Pierpaolo Zuddas

Effect of carbonate ions on pyrite (FeS2) dissolution

Neutralization by carbonate of acidification generated by pyrite (FeS2) oxidation was investigated by both solution (iron and sulfur speciation, pH and Eh) and solid (FT-IR) characterizations. Batch dissolution experiments were carried out in contact with atmospheric oxygen (20 %) in four different bicarbonated solutions ((NaHCO3)=10 -3 , 1,12.10 -2 ,1 0 -1 and 1 mol/L). Five different contact duration were selected : 6 hours, 1, 3, 8 and 30 days. Ferrous carbon- ate complexes (FeOHCO 3 and Fe(CO 3 ) - 2 2 ) tend to maintain iron in solution (up to 152.2 µmol/L in (NaHCO3)=1 mol/L solution) and to increase pyrite oxidation rate by preventing surface coating. Acidification is thus more intense in di- luted and concentrated carbonate medium ((NaHCO 3 ) =10 -3 and 1 mol/L) with respectively ∆ pH=5.06 and ∆ pH=1.99 at 30 days whereas pH remains buffered in () NaHCO 3 =1.12.10 -2 and 0.1 mol/L solutions. Siderite appears to be the first solid precipitating, transforming into gœthite, oxyhydroxy ferric sulfate incorporating sulfite and thiosulfate, and then lepidocrocite. Sulfur chemistry controls the acidification observed. Thiosulfate is the first sulfoxyanion released in solution and its oxidation into sulfite then sulfate seems to be the key of acidification production. Thus, carbonate pH buffer properties seem to be limited and effective for moderated carbonate concentrations.

Nature and distribution of chemical species on oxidized pyrite surface: Complementarity of XPS and nuclear microprobe analysis

Abstract The coupling of X-ray photoelectron spectroscopy (XPS) and nuclear microprobe analysis (NMA) using resonant reactions 16 O ( α , α ) 16 O and 12 C ( p , p ) 12 C at 3.05 and 1.725 MeV, respectively, is particularly adapted to the characterization of thin oxidation layers onto pyrites. XPS permits to determine both oxidation state and chemical environment of S and Fe. NMA gives an information about spatial distribution and chemical composition heterogeneity of oxidation products. Pyrites oxidized in acidic medium produce few solid components. Only FeII sulfate is detected on the oxidized pyrite surface. In carbonate medium, oxidation layer is more complex. Iron is mainly with a (+II) oxidation state under siderite or FeII sulfate form. As illustrated by the comparison of Fe3p and Fe2p3/2 peaks, iron has an (+III) oxidation state to a minor extent under α-FeOOH and FeIII sulfate forms from the first oxidized pyrite layers. Sulfur oxidation induces intermediate species (polysulfides and sulfoxyanions as S2O32− also evidenced in solution) indicating that oxidation occurs at solid state. NMA has shown that oxidation occurs only on localized points of pyrite surface, with oxidation layers showing spatial distribution and thickness heterogeneities.

Percolation of CO2-rich fluids in a limestone sample: Evolution of hydraulic, electrical, chemical, and structural properties

Percolation of CO2-rich fluids in limestones causes the dissolution (and eventual reprecipitation) of calcium carbonate minerals, which affect the rock microstructure and change the rock petrophysical properties (i.e., hydraulic, electrical, and elastic properties). In addition, microstructural changes further feed back to affect the chemical reactions. To better understand this coupled problem and to assess the possibility of geophysical monitoring, we performed reactive percolation laboratory experiments on a well-characterized carbonate sample 35 cm in length and 10 cm in diameter. In a comprehensive study, we present integrated measurements of aqueous chemistry (pH, calcium concentration, and total alkalinity), petrophysical properties (permeability, electrical formation factor, and acoustic velocities), and X-ray tomography imaging. The measured chemical and electrical parameters allowed rapid detection of the dissolution of calcite in the downstream fluid. After circulating fluids of various salinities at 5mL min−1 for 32 days (about 290 pore sample volumes) at a pCO2 of 1 atm (pH = 4), porosity increased by 7% (from 0.29 to 0.31), permeability increased by 1 order of magnitude (from 0.12 D to 0.97 D), and the electrical formation factor decreased by 15% (from 15.7 to 13.3). X-ray microtomography revealed the creation of wormholes; these, along with the convex curvature of the permeability-porosity relationship, are consistent with a transport-controlled dissolution regime for which advection processes are greater than diffusion processes, confirming results from previous numerical studies. This study shows that nonseismic geophysical techniques (i.e., electrical measurements) are promising for monitoring geochemical changes within the subsurface due to fluid-rock interactions.

Estimation of biomass and carbon stock in Para rubber plantations using object-based classification from Thaichote satellite data in Eastern Thailand

Abstract. This paper deals with the efficiency of measurements of carbon stock by remote sensing techniques on Para rubber plantations in Thailand. These plantations could play an important role in carbon budget and thus are part of the Clean Development Mechanism of the Kyoto Protocol. Current methods of carbon stock estimations use middle resolution images and produce results with a large uncertainty. We use very high resolution images from the Thaichote satellite, associated with field measurements to estimate the carbon stock and its evolution in the Mae num Prasae watershed, Eastern Thailand. Using object-based classifications, the plantations have been mapped and their age has been estimated from a parametric model derived from both spectral and textural information and field data. The total biomass and carbon stocked are 2.23 and 0.99 Megaton with an uncertainty of 11%. One hundred and twenty one tons of carbon are sequestered annually in the Para rubber plantations of the studied area.

Behavior of trace elements during feldspar dissolution in near-equilibrium conditions: Preliminary investigation

Abstract Synthetic sanidine, anorthite and labradorite dissolution runs are carried out in aqueous solutions close to equilibrium with secondary phases at 180°C. K-feldspar and plagioclases were spiked with Rb and Sr, respectively. The following results were obtained: 1. (1) at the beginning, dissolution is stoichiometric for the major (K or Ca) - trace element (Rb or Sr) pair; 2. (2) mineralogical buffers (adularia-kaolinite-quartz or prehnite-kaolinite-quartz) controlling the major-element content in solution are formed; 3. (3) during coprecipitation of secondary phases, for the Rb/1bK pair, the aqueous solution is enriched in Rb; for the Sr/1bCa pair, the solution is slightly depleted in Sr: 4. (4) solution composition reaches a steady state after 2 months; 5. (5) an equilibrium distribution coefficient for Rb/1bK between adularia and solution (D = 0.28) can be estimated.

Geochemistry of Zr, Hf, and REE in a wide spectrum of Eh and water composition: The case of Dead Sea Fault system (Israel)

Along the Jordan Valley-Dead Sea Fault area several natural waters in springs, wells, and catchments occur. The chemical-physical characters of the studied waters allowed for the first time the investigation of the Zr and Hf geochemical behavior, apart from REE, extended to a wide range of Eh, temperature, salinity, and pH conditions. The results of this study indicate that the dissolved Zr and Hf distribution in natural waters is strongly influenced by redox conditions since these in turn drive the deposition of Fe-oxyhydroxides or pyrite. In oxidizing waters saturated or oversaturated in Fe-oxyhydroxides (Group 1), superchondritic Zr/Hf values are measured. On the contrary, in waters where Eh< 0 values occur (Group 2), chondritic Zr/Hf values are found. Superchondritic Zr/Hf values are produced by the preferential Hf scavenging onto Fe-oxyhydroxides that is inhibited under reducing conditions consistent with the water oversaturation relative to pyrite. Redox conditions also influence the amplitude of Ce and Eu anomalies. Oxidized Group-1 waters show negative Ce anomalies related to the oxidative Ce scavenging as CeO2 onto Fe-oxyhydroxide. Reduced Group-2 waters show positive Eu anomaly values consistent with the larger Eu concentration relative to Eu in these waters suggested by model calculations. The higher stability of Eu with respect to its trivalent neighbors along the REE series can explain the above mentioned positive Eu anomaly values. The middle-REE enrichment observed in shale-normalized REE patterns of studied waters can be ascribed to carbonate and/or gypsum dissolution.