Pivotal effects of external Fe²⁺ on remediation of arsenite by zero-valent iron/persulfate: Efficiencies and mechanism

Persulfate could be activated by zero-valent iron (ZVI) leading to the rapid removal of various contaminants. However, quick consumption of Fe²⁺ largely constrained the removal (%) of target pollutants. Here it was reported that Na₂S₂O₈ (SP) combined with ZVI, as an external source of Fe²⁺, was activated by Fe²⁺ to quickly (minutes scale) and efficiently (more than 90%) remove As (III) from aqueous solution at an initial pH value from 1.0 to 9.0. As (III) removal was obviously improved by an increase of Fe²⁺ rather than Na₂S₂O₈ dosage. The removal of As (III) using Fe²⁺-SP-ZVI system followed the pseudo-second-order kinetic and pseudo-first-order kinetic expression. Fe²⁺ from ZVI oxidization could improve the efficient generation of [Formula presented], which obviously boosted ZVI corrosion. The production of [Formula presented] could be manipulated by oxalic acid, ethylenediaminetetraacetic acid (EDTA), citric acid and phosphates through controlling the concentration of dissociative Fe²⁺, leading to an obvious repression on As (III) removal. The fitting of X-ray absorption fine structure (XAFS) spectra illustrated that the interatomic distance of As–O shell was located between As(III)–O and As(V)–O shell and external Fe²⁺ could promote the oxidation of As (III) to As (V) from 35.6% in 1.0 min–44.5% in 10.0 min. Goethite as the main component of iron oxyhydroxides might play a significant role of As (III) adsorption in Fe²⁺-SP-ZVI system. These findings are crucial for knowing the fate and transport of arsenic under permeable reactive barriers.