The site at Gyöngyösoroszi (Hungary) is heavily polluted with toxic metals, due to former mining in the area. According to our remediation strategy, the identified diffuse pollution sources and the residual pollution remaining after the removal of point sources will be remedied using combined chemical- and phytostabilization. To select the best chemical immobilizing agent, which can reduce the mobility and biological availability of toxic metals in the soil and waste materials over the long term, laboratory microcosm tests were performed. The stabilizing additives were tested in various concentrations between 1 and 10% by weight: four types of fly-ash and their combination with lime, two types of Fe–Mn-hydroxide precipitate, red mud, lignite, alginite, hydrated lime, phosphate and a mixture of the latter four. The stabilization process was observed by integrated monitoring, which combined physico-chemical analysis with biological and ecotoxicity testing. Based on the chemical analytical, bacterial and plant toxicity test results, including plant accumulation, the alkaline fly-ash treatment showed the highest efficiency for the mine-waste-contaminated Gyöngyösoroszi soil. The decrease in the mobile Cd and Zn concentrations exceeded 99% in the water extracts and 45-50% in the acetate extracts of the soil. A combination of non-alkaline fly-ash and lime was as effective as alkaline fly-ash. The mixture of lignite, alginite, hydrated lime and raw phosphate was also advantageous over the long term, resulting in a 68% decrease in the acetate and a 99% decrease in the water extract.

Key words: fly-ash, microcosm testing, remediation, soil, stabilization, stabilizing agents, toxic metals