Leaching Study of Thermally Treated Cadmium-Doped Soils

Abstract

Soils doped with cadmium were effectively immobilized by thermal treatment in a bench-scale fluidized- and fixed-bed incinerator. Toxicity characteristics leachability procedure (TCLP) was conducted to evaluate the effectiveness of Cd immobilization by thermal treatment of the doped soils. USEPA SW846 Methods 3050 and 3051 were performed to determine the presumptive total Cd content in the untreated and treated soils. Experimental parameters include thermal treatment temperature and time. Cd content in acidic and noncalcareous soils, sorbent type and amount added, soil particle diameter, and incinerator mode. Results illustrate that Cd in soils of smaller diameter is less effectively immobilized. At a temperature of 600°C, a treating time of 30 minutes, and a Cd content of 91.3 ppm in pre-treated sample, thermal treatment in fluidized-bed mode reduces Cd leaching percentage from 47% and 75% (un-treated soil) to 12 and 22% for soils of particle size 16-20 mesh and 45-50 mesh, respectively Data of treating 150-ppm Cd-doped soils in fluidized-bed mode at 600° for different times, viz., 10, 20, and 30 minutes indicate only a minimal difference in Cd leaching percentage that are between 7.6% and 8.5%. The Cd content in soils does not affect the leaching percentage of fluidized-bed treated soils, while the results of fixed-bed treatments of Cd soils at 600° for 30 minutes show an increase of leaching percentage from 8.8 to 13% as Cd content in soils is increased from 75.0 to 175.0 ppm. The constancy of leaching percentage from soils treated in fluidized-bed mode is obviously due to high degree of mixing. As the treating temperature increases (from 150°C to 750°C ). Cd is better immobilized in mixtures. Excessive addition of sorbents will reduce their adsorption effectiveness of Cd for each unit mass of sorbem. Both the Langmuir and Freundlich isotherms fit the sorption data quite successfully with all correlation coefficients (r) greater than 0.984.