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Comments pared to that of a pure g ~ e t h i t e . ~Furthermore, ,~ the presence of calcium (TOTCa = 0.01 M) may result in competition reactions for Zn adsorption especially at higher pH6 though the extent of this competitive effect is less than observed with Cd and Ni by Barrow et aL6 Finally, from the experimental setup reported by Barrow et a1.F we assume that carbon dioxide had not been In the article “On the Nature of the Energetic Surface excluded. This may induce further surface heterogeneity. Heterogeneity in Ion Adsorption at a WaterlOxide InterCarbon dioxide may also affect aqueous solution speciation face: Theoretical Studies of Some Special Features of Ion of Zn, which is not addressed in the paper by Rudzinski Adsorption at Low Ion Concentrations”, Rudzinski and and co-workers.’ Fortunately, our own calculations have co-workers’ put forward that the knowledge acquired in shown that there is practically no effect of carbonate and adsorption of gases on heterogeneous surfaces has not hydroxide on Zn speciation (in equilibrium with atmobeen appropriately realized by most people interested in spheric carbon dioxide)under the respective experimental adsorption of inorganics at waterloxide interfaces. While conditions. this may be true, we believe that the examplesRudzinski We state that, in this case, there might be more and co-workers’ have chosen to illustrate their point of phenomenological reasons for the observed surface hetview and to criticize discrete site models by Morel et al.2-4 erogeneity in Zn adsorption besides the heterogeneity of do not seem to be the best ones. a pure goethite ample.^ The heterogeneity of a pure We would like to make the followingremarks especially on the comments on studies by Morel and co-~orkers.~-* goethite could be successfullypredicted assuming discrete sitesa7In any case, the comparison between isotherms 1. In Figures 5 and 6, Rudzinski and co-workers’ refer calculated with the discrete site model4 for a pure solid to data by Benjamin and L e ~ k i e ,that ~ , have been in part sorbent and isotherms measured with an a priori hetobtained for high sorbatdsorbent ratios, where sorption kinetics may have limited the achievement of equilibri~m.~ erogeneous sorbent is problematic. Furthermore, the constellation of parts A and B of Figure Therefore, these data were not used in the development 8 may induce misunderstandingsbecause neither abscissa ofthe data base for amorphous hydrous ferric oxide (HFOIa4 nor ordinates are of the same scale. As a consequence, the parameters of the data base cannot describe these data. If kinetics is important for those 4. There is some evidence that sorption of metals on data, the Freundlich-like behavior at higher sorbate/ hydrous ferric oxide can be modeled satisfactorily using sorbent ratios might be due to the fact that equilibrium only one site and (surface) precipitation? especially in is not reached. Hence isotherms at constant pH conthe case where sorption data have been obtained with structed with these data may only be apparently Freundlonger equilibration times. While there may be some lich-like. exceptions to this statement, we would like to give the 2. In Figure 8A, Rudzinski and co-workers’ reproduce following recent examples: (i) Charlet and Manceau6 constant pH isotherms as given by Dzombak and Morel.4 reported a single site model for Cr(II1) sorption on HFO; Rudzinski and co-workers’ argue that these isotherms do (ii) Tiffreau et aL4modeled experimental data on Hg(I1) not exhibit Freundlich-like behavior over a sufficientrange sorption on HFO by Avotinslousing only one site together of free metal concentration. Here, we would like to add with surface precipitation; (iii) Charlet and Manceau” that by using surface precipitation reactions the range of and Spadini12reported surface spectroscopicresults that Freundlich-like behavior can be extended at least for the suggested that Cd adsorption on HFO predominantly higher pH valuesa4 takes place on one site, which is in agreement with the model for the respective potentiometric titrations.12 For 3. In Figure BB, Rudzinski and co-workers’ compare a well-defined and pure goethite, the behavior of several data from Barrow et ale6to isotherms at constant pH sorbates has been investigated by the group of Professor calculated by Dzombak and Morel4 with their two-site Sjoberg. Their results of potentiometric titrations of the surface complexation model (Figure 8A as discussed above). In that case, Rudzinski and co-workers’ do not pure goethite could be convincingly modeled using only point out on the one hand that the goethite used for the one site with about the same site density (see e.g. adsorption experiments (from which the data in Figure Gunneriusson’sthesis13 1. 