Engineering Pine Wood Biochar via Steam Activation for Enhanced Methylene Blue Adsorption: Linking Surface Reconstruction to Adsorption Performance

The widespread use of methylene blue (MB) has contributed to water pollution. This study evaluates steam-activated pine wood biochar for efficient MB removal from aqueous solutions. Biochar samples were produced by pyrolysis at temperatures ranging from 400 to 850 °C (BC4–BC8.5), followed by steam activation at 850 °C to prepare steam-modified biochars (SM-BC4–SM-BC7). The physicochemical properties of the biochars were characterized, and batch adsorption experiments were conducted to investigate adsorption equilibrium, kinetics, and mechanisms. Steam activation markedly enhanced the specific surface area from 321.32 to 635.20 m2 g−1 for SM-BC7, while improving pore development, promoting partial graphitic ordering, and inducing a more negative surface charge, as indicated by the zeta potential of −32.84 mV before adsorption, which shifted to −7.51 mV after MB adsorption. Among all samples, SM-BC7 exhibited the highest removal efficiency. The adsorption process was best described by the Langmuir isotherm, yielding a maximum adsorption capacity (Qmax) of 133.50 mg g−1. Among the kinetic models evaluated, the pseudo-second-order (PSO) model provided the best fit to the experimental data. Overall, steam-activated pine wood biochar showed strong adsorption performance for the removal of MB from aqueous solutions, highlighting its potential as an adsorbent for dye removal.