Surface reactions in aqueous metal sulfide systems: 2. Ion exchange and acid/base reactions at the ZnS [sbnd]H 2O interface

Abstract

The amphoteric properties of hydrous ZnS surfaces as well as an ion exchange reaction between surface coordinated Zn 2+ ions and H + ions of the aqueous phase have been established. The measurements were performed as potentiometric titrations in 0.1 M NaClO 4 medium at 25°C. Besides measuring −log[H +] by using a glass electrode, −log[S 2−] was recorded (Ag/Ag 2S selective electrode). In addition, the total concentrations of H +, Zn 2+, and S were analyzed as a function of −log[H +] of the bulk solution. Synthetically prepared ZnS and the mineral sphalerite were studied. Bro¨nsted acidity constants pK 1 s = 6.91 and pK 2 s = 10.28 for synthetic ZnS, and pK 1 s = 7.14 and pK 2 s = 10.29 for sphalerite, were established, which implies −log[H +] of zero proton condition equal to 8.60 and 8.72, respectively. Furthermore, the ion exchange reaction SZn + 2H + ⇆ SH 2 + Zn 2+ was characterized within the range 4 ⩽ −log[H +] ⩽ 7, with log K = 9.59 and 9.65. Experimental data were evaluated on the basis of the constant capacitance model by using the computer program FITEQL. The “best” fit to experimental data was obtained by assuming no charge dependence of K 1 s and K 2 s.