Closing the loop to minimize the generation of liquid waste: recycling phosphorus species from solid residue to activate with H3PO4 for next-round activating biomass

Abstract

After H₃PO₄ activation, post-treatment of activated carbon (AC) via water washing generates liquid waste. H₃PO₄-derived phosphoric oxides can be converted back to H₃PO₄ in the washing filtrate, which could subsequently be recycled for further use. This hypothesis was verified by conducting poplar activation with H₃PO₄ and a second-round activation with the washing filtrate. The results showed that filtrate activation could facilitate the formation of AC (yields: 39.1% at 500 °C versus 28.2% from pyrolysis). The recycled P-containing species in the washing filtrates promoted the development of micropores at 500 °C (S_BET: 847.7 m² g⁻¹ versus 952.6 m² g⁻¹ from H₃PO₄ activation) or 700 °C (823.4 m² g⁻¹ versus 948.3 m² g⁻¹ from H₃PO₄ activation). In situ IR characterization confirmed the conversion of H₃PO₄ to H₄P₂O₇, HPO₃, PO₂, P₂O₅, C–O–P, and P–O–P species. Oxidation of the carbon skeleton led to contraction of the carbon framework and collapse of mesopores into micropores, but rendered a hydrophilic surface. The use of the washing filtrate for activation reduced the overall environmental impact.