Direct conversion of non-alkali-activated spent fluid catalytic cracking (FCC) catalyst into heulandite (HEU) zeolite via K+-directed seed-assisted crystallization for efficient Pb2+ removal

Direct synthesis of HEU zeolite from non-alkali-activated spent FCC catalyst. • K + -directed seed-assisted crystallization enables selective HEU formation. • Seed crystals accelerate nucleation by shortening the induction period. • FCC-derived HEU exhibits abundant exchange sites for Pb 2+ removal. • Waste-derived HEU provides a sustainable platform for ion-exchange applications. The direct valorization of spent fluid catalytic cracking (FCC) catalysts into high-value zeolitic materials remains challenging because conventional conversion routes generally require alkali activation, complex pretreatments, or organic structure-directing agents. Herein, we report a direct and reproducible synthesis of HEU -type zeolite from non-alkali-activated spent FCC catalyst through a K + -directed seed-assisted hydrothermal strategy. In this approach, KOH serves as the sole alkali source, while synthetic HEU seeds promote nucleation and regulate framework formation. The optimized synthesis conditions (2.3 K 2 O: Al 2 O 3 : 8.0 SiO 2 : 440 H 2 O, 13 wt% seed loading, 160 °C, 72 h) yield highly crystalline HEU zeolite without the use of organic structure-directing agent (OSDA) or alkali activation. Systematic investigation of the crystallization field reveals that K + plays a critical structure-directing role by stabilizing the building units required for HEU formation, whereas seed crystals shorten the induction period and accelerate nucleation. After Na + exchange, the resulting Na-FCC-HEU exhibits excellent Pb 2+ adsorption performance, including >99.8% removal efficiency at a low solid-to-liquid ratio of 1/1000, rapid adsorption equilibrium within 35 min, broad pH tolerance (3–7), strong resistance toward competing ions, and a maximum adsorption capacity of 215.1 mg g −1  at 80 °C. The superior adsorption behavior originates from the low Si/Al ratio and abundant exchangeable sites of FCC-derived HEU . This work provides a sustainable route for converting industrial waste into functional zeolitic materials and offers new insights into the crystallization chemistry of HEU zeolite. We developed an energy-efficient, economical, and reproducible route for high value-added HEU zeolite synthesis from non-alkali-activated spent FCC catalyst, which exhibited excellent performance in removing Pb 2+ pollution after Na  +  modification by ion-exchange. Download: Download high-res image (276KB) Download: Download full-size image