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In this paper, we report on the extraction of zinc, lead and arsenic from goethite sludge, a waste material produced by the Zn refining industry. Goethite sludge is an iron-rich waste material which is currently landfilled due to the high concentrations of heavy metals (Zn, Pb, As, etc.), which labels the waste as hazardous. Valorisation of goethite sludge as a secondary raw material is challenging since many heavy metals need to be removed form its matrix, before this matrix material could be reused (e.g. as Fe-ore or pigment material) or more safely stored. In this work, microwave assisted alkaline roasting of the waste material was investigated in order to convert the stable franklinite (ZnFe2O4) spinel into more soluble ZnO, which could be leached, together with Pb and As, in a subsequent water-based leaching step. Furthermore, this study yielded insights in the intricate chemical conversions of different metal species as a function of the applied alkali source (NaOH or KOH). In particularly, the influence of Mn conversion during KOH roasting and subsequent leaching as permanganate has been shown to influence the Zn leachability. Finally, the application of microwave power instead of conventional heating has shown to lower the roasting time and overall furnace temperature, because the samples absorbed MW power and locally heated to much higher temperatures than the set furnace temperature.

Secondary treatment of heavy metal bearing solutions requires highly expensive procedures including the application of ionic exchange resins or activated carbon packed in fixed bed reactors. The use of nanoparticles with magnetic properties as adsorbents can improve metal removal performances allowing for the achievement of high specific surface area. In addition, the simplification of the final solid-liquid separation by magnetic field can avoid the application of packed bed columns. In this study a simple synthetic pathway was optimized to produce copper nanoferrites (CuFe2O4), stable in water, magnetically active and with high specific area, to be further used as sorbent material for heavy metal removal in water solution. The hydrothermal route included surfactant-assisted coprecipitation (performed at different pH), hydrothermal treatment (1h at 120°C), washing with water and hexane, drying, and sintering (performed at 100 and 200°C for 1h). Structure and sizes of CuFe2O4 crystallites were studied as function of coprecipitation pH (8, 10, and 12.5) and sintering temperature (100-200°C). CuFe2O4 powders were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), Brunauer-Emmett- Teller (BET) analysis of porosimetric data. Releasing tests of Fe and Cu at different pH were performed to define the pH range of stability in water. Potentiometric titrations were performed to determine the net charge depending on bulk solution pH. Best samples in terms of magnetic characteristics were obtained at pH 12.5 not depending on the sintering temperature. Mean size of nanoparticles obtained in such conditions was estimated by SEM images as 35-45 nm. BET analysis gave specific surface area of 147.8 ± 0.2 m2/g. CFNs have shown chemical stability in water solutions from pH 6 to 10. Zero charge point was estimated as pH 5.5. Then in the stability range of pH, CFNs present negative surface charge being able to coordinate positively charged heavy metal species.