Characteristics of Surface Iron(II) and Nitrite at the Gibbsite-Water Interface.

The microbial reduction of solid Fe(III) hydr(oxide) minerals is an important process in the biogeochemical cycling of nutrients and metal contaminants.  The presence of nitrate can inhibit the net Fe(III) reduction to Fe(II) under anoxic conditions.  One proposed mechanism involves simultaneous nitrate and Fe(III) reduction coupled to chemical reoxidation of Fe(II) to Fe(III) by nitrite, the intermediate of nitrate reduction.  We have previously shown that mineral iron(II) forms such as siderite and wüstite are effective in reducing nitrite, however, very little is known about the reactivity of adsorbed iron(II) forms.  Therefore, we have further addressed the chemical pathway by studying the behavior of adsorbed iron(II) on gibbsite, the most commonly identified aluminum hydroxide mineral in soils.  The kinetics of iron(II) adsorption on gibbsite were characterized by a fast reaction followed by a slower reaction with time.  The rates of iron(II) adsorption increased with an increase in gibbsite surface area.  The adsorption of iron(II) on gibbsite was strongly pH-dependent as well.  Additional experiments will be performed using optical and Mössbauer spectroscopies that bear on the question of the chemical structure of iron(II) at the gibbsite-water interface.  The next step will be to test the reactivity of adsorbed iron(II) surface complexes with nitrite.  This research is a timely pursuit given the high costs of nitrate fertilizer and the desire to protect water resources from elevated nitrate levels.