8B1or Figure 46were obtained) was a goethite containing We note that the above cited observations and simulasilica and on the other hand that the experiments were tions could be explained by the compensation of heterocarried out in the presence of calcium.6 The presence of geneity effects described in the previous article by silica changes the surface chemical properties of goethite and thus creates additional surface heterogeneity com(7) Hiemstra, T.; de Wit, J. C. M.; van Riemsdijk, W. H. J . Colloid Comment on the Article “On the Nature of the Energetic Surface Heterogeneity in Ion Adsorption at a Water/Oxide Interface: Theoretical Studies of Some Special Features of Ion Adsorption at Low Ion Concentrations”
(1)Rudzinski, W.; Charmas, R.; Partyka, S.;Bottero, J.Y. Langmuir 1993. . 2641. ,9 ~, (2) Farley, K.J.;Dzombak, D. A.; Morel, F. M. M. J. Colloid Interface Sci. 1985,106,226. (3) Dzombak, D. A.; Morel, F. M. M. J. Colloid Interface Sci. 1986, 112. 588. (kYDzombak,D. A.; Morel, F. M. M. Surface Complexation Modeling, Hydrous Ferric Oxide; John Wiley & Sons, Inc.: New York, 1990. ( 5 ) Benjamin, M. M.; Leckie, J. 0.J . Colloid Interface Sci. 1981,79, 209. (6) Barrow, N. J.; Gerth, J.; Briimmer, G. W. J . Soil Sci. 1989,40, 437. ~~
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Interface Sci. 1989,133,105. (8) Charlet, L.; Manceau, A. A. J . Colloid Interface Sci. 1992,148, 443. (9) Tiffreau, Ch.; Lutzenkirchen, J.; Behra, Ph. Proceedings of the International Symposium on “Transport and Reactive Processes in Aquifers”, ETH Zurich, Switzerland, April 11-15, 1994. (10)Avotins, P. Ph.D. thesis, Stanford University, 1975. (11)Charlet, L.; Manceau, A. Proceedings of the workshop “Chemodynamics of Groundwaters: From Molecular to Regional Scale”,Mont Sainte Odile, France, October 5-8, 1993, org. Ph. Behra et al. (12) Spadini, L. Ph.D. thesis, University of Bern, Switzerland, 1993. (13) Gunneriusson, L. Ph.D. thesis, University of Umea, Sweden, 1993.
0743-746319412410-3916$04.50/00 1994 American Chemical Society
Comments
Rudzinski and co-w~rkers.'~ In any case, it is apparent that, apart from the actual surface heterogeneity, we may consider several phenomena that can account for slopes that are not equal to 1 in isotherms at constant pH (especially at higher free metal concentrations). For example, polynuclear surface complexes for which there is more and more evidence13J6 may lead to nonlinear effects. We would like to point out that the acquisition of experimental data should be critically reviewed, before theoretical models on adsorption are assessed or one model is consideredto be better than another one. In this context, it must be realized that many details on experimental procedures are often not available to modelers, because the description of the experimental setup is not always sufficiently detailed. Dzombak and Morel4clearly stress (14)Rudzinski, W.; Charmas,R.; Partyka, S.; Bottero, J.Y.Langmuir 1992, 8,1154. (15)Chisholm-Brause, C. J.; O'Day, P. A.; Brown, G. E., Jr.; Parks, G. A. Nature 1990,348, 528.
Langmuir, Vol. 10, No. 10, 1994 3917
this point with respect to possible silica contaminations from silica-containing reaction vessels and in their comments on carbonate contamination, which they finish with the somehow discouraging sentence: W e have chosen to take the authors' word for it if they claimed to have excluded COZ." In our opinion, this clearly shows the problems related to working on other people data. Acknowledgment. J. L. is supported by CEC Grant ref. 910959 STEP. The support by the Korber-Stiftung is acknowledged. Johannes Littzenkirchen and Philippe Behra. Institut de Micanique des Fluides de I'Universiti Louis Pasteur URA CNRS 854 2, rue Boussingault 67000 Strasbourg, France Received February 3, 1994 In Final Form: June 3, 1994
