BayCEER Workshop 2008

Iron-sulfide interaction: The reactivity of ferric (oxy)hydroxides in aquifers toward dissolved sulphide

Poster in Postersession

The reaction between H₂S and ferric (oxy)hydroxides exerts a major role for the sulphur and iron cycle as for the electron and carbon flow in many aquatic systems (e. g. Canfield, 1992). The reaction mechanism is reasonably well understood (Dos Santos Afonso et al, 1992; Peiffer et al., 1992). The reaction rate depends on pH which can be explained by a surface speciation model according to which the electron transfer is preceded by an adsorption step of a sulphide species to the neutral ferric oxide surface >FeOH.
In a recent study, steady-state experiments have been performed at low pH (< 5) which covers conditions typically met in the sediment of the mining lakes. Therefore a fluidized-bed reactor was developed that is supplied with a constant flow of electrochemically generated hydrogen sulphide (Peiffer & Gade, 2007). The surface area normalized experimental reaction rates depended on bulk properties of the used minerals and decreased in a sequence Gt > 2lfh > 6lfh. These observations are in contrast to results from batch experiments obtained at pH 7.5 where mineral reactivity seemed to be related to the free energy of their formation (Poulton et al, 2004). Under these conditions Fe²⁺ strongly adsorbs to the mineral surface and thereby interacts with dissolved sulfide.

In this study we attempt to resolve these contradictions. Using the same experimental approach as decribed in Peiffer & Gade (2007), we have extended the experimental pH range to study the steady-state reactivity of various synthetic iron (hydr)oxides to values between 3 and 9 to resolve the following issues: ·

• Different pH, different sulfide concentration, presence of organic matter: Does the rate change? ·
• Various identical minerals of different surface properties are available: Does the rate change?
  
• Fate of Fe(II): Does the Fe(II), which is not bound to FeS at pH> 6, lead to the transformation of synthetic iron oxides into other iron oxides like goethite, magnetite, green rust? Therefore the product formation both in the solid and dissolved phase during the reaction will be study.
  
• To researched the sulfur isotopic fractionation during this reaction batch experiments will be performed.

The overall aim of the study is the envelopment of a generalized kinetic model for the abioric anaerobic H₂S-oxidation by ferric (oxyhydr)oxides for the full pH range in consideration of mineral properties, surface complexes and the competition with other ions.

The poster will show the concept, the experimental setup and first results from these experiments.

References
Canfield D. E. and Raiswell R. and Bottrell S. (1992), The reactivity of sedimentary iron minerals toward sulphide. Amer. J. Sci. 292, 659-683.
Dos Santos Afonso M. and Stumm W. (1992), Reductive Dissolution of Iron(III) (Hydr)oxides by Hydrogen Sulfide. Langmuir 8, 1671-1675.
Peiffer S. and Dos Santos Afonso M. and Wehrll B. and Gächter R. (1992), Kinetics and mechanism of the reaction of H₂S with lepidocrocite. Environ. Sci. Technol. 26(12), 2408-2413.
Peiffer S. and Gade W. (2007), Reactivity of ferric oxides toward H₂S at low pH. Environ. Sci. Technol. In press.
Poulton, S. W. and Krom M. D. and Raiswell R. (2004), A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulphide. Geochim. Cosmochim. Acta. 68, 3703-3716.