Degradation of Phosphonate Antiscalants in Reverse Osmosis Concentrate through Oxygenation of Fe(II)-Bearing Biotite

Phosphonates are widely used in reverse osmosis (RO) and other filtration processes, making their removal critical for effective management of RO concentrate. While oxygenation of Fe(II)-bearing minerals can generate powerful oxidants, such as hydroxyl radical (•OH) and high-valent Fe species, their efficacy in degrading phosphonates under the complex water chemistry of RO concentrate remains unclear. Here, we investigated the degradation of two representative phosphonates─nitrilotri(methylphosphonic acid) (NTMP) and iminodi(methylphosphonic acid) (IDMP)─using oxygenation of Fe(II)-bearing biotite. NTMP degraded rapidly in the presence of biotite under oxic conditions, while IDMP showed negligible degradation. Mechanistic analyses indicate that both •OH and high-valent Fe species contributed substantially to NTMP degradation, driven by the oxygenation of biotite structural Fe(II). Despite the complex composition of RO concentrate, NTMP degradation remained relatively consistent in the presence of biotite under neutral pH, although with slower kinetics. Moreover, the addition of acetate and oxalate accelerated NTMP degradation, even in RO concentrate. Overall, these findings reveal a mineral-mediated, sustainable Fenton-like pathway for phosphonate degradation and highlight the potential of Fe(II)-bearing minerals as a greener strategy for managing phosphonates in RO concentrate, thereby reducing associated environmental impacts.