Sorption of As and Se on mineral components of fly ash: Relevance for leaching processes

The reductive dissolution of hematite (alpha-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone-2,6-disulfonate (AQDS), which is often used as a model electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow rate, pH, and Fe(II) and phosphate concentrations were varied to investigate the redox kinetics in a flow-through reactor. The hematite reduction rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and ADDS sorption on hematite surfaces were independently investigated for interpreting hematite reduction kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to alpha-Al2O3, a redox stable analog of alpha-Fe2O3. Decreasing Fe(ll) and increasing AH2DS sorption by controlling flow rate, influent pH, and Fe(II) and phosphate concentrations increased the rates of reductive dissolution. The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium. This study demonstrated the importance of the geochemical variables for the reductive dissolution kinetics of iron oxides.

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Kinetics of Reductive Dissolution of Hematite by Bioreduced Anthraquinone-2,6-disulfonate

Quantum chemical calculations were performed to estimate relative Gibbs free energies of sulfate adsorption on variably charged Al- and Fe-(hydr)oxide clusters. Inner-sphere bidentate bridging and monodentate adsorption were predicted to be exergonic on positively charged Al- and Fe-(hydr)oxides (ranging from −19 to −124 kJ mol-1). However, inner-sphere and H-bonded adsorption on neutral Al- and Fe-(hydr)oxides was predicted to be endergonic (ranging from +5 to +61 kJ mol-1). At the highest positive surface charge, bidentate bridging adsorption was most thermodynamically favorable. At intermediate positive surface charge, bidentate bridging and monodentate adsorption energies were equivalent on Al-(hydr)oxides; monodentate adsorption was more thermodynamically favorable on Fe-(hydr)oxides as compared with bidentate bridging adsorption. The predicted thermodynamic favorability of sulfate adsorption on Al- and Fe-(hydr)oxides was directly related to positive surface charge and indirectly related to the HO-/...

Quantum Chemical Calculations of Sulfate Adsorption at the Al- and Fe-(Hydr)oxide-H2O InterfaceEstimation of Gibbs Free Energies

Batch experiments were used to investigate the release of sulfate sorbed on X-ray amorphous iron oxyhydroxide precipitates that formed in acid mine drainage (AMD) (pH = 2.8-3.2) from the Stearns Coal Belt in southeastern Kentucky. The sediments were characterized by high sulfate concentrations (∼600-1000 mmol/kg) and iron/sulfate ratios that ranged from 6.6to 8.6 as determined by dissolution in 10 M hydrochloric acid. The results of a ligand exchange experiment (using separate preparations of 0-0.25 N sodium nitrate, chloride, phosphate, bicarbonate, and oxalate) indicated that ∼60-70% of the total sulfate will be retained in the presence of monovalent ligands. This may be indicative of a bidentate bridging mechanism bonding iron and sulfate. Most of this sulfate would likely be stable upon the secondary iron oxyhydroxides associated with acid mine drainage. Sulfate desorption increased directly with pH. At neutral pH, approximately 33-50% of the total sulfate present in these precipitates was released to solution. The bicarbonate ion released ∼60% of the total sulfate from one of the AMD precipitates. These results imply that acid neutralization methods (e.g., application of crushed limestone) used to stabilize metals in AMD can have the unwanted effect of raising sulfate concentrations within impacted watersheds.

Release of Sorbed Sulfate from Iron Oxyhydroxides Precipitated from Acid Mine Drainage Associated with Coal Mining

Wastes from the sulfuric acid industry are an environmental concern, because of the emission of acids, heavy metals, and sulfate to the environment. The wastes in Falun consist of 70–80% iron oxides, 10–20% silicates, less than 10% residual sulfides, and small amounts of secondary precipitates (iron hydroxides and Fe-, Zn- and Cu-sulfates). Due to the different behavior of sulfides during the roasting process, pyrrhotite and sphalerite are the major sulfide residues associated with lesser amounts of pyrite, chalcopyrite, and galena. The leachates are low-pH and enriched in Zn, Fe, and SO4. The acid ferric Fe-rich solution promotes the dissolution of sphalerite and favors the formation of Pb-sulfate coatings on galena, providing an armoring effect which slows down the further oxidation of the galena. The residual sulfides are the potential source for acid generation and metal release. During the roasting process, iron oxides retain small amounts of sulfur and sphalerite forms alteration rims containing Zn-oxides. The iron oxides and Zn-oxides are important contributors to SO4 and Zn in the leachates. The conditions in the waste deposit are favorable for the precipitation of Zn-, Cu-Fe-sulfates (e.g. gunningite, chalcanthite, Zn-copiapite). The highly soluble sulfates play important roles in controlling the concentrations of Cd, Cu, Fe, Zn, and SO4 in the leachates. The mineralogical and geochemical data help to develop the reclamation strategies of this type of industrial wastes.

Mineralogical and chemical characterization of wastes from the sulfuric acid industry in Falun, Sweden

Sulfate ion adsorption and desorption experiments carried out on synthetic goethite and hematite and natural hematite show adsorption to be a highly irreversible reaction. All oxides showed an increase in sulfate ion adsorption with decrease in pH. Only a small fraction of sorbed sulfate was desorbable after 48 hr, and only at a pH of 3. Extreme irreversibility of sulfate sorption on these common soil minerals suggests that adsorbed sulfate is more immobile in watersheds than previously considered and that recovery models which inherently assume reversibility may need to be modified.

Laurie J. Turner

Papers

Irreversibility of sulfate sorption on goethite and hematite