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.