Impact of organic carbon on the leachability of vanadium, manganese, iron and molybdenum from shale residues

•Addition of wood chips to shale residues alters the composition of produced leachates.•Mixing shale residues and wood chips decreased V and Mo in the leachates.•Covering shale residues with wood chips increased V and Mo in the leachates.•Overgrowth of shale residues may increase environmental impact from V and Mo.

From 1942 to the 1966, oil was produced by pyrolysis of shale, in Kvarntorp, Sweden. This generated some 40 million m3 of metal rich pyrolyzed shale and discarded fines that were piled on site with its original metal content almost intact. The present study focuses on the leaching of vanadium, manganese, iron and molybdenum from fines after addition of wood chips and steel slag, in outdoor 1 m3 reactor systems at low liquid to solid ratio, in order to evaluate the potential environmental impact and recovery of the elements from the leachates. Seasonal variations were observed, with increased leaching during peak summer. For vanadium and molybdenum, high addition of wood chips decreased the leaching, probably due to adsorption. Manganese showed the opposite behavior while leaching of iron was almost independent of the amount of wood chips. Depending on the systems, up to 2200 μg L−1 vanadium, 90 μg L−1 molybdenum, 25 mg L−1 manganese and 500 mg L−1 iron was found in the aqueous phase. Applied to the 40 million m3 pile, the annual leaching of those elements may reach 14 ton, 0.6 ton, 200 ton and 2400 ton, respectively.