Fabien Thomas

Mechanism of adsorption and desorption of water vapor by homoionic montmorillonites; 2, The Li (super +) , Na (super +) , K (super +) , Rb (super +) and Cs (super +) -exchanged forms

Methods previously used to distinguish between water adsorbed on external surfaces and in the interlamellar space of Na-montmorillonite during adsorption and desorption of water vapor have been extended to a set of homoionic Li-, Na-, K-, Rb- and Cs-montmorillonite. The textural and structural features have been investigated at different stages of hydration and dehydration using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption gravimetry, immersion microcalorimetry and X-ray powder diffraction under controlled humidity conditions. During hydration, the size of the quasi-crystals decreases from 33 layers to 8 layers for Na-montmorillonite and from 25 layers to 10 layers for K-montmorillonite, but remains stable around 8–11 layers for Cs-montmorillonite. Each homoionic species leads to a one-layer hydrate, which starts forming at specific values of water vapor relative pressure. Li-, Na- and K-montmorillonite can form a two-layer hydrate. By comparing experimental X-ray diffraction patterns with theoretically simulated ones, the evolution of structural characteristics of montmorillonites during hydration or desorption can be described. Using structural and textural data, it is shown that during adsorption: (1) the rate of filling of interlamellar space of the one layer hydrate increases with the relative pressure but decreases with the size of the cations; and (2) the different hydrated states are never homogeneous.

Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite; 3, The Mg (super 2+) , Ca (super 2+) , and Ba (super 3+) exchanged forms

The swelling of some well-defined Mg-, Ca-, Sr- and Ba- homoionic montmorillonites was studied in the domain of water relative pressures lower than 0.95. This involves the expansion of the crystal lattice itself, commonly known as the “interlamellar expansion” or “inner crystalline swelling”. The initial freeze-dried clays were characterized by nitrogen adsorption-desorption volumetry and controlled transformation rate thermal analysis. The evolution of the structural and textural properties of these different clays at different stages of hydration and dehydration was investigated using water adsorption gravimetry, immersion microcalorimetry at different precoverage water vapor relative pressures and X-raydiffraction (XRD) under controlled humidity conditions. Large textural variations are observed in the dry state depending on the exchangeable cations. The 2-layer hydrate exhibits the most ordered layer stacking. Water is mainly adsorbed in the interlamellar space. With increasing water pressure, each homoionic species leads to a 1-layer hydrate and, with the exception of Ba-montmorillonite, to a predominant 2-layer hydrate. The relative pressure corresponding to the formation of the 2-layer hydrate decreases with increasing hydration energy of the interlayer cation. For Ca-, Sr- or Mg-montmoriHonites, simulation of XRD patterns leads to the definition of successive homogeneous states corresponding to the 2-layer hydrate. Furthermore, it yields the water filling ratio corresponding to the different hydration states during adsorption and desorption of water vapor.

Dolomite surface speciation and reactivity in aquatic systems

Abstract The surface charge of dolomite (CaMg(CO 3 ) 2 ) was measured as a function of pH (6.5–11.5), pCO 2 (10 −3.5 , 0.01, and 0.96 atm) and ionic strength (0.01, 0.1, and 0.5 M NaCl) using potentiometric titrations in a limited residence time reactor. Dolomite zeta potential (ζ) was determined using streaming potential and electrophoresis techniques at pH 2 to 12 in solutions having ionic strengths from 0.001 to 0.1 M NaCl as a function of aqueous Ca 2+ , Mg 2+ , and CO 3 2− concentrations. The point of zero charge (PZC) and isoelectric point (IEP) of dolomite are the same (pH ∼8 at pCO 2 ∼10 −3.5 atm) and very close to those of calcite and magnesite. On the basis of these results, a surface complexation model (SCM) is proposed that postulates the presence of three distinct primary hydration sites: >CO 3 H°, >CaOH°, and >MgOH°. The intrinsic stability constants of dolomite surface reactions were determined by fitting the pH dependence of the surface charge and taking into account the isoelectric points and ζ-potential values for a wide range of solution compositions. In most natural aquatic environments, dolomite surface speciation can be modeled using the following species: >CO 3 − , >CO 3 Me + , >MeOH 2 + , >MeHCO 3 o , and >MeCO 3 − , where Me = Ca, Mg. The speciation model presented in this study allows description of metal and ligand adsorption onto dolomite surface and provides new insights on the mechanisms that control dolomite dissolution/crystallization in aqueous solutions. In particular, it is shown that dolomite dissolution is controlled by the protonation of >CO 3 H° surface complexes at pH MeOH 2 + groups at higher pH.

