Lanthanum-loaded biochar-based alginate hydrogels for efficient phosphate adsorption from wastewater

In this study, a lanthanum-loaded biochar-based alginate hydrogel (La@SA@BC) was successfully fabricated at an optimal 1:1 mass ratio of lanthanum-loaded biochar to sodium alginate. A maximum phosphate (P) adsorption capacity of 56.5 mg/g was achieved under an adsorbent dosage of 0.9 g/L. The hydrogel demonstrated 90 % P removal efficiency at a pH of 3 and showed minimal adsorption reduction (<10 %) in the presence of competing ions (Cl^-, SO₄^2-, NO₃^-). Kinetic analysis indicated a higher pseudo-second-order model fit (R² = 0.95) in the adsorption process, suggesting chemisorption as the dominant adsorption mechanism. In addition, the morphology, elemental composition, and surface functional group type analysis confirmed the La@SA@BC hydrogel had a rough, folded surface structure with internal cavities and uniform lanthanum distribution. The diversified characterization results identified electrostatic interactions, ligand exchange, precipitation, Lewis acid-base interactions, and pore filling were the primary routes for P removal. These findings highlight La@SA@BC hydrogel's high selectivity, robust ion tolerance, and strong adsorption capacity, enabling it a promising candidate for remediating P-contaminated water.