Formation of Layered Fe(II)-Hydroxides during Fe(II) Sorption onto Clay and Metal-Oxide Substrates

Sorption of Fe­(II) in anoxic aqueous suspensions of γ-Al₂O₃, smectitic clay and amorphous silica was studied as a function of pH (5.0–10.0) and reaction time (up to 110 days), using batch experiments complemented with synchrotron X-ray absorption spectroscopic analyses. Formation of secondary Fe­(II) precipitates was observed at pH > 7 in all systems, with the rate of precipitation and the types of precipitates formed varying with pH and substrate type. Sorption of Fe­(II) on γ-Al₂O₃ at pH ≥ 7.0 and onto clay at pH 7.0 and 7.5 led to formation of Fe­(II)–Al­(III) layered double hydroxides, whereas poorly crystalline trioctahedral Fe­(II)-phyllosilicates formed in the amorphous SiO₂ suspensions at pH > 7.5 and in the clay suspensions at pH 8.0. The rate and extent of Fe­(II) sorption increased with pH, underscoring the importance of pH in regulating precipitate formation. Notably slower Fe­(II) precipitation in the clay suspensions compared to γ-Al₂O₃ and SiO₂ is attributed to relatively low availability of substrate-derived Al and Si. Our findings demonstrate that sorbent type, pH and reaction time are important factors affecting precipitation of secondary Fe­(II) minerals in anoxic environments, and suggest substantial complexity in the type and reactivity of Fe­(II) sorption products that may form