THE COORDINATION CHEMISTRY OF WEATHERING .4. INHIBITION OF THE DISSOLUTION OF OXIDE MINERALS

The surface coordinative chemical model of mineral dissolution-surface chemical interaction with H+, OH-, metal ions, and ligands to form an array of surface complexes whose reactivities determine the mechanisms of many surface-controlled processes-has been successfully applied to interpret and quantify proton and ligand dependent dissolution rates of oxides and silicates. Here this model is extended to include the effects of inhibition. Experimental data are presented on the various factors which enhance and inhibit the nonreductive dissolution and the reductive dissolution (by H2S) of various Fe(III)(hydr)oxides (alpha-Fe2O3, alpha-FeOOH, and gamma-FeOOH). The effects observed with iron oxides are similar to those expected to occur with other oxide and aluminumsilicate minerals. Special attention is paid to the effects of changes in surface protonation brought on by the binding of cations and anions to the surface. Binuclear surface complexes, especially those formed by the adsorption of oxoanions such as phosphate, arsenate, borate, and sulfate, are particularly good at inhibiting both the reductive and nonreductive dissolutions of a reference system of iron(III) oxides. This phenomenon is due to (1) the large activation energy which must be overcome to simultaneously detach two surface metal centers and to (2) the lack of additional surface protonation when uncharged binuclear or multinuclear complexes are formed.