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Scheckel, K.G., and D.L. Sparks. 1998. Assessment of Ni sorption mechanisms on soil mineral surfaces using time-resolved x-ray absorption fine structure (XAFS) spectroscopy. V.M. Goldschmidt Conference, Toulouse, France. Mineralogical Mag. Vol., 62A:1330--1331.

Introduction: Many kinetic studies have shown that sorption of metal cations onto soil, clay mineral and metal (hydr)oxide surfaces is described by a two step process comprising a rapid stage and a slow stage (Ainsworth et al., 1994). The rapid initial stage, occurring on time scales of seconds, minutes or hours, has been interpreted as an adsorption phenomenon (Sparks, 1995). However, the slow reaction stage, associated with time scales on the order of days, weeks or months is not clearly understood. Three possible mechanisms have been attributed to the slow sorption stage including: 1)adsorption onto sites of lower reactivity (Dzombak and Morel, 1986), 2) diffusion of the adsorbate into the adsorbent via micropores and/or directly in the solid material (absorption) (Pignatello and Xing, 1995), and 3) surface precipitation formation (Scheidegger et al., 1996). However, until recently, the proposed mechanisms for slow metal sorption were based strictly on macroscopic measurements via sorption isotherms and kinetic studies. To definitively prove sorption mechanisms one must employ molecular approaches. With the advent of in-situ spectroscopic and microscopic techniques, it is possible to identify the cause of slow metal sorption mechanisms.

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