Chapter 8
Bound (Nonextractable) Pesticide Degradation Products in Soils
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Bioavailability to Plants
Shahamat U. Khan Land Resource Research Centre, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6, Canada A significant proportion of degradation products from certain pesticides applied in agriculture remains in soils as bound (nonextractable) residues. S o i l organic matter i s largely responsible for the formation of bound residues. In addition to chemical binding, the degradation products of pesticides are also firmly retained by s o i l organic matter fractions to form bound residues by a process that more likely involves adsorption on external surfaces and entrapment in the internal voids of molecular sieve-type structural arrangements. The bound residues of pesticides degradation products i n s o i l are bioavailable to plants.
Although the formation of bound (nonextractable) pesticide residues i n s o i l and plants have been known to occur for over two decades, their significance has been c r i t i c a l l y addressed only recently when i t became obvious that these residues are not excluded from environmental Interactions. The true nature of bound residues of pesticides and their degradation products i n s o i l and plants i s s t i l l poorly understood. However, i t i s well established that various pesticides and/or metabolites can form appreciable amounts of bcund residues. This was revealed primarily by use of radiolabeled pesticides, which after application, led to the detection and quantitation of residues undetectable by any conventional analytical techniques. Thus, for a long time the possible s o i l or plant burden of total pesticide and/or i t s degradation products residues has been underestimated. The question of the significance of residues of bound pesticides and/or their degradation products can be evaluated i n terms of their availability to plants i f they should be released from s o i l . Recent evidence indicates that bound residues may be released frcm s o i l and absorbed by plants. The formation of bound residues of pesticides i n s o i l and plants, and their bioavailability has been reviewed by Klein and 0097-6156/91Α)459-0108$06.00Λ) Published 1991 American Chemical Society Somasundaram and Coats; Pesticide Transformation Products ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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Bound Degradation Products: Bioavailability
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Scheunert (14), Khan (7), Roberts (16) and Calderbank (3). The present paper summarizes the information on bound residues of the degradation products of certain pesticides i n s o i l and their uptake by plants* The mechanisms(s) of the formation of bound residues i n s o i l i s also discussed. Most of the data reported i n this paper are based on research efforts i n our laboratory at the Land Resource Research Centre, Ottawa.
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Formation of Bound Residues i n Soil Figure 1 illustrates several of the direct and indirect routes by which pesticides and their degradation products form bound residues i n s o i l (3). The majority of these bound residues i n s o i l originate f ran deliberate application of pesticides to the s o i l or to the foliage of crop plants and weeds. Very often substantial quantities of the applied pesticide reach the s o i l by either missing the target or by run-off from leaves and stems. Death of plants or root exudation may also contribute to the incorporation of pesticide i n s o i l . Although leaching, volatilization and biota aœunulation may result i n the loss of pesticide from s o i l , a proportion of many of these chemicals on or i n s o i l i s subjected to biological, chemical and photochemical degradation of the parent chemical. A portion of the pesticide and/or i t s degradation products then becomes much more firmly held by the s o i l component than the average and i s now referred to as a "bound" residues. The mechanism of binding processes by which the pesticide and/or metabolites become bound i n s o i l has been discussed by Calderbank (3). In studying bound residues i n s o i l i t often becomes very d i f f i c u l t to differentiate between bound residues of parent pesticides and their degradation products. Drastic experimental methods utilized to extract the bound radiolabeled residues or combustion to CD to determine the total bound C residues destroy or alter the chemical nature of the residues. Nevertheless, identification of bound residues i n s o i l as degradation products of the parent pesticide has been reported i n many instances. Katan et a l (6) and Katan and Lichtenstein (5) demonstrated rapid binding of the amino analogue of parathion. These s o i l bound residues of aminoparathion were unextractable and therefore not detected i n routine residue analyses. Wheeler et a l (20) also observed a significant relationship between the amount of binding and the nature of substitution on the amino nitrogen. These authors postulated that some of the metabolites containing secondary or primary amino functional groups may have become part of the bound residue i n s o i l . Spillner et a l (19) implicated 2nnethylh^roquinone, an oxidative product of 3nmethyl-4-nitrophenol, as the precursor to the formation of bound residues of fenitrothion i n aerobic s o i l . However, under anaerobic conditions binding was thought to proceed through the intermediate. Golab et a l (4) suggested that α, α, α-trifluorotoluene- 3,4,5-triamine, a degradation product of t r i f l u r a l i n , may be a key conpound i n the formation of s o i l bound residues. In other studies concerned with the formation of bound residue i n s o i l treated with fenitrothion i t was suspected that part of the bound residue was 3-inethyl-4-nitrqphenol (15). Klein and Scheunart (14) presented data on bound residues of pesticides which have been shown to form anilines and phenols i n s o i l . 14
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a
Somasundaram and Coats; Pesticide Transformation Products ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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PESTICIDE TRANSFORMATION PRODUCTS
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Pesticide
i Foliar Application
Seed Dressing
Soil Application
Degradation Products
Bound Residues
Soil Residues
Bound Residues i n Soil (Parait and/or Dégradation Products) Figure 1. Possible routes for the formation of bound residues i n s o i l following pesticide application.
Somasundaram and Coats; Pesticide Transformation Products ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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Bound Degradation Products: Bioavailability
In an earlier study (10) i t was shown that bound residues of the degradation products of prometryn were formed when an organic s o i l was Incubated with the herbicide for one year (Table I ) . At the end of Incubation period i t was observed that 57.4% of the C i n i t i a l l y added was not retractable with solvents, constituting a bound residue. While more than 50% of the total bound C residues constituted the parent herbicide, measurable amounts of the mono-N-dealkylated and hydroxy analogues of prometryn were also present i n the form of bound C residues (Table I ) . I t was shown later that Incubation of this s o i l containing bound residues with a fresh s o i l inoculum resulted i n a release of C (27%) which had been i n i t i a l l y unextractable (11). Examination of the extractable material indicated the presence of prometryn and several metabolites including hydroxyprx>pazine (9%) and partially N-dealkylated compounds (2%). The radioactivity which remained bound consisted of prometryn (31%), hydroxypropazine (7%) and mono-N-dealkylated prometryn (
