Leaching and immobilization behavior of Zn and Cr from cement-based stabilization/solidification of ash produced from incineration

In this paper, stabilization/solidification of ash derived from real refinery oily sludge samples was applied using two types of cement (CEM I42.5N and II42.5N) in various additions. Leaching behavior of Cr and Zn was investigated, by means of a five-point sequential Toxicity Characteristic Leaching Procedure (TCLP) test. Sequential TCLP extractions of incinerated refinery sludge samples resulted in very low leachability ( < 0.2 mg/L at 8.6 < pH < 11.6) of heavy metals (Ni, Pb, Co, Cd, Cu, and Fe) after stabilization/solidification with cement, except for Cr (< 40 mg/L at 8.6 < pH < 12.1) and Zn (< 6 mg/L at 8.6 < pH < 12.1). Furthermore, the leaching behavior of Cr and Zn was modeled using the chemical equilibrium program Visual MINTEQ. For that purpose the Diffuse Layer Model (DLM) coupled with 2-pK formalism was employed to model the suspected formation of CaCrO _4(s), Cr(VI)-ettringite and zinc hydroxide. The results showed that Cr leachability was mainly controlled by the speciation of Cr(VI) in the alkaline matrix of the solidified ash and depended on the dissolution of Cr(VI)-bearing minerals, such as CaCrO_4(s) at 8.7 < pH < 11.5 and Cr(VI)-ettringite at pH > 11.5, as described by chemical equilibrium. Zn leachability in the TCLP extracts was controlled by dissolution of Zn(OH) _2(s) and/or ZnO_(s), in combination with surface complexation and precipitation on a ferrihydrite surface.