The application of circular economy principles to the sustainable management of waste from the pulp industry presents significant environmental challenges. In this context, using biological sludge as a raw material for producing activated biochar (BC) emerges as a promising and sustainable alternative. This study evaluated the valorization of biological sludge through the synthesis of activated BC for the removal of color, chemical oxygen demand (COD), and conductivity from the industry’s effluent. BC was produced using chemical activation with phosphoric acid (H₃PO₄) and potassium hydroxide (KOH), followed by pyrolysis at 500 °C and 450 °C, respectively. A central composite rotational design (CCRD) was applied to optimize the process. The optimized BCs were characterized by proximate analysis, FTIR, BET surface area, higher heating value (HHV), and SEM. Adsorption assays showed that H₃PO₄-activated BC achieved removal efficiencies of 52.2% for color, 23.9% for COD, and 46.2% for conductivity at a dosage of 5 g L⁻¹. Conversely, KOH-activated BC did not perform effectively. The results highlight the influence of activation and pyrolysis on BC properties and confirm the potential of this approach for the tertiary treatment of industrial effluents, contributing to waste valorization and environmental sustainability.

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 (R ² = 0.996, R ² _adj = 0.995, and χ² = 1.62). The adsorption kinetics followed the pseudo-second-order model (R ² = 0.982, R 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.

Sewage sludge serves as a promising raw material for producing activated biochar (BC), offering both environmental and economic benefits. This work aimed to optimize the synthesis of biochar (BC) from cosmetic industry sludge using response surface methodology (RSM) with central composite rotational design (CCRD) and to assess its effectiveness in removing the antibiotic enrofloxacin (ENR) from synthetic effluents. The input variables considered were (i) residence time in the reactor and (ii) impregnation ratio (NaOH/biomass). The response for optimization was the iodine number. The sludge underwent pyrolysis at 550 °C under a nitrogen atmosphere. The optimized biochar (OBC) had a macroporous structure with a specific surface area of 46.1 m2 g–1 and a calorific value of 4146 kcal kg–1. The bio-oil contained nitrogenous compounds and alcohols. The effectiveness of the OBC in adsorbing ENR in aqueous systems was assessed. Kinetic assays showed an equilibrium time of 90 min, with the pseudo-second-order model best fitting the experimental data. The Langmuir isotherm model best described the adsorption (qmax = 15.83 mg g–1). The material is competitive with commercial adsorbents. This work underscores the potential of industrial sludges as a sustainable source for BC, contributing to pollution reduction and promoting circular economy practices.

The catalytic hydrolysis of ammonia borane represents a pathway for controlled hydrogen evolution. To address this, palladium nanoparticles supported on porous biochar derived from zinc chloride-activated banana pseudostem agricultural waste (BC-PdNPs) were prepared and evaluated. Material characterization was performed using multiple techniques. FTIR analysis confirmed biomass carbonization with the retention of residual oxygenated functional groups, while Raman spectroscopy revealed a disordered carbon matrix with an I_D/I_G ratio of 1.03. Surface charge characterization via zeta potential measurements showed an increase in negative charge density under alkaline conditions. X-ray diffraction indicated an amorphous carbon structure and confirmed palladium deposition via a characteristic peak at 40.1 degrees. SEM–EDS and TEM analyses showed excellent metal dispersion, displaying crystalline face-centered cubic metallic palladium nanoparticles with an average size of 3.92 nm. Nitrogen physisorption isotherms confirmed a mesoporous structure with a specific surface area of 1493.09 m² g⁻¹. XPS analysis corroborated that metallic Pd⁰ was the predominant surface species along with minor Pd²⁺ contributions. Multivariate optimization conducted via a Central Composite Rotatable Design established operating conditions at a support dosage of 15.96 mg and a metal loading of 2.81 mmol%, yielding a hydrogen generation rate of 24,396 mL min⁻¹ g⁻¹. Kinetic studies determined an activation energy of 56.98 kJ mol⁻¹, while a primary kinetic isotope effect verified that the reaction rate is governed by water O–H bond cleavage. Furthermore, the catalyst maintained its performance across 12 consecutive reaction cycles. These findings demonstrate the viability of utilizing lignocellulosic agro-industrial residues as functional supports for hydrogen-production nanocatalysts. Graphical abstract