Co-pyrolysis of pine cones and natural resin: Tailoring microporous architecture and interfacial alkalinity for enhanced heavy metal immobilization

Abstract

This study presents a novel approach for the highly efficient separation of Cu 2+ and Pb 2+ from aqueous phases using advanced activated carbons synthesized via the co-pyrolysis of pine cones and natural resin, followed using CO 2 physical activation. The strategic incorporation of an optimal resin dosage functioned as a structural template, significantly upgrading the porous architecture of the PC/R-0.5 and PC/R-1 samples. This modification maximized the micropore volume up to 0.316 cm 3 ⋅g −1 and expanded the specific surface area up to 858 m 2 ⋅g −1 . Phase-boundary immobilization exhibited rapid mass transfer, adhering precisely to the pseudo-second-order kinetic model and Sips isotherm. Mechanistic insights revealed that the resin matrix enhanced the degree of carbonization and active binding site density. Native alkaline ash components created a highly localized pH gradient at the solid-liquid interface. This robust interfacial alkalinity triggered distinct, surface-induced heterogeneous nucleation pathways: Pb 2+ removal was governed by extensive precipitation of cerussite PbCO 3 and lead hydroxyapatite Pb 10 (PO 4 ) 6 (OH) 2 , while Cu 2+ uptake transitioned from surface complexation into the localized precipitation of Cu 2 (NO 3 )(OH) 3 . The novel PC/R-0.5 and PC/R-1 variants achieved outstanding maximum immobilization capacities of 31 and 30 mg⋅g −1 for Cu 2+ and 150 and 149 mg⋅g −1 for Pb 2+ , significantly outperforming commercial activated carbons (13 and 46 mg⋅g −1 ). Findings demonstrate that this synergistic chemical pathway anchors the heavy metals into solid crystalline phases, fundamentally minimizing the risk of spontaneous desorption and providing a highly stable intermediate state for subsequent safe end-of-life management.