Metalliferous mines are commonly associated with elevated concentrations of potentially toxic elements (PTEs) in both the wastes that they produce or in the soils surrounding them. PTEs are elements that may or may not be beneficial to organisms at very low concentrations, but are toxic at relatively low concentrations. In this report the term PTE refers to the elements Al, As, Cd, Cu, Mn and Zn.
Pilot scale laboratory-based column-leaching experiments were set-up to investigate the effects of different diatomite application rates and drainage regimes on the leachate properties of two different types of mine waste. The main objectives of the experiments were to: define optimal diatomite application rates; investigate the efficacy of diatomite in reducing PTE mobility; compare the efficacy of diatomite in mine wastes of different properties; study the effects of waterlogging and evaluate the long-term stability of PTE sequestration.
These investigations identified major differences between the two samples. The Ron Phibun waste was a lightly weathered, highly complex, heterogeneous mixture of sulphides, silicates and oxides with high total concentrations of As and Cd. A large proportion of the PTEs were also present in highly soluble forms. By contrast the Pin Yo waste was a relatively simple silica-dominated substrate with high concentrations of some PTEs, especially Cu, Mn and Zn. The non-silica mineralogy was dominated by secondary minerals indicating a high degree of weathering.
Duplicate sets of column experiments were set-up: one at CSM in the UK and one at DMR in Bangkok. Each experiment consisted of two kinds of mine waste (Ron Phibun and Pin Yo), three diatomite concentrations (1%, 5% and 10%) and a control (0%), and two drainage regimes (saturated and free-draining). Ron Phibun waste leachate concentrations of Fe and Mn were decreased by 5% diatomite. 10% diatomite decreased the concentrations of Al, As, Ca, Cu, Fe, Mn, and Zn, while increasing the pH and the concentration of Mg. The Pin Yo waste leachate Ca concentration was decreased by 1% diatomite, although Mg concentrations increased; 5% diatomite decreased Ca and Mn concentrations and the conductivity, but increased K and Mg. 10% diatomite decreased Pin Yo leachate concentrations of Ca, Mn and Zn concentrations and conductivity, but increased K and Mg concentrations and the pH. Increased leachate K and Mg concentrations resulted from the leaching of these elements from the diatomite itself which is enriched in these elements relative to the mine wastes.
Saturation decreased PTE concentrations in Ron Phibun leachates and increased the pH substantially. However, when interacting with the Pin Yo mine waste, saturation increased leachate concentrations of As, Fe and Mn. A possible mechanism for this unexpected result is discussed in the report.
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