Properties and Uses of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 15

microbial degradation. Volatilization dominates dissipation from foliage in the
initial 12 h after application, but decreases as the formulation adsorbs to foliage or
soil (Mackay et al. 2014 ). In the days after application, CPY adsorbs more strongly
to soil, and penetrates more deeply into the soil matrix, and becomes less available
for volatilization; other degradation processes become important.
Dissipation from soil. Factors affecting degradation of CPY in soil have been
reviewed by Racke ( 1993 ). The key values that affect soil dissipation have been
updated and are presented in SI Table A-1. Photolysis and oxidation are known to
form CPYO in air (Mackay et al. 2014 ) and on foliar surfaces. These routes are
either insignificant in soil or CPYO degrades as quickly as it is formed, since CPYO
has only been formed in undetectable or small amounts in studies that have used
radiotracers to investigate degradation in soils in the laboratory (de Vette and
Schoonmade 2001 ; Racke et al. 1988 ) or field (Chapman and Harris 1980 ; Rouchaud
et al. 1989 ). The primary degradation pathway in soil involves hydrolysis to yield
3,5,6-trichloropyridinol (TCP, Fig. 1 ) from either CPY or CPYO. Results of several
studies have shown that this step can be either abiotic or biotic, and the rate is 1.7- to
2-fold faster in biologically active soils. Both modes of hydrolysis can occur in
aerobic and anaerobic soil. The rate of abiotic hydrolysis is faster at higher pH.
Hydrolysis is also faster in the presence of catalysts such as certain types of clay
(Racke 1993 ). Degradation of the intermediate, TCP, is dependent on biological
activity in soil, and leads to formation of bound residues and reversible formation
of 3,5,6-trichloro-2-methoxypyridinol (TMP; Fig. 1 ). Under aerobic conditions, the
primary, terminal degradation product of CPY is CO 2 . Since TCP and TMP are not
considered to be residues of concern (USEPA 2011b ), they were not included in
characterizations of exposures presented here or the assessment of risk in the com-
panion papers. Because of rapid degradation in soil (see above), CPYO (Fig. 1 ) was
not included in the characterization of exposures via soil.
The half-life for degradation of CPY in soils, based on results of studies con-
ducted under standardized laboratory conditions, ranged from 2 to 1,575 d (n = 68,
next highest value is 335 d; SI Table A-1). This range in rates of degradation was
attributed to differences in soil organic carbon content, moisture, rate of application,
and microbial activity in the reported studies (Racke 1993 ); however, quantitative
relationships between these potential drivers and rates of degradation have not been
developed. Greater rates of application resulted in slower degradation, possibly due
to the concentration in soil-water reaching the solubility limit of approximately
1 mg L −1 , which affects bioavailability to microbiota. The formulation applied can
affect results; dissipation from material applied as the granular product is slower
(Racke 1993 ). Half-lives for dissipation from soils determined under field condi-
tions have been reported to range from 2 to 120 d (N = 58; SI Table A-2).
Biphasic dissipation. Results of studies of aerobic degradation of CPY in soils
under laboratory conditions exhibit bi-phasic behavior in most soils. Initial rates of
degradation are greater than overall degradation rates by factors of 1.1 to 2.9 (Racke
1993 ). This behavior of CPY is also variable and not as apparent for some of the
soils studied, for which half-lives were calculated by using simple first-order