# Solid-Liquid Partitioning

Pesticides will partition into particulate and dissolved forms. The fraction of pesticide in each phase is a function of the pesticide’s partition coefficient and the water body’s suspended solid concentration:

      $$F\_ d=\frac{1}{1+K\_d\*conc\_{sed}}$$                                                                     8:4.1.1

      $$F\_p=\frac{K\_d*conc\_{sed}}{1+K\_d*conc\_{sed}}=1-F\_d$$                                                   8:4.1.2

where $$F\_d$$ is the fraction of total pesticide in the dissolved phase, $$F\_p$$ is the fraction of total pesticide in the particulate phase, $$K\_d$$ is the pesticide partition coefficient (m$$^3$$/g), and $$conc\_{sed}$$ is the concentration of suspended solids in the water (g/m$$^3$$).

            The pesticide partition coefficient can be estimated from the octanol-water partition coefficient (Chapra, 1997):

       $$K\_d=3.085\*10^{-8}\*K\_{ow}$$                                                    8:4.1.3

where $$K\_d$$ is the pesticide partition coefficient (m$$^3$$/g) and $$K\_{ow}$$ is the pesticide’s octanol-water partition coefficient (mg $$m^{-3}*{octanol}$$(mg $$m^{-3}*{water})^{-1}$$). Values for the octanol-water partition coefficient have been published for many chemicals. If a published value cannot be found, it can be estimated from solubility (Chapra, 1997):

       $$log(K\_{ow})=5.00-0.670\*log(pst'\_{sol})$$                           8:4.1.4

where $$pst'\_{sol}$$ is the pesticide solubility ($$\mu$$moles/L). The solubility in these units is calculated:

        $$pst'*{sol}=\frac{pst*{sol}}{MW}\*10^3$$                                                           8:4.1.5

where $$pst'*{sol}$$ is the pesticide solubility ($$\mu$$moles/L), $$pst*{sol}$$ is the pesticide solubility (mg/L) and $$MW$$ is the molecular weight (g/mole).


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