ROTATING-DISC ELECTRODE VOLTAMMETRY AS A PROBE OF ADSORPTION RATES ONSOLID PARTICLES IN LIQUIDS - APPLICATION TO ZN-II ADSORPTION AT THE HYDROXYAPATITE AQUEOUS INTERFACE

A simple new approach for the measurement of the extent and kinetics o f adsorption at the interface between a solid particle and a liquid is proposed. The technique employs a rotating-disc electrode to suspend the particles fully in the liquid under study, while the temporal depe ndence of the transport-limited current for the electrolysis of the so lution component of the adsorbate serves as a concentration probe. The technique is used to study Zn-II adsorption on hydroxyapatite (HAP) f rom aqueous solution (containing 0.1 mol dm(-3) KCl as a supporting el ectrolyte). In the absence of dissolved Ca2+, adsorption is effectivel y irreversible with Zn-II adsorbed on HAP at a coverage of 7.5 x 10(-1 0) mol cm(-2). Adsorption appears to involve at least two rate process es: ca. 70% of adsorption sites (fraction theta(1)) are filled in an i nitial rapid process which is complete on the timescale (ca. 5 s) of m ixing the adsorbate solution with the suspension of hydroxyapatite par ticles. The remaining sites (fraction theta(2)) are filled in an irrev ersible, surface-controlled process governed by the following kinetic rate law: d theta(2)/dt = {k'[Zn2+(aq)](1 - theta(1) - theta(2))}/N, w here N is the surface density of (total) adsorption sites (7.5 x 10(-1 0) mol cm(-2)), [Zn2+(aq)] is the bulk solution concentration, t is ti me and the adsorption rate constant, k', has a value of 2.6 (+0.6, -0. 3) x 10(-5) cm s(-1).