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Kinetics and Mechanisms of Molybdate Adsorption/Desorption at the Goethite/Water Interface Using Pressure-Jump Relaxation

Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303

*Corresponding author.

ABSTRACT

Pressure-jump (p-jump) relaxation with conductivity detection was used to ascertain the kinetics and mechanisms of MoO₄ adsorption/desorption on goethite. A postulated reaction mechanism consisting of two consecutive elementary steps was examined and verified through kinetic and equilibrium studies. The first step is the formation of an ion-pair complex through the electrostatic attraction between the protonated surface and the MoO₄ anion. The second step involves a ligand exchange process, whereby 1 mol of H₂O is replaced by 1 mol of adsorbed MoO₄ from the surface. It is much slower than the first step. The forward and backward intrinsic rate constants for steps 1 and 2 are: k^int₁ = 4019.2 mol^–1 L s^–1, k^int_-1 = 391.5 s^–1, k^int₂ = 1.888 mol^–1 L s^–1, and k^int_-2 = 42.34 s^–1. A slightly modified triple layer model (TLM) was employed to calculate the distribution of ionic species on the goethite surface, in the and ß layers, and in the bulk solution at equilibrium and electrical parameters for the charged surface. Both equilibrium and kinetic data fit the postulated mechanism for the reaction steps and the modified adsorption model well.

Contribution from the Delaware Agric. Exp. Stn. no. 1064 and the Dep. of Plant Science no. 243.

Received for publication October 17, 1988.

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