The depletion and regeneration of dissolution-active sites at the mineral-water interface: II. Regeneration of active sites on alpha-Fe2O3 at pH 3 and pH 6

Periods of transient nonsteady state dissolution can contain much informati on about dissolution mechanisms. Here, pH-jump-induced dissolution transien ts are used to explore the kinetics of production, at pH 3 and pH 6, of alp ha -Fe2O3 surface sites active for dissolution at pH 1. We find that such s ites are generated in a matter of minutes or less at higher pH. The steady state dissolution rate of hematite at pH 1 is less than or equal to 10.7 pm ol (-2) s(-1), whereas the rate of;active site production at pH 6 in the fi rst 30 min, of aging is at least 119 pmolm(-2) s(-1). Apparently, active si tes are produced relatively slowly at low pH and relatively rapidly at circ umneutral pH, despite the fact that dissolution rates are near a minimum at circumneutral pH. Using aqueous water exchange rates as a proxy for surfac e ligand exchange rates, this is qualitatively consistent with relatively s low water exchange by aqueous Fe3+ ions at low pH and relatively rapid wate r exchange by Fe3+ hydrolysis products (e.g., Fe(OH)(2+)) at circumneutral pH. Consequently, the highest overall dissolution rates are achieved not at steady state at low pH, but by cycling between neutral and low pH. Our res ults call into question the assumption that oxide mineral surfaces, particu larly those of iron and aluminum oxides, are inert on the time scale of pro ton or ligand adsorption (e.g., during the acid-base titrations typically u sed to measure oxide surface charge due to proton adsorption). Copyright (C ) 2000 Elsevier Science Ltd.