Anal. Chem. 1900, 6 0 , 1477
1477
Response to Comments Sir: Tessier and Campbell have criticized our recently published research-”Accuracy of Selective Extraction Procedures for Metal Speciation in Model Aquatic Sediments“ (I). Specifically, they objected to the validity of the experimental approach, to our choice of model sediment compositions, and to the adequacy of some of our experimental procedures. Our replies to these specific criticisms are discussed below. First, however, a general comment is necessary concerning the heart of this controversy. General Comments. Tessier and Campbell’s response contains one principal message-that selective extraction procedures can provide “useful” elemental distribution data. They contend that the evidence presented by us and other authors for significant redistribution is weak because of inappropriate experimental design. Furthermore, they imply that careful selection of experimental procedures (e.g. stable sample storage conditions and adequate capacities of extraction reagents) leads to useful, albeit operationally defined, elemental distribution data. “Operationally defined” means that experimental distributions are affected by extraction reagent type and concentration, sequence of application, extraction duration, and solid/solution ratio. Our position is that “operationally defined” methods may offer empirical guidance but offer no fundamental understanding of technique or substrate chemistry. Without some kind of experimental control, e.g. model sediments, “usefulness” takes on a nebulous quality. Of what geochemical use is a method that segregates trace elements into fractions whose relationship to actual substrates is poorly known. In particular, in the selective extraction literature the problem of redistribution is sometimes acknowledged but generally is ignored during interpretation of results. We disagree with Tessier and Campbell’s claim that the evidence for redistribution is ambiguous. Several authors (2-5), using a variety of experimental approaches, warned about this phenomenon. In contrast, we know of no evidence showing that redistribution does not occur. We believe it is unwise to assume the problem is insignificant. Tessier and Campbell disagree with our statement that poor extraction performance on model sediments probably implies even poorer performance on real sediments. They interpreted this as a conclusion derived from our results. Actually, this was intended as a statement in support of the need for testing model sediments in general. We concede that “poorer” does overstate the case based on the evidence we present. If, however, we assume a model is developed that meets the objections of Tessier and Campbell-representative and not “biased toward” redistribution-poor extraction performance on this certainly would imply questionable (if not poorer) performance on real sediments. Specific Criticisms. With respect to the specific criticism concerning model sediment composition, we acknowledge that including equal amounts of reduced and oxidized iron species in the model sediments was not representative of most properly collected materials. This, however, does not necessarily preclude testing such models. We carried out our extractions in air, anticipating that iron sulfides may oxidize during the procedure. Our intent was to demonstrate how a labile sulfide substrate might affect results if the possibility of oxidation were ignored. In essence, our work on models was similar to the study on real sediments which Tessier and co-workers (6) published after our paper had been submitted. Both studies agree that labile sulfides are not extracted se-
0003-2700/88/0360-1477$01.50/0
lectively by this extraction scheme. Tessier and Campbell also claim that our experimental procedure-doping one trace element per major substratebiases results toward redistribution. This contrasts with “natural sediments where the metal would have already been distributed among the various sediment phases and where the driving force for redistribution during extraction would correspondingly be much lower.”. They recommend an alternate procedure in which all phases are separately “equilibrated” with each metal. This procedure may be advantageous, and by all means should be investigated. However, there is a distinct disadvantage. Since a given element is initially distributed among all phases, any changes that occur in the distribution may be difficult to rationalize-the elemental content of a given phase could be affected by redistribution from any earlier extracted phase and by nonselective attack of any later extracted phase. In other words, one may be able to see differences but not reasons for those differences. Our preference is to challenge extraction methods with model sediments that clearly expose artifacts so that their significance may be understood and thereby minimized. Lastly, Tessier and Campbell objected to some of our synthetic and extraction procedures. Since this was our first venture into this area, we selected phase synthesis procedures with simplicity and stability in mind. For instance, we justified in our original article the decision to use freeze drying as an experimental compromise at this stage of the game. Granted, some substrates, like iron oxides, may be altered by this process, but our interpretations did not discount this possibility. As Tessier and Campbell point out, sediment P in our original article clearly pushes the capacity of the extraction reagents for calcite and iron oxides. We acknowledged this limitation in presenting a speculative interpretation of the results for this sediment. The capacity problem is clearly not a concern for sediments NC and NA. Yet trace element distribution patterns of P b and Zn, for instance, are similar for both sediments NC and P. This indicates that perhaps reagent exhaustion is not a major factor for sediment P. Concluding Remarks. It is clear that there are a number of questions about the fundamental and practical aspects of selective extraction methods that our work has only begun to address. We hope that this discussion promotes further careful investigation. LITERATURE C I T E D (1) Kheboian, C.; Bauer, C. F. Anal. Chem. 1987, 59, 1417-1423. (2) Guy, R. D.; Chakrabarti, C. L.; McBain, D. C. Water Res. 1978. 12, 21-24. (3) Rendell, P. S.; Batley, G. E. Environ. Sci. Technoi. 1980, 1 4 , 3 14-3 18. (4) Tipping, E.; Hetherington, N. B.; Hiiton, J.: Thompson, D. W.; Bowles, E.; Hamilton-Taylor, J. Anal. Chem. 1985, 57, 1944-1946. (5) Dominik, J.; Rapin, F.; Vernet, J.-P. Proceedings of the 4th International Conference on Heavy Metals in rhe Environment, Heidelberg; CEP Consultants Ltd.: Edinburgh, U.K., 1983; p 1078. (6) Rapin, F.; Tessier, A.; Campbell, P. G. C.; Carignan, R. Environ. Sci. Technoi. 1986, 20, 836-840.
C. F. Bauer* C. Kheboian Department of Chemistry University of New Hampshire Durham, New Hampshire 03824
RECEIVED for review February 8,1988. Accepted March 22, 1988.
0 1988 American Chemical Society
