LEACHING STUDY OF THERMALLY TREATED CADMIUM-DOPED SOILS

Soils doped with cadmium were effectively immobilized by thermal treat ment in a bench-scale fluidized- and fixed-bed incinerator. Toxicity c haracteristics leachability procedure (TCLP) was conducted to evaluate the effectiveness of Cd immobilization by thermal treatment of the do ped soils. USEPA SW846 Methods 3050 and 3051 were performed to determi ne the presumptive total Cd content in the untreated and treated soils . Experimental parameters include thermal treatment temperature and ti me, Cd content in acidic and noncalcareous soils, sorbent type and amo unt added, soil particle diameter, and incinerator mode. Results illus trate that Cd in soils of smaller diameter is less effectively immobil ized. At a temperature of 600 degrees C, a treating time of 30 minutes , and a Cd content of 91.3 ppm in pre-treated sample, thermal treatmen t in fluidized-bed mode reduces Cd leaching percentage from 47% and 75 % (untreated 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 degrees C for different times, viz. 10, 20, and 30 minutes indicate only a minimal difference in Cd leaching p ercentage that are between 7.6% and 8.5%. The Cd content in soils does not affect the leaching percentage of fluidized-bed treated soils, wh ile the results of fixed-bed treatments of Cd soils at 600 degrees C f or 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 consta ncy of leaching percentage from soils treated in fluidized-bed mode is obviously due to high degree of mixing. As the treating temperature i ncreases (from 150 degrees C to 750 degrees C). Cd is better immobiliz ed in mixtures. Excessive addition of sorbents will reduce their adsor ption effectiveness of Cd for each unit mass of sorbent. Both the Lang muir and Freundlich isotherms fit the sorption data quite successfully with all correlation coefficients (r) greater than 0.984.