Sorption of Eu<Superscript>3+</Superscript>onto nano-size silica-water interfaces

The sorption of Eu species onto nano-size silica-water interfaces is investigated at pH range of 1―8.5 and the initial Eu concentrations (C_Eu) of 2×10⁻⁵, 2×10⁻⁴ and 2×10⁻³ M using fluorescence spectroscopy. The sorption rate of Eu is initially low, but significantly increases at pH > 4. For the initial C_Eu of 2×10⁻⁵, 2×10⁻⁴ and 2×10⁻³ M, the dissolved Eu species are completely sorbed onto silica-water interfaces at pH = 4.75, −5.8 and 6.6, respectively, with the respective sorption densities of −1.58×10⁻⁸, 1.58×10⁻⁷ and 1.58×10⁻⁶ mol/m². The sorbed Eu species at pH < 6 is aquo Eu³⁺, which is sorbed onto silica-water interfaces as an outer-sphere complex at pH < 5, but may be sorbed as an inner-sphere bidentate complex at 5 < pH < 6, due to the decrease of the N_H2O to −6 at pH = 6. At pH = 6 – 8, Eu(OH)²⁺, Eu(CO₃)⁺and Eu(CO₃)₂ ⁻ form in the solutions, and Eu(CO₃)⁺is dominant at pH = −7.5. These ions may be sorbed onto silica-water interfaces as inner-sphere bidentate complexes or multi-nuclear pre-cipitates.     

The behaviour of dissolved silica during the mixing of river and sea waters in Tokyo Bay

The behaviour of dissolved silica was studied in Tokyo Bay during six surveys in 1979 and 1980. The data from late spring and mid-summer samples showed concave mixing curves of silica versus salinity, whereas for the winter samples a simple conservative dillution curve was obtained. Plots of particulate silicon to particulate aluminium showed that even for winter samples as well as summer ones there were some processes removing silica from solution. The processes could not be adequately explained by adsorption onto suspended solids. The data are indicative of uptake by diatoms as the principal removal mechanism. Most of the diatom skeletons settled to the bottom where dissolution was rapid. The silica-salinity curves in this study thus demonstrate an apparent removal process for dissolved silica. This is because during summer the displacement rate of biogenic silica from a unit water column to the bottom as fecal pellets or by sinking is greater than the supply rate of dissolved silica by the action of diffusion and mixing of bottom water enriched with dissolved silica whereas in winter these rates are reasonably balanced.     

Phase de Laves dans la première opale CT bidisperse

A Mexican opal from the state of Jalisco has a bidisperse, perfectly ordered structure with framboid silica lepispheres of two different diameters. Its structure has been determined by the observation of a vicinal section, close to a dense plane. Fresh breaks along a vicinal plane emphasized regular steps, which were studied by means of SEM. We have been able to propose unit cell models, theoretical positions of the two sets of spheres, and probable space group, P6₃/mmc. This structure arranges similarly to that of a diatomic compound having an AB₂ stoichiometry and corresponds to a cubic (MgCu₂-type) or hexagonal (MgZn₂-type) Laves phase. This last variety seems to be the most probable structure. To cite this article: J.-P. Gauthier et al., C. R. Geoscience 336 (2004).     

Silica and the cycle of carbon in the ocean

Silicon is a key element whose major role in the control of the cycle of carbon in the ocean has been recently revealed. This is first illustrated through the resolution of the ‘opal paradox’ in the modern Southern Ocean. Second, the ‘silica hypothesis’ explains why, during the Last Glacial Maximum, the atmospheric concentration of CO₂ was about 40 % below that of the interglacial period. Increased deliveries of wind-borne silica to the surface ocean as well as of riverine inputs favoured the dominance of diatoms over coccolithophorids, resulting in a net emission decrease of CO₂ by coccolithophorids from the ocean surface to the atmosphere. To cite this article: P. Tréguer, C. R. Geoscience 334 (2002) 3–11     

Non-ionic surfactants adsorption: structure and thermodynamicsAdsorption des tensioactifs non ioniques : structure et thermodynamique

