Surface area, porosity and water adsorption properties of fine volcanic ash particles

Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash (500 Å. All the specimens had similar pore size distributions, with a small peak centered around 50 Å. These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption as values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be ~10–2 g. Some volcanic implications of this study are discussed

Chemical coagulation of combined sewer overflow: Heavy metal removal and treatment optimization

International audienceThe coagulation of combined sewer overflow (CSO) was investigated by jar-testing with two commercial coagulants, a ferric chloride solution (CLARFER) and a polyaluminium chloride (WAC HB). CSO samples were collected as a function of time during various wet-weather events from the inlet of Boudonville retention basin, Nancy, France. Jar-tests showed that an efficient turbidity removal can be achieved with both coagulants, though lower optimum dosages and higher re-stabilization concentrations were obtained with the aluminum-based coagulant. Optimum turbidity removal also yielded effective heavy metal elimination. However, the evolution with coagulant dosage of Cu, Zn, Pb, Cr, soluble and suspended solids contents followed various patterns. The removal behaviors can be explained by a selective aggregation of heavy metal carriers present in CSO and a specific interaction between hydrolyzed coagulant species and soluble metals. Stoichiometric relationships were established between optimal coagulant concentration, range of optimal dosing, and CSO conductivity, thus providing useful guidelines to adjust the coagulant demand during the course of CSO events

Surface Heterogeneity at the Solid-Gas Interface of Hydrophilic Solids Modified by Water-Repellent Molecules

The surface heterogeneity of a hydrophilic calcium carbonate (calcite) at the solid-gas interface was studied before and after modification by the adsorption of different amounts of a water-repellent molecule (WRM). The surface heterogeneity was analysed using nitrogen, argon and water as molecular probes. Low-pressure adsorption techniques coupled with derivative isotherm summation (DIS) analysis of the experimental curves gave quantitative information on the decrease in the calcite surfaces and the increase in the WRM surfaces. The modelling results enabled the monolayer capacity for WRMs as measured by argon adsorption to be correlated with the disappearance of calcite surfaces and the disappearance of high-energy adsorption sites as measured by nitrogen adsorption. In addition, the argon results suggested that WRMs first adsorb on argon low-energy carbonate faces and then on low- and high-energy faces. With water, the adsorption energy distribution remained unchanged in shape, indicating that this molecule can diffuse into the adsorbed layer between and around the adsorbed hydrophilic heads of the WRMs and thereby interact directly with the carbonate surface

A comparative study of some kaolinites surface properties

International audienceThe surface properties of five kaolinites of various origins were analyzed using potentiometric titration, electrokinetic measurements and low-pressure gas adsorption associated to derivative isotherm summation (DIS) modeling. The data show that sample structure is important in determining the surface properties. The combined analytical results clearly shows the existence of a permanent negative layer charge for all the samples. The general features that could be observed for the five kaolinites are: the titration curves are shifted to lower pH with increasing ionic strength due to the layer charge; the amount of consumed proton is influenced by the permanent layer charge; although low, the permanent charge determines the electrokinetic behavior. The shape anisotropy and the charge distribution on the basal and edge surfaces are crucial parameters to understand the behavior of kaolinite particles in aqueous media. The Cameroonian sample of the set exhibits a significant permanent charge together with large specific surface area associated to fine particle size and considerable shape anisotropy

Adsorption of humic acid onto a kaolinitic clay studied by high-resolution argon adsorption volumetry

28 ref. doi: 10.1180/0009855033840107International audienc

Novel Insights on the Transport of HPAM Solutions in Low Permeability Porous Media: Impacts of Brine and Reservoir Properties

