Kinetics goethite-water

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Zhang, P.C. Sparks, D.L. (1989) Kinetics and mechanism of molybdate adsorption/desorp-tion at the goethite/water interface using pressure-jump relaxation. Soil Sci. Soc. Am. 

Zhang, P.C. Sparks, D.L. (1990) Kinetics and mechanism of sulfate adsorption and desorption on goethite using pressure jump relaxation. Soil Sci. Soc. Am. J. 54 1266-1273 Zhang, P.C. Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface. Environ. Sci. Technol. 24 1848-1856... 

Zhang, R-C. and Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/ desorption at the goethite/water interface. Environ. Sci. Technol., 24, 1848-1856. 

Precipitation, dissolution and reprecipitation of the various Fe oxides in the environment depend predominately on factors such as pH, Eh, temperature and water activity. For this reason, the different Fe oxides may serve as indicators of the type of environment in which they formed. Goethite and hematite are thermodynamically the most stable Fe oxides under aerobic surface conditions and they are, therefore, the most widespread Fe oxides in soils and sediments. Other Fe oxides are, however, also found in the enviroirment because, although they are thermodynamically less stable, their formation is kinetically favored and their transformation to more stable forms proceeds sluggishly. 

Maurice PA, Lee YJ, Hersman LE (2000) Dissolution of Al-substituted goethites by an aerobic Pseudomonas mendocina var. bacteria. Geochim Cosmochim Acta 64 1363-1374 McCoy JM, LaFemina JP (1997) Kinetic Monte Carlo investigation of pit formation at the CaCOs (1014) surface-water interface. Surface Sci 373 288-299... 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

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