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The present work deals with developing a method for revalorizing steel residues to create sunlight-active photocatalysts based on iron oxides. Commercial-grade steel leftovers are oxidized under different combinations of pH and temperature (50–90 °C and 3 ≥ pH ≤ 5) in a low energy-intensive setup. The material with the highest production efficiency (yield > 12%) and magnetic susceptibility (χm = 387 × 10−6 m3/kg) was further explored and modified by diffusion of M2+ (Zn and Co) ions within the structure of the oxide using a hydrothermal method to create ZnFe2O4, CoFe2O4 and combined Co–Zn ferrite. (Co–Zn)Fe2O4 displayed a bandgap of 2.02 eV and can be activated under sunlight irradiation. Electron microscopy studies show that (Co–Zn)Fe2O4 consists of particles with diameters between 400 and 700 nm, homogeneous size, even distribution, and good dispersibility. Application of the developed materials in the sunlight catalysis of black liquors from cellulose extraction resulted in a reduction of the Chemical Oxygen Demand (− 15% on average) and an enhancement in biodegradability (> 0.57 BOD/COD) after 180 min of reaction. Since the presented process employs direct solar light, it opens the possibility to large-scale water treatment and chemical upgrading applications. [ABSTRACT FROM AUTHOR]

Copper (Cu) is a low cost and efficient filler for nanofluids. However, commercially available copper nanopowders are as costly as silver. A few methods are available for synthesis of copper nanopowder in the lab, but the amount of powder produced per batch is very small, and it is impractical to produce a large volume of nanofluid using these methods. Solution based high concentration synthesis for production of copper nanopowder using batch reactor has been reported in this work. Cost of production is minimal in this method and nanoparticles are separated from the suspension by a controlled flocculation process. The proposed method relies on the higher solubility of cuprammonium complex in an aqueous medium and its chemical reduction using hydrazine hydrate. The colloid has been stabilised using sodium citrate and obtained as sub 50 nm powder re-dispersible in water and ethylene glycol (EG). Continuous synthesis of this product using a Continuous Stirred Tank Reactor (CSTR) has also been demonstrated. The quality of particles from the CSTR is similar to that of the batch method. Stable nanofluids have been prepared by re-dispersing the nanopowder in water and EG. Moderate enhancement (∼5%) in thermal conductivity for Cu-EG nanofluids and small enhancement (∼2%) for Cu-water nanofluid have been observed with 0.6 vol% of copper loading. [ABSTRACT FROM AUTHOR]