Redox Transformation of Arsenic by Fe(II)-Activated Goethite (α-FeOOH)

Abstract

The redox state and speciation of the metalloid arsenic (As) determine its environmental fate and toxicity. Knowledge about biogeochemical processes influencing arsenic redox state is therefore necessary to understand and predict its environmental behavior. Here we quantified arsenic redox changes by pH-neutral goethite [alpha-Fe(III)OOH] mineral suspensions amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis. Goethite itself did not oxidize As(III) and, in contrast to thermodynamic predictions, Fe(II)-goethite systems did not reduce As(V). However, we observed rapid oxidation of As(III) to As(V) in Fe(II)-goethite systems. Mössbauer spectroscopy showed initial formation of (57)Fe-goethite after (57)Fe(II) addition plus a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer, EXAFS, SEM, XRD, or HR-TEM. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite. In summary this study indicates that in the simultaneous presence of Fe(III) oxyhydroxides and Fe(II), as commonly observed in environments inhabited by iron-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.

Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

“…During data collection, As samples were maintained at a temperature of 5 K to prevent beam-induced redox reactions using an Oxford Instruments CF1208 continuous flow liquid helium cryostat (Oxford Instruments, Abingdon, UK). [58] Energy selection was accomplished with a Si (220) monochromator. All Fe EXAFS spectra were collected in fluorescence mode with a wide-angle collection ionisation chamber (Lytle detector).…”

“…[59] Uranium reference compounds included uranyl acetate and uraninite. [60] Arsenic reference compounds included As III adsorbed to goethite, As V adsorbed to goethite [58] and orpiment (As 2 S 3 ). Standards of arsenic adsorbed to goethite were prepared according to Amstaetter et al by adding 1.2 mg L À1 of sodium arsenite and sodium arsenate to a 5.4 g L À1 suspension of goethite (Bayferrox 920 Z, LANXESS Deutschland GmbH, Leverkusen, Germany).…”

“…Standards of arsenic adsorbed to goethite were prepared according to Amstaetter et al by adding 1.2 mg L À1 of sodium arsenite and sodium arsenate to a 5.4 g L À1 suspension of goethite (Bayferrox 920 Z, LANXESS Deutschland GmbH, Leverkusen, Germany). [58] Orpiment was obtained from the RRUFF database at University of Arizona Mineral Museum and was originally from Mercur, Utah. Linear combination fitting of k 3 -weighted Fe EXAFS spectra was performed from 3 to 12 k (Å À1 ) using Athena.…”

Evaluation of chemical immobilization treatments for reducing arsenic transport in red mud

“…Green rust was formed under anaerobic conditions with ferrous added. Some part of green rust could transform to goethite after oxidation (Amstaetter et al 2010). Part of hematite could dissolve in WA at first, then the Fe 3?…”

The fate of arsenic adsorbed on iron oxides in the presence of arsenite-oxidizing bacteria

“…Iron(hydr) oxides could only slightly oxidize As(III), when they were induced or catalyzed, for example, by Fe(II) or light. Early studies showed the rapid oxidation of As(III) to As(V) at the surface of goethite, magnetite and ferrihydrite in Fe(II)-mediated reactions (Amstaetter et al, 2010;Ona-Nguema et al, 2010). It is reported that goethite interacting with dissolved Fe(II) produced the highest redox activity compared with other Fe(III) (oxy)hydroxides (Elsner et al, 2004).…”