Document XO2yenM1617DjG92GjBZoz9ox

UNIVERSITY OF CALIFORNIA, RIVERSIDE SANTA BARBARA * SANTA CRUZ COLLEGE OF NATURAL AND AGRICULTURAL SCIENCES CITRUS RESEARCH CENTER AND AGRICULTURAL EXPERIMENT STATION DEPARTMENT OF SOIL AND ENVIRONMENTAL SCIENCES RIVERSIDE. CALIFORNIA 92521-0424 September 25, 1984 Mr. Raymond A. Freeman Mr. Jerry M. Schroy MONSANTO COMPANY * 800 N. Lindbergh Boulevard St. Louis, Missouri 63166 Dear Sirs: When Dr. Farmer returned from his meeting with you last week, he gave me a copy of several draft publications which you have written concerning the mathematical simulation of dioxin volatilization. I must confess that all of us (Spencer, Farmer and myself) were asto nished to find that a compound with the physical-chemical charac teristics of dioxin would volatilize to the extent predicted in your model. We therefore undertook to check the model assumptions, equations, and also reran isothermal simulations on our screening models. I am afraid that I at least have become convinced that you have made a huge^error in the formulation of your equations, which has the effect of allowing the chemical to diffuse in the vapor phase without a reduction in mobility due to adsorption. By sorting through your publications, I believe I have found the place in the derivation where this error occurred. My line of reasoning is as follows: 10In the paper "Environmental mobility of dioxin" the Theory section begins on page with a continuity equation for the gaseous con centration which is equivalent to the following: (1 ) 2 1which is identical to your equation (1) where the expression for R in equation ( ) is plugged into equation ( ) and Ri is deleted as it was in your paper. (continued) . Raymor..: Freeman . Jerry M, Schroy je 2 September 25, lai~ /' J.f f/ J 1984 a quantity in brackets represents the total concentration of dioxin in ss per volume of soil. The equation states that the time rate of ange of this material is entirely due to the gradient of the vapor jx which is essentially correct. There is a small e r r o r , t h i s point the formulation of the paper in that h is set equal to/x/e and t is signed.a value of 2. If t is considered to be a tortuosity it should 11duce the available vapor space compared to air. In fact, we use the ington'-Qui rk model for the combined terms eh Dab = D soil ,10/3 ab (2 ) ?- ere a is air content. This has the effect of reducing the gaseous ffusion coefficients significantly compared to its value in free air, rticularly when the soil is moist. Your model, if formulated with the 7rameter values that are indicated in the back of the paper, would tually increase this over its value in free space which is not ---J j . rrect. T Ir * - wever, the principal error in the model formulation occurs when you G gin with a rate limited expression for the concentration of chemical i the adsorbed phase A It Cd ` " <Ca - (3 ) 1iere my parameters a and K have replaced many of the symbols in your iper. Although you do not state it explicitly in your other paper, in paper "Modeling the tranport of 2,3,7,8-TCDO.. you state at the }ttom of page 5 that the vapor phase transport is so rapid that it ways equilibrates with the adsorbed phase from which you set equation 3) equal to 0. However, this is equivalent to the assumption that Ca = K'Cd nd hence, that at3Cd _ i1r~33Cta (4) ius, equation (1) should be modified by plugging in equation (5) and ot by dropping the term involving C(j. Thus, instead of the equation at ne bottom of page 5 of the second paper, the equation should read _ hD^ 32Ca _ n 32Ca 3t ' U + l/K* ) I z 2 = E 022 (6 ) ed n the rbon ch is is aper. y for a a we pi- , has e ides d and nd rom ic. 1p2l.i- 'oui d ons 24 il.66% fi rst 2\! cm 1 phys- 0-9 (continued) Mr. Raymond A. Freeman Mr.Jerry Schroy Page 4 September 25, 1984 I don't- know the status of this paper or of your work published in other areas. I do believe, however, that the information available for dioxin is so scarce that papers such as you have presented would have a signi ficant influence on the thinking of scientists and non-scientists alike. I will be happy to talk with you in further detail about this if you are either uncertain about my reasoning or in disagreement with it. I would hope that we could resolve this before any mistaken information reaches the open literature. Sincerely, William A. Jury Professor of Soil Physics WAJ:dem cc: W. F. Spencer W. J. Farmer