Kinetic evidences of the existence of positively charged species at the quartz-aqueous solution interface.
To probe the surface speciation of quartz in strong acidic solutions (pH 0-3), where surface titration and electrophoresis are extremely difficult to perform, dissolution rates of this mineral were measured at 25 degrees C and constant ionic strength (1.0 M) using mixed-flow and batch reactors. Dissolution rates increase with activity of protons at 0 < or = pH < or = 3, which suggests the adsorption of H+ on the mineral surface, leading to polarization of SiO bonds and detachment of the silicon atom from the structure. This scheme is consistent with the presence of a non-negligible amount (i.e., up to 30-50% at pH close to 0) of protonated >SiOH2+ species on the surface, as was recently demonstrated using X-ray photoelectron spectroscopy (XPS) analysis of exactly the same quartz sample [Y. Duval, J. Mielczarski, O.S. Pokrovsky, E. Mielczarski, J.J. Ehrhardt, J. Phys. Chem. B 106 (2002) 2937-2945]. A 2-pK electrical double layer (EDL) constant capacitance surface speciation model has been used to model the obtained kinetic data. A set of surface stability constants consistent with previous spectroscopic XPS measurements (pK1 = -1.0 and pK2 = 4.0) and EDL capacitance of 1.5 F/m2 provide adequate description of the dissolution rate with reaction order with respect to [>SiOH2(+)] close to 1. Although the CCM model used in this study presents some limitations on surface charge versus pH dependences, the developed kinetic approach opens new possibilities of probing the surface speciation at the SiO2-aqueous solution interface under extreme solution conditions.
Pokrovsky
O S
OS
Experimental Aqueous Geochemistry Group, LMTG, UMR 5563, CNRS-UPS Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 314000 Toulouse, France. oleg@lmtg.obs-mip.fr
Golubev
S V
SV
Mielczarski
J A
JA