Photocontrol of Goethite Crystal Facet Exposure: Implications for Phosphate Binding in Water and Sediment Systems

The environmental fate of phosphorus in shallow aquatic systems is closely linked to the properties of iron (oxyhydr)oxide minerals such as goethite, which can bind phosphate and influence its mobility. This study investigates how light exposure during mineral formation affects goethite crystal habit and subsequent phosphate adsorption behavior. Humic-acid (humic acid)-mediated transformation of siderite or dissolved Fe(II/III) was conducted under simulated shallow-water irradiance (113 mW·cm^-2) or under dark conditions. We found sunlight-driven changes in the crystal habit of goethite (α-FeOOH) were quantitatively linked to a 33% reduction in phosphate (P) sequestration capacity compared with dark-formed phases. In the dark, oriented attachment along the c-axis yielded (100)-faceted nanorods (100-500 nm, surface energy 1.22 J·m^-2) that adsorbed 1.5-fold more P (5 mg P·L^-1, 24 h) than light-exposed crystals. Light generated ^•OH/^•O₂^- radicals that lowered the electron density of the (100) plane, redirected growth along the a-axis, and produced highly crystalline (001)-terminated rods (1-2 μm). XANES fingerprinting showed that dark phases formed predominantly bidentate Fe-PO₄ complexes (pre-edge intensity 0.25), whereas light phases favored monodentate coordination (pre-edge 0.15). These results suggest that diel irradiation cycles in oxic surface waters may influence goethite reactivity and phosphorus mobility, although extrapolation to natural systems requires consideration of additional environmental complexities.