An investigation into the use of synthetic zeolites for in situ land reclamation
This thesis describes an experimental study to determine the feasibility of using zeolite addition for the in situ treatment of soils contaminated with heavy metals. The aim of the present work was to examine the effectiveness of three synthetic zeolites to reduce plant available metal pools in contaminated soils. Three contaminated soils were studied, which are representative of typical contamination sites in the UK: Prescot, site of a copper refmery, Trelogan, an old lead/zinc mine spoil, and Gateacre, a sewage sludge treated field. The action of zeolites to reduce available metal concentrations in soils is due to their ion-exchange properties. To investigate the decrease of metal bioavailability by zeolites, laboratory and greenhouse trials were performed to clarify the mechanism of heavy metal fixation by synthetic zeolites and to quantify the effect of different zeolites for land remediation. For this approach, it was necessary to measure the metal concentration in the soil and the soil solution in zeolite-amended soils and to determine the zeolite specific isotherms of all the metals studied. Cation exchange studies involved exchanging the sodium form of the zeolites with different metals in solution, in order to determine the zeolite affinity for the metals copper, cadmium, zinc and lead. The resulting isotherms demonstrated that all three zeolites showed a preference for the heavy metal ions over sodium ions. The changes in metal speciation in zeolite-treated contaminated soils were evaluated using sequential extraction procedures. After incubation with synthetic zeolites, metals extracted with ammonium acetate were significantly decreased (31.4 % - 72.4 %) in amended soils compared to the controls. This decrease in heavy metal availability is extremely significant. The exchangeable metal fraction is the most available for uptake by plants. Long term soil solution experiments with zeolite-amended soils showed that the metal concentrations in the aqueous leachate were significantly reduced than in the leachate from the same substrates without zeolite addition. Greenhouse pot trials were carried out with sunflower (Helianthus annuus), maize (Zea mays), willows (Salix viminalis) and ryegrass (Lolium perenne) plants grown in zeolite amended contaminated soils. There were significant improvements in visual appearance and growth of plants from the zeolite-treated soils compared to the controls. In addition, metal content of plant tissues was reduced when compared to the controls. Optimum zeolite concentrations were noted for each zeolite. Zeolite P and 4A were more effective at reducing the phytotoxicity at 0.5% and 1%, whilst zeolite Y had to be added at 5% to achieve a similar effect. In order to link the laboratory test results and soil data to a pilot field scale, in which the actual soil and environmental conditions are required to give a complete evaluation of the proposal technique when applied to a given hazardous waste site, a field trial was initiated, at a copper contaminated site at B.I.C.C., Prescot to examine the effectiveness of zeolite amendments under field conditions. Zeolites P and 4A applied at 1% level proved to be an effective treatment for the remediation of the contaminated site, as indicated by improved plant growth and low metal concentrations in the water soluble fraction of the soil. The results show that zeolite addition, particularly zeolites P and 4A, provide an effective method for decreasing plant heavy metal bioavailability in polluted soils, under glasshouse and field conditions.