Optimization of pyrolysis temperature and Fe-doped K-feldspar catalysis for enhancing sp²-carbon network and soil CO₂ capture of biochar

This study investigated the synergistic effects of pyrolysis temperature and in situ Fe-doped K-feldspar catalysis on the physicochemical properties and soil CO₂ adsorption kinetics of lignocellulose-derived biochar. The objective was to identify process conditions that optimize biochar yield, enhance CO₂ uptake, and improve carbon stability. Biochars were synthesized at 400–1200 °C with catalyst loadings of 0–2 wt% and characterized using elemental analysis, solid-state ¹³C NMR, XPS, N₂-BET, and CO₂ adsorption isotherm and kinetic models. At 400–600 °C, oxygen-rich biochars displayed chemisorption-dominated pseudo-second-order kinetics with multilayer binding (Freundlich/Sips). At 600–800 °C, Fe catalysis pruned polar groups, enhanced sp²-carbon domains, and shifted kinetics to pseudo-first-order physisorption, consistent with dual-site Langmuir or Toth behavior. At 1000–1200 °C, Fe-induced graphitization and micropore generation led to intraparticle diffusion-controlled uptake, captured by Weber–Morris modeling and equilibrium fitting with Langmuir and Dubinin–Astakhov equations. Mechanistically, Fe³⁺ Lewis acid sites promoted aldol-type condensation, water-gas shift reaction, and dehydration to α,β-unsaturated carbonyls, while Fe²⁺/Fe³⁺ redox cycling enabled single-electron transfer that cleaved C–O bonds in alcohols and ethers, accelerating deoxygenation and aromatization. The resulting sp²-carbon frameworks, enriched with pyridinic, pyrrolic, and phenolic groups, strengthened CO₂ binding via π–quadrupole and acid–base interactions. Multi-response optimization identified 400 °C with 2 wt% catalyst as the best trade-off, producing 43.6 wt% biochar with 108 mg g⁻¹ CO₂ uptake, 49.1 % sp²-carbon, and low O/C (0.23) and H/C (0.54) ratios, alongside co-production of biocrude oil and syngas. This work provides a mechanistic and statistical framework for engineering multifunctional biochars for CO₂ capture and sustainable energy applications.