Promoted As(III) redox transformation on zero-valent iron by co-occurring phosphate: The role of interfacial phosphatization

Phosphate inhibits As sequestration while redirecting interfacial redox pathways. • Passive phosphatization forms a phosphate-enriched mixed corrosion shell. • Surface-associated ROS contribute to enhanced oxic As(III) oxidation. • Surface Fe(II) retention is associated with an anoxic As(0) fraction up to 32.7%. Surface corrosion layers govern the reactivity of zero-valent iron (ZVI), yet their dynamic reconstruction by co-occurring phosphate and its consequences for arsenic (As) transformation remain poorly understood. In this work, we demonstrate that in situ passive phosphatization restructures the ZVI interface into a phosphate-enriched mixed corrosion shell. This interfacial reconstruction decreased overall As sequestration through competitive phosphate occupancy but promoted distinct As(III) redox pathways depending on oxygen availability. Under oxic conditions, phosphate promoted aqueous As(V) accumulation and increased the XPS-derived near-surface As(V) fraction from 52.1% to 78.1% on the finer ZVI. Oxygen-exposure, scavenger, and probe experiments indicated that this enhancement involved surface-associated reactive oxygen species. Under anoxic conditions, phosphate incorporation increased the near-surface Fe(II) fraction to 70.6% and the As(0) fraction from 6.7% to 32.7%, indicating enhanced interfacial As reduction associated with Fe(II) retention. Spectroscopic and electrochemical analyses further showed altered ZVI corrosion behavior and charge-transfer resistance. These findings establish co-occurring phosphate as an active regulator of ZVI interfacial reactivity rather than merely a competitive inhibitor, revealing a trade-off between As redox transformation and sequestration and highlighting its potential for ZVI-based pretreatment coupled with downstream As capture. Download: Download high-res image (212KB) Download: Download full-size image