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An Inter-loop Approach on Hydrothermal Carbonization of Sewage Sludge for the Production of Biochar and Its Application as an Adsorbent for Lead-acid Battery Recycling


 This work presents an inter-loop approach in which the hydrothermal carbonization (HTC) of sewage sludge allows the production of biochars capable of removing iron ions, which usually harm the lead-acid batteries performance, from spent sulfuric acid without any activation step. The HTC process was performed at three different water/biomass ratios under 180 ºC by 24 h, and except for water, no additional chemical input was used. The moisture content of the sludges ranged from 76 to 91 wt.%. Biochars were characterized by XRD, FTIR, SEM, TGA and N2 adsorption-desorption. Results suggest a high dependence of their textural and surface properties on the water amount inside the reactor vessel. The expressive presence of multiple mineral phases in the sewage sludge allowed the formation of a hydrophilic surface, which was fundamental for the iron ions adsorption at strong acidic conditions. Porosity was strongly influenced by the water/biomass ratio, with biochar’s surface displaying pore dimensions in nano and micro domains. Furthermore, the non-activated biochar presented an adsorption capacity up to 148 mgFe g-1, whereas the commercial activated carbon “as received” achieved 178 mgFe g-1. Results show the potential of the HTC technique for sewage sludge conversion into biochar 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 circular economy.