The leaching behavior of cement stabilized air pollution control residues:A comparison of field and laboratory investigations

The factors controlling leachate composition of cement stabilized air pollu tion control (APC) residues (41% APC residues, 22% cement, 3% Na2CO3, and 3 2% water, w/w) have been investigated both in the laboratory ansi in a pilo t landfill. Batch leaching and tank leaching tests were carried out in the laboratory in order to determine solubility controlling phases and diffusio n controlled species. The major species Ca, SO4, Al, and Si could be partia lly modeled by assuming calcium silicate hydrate (C-S-H), portlandite, and ettringite to be the solubility controlling phases both in field and labora tory. There were obviously additional minerals that could not be taken into account in calculations because of the lack of data. The determined effect ive diffusion coefficients (D-e) for Na and K (2.18e(-12) and 5.43e(-12) m( 2)s(-1)) were used to model field concentrations. Agreement with field data was good. Heavy metal concentrations were in the range of 10(-8) mol dm(-1 ) (Cd, Co, Cu, Mn, Ni) to 10(-6) mol dm(-1) (Mo, Pb, W, Zn) in all experime nts and often lower in the field leachate than expected from batch experime nts. In laboratory experiments, the solubility of Mo and W was most probabl y controlled by their calcium metalates, Cu by CuO, Ni by Wi(OH)(2), and Zn probably by a Zn containing C-S-H phase. In the field, diffusion seems to control Mo and W leachability, with calculated D,values of 3.49e(-14) and 1 .35e(-15) m(2) s(-1).