Processes at the magnesium-bearing carbonates/solution interface. I. A surface speciation model for magnesite

Abstract The surface charge of magnesite (MgCO3) was measured at 25°C as a function of pH (4.6 to 11) and ionic strength (0.01, 0.1 and 0.5 M NaCl) under pCO2 from 10−3.5 to 0.96 atm. The acid-base titrations were performed in a limited residence time reactor following the approach developed by Charlet et al. (1990) for rhodochrosite and siderite. Magnesite zeta potential (ζ) was determined as a function of pH (1.5 to 12), ionic strength (0.001 to 0.1 M) and Mg2+ and HCO3− concentrations. Based on these results and spectoscopic data on the calcite and dolomite/water inerfaces, a surface complexation model is presented which postulates the formation of the primary hydration species >CO3H° and >MgOH°. Within this scheme, magnesite surface speciation is governed by the following species: >CO3H°, >CO3−, >CO3Mg+, >MgOH°, >MgO−, >MgOH2+, >MgHCO°3, and >MgCO3−. The intrinsic stability constants of these species were determined by fitting the pH dependent surface charge over a wide range of solution composition. For the conditions investigated in this study, the measured ζ-potential values of magnesite are in good agreement with the sign of the surface potential and its dependence on solution composition calculated using the surface complexation model generated in this study.

Layer charge and electrophoretic mobility of smectites

The aim of this study is to determine the effect of the layer charge of smectites on their electrophoretic mobility (EM), using electrophoresis measurements. In order to cover the charge domain from 0 to 2 charges per unit structural cell, two clay series were used: thermally treated Cu-montmorillonite (0-0.7) and synthetic saponites (0.7-2). All the studied samples are negatively charged in a pH range from 2 to 12. Neither layer charge nor ionic strength influences the EM of smectites at neutral to alkaline pH. At acidic pH, the EM of smectites ranges from −3 to −0.4 10−8 m2 s−1 V−1 when the layer charge ranges from 0.8 to 2. High charge and low charge smectites, which are not expandable, tend to aggregate in aqueous suspension, and the measured EM corresponds then to the amphoteric sites of the edge faces of the layer stacks. These sites are negatively charged at pH values above 3.

Fe, Mg and Al distribution in the octahedral sheet of montmorillonites. An infrared study in the OH-bending region

Abstract Ten montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤ 0.56 per Si8 formula unit), a linear relationship between the position of the δAlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δAlAlOH, δAlMgOH and δAlFeOH) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.

N2-BET SPECIFIC SURFACE AREAS OF SOME LOW ACTIVITY CLAY SOILS AND THEIR RELATIONSHIPS WITH SECONDARY CONSTITUENTS AND ORGANIC MATTER CONTENTS

Size fractions (2000–20 μm, 20–2 μm, 2–0.2 μm, and 0.2–0 μm) were obtained after Na-resin treatment of five top horizons from low-activity clay soils. Their specific surface areas (SSA) were measured by the N2-BET method. The same method was applied after organic matter removal by H2O2 treatment to two of the bulk soils and their size fractions. The weighted sum of the SSA of the fractions fitted well to the SSA of the bulk soils, indicating that the association of particles, destroyed by the fractionation procedure had no effect on the SSA. The SSA of the coarsest fractions were greater than theoretical values from a sphere model, because of the presence of microaggregates. A good relationship was found between the SSA of the soils and subfractions and their total contents in R2O3 (R2O3 = Al2O3 + Fe2O3), i.e., an estimation of the importance of the secondary minerals present. Organic matter removal from two of the soil samples and their subfractions resulted in systematical but weak increase in SSA (≤ 8 m2·g−1). This increase was not correlated to the amount of C removed from the samples. It was mainly due to changes in SSA occurring in the clay fractions where amorphous organic matter was assumed to act as a glue binding some clay plates together.

Transfer of organic matter between wastewater and activated sludge flocs

The organic matter of wastewater was fractionated into settleable (i.e., particulate) and non-settleable (i.e., colloidal + soluble) fractions by settling followed by 0.22 micron filtration. Particulate, colloidal and soluble proportions were found to be relatively constant (45, 31 and 24% of the total COD, respectively). Transfer of soluble fraction always occurred from the wastewater to the activated sludge flocs, whereas bidirectional transfer occurred for the colloidal fraction. The transfer of soluble and colloidal matter reached a steady state after 40 min-mixing and 20 min-mixing, respectively. Desorption of a part of the colloidal organic matter pre-sorbed on the activated sludge flocs was evidenced. The biosorption capacity of activated sludge was around 40-100 mgCODg-1TSS. The biosorbable fraction of wastewater represented on average 45% of the non-settleable fraction.

Adsorption properties of TiO2 related to the photocatalytic degradation of organic contaminants in water

Abstract The purpose of this study was to correlate the photocatalytic degradation of water contaminants with the adsorption phenomena, taking both the equilibrium state and kinetics into consideration. 1,2-Dichloroethane (DCE) was chosen as a test pollutant. The adsorption isotherm was found to follow the Langmuir model. The kinetics of adsorption were studied and interpreted as a mass transfer process. The kinetics of photocatalytic degradation were analysed with special attention given to the influence of the reactant concentration. A kinetic model is proposed, which includes mass transfer, leading to an apparent Langmuir-Hinshelwood equation for the degradation rate, even when the kinetics are controlled by mass transfer. The experimental results show an agreement with the model at concentrations above 0.5 mmol l −1 , but at lower concentrations the rate is lower than expected, possibly due to a loss of DCE or hydroxyl radicals.

Toxicity of copper oxide nanoparticle suspensions to aquatic biota

Toxicity effects induced by nanosuspensions of CuO (<50 nm; Sigma-Aldrich) on macrophytic algae cells of Nitellopsis obtusa (96-h median lethal concentration [LC50]), microphytic algae Chlorella (30-min median inhibitory concentration [IC50]), shrimp Thamnocephalus platyurus (24-h LC50), and rotifer Brachionus calyciflorus (24-h LC50) were investigated. No substantial differences between the effects of nonsonicated and sonicated nCuO suspensions were observed. The particle size distribution analysis accomplished by the laser diffraction technique at suspension concentration from 3 to 100 mg/L revealed rapid (within 5 min) reagglomeration of the particles after the sonication. The observed adverse effects on N. obtusa cells may be attributed to nanoparticles per se, but not to ionic Cu, because neither chemical analysis nor biological testing (algae survival in the supernatants of suspensions) confirmed the presence of cupric ions in toxic amounts. Contrary to ionic Cu form, nCuO delayed the initial phase of N. obtusa cell membrane depolarization. Lethality tests with rewash demonstrated that the least used 5-min exposure in 100 mg/L nCuO sonicated suspension induced 70% mortality in charophyte cells after 8 d, whereas the rewash after a short exposure to a noticeably toxic concentration of Cu(2+) prevented cell mortality. The obtained data suggested the possible influence of a thick charophyte cell wall on the dynamics of nanotoxicity effects.