Langmuir 1995,11, 377-378
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Comments Reply to Comments 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” The authors’ have undertaken an attempt to defend the two-site model launched by Dzombak and MoreP4to represent the surface heterogeneity of the actual ferric oxides. This, surely, is not an easy job, mainly because of the investigation strategy accepted by Dzombak and M0re1.~-~ The main point is their way of handling the experimental data of Benjamin and L e ~ k i ewho , ~ carried out an extensive and careful experimental study of the adsorption of the heavy ions (Cd2+,Cu2+,Zn2+,Pb2+)on an amorphous ferric oxy-hydroxide at low and higher ion concentrations (surface coverages). Namely, much of the data by Benjamin and Leckie5 has been excluded by Dzombak and MoreP4 from their analysis. Dzombak and MoreP4and Lutzenkirchen and Behra’ explain why much of the data should be excluded from their theoretical analysis. Firstly “because equilibrium had possibly not been reached. To us, there are good reasons to believe what Benjamin and Leckie5wrote about their own experiment: “Preliminary kinetic experiments indicated that adsorption of all four metals is rapid initially (1 hr), followed by a much slower second step, possibly related to solid-state difision (Fig 1). Similar results have been reported for adsorption in several other metal I oxide systems (16-181.” The intrasolid diffusion occurring after a long time was considered neither by us nor by Dzombak and M0re1.~-~ The initial adsorption process is a fast one. As the problem of adsorption equilibrium is one of the leitmotifs in the comment by Lutzenkirchen and Behra,’ we would like to draw attention to the more recent extensive experimental studies of the kinetics of ion adsorption on goethite published by Briimmer and co-workers.6 In the summary of their article they write: “Theinitially rapid adsorption of metals within a few hours was followed by a much slower reaction linearly related to time*, interpreted as difisioncontrolled penetration of goethite”. The data published by Benjamin and Leckie5 must be partly the same as those presented in the Ph.D. Thesis of Benjamin. What is absolutely sure, all these data were measured in the same laboratory at the Stanford University, by using the same experimental set. Thus, it is difficult to understand why much ofthe data by Benjamin and Leckie5 had been excluded by Dzombak and Morel4 (for the hypotheticallack of equilibrium),while some other part of these data was used by Dzombak and MoreP4 to demonstrate the applicability of their precipitation model (Figure 6 in the work by Morel and co-workers2). As the precipitation approach refers to equilibrium, and the range
* Author to whom t h e correspondence should be addessed. (1)Liitzenkirchen, J.; Behra, P. Langmuir 1994,10, 3916. (2)Farley, K.J.;Dzombak, D. A.; Morel, F. M. M. J . ColZoidInterface Sci. 1985,106, 226. (3)Dzombak, D. A.; Morel, F. M. M. J . Colloid Znteface Sci. 1986, 112, 588. (4) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling, Hydrous Ferric Oxides; John Wiley & Sons, Inc.: New York, 1990. (5)Benjamin, M. M.; Leckie, J. 0.J . Colloid Inteface Sci. 1981,79, 209. (6)Briimmer, G.W.; Gerth,J.;Tiller, K. G. J . Soil Sci. 1988,39,37.
of the heavy metal concentration is the same as that investigated by us, Lutzenkirchen and Behral create a somewhat confusing uncertainty as to whether the equilibrium in this adsorption regime was achieved (is to be believed) or not. Another leitmotif in the comments by Lutzenkirchen and Behra is the effect of carbon dioxide present in the experimental set. They guess that “carbon dioxide had not been excluded” in the experimental set-up used to obtain the results reported by Barrow et al.7 On the contrary, any serious role of carbon dioxide must be excluded in the experiment of Benjamin and LeckieS5 They describe their experiment as follows: “All reagents used in this study were analyticalgrade or better. ... Solutions were made with deionized, conductivity-grade water and were adjusted to 0.1Mionic strength with NaNO3. They were maintained at 20 “Cunder a nitrogen atmosphere”. Such experimental conditions must eliminate any guess about serious effects due to the presence of C02 and to competing ions other than those of the basic electrolyte. Thus, the Freundlich log-log plots covering many orders of concentration and being parallel for different pH values observed by both Barrow7 and Benjamin and Leckie5 cannot be ascribed to complications arising from the presence of COZor competing ions introduced by poor quality reagents. Now let us remark that the data by Benjamin and Leckie6(Figure 6 in our papers), and the data reported by Barrow’ (Figure 8B in our papeP), exhibit essentially the same behavior. It means that the small amount of silica and calcium impurities is not the source of the two fundamental features of these systems: the linearity of the Freundlich log-log plots over several orders of magnitude; the fact, that the log-log plots corresponding to various pH are parallel. Further, the data from Benjamin and Leckie6have been short term data measured after 4 h of equilibration. On the contrary, the data from Barrow7 are long term data measured 7 days after equilibration. Thus, these two fundamental features of the Freundlich log-log plots cannot be ascribed to kinetic effects. Therefore, we cannot accept the thesis launched in the first point of the comments by Lutzenkirchen and Behra’ who write: “the Freundlich-like behaviour at higher sorbatelsorbent ratios might be due to the fact that equilibrium is not reached. So far, the Freundlich-like behavior has always been related to adsorption equilibria, and exponentially decreasing adsorption energy distribution. In point 2 oftheir comments, Lutzenkirchen and Behra’ claim that “by using surface precipitation reactions the range ofFreundlich-like behaviour can be extended at least for the higher values”. So far, we have not been able to find a model calculation in the works by Dzombak and Morel that would confirm such a guess. As an example we consider that part of Figure 6 in the work by Farley, Dzombak, and Morel2in which cadmium (7)Barrow, N.J.;Gerth, J.; Briimmer, G. W. J . Soil Sci. 1989,40, 437. (8)Rudzifiski, W.; Charmas, R.; Partyka, S.;Bottero, J. Y. Langmuir 1993,9,2641. (9)Benjamin, M. M. Ph.D. Thesis, Stanford University, Stanford, CA, 1978. ~~
0743-746319512411-0377$09.00/0 0 1995 American Chemical Society
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log[Cd2+] Figure 1. (A) The isotherm tit (-) presented by Farley, Dzombak, and Morel2using their precipitationmodel to describe cadmium adsorption on amorphous iron oxide. (Data ( 0 )from Benjaming). (B) The linear regression made by us (-1 rep-
resents the Freundlich log-log plot.
