Immobilization of Ni by synthesising zeolite at low temperatures in a polluted soil

Over the last few years a great deal of research has been carried out in order to develop remediation methods for reducing environmental risks due to polluting metals. Zeolite formation in contaminated soils mixed with coal fly ash could be a useful method to reduce both the availability and the mobility of metals in contaminated areas. In this study a soil sample--treated with coal fly ash and artificially contaminated with a high concentration of Ni--was used for synthesizing zeolite at low temperatures. The role played by this mineral in the immobilization of heavy metal was investigated. The materials were analysed chemically (sequential extraction) and by XRD and SEM-EDS analyses. The synthesis was carried out both in the laboratory and on a bench-scale for 1 year. Zeolite crystallization readily occurred after a month. The presence of Ni does not exert any influence on zeolite formation. On the other hand newly-formed zeolites reduce the toxicity of the element in the polluted soil. A reduction in heavy metal availability was observed after ammonium acetate extraction. The use of the modified BCR three-step sequential extraction (sequential extraction protocol developed by Community Bureau of Reference of the Commission of the European Communities) suggests that Ni mobilization takes place when zeolite structure collapses after the BCR second step. The Ni thus available was mobilized in the third step.