Department of Agronomy, Kansas State University, Manhattan, KS, 66506, 913-532-7215
Disagreement often exists between degradation rate constants obtained from batch degradation experiments and leaching experiments. Although degradation can be quantified successfully in a batch-type experiment, no satisfactory method exists for this purpose in a leaching experiment, apart from curve-fitting. During leaching, the simultaneous occurrence of sorption, dispersion and convection confounds the duration for which true degradation takes place within the soil. A correct description of the true degradation opportunity time is needed for computing the degradation rate constant in this case. We have used statistical moments to show that when degradation follows first-order kinetics, the time of degradation is an adjusted convection time (ACT). It is defined as the harmonic mean of m1/R and the convection time (L/v), where m1 is the first moment, R is the retardation factor, L is the length of soil column, and v is the pore water velocity. The adjustment in the convection time scale is a direct result of both dispersion and degradation occurring simultaneously in soil. We show that these two processes tend to slightly reduce the true convection time for the leaching event. Therefore, the ACT is smaller than L/v when both dispersion and degradation occur. We have tested our result by generating artificial breakthrough curves with known first-order degradation rate constants and then successfully estimating the rate constant from the breakthrough curve by using our approach. We have outlined a detailed procedure to estimate the degradation rate constant from the information generated in a typical leaching experiment.
degradation, breakthrough curve, moment, travel time, convection time
This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.