The role of sawdust Ca-biochar on the phosphate adsorption: an optimization through a 2³ experimental design

Phosphorus (P) is a vital element for sustaining life; however, its excessive discharge into aquatic systems has accelerated eutrophication, while global reserves of phosphate rock approach depletion. Developing efficient materials for phosphate removal and recovery is essential to ensure food and water security. In this study, pine sawdust, an abundant industrial by-product in Brazil, was converted into calcium-modified biochars using both batch (tubular furnace) and continuous-flow pyrolysis. A 2³ factorial design was applied to optimize modification parameters (time, concentration, and biomass mass). Phosphate removal efficiencies ranged from 13 to 97%, with the statistical model explained 99% of the response variation. Characterization by SEM/EDS and BET revealed fissures, porosity, and effective Ca incorporation, while FTIR identified persistent oxygen-containing functional groups (-OH, C = O, C-O-C), Raman spectroscopy indicated partially graphitized carbon structures with defect-rich surfaces (I_D/I_G = 2.01-2.62), and XRD confirmed the presence of CaO, Ca(OH)₂, and CaCO₃. Equilibrium data fitted Langmuir, Freundlich, and Sips models, with maximum adsorption of 158.4 mg g⁻¹. The combined characterization and adsorption results indicated that phosphate removal occurred predominantly through calcium-mediated mechanisms, including surface complexation and calcium-phosphate precipitation. Biochars produced in the pilot-scale continuous-flow furnace exhibited adsorption efficiencies comparable to those obtained in the laboratory-scale tubular furnace, demonstrating the feasibility of continuous production while maintaining adsorption performance. The results highlight calcium-modified sawdust biochars as promising materials for phosphate removal and recovery, while the formation of phosphorus-enriched biochars creates opportunities for nutrient recycling within a circular economy framework.