La-modified alkali-activated substrate derived from fly ash with biochar incorporation for phosphorus removal: Morphology control and La bonding mechanisms

ABSTRACT Developing bulk substrates with high phosphorus (P) removal efficiency remains a critical bottleneck in constructed wetlands (CWs). In this study, alkaline-activated substrates (FBG) modified by lanthanum (La) for effective P removal were synthesized from fly ash and sludge biochar utilizing over 80 wt% of industrial solid wastes. The incorporation of biochar enlarged the mesoporous structure (D = 23.66 nm) of FBG while maintaining a high compressive strength (36.16 MPa). Two La modification strategies, oscillation-assisted precipitation (O-La-FBG) and ultrasound-assisted dispersion (U-La-FBG), were systematically compared. O-La-FBG yielded La(OH) 3 agglomerates favoring a higher P adsorption capacity (3.34 mg/g), whereas U-La-FBG generated well-defined La(OH) 3 nanoflowers with a more exceptional La utilization efficiency (209 mg P/g La). Theoretical calculations of the bonding state of La on the substrates indicated that La(OH) 3 species were significantly more reactive toward silicon-oxygen species than structurally stable aluminum-oxygen tetrahedrons. XPS and FTIR analyses revealed the P adsorption mechanism dominated by electrostatic attraction and inner-sphere ligand exchange between protonated La(OH) 2 + and H 2 PO 4 - . Furthermore, a 30-day dynamic column study validated that although U-La-FBG offered higher La utilization efficiency, O-La-FBG was more suitable for long-term applications in CWs, with both consistently reducing effluent P concentration to below 0.2 mg/L. Overall, this study provides a sustainable management option for recycling solid wastes into functional materials for P removal and realizing the "waste to resources" strategy. The bulk La-modified substrates could mitigate the bed clogging observed in continuous-flow systems, while providing P adsorption capacity for treating micro-polluted water bodies in the actual operation of CWs.