Immobilization of Lead in Fly Ash by Phosphate Compounds

At the present time, organic chelating agents are mainly used as a method for immobilizing harmful metals in fly ash at waste treatment facilities. However it is a problem that lead may be re-eluted by chemical decomposition of the chelate complex in a long-term landfill period. In this study, immobilization of lead in fly ash from a general waste was carried out by addition of inorganic compounds such phosphates. We also conducted a long-term stability test assuming a long-term landfill period, and a pH dependence test assuming pH changed of soil. In the result, it was found that potassium dihydrogen phosphate and sodium dihydrogen phosphate dihydrate had high immobilizing ability, because they had much PO43- released by hydrolysis in elution pH 6 - 9. In the simplification of the system using the model compound containing lead chloride in place of fly ash, where the Pb concentration is assumed to be 10 times higher, Pb elution concentration is less than 0.01 mg L-1 in the model compound mixed with potassium dihydrogen phosphate. It was found out that the potassium dihydrogen phosphate had the highest immobilizing ability in elution pH 6 - 10.