Lanthanum carbonate nanoparticles confined within anion exchange resin for phosphate removal from river water: Batch and fixed-bed column study

The use of lanthanum for phosphate removal has gained increasing attention due to its relative abundance, non-toxicity, and strong affinity toward phosphate. In this regard, although lanthanum hydroxide exhibits promising phosphate removal ability, its practical application remains limited due to certain technical issues including its structural instability and leaching. To circumvent these issues, a lanthanum carbonate-based adsorbent was developed in this study. Lanthanum carbonate@anion exchange resin (LC@AER) and lanthanum hydroxide@anion exchange resin (LH@AER) beads were first prepared through in-situ precipitation using identical bead-to-precursor mass ratios. LC@AER beads were chosen for further study as they displayed better adsorption capacity and stability, and the bead-to-precursor mass ratio was further optimized to improve performance and stability. LC@AER (1:2) beads exhibited a maximum adsorption capacity of 77.43 mg-P/g and excellent selectivity toward phosphate in the presence of various co-existing anions. Experiments using river water indicated high phosphate removal efficiency, demonstrating potential for treating river water. Investigations revealed key differences in phosphate binding mechanisms for batch and column experiments. In batch setting, phosphate is primarily captured through ligand exchange and inner-sphere complexation. However, over prolonged column operation, surface precipitation and electrostatic attraction (between phosphate and quaternary ammonium) become increasingly important for binding phosphate, which may affect phosphate recovery efficiency and must be accounted for in process design. Overall, the findings indicate that lanthanum carbonate serves as a good alternative to lanthanum hydroxide and that LC@AER (1:2) beads are promising for phosphate removal.