Anion leaching from refinery oily sludge and ash from incineration of oily sludge stabilized/solidified with cement. Part II. Modeling

This paper presents the modeling of anion leaching (SO₄ ^2- and CrO₄^2-) from refinery oily sludge and ash produced by incineration of oily sludge, stabilized/solidified (s/s) with two types of cement, I42.5 and II42.5. Anion leaching was examined using a sequential toxicity characteristic leaching procedure (TCLP) test. To elucidate the mechanisms of sulfate and chromate leaching, we employed Visual MINTEQ, incorporating a multiple-problem setup. Specifically, 10-14 different problems, depending on the pH range of the leachates, were connected together in the same run. Each problem corresponded to one pH value of the leachate and the model run covered the pH range of the five sequential TCLP extractions. This modeling approach was tested using chemical equilibrium with or without sorption onto ferrihydrite. Good agreement between experimental and modeling results was obtained for sulfate leaching from solidified oily sludge and ash, considering surface complexation onto ferrihydrite on top of chemical equilibrium controlled by gypsum atpH <11 and ettringite at pH >11. Chromate leaching was described by chemical equilibrium, controlled by CaCrO₄(s) (at pH <11) and Cr(VI)ettringite (at pH >11).