Novel Insight into Organoarsenic Degradation and Inorganic Arsenic Sequestration in Ferrate Treatment: Unraveling the Role of Active Fe–As Complexes at the Molecular Level

Efficiently removing organoarsenic contaminants like p-Aminophenylarsonic acid (pASA) with their toxic inorganic byproducts remains a major challenge. Ferrate [Fe(VI)] can oxidize organoarsenic and immobilize inorganic arsenic, but the molecular mechanisms linking these functions are unclear. Here, using theoretical computational spectroscopy and stopped-flow analysis, we identified a critical pathway mediated by Fe(V/IV)–O-As(III) intermediates (INTs) that bridges oxidative degradation and heterogeneous sequestration. X-band EPR further confirmed the INTs (g-factors = 2.006, 1.928, 1.712), with rhombic peaks and quadruple hyperfine splitting. These INTs form barrierlessly between Fe(VI/V) and ·As(III)O(OH)2 released upon C–As bond cleavage in pASA. The thus-formed INTs exhibit superior reactivity to Fe(VI/V/IV), enabling more effective organoarsenic degradation and As(III) oxidation via outer-sphere electron transfer. Additionally, these INTs facilitate highly efficient inner-sphere electron transfer, forming Fe(III/II)–O-As(V) complexes that directly evolve into As-bearing Fe nanoparticles via Fe–O–As bond formation, a process with a lower energy barrier. The abundance of INTs during pASA degradation can be regulated by modulating Fe(VI) decomposition kinetics using common reagents: H2O2 accelerates and NaClO delays Fe(VI) decomposition. The Fe(VI)/NaClO system achieved 88% total As removal, higher than the 72% for Fe(VI)/H2O2 and 78% for Fe(VI) alone, and exhibited enhanced As–Fe coordination, indicating improved sequestration. Overall, this work reveals a pivotal mechanistic bridge in metalloid contaminant removal, offering new insights for designing oxidation-sequestration synergy in environmental remediation.