Direct Nano-Imaging Reveals the Underestimated Role of Lanthanum Phosphate Formation in Phosphorus Sequestration by Lanthanum Carbonate

Lanthanum-based materials are recognized as highly effective adsorbents for advanced phosphorus removal, with a prevailing belief that acidic conditions promote phosphorus uptake via enhanced surface complexation. Herein, we demonstrate that lanthanum carbonate exhibits a 1.8-fold higher phosphate adsorption capacity at pH 7 (33.2 mg-P/g) compared to pH 4 (18.5 mg-P/g), attributed to the enhanced formation of LaPO4 nanocrystals. Leveraging in situ atomic force microscopy (AFM), we resolve real-time phosphorus sequestration dynamics, capturing LaPO4 nucleation within minutes, contradicting prior reports that LaPO4 formation is time-intensive. This discrepancy arises because conventional characterization techniques (e.g., X-ray diffraction) overlook transient amorphous LaPO4 intermediates due to insufficient sensitivity, whereas the nanoscale resolution of AFM directly tracks interfacial transformations. A dissolution-nucleation-growth mechanism was then proposed for the interfacial formation of LaPO4. This study revises the mechanistic framework for phosphorus removal and highlights the crucial role of LaPO4 formation in maximizing the utilization efficiency of lanthanum active sites for enhanced phosphate sequestration by La-based materials.