HEAVY-METAL STABILIZATION IN MUNICIPAL SOLID-WASTE COMBUSTION DRY SCRUBBER RESIDUE USING SOLUBLE PHOSPHATE

The mechanisms of heavy metal stabilization of calcium-based dry scrub ber residue using soluble PO43- were investigated. This stabilization technology is presently used in the U.S. and Japan to reduce metals le aching from municipal solid waste combustion residues. At an experimen tal dose of 1.2 mol of H3PO4/kg of residue and using a relatively dry mixing system, the reduction in the operationally defined fraction ava ilable for leaching (using the Dutch Total Availability Test) is 38% f or Cd, 58% for Cu, 99% for Pb, and 28% for Zn. pH-dependent leaching ( pH 4, 6, 8) showed that the treatment was able to reduce equilibrium c oncentrations by 0.5-2 log units for many of these metals, particularl y Pb and Zn. Depth profiling of particles using secondary ion mass spe ctroscopy suggests that stabilization is by precipitation of metal pho sphate reaction products rather than by adsorption of metals to phosph ate particle surfaces. Bulk and surface spectroscopies show that the i nsoluble reaction products are nanometer-sized, crystalline and amorph ous calcium phosphates, tertiary metal phosphates, and apatite family minerals. The geochemical thermodynamic equilibrium model MINTEQA2 was modified to include both extensive phosphate minerals and simple idea l solid solutions for modeling pH-dependent solid phase control of lea ching. Both apatite family and tertiary metal phosphate end members an d ideal solid solutions act as controlling solids for Ca2+, Zn2+, Pb2, Cu2+, and Cd2+. The prevalence of small, nanometer-sized reaction pr oducts suggest that Ostwald ripening and precipitate maturation has no t completely occurred during initial mixing. Nevertheless, soluble pho sphate is an effective stabilization agent for divalent heavy metals i n waste materials such as scrubber residues.