Comparative Study on the Performance and Mechanism of Adsorption-Oriented Phosphorus-Modified High-Efficiency and Durable Activated Biochar from Fast Pyrolysis

Extensive research has demonstrated the advantageous utilization of medium-low temperature fast pyrolysis (FP) for biomass, yielding high-grade liquid-phase chemicals or fuels. However, the field of FP-based high-performance solid biochar research still presents several gaps. Herein, a one-step versus two-step method for biomass H3PO4 activation under FP was comparatively analyzed for the first time, and efficiently activated carbons (ACs) for dye removal were successfully synthesized at a low temperature (723 K). Investigation of methylene blue (MB) adsorption revealed that the one-step sample P-H-0.5, possessing a specific surface area of 1004 m(2)& BULL;g(-1), exhibited a remarkable adsorption capacity of 695.54 mg & BULL;g(-1) with an ultra-high removal rate (99.94%, C-0 = 150 mg & BULL;L-1). The two-step sample P-2-H-2, a modified byproduct of FP, achieved efficient dye adsorption in the shortest time (2 min, 383.91 mg & BULL;g(-1)). This originated from the well-developed surface macropores and elevated group content derived from phosphorus (P)-modification. Both adsorption data were well-fitted with pseudo-second-order kinetics and the Langmuir model, revealing the presence of chemical effects and the dominance of monolayer adsorption. A more detailed kinetic study suggested intrapore transport primarily governed the adsorption process on P-H-0.5, whereas P-2-H-2 relied on surface diffusion. FTIR and XPS revealed notable differences in the active sites between the two methods. Aside from -OH, -COOH with C-O-P, the P elements of P-H-0.5 were classified as C-P-O3 and C2-O-P2, demonstrating the ability of one-step FP to introduce heteroatoms into carbon defects. The basic interactions of ACs with MB were & pi;-& pi; stacking and hydrogen bonding established by -OH-containing groups. At a suitable pH (>5), most H+ was removed from the surface, and the electrostatic attraction became the strongest linking force. Both ACs exhibited exceptional reusability, with removal rates surpassing 90% of the initial rate after four cycles of regeneration.