Adsorption of calcium at the zinc sulphide-water interface

Abstract

The uptake of aqueous calcium ions by pure zinc sulphide colloids has been measured by direct adsorption and microelectrophoresis techniques. This uptake is pH dependent, with significant adsorption commencing at approximately pH 7 and increasing up to pH 11. The electrokinetic measurements also reflect the interfacial presence of Ca²⁺. The zeta-potential of zinc sulphide particles is more positive in the presence of Ca(NO₃)₂ solutions than in simple background electrolyte (KNO₃) solutions. If the concentration of calcium ion is sufficiently high, the zeta-potential changes sign c. pH 8.

Adsorption and simple ion-exchange models which have been successfully applied to other oxide and sulphide minerals are not particularly successful. However, reasonable agreement between direct adsorption and electrophoresis data has been obtained from the simple Gouy-Chapman-Stern double layer model, provided that: (i) the surface potential is approximately given by the Nernst equation; (ii) the specific adsorption potential of Ca²⁺ on ZnS increases from about 0 k_BT/ion, at low pH values below pH 7, up to about - 6.5 kBT/ion, at pH 8 and higher.

An ion-exchange model could only be used to give reasonable agreement with experiment at low adsorption densities. This exchange reaction is not strongly pH dependent and involves the release of one proton for every three Ca²⁺ ions adsorbed.

These results would indicate that Ca²⁺ adsorption at the ZnS-H₂O interface involves the movement of calcium ions into the Stern plane without the release of an equivalent number of protons into the diffuse layer and bulk solution. The subsequent decrease in zeta-potential caused by Ca²⁺ adsorption may be related to deteriorating flotation recoveries and grades observed for sphalerite in the presence of Ca²⁺ solutions when lime is used as a pH modifier and/or suppressant for pyrite.