Environ. Sci. Technol. 2005, 39, 821-825
Oxidation of Ferrocyanide by Birnessite THILO RENNERT,† ANDREAS POHLMEIER,‡ AND T I M M A N S F E L D T * ,† Arbeitsgruppe Bodenkunde und Bodeno¨kologie, Fakulta¨t fu ¨ r Geowissenschaften, Ruhr-Universita¨t Bochum, D-44780 Bochum, Germany, and Geospha¨re IV, Agrospha¨re, Forschungszentrum Ju ¨ lich, D-52425 Ju ¨ lich, Germany
matter formation (7). It has been frequently reported that inorganic contaminants in soil such as AsO33- (8), SeO32- (9), or Cr3+ (10) are oxidized by birnessite. These processes are associated with reductive dissolution of the manganese oxide releasing Mn2+/3+ ions. The oxidized reaction products may then remain in solution or are adsorbed on the birnessite surface (e.g., AsO43-; 8) or precipitate on the surface (e.g. as krautite, MnIIHAsO4; 11). The comparison of the redox potentials (EH) of the halfreactions indicates that birnessite should oxidize [FeII(CN)6]4to [FeIII(CN)6]3- (12, 13):
/2MnO2 + e- + 2H+ a 1/2Mn2+ + H2O
1
The Fe-CN complexes ferrocyanide, [FeII(CN)6]4-, and ferricyanide, [FeIII(CN)6]3-, which are contaminants in soil and groundwater, form a redox couple, [FeII(CN)6]4- a [FeIII(CN)6]3- + e-, EH ) 356 mV. We studied the oxidation of [FeII(CN)6]4- by birnessite, δ-MnIVO2, in batch experiments as influenced by [FeII(CN)6]4- concentration, pH, and reaction time. Additionally, stopped-flow experiments were carried out at five temperatures (10-30 °C) and four pH values (pH 4.1-5.3). In the batch experiments, [FeII(CN)6]4was completely oxidized to [FeIII(CN)6]3-, and oxidation did neither depend on time for t > 2 min, nor on concentration (0.12-0.47 mM), nor on pH (pH 3.3-9.9). Lasting adsorption of Fe-CN complexes on the birnessite surface or precipitation of manganese ferricyanide were not detected. Manganous ions resulting from the reductive dissolution of birnessite did not precipitate as manganese oxide because an identical decrease of Mn solution concentrations was observed under air and under a N2 atmosphere. Two processes were detected by the stopped-flow experiments. The first rapid one with an activation energy of approximately 60 kJ mol-1 was attributed to short-term adsorption and simultaneous oxidation of [FeII(CN)6]4- on the birnessite surface. The second slower process with an activation energy of approximately 20 kJ mol-1 was attributed most probably to diffusion of the reaction product Mn2+ into the interior of the birnessite, which creates fresh reaction sites at the outer surface.
Introduction The Fe-CN complexes ferrocyanide, [FeII(CN)6]4-, and ferricyanide, [FeIII(CN)6]3-, are contaminants present in soil and groundwater caused by anthropogenic inputs (1). An environmental concern is given by the tendency of [FeII(CN)6]4to release extremely toxic free CN- in the presence of light (2). Since Fe is present in two different oxidation states in the complexes, they may undergo redox reactions with the soil matrix. For example, it has been observed that [FeIII(CN)6]3- was reduced to [FeII(CN)6]4- during the transport in a humic topsoil horizon and subsequently precipitated as Berlin Blue particles, Fe4[FeII(CN)6]3 (3). Manganese oxides such as birnessite, δ-MnIVO2 (4), take part in many soil chemical processes such as cation adsorption (5), redox reactions with metal ions (6), or the catalysis of soil organic * Corresponding author phone: +49-(0)234-3223439; fax: +49(0)234-3214469; e-mail:
[email protected]. † Arbeitsgruppe Bodenkunde und Bodeno ¨ kologie. ‡ Geospha ¨ re IV. 10.1021/es040069x CCC: $30.25 Published on Web 12/29/2004
2005 American Chemical Society
EH ) 520 mV (1)
[FeII(CN)6]4- a [FeIII(CN)6]3- + e- EH ) 356 mV
(2)
Consequently, oxidation of [FeII(CN)6]4- to [FeIII(CN)6]3- by manganese oxides was assumed to affect the speciation of Fe-CN complexes in soil under varying redox conditions (14). A redox reaction, although thermodynamically possible, does not have to occur under natural conditions necessarily due to kinetic stability of the redox process (12). Ferricyanide is more toxic than [FeII(CN)6]4- as demonstrated in fish test (15). In soil, [FeIII(CN)6]3- is more mobile than [FeII(CN)6]4-, since the latter is sorbed more strongly on iron oxides (16). Therefore, the geochemical behavior of Fe-CN complexes and possible environmental hazards caused by them in soils on contaminated sites is affected by the speciation of the Fe-CN complexes between [FeIII(CN)6]3- and [FeII(CN)6]4-. Oxidation of [FeII(CN)6]4- by soil manganese oxides might influence the species distribution, but reactions between these oxides and Fe-CN complexes have not been studied yet. Therefore, the aim of this study was to investigate the extent and the rate of [FeII(CN)6]4- oxidation by a synthetic birnessite by batch and stopped-flow experiments.
Materials and Methods Birnessite. Birnessite was prepared by adding concentrated HCl to a boiling MnO4- solution (17):
MnO4- + H+ a δ-MnO2 + 3/4O2 + 1/2H2O
(3)
Subsequently, the precipitate was cleaned by pressure filtration until the electrical conductivity was
