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Comment on “Nonreversible Adsorption of Divalent Metal Ions (MnII, CoII, NiII, CuII, and PbII) onto Goethite: Effects of Acidification, FeII Addition, and Picolinic Acid Addition” SIR: In a recent paper, Coughlin and Stone (1) report on the adsorption of picolinic acid onto goethite. Inner-sphere and outer-sphere configurations for the binding of picolinic acid to goethite were modeled using the program HYDRAQL and the triple layer surface complexation model, and the results were compared to experimental data. Using the best-fit criteria of minimizing the sum of the square of the residuals, it was concluded that adsorption to the zero plane occurred, forming the SPIC0 complex. In contrast, formation of the SOH2+PIC- complex was found to be less likely. We feel that this best-fit criteria does not provide conclusive evidence in this particular application, which appears to require additional consideration of the overall shape of the data set and model results. It is possible that the surface interaction portrayed by Davis and Leckie (2), in which the picolinic acid molecule is bound to the surface via the carboxylic acid functional group and N forms a coordinative bond to a neighboring surface site, is a more accurate representation. Our visual inspection of the picolinate sorption and modeling data graphically presented in the paper suggests to us that the outer-sphere surface complex model yields a far more acceptable fit, in terms of shape and trends, to the experimental data. We feel that the sorption maxima exhibited at about pH 5 is real and, therefore, consistent with the SOH2+PIC- model fit. The sorption maxima at about the same pH was also exhibited by picolinic acid sorption data in experiments by Davis (3) on Fe(OH)3(am) and Pope et al. (4) on R-Fe2O3 and Cr(OH)3. In lieu of direct spectroscopic evidence, further sorption experimental data of the picolinic acid-goethite system at different
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1996 American Chemical Society
ionic strengths would be a helpful and perhaps decisive indicator of which surface complex forms (5). An appropriate additional test for the model parameters would have been to apply them to the other sorption data set comprising 1 g/L FeOOH. With the data of Davis (3) and using HYDRAQL, we modeled picolinic acid sorption onto Fe(OH)3(am) with the following conditions: 4 × 10-5 M picolinate, 6.9 × 10-4 M Fe(OH)3(am), and 0.1 M NaClO4. Protonation stoichiometry with an outer-sphere picolinic acid surface complex product yielded a good fit. With further data from Davis for the adsorption of 10-6 M Cu and 4 × 10-5 M picolinate on 10-3 M Fe(OH)3(am) in 0.1 M NaNO3, we obtained by extension of the stoichiometries including an inner-sphere Cu surface complex a satisfactory fit using SOH2+PIC- in the reaction stoichiometry set. Application of the outer-sphere picolinate surface complex in the reaction stoichiometry might improve the HYDRAQL model prediction to the experimental data of the multicomponent systems.
Literature Cited (1) Coughlin, B. R.; Stone, A. T. Environ. Sci. Technol. 1995, 29, 2445-2455. (2) Davis, J. A.; Leckie, J. O. Environ. Sci. Technol. 1978, 12, 13091315. (3) Davis, J. A. Ph.D. Dissertation, Stanford University, Stanford, CA, 1978. (4) Pope, C. G.; Matijevic, E; Patel, R. E. J. Colloid Interface Sci. 1981, 80, 74-83. (5) Hayes, K. F.; Papelis, C.; Leckie, J. O. J. Colloid Interface Sci. 1988, 125, 717-726.
Colin G. Ong* and James O. Leckie Department of Civil Engineering Stanford University Stanford, California 94305 ES950835N
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