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James I. Drever - One of the best experts on this subject based on the ideXlab platform.

  • THE EFFECT OF LAND PLANTS ON WEATHERING RATES OF Silicate Minerals
    Geochimica et Cosmochimica Acta, 1994
    Co-Authors: James I. Drever
    Abstract:

    Abstract Land plants and their associated microbiota directly affect Silicate mineral weathering in several ways: by generation of chelating ligands, by modifying pH through production of CO2 or organic acids, and by altering the physical properties of a soil, particularly the exposed surface areas of Minerals and the residence time of water. In laboratory experiments far from equilibrium, 1 mM oxalate (a strong chelator of Al) has a negligible effect on the dissolution rate of alkali feldspars, but some effect on calcic feldspars and olivine. By analogy to oxalate, the overall effect of organic ligands on the weathering rate of Silicate Minerals in nature is likely to be small, except perhaps in microenvironments adjacent to roots and fungal hyphae. The effect of pH on Silicate mineral dissolution rate depends on pH: below pH 4–5, the rate increases with decreasing pH, in the circumneutral region the rate is pH-independent, and at pH values above around 8 the rate increases with increasing pH. Vegetation should thus cause an increase in weathering rate through the pH effect only where the pH is below 4–5. As an overall generalization, the effect of plants on weathering rate through changes in soil-solution chemistry is probably small for granitic rocks; it may be greater for more mafic rocks. It is the release of Ca and Mg from mafic rocks that has the greatest influence on the global CO2 budget. The effect of changes in soil physical properties on weathering rate can be major. By binding fine particles, plants can greatly increase weathering rates in areas of high physical erosion. Where erosion rates are lower, the effect of plants is less clear. On long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered Minerals from meteoric water. A major unknown in estimating the effect of the advent of land plants on weathering rates is the nature (thickness, particle size distribution, permeability) of the regolith on the pre-Silurian continents. The indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects.

Christof Holliger - One of the best experts on this subject based on the ideXlab platform.

  • Use of Silicate Minerals for pH control during reductive dechlorination of chloroethenes in batch cultures of different microbial consortia
    Applied and Environmental Microbiology, 2014
    Co-Authors: Elsa Lacroix, Alessandro Brovelli, David Andrew Barry, Christof Holliger
    Abstract:

    In chloroethene-contaminated sites undergoing in situ bioremediation, groundwater acidification is a frequent problem in the source zone, and buffering strategies have to be implemented to maintain the pH in the neutral range. An alternative to conventional soluble buffers is Silicate mineral particles as a long-term source of alkalinity. In previous studies, the buffering potentials of these Minerals have been evaluated based on abiotic dissolution tests and geochemical modeling. In the present study, the buffering potentials of four Silicate Minerals (andradite, diopside, fayalite, and forsterite) were tested in batch cultures amended with tetrachloroethene (PCE) and inoculated with different organohalide-respiring consortia. Another objective of this study was to determine the influence of pH on the different steps of PCE dechlorination. The consortia showed significant differences in sensitivities toward acidic pH for the different dechlorination steps. Molecular analysis indicated that Dehalococcoides spp. that were present in all consortia were the most pH-sensitive organohalide-respiring guild members compared to Sulfurospirillum spp. and Dehalobacter spp. In batch cultures with Silicate mineral particles as pH-buffering agents, all four Minerals tested were able to maintain the pH in the appropriate range for reductive dechlorination of chloroethenes. However, complete dechlorination to ethene was observed only with forsterite, diopside, and fayalite. Dissolution of andradite increased the redox potential and did not allow dechlorination. With forsterite, diopside, and fayalite, dechlorination to ethene was observed but at much lower rates for the last two dechlorination steps than with the positive control. This indicated an inhibition effect of Silicate Minerals and/or their dissolution products on reductive dechlorination of cis-dichloroethene and vinyl chloride. Hence, despite the proven pH-buffering potential of Silicate Minerals, compatibility with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site.

  • Evaluation of Silicate Minerals for pH Control During Bioremediation: Application to Chlorinated Solvents
    Water Air & Soil Pollution, 2012
    Co-Authors: Elsa Lacroix, Alessandro Brovelli, Christof Holliger, D A Barry
    Abstract:

    Accurate control of groundwater pH is of critical importance for in situ biological treatment of chlorinated solvents. This study evaluated a novel approach for buffering subsurface pH that relies on the use of Silicate Minerals as a long-term source of alkalinity. A screening methodology based on thermodynamic considerations and numerical simulations was developed to rank Silicate Minerals according to their buffering efficiency. A geochemical model including the main microbial processes driving groundwater acidification and Silicate mineral dissolution was developed. Kinetic and thermodynamic data for Silicate Minerals dissolution were compiled. Results indicated that eight Minerals (nepheline, fayalite, glaucophane, lizardite, grossular, almandine, cordierite, and andradite) could potentially be used as buffering agents for the case considered. A sensitivity analysis was conducted to identify the dominant model parameters and processes. This showed that accurate characterization of mineral kinetic rate constants and solubility are crucial for reliable prediction of the acid-neutralizing capacity. In addition, the model can be used as a design tool to estimate the amount of mineral (total mass and specific surface area) required in field applications.

  • Numerical modeling of field scale application of ground Silicate Minerals for groundwater pH buffering
    2012
    Co-Authors: Alessandro Brovelli, Elsa Lacroix, David Andrew Barry, Christof Holliger
    Abstract:

    Recently, it was shown that ground Silicate Minerals are a potentially suitable pH buffering material for contaminated soils undergoing acidification, for example due to acid mine leachate and coal pile runoff infiltration, as well as during the degradation of chlorinated ethenes (Lacroix et al., 2012, Doi: 10.1007/s11270-011-1058-4). Compared to traditional buffering methods, such as the circulation of an alkaline solution, Silicate Minerals are appealing because they are long-term sources of buffering capacity. In this work, the applicability of ground Silicate Minerals to a realistic field scale scenario was examined, and possible solutions to deliver the Silicates in the contaminated area were explored. To this end, a reactive transport model was developed using PHAST to study particle filtration and dissolution, pH evolution and the effect of soil and groundwater geochemistry. The model accounts for particle advection and dispersion, deep-bed filtration, porosity and hydraulic conductivity changes associated with deposition and mobilization. The deep-bed filtration coefficients vary with the flow rate and the composition of the pore-solution, ionic strength and, in particular, pH. Experimental data taken from the literature were used to calibrate and validate the deep-bed filtration model and the relationships that describe porosity and hydraulic conductivity variations. A satisfactory comparison was found in most situations. A two-dimensional model was setup to study the delivery and spreading of Silicate Minerals in a hypothetical contaminated site. Different injection scenarios were tested. It was found that the injection flow rate and well configuration strongly affect the distribution of Silicates and therefore the buffering efficiency. In general, it was observed that the distance between two injection wells should not exceed 15 m to ensure a sufficiently homogeneous distribution of the substrate. It was further observed that the optimal size of the injected particles is around 5 µm. Since ground Minerals have a rather large reactive surface area, relatively small amounts of Silicate Minerals are needed to guarantee sufficient buffering in most situations. For example, to degrade 40 mM of TCE to ethene in 100 d, 10 g (kg soil)-1 of Silicates are sufficient. With this amount, the variation of the soil porosity is less than 2%, and the associated hydraulic conductivity change predicted is also small. In summary, the numerical experiments performed confirmed that the injection of Silicate Minerals can be a viable strategy to provide pH buffering capacity to soils.

  • Utilization of Silicate Minerals for pH control during in situ bioremediation of chlorinated solvent
    2011
    Co-Authors: Elsa Lacroix, Alessandro Brovelli, David Andrew Barry, Christof Holliger
    Abstract:

    Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater due to their extensive use in industrial processes. In situ bioremediation (ISB) is an attractive technology for removal of these compounds. It relies on an anaerobic process in which specialized bacteria obtain energy for growth using chloroethenes as an electron acceptor via organohalide respiration. Engineered bioremediation is achieved by stimulating these microorganisms through the addition of electron donor in the subsurface. This technology has been widely used for bioremediation of chloroethene plumes and recent studies have indicated promising results for bioremediation of chlorinated solvent source zones. However, application of source zone ISB is still a significant technical challenge. One of the main issues is the groundwater acidification due to organohalide respiration and fermentation processes, which can inhibit the activity of dehalogenating micro-organisms. The main objective of this work was to develop an efficient pH control strategy for chloroethene ISB by using the acid neutralizing potential of Silicate Minerals. To do so, modeling and experimental approaches were combined. A geochemical model, implemented within PHREEQC, was developed to select appropriate buffer candidates and to help determine main parameters influencing mineral buffering capacity. The model included chloroethene microbial degradation kinetics, mineral dissolution kinetics and chemical speciation. Second, anaerobic microcosm experiments were performed to determine the influence of pH on dehalogenation. These microcosms were inoculated with enriched consortia of dehalogenating bacteria and fed with PCE and hydrogen. Another set of microcosm experiments was carried out to compare the buffering capacity of ten Silicate Minerals and to investigate interactions between Minerals and dehalogenating bacteria, e.g., the potential inhibitory effect of Minerals on the dehalogenating activity. These microcosms were amended with 5 mmol l-1 of PCE and 4 g l-1 of mineral with grain sizes between 50 and 100 μm. The cultivation medium was modified such that the Silicate mineral powder was the sole pH buffer present. Chloroethenes, pH and dissolved cation measurements were conducted to determine the system efficiency. Abiotic dissolution experiments were also performed to determine mineral dissolution rates in the absence of bacteria. The model confirmed that the efficiency of the system is dependent mainly on mineral dissolution kinetic constants, equilibrium constants and reactive surface area. The geochemical model and literature parameter data were used to pre-select Minerals with a buffering capacity sufficient to counterbalance acidity produced by dehalogenating bacteria at a rate of 4 mmol l-1.d-1of chloride. Of the 31 Silicate Minerals for which there were published kinetic data, 10 were identified as suitable candidates. The inhibitory pH for the dehalogenating consortia was found to vary between 5 and 6. The last steps of the dechlorination from DCE to ethene were more sensitive to pH than the first steps from PCE to DCE, as has been noted in other dechlorination studies. Results of microcosm experiments with Silicate Minerals demonstrated that, under the selected conditions, the pH control behavior and the impact on bacterial activity exhibited strong variations depending on the mineral. Of the ten Minerals tested experimentally, three (olivine, fayalite and diopside) maintained the pH in the appropriate range, i.e., between 5.5 and 6.5 and led to complete transformation of PCE. For the other Minerals tested, either the acid neutralization capacity was insufficient due to slow dissolution kinetics (glaucophane and staurolite) or dechlorination activity was inhibited by (unmeasured) compounds released during mineral dissolution. Both modeling and experimental results demonstrated the feasibility of using selected Silicate Minerals as a buffering agent during ISB. However, the experimental results also revealed a mineral-induced potential inhibitory effect that should be investigated prior to application at a contaminated site.

Pere Enrique - One of the best experts on this subject based on the ideXlab platform.

Jiwchar Ganor - One of the best experts on this subject based on the ideXlab platform.

  • a new approach for measuring dissolution rates of Silicate Minerals by using silicon isotopes
    Geochimica et Cosmochimica Acta, 2013
    Co-Authors: Chen Gruber, Liat Harpaz, Chen Zhu, T D Bullen, Jiwchar Ganor
    Abstract:

    Abstract The two major problems in measuring dissolution rates under close-to-natural conditions in laboratory experiments are: (1) our inability to measure small differences in concentration between solutions with relatively high concentrations and (2) the inherent problem that the change in solution concentration is affected by both the dissolution of the primary mineral and the precipitation of secondary Minerals. The present manuscript proposes and tests a novel method, “the isotope ratio method”, for measuring slow dissolution rates of Silicate Minerals by measuring the change in the ratios between stable isotopes of silicon of a spiked solution. Based on mass balance calculations, two equations that describe the dissolution rate of a Silicate mineral in a batch reactor and in a flow-through reactor at steady-state are developed. The precipitation rate of the secondary mineral may be calculated by subtracting the release rate of Si that was calculated using isotope dilution from the rate that was calculated using the proposed isotope ratio method. Numerical simulations of flow-through and batch experiments demonstrate that the “isotope ratio method” is significantly more precise than conventional methods. The analytical uncertainty for the determination of dissolution rates was found to be low for the entire range of reported field-based dissolution rates. The calculation showed that even relatively large isotopic fractionations (up to e values of 20‰), introduce insignificant uncertainties. Preliminary flow-through experiments support the above conclusion that dissolution rate may be obtained accurately and with small uncertainty using the proposed “isotope ratio method”.

Giovanni Ferraris - One of the best experts on this subject based on the ideXlab platform.

  • A survey of hybrid twins in Silicate Minerals
    European Journal of Mineralogy, 2011
    Co-Authors: Isabella Pignatelli, Massimo Nespolo, Giovanni Ferraris
    Abstract:

    This article is the second part of the survey presenting a systematic crystallographic analysis of hybrid twins inMinerals. Here we deal with Silicate Minerals and in particular the hybrid twins of allanite, augite, axinite, beryl, chloritoid, clinoenstatite, clinozoisite, cordierite, cummingtonite-grunerite, diopside-hedenbergite, enstatite-ferrosilite, epidote, forsterite-fayalite, johannsenite, natrolite, piemontite, pigeonite, protoenstatite, pumpellyite, pyroxmangite, quartz, titanite, spodumene, staurolite, topaz, tourmaline, tremoliteactinolite-ferroactinolite, tridymite. The comparison with the twin analysis of non-Silicate Minerals shows that the number of hybrid twins for non-Silicates is higher than that of the Silicates. This well agrees with the fact that the twin formation is favoured by the high symmetries of the close-packing topologies, typical of many non-Silicates, especially oxides and simple sulphides, whereas the Silicate structures are more complex and characterized by different degrees of polymerization of Si-O tetrahedra.

  • A survey of hybrid twins in non-Silicate Minerals
    European Journal of Mineralogy, 2009
    Co-Authors: Massimo Nespolo, Giovanni Ferraris
    Abstract:

    This article is the second part of the survey presenting a systematic crystallographic analysis of hybrid twins in Minerals. Here we deal with Silicate Minerals and in particular the hybrid twins of allanite, augite, axinite, beryl, chloritoid, clinoenstatite, clinozoisite, cordierite, cummingtonite-grunerite, diopside-hedenbergite, enstatite-ferrosilite, epidote, forsterite-fayalite, johannsenite, natrolite, piemontite, pigeonite, protoenstatite, pumpellyite, pyroxmangite, quartz, titanite, spodumene, staurolite, topaz, tourmaline, tremolite-actinolite-ferroactinolite, tridymite. The comparison with the twin analysis of non-Silicate Minerals shows that the number of hybrid twins for non-Silicates is higher than that of the Silicates. This well agrees with the fact that the twin formation is favoured by the high symmetries of the close-packing topologies, typical of many non-Silicates, especially oxides and simple sulphides, whereas the Silicate structures are more complex and characterized by different degrees of polymerization of Si-O tetrahedra.

  • A survey of hybrid twins in non-Silicate Minerals
    European Journal of Mineralogy, 2009
    Co-Authors: Massimo Nespolo, Giovanni Ferraris
    Abstract:

    Hybrid twins are twins in which two or more concurrent sublattices based on the same twin element exist. Each sublattice contributes, with its twin index and obliquity, to the overall degree of lattice restoration, which is measured by the effective twin index nE, a generalization of the classical twin index. A systematic analysis of twins in the following non-Silicate Minerals is presented and the hybrid nature of several of them is pointed out: apatite, anhydrite, aragonite, aramayoite, arsenic, atacamite, baddeleyite, becquerelite, bellingerite, brookite, butlerite, calcite, cassiterite, celestine, chalcostibite, corundum, cryolite, derbylite, diaphorite, dolomite, euxenite-(Y), columbite-(Fe) (also known as ferrocolumbite), glaucodot, gudmundite, gypsum, haindingerite, hausmannite, hematite, ilmenite, iodargyrite, linarite, kotoite, marcasite, maucherite, metarossite, millerite, monazite, nickeline, nitratine, proustite, pyrargyrite, raspite, sassolite, scorzalite, stannite, symplesite, ulexite, xenotime-(Y).