Distinct phosphate adsorption mechanisms of LDH- versus spinel-modified biochar: A structure-participating vs. surface-site-dominating pathway

The key to designing modified biochar-based phosphate adsorbents lies in clearly understanding how different surface modification strategies influence adsorption performance, which is of great significance. Using the same Ulva biomass precursor, layered double hydroxide-modified biochar (LDH@BC) and spinel-modified biochar (Spinel@BC) were fabricated via co-precipitation routes tailored to the respective phase formation requirements, and their adsorption behaviors and underlying mechanisms were systematically compared. LDH@BC retained a well-defined layered structure with a specific surface area of 78.22 m 2 ·g −1 , whereas Spinel@BC converted into a dense, highly crystalline spinel phase with a lower surface area of 56.42 m 2 ·g −1 . LDH@BC exhibited a higher phosphate adsorption capacity (Q m = 97.1 mg·g −1 ) and reached equilibrium rapidly within 200 min. It also maintained over 88% removal efficiency in the presence of coexisting anions. However, its performance was more pH dependent than that of Spinel@BC. In contrast, Spinel@BC delivered a lower capacity (Q m = 56.5 mg·g −1 ) but more stable adsorption across a broader pH range. Real wastewater tests showed that LDH@BC achieved 85%–95% phosphate removal at 0.3–0.5 g·L −1 . FTIR and XPS analyses revealed that LDH@BC immobilizes phosphate through a structure-participating pathway involving interlayer anion exchange, surface hydroxyl substitution, and Zn/Fe site complexation, whereas Spinel@BC operates via a surface-site-dominating mechanism through inner-sphere Zn/Fe-O-P complexation on surface metal sites. Regeneration experiments demonstrated that LDH@BC retained 78.5% of its initial removal efficiency after ten cycles, outperforming Spinel@BC (54.3%). These findings offer a basis for the rational design and application of biochar-based phosphate adsorbents.