Synthesis of hydrochars derived from industrial laundry sludge and its application in the removal of cationic dye

•Tannin-based sludge as precursor of activated carbons was never evaluated before.•The best coagulant dose was determined to produce the sludge and the adsorbents.•Hydrothermal carbonization was used to produce the adsorbents.•The produced activated hydrochar had a high specific area (3005.57 m² g−1).•The adsorbent showed a high methylene blue adsorption capacity (808.83 mg g−1). The textile industry generates large amounts of effluents, often treated by coagulation/flocculation. The tannin-based coagulants produce a high-quality treated effluent and a more biodegradable sludge. Efforts have been made to reuse the sludge, however, the synthesis of hydrochars and activated hydrochars from this residue was never evaluated before. Therefore, this study aimed to produce a novel sludge-based adsorbent, through hydrothermal carbonization (HTC) and KHCO₃ activation, from real laundry wastewater treated with tannin (Tanfloc SG), in order to remove methylene blue (MB) from aqueous solutions. Tanfloc SG coagulant was more effective to reduce color and turbidity than chemical oxygen demand. The best coagulant concentration (140 mg L−1) was selected to produce sludge, the hydrochar (HC-S), and activated hydrochar (AHC-S). The characterization demonstrated that activation volatilized the organic matter, leaving coke and inorganic solids. However, despite the lower carbon content, AHC-S presented a notorious surface area (3005.57 m² g−1). Minerals were presented in the adsorbents and AHC-S showed graphene-like layers. Regarding MB adsorption, pseudo-second-order and Langmuir models fitted kinetic and equilibrium data, respectively. HC-S and AHC-S obtained qmax of 69.77 and 808.83 mg g−1, respectively, and the latter was higher than other activated hydrochars, demonstrating the potential of the adsorbent.