The rate of degradation of pesticides is often expressed as half-life (DT50), expressed in years, months or days. Every pesticide has its own half-life value. After this period only half of the original amount of pesticide is left, the other half having been degraded away.

The rate of degradation of various pesticides is influenced by external factors such as temperature, light and soil acidity. As a rule, degradation of a compound is considered complete after a period of five times the half-life of that compound.

Probably because it is mathematically simple and can be easily fit to experimental data (as in the days before everyone had a high-speed computer), the first-order model has been widely used to describe pesticide degradation kinetics.

Applicability of the first-order model makes several assumptions and, in some cases, either the assumptions do not hold or, for some other reason, the degradation data cannot be adequately described by first-order kinetics. Pesticides may exhibit deviation from first-order kinetics.

Some researchers have found that such degradation behavior is adequately described by an Nth-order model dC / dt = -k C^N

where N is an empirical constant. Although the Nth-order model may adequately describe degradation kinetics, it is not founded on any underlying mechanism. However, the dependence of C on t in this model is similar to the dependence of C on t in an extension of the first-order model in which the pesticide is presumed to be partitioned into two physically separate compartments,1 and 2, each with different degradation rate constants

dC / dt = (-k1C1) + (-k2C2)

the dependence of C on tin this model is similar to the dependence of C on t in an extension of the first-order model in which the pesticide is presumed to be partitioned into two physically separate compartments, 1 and 2, each with different degradation rate constants.

C= -k1 C10 exp (-k1 t) –k2 C20 exp (-k2 t)

And since C0= C10+ C20, the initial concentrations in these compartments,and there is no transfer between compartments

C = -k1 f C0 exp (-k1 t) –k2 (1-f) C0 exp (-k2 t)

The number of such separate compartments can be increased without bound and the frequency of different rate constants statistically distributed, leading the same type of degradation behavior described by the Nth-order model.In other words, the Nth-order model is consistent with a large number of quire different degradation micro-environments.

The rate of degradation of pesticides is often expressed as half-life (DT50), expressed in years, months or days. Every pesticide has its own half-life value. After this period only half of the original amount of pesticide is left, the other half having been degraded away.

The rate of degradation of various pesticides is influenced by external factors such as temperature, light and soil acidity. As a rule, degradation of a compound is considered complete after a period of five times the half-life of that compound.

Probably because it is mathematically simple and can be easily fit to experimental data (as in the days before everyone had a high-speed computer), the first-order model has been widely used to describe pesticide degradation kinetics.

Applicability of the first-order model makes several assumptions and, in some cases, either the assumptions do not hold or, for some other reason, the degradation data cannot be adequately described by first-order kinetics. Pesticides may exhibit deviation from first-order kinetics.

Some researchers have found that such degradation behavior is adequately described by an Nth-order model dC / dt = -k C^N

where N is an empirical constant. Although the Nth-order model may adequately describe degradation kinetics, it is not founded on any underlying mechanism. However, the dependence of C on t in this model is similar to the dependence of C on t in an extension of the first-order model in which the pesticide is presumed to be partitioned into two physically separate compartments,1 and 2, each with different degradation rate constants

dC / dt = (-k1C1) + (-k2C2)

the dependence of C on tin this model is similar to the dependence of C on t in an extension of the first-order model in which the pesticide is presumed to be partitioned into two physically separate compartments, 1 and 2, each with different degradation rate constants.

C= -k1 C10 exp (-k1 t) –k2 C20 exp (-k2 t)

And since C0= C10+ C20, the initial concentrations in these compartments,and there is no transfer between compartments

C = -k1 f C0 exp (-k1 t) –k2 (1-f) C0 exp (-k2 t)

The number of such separate compartments can be increased without bound and the frequency of different rate constants statistically distributed, leading the same type of degradation behavior described by the Nth-order model.In other words, the Nth-order model is consistent with a large number of quire different degradation micro-environments.

Please find the papers attached

Good luck

All above is right. Anyway take care of metallic ion pesticides.

Metallic ions are not degradable. (Cu, Zn, Al...)

Dear Hakima,

Thank you for your effort and attaching two important files on LD50. But, I am looking for DT50 or Half life of Pesticides papers. -Aniruddha

Dear Valentina,

Thank you for your detail answer. May I go with software or manual input in excel to calculate the value? -Aniruddha

See USDA link: https://www.ars.usda.gov/pacific-west-area/parlier/sjvasc/cpq/docs/probit-download/

Sure, there are many. I found this useful https://19january2017snapshot.epa.gov/pesticide-science-and-assessing-pesticide-risks/guidance-calculate-representative-half-life-values_.html

If you want to determine DT50 (dissipation half-life) in standard laboratory conditions, you could use the OECD guideline no. 307 (https://www.oecd-ilibrary.org/environment/test-no-307-aerobic-and-anaerobic-transformation-in-soil_9789264070509-en).

If you want to determine field DT50 values, use the OECD guideline on terrestrial DT50 studies http://www.oecd.org/officialdocuments/publicdisplaydocumentpdf/?cote=ENV/JM/MONO(2016)6&docLanguage=En

If you strictly want to determine degradation losses (DegT50; soil degradation half-life), you should dig into the FOCUS-guideline from 2006: https://esdac.jrc.ec.europa.eu/public_path/projects_data/focus/dk/docs/finalreportFOCDegKinetics.pdf.

You may also like to read EFSA's guidances, e.g. :

https://www.efsa.europa.eu/en/press/news/140508

I recommend using the free CAKE software from Tessella, it is very easy to use and produces all relevant parameters (as described in the FOCUS guideline). Download here: https://tessella.com/products/demonstrate-chemical-satisfies-environmental-regulations/

Thank you Marit, for your kind assistance in this regard. I think these links and protocols trully helps me out to do so.

Regards,

A. Sarker