School of Chemistry - Theses

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    Heavy metal ion activation of zinc sulphide
    Ralston, John ( 1978)
    This thesis deals with the activation of zinc sulphide by the heavy metal ions Cu ll, Cd 11 and Pb II over a wide range of pH, metal ion concentration, zinc sulphide concentration and incident light conditions. At acid to neutral pH values the uptake of a heavy metal ion has been observed to take place in two stages: a fast, initial step followed by a second, slower step. Kinetic data have shown that heavy metal ion uptake generally follows a logarithmic dependence on time. At alkaline pH values heavy metal ion uptake is complicated by hydrolysis effects. A sensitive mass spectrometric technique has been developed and is capable of quantitatively detecting down to 1% of a nominal monolayer of elemental sulphur on a mineral of surface area 0.7 m2 g-1. Elemental sulphur was detected on sphalerite surfaces activated by Cu11 and Pb II up to pH 6.6, both in the dark and under UV irradiation. Elemental sulphur was only detected on Cd II activated and unactivated zinc sulphide surfaces under UV irradiation. A mechanism for the activation of zinc sulphide is advanced at acid, neutral and alkaline pH values. At acid to neutral pH values, metal ion uptake occurs by an exchange reaction This is coupled with a surface redox process, which is linked with the semiconductor properties of zinc sulphide. As a result of this surface redox process, elemental sulphur may form on the zinc sulphide surface, depending on the type of metal ion involved and on the incident light conditions. It is proposed that the rate determining step in the exchange reaction is the transfer of an adsorbed metal ion from a surface, adsorbed site to a lattice site. This leads to a logarithmic dependence of metal ion uptake on time, as observed experimentally. At alkaline pH values, the mechanism at acid to neutral pH values is complicated by metal ion hydrolysis, adsorption and precipitation effects. An overall mechanism is proposed which accounts for the observed behaviour.