Optimized valorization of spent coffee grounds into biochar for lead removal using Box–Behnken design

Spent coffee grounds (SCGs), representing a significant source of food waste, were transformed into functional biochar for the sustainable removal of lead (Pb 2+ ) from water systems. Utilizing a Box–Behnken design (BBD) within the response surface methodology framework, the parameters for pyrolysis, namely, temperature, heating rate and heating time were refined to enhance adsorption efficiency. The optimal parameters identified were 469.3 °C, 15.9 °C per min, and 1.28 h, producing biochar (O-SCGBC) that achieved a 97.5 % Pb 2+ removal rate and an adsorption capacity of 4.85 mg/g, aligning closely with predicted results (98.3 %, 4.91 mg/g). Adsorption processes adhered to both pseudo-first- and pseudo-second-order kinetics (R 2 ≈ 0.99) and were consistent with the non-linear Langmuir isotherm (R 2 = 0.98), indicating chemisorption occurring in a monolayer. Detailed characterization (BET, FE-SEM-EDS, FTIR, XRD) showed increased porosity, surface oxygenation, and the formation of Pb complexes on O-SCGBC compared to non-optimized materials. A continuous syringe-pump system confirmed equilibrium kinetics under continuous flow conditions, bridging the gap between lab optimization and practical application. Multilinear regression analysis independently validated the significance of the parameters (R 2 = 0.65) and the model's robustness. This integration of statistical design, mechanistic validation and continuous-flow testing offers a scalable method for converting coffee waste into effective, low-carbon adsorbents. This work advances circular-economy strategies for heavy-metal remediation by optimizing waste-derived biochar using a data-driven approach. • Optimized SCG biochar exhibited high Pb 2+ adsorption efficiency in aqueous solution. • Adsorption data for O-SCGBC fit the PFO and PSO kinetic models with r 2 = 0.99. • The multilinear regression model predicted Pb 2+ removal efficiency (R 2 = 0.65) based on heating temperature, rate, and time.