Secondary Mobility of Arsenic Due to In Situ-Generated Iron–Arsenic Colloids upon Iron Oxide Amendments

Applying iron (Fe) amendments to arsenic (As)-contaminated groundwater to form As-laden Fe precipitates represents a promising in situ remediation approach. However, less is known about the secondary risk of As mobilization due to formation of Fe-As colloids released from these precipitates. Here, we systematically explored the nanoscale formation, composition, and stability of Fe-As colloids in high-As contaminated groundwater following the addition of Fe salts under oxic conditions. Batch experiments show that total aqueous As is incorporated into settled particles upon adding Fe(II) and Fe(III). However, we also observe substantial release of Fe-As colloids (168-719 nm), which can remain stably suspended in water for more than 9 days. Higher colloid concentrations and stability occurred at lower molar Fe/As ratios (Fe/As < 5.8), suggesting that colloid-facilitated transport may dominate secondary As mobilization. Colloid characterization confirmed that low Fe/As ratios promote the formation of stable colloids enriched with surface-bound As, with electrostatic interactions stabilizing the colloids. Using advanced analytical and computational techniques, we elucidate the nanoscale structural properties of colloids, showing that As(V) inhibits Fe octahedral polymerization and favors the formation of smaller particles. These mechanistic insights are crucial for evaluating the long-term fate and transport of sequestered As and designing effective groundwater remediation strategies.