The surface chemistry of multi-oxide silicates

The surface chemistry of natural wollastonite, diopside, enstatite, forsterite, and albite in aqueous solutions was characterized using both electrokinetic techniques and surface titrations performed for 20 min in batch reactors. Titrations performed in such reactors allow determination of both proton consumption and metal release from the mineral Surface as a function of pH. The compositions, based on aqueous solution analysis, of all investigated surfaces vary dramatically with solution pH. Ca and Mg are preferentially released from the Surfaces of all investigated divalent metal silicates at pH less than similar to 8.5-10 but preferentially retained relative to silica at higher pH. As such, the surfaces of these minerals are Si-rich and divalent metal poor except in strongly alkaline solutions. The preferential removal of divalent cations from these Surfaces is coupled to proton consumption. The number of protons consumed by the preferential removal of each divalent cation is pH independent but depends on the identity of the mineral; similar to 1.5 protons are consumed by the preferential removal of each Ca atom from wollastonite, similar to 3 protons are consumed by the preferential removal of each Mg or Ca atom front diopside or enstatite, and similar to 4 protons are consumed by the preferential removal of each Mg from forsterite. These observations are interpreted to stem from the creation of additional 'internal' adsorption sites by the preferential removal of divalent metal cations which can be coupled to the condensation of partially detached Si. Similarly, Na and Al are preferentially removed from the albite surface at 2 > pH > 11; mass balance calculations suggest that three protons are consumed by the preferential removal of each Al atom from this surface over this entire pH range. Electrokinetic measurements on fresh mineral powders yield an isoelectric point (pH(IEP)) 2.6, 4.4, 3.0, 4.5, and <1, for wollastonite, diopside, enstatite, forsterite, and albite, respectively, consistent with the predominance of SiO2 in the surface layer of all of these multi-oxide silicates at acidic pH. Taken together, these observations suggest fundamental differences between the surface chemistry of simple versus multi-oxide minerals including (1) a dependency of the number and identity of multi-oxide silicate surface sites on the aqueous solution composition, and (2) the dominant role of metal-proton exchange reactions on the reactivity Of multi-oxide mineral surfaces including their dissolution rate variation with aqueous solution composition. (C) 2009 Elsevier Ltd. All rights reserved.