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<su