The role of structural iron oxidation in the weathering of trioctahedral micas by aqueous solutions

An integrated approach involving several experimental and analytical techniques was used to study the role of structural iron oxidation in the weathering of trioctahedral micas by aqueous solutions. Analytical techniques for the assay of Fe(II) by oxidimetry and for the estimation of octahedral sheet cation occupancies by x-ray diffraction were developed. A weathering apparatus to allow the continuous treatment of micas by fresh aqueous solutions was also developed. This apparatus was used to treat the 10- to 20-[mu]m size-fractions of three trioctahedral micas (a phlogopite, biotite, and siderophyllite) by aqueous solutions that varied in their tendency to promote oxidation, dissolution, and interlayer expansion reactions in the micas. The treatments generally involved 1 M KOAc(pH 4.7)-0.1 M K[subscript]2 EDTA solutions with or without 25% H[subscript]2O[subscript]2 as an oxidant, and were applied at 80°C for periods as long as 36 days. Experiments with deuterated mica samples and with K-depleted samples in 1 M Na solutions were also conducted. The time-dependence of the dissolution of the micas was determined by measuring the amounts of Al, Mg, and Fe in the effluent solutions after different periods and calculating apparent rate constants (ca. 10[superscript]-4 s[superscript]-1) using a heterogeneous kinetic model. Ratios of the rate constants obtained in different solutions were used to estimate the extent of exfoliation of the micas by the weathering treatments. Structural and chemical analyses of the micas before and after treatment were performed by conventional techniques (x-ray diffraction, infrared spectroscopy, Mossbauer spectroscopy) in addition to the two techniques that were developed. These analyses provided strong evidence for the deprotonation of structural hydroxyls and ejection of octahedral cations during the oxidation treatments and for the existence of vacant tetrahedral sites in micas. The main conclusions reached were (1) oxidation of structural Fe(II) in trioctahedral micas does not occur in aqueous solutions without an associated expansion or exfoliation of the interlayer region; (2) the charge created by oxidation is balanced by a combination of the deprotonation of structural hydroxyls, the ejection of octahedral cations (trivalent preferentially to divalent), and the loss of interlayer cations, and (3) the K-selectivity of the mica plays a key role in determining whether oxidation will occur in high-K solutions

Influence of soil minerals on chromium(VI) reduction by sulfide under anoxic conditions

The effects of soil minerals on chromate (Cr(VI)O(4)(2-), noted as Cr(VI)) reduction by sulfide were investigated in the pH range of 7.67 to 9.07 under the anoxic condition. The examined minerals included montmorillonite (Swy-2), illite (IMt-2), kaolinite (KGa-2), aluminum oxide (γ-Al(2)O(3)), titanium oxide (TiO(2), P-25, primarily anatase), and silica (SiO(2)). Based on their effects on Cr(VI) reduction, these minerals were categorized into three groups: (i) minerals catalyzing Cr(VI) reduction – illite; (ii) minerals with no effect – Al(2)O(3); and (iii) minerals inhibiting Cr(VI) reduction- kaolinite, montmorillonite, SiO(2 )and TiO(2 ). The catalysis of illite was attributed primarily to the low concentration of iron solubilized from the mineral, which could accelerate Cr(VI) reduction by shuttling electrons from sulfide to Cr(VI). Additionally, elemental sulfur produced as the primary product of sulfide oxidation could further catalyze Cr(VI) reduction in the heterogeneous system. Previous studies have shown that adsorption of sulfide onto elemental sulfur nanoparticles could greatly increase sulfide reactivity towards Cr(VI) reduction. Consequently, the observed rate constant, k(obs), increased with increasing amounts of both iron solubilized from illite and elemental sulfur produced during the reaction. The catalysis of iron, however, was found to be blocked by phenanthroline, a strong complexing agent for ferrous iron. In this case, the overall reaction rate at the initial stage of reaction was pseudo first order with respect to Cr(VI), i.e., the reaction kinetics was similar to that in the homogeneous system, because elemental sulfur exerted no effect at the initial stage prior to accumulation of elemental sulfur nanoparticles. In the suspension of kaolinite, which belonged to group (iii), an inhibitive effect to Cr(VI) reduction was observed and subsequently examined in more details. The inhibition was due to the sorption of elemental sulfur onto kaolinite, which reduced or completely eliminated the catalytic effect of elemental sulfur, depending on kaolinite concentration. This was consistent with the observation that the catalysis of externally added elemental sulfur (50 μM) on Cr(VI) reduction would disappear with a kaolinite concentration of more than 5.0 g/L. In kaolinite suspension, the overall reaction rate law was: -d[Cr(VI)]/dt = k(obs)[H(+)](2)[Cr(VI)][HS(-)](0.70

Interfacial Reduction-Oxidation Mechanisms Governing Fate and Transport of Contaminants in the Vadose Zone

Immobilization of toxic and radioactive metals in the vadose zone by In Situ Gaseous Reduction (ISGR) using hydrogen sulfide (H2S) is a promising technology for soil remediation. Earlier laboratory and field studies have shown that Cr(VI) can be effectively immobilized by treatment with dilute gaseous H2S. The objective of this project is to characterize the interactions among H2S, the metal contaminants, and soil components. Understanding these interactions is needed to assess the long-term effectiveness of the technology and to optimize the remediation system

Interfacial Reduction-Oxidation Mechanisms Governing Fate and Transport of Contaminants in the Vadose Zone

Immobilization of toxic and radioactive metals (e.g., Cr, Tc, and U) in the vadose zone by In Situ Gaseous Reduction (ISGR) using hydrogen sulfide (H2S) is a promising technology for soil remediation. Earlier laboratory and field studies have shown that Cr(VI) can be effectively immobilized by treatment with dilute gaseous H2S. The objective of this project is to characterize the interactions among H2S, the metal contaminants, and soil components. Understanding these interactions is needed to assess the long-term effectiveness of the technology and to optimize the remediation system. Proposed research tasks include: (A) Evaluation of the potential catalytic effect of mineral surfaces on the rate of Cr(VI) reduction by H2S and the rate of H2S oxidation by air; (B) Identification of the reactions of soil minerals with H2S and determination of associated reaction rates; (C) Evaluation of the role of soil water chemistry on the reduction of Cr(VI) by H2S; (D) Assessment of the reductive buffering capacity of H2S reduced soil and the potential for emplacement of long-term vadose zone reactive barriers; (E) Evaluation of the potential for immobilization of Tc and U in the vadose zone by reduction and an assessment of the potential for remobilization by subsequent reoxidation. Through a collaborative effort in the last three years, Tasks A, B, C, and E have been completed, resulting in a much improved understanding of reaction kinetics and mechanisms involved in the Cr(VI)-H2S-O2-Soil System and the treatability for Tc and U. Research on Task C will continue in the one-year period of no-cost extension granted to this project. The result will be submitted to the Department of Energy by October 2003 as a supplement to this report