A model for the zeta potential of copper sulphide

A model is proposed for the zeta-potential versus pH curve of Covellite (CuS). The classical Gouy–Chapman–Stern treatment of the adsorption of protons is applied to a model surface derived from the crystallographic data. The surface charges are obtained by an original adaptation of the concepts from the MUSIC model to a sulphide, i.e. the analysis of the chemical bonding in Paulingʹs electrostatic valence framework to derive partial surface charges. The authors develop a software for the fitting of pζ–pH curves using IPHREEQC for the calculation of the zeta potential at each pH point and Matlab to drive the calculation of the whole pH set and the optimisation of the parameters: capacitances of the double layer, site densities, charges and acidity constants. The model successfully reproduces the experimental data, considering only two sulphided surface sites: introduction of an elemental sulphur layer or consideration of hydroxyl surface sites is not required to explain the charging behaviour of Covellite when carefully precipitated under nitrogen.