Arsenite sequestration at the surface of nano-Fe(ОН)_2, ferrous-carbonate hydroxide, and green-rust after bioreduction of arsenic-sorbed lepidocrocite by Shewanella putrefaciens

摘要

X-ray Absorption Fine Structure (XAFS) spectroscopy was used in combination with high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), X-ray energy dispersive spectroscopy (XEDS), X-ray powder diffraction, and M_ssbauer spectroscopy to obtain detailed information on arsenic and iron speciation in the products of anaerobic reduction of pure and A_s(V)- or A_s(III)-adsorbed lepidocrocite ('y-FeOOH) by Shewanella putrefaciens ATCC 12099. We found that this strain of S. putrefaciens is capable of using Fe(III) in lepidocrocite and As(V) in solution or adsorbed on lepidocrocite surfaces as electron acceptors. Bioreduction of lepidocrocite in the absence of arsenic resulted in the formation of hydroxycarbonate green rust 1 [Fe~II_4F_eIII_2(OH)_(I2)СO_3: GRI(CO_3)], which completely converted into ferrous-carbonate hydroxide (Fе~II_2(ОН)_2CO_3: FCH) over nine months. This study thus provides the first evidence of bacterial reduction of stoichiometric GRI(CO_3) into FCH. Bioreduction of A_s(III)-adsorbed lepidocrocite also led to the formation of GRI(CO_3) prior to formation of FCH, but the presence of A_s(III) slows down this transformation, leading to the co-occurrence of both phases after 22-month of aging. At the end of this experiment, As(III) was found to be adsorbed on the surfaces of GRI(CO_3) and FCH. After five months, bioreduction of A_s(V)-bearing lepidocrocite led directly to the formation of FCH in association with nanometer-sized particles of a minor As-rich Fе(ОН)_2 phase, with no evidence for green rust formation. In this five-month experiment, A_s(V) was fully converted to A_s(III), which was dominantly sorbed at the surface of the Fе(ОН)2 nanoparticles as oligomers binding to the edges of F_е(ОН)_6 octahedra at the edges of the octahedral layers of F_е(ОН)_2. These multinuclear A_s(III) surface complexes are characterized by A_s_As pairs at a distance of 3.32 ± 0.02 A and by A_s_Fe pairs at a distance of 3.50 ± 0.02 A and represent a new type of A_s(III) surface complex. Chemical analyses show that the majority of A_s(III) produced in the experiments with As present is associated with iron-bearing hydroxycarbonate or hydroxide solids