International audienceChemical EOR is now considered as an attractive option for low permeability reservoirs, in particular where lack of gas supply does not allow gas injection processes. However, its application can be challenging for permeabilities below 100 mD as poor injectivity and high chemical retention are frequently observed. This work aimed at investigating the impact of both chemical and mineralogical parameters on the transport of polymer solutions in well-controlled low permeability porous media. Selected polyacrylamide (HPAM) solubilized in brines of variable strengths and hardnesses were injected in granular sand and clays packs having similar petrophysical characteristics (permeability around 60-80 mD) but variable and well controlled mineralogical compositions. The granular packs were characterized in terms of structure (SEM) and specific surface area (BET) before and after polymer injections. The main observables of the coreflood tests were the resistance and residual resistance factors generated by the polymer, the polymer inaccessible pore volume and its irreversible retention. Viscometric analysis showed that the HPAM solutions intrinsic viscosity decreased with increasing total salinity, as expected from charge screening, with a sharp decrease in presence of divalent cations, even at low ionic strength, which was less expected. Coreflood experiments revealed that polymer retention, resistance factor and irreversible resistance factor increased significantly: (a) with increasing ionic strength and hardness for porous media of a given mineralogical composition (this appeared consistent with the outcomes of the viscometric study and confirmed the major impact of hardness); (b) in presence of clays, even at low ionic strength and hardness. The polymer inaccessible pore volume was significantly impacted by the presence of clays, but not by the brine composition. Assuming that polymer retention originated in polymer adsorption, irreversible resistance factors were translated into adsorbed layer thicknesses according to a simple capillary bundle model. This allowed discussing the results in terms of adsorbed layer density, which was showed to increase if brine hardness was increased and to be lower in presence of illite than kaolinite and pure quartz. These findings indicate that complex polymer adsorption/retention mechanisms occur depending on the clay type (layer charge and expandability). This systematic study allowed dissociating the impacts of salinity, hardness and clay contents/types on the transport properties of polymer solutions in low permeability porous media. Its results should be of interest to the chemical EOR industry as they provide guides to help tuning the injection brine composition and polymer concentration to the reservoir properties

Evidence of a critical content in Fe(0) on FoCa7 bentonite reactivity at 80 °C

In order to assess the evolution of the confinement properties of clay engineered barriers (EBS) when in contact with metallic canisters containing radioactive wastes, Fe(0)-bentonite interactions need to be assessed. "45 days-80 °C" tests were performed using powdered FoCa7 bentonite and metallic iron. Since one fundamental parameter may be the available quantity of Fe(0), a wide range of Iron/Clay mass ratios (I/C) from 0 to 1/3 is used. The confinement power of clay material results from the swelling properties and the retention capacity. Thus, the major criterion which is chosen to assess the evolution of the confinement properties in this study is the variation of Cation Exchange Capacity (CEC). In parallel, the physico-chemical evolution of bentonite is studied using XRD and EDS-TEM microanalyses. The evolution of the distribution of iron environments is obtained by 57Fe Mössbauer spectroscopy. This study evidences that both kaolinite and smectite from the bentonite are altered into SiAlFe gels when in contact with Fe(0). These gels maturates into Fe-rich di-trioctahedral phyllosilicates, whose composition is bounded by the one of odinite and greenalite in a Fe-M+-4Si diagram when I/C = 1/3. Most of all, it is evidenced that the reaction depends on the available quantity of Fe(0). When the I/C ratio is between 1/30 and 1/7.5, the exchange capacity of FoCa7 bentonite starts decreasing, the consumption of Fe(0) becomes significant, the alteration of smectites occurs and secondary oxides are formed. The crystallization of Fe-rich phyllosilicates is observable when I/C ratio is higher, from a threshold between 1/7.5 and 1/5. Above I/C = 1/3.75, initial iron oxides are strongly consumed and participate in the incorporation of Fe2+ and Fe3+ in gels or new phyllosilicates octahedra. These experimental results were used as input data for the prediction of the long-term evolution of the EBS using Crunch reaction-transport model. © 2007 Elsevier B.V. All rights reserved

