Rapid Kinetics of Cu(II) Adsorption/Desorption on Goethite

Abstract

The kinetics of Cu[sup 2+] adsorption/desorption on goethite ([alpha]-FeOOH) was evaluated using the pressure-jump (p-jump) relaxation technique. This technique provides both kinetic and mechanistic information for reactions occurring on millisecond time scales. A double relaxation event was observed for Cu[sup 2+] adsorption/desorption on goethite. The rate of these relaxations ([tau]) decreased with an increase in pH, along the adsorption edge. The mechanism ascribed to the relaxations is the formation of a monodentate innersphere Cu[sup 2+]/goethite surface complex. The calculated intrinsic rate constant for adsorption (k[sub 1][prime]int) was 10[sup 6.81] L mol[sup [minus]1] s[sup [minus]1] and was about 2 orders of magnitude larger than the intrinsic rate constant for desorption (k[sub 1][prime]int = 10[sup 4.88] L mol[sup [minus]1] s[sup [minus]1]). 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. 30 refs., 7 figs., 1 tab.