Sustainable Black Liquor‐Derived Magnetic Hydrochar/Chitosan Composite Hydrogel for Naphthol Green Dye Elimination: Kinetics, Rheology, and Adsorption Mechanisms

In this study, the magnetic hydrochar particles (MHPs) were prepared from industrial waste black liquor (hydrothermal method) and used to synthesize a sustainable magnetic hydrochar/chitosan composite hydrogel (MHCH), and were tested for the removal of naphthol green (NG) dye from aqueous solutions. Structural and physicochemical studies revealed the successful incorporation of magnetic hydrochar into the chitosan structure, resulting in a porous and magnetically recoverable adsorbent. As‐prepared MHCH exhibited saturation magnetization of 12.38 emu/g, hydrophobic properties, and improved mechanical stability, which allowed simple separation by an external magnetic field. Rheological studies indicated that the improved MHCH exhibited a higher mechanical strength (∼200,000 Pa) than conventional chitosan hydrogels (CHs), thereby demonstrating MHCH with good structural stability for recycling use. The adsorption studies revealed that the equilibrium adsorption capacity of NG dye was high, 1050 mg/g. The adsorption isotherm model was best fitted by the Freundlich isotherm (multilayer adsorption). Kinetic adsorption was best fitted with the pseudo‐second‐order kinetic model (chemical adsorption occurs). The reusability of MHCH was good, even after ten cycles. After multiple adsorption‐desorption cycles, both rheological and FTIR studies demonstrated the mechanical and chemical stability of MHCH, making it a feasible and sustainable adsorbent for dye wastewater treatment.