Mixture design optimization considering three disposal options for solifified heavy metal sludges

Abstract

This research evaluated the feasibility of three different disposal options such as landfill, reuse in construction, and ocean disposal for solidified materials containing synthetic heavy metal sludges - copper and chromium. Three factors - binder/sludge, fly ash/binder, and sodium silicate (Na2SiO3) sludge ratios - were investigated in terms of physical and leaching properties. Considering the actual disposal environment, three different leachants, namely, acid water, distilled water, and seawater, were used in the leaching tests. The inhibition of cement hydration by copper sludge significantly affected the physical properties of solidified materials. Fly ash additions deteriorated the overall properties of solidified material, but the use of a sodium silicate solution had no significant effect. Leachate characteristics after 24-h extraction were strongly affected by the nature of leachant, and leached metal concentrations were well matched with theoretical solubility according to leachate pH. A comparison of leaching characteristics indicated that acidic water was the most aggressive toward leaching the solidified matrices and that distilled water was slightly more aggressive than seawater. Leachate alkalinity after seawater leaching decreased less than initial seawater alkalinity, which would be caused by carbonate influx and magnesium hydroxide [Mg(OH)2] formation onto solidified matrices. Results of triplicate tests and statistical analysis indicated that the binder/sludge ratio had the greatest effect on physical properties while the type of leachant had the greatest effect on leaching properties. Mixture design optimization using linear programming was attempted for the economic comparison of three disposal options. As a result, ocean disposal as well as reuse in construction were estimated as the preferred disposal alternatives for solidified heavy metal sludge.