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What is the relation between kdeg (s-1) and half-life (days/hrs)?

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SandraR
SandraR's picture
What is the relation between kdeg (s-1) and half-life (days/hrs)?

Hello,

In EPIsuite, the degradation is expressed as half-life in days/hours. Is it only to reverse it?
When I checked with the values in the USEtox database it was not.

For example for P-HYDROXYBENZOIC ACID, the EPIsuite data for athmospheric degradation reversed is 1/(9.873 (hrs)*3600)=2.81E-05 and in the USEtox database the value for kdegA is 9.75E-06.

Best regards,

Sandra

USEtox Team
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Conversion degradation half-life into bulk rate constant

Dear Sandra,

Thank you for your question. USEtox does not take the half-life associated with the hydroxyl radicals reaction in air given in EPIsuite. Instead, as stated in the comment of cell 'J2' of sheet 'Epi-est' in USEtox file 'Database_organics.xls', the bulk degradation rate constant in air is based on the "OVERALL OH Rate Constant in cm³/molecule/sec multiplied with 1.5E+06 OH/cm³ and assuming 12 hours effective removal per day".

Example for  P-HYDROXYBENZOIC ACID (CAS: 99-96-7):

  1. EPIsuite gives a (second order) OVERALL OH Rate Constant = 13.0001E-12 cm³/molecule/sec
  2. Multiplying this (second order) rate constant with 1.5E+06 OH molecules/cm³ gives a (first order) rate constant of 1.95E-05 1/sec
  3. Assuming that only half of the day is affected by this removal pathway (i.e. 12 hours), we divide this (first order) rate constant by 2 and get a (first order) rate constant of 9.75E-06 1/sec -- this value is finally stated as rate constant in USEtox (see cell 'J3077' in sheet 'Epi-est')

Kind regards,

the USEtox Team

SandraR
SandraR's picture
Thank's for quick reply!

Really fast, thank's!

Sandra

SandraR
SandraR's picture
Water degradation

Hello again,

now I got stuck on the water of course, but I read this time at least the instruction:

Based on ultimate degradation estimates with Biowin and converted to degradation rates following Episuite suggested relationships.

 

Does this mean the value from EPIsuite on Biowin3 (Ultimate Survey Model)?

For P-HYDROXYBENZOIC ACID this value is 3.0382 weeks, and reversed that becomes 5,44E-07 which is very similar to 5,34836E-07 in the database...

 

The soil and sediment seems easy at least :)
Thanks again,

Sandra

 

USEtox Team
USEtox Team's picture
Degradation in water: converting half-lives into rate constants

Thanks again for your question.

EPIsuite values from the BioWin module are not half-lives, but scores to which respective half-life ranges (e.g. "weeks") are assigned. Consequently, the values themselves cannot be used to obtain a rate constant for degradation.

Instead, you find the assumed average half-life for each range in Table 1 of the USEtox manual for Chemical database: organics. For the range of "weeks", a half-life of HL = 15 days is assigned. After conversion from days to seconds, i.e. 15 days x 24 hr/day x 3600 sec/hr = 1,296,000 sec, you can obtain from this half-life a corresponding rate constant k [1/sec] via the general relationship k = ln(2) / HL assuming first order kinetics with "ln" as natural logarithm, i.e. ln(2) / 1,296,000 sec = 5.348E-07 1/sec, which is stated also in USEtox (cell 'K3077' in sheet 'Epi-est' of file 'Database_organics.xls' for P-HYDROXYBENZOIC ACID).

This equation is stated in many text books and you can also find in Fantke and Jurske (2013) a description of the relationship for first order kinetics (ibid., equations 1 and 2) along with an overview of how to obtain rate constants or half-lives for other kinetics, such as 2nd order (ibid., Table 2).

Kind regards,

The USEtox Team