The dissolution kinetics of major elements in municipal solid waste incineration bottom ash particles

Leaching and tracer experiments in batches at L/S 20 were performed with 3-month-old MSWI bottom ash separated into eight different particle sizes. The time-dependent leaching of major elements (Ca2+, K+, Na+, Cl− and SO4− 2) was monitored for up to 747 h. Physical properties of the particles, the specific surface (BET), pore volume and pore volume distribution over pore sizes (BJH) were determined for all particle classes by N2 adsorption/desorption experiments. Some common features of physical pore structure for all particles were revealed. The specific surface and the particle pore volume were found to be negatively correlated with particle size, ranging from 3.2 m2/g to 25.7 m2/g for the surface area and from 0.0086 cm3/g to 0.091 cm3/g for the pore volume. Not surprisingly, the specific surface area was found to be the major material parameter that governed the leaching behavior for all elements (Ca2+, K+, Na+, Cl− and SO4− 2) and particle sizes. The diffusion resistance was determined independently by separate tracer (tritium) experiments. Diffusion gave a significant contribution to the apparent leaching kinetics for all elements during the first 10–40 h (depending on the particle size) of leaching and surface reaction was the overall rate controlling mechanism at late times for all particle sizes. For Ca2+ and SO4− 2, the coupled effect of diffusion resistance and the degree of undersaturation in the intra particle pore volume was found to be a major rate limiting dissolution mechanism for both early and late times. The solubility control in the intra particulate porosity may undermine any attempt to treat bottom ash by washing out the sulfate. Even for high liquid/solid ratios, the solubility in the intra-particular porosity will limit the release rate.