Kinetics and modeling of trace metal leaching from bottom ashes dominated by diffusion or advection

Leaching kinetics of trace metals from incineration bottom ashes (IBA) under diffusion and advection were investigated through leaching tests of compacted granulars of IBAs and their packed columns with seawater eluent for 64 days and 26 days, respectively. Metal fluxes were distinct among species while linearily decreased at log-log scales as a function of time. Short-term environmental risks for Cu, Ni and Pb were identified under advection. The metal leaching behavior generally followed the pseudo-second order under diffusion, while the pseudo-first order kinetics under advection. Investigated metals may be further identified as diffusion- (As, Cd, Cr, Sb) and advection-dominant species (Ba, Cu, Ni, Pb, Zn) according to their fluxes, which interestingly corresponded to the low- (5.19-147.90 mg·kg^-1) and high-value (116.46-2398.44 mg·kg^-1) of their metal distribution from IBAs, respectively. Considering the general higher metal release, decay models were employed to simulate the column leaching results. Particularly, Type-II model based on two-site assumptions fit much better to the experimental data, unveiling significant yet retarded release (in 1-2 pore volumes) of certain metals from the slow-reaction sites. Further investigation on the release of bulk parameters unveiled that, there existed rebounded leaching rates primarily ascribed to the IBA heterogeneity.