Surface charge concentrations on silica in different 1.0 m metal-chloride background electrolytes and implications for dissolution rates

The empirical rate laws formulated to describe the dissolution rates of oxi de minerals include the surface charge concentration that results from the protonation and deprotonation of surface functional groups. Previous experi ments on quartz and silica have shown that dissolution rates vary as a func tion of different background electrolyte solutions, however, such experimen ts are often conducted at elevated temperatures where it is difficult to es timate surface charge along with the dissolution rates. In the present stud y we measure surface charge concentrations for silica in different electrol yte solutions at 298 K in order to quantify the extent to which the differe nt counterions could affect the dissolution rates through their influence o n the surface charge concentrations. The experimental solutions in the elec trolyte series: LiCl, NaCl, KCl, RbCl, CaCl2, SrCl2 and BaCl2 were prepared to maintain a constant metal concentration of 1.0 M. For the alkali-metal chlorides, the surface charge concentrations correlate with the size of the hydrated alkali metal, consistent with the idea that these counterions aff ect charge via outer-sphere coordination that shield proton surface complex es from one another. The reactivity trend for alkaline-earth cations is les s clear, but the data demonstrate distinct differences in the acid-base pro perties of the silica surface in these different electrolytes. We then disc uss how these trends are manifested in the rate equations used to interpret dissolution experiments.