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Response to Comment on “Arsenite and Arsenate Adsorption on Ferrihydrite: Surface Charge Reduction and Net OH- Release Stoichiometry” SIR: In the comment on our paper (1), the author stated that the use of proton and hydroxyl release data in the assignment of the surface complexation reactions is unjustified because of the precipitation of ferric arsenate or ferric arsenite during the reaction of arsenate or arsenite with ferrihydrite. We maintain that the use of net OH--release and surface-charge stoichiometry for comparing and assigning probable surface complexation reactions as in our paper (1) is justified. In a discussion of arsenic sorption on ferrihydrite, arsenate and arsenite must be considered separately because of their different sorption behavior. The maximum arsenate sorption density of 0.25 molAs molFe-1 observed in our study (2) and a previous study (3) was approximately the same as the maximum inner-sphere adsorption predicted from total surface site density. Extended X-ray absorption fine structure (EXAFS) spectroscopy results supported a surface innersphere ligand bonding mechanism(s) for the retention of arsenate on ferrihydrite and did not show any evidence of reprecipitation of ferric arsenate or other Fe-As phase (4). On the contrary, the maximum arsenite sorption density of 0.60 molAs molFe-1 was much higher than the total surface site density of ferrihydrite (2, 5, 6). The inner-sphere ligand bonding of arsenite to iron oxide has been confirmed by EXAFS (7) and FTIR (8) spectroscopy. But the high sorption capacity of arsenite on ferrihydrite cannot be explained by the inner-sphere surface complexation model. We had originally postulated that ferrihydrite, at high arsenite concentrations, might be reordered to a ferric arsenite phase (2). In recent experiments (unpublished), we have conducted EXAFS and powder X-ray diffraction (XRD) of ferrihydrite samples with low and high sorbed arsenite concentrations. EXAFS spectra did not provide evidence of a change in second-shell bonding of structural Fe or of precipitation of a ferric arsenite phase. The powder XRD patterns showed only the two broad peaks characteristic of 2-line ferrihydrite, with no significant change in peak position upon reaction with arsenite, again providing no evidence of precipitation of a ferric arsenite phase. Also, the sorption reaction approached approximate equilibrium within minutes (2). The rapid reaction might not be expected for a process involving dissolution and reprecipitation. There is a strong possibility that the high sorption of arsenite on ferrihydrite is attributable instead to a polymerization of ligand-bound arsenite on the iron oxide surface as suggested by Edwards
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(9), although we do not presently have any direct structural evidence thereof. Condensed arsenite anions do not exist in solution, but they have been found to exist in solid phase (10). It is possible that the iron oxide might promote a surface polymerization of arsenite. These reactions deserve further study, but at this time, we suspect that the surface polymerization of ligand-bound arsenite is the most probable reaction contributing to the high sorption density of arsenite on ferrihydrite. In our studies (1), the surface complexation reactions were compared and assigned based on the net OH--release and surface charge stoichiometry data obtained during the interaction of 0.53 mM (corresponding to 0.267 molAs kgfer-1 and As:Fe molar ratio of 1:39) and 1.60 mM (0.801 molAs kgfer-1 and As:Fe molar ratio of 1:13) arsenite or arsenate with ferrihydrite. At these concentrations, surface ligand bonding of arsenite is predominant (2), and surface polymerization, if this process occurs, would be negligible. Therefore, the interpretation of net OH--release and surface charge stoichiometry in comparing and assigning probable surface complexation reactions is appropriate. Polymerization of adsorbed As(III) is likely to be appreciable only after the high-energy ligand bonding sites are largely filled, in which case net OH--release and surface charge stoichiometry would be influenced by the polymerization reaction.
Literature Cited (1) Jain, A.; Raven, K. P.; Loeppert, R. H. Environ. Sci. Technol. 1999, 33, 1179-1184. (2) Raven, K. P.; Jain, A.; Loeppert, R. H. Environ. Sci. Technol. 1998, 32, 344-349. (3) Fuller, C. C.; Davis, J. A.; Waychunas, G. A. Geochim. Cosmochim. Acta 1993, 57, 2271-2282. (4) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A. Geochim. Cosmochim. Acta 1993, 57, 2251-2269. (5) Ferguson, J. F.; Anderson, M. A. Chemical forms of arsenic in water supplies and their removal. In Chemistry of water supply, treatment, and distribution; Rubin, A. J., Ed.; Ann Arbor Science: Ann Arbor, MI, 1974; pp 137-158. (6) Pierce, M. L.; Moore, C. B. Water Res. 1982, 16, 1247-1253. (7) Manning, B. A.; Fendorf, S. E.; Goldberg, S. Environ. Sci. Technol. 1998, 32, 2383-2388 (8) Sun, X.; Doner, H. E. Soil Sci. 1996, 161, 865-872. (9) Edwards, M. J. Am. Water Works Assoc. 1994, 86, 64-78. (10) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1988; p 430.
Amita Jain, Klaus Richard H. Loeppert*
P.
Raven,
and
Soil and Crop Sciences Department Texas A&M University College Station, Texas 77843-2474 ES992023N
10.1021/es992023n CCC: $18.00
1999 American Chemical Society Published on Web 09/03/1999
