CLOSURE Closure to Discussion of: Model for predicting PCE desorption from contaminated soils, N. Biswas et al., 64, 170 (1992); Discussion by J.-F. Kuo, 65, 85 (1993). Nihar Biswas, Richard G. Zytner, Jatinder K. Bewtra The authors wish to thank Kuo for his comments and the following discussion addresses them point by point. Kuo is correct on his thoughts concerning C e in equation 2. We implied that C e is the equilibrium concentration after de- sorption. A less confusing approach would have been the use of C d , the equilibrium concentration after desorption. Defining C d as the equilibrium concentration should also clarify Kuo's second point concerning desorption equilibrium. For each soil tested, desorption equilibrium times were evaluated to determine the maximum desorption of PCE from soil. Pre- liminary studies showed that they were 110, 88,48, and 24 hours for organic top soil, peat moss, sandy loam, and GAC, respec- tively (Zytner et al, 1989). Determination of the equilibrium times showed a pattern similar to that reported by Pavlostathis and Mathavan (1992). They also concur that desorption equi- librium conditions must be accounted for when considering field applications, but usually the necessary data is not available. The desorption values reported were mass of PCE desorbed (released) from the soil. This is important information when considering remediation options, for example, soil washing. Equation 2 was presented as the isotherm that expressed the desorption of PCE from the four soils tested in the laboratory. The remaining parts of the paper show how the desorption data can be obtained without running desorption experiments. For equation 19, using equations 2 and 18c, and with X des = V d C d , equation 19 can be written X des /M = K fd C d Vm or VdC d l M = Kf d C d l/m . This is possible when mass of chemical desorbed is reported on a per mass basis. Figures 2 and 3 showed the validity of using the Freundlich isotherm equation to express desorption. For Equation D l by Kuo, it appears that 1 /n {d is assumed to be 1. This is not always the case as shown in Table 3 for the observed 1 /n fd values. However, by using equation 21a, the de- sorption values can be determined without making this as- sumption. K fd is defined as the equilibrium constant indicative of desorptive capacity (mg/kg)(mg/L) 1/nfd . The authors agree that the problem has been simplified by assuming no hysteresis, but when evaluating models for field application it is important that they be simple because data may not always be available. In such cases it may be necessary to assume no hysteresis and approximate the amount of chemical to be desorbed and remediate accordingly. Figures 4 through 7 were developed for Case II, based on the Freundlich adsorption coefficients for organic top soil and for the conditions given in the example. Equation D4 does not seem to use the adsorption coefficients. It should be noted that when the Freundlich adsorption coefficients change, that is, A^and 1 / rif, the nomographs will also change. The original paper clearly states that ". . . plots similar to those shown in Figures 4 to 7 can be prepared for situations described in case II." In other words, a family of curves is to be prepared for each scenario. Kuo came to the same conclusion. The authors are grateful for the contribution made by Kuo in his discussion, which definitely has strengthened this paper. References Pavlostathis, S. G., and Mathavan, G. N. (1992) Desorption Kinetics of Selected Volatile Organic Compounds from Field Contaminated Soils. Environ. Sci. Technol, 26, 3, 532. Zytner, R. G., et al. (1989) Adsorption and Desorption of Perchloro- ethylene in Soils, Peat Moss and Granular Activated Carbon. Can. J. Civ. Eng., 16, 6, 798. Water Environment Research, Volume 65, Number 1 88