Adsorption of non-ionic surfactants from aqueous solutions has been studied for years because of its industrial relevance. These surfactants, made of an aliphatic part attached to a hydrophilic and polar chain, play an important role in diverse applications. Modelling of the adsorption mechanisms of surfactants has traditionally been based on the interpretation of adsorption isotherms. However, this first level of investigation is not sufficient for a complete understanding. Direct inspection of the adsorbed layer of non-ionic surfactant molecules at the solid–liquid interface is needed. This is an active field of research for almost 15 years, even if the subject is not completely settled down. The aim of this paper is to present a short review of this topic. I will restrict this paper to the case of non-ionic adsorption onto hydrophilic or polar solid interfaces. In the first part, I comment some experimental studies of non-ionic adsorption layer, including fluorescence spectroscopy, neutron scattering and atomic force microscopy. In the second part, a theoretical discussion about thermodynamics of adsorption is presented. A model involving an aggregative adsorption process is proposed. This last part will permit to raise some interesting questions concerning the role of the so-called ‘normal interaction’ (surfactant/solid interaction) to select the optimal surface aggregate curvature. To cite this article: P.E. Levitz, C. R. Geoscience 334 (2002) 665–673.     

Biogeochemistry of silica in Devils Lake: implications for diatom preservation

Diatom-salinity records from sediment cores have been used to construct climate records of saline-lake basins. In many cases, this has been done without thorough understanding of the preservation potential of the diatoms in the sediments through time. The purpose of this study was to determine the biogeochemistry of silica in Devils Lake and evaluate the potential effects of silica cycling on diatom preservation. During the period of record, 1867–1999, lake levels have fluctuated from 427 m above sea level in 1940 to 441.1 m above sea level in 1999. The biogeochemistry of silica in Devils Lake is dominated by internal cycling. During the early 1990s when lake levels were relatively high, about 94% of the biogenic silica (BSi) produced in Devils Lake was recycled in the water column before burial. About 42% of the BSi that was incorporated in bottom sediments was dissolved and diffused back into the lake, and the remaining 58% was buried. Therefore, the BSi accumulation rate was about 3% of the BSi assimilation rate. Generally, the results obtained from this study are similar to those obtained from studies of the biogeochemistry of silica in large oligotrophic lakes and the open ocean where most of the BSi produced is recycled in surface water. During the mid 1960s when lake levels were relatively low, BSi assimilation and water-column dissolution rates were much higher than when lake levels were high. The BSi assimilation rate was as much as three times higher during low lake levels. Even with the much higher BSi assimilation rate, the BSi accumulation rate was about three times lower because the BSi water-column dissolution rate was more than 99% of the BSi assimilation rate compared to 94% during high lake levels. Variations in the biogeochemistry of silica with lake level have important implications for paleolimnologic studies. Increased BSi water-column dissolution during decreasing lake levels may alter the diatom-salinity record by selectively removing the less resistant diatoms. Also, BSi accumulation may be proportional to the amount of silica input from tributary sources. Therefore, BSi accumulation chronologies from sediment cores may be effective records of tributary inflow.     

Aquatic system response to climatic and human changes: Productivity,bottom water oxygen status,and sapropel formation in Lake Lugano over the last 10 000 years

The Holocene record of Lake Lugano (southern basin: surface area 20.3 km², maximum depth 87 m) comprising organic carbon-rich sediments (sapropels), is divided into eight intervals based on radiocarbon- and varve-dating. The content of organic carbon, inorganic carbon, and biogenic silica, as well as the benthic remains of ostracods and oligochaetes, are converted into accumulation rates and benthic abundances in order to assess past production rates and bottom water oxygen status, respectively. The results suggest three periods of distinct palaeolimnological character: (i) low primary production combined with shifts between aerobic and anaerobic profundal conditions (prior to ca. 3000 BC), (ii) moderate rates of production combined with a relatively high profundal oxygen content (after ca. 1500 BC), and (iii), high production rates (460 g C m^–2 a^–1) combined with anaerobic profundal conditions (present eutrophic state). Corresponding organic carbon contents in the sediments are: up to 5% (i), 4% (ii), and 8% (iii). Until the beginning of this century, the flux of autochthonous sediments