Post-activation exposes hidden Fe-N sites in sludge-Chlorella biochar for synergistic tetracycline adsorption: A pore-site decoupling strategy.

Conventional biochar modification methods suffer from a fundamental pore-site trade-off: pore-forming treatments (e.g., KOH activation) disrupt pre-loaded metal centers, while metal loading blocks existing pores. Here, we resolve this dilemma by temporally decoupling pore creation from active site exposure. Fe and N species are first pre-loaded into sludge-Chlorella precursors via co-pyrolysis, which embeds them within the carbon matrix but leaves them inaccessible beneath the surface. Then, KOH post-activation chemically etches the carbon framework, simultaneously generating abundant micropores/mesopores (specific surface area increases from 29.3 to 64.2 m2/g) and, crucially, exposing the previously hidden Fe3O4 crystallites and pyridinic N sites to the biochar surface. The resulting KFe2NSCBC achieves a tetracycline adsorption capacity of 193.1 mg/g at 25°C (315.8 mg/g at 45°C), far surpassing single- or dual-modified counterparts. Mechanistic and DFT analyses reveal a clear pore-site synergy: KOH-created pores serve as diffusion highways, while the exposed Fe3O4 and pyridinic N act as high-affinity binding stations for surface complexation and π-π electron donor-acceptor interactions, respectively. This pore-site decoupling strategy-where pore creation and active site exposure are temporally separated-transforms waste-derived biochar from a structure-limited or function-limited material into a high-performance adsorbent, offering a generalizable design principle for upgrading biomass waste into value-added environmental materials.