adsorption is shown at the highest pH = 7.2. It is shown in Figure 1. The four experimental points can be very well correlated by the linear log-log Freundlich plot drawn by us, indicating the linearity over 3 orders of bulk concentration a t least. The best fit obtained by using the precipitation model can therefore be challenged by the linear log-log plots corresponding to the continuous Gaussian-like adsorption energy distribution. The case of Cr(II1) sorption on HFO investigated by Charlet and ManceaulO has not been luckily chosen to advocate for the model of a homogeneoussurface adsorbing heavy metal ions. The mechanism of Cr(II1) adsorption is totally different. One of its striking features is that the adsorption of one C13+ion causes a release of three protons, so, Charlet and ManceaulO conclude it in this way: “This adsorption therefore does not induce any change in the surface charge, a result which contrasts that obtained in studies of bivalent heavy metal ion adsorption on goethite and hematite, where adsorption was never electrically balanced by an equal release of protons.” We had n9 chance to get familiar with the results presented in the Ph.D. Thesis by Gunneriusson,ll defended in the Laboratory of Professor Sjoberg in Umea, Sweden, and with the results presented in the Ph.D. Thesis by Spadini,12defended at the University of Bern, Switzerland. While taking into consideration the two papers by Gunn e r i u ~ s o n ’ ~on J ~Hg2+ and Cd2+ adsorption on Goetite which have already been published, it is difficult to understand why they should support the DSM model. Gunneriusson considers simply three or four kinds of heavy metal complexes formed on only one kind of adsorption sites SFeOH. Looking on the data which have already been published, we may quote a number of papers suggesting that the ~
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two-site model may be too crude a representation of the surface heterogeneity of many ferric oxides. First of all, we would like to draw attention to the experimental and theoretical works by Barrow,15-” who presented an impressive support for the “multisite”model. There, one can find also references to other works of that kind. Hiemstra and Van Riemsdijkls detected three types of surface groups at the Fe (hydrloxide. Some goethites formed from rapidly precipitated iron hydroxide are partially disordered and show a large variety of adsorbing sites. While concludingtheir work they wrote: “Themodel constants obtained (affinity constant, Stern layer capacitance) when applying the classical models are difficultto interpret physically since these models ignore the presence of different types of reactive surface groups and other differences in surface structure”. Very recently Bottero et al.19 have carried out a combined experimental study of the surface heterogeneity of some ferric oxyhydroxide. The analysis of argon adsorption, surfactant adsorption, and FTIR spectra has led them to the conclusion that there must be three or four different types of adsorption sites on the surface of their ferric oxyhydroxide sample. Then, it is to be expected, that even within a certain type of adsorption sites there will exist some dispersion of their adsorptive properties. Thus, the spectrum of adsorption energies will not consist of a certain number of Dirac delta functions but of a number of overlapping diffuse peaks. Such a spectrum may be well represented by a certain “smoothed” form, like the Gaussian-like function assumed by us. Further, we do not deny the role of the precipitation reactions which may become an important phenomenon a t higher surface coverages (ion concentrations). While concluding our considerations we would like to emphasize that we see the two-site model as a substantial progress toward more realistic treatment of heavy metal ions on ferric oxides. Morel and ~ o - w o r k e r s ~demon-~ strated its applicability for a quite large number of systems. Ferric oxides prepared in different ways may exhibit much different degree of surface heterogeneity. Thus, in many cases the two-site model may be too crude to represent the surface heterogeneity of ferric oxides. This was a t least the case of the systems studied by us.
Acknowledgment. The research is carried out within the frame of the project “Badania Statutowe 1994”. W. Rudzihski* and R. Charmas Department of Theoretical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, PI. M. Curie Sktodowskiej 3, Lublin 20-031, Poland Received June 29, 1994 In Final Form: October 21, 1994 LA940513H
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(10)Charlet, L.; Manceau, A. A. J . Colloid Intefuce Sci. 1992,148, 443. (11)Gunneriusson, L. Ph.D. Thesis, University of Umea, Umea, Sweden, 1993. (12)Spadini,L. Ph.D. Thesis, UniversityofBern, Bern, Switzerland, 1993. (13) Gunneriusson, L.; Sjoberg, S. J.Colloid Inteface Sci. 1993,156, 121. (14) Gunneriusson, L.J . Colloid Inteface Sci. 1994,163,484.
(15) Barrow, N.J. Reactions with Variable Charge Soils; Martinus NijhofE Dordrecht, 1987. (16) Barrow, N. J.A u t . J . Soil Res. 1989,27,457. (17) Barrow, N.J.;Briimmer, G. W.; Strauss, R. Langmuir 1993,9, 2c;nfi.
(18)Hiemstra, T.; Van Riemsdijk, W. H. Colloid Surf. 1991,59, 7. (19) Bottero, J.Y.; Amaud, M.; Villihras, F.; Michot, L. J.;de Donato, P.; FranGois J . Colloid Intefuce Sei. 1993,159, 45.
