A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide

TLDR: In this article, the reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C.

About: This article is published in Geochimica et Cosmochimica Acta.The article was published on 2004-09-15 and is currently open access. It has received 550 citations till now. The article focuses on the topics: Hydrous ferric oxides & Sulfide.

Figures

Table 2 Rate constants for the oxidation of sulfide and the reductive dissolution of Fe(II) in seawater at pH 7.5. n = number of experiments. 1assuming a surface area in the range 300- 600 m 2 g -1 (see text for details).

Figure 3 Logarithmic plots of sulfide oxidation rates (Rs; A) and Fe(II) dissolution rates (RFe; B) as a function of initial solid phase Fe(III) concentration. Experiments were performed at pH 7.5 in seawater, with initial sulfide concentrations of 1055 ± 32 µM for lepidocrocite, goethite, magnetite, and hematite, and 249 ± 3 µM for HFO (see Table 1).

Figure 7 Rate constants for the dissolution of Fe(II) (kFe in mol 0.5 l0.5 m-2 min-1) in 0.1 M NaCl as a function of the free energy (kJ mol-1) for the reactions given in Equations 9-12. Free energy was determined at pH 4; 25°C; [H2S] = 1 mM. The regression line is plotted for the data excluding HFO. Error bars represent the standard deviations reported in Table 2, with the exception of the error bars for HFO, which represent the range in kFe obtained assuming a surface area of 300-600 m2 g-1. Errors in 〉G were calculated based on the ranges in 〉Gf0 reported in Schwertmann and Cornell (1996), but were within the size of the data points.

Table 3 Half lives (t1/2) for the reductive dissolution of Fe (oxyhydr)oxides in seawater at pH 7.5. Data for magnetite is calculated with the inclusion of structural Fe(II). Half lives are for a dissolved sulfide concentration of 1000 µM. 1 Poulton (2003).

Figure 2 Representative data for the reductive dissolution of Fe(II) in seawater at pH 7.5 (goe = goethite; mag = magnetite).

Figure 6 Effect of competitive adsorption of major seawater solutes on the rate of sulfide oxidation by different iron (oxyhydr)oxide minerals at pH 7.5. kS(sea) = sulfide oxidation rate constant in seawater, kS(NaCl) = sulfide oxidation rate constant in 0.1 M NaCl.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide" ?

The reaction between dissolved sulfide and synthetic iron ( oxyhydr ) oxide minerals was studied in artificial seawater and 0. The rate constants derived from the above equation suggest that the reactivity of Fe ( oxyhydr ) oxide minerals varies over two orders of magnitude, with increasing reactivity in the order, goethite < hematite < magnetite < < lepidocrocite ≈ HFO. The derivation of half lives for the sulfide-promoted reductive dissolution of Fe ( oxyhydr ) oxides in seawater, suggests that mineral reactivity can broadly be considered in terms of two mineral groups. 

Q2. What is the effect of dissolution of iron minerals in organic-rich environments?

The reductive dissolution of these minerals by hydrogen sulfide in organic-rich environments exerts a major control on dissolved sulfide profiles (e.g. Canfield, 1989; Canfield et al., 1992), and results in the release to solution of associated trace metals, organics, and ligands such as phosphate (e.g. Krom and Berner, 1981; Morse, 1994). 

Q3. How was the pH of the solution maintained?

During oxidation the solution was rapidly stirred with a magnetic stirrer, and the pH was maintained at a constant 8.0 ± 0.05 by the addition (via a pH-stat) of 1 M NH3. 

Q4. What is the sulfide oxidation rate for goethite?

Thus the goethite reduction rate essentially equates to double thesulfide oxidation rate (given that elemental S was the dominant product), and hence provides support for the 0.5 order dependency of sulfide oxidation rate on initial sulfide concentration determined for all minerals in the present study. 

Q5. What was the mean of the sulfide concentrations measured on unfiltered samples?

Replicate measurements of a stock sulfide solution gave a mean of 1088 ± 12 µM (2j, n = 8). Filtered(0.2 µm) samples were periodically analysed for thiosulfate, sulfite, and sulfate. 

Q6. Why does the sulfide promoter dissolve faster than pure lepidocrocite?

These values suggest that Al-substituted lepidocrocite actually dissolves faster than pure lepidocrocite (t1/2 = 10.9 h) due to the increased surface area available for reaction. 

Q7. What is the kinetics of the reaction between dissolved sulfide and synthetic?

Reaction kinetics were expressed in terms of an empirical rate equation of the form:AkR ii 0.5 0t2S)H( ==where Ri is the rate of Fe(II) dissolution (RFe) or the rate of sulfide oxidation (RS), ki is the appropriate rate constant (kFe or kS), (H2S)t=0 is the initial dissolved sulfide concentration, and A is the initial mineral surface area. 

Q8. Why is the increased effect of competitive adsorption more likely?

the increased effect of competitive adsorption for the less reactive minerals is more likely due to competition with dissolved sulfide during the slower adsorption stage. 

Q9. What is the effect of the reaction order on the surface area of the mineral?

there is also some suggestion that this approximation may mask a systematic variation in reaction order in relation to the specific surface area of each mineral, whereby the reaction order tends to progressively increase with relative surface area. 

Q10. What is the effect of competitive adsorption on the reaction rates of the different minerals?

Competitive adsorption may clearly exert an important influence on reaction rates for the less reactive Fe (oxyhydr)oxides (i.e. magnetite, goethite, hematite), but has little effect on the reactivity of HFO and lepidocrocite (Figure 6). 

Q11. What is the effect of competitive adsorption on reaction rates for the reactive minerals?

At pH 7.5, seawater solutes have little effect on reaction kinetics for the most reactive minerals (i.e. HFO and lepidocrocite), which is consistent with previous studies of competitive adsorption during the reaction of dissolved sulfide with ferrihydrite (Poulton, 2003). 

Q12. What is the number of reduced Fe monolayers at the lepidocrocite surface?

and given that adsorption reactions are most likely to involve only singly coordinated OH groups (Cornell and Schwertmann, 1996), the number of reduced Fe monolayers at the lepidocrocite surface lies in the range 3 to 16.