Environ. Sci. Technol. 2000, 34, 4255
Comment on “Integrated Chemical-Biological Treatment of Benzo[a]pyrene” SIR: In a recent paper, Zeng et al. (1) report on integrated chemical-biological treatment of benzo[a]pyrene. The authors claim that “this paper provides mechanistic details on the degradation of benzo[a]pyrene subject to ozonation, identifies intermediates and byproducts formed during ozonation ...”. The authors relied on GC/MS analyses of treated samples to identify and quantify intermediates and byproducts from “interpretations of ion fragmentation information” after ozonation of the compound. Among the compounds claimed to be identified and derived from the ozonation of benzo[a]pyrene in the aqueous phase are three compounds with a PAH structure, and the rest are predominantly aliphatic hydrocarbons and aromatic carboxylic esters including many phthalates that are common plasticizers. Many of the identifications reported and the pathways deduced by the authors are doubtful; therefore, the paper requires comment. In Table 2, the authors list various intermediates and products from the ozonation of benzo[a]pyrene together with retention times and some (EI) mass spectral data (m/z values). Among the 61 compounds, the authors believe to have identified 22 compounds as “1,2-benzenedicarboxylic acid, diisooctyl” (compounds 36-57) and a further compound as bis(2-ethylhexyl) phthalate (compound 35). Phthalates are important plasticizers for poly(vinyl chloride) (PVC), and dioctyl, diisooctyl, and bis(ethylhexyl) phthalate are among the most common ones used. Phthalates are ubiquitously found almost everywhere in the environment. They are often present in solvents and chemicals as contaminants and are contained and leached from plastic materials such as tubing, containers, sealings, etc. Unfortunately, the authors did not report results of control experiments (blanks) and whether these phthalates were present prior to ozonation or not. In fact, the authors report the presence of phthalates prior to ozonation of another PAH compound (pyrene) in a parallel study (2). Several of the identifications reported by the authors are incorrect or doubtful. Whereas some of the m/z values in Table 2 might be considered as characteristic of the compound listed, others simply are not; e.g., the authors list ions of higher mass than the molecular ion of the compound (compounds 6, 11, 29, 31, and 32) or ions that simply cannot be formed from the compound itself via rational fragmentation pathways (compounds 22 and 33). Even the ion m/z 293 listed for 20 of the “1,2-benzenedicarboxylic acid, diisooctyl” (see Table 2) is not a fragment ion of this compound according to the National Institute of Standards and Technology (NIST) mass spectral library, although it is an important fragment ion of other phthalates, also plasticizers. For 17 and 19, the authors list the same chemical name, and for 22 the authors suggest three chemical names and/or structures (see Table 2 and Figures 4 and 5). Furthermore, the authors list several structures in Figure 4 that do not correspond to the chemical names given (see compounds 9, 11, 16, 17, and 58); therefore, * Corresponding author telephone: ++41 1 783 6286; fax: ++41 1 783 6439; e-mail: hans-rudolf.buser@ faw.admin.ch.
10.1021/es000104+ CCC: $19.00 Published on Web 09/06/2000
2000 American Chemical Society
it is not clear whether the reader should rely on the structures or the names given. The authors also used non-IUPAC names such as nondecane, henicosane, hextenes (compounds 1214, 18, 60, and 61; Table 2). The authors claim that tridecane (compound 34) eluted later than pentadecane (compound 31) on their nonpolar GC column, which is contrary to ordinary elution orders. In pathway VI (Figure 6), the formation of hexadecane from the decarboxylation of hexadecanoic acid is postulated. Furthermore, it is not clear how the authors were able to distinguish and identify the various branched alkanes/alkenes for which there are many isomers with often similar mass spectra. According to ref 2 all “identifications” were simply matches with entries of a mass spectral library, and no reference compounds were analyzed. In the Materials and Methods section, the authors claim to have identified three key intermediates via “comparison of parent compound structure and interpretation of mass spectra of the intermediates from ion fragmentation information” (7-propanal-8-methylpyrene, 7-ethyl-8-ethanalpyrene, and 4-methyl-5-hydroxylchrysene). However, in Table 2 they actually list only one of the three compounds as found (7-ethyl-8-ethanalpyrene), but later they do not mention it anymore in the degradation scheme in Figure 4. It is unfortunate that the authors did not consider the formation of (di)carboxylic acids, potential ozonation products of PAHs (3). Otherwise they would have analyzed their samples after suitable derivatization when using GC/MS. The authors also postulate pathways for the formation of n-alkanes (e.g., C21) via phthalic anhydride. In a parallel paper (2), they had postulated the formation of even higher molecular weight n-alkanes (C21-C26) from the ozonation of the lower molecular weight pyrene. The apparent formation of saturated alkanes from an unsaturated PAH under strongly oxidative conditions (O3) is not easy to rationalize. In a comment to the other paper, we pointed out evidence why these alkanes should be considered as contaminants rather than ozonation products (4). In summary, the mechanistic degradation pathways for benzo[a]pyrene presented by the authors should not be followed without question because many of the identifications reported are questionable and other potential ozonation products were not considered.
Literature Cited (1) Zeng, Y.; Hong, P. K. A.; Wavrek, D. A. Environ. Sci. Technol. 2000, 34, 854-862. (2) Zeng, Y.; Hong, P. K. A.; Wavrek, D. A. Water Res. 2000, 34, 1157-1172. (3) Bailey, P. S. Ozonation in Organic Chemistry; Academic Press: New York, 1982. (4) Poiger, T.; Buser, H. R.; Mu ¨ ller, M. D. Water. Res. Manuscript submitted for publication.
Hans-Rudolf Buser,* Markus D. Mu1 ller
Thomas
Poiger,
and
Swiss Federal Research Station (FAW) CH-8820 Wa¨denswil, Switzerland ES000104+
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