Influence of pyrolytic residence time on Caesalpinia pulcherrima seed biochar for optimized removal of crystal violet and lead ions from simulated textile effluent

The influence of pyrolytic residence time (1–5 h) at 800 °C on the textural properties of Caesalpinia pulcherrima seed biochar (CPSB 1–5 ) was investigated using nitrogen adsorption–desorption isotherms. The biochar produced after 4 h (CPSB 4 ) exhibited the highest surface area (S BET = 1035.26 m 2 g −1 ) and was selected for the removal of crystal violet dye (CVD) and Pb 2+ from simulated textile effluent. CPSB 4 was characterized using BET, SEM, EDX, and FTIR analyses. Under optimal conditions (70 min, 50 mg L −1 , pH 5.4, 30 °C), removal efficiencies of 98.20% for CVD and 94.25% for Pb 2+ were achieved. Equilibrium data followed the Freundlich isotherm (SNE = 1.109) for CVD and the Langmuir isotherm (SNE = 1.182) for Pb 2+ , while the Dubinin–Radushkevich isotherm indicated physisorption with low mean adsorption energies (<8 kJ mol −1 ). Adsorption kinetics were best described by the Pseudo-second-order kinetic model (SNE = 1.0701, 1.1797). Diffusion analysis using the Weber–Morris intraparticle diffusion model revealed a multi-stage adsorption process with relatively high values of boundary layer thickness and a deviation of the plots from the origin, indicating that intraparticle diffusion was not the sole rate limiting mechanism. The Boyd kinetic model indicated that liquid-film diffusion predominantly controlled the adsorption rate since the plots did not pass through the origin and the intraparticle diffusion coefficients (2.4 × 10 −7 cm 2 s −1 for CVD and 1.9 × 10 −7 cm 2 s −1 for Pb 2+ ) fall within the typical adsorption diffusion range. Thermodynamic parameters (ΔH° > 14 kJ/mol, ΔG° > 2.6 kJ/mol, ΔS° > 0.039 kJ/mol· K) confirmed an endothermic process with increased randomness at the solid–solution interface. CPSB 4 retained over 80% removal efficiency after four regeneration cycles. Optimization using RSM with CCD validated the predictive model and demonstrated that CPSB 4 is a reusable and eco-friendly adsorbent for treating textile wastewater containing mixed contaminants. • Caesalpinia pulcherrima seeds were pyrolyzed to produce high-surface-area biochar. • Biochar showed strong adsorption for crystal violet dye and Pb(II) ions. • Adsorption parameters optimized via central composite design of RSM. • Freundlich and Langmuir isotherms confirmed multilayer adsorption behavior. • Biochar retained >80% removal efficiency after four reuse cycles.