Environ. Sci. Technol. 19Q4, 28, 1422-1429
Rapid Kinetics of Cu( I I ) Adsorption/Desorption on Goethite Paul R. Grossl’ and Donald L. Sparks
Department of Piant and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303 Calvin C. Alnsworth
Battelle, Pacific Northwest Laboratory, Richland, Washington 99352 The kinetics of Cu2+adsorption/desorption on goethite (a-FeOOH) was evaluated using the pressure-jump (pjump) relaxation technique. This technique provides both kinetic and mechanisticinformation for reactions occurring on millisecondtime scales. A double relaxation event was observed for Cu2+adsorption/desorption on goethite. The rate of these relaxations (7)decreased with an increase in pH, along the adsorption edge. The mechanism ascribed to the relaxations is the formation of a monodentate innersphere Cu2+/goethite surface complex. The calculated intrinsic rate constant for adsorption (kl’int) was 106.81 L mol-l5-l and was about 2 orders of magnitude larger than the intrinsic rate constant for desorption (k-l’int = 104.88 L mol-’ s-l). Using results from this study and others, it was established that the rate of adsorption of divalent metal cations on goethite was directly related to the rate of removal of a water molecule from the primary hydration sphere of a particular divalent metal cation.
Introduction Copper contamination of the environment arises from industrial and agricultural emmissions. It is found in municipal wastes as a byproduct from the metal mining and processingindustry and from agriculturalsourcessuch as fertilizers, fungicidal sprays, and animal wastes (1,2). Copper is an essential nutrient for plants and animals; however, at sufficiently high concentrations and when in a mobile form, it becomes toxic (3). It has been suggested that Cu toxicity in aqueous systems may be a function of the free ion concentration [Cu(H&),j2+,or for simplicity Cu2+lrather than of the total Cu concentration (3). As a result, there is concern regarding the fate and transport of Cu2+and other divalent metal cations in soils and aquatic systems. To appraise the potential hazard of Cu(I1) and other metals to the environment, it is necessary to understand the interaction of these metal ions with soils and soil constituents. Metal ion adsorption on solid surfaces is an important process regulating metal concentrations in natural waters ( 4 ) . This is one reason why metal concentrations in these waters are often much lower than values expected from equilibrium solubility relationships. Numerous equilibrium studies have been conducted measuring the adsorption of divalent metal cations onto oxide surfaces (4-7). Sorption is strongly pH dependent, increasing with an increase in pH (4,6-8). Typically, there is a narrow pH range (the adsorption edge) where adsorption increases from 0 to nearly 100% ( 4 , 5 , 9 ) . The pH position of the adsorption edge for a certain metal cation is related to its hydrolysis or acid/base properties. Although equilibrium
* Address correspondence to this author at his present address: Department of Plants, Soils, and Biometeorology, Utah State University, Logan, UT 84322-4820. 1422
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adsorption studies are able to relate the total concentration of a metal in aqueous solution to its total sorbed concentration, they are limited in their ability to describe the actual rates and binding mechanisms of adsorption. In order to understand the behavior of metal ions in soil systems and to determine their potential release into aqueous environments, it is necessary to understand their interaction with solid surfaces. For example, are metals specifically adsorbed onto the surfaces forming innersphere surface complexes, or is the binding more electrostatic forming outer-sphere surface complexes? An inner-sphere surface complex is more stable than an outersphere complex since an inner-sphere surface complex is one where the metal ion binds directly with the surface functional group (10). No coordinated waters lie between these two units, and the surface complexbond can be either covalent or ionic in nature. Whereas, an outer-sphere complex is coulombic in nature, with at least one of the metals’ coordinated waters interposed between it and the surface functional group (10). Kinetic techniques can be employed to ascertain mechanisms for metal ion adsorption on oxides and also to predict the rate of retention and release of free metal ions on soils as it relates to water quality and ultimately human health (5,11,12). However,definitive mechanistic information can only be obtained in conjunction with spectroscopic experiments. The kinetics of metal ion adsorption/desorption reactions are extremely rapid (
