Correspondence Comment on “Reactions of Polynuclear Aromatic Hydrocarbons on Soil” SIR: In a recent paper of Gray et al. (1) the interactions of some PAHs with soils and montmorillonite clay were studied. The authors found that anthracene and pyrene were polymerized on the dry mineral fraction to higher molecular weight aromatic products. Based upon their studies of abiotic PAH reactions on artificially contaminated soils they made implications about the fate of PAHs in the environment. We have an objection to the experimental procedure applied by Gray et al. and a supplement to their conclusions. The objection: We believe that leaving the solvent methylene chloride in the soil sample (200 mL/kg) may cause biased conditions in that the solvent phase greatly increases the sorbate mobility. This may significantly enhance polymerization reactions, because the proposed reaction mechanism (chain reactions initiated by PAH radical cations) needs mobile reactants. Usually, solvents used for homogeneous spiking of a sample are removed before further treatment. In the absence of an organic solvent phase, as it is the case in the environment, the soil organic matter binds hydrophobic compounds by sorption. This will reduce their mobility and strongly decrease the reaction rate compared with the conditions applied by Gray et al. It seems to us that extrapolating from activated soils (pretreatment at 140 °C for 48 h) to air-dried soils overestimates the reaction rates. Moreover, the masking effect of soil organic matter on the availability of hydrophobic compounds such as PAHs, caused by very slow intraorganic matter diffusion (2), is not taken into account. The supplement refers to the reactions of PAHs at elevated temperatures: In some recent papers (3-6), we studied the conversion of various hydrocarbons, among them PAHs, during a low-temperature thermal treatment (300 °C) on various organic and inorganic matrices. Our results give evidence that biopolymers such as humic acids or hemicellulose are able to stimulate the conversion of reactive PAHs into nonvolatile products (3). This conversion is more pronounced with some inorganic matrices such as an acidactivated clay and an oxidatively activated sediment. However, organic matter reduced the catalytic activity of dry minerals, as was also observed by Gray et al. We obtained evidence that redox processes play an important role in PAH reactions. A pretreatment of the
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minerals under reducing conditions (e.g., flow of hydrogen) strongly decreased their activity. Large differences in the reactivity of various PAHs clearly correlate with their chemical structure. PAHs with a structure count ratio (SCR) (7, 8) g 3 are reactive, whereas those having a lower SCR (e.g., polyphenyls, naphthalene, phenanthrene) are not (4). The SCR is considered to characterize the reactivity of PAHs with respect to radical reactions. Methyl substituents increase the reactivity of the corresponding aromatic backbone. These findings point to radical or radical-ion reactions between the sorbed PAHs and sorbent active centers. In this respect, we fully agree with the proposal of Gray et al. and others for the polymerization mechanism, where radical cations are postulated as intermediates. By means of HPLC analysis as well as by thermoanalytical methods, we could prove that PAHs form nonvolatile, coke-like products, which can be decomposed but not evaporated. From the viewpoint of soil remediation, such transformations of toxic products into chars can be evaluated as a desirable completion of widely used thermodesorption processes.
Literature Cited (1) Karimi-Lotfaband, S.; Pickard, M. A.; Gray, M. R. Environ. Sci. Technol. 1996, 30, 1145-1151. (2) Pignatello, J. J.; Xing, B. Environ. Sci. Technol. 1996, 30, 1-11. (3) Remmler, M.; Kopinke, F.-D. Thermochim. Acta 1995, 263, 113121. (4) Kopinke, F.-D.; Remmler, M. Thermochim. Acta 1995, 263, 123139. (5) Remmler, M.; Kopinke, F.-D.; Stottmeister, U. In Contaminated Soil ’93; van den Brink, W. J., Bosman, R., Arendt, F., Eds.; Kluwer Academic Publishers: Dordrecht, 1993; pp 1461-1462. (6) Remmler, M.; Kopinke, F.-D.; Ondruschka, B.; Juetterschenke, P.; Rippl, G. In Contaminated Soil ’95; van den Brink, W. J., Bosman, R., Arendt, F., Eds.; Kluwer Academic Publishers: Dordrecht, 1995; Vol. 2, pp 1329-1330. (7) Herndon, W. C. J. Org. Chem. 1981, 46, 2119-2125. (8) Herndon, W. C. Tetrahedron 1982, 38, 1389-1396.
Frank-Dieter Kopinke* and Katrin Mackenzie Center for Environmental Research Department of Remediation Research Permoserstrasse 15 D-04318 Leipzig, Germany ES960627A
S0013-936X(96)00627-X CCC: $14.00
1996 American Chemical Society
