La(CO3)(OH) magnetic porous biochar for low concentrations of phosphate advanced removal from wastewater

Lanthanum (La)-based hydroxide materials are considered promising candidates for phosphate removal, but their adsorption efficiency is often limited by the hydroxide (OH – ) release and slow mass transfer. In this study, we synthesized lanthanum hydroxyl carbonate supported onto magnetic porous biochar (La-FPBC 0.5 ) for advanced phosphate removal from actual secondary effluent of wastewater. La-FPBC 0.5 significantly enhanced phosphate removal by releasing more carbonate (CO 3 2– ) rather than hydroxy (–OH) during adsorption. Density functional theory calculations revealed that the absolute value of absorb adsorption energy for ion exchange in La-FPBC 0.5 is 4.48 eV, higher than that for ligand exchange (3.11 eV). The adsorption kinetic rate constant of La-FPBC 0.5 reached 1.7 × 10 -3 g/(mg −1 min), which is 1.8 times that of La(CO 3 )(OH). Furthermore, La-FPBC 0.5 removed up to 151.5 mg P/g of phosphate from wastewater, representing a 1.6-fold increase compared to La(CO 3 )(OH), surpassing not only commercial La-based materials but also most reported adsorbents. It maintained over 85 % of its phosphate adsorption capacity in the presence of anions or humic acid and showed effective adsorption across various water sources, including river, campus lake, and tap water, with a capacity exceeding 120 mg P/g. La-FPBC 0.5 also exhibited excellent stability, retaining 95 % of its capacity after five adsorption–desorption cycles. Notably, La-FPBC 0.5 treated approximately 1800 bed volumes of phosphate-containing wastewater to below 0.1 mg/L in a fixed-bed adsorption mode, with the recovered phosphate suitable for plant growth. These results suggest that La-FPBC 0.5 is an ideal candidate for phosphate removal in practical applications.