Abstract: Bottom and fly ashes from a hazardous waste incineration plant, which showed a high concentration of Mo in a leaching test, have been treated to decrease its leachability to below the limit set by the laws in force for admittance to a non-hazardous waste landfill. Both ashes have a basic pH, that is, of around 11–13, therefore most of the molybdenum species were expected to be in the form of molybdate ions (MoO42−), the most soluble molybdenum compound. The aim of the study was to immobilize the molybdate ions using the reducing properties of the Fe2+ ion, in compliance with specific pH and redox potential conditions, which resulted to be crucial for the partial stabilization and immobilization of Mo. The bottom ash was treated both by adding a cost-effective ferrous sulfate-based additive alone and by adding it together with calcium oxide. The fly ash was initially tested with the same ferrous sulfate-based additive; a mix of the former additive and a concentrated solution of sulfuric acid (96%) was then tested to increase the stabilization efficiency. The conducted experiments revealed that the molybdenum leaching behavior can be controlled efficiently by adding ferrous sulfate, under specific pH and redox potential conditions, thereby providing further insight into the sustainable chemical conditions necessary to partially stabilize Mo-containing hazardous waste. Graphic Abstract: [Figure not available: see fulltext.]. © 2020, Springer Nature B.V.

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