<i>In Situ</i> Observation of pH and Phosphorus Distribution at Apatite–Solution Interface

Phosphorus (P) availability in soils is a critical environmental challenge affecting agricultural productivity and ecosystem health. Apatite minerals serve as a key P reservoir, yet their dissolution mechanisms at the microscale remain poorly understood due to technical limitations in directly observing mineral–solution interfaces. Here, we demonstrate an innovative dual-imaging approach that combines fluorescence-based pH monitoring with diffusive gradients in thin-films (DGT) to visualize apatite dissolution dynamics at a submillimeter resolution. Using the pH-sensitive fluorescent probe HPTS, we revealed distinct spatial patterns of pH increase at the apatite–solution interface after 2.5 h exposure in acidic solution (pH 6.0), with the highest pH observed within a 70 μm interfacial layer. Concurrent DGT measurements showed P release peaked at 0.15 ng cm–2 s–1 at the interface, with radial diffusion patterns indicating reaction hotspots. This methodology provides unprecedented insights into the coupled dynamics of proton consumption and P release during mineral weathering. Our findings suggest that manipulating interfacial pH could enhance P mobilization from mineral sources, offering new strategies for optimizing nutrient availability in agricultural systems and understanding P cycling in natural environments.