Maria L. Peterson

Chemical weathering rates of a soil chronosequence on granitic alluvium: I. Quantification of mineralogical and surface area changes and calculation of primary silicate reaction rates

Mineral weathering rates are determined for a series of soils ranging in age from 0.2–3000 Ky developed on alluvial terraces near Merced in the Central Valley of California. Mineralogical and elemental abundances exhibit time-dependent trends documenting the chemical evolution of granitic sand to residual kaolinite and quartz. Mineral losses with time occur in the order: hornblende > plagioclase > K-feldspar. Maximum volume decreases of >50% occur in the older soils. BET surface areas of the bulk soils increase with age, as do specific surface areas of aluminosilicate mineral fractions such as plagioclase, which increases from 0.4–1.5 m2 g−1 over 600 Ky. Quartz surface areas are lower and change less with time (0.11–0.23 m2 g−1). BET surface areas correspond to increasing external surface roughness (λ = 10–600) and relatively constant internal surface area (≈ 1.3 m2 g−1). SEM observations confirm both surface pitting and development of internal porosity. A numerical model describes aluminosilicate dissolution rates as a function of changes in residual mineral abundance, grain size distributions, and mineral surface areas with time. A simple geometric treatment, assuming spherical grains and no surface roughness, predicts average dissolution rates (plagioclase, 10−17.4; K-feldspar, 10−17.8; and hornblende, 10−17.5 mol cm−1 s−1) that are constant with time and comparable to previous estimates of soil weathering. Average rates, based on BET surface area measurements and variable surface roughnesses, are much slower (plagioclase, 10−19.9; K-feldspar, 10−20.5; and hornblende 10−20.1 mol cm−2 s−1). Rates for individual soil horizons decrease by a factor of 101.5 over 3000 Ky indicating that the surface reactivities of minerals decrease as the physical surface areas increase. Rate constants based on BET estimates for the Merced soils are factors of 103–104 slower than reported experimental dissolution rates determined from freshly prepared silicates with low surface roughness (λ < 10). This study demonstrates that the utility of experimental rate constants to predict weathering in soils is limited without consideration of variable surface areas and processes that control the evolution of surface reactivity with time.

Differential redox and sorption of Cr (III/VI) on natural silicate and oxide minerals: EXAFS and XANES results

Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to investigate the reduction of aqueous Cr(VI) to Cr(III) in magnetite-bearing soils from Cr-contaminated sites. Soils from two field sites were examined, showing that mixed-valence Cr(III/VI) effluent is reduced to Cr(III) when associated with the magnetite fraction of the soil, whereas the Cr effluent associated with non-Fe(II)-bearing minerals results in mixed Cr(III/VI) adsorbates or precipitated phases. The Fe{sup 2+} in magnetite, Fe{sup 2+}Fe{sub 2}{sup 3+}O{sub 4}, may act as an electron source for heterogeneous Cr(VI)-to-Cr(III) reduction, converting magnetite topotactically to maghemite, {gamma}-Fe{sub 2}{sup 3+}. The ratio of Cr(VI)/total Cr was determined by the height of the Cr(VI) XAFS pre-edge feature, which is due to a Is to 3d electronic transition. This pre-edge feature was calibrated as a function of Cr(VI)/Cr(III) using mixtures of Cr(III) and Cr(VI) model compounds. Environmental remediation of Cr-contaminated sites requires knowledge of chromium oxidation and speciation, and XAFS spectroscopy may be used to supply both types of information with minimal sample processing or data analysis. 36 refs., 9 figs., 2 tabs.

REDUCTION OF AQUEOUS TRANSITION METAL SPECIES ON THE SURFACES OF FE( II)-CONTAINING OXIDES

Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1–7 at 25°C. For an aqueous transition metal m, such reactions are 3[Fe2+Fe23+]O4(magnetite)+2/nmz→4[Fe23+]O3(magnetite)+Fe2++2/nmz−n and 3[Fe2+Ti]O3(ilmenite)+2/nmz→Fe23+Ti3O9(pseudorutile)+Fe2++2/nmz−n, where z is the valance state and n is the charge transfer number. The half cell potential range for solid state oxidation [Fe(II)] → [Fe(III)] is −0.34 to −0.65 V, making structural Fe(II) a stronger reducing agent than aqueous Fe2+ (−0.77 V). Reduction rates for aqueous metal species are linear with time (up to 36 h), decrease with pH, and have rate constants between 0.1 and 3.3 × 10−10 mol m−2 s−1. Iron is released to solution both from the above reactions and from dissolution of the oxide surface. In the presence of chromate, Fe2+ is oxidized homogeneously in solution to Fe3+. X-ray photoelectron spectroscopy (XPS) denotes a Fe(III) oxide surface containing reduced Cr(III) and V(IV) species. Magnetite and ilmenite electrode potentials are insensitive to increases in divalent transition metals including Zn(II), Co(II), Mn(II), and Ni(II) and reduced V(IV) and Cr(III) but exhibit a log-linear concentration-potential response to Fe(III) and Cu(II). Complex positive electrode responses occur with increasing Cr(VI) and V(V) concentrations. Potential dynamic scans indicate that the high oxidation potential of dichromate is capable of suppressing the cathodic reductive dissolution of magnetite. Oxide electrode potentials are determined by the Fe(II)/Fe(III) composition of the oxide surface and respond to aqueous ion potentials which accelerate this oxidation process. Natural magnetite sands weathered under anoxic conditions are electrochemically reactive as demonstrated by rapid chromate reduction and the release of aqueous Fe(III) to experimental solution. In contrast, magnetite weathered under oxidizing vadose conditions show minimum reactivity toward chromate ions. The ability of Fe(II) oxides to reduce transition metals in soils and groundwaters will be strongly dependent on the redox environment.

Electrochemistry and dissolution kinetics of magnetite and ilmenite

Natural samples of magnetite and ilmenite were experimentally weathered in pH 1–7 anoxic solutions at temperatures of 2–65 °C. Reaction of magnetite is described as [Fe2+Fe23+]O4(magnetite) + 2H+ → γ[Fe23+]O3(maghemite) + Fe2+ + H2O. Dynamic polarization experiments using magnetite electrodes confirmed that this reaction is controlled by two electrochemical half cells, 3[Fe2+Fe23+]O4(magnetite) → 4γ[Fe23+]O3(maghemite) + Fe2+ + 2e− and [Fe2+Fe23+]O4(magnetite) + 8 H+ + 2e− → 3Fe2+ + 4H2O, which result in solid state Fe3+ reduction, formation of an oxidized layer and release of Fe(II) to solution. XPS data revealed that iron is present in the ferric state in the surfaces of reacted magnetite and ilmenite and that the TiFe ratio increased with reaction pH for ilmenite. Short-term (<36 h) release rates of Fe(II) were linear with time. Between pH 1 and 7, rates varied between 0.3 and 13 × 10−14 mol · cm−2 · s−1 for magnetite and 0.05 and 12.3 × 10−14 mol · cm−2 · s−1 for ilmenite. These rates are two orders of magnitude slower than electrochemical rates determined by Tafel and polarization resistance measurements. Discrepancies are due to both differences in geometric and BET surface area estimates and in the oxidation state of the mineral surface. In long-term closed-system experiments (<120 days), Fe(II) release slowed with time due to the passivation of the surfaces by increasing thicknesses of oxide surface layers. A shrinking core model, coupling surface reaction and diffusion transport, predicted that at neutral pH, the mean residence time for sand-size grains of magnetite and ilmenite will exceed 107 years. This agrees with long-term stability of these oxides in the geologic record.

Direct XAFS evidence for heterogeneous redox reaction at the aqueous chromium/magnetite interface

Abstract Hexavalent chromium is a highly toxic, carcinogenic, and mobile contaminant present in wastewaters from mining and industrial operations. Its reduction to trivalent chromium, both less toxic and less soluble over the pH range of most natural waters, has previously been observed in solutions in contact with the redox-sensitive iron oxide magnetite (Fe2+Fe23+O4), and occurs via electron transfer from Fe2+ in the magnetite structure. This study presents direct in situ X-ray absorption fine structure (XAFS) evidence for the presence of Cr(III), initially resulting from the reduction of Cr(VI)aq in solution, at the surface of synthetic magnetite at near-neutral pH. Cr(VI) reacts with freshly-synthesized magnetite at a surface coverage of 4.5 μmol m−2 to be entirely reduced to Cr(III), as evidenced by the Cr absorption edge position and by the absence of a 1s → 3d pre-edge peak. XAFS spectra of Cr on progressively oxidized magnetite surfaces, however, show increasing pre-edge peak height indicating the presence of Cr(VI), i.e. a decrease in the Cr-reducing capacity of altered (maghemite-coated) magnetite grains. As expected, Cr(VI) sorbed to a ferric oxide, synthetic maghemite (γ-Fe23+O3), is not significantly reduced. The Cr pre-edge peak height for Cr(VI) reacted with maghemite is comparable to pre-edge peak heights of Cr(VI) model compounds. XAFS spectra for Cr model compounds compare well with theoretical XAFS spectra for the same compounds calculated using the ab initio, single- and multiple-scattering code FEFF. Fit parameters derived from FEFF models of Cr(III)- and Cr(VI)-containing compounds were used to determine the coordination environment (number and chemical identity of neighboring shells of atoms and their interatomic distances) of Cr sorbed to magnetite and maghemite samples.

Sources and fractionation processes influencing the isotopic distribution of H, O and C in the Long Valley hydrothermal system, California, U.S.A.

Abstract The isotopic ratios of H, O and C in water within the Long Valley caldera, California reflect input from sources external to the hydrothermal reservoir. A decrease in δD in precipitation of 0.5‰ km −1 , from west to east across Long Valley, is caused by the introduction of less fractionated marine moisture through a low elevation embayment in the Sierra Nevada Mountain Range. Relative to seasonal fluctuations in precipitation (−158 to −35‰.), δD ranges in hot and cold surface and groundwaters are much less variable (−135 to −105‰.). Only winter and spring moisture, reflecting higher precipitation rates with lighter isotopic signatures, recharge the hydrological system. The hydrothermal fluids are mixtures of isotopically heavy recharge (δD = − 115‰, δ 18 O = − 15‰) derived from the Mammoth embayment, and isotopically lighter cold water (δD = −135‰, δ 18 O = −18‰). This cold water is not representative of current local recharge. The δ 13 C values for dissolved carbon in hot water are significantly heavier (− 7 to − 3‰) than in cold water (−18 to −10‰) denoting a separate hydrothermal origin. These δ 13 C values overlie the range generally attributed to magmatic degassing of CO 2 . However, δ 13 C values of metamorphosed Paleozoic basement carbonates surrounding Long Valley fall in a similar range, indicating that hydrothermal decarbonization reactions are a probable source of CO 2 . The δ 13 C and δ 18 O values of secondary travertime and vein calcite indicate respective fractionation with CO 2 and H 2 O at temperatures approximating current hydrothermal conditions.

Chemical equilibrium and mass balance relationships associated with the Long Valley hydrothermal system, California, U.S.A.

Abstract Recent drilling and sampling of hydrothermal fluids from Long Valley permit an accurate characterization of chemical concentrations and equilibrium conditions in the hydrothermal reservoir. Hydrothermal fluids are thermodynamically saturated with secondary quartz, calcite, and pyrite but are in disequilibrium with respect to aqueous sulfide-sulfate speciation. Hydrothermal fluids are enriched in 18 O by approximately 1‰ relative to recharge waters. 18 O and Cl concentrations in well cuttings and core from high-temperature zones of the reservoir are extensively depleted relative to fresh rhyolitic tuff compositions. Approximately 80% of the Li and 50% of the B are retained in the altered reservoir rock. Cl mass balance and open-system 18 O fractionation models produce similar water-rock ratios of between 1.0 and 2.5 kg kg −1 . These water-rock ratios coupled with estimates of reservoir porosity and density produce a minimum fluid residence time of 1.3 ka. The low fluid Cl concentrations in Long Valley correlate with corresponding low rock concentrations. Mass balance calculations indicate that leaching of these reservoir rocks accounts for Cl losses during hydrothermal activity over the last 40 ka.