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Soil Adsorption Coefficient (Kd/Koc)

By Little Pro on 2016-02-01 Views:  Update:2017-01-18

Soil adsorption coefficient (Kd) measures the amount of chemical substance adsorbed onto soil per amount of water.

  • Kd = Concentration of chemical in soil/Concentration of chemical substance in water

Values for Kd vary greatly because the organic content of soil is not considered in the equation. Since adsorption occurs predominantly by partition into the soil organic matter, it is more useful to express the distribution coefficient in Koc. Koc is also known as organic carbon-water partition co-efficient. 

  • Koc = (Kd * 100)/ % Organic carbon

Koc is also frequently estimated based on octanol-water partition coefficient Kow and water solubility.

Soil Adsorption Coefficient (Kd/Koc) and Chemical Risk Assessment

Kd or Koc measures the mobility of a substance in soil. A very high value means it is strongly adsorbed onto soil and organic matter and does not move throughout the soil. A very low value means it is highly mobile in soil. Koc is a very important input parameter for estimating environmental distribution and environmental exposure level of a chemical substance. Read: how to use EUSES to estimated predicted environmental concentrations.

For pesticides, higher Koc or Kd is better because such pesticides are less likely to leach or occur as surface runoff to contaminate ground water. However, if Koc of a substance is very high (for example, log Koc >4.5), we should then be concerned with the potential adverse effects of the substance on terrestrial organisms such as earthworms. Additional terrestrial toxicology tests can be conducted to confirm the toxicity of a substance to soil organisms.

Testing Guidelines

OECD 106 vs OECD 121

The experimental method in the Test Guideline OECD 121 uses HPLC for the estimation of the adsorption coefficient Koc in soil and in sewage sludge.The estimates are of higher reliability than those from QSAR calculations. However, as an estimation method it cannot fully replace batch equilibrium experiments used in the OECD Test Guideline 106.

In China, only OECD 106 is accepted for China REACH registration. OECD 121 is accepted only when OECD 106 is not applicable or possible (i.e, low water solubility).

References

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You have learned the definition of Soil adsorption coefficient (Kd, or Koc), testing guidelines, and how it is relevant to environmental risk assessment. You have also learned the difference between OECD 106 and OECD 121.

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