Chapter 7
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Factors Affecting the Degradation of 3,5,6-Trichloro-2-Pyridinol in Soil Kenneth D. Racke and Susan T. Robbins Environmental Chemistry Laboratory, DowElanco, 9001 Building, Midland, MI 48641-1706 The degradation and sorption of 3,5,6-trichloro-2pyridinol (TCP), a primary metabolite of the insecticide chlorpyrifos and the herbicide triclopyr, was examined in 25 different soils in order to better predict its environmental fate and significance. TCP exhibited sorption (Kd) coefficients of between 0.3 and 20.3 ml/g (mean 3.1) and calculated mean Koc coefficients for the neutral and anionic forms of 3344 and 54 ml/g, respectively. Mineralization was used as an indicator of the degradation in soil of TCP and similar organic compounds chosen for comparative study (3-chloro5-trifluoromethyl-2-pyridinol, 2-hydroxypyridine, 2,4-dichlorophenol). Although rates of TCP degradation varied between soils, multiple regression analyses revealed poor correlation of degradation with commonly measured soil properties. However, inclusion of soil degradation rates of related organic compounds or glucose, and sorption information in the regression model resulted in a significant improvement in the ability to predict TCP degradation. The mineralization of TCP is microbially-mediated, yet i t is unclear whether catabolic or cometabolic processes are predominant. Only 2 soils contained microbial populations that could utilize TCP as a sole carbon source in mixed culture.
Most environmental f a t e studies have focused strongly i f not e x c l u s i v e l y on the parent p e s t i c i d e molecule and have devoted l i t t l e a t t e n t i o n to the f a t e of p e s t i c i d e metabolites. A number of p e s t i c i d e s contain halogenated pyridine r i n g systems, and various halogenated p y r i d i n o l s may be produced from these compounds during degradation. One o f these p y r i d i n o l s o f i n t e r e s t i s 3,5,6-trichloro-2-pyridinol (TCP). TCP i s a primary metabolite o f
0097-6156/91/0459-0093$06.00/0 © 1991 American Chemical Society In Pesticide Transformation Products; Somasundaram, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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the i n s e c t i c i d e c h l o r p y r i f o s and the herbicide t r i c l o p y r i n s o i l and water and on plant surfaces. Formation of TCP occurs v i a both h y d r o l y t i c and p h o t o l y t i c mechanisms, and measurable q u a n t i t i e s o f TCP can be produced i n s o i l f o l l o w i n g a p p l i c a t i o n of e i t h e r of these pesticides. Maximum TCP concentrations i n s u r f i c i a l s o i l s f o l l o w i n g a p p l i c a t i o n of c h l o r p y r i f o s or t r i c l o p y r have been measured as
Downloaded by NORTH CAROLINA STATE UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: March 21, 1991 | doi: 10.1021/bk-1991-0459.ch007
0.07-0.7 and 0.4-11 μg/g, r e s p e c t i v e l y
(1-3).
Although many halogenated phenols have been studied quite extensively, few halogenated p y r i d i n o l s have been as thoroughly investigated. In contrast, TCP has been f a i r l y well characterized. It i s an a c i d i c compound with a pka of 4.55, a water s o l u b i l i t y o f 117 μg/ml (pH 2-3), and a n e g l i g i b l e vapor pressure o f
