Environ. Sci. Technol. 1995, 29, 285
SIR: We appreciate the comments made by Phillips and Jafvert and would like to address some of the points raised in their discussion. Phillips and Jafvert suggest that a diffusion limiting sphere-equivalent diameter (4of 3 x cm is too small and that 1 pm is a more reasonable estimate. Recently, Pignatello ( I ) concluded that the diffusive length scale for atrazine and metalochlor in a Merrimac fine sandy loam is less than 1pm since diffusionfrom a clay ( 52pm) fraction was not sigmficantlydifferentthan from other size fractions. Pignatello further suggested that a coating of organic microparticles over all particle size fractionsis the diffusive medium, similar to the concept proposed in our paper. Very small particles are reported in synthetic polymer literature, where phase domains in polymer blends may be as small as 2 x lo-' cm (2). Macroreticular adsorbents consist of beads composed of polymeric microspheres as small as 3 x lo-' cm (3). Clearly, there is precedence for submicron dimensions in polymers, both natural and synthetic. As Phillips and Jafvert point out, independent measurement of humic polymer dimensions (e.g., microscopy or surface area measurements) would be useful to verify these model-predicted lengths. Phillips and Jafvert also suggest that a Permachor value of 30 callmL (which would result in a 1pm diffusive path length) would be an improvement over our estimate of 50 callmL. A value of 30 cal/mL is the Permachor value of polycarbonate, a polymer consisting of both polar and nonpolar groups but with limited water sorption capacity [0.4%water at 100%RH at 20 "C (411. Chiou (5) reports soil humic acid has a high sorption capacity for water of 245 g/kg, indicating the humic polymer is considerably more polar than polycarbonate. Highly hydrogen-bonded synthetic polymers have Permachor values in the 60-70 call mL range when saturated with water (6). Our estimate of 50 cal/mL is intermediate between these extremes and seems to us to be a reasonable estimate pending further investigation. The model in our paper ( 7)assumesthat Fickian diffusion with constant diffusion coefficients in spherical geometry occurs in both the swollen and condensed humic polymer
0013-936x195/0929-0285$09.00/0
0 1994 American Chemical Society
phases. In the study of the desorption of vinyl chloride monomer (VCM) from glassy WC submicron spheres, Berens (8) found only Fickian diffusion if the desorption was to an infinite sink, which is quite analogous to our experimental conditions. Any non-Fickian contributions to the desorption process are due to a deswelling or relaxation process of the polymer, which is comparable to the rate of permeant diffusion (9). In the case of environmental humic polymer, the major swelling agent of humic polymer is water, which remains at constant saturation. Permeant levels generallymust be apercent or more in the polymer to produce significant swelling or plasticization. PCB concentrations in the resistant phase were an order of magnitude lower than this value in our sediment. We agree that temperature changes could result in phase changes in humic polymer due to traversing an effective glass transition temperature. Glass transition temperatures for water-saturated lignins (72- 100 "C) are reported in the literature (10). Similar work is recommended for humic polymers in sediments.
Literature Cited (1)Pignatello, J. J.; Ferrandino, F. I.; Huang, L. Q.Environ. Sci. Technol. 1993,27,1563-1571. (2) Wilkes, G. Chemtech. 1983,13, 174-179. (3)Gustafson,R. L.; Albright, R. L.; Heisler, J.; Lirio, J. A.; Reid, 0. T.Ind. Eng. Chem. Prod. Res. Dev. 1968,7, 107-115. (4)Narkis, M.; Sibony,S.;Niclolais,L.; Apicella,A.; Bell, J. P. Polym. Commun. 1985,26,339-344. (5)Chiou, C. T.;Kile, D. E.; Malcolm, R. L. Environ. Sci. Technol. 1988,22,298-303. (6)Salame, M. Polym. Eng. Sci. 1986,26,1543-1546. (7)Carroll, K. M.;Harkness, M. R.; Bracco, A. A.; Balcarcel, R. R. Environ. Sci. Technol. 1994,28, 253-258. (8) Berens, A. R. Polymer 1977,18, 697-704. (9)Frisch. H.L. I. Polym. Eng. Sci. 1980,20, 2-13. (10)Salmen, L.I. Muter. Sci. 1984,19, 3090-96.
Kenneth M. Carroll* and Mark R. Harkness General Electric Corporate Research and Development Schenectady, New York 12301 ES9409570
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