n the types of functional groups present. They are believed to be responsible for the cation- and anion-exchange properties exhibited by soil organic matter. About one-third of the humus is removed from soil by extracting with alkali. After neutralizing alkaline extracts, humic acid is precepitated, and polymeric fulvic acid remains i n solution. Both acidic substances contain chemically bound nitrogen, phosphorus, and sulfur. Molecular weight estimates indicate a range of 5000-100,000 for humic acids and 2000-9000 for fulvic acids with considerable overlap. H u m i n is that fraction of soil organic matter which cannot be readily extracted with cold alkali. It is a chemically heterogeneous fraction and consists of relatively unchanged organic material not immediately soluble i n alkali and of humified material. H u m i c acids are usually polymers of aromatic compounds. The extent of aromaticity and whether aliphatic and alicyclic structures are also incorporated is not well understood. Carboxyl, phenolic hydroxyl, alcoholic hydroxyl, carbonyl, and methoxy groups are present, but the relative proportions of each are not consistent and vary with soil type, horizon, and method of isolation of the fraction. Oxygen is present i n ether linkages. The nitrogen content of humic acid varies from 0.4-5%. H u m i c acid is a heterogeneous fraction even after purifying extensively to remove inorganic material, carbohydrates, and nitrogenous compounds. Some soils (Rendzina and mull humus) yield lignohumate humic acid, while others (moss humus and Podzel) are predominantly flavonoid. A hypothetical structure of a humic acid molecule is given i n Figure 4. Additional details concerning soil organic matter are given b y Bear (32), Kononova (36), M c L a r e n and Peterson (39), and Eglinton and Murphy (38).

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Interaction of Organic Pesticides with Particulate ... - ACS Publications

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