Co-Hydrothermal Carbonization of Sewage Sludge and Waste Pickling Acid to Produce a Novel Adsorbent for Hydrogen Sulfide Removal From Biogas

In this work, a novel approach to the co-hydrothermal carbonization (HTC) process is presented. A laboratory-scale autoclave reactor was used as an open-loop recycling station (OLORS) operating with an unusual blend formed by sewage sludge and an acidic residue from the electroplating industry with a high concentration of iron and zinc chlorides. An in situ impregnated hydrochar was successfully produced and evaluated as adsorbent in H₂S removal from a synthetic biogas achieving performance of up to 65% in reducing H₂S concentration after 260 min of contact in a static system. The standard gaseous mixture containing 3000 ppmv of H₂S was placed into contact with the powdered hydrochar in a batch system, and the decrease in the area ratio between H₂S area peak and total area peak was monitored by gas chromatography as a function of time. The best performance was achieved by the hydrochar HC-5 and may be related to the type of chemical species formed on the surface of the adsorbent. The hydrochars were characterized by XRD, SEM/EDS, N₂ adsorption–desorption, and elemental analysis (CHN). Although the hydrochars did not present a high specific area (3.0–22.0 m² g⁻¹), in general, the massive deposition of iron and zinc chlorides on its surface, reaching up to 13.6 and 6.41 wt.%, respectively, rules the performance of the novel adsorbents. The OLORS approach highlights some aspects of sustainability due to the co-processing of two complex residues to produce a material capable of removing H₂S from a synthetic biogas mixture in a static batch system.