From source to sink via a ferronickel smelting process: Chromium, nickel, and mercury in sediments of the Evoikos Gulf, Greece

Ferronickel smelters are highly waste intensive industrial processes. In the Larymna Bay (LB) of the Evoikos Gulf (Greece), the slag produced after the processing of lateritic ores for nickel extraction used to be discharged to an authorized marine area (slag disposal area; SDA). This study assessed the distribution of Cr, Ni, Fe and Mn, largely related to ultramafic parent rocks, laterites, and the slag, as well as Hg as a tracer of combustion processes in the sediments of Evoikos Gulf. A very severe (Cr) and moderate (Ni) enrichment was found both in LB and SDA in relation to local background levels. Mercury was severely enriched in LB. LB becomes enriched with magnetic particles and metals through washout and deposition of dust. Magnetic susceptibility was found to be a good proxy for Cr and Ni distributions, as well as the dispersal of slag because both parent rocks and slag contain magnetite-like magnetic minerals. The strong tidal currents of the area do not cause the dispersal of slag particles southwards, beyond SDA, yet they are responsible for the accumulation and focusing of Hg in the South Evoikos Gulf, close to the Chalkida city. Metals mobilization potential from the slag was higher compared to laterites, but lower compared to SDA's sediments, due to reduction processes in the plant and oxidation after its disposal, respectively. Oxidation processes result in the genesis of Cr(VI) in SDA and LB, through oxidative reactions with Mn oxides. This is the first time (to the best of our knowledge) that the formation of Cr(VI) from slags is reported. This finding is important for settings rich in Mn oxides, receiving Cr-rich wastes from Fe–Ni smelter plants. •Moderate (Ni), very severe (Cr) and severe (Hg) modification of sediments.•Dust fallout is responsible for sediments contamination close to the smelter.•Magnetic susceptibility is a proxy for Cr and Ni spatial distribution.•The potential mobility of metals increases in the slag compared to parent rocks.•Oxidative formation of Cr(VI) in slag and dust deposition areas.