Environ. Sci. Technol. 1996, 30, 1412
Response to 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: We appreciate the comments of Ong and Leckie regarding the modeling of picolinic acid adsorption onto goethite in August 1995 Environ. Sci. Technol. paper (1). Although both models employed the same protonation stoichiometry, the electrostatic correction terms for H+ and PIC- (the picolinate anion) exactly cancel out in the innersphere model but yield an exp((Fψβ - Fψo)/RT) correction term in the outer-sphere model. The inner-sphere model (full line in Figure 1B) predicts that adsorption increases with decreasing pH until a plateau is reached slightly below the pKa for picolinic acid, which extends for almost 2 additional pH units. The outer-sphere model (dashed line) also yields an increase in adsorption with decreasing pH, but adsorption diminishes significantly below the pKa. As pointed out by Ong and Leckie, results from experiments employing 10 g/L FeOOH (open symbols in Figure 1B) can be described by either model. Results from experiments employing 1 g/L FeOOH (filled symbols in Figure 1B) however yield the plateau predicted by the inner-sphere
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model. We agree with Ong and Leckie that the available experimental information is not sufficient to distinguish inner-sphere from outer-sphere picolinic acid adsorption. New experimental measurements that include the effects of ionic strength on picolinic acid adsorption could contribute substantially to our understanding of the processes governing adsorption. An analogy to complex formation in solution is perhaps appropriate here; spectroscopic studies (most notably NMR) or indirect methods (e.g., pressure-jump studies and kinetic analyses) are usually required for the distinction between inner-sphere and outer-sphere complexes to be made unambiguously.
Literature Cited (1) Coughlin, B. R.; Stone, A. T. Environ. Sci. Technol. 1995, 29, 2445-2455.
Alan T. Stone* and Barbara R. Coughlin† Department of Geography and Environmental Engineering G. W. C. Whiting School of Engineering The Johns Hopkins University Baltimore, Maryland 21218 ES9510119 † Present address: S. S. Papadopulos & Associates, Bethesda, MD 20814.
0013-936X/96/0930-1412$12.00/0
1996 American Chemical Society
