Solidification and ultimate disposal options of heavy metal sludge

Abstract

This research evaluated the feasibility of three different disposal options such as landfill, constructive reuse, and ocean disposal for the solidified materials containing synthetic heavy metal sludges - Cu and Cr. Three factors of binder/sludge (B/S) ratio, fly ash/binder (F/B) ratio, and sodium silicate/sludge (Si/S) ratio, were investigated in terms of physical and leaching properties. Considering the actual disposal environment, three different leachants, namely, acidic water, distilled water, and sea water were used in 24-hr extraction and dynamic leach test. Due to the cement hydration inhibition of cu sludge, the physical properties of solidified materials were significantly affected while negligible effect was observed in Cr sludge. Leaching characteristics depended mainly on the type of leachant, and heavy metal concentrations leached in 24-hr extraction were well matched with their solubility. Fly ash addition was to deteriorate the overall properties of solidified materials, but use of sodium silicate solution had no significant effect. A leachability comparison indicated that acidic water is the most aggressive toward leaching the solidified matrices and distilled water is slightly more aggressive than sea water. During dynamic leaching periods, almost matrix constituents of solidified materials leached out under acidic water indicating that leaching mechanisms were controlled by matrix decompositions rather than by bulk diffusions. Under distilled water and seawater, dynamic leaching of solidified haevy metal sludges seemed to be controlled by bulk diffusions. Leachate alkalinity under seawater decreased less than leachant alkalinity, which would be caused due to carbonate influx and magnesium hydroxide formation onto solidified matrices. Results of triplicate tests and statistical analysis indicated that B/S ratio had he greatest effect on physical properties while the type of leachant on leaching properties. Multiple linear regression was perfor...