Rotating-disc electrode voltammetry as a probe of adsorption rates on solid particles in liquids. Application to ZnII adsorption at the hydroxyapatite/aqueous interface

Abstract

A simple new approach for the measurement of the extent and kinetics of 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 dependence of the transport-limited current for the electrolysis of the solution component of the adsorbate serves as a concentration probe. The technique is used to study Zn^II adsorption on hydroxyapatite (HAP) from aqueous solution (containing 0.1 mol dm^–3 KCI as a supporting electrolyte). In the absence of dissolved Ca²⁺, adsorption is effectively irreversible with Zn^II adsorbed on HAP at a coverage of 7.5 × 10^–10 mol cm^–2. Adsorption appears to involve at least two rate processes: ca. 70% of adsorption sites (fraction θ₁) are filled in an initial rapid process which is complete on the timescale (ca. 5 s) of mixing the adsorbate solution with the suspension of hydroxyapatite particles. The remaining sites (fraction θ₂) are filled in an irreversible, surface-controlled process governed by the following kinetic rate law: dθ₂/dt={k′[Zn²⁺(aq)](1 –θ₁–θ₂)}/N, where N is the surface density of (total) adsorption sites (7.5 × 10^–10 mol cm^–2), [Zn²⁺(aq)] is the bulk solution concentration, t is time and the adsorption rate constant, k′, has a value of 2.6 (+0.6, –0.3)× 10^–5 cm s^–1.