The pesticide Atrazine (C₈H₁₄C1N₅, mol wt. 216 g/ mol) degrades in soil by a first-order reaction process. Consider the situation shown in the figure below, where there is a spill of solid Atrazine rests on top of a 10-cm-thick layer of water- saturated soil at 20°C. The solid Atrazine dissolves into the water, diffuses through the water-saturated soil and then degrades in the water-saturated soil with a homogeneous first-order rate constant (k₁) of 5.0 10^-4hr^-1at 20°C. Underneath the water-saturated soil layer is an impermeable clay barrier. The maximum solubility of Atrazine in water is 30 mg/L (0.139 mmole/L) at 20°C. 

a. What is the molecular diffusion coefficient of Atrazine in water (D_AB) at 20°C, and the effective diffusion coefficient (D_Ae) in the water-saturated soil? The specific molar volume of Atrazine is 170 cm³/gmole, at its normal boiling point. The effective diffusion coefficient is estimated by D_Ae = ε² D_AB (A = Atrazine, B = water), with a void fraction (ε) of 0.6. The effective diffusion coefficient accounts for the fact that not all of the soil is liquid water. 

b. What is the concentration of Atrazine (mmole/L) in the water-saturated soil at the clay barrier (z = L)? It may be assumed that the process is at steady state with a one dimensional flux of A in the z-direction.

Transcribed Image Text:

Solid Atrazine L = 10 cm Water-saturated soil 20°C Clay barrier