An inter-loop approach for hydrothermal carbonization of sewage sludge to produce hydrochars and their use as an adsorbent for iron removal from spent sulfuric acid

This work presents an inter-loop approach as an environmental solution in which the hydrothermal carbonization (HTC) of sewage sludge allows the production of hydrochars capable of removing iron ions, which usually harm the lead-acid batteries performance, from spent sulfuric acid. The HTC process was performed at three different water/biomass ratios under mild experimental conditions, and except for water, no additional chemical input was used. The moisture content of the sludges was set to range between 76 and 91 wt.%. Hydrochars were characterized by XRD, FTIR, SEM, TGA, and N₂ adsorption–desorption techniques. Results suggest a high dependence of their textural and surface properties on the water/biomass weight ratio inside the reactor. The expressive presence of multiple mineral phases in the sewage sludge allowed the formation of a hydrophilic surface, which was fundamental for the adsorption of iron ions under strong acidic conditions. Porosity was also strongly influenced by the water/biomass weight ratio, with the hydrochar’s surface displaying pore dimensions in nano- and micro-domains. Furthermore, the hydrochar presented an adsorption capacity up to 148 mgFe g⁻¹ without any activation step, whereas the ordinary commercial activated carbon achieved 178 mgFe g⁻¹. Results show the potential of the HTC process for sewage sludge conversion into hydrochars without pre-drying, and the possibility of interconnecting two or more industrial processes in order to make them cleaner and more sustainable, matching the principles of the circular economy.