Sorption of Iron(II) at the Surface of Gibbsite.

The microbial reduction of soil iron(III) to iron(II) impacts the biogeochemical cycling of nutrients and soil physicochemical properties.  It is generally observed that a majority of the iron(II) released during microbial iron(III) reduction is in a mineral or surface-associated form.  The mechanisms of iron(II) interaction with aluminum hydroxide minerals are poorly understood.  In this study, the sorption behavior of iron(II) was investigated using kinetic, thermodynamic, and spectroscopic studies on a high (76 m² g^-1) surface area gibbsite.  In general, the kinetics of iron(II) sorption were characterized by a fast reaction step followed by a slower reaction with time.  An estimated rate coefficient for the rapid sorption step was 7.5 ×10^-3 s^-1 (± 4.7 ×10^-5 s^-1) at 12 g gibbsite L^-1 and pH 6.5.  The sorption data of the slower stage were fitted with pseudo-first order kinetics and the rate coefficient was 2.21 ×10^-3 s^-1 (± 2.2 ×10^-4 s^-1).  The sorption of iron(II) on gibbsite was strongly pH-dependent and was not affected by ionic strength.  This indicates a minimal contribution of coulombic forces to the overall free energy change of Fe(II) sorption. Additional experiments will be performed using optical spectroscopy and measuring particle mobilities that bear on the question of the chemical structure of iron(II) at the gibbsite-water interface.