Environ. Sci. Technol. 1996, 30, 360
1995, Volume 29, Pages 2333-2338 Stefan Trapp* and Michael Matthies: Generic OneCompartment Model for Uptake of Organic Chemicals by Foliar Vegetation There is a problem with eq 1 in this paper. The partitioning of lipophilic chemicals between plant tissue and water was calculated with a regression to the octanolwater partition coefficient KOW:
KPW ) (W + lKOWb)ρplant/ρwater
log KSW ) b log KOW + c
(2)
KSW ) aKOWb
(3)
transformed to
Equation 1 is wrong both theoretically and dimensionally because the coefficient a is missing. The correct equation is
(4)
The coefficient a has two functions: It corrects the units, here of the KOW (mL of water/mL of octanol) to (g of water/g of octanol) (neglecting the effect of the exponent b on the dimension). Second, a considersstogether with the exponent bsthe different sorption capacities of plant lipids compared to octanol. Thus, the dimension of a is
a ) [ρwater/ρoctanol] [g of substance/g of octanol:g of substance/g of lipid] (5) The first term is the density correction (the density of octanol
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a ) ρwater/ρoctanol × 1
(6)
Another correct form of the equation is
(1)
where W is the water content [mass water/mass plant]; l is the lipid content [mass lipid/mass plant]; Fplant and Fwater are the densities of plant tissue and water [mass per volume]. The unit of the KPW is volume of water/volume of plant. This equation has the form of a ‘solvent regression equation’. In the Handbook of Chemical Property Estimation Methods, 37 solvent regression methods are given (1). From theoretical considerations (2), it follows that the form (Collander equation) of the solvent regression is
KPW ) (W + laKOWb)ρplant/ρwater
is 0.822 g/mL). The second term is the ‘solvent regression’, and values have to be determined experimentally. Briggs et al. (3, 4) determined RCF and SCF and fitted la. Factor a was not determined separately, and indeed no values are known at the present. This is why we set the coefficient to 1. By this, we had overlooked its function as a density correction; it would have been better as
KPW ) W + laKOWb
(7)
where W is the water content (volume of water/volume of plant) and l is the lipid content (volume of lipid/volume of plant). Usually, water and lipid contents are measured on a mass per mass basis and may then be transfered to volume per volume by multiplying with density. Densities of plant tissue and lipids need to be known. In eq 7, factor a is not needed for the correction of the KOW dimension. Now, a has the dimension (kg of substance/m3 of octanol:kg of substance/m3 of lipid). We hope that we shed some light on this tricky question. The problem is very similar to the sorption on soil matrix that is described by the KOC and the organic carbon content (in the unit mass per mass). We encourage our readers to carry out experiments to determine the form of the solvent regression between octanol and plant lipids too. The error in eq 1 does not lead to subsequent errors. The other equations and the conclusions remain as they were. Many thanks to Tjalling Jager (RIVM, NL) for the advice on this problem.
Literature Cited (1) Lyman, W. J. In Handbook of Chemical Property Estimation Methods; Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., Eds.; American Chemical Society: Washington, DC 1990. (2) Briggs, G. G. J. Agric. Food Chem. 1981, 29, 1050-1059. (3) Briggs, G.; Bromilow, R.; Evans, A. Pestic. Sci. 1982, 13, 495. (4) Briggs, G.; Bromilow, R.; Evans, A.; Williams, M. Pestic. Sci. 1983, 14, 492.
ES951008P
0013-936X/96/0930-0360$12.00/0
1995 American Chemical Society
