Enhanced methylene blue adsorption and wastewater treatment using hydrochar derived from urban sludge: Response surface methodology optimization, cost estimation, regeneration, and mechanism study

• Hydrochar was produced from urban sewage sludge via hydrothermal carbonization. • The hydrochar effectively adsorbs methylene blue from aqueous solutions. • Response Surface Methodology (RSM) was used to optimize the adsorption conditions. • Adsorption isotherms and kinetics were analyzed using SSE to evaluate model fitting. Hydrothermal carbonization (HTC) has emerged as a promising and sustainable approach for the valorization of sewage sludge into functional carbonaceous materials. In this study, urban sewage sludge was converted into hydrochar via hydrothermal carbonization and subsequently applied for the removal of methylene blue (MB), used as a model cationic dye, from aqueous solutions and real wastewater. The adsorption process was optimized using a central composite design (CCD) combined with response surface methodology (RSM), considering pH, adsorbent dosage, initial dye concentration, and contact time as independent variables. The optimization results indicated that the maximum adsorption capacity was achieved at ambient temperature, pH 9, a hydrochar dosage of 0.2 g L⁻¹, an MB concentration of 100 mg L⁻¹, and a contact time of approximately 100–120 min, with an experimental adsorption capacity of 360.7 mg g⁻¹, in close agreement with the predicted value (Δ = 2.8%). Adsorption equilibrium was best described by the Sips and Hill isotherm models (R² = 0.99), revealing surface heterogeneity and positive cooperative adsorption, with a high maximum adsorption capacity reaching 718.75 mg g⁻¹. Kinetic studies showed that MB adsorption followed the pseudo-first-order model (R² = 0.999), indicating a physisorption-controlled mechanism, supported by intraparticle diffusion analysis. The hydrochar also demonstrated effective removal of organic matter from real urban wastewater, achieving COD removals of 59.15% after 3 h and 71.24% after 24 h. Regeneration experiments showed acceptable reusability over two adsorption cycles. The characterization of the hydrochar produced by hydrothermal carbonization was conducted using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and differential thermal analysis.