Sustainable Conversion of Pulp Industry Sludge into Activated Biochar for High-Performance Methylene Blue Removal

The pulp and paper industry generates large amounts of biological sludge, which can be valorized into activated biochar (A-BC), offering environmental and economic benefits. In this work, A-BCs were produced from this residue using H₃PO₄ as an activating agent and subjected to different pyrolysis temperatures (400 to 550 °C). H₃PO₄ was selected for its superior activation performance over KOH, enhancing porosity and surface functionalization, while the temperature was chosen to match the main thermal degradation of the sludge's lignocellulosic matrix. The A-BCs were characterized by proximate and elemental analysis (fixed carbon content ∼30%), FTIR (CO, O-H, O-Si-O, and PO functional groups), XRD (predominantly amorphous structure), and Raman spectroscopy (D and G bands). Furthermore, BET surface areas from 7.68 to 1.52 m² g^-1, a higher heating value (HHV) from 3788 to 4750 kcal kg^-1, and a point of zero charge (pH_PZC) from 3.31 to 6.15 were obtained. Increasing the temperature from 400 to 450 °C increases surface area via pore formation, while higher temperatures reduce porosity due to pore collapse and lignin condensation. The A-BC produced at 450 °C (A-BC2) exhibited more than double the surface area and higher methylene blue (MB) removal efficiency than the other samples, consistent with the characterization results. The adsorption assays indicated that the maximum adsorption capacity was 390.73 mg g^-1, with the Langmuir model fitting the experimental data best (<i>R</i> ² = 0.996, <i>R</i> ² _adj = 0.995, and χ² = 1.62). The adsorption kinetics followed the pseudo-second-order model (<i>R</i> ² = 0.982, <i>R</i> ² _adj = 0.981, and χ² = 7.78), indicating a chemisorption-controlled mechanism involving electron sharing or exchange between cationic dyes and oxygenated biochar surface groups. The study demonstrates that A-BC from cellulose industry sludge is a viable, sustainable option for dye-containing effluent treatment, supporting circular economy principles.