Self-enhanced phosphonate degradation in mZVI/air-Fenton process: The role of coordination for mZVI corrosion and iron-sludge reduction

The widespread discharge of phosphonates into aquatic ecosystems has triggered a wave of global environmental concerns, primarily due to their recognized persistence and significant ecological risks. In order to avoid the potential environmental risks associated with phosphate, the coupled strategy of oxidation and precipitation to mineralize phosphonates and immobilize the orthophosphate (o-PO 4 ) has become the main focus of current research. This strategy can be achieved through the morphological changes of iron species in the Fenton process. However, the conventional Fenton process is limited by the slow reduction rate of Fe(III), resulting in high iron-sludge output. Here, we use microscale zero-valent iron (mZVI) as an electron donor and exploit the strong iron-phosphorus coordination effect to develop a mZVI-based two-stage oxidation process (mZVI/air-Fenton). This process can achieve 75% to 100% removal of various phosphonates, and the iron sludge production is 49.6% lower than that of the Fenton process. The experimental results combined with theoretical calculations indicate that phosphonates coordinate with iron to form complexes, which induce electron redistribution on phosphonates through the ligand to metal charge transfer (LMCT) effect, reduce C-P bond energy, and form intermediates susceptible to radical attack. This coordination effect can dynamically remove the passivation layer on mZVI and promote the continuous activation and release of mZVI. In addition, this coordination effect reduces the redox potential of Fe(III)/Fe(II) by 0.14 V through intramolecular charge transfer, which greatly speeds up the cycle of iron, improves the utilization efficiency of iron electrons, and increases the yield of hydroxyl radical ( • OH) by 3.1 times. Therefore, this study provides an innovative improved Fenton oxidation process for the efficient treatment of organophosphorus wastewaters and significant iron-sludge reduction.