Production of microalgal-based carbon encapsulated iron nanoparticles (ME-nFe) to remove heavy metals in wastewater

Abstract

The integration of microalgae-bacteria consortia within existing wastewater treatment plants as alternative biological treatment could be an interesting option to improve the sustainability of these facilities. However, the fate of the produced biomass is decisive to make that option economically attractive. The present study aimed to valorize the microalgae grown at a pilot scale and used for the treatment of the centrate from municipal sewage sludge, producing microalgal-based iron nanoparticles (ME-nFe), by hydrothermal carbonization. The final product had high carbon content, strong sorbent power, and reducing properties, due to the presence of zerovalent iron. Different synthesis conditions were tested, comparing iron (III) nitrate nonahydrate (Fe (NO ₃) ₃·9H ₂O) and ammonium iron (III) sulfate dodecahydrate (NH ₄ Fe (SO ₄) ₂·12 H ₂O) as iron sources, four different Fe/C molar ratios (0.02, 0.05, 0.1, 0.2), and three process temperatures (180, 200, 225 °C). Based on the characterization of all the prototypes, the best one (having a specific area of 110 m ²g ⁻¹) was chosen and tested for the removal of selected heavy metals by Jar tests. The removal of copper, zinc, cadmium, and nickel from the treated effluent from the wastewater treatment plant was 99.6%, 97.8%, 96.4%, and 80.3%, respectively, also for very low starting concentrations (1 mg L ⁻¹ ). The removal of total chromium, on the contrary, was only 12.4%. Thanks to the magnetic properties, the same batch of ME-nFe was recovered and used effectively for three consecutive Jar tests.