Bentonite–iron interactions under alkaline condition: An experimental approach

International audienceThe clay–iron interactions in presence of an alkaline (Ca, Na) fluid were experimentally investigated at three temperatures (80, 150 and 300°C) for three durations of 3, 6 and 9 months. The starting product, a bentonite (MX-80), was mixed with iron powder (metallic iron and magnetite) and placed in contact with a metallic iron plate. Mineralogical changes have been characterised by XRD and SEM imaging, and the crystal chemistry of clay particles and by-products has been determined by EELS- and EDS-TEM. The rate and nature of the clay transformations depend on two critical parameters: temperature and distance to metallic iron plate. At 300°C, run products are dominated by zeolite (mordenite-type zeolite) and Fe-rich vermiculite, the iron/magnesium ratio of vermiculite increasing at proximity of the iron plate. At 80 and 150°C, di-octahedral smectite remains the predominant clay, but is progressively enriched in Fe, especially at proximity of the iron plate. Besides the clay transformation, the progressive dissolution of feldspars and of the metallic iron powder is observed. Thus, the dissolution of metallic iron results in the formation of magnetite and Fe-rich clays (Fe-rich smectite, Fe-vermiculite), which are different from those (Fe-saponite, chlorite) produced in experiments carried out with neutral pH solutions

Pre-collisional geodynamic context of the southern margin of the Pan-African fold belt in Cameroon

International audienceWe reassess the geodynamic context close to the Congo craton during the pre-collisional period of the Pan-African orogeny from whole-rock major and trace element compositions and isotopic data obtained in the westward extension of the Yaounde series (Boumnyebel area, Cameroon). The series consists of metasediments (micaschists, minor calc-silicate rocks and marbles) and meta-igneous rocks (hornblende gneisses, amphibolites, metagabbros, pyroxenites and talcschists) recrystallized under high-pressure conditions.Chemically, the micaschists correspond to shales and greywackes similar to the Yaounde high-grade gneisses. 87Sr/86Sr initial ratios (0.7084–0.7134), moderately negative εNd(620 Ma) values (−5.75 to −7.81), Nd model ages (1.66 < TDM < 1.74 Ga) and radiometric ages point to the conclusion that the Yaounde basin was filled with siliciclastic sediments derived from both reworked older continental crust (Palaeoproterozoic to Archaean in age) and Neoproterozoic juvenile volcanogenic material. This occurred in the same time span (625–1100 Ma) as the deposition of the Lower Dja, Yokadouma, Nola and Mintom series (Tonian–Cryogenian). Dolomitic marble associated with mafic/ultramafic rocks and characterized by high Cr (854–1371 ppm) and Ni (517–875 ppm) contents, are considered to result from chemical precipitation in relation with submarine magmatic activity.Talcschists (orthopyroxenitic to harzburgitic in composition) show primitive-mantle-normalized multi-element patterns with significant negative Nb–Ta anomalies, and slopes similar to that of average metasomatically altered lithospheric mantle. These rocks could be mantle slices involved in the collision tectonics. Amphibolites show the compositions of island-arc basalts with systematic negative Nb–Ta anomalies, 87Sr/86Sr initial ratios mostly <0.7047 and positive εNd(620 Ma) values (+1.41 to +6.58). They are considered to be the expression of incipient oceanisation to the north of the Congo craton during the early Neoproterozoic. Hornblende gneisses show andesitic compositions, with high 87Sr/86Sr initial ratios (0.7105 and 0.7125) and low εNd(620) values (−14.0 and −20.7) suggesting that their genesis involved juvenile and recycled older crustal materials. Syn-metamorphic metagabbro (Mamb) and metadiorite (Yaounde) intrusions show negative Nb–Ta negative anomalies but enrichment in light rare-earth and large-ion lithophile elements, suggesting a metasomatized mantle source. Overall, meta-igneous rocks seem to be representative of distinct magmatic events that accompanied the evolution of the Yaounde sedimentary basin, from opening and oceanisation to convergence and closure in relation with the collisional process.These data suggest that the Yaounde basin should not be considered as a back-arc basin, but more likely represents the expression of extensional processes to the north of the Congo craton, which led to rifting, fragmentation and limited oceanisation. In this view, the Adamawa-Yade block may represent a micro-continent detached from the Congo craton during the early Neoproterozoic