An electrokinetic study of ZnS and CdS surface chemistry

Abstract

ζ-potential-vs-pH measurements on ZnS and CdS powders were performed in oxygen-free, nonoxidizing indifferent supporting electrolytes, in Zn and Cd salt solutions, respectively, and in (NH 4) 2S solutions of different concentrations. ZnS and CdS ζ-potential-vs-pH curves in Zn and Cd salt solutions, respectively, are S-shaped, showing three charge reversal points: CR1, CR2, and CR3. Up to CR1 points, ζ potentials are positive due to H + adsorption on S Lewis sites; between CR1 and CR2 points, they are negative owing to ionization of S Brønsted sites; between CR2 and CR3 points, they are positive as a result of Zn 2+ and ZnOH + or Cd 2+ and CdOH + chemisorption on S Brønsted sites; they are negative beyond CR3 due to OH − adsorption on Zn or Cd Lewis sites and due to the surface precipitation of hydroxides. ZnS and CdS ζ-potential-vs-pH curves in (NH 4) 2S solutions show minima owing to adsorption of HS − anions on Zn or Cd Lewis sites. ZnS and CdS ζ potentials show a logarithmic dependence(i) on [HS-] in the pH range 6 to 9, (ii) on [Zn 2+] in the pH range 4 to 6, and (iii) on [Cd 2+] in the pH range 4 to 7. H +, OH −, HS −, Zn 2+, ZnOH +, Cd 2+, and CdOH + are potential-determining ions. The ZnS pH iep is 2.3; the CdS pH iep is 1.8.