Leaching Behavior of Metals Released from Cement-Stabilized/Solidified Refinery Oily Sludge by Means of Sequential Toxicity Characteristic Leaching Procedure

In this study, stabilization/solidification (S/S) of refinery oily sludge was applied using two types of cement (I and II), in cement-to-waste ratio from 0.1 to 0.7. The leaching behavior of heavy metals was investigated, by means of a five-point sequential toxicity characteristic leaching procedure (TCLP) test. Sequential TCLP was used to provide an improved assessment of long-term contaminant potential leachability, because the acidic leachant is renewed, whereas in the single TCLP, contaminant leachability is limited by the pH neutralization of the alkaline binder. Cement-based S/S of real refinery oily sludge resulted in very low leachability of heavy metals. Pb and Cd were not detected in any TCLP leachate. The maximum leachability of Fe, Zn, and Ni occurred in the pH range between 5.5 and 6.5. The leachability of Cu and Cr increased with increasing pH. Maximum cumulative percentage of Fe, Ni, and Zn leaching after five consecutive TCLP extractions (for worst case conditions, i.e., 10% II42.5) were 0.01, 22, and 1.2%, respectively, on the basis of metal content of each solidified sample. Further, the leaching behavior of Zn and Ni was modeled using the chemical equilibrium program Visual MINTEQ. Using several combinations of suspected solid phases of Zn and Ni hydroxides, carbonates and sulfides, and surface complexation onto ferrihydrite the diffuse double layer model did not accurately describe the leaching behavior of Zn and Ni.