ON THE RELATIONSHIP BETWEEN SORPTION AND DEGRADATION OF MOLECULES IN SOILS, AND THE WALKER MOISTURE EXPONENT

Authors: Wauchope, Robert - Don

Submitted to: American Chemical Society National Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 12/31/2003
Publication Date: 3/20/2004
Citation: Wauchope, R.D. 2004. On the relationship between sorption and degradation of molecules in soils, and the walker moisture exponent. American Chemical Society National Meeting, Anaheim, CA. Picogram 66, Abs. #14 (2003), AGRO Division, American Chemical Society, Washington, DC.

Interpretive Summary:

Technical Abstract: On the relationship between sorption and degradation of molecules in soils, and the Walker moisture exponent. R. Don Wauchope, Research Chemist USDA-Agricultural Research Service 2316 Rainwater Road, POB 946 Tifton, GA 31794 Molecules which are strongly sorbed to the solid phase of geosolids are typically persistent in the environment, and it is usually assumed that degradation processes take place exclusively in the aqueous phase of soil. A simple relationship between soil sorption equilibrium constant Kd (ml.g-1) and the degradation rate for chemicals in soil may be developed (Paris, et al., 1978; Guo, et al., 2000): where CT is the total chemical concentration per mass moist soil at time t (d), is the initial concentration (at t = 0), kw (d-1) is the degradation rate constant for nonabsorbed molecules, and ' (g.cm-3) and ' (cm3.cm-3) are soil bulk density and volumetric soil moisture fraction. This equation can be transformed into a half-life form: where H (d) is the half-life of the total residue. This equation indicates that a plot of half-lives measured in soil at various moisture contents will give a straight line when plotted vs. 1/', with an intercept equal to ln2/kw and a slope equal to the intercept times 'Kd. We will explore the application of this equation to laboratory data.