Zeolite-A as a Remediation Technique for Soil Contaminated with Lead

The aim of the current project is to determine if Zeolite-A is suitable for lead remediation in pyrite ash contaminated soils. Pyrite ash is the waste product formed by roasting sulphide-bearing minerals for the production of sulphuric acid. The main sulphide mineral in the original material is pyrite, FeS2. After roasting, the soils contain a large amount of iron oxide in the form of hematite (?-Fe2O3), as this is the main product in the roasting process of pyrite. It was found that concentrations of 23000 to 26000 ppm of lead were present in the pyrite ash layers.

Zeolite-A was chosen as a method of remediation because it is a synthetic zeolite that is easy to synthesise, has a small pore size, and has a high affinity towards lead as shown by other studies on wastewater treatment.

Zeolite-A was added to soil samples and washed with water and dilute nitric acid to simulate rain and acid rain conditions. It was found that the addition of Zeolite- A to all soil samples investigated resulted in a pH increase by over 2 pH units. In soils washed with acid, having a pH of around 0.30, the pH increased to about 3.00. In soils with a pH of around 4.50, the addition of Zeolite-A increased the pH to nearly 8.00.

It was demonstrated that the addition of Zeolite-A to the samples resulted in a reduction of lead ions in all the leachates, independent of initial pH. The effect of Zeolite-A varied as a function of pH, but was found to reduce lead concentration in the leachate by approximately 82% to 99%. It was concluded to be successful for lead remediation.

One explanation for the successful result of lead remediation by Zeolite-A was the formation of a natural zeolite, gismondine (CaAl2Si2O8?4H2O). It was found that gismondine was formed both when the soils were washed with water or with acid. Hence, the gismondine formation was independent of pH. It is also proposed by this study that Zeolite-A stimulated the crystalline formation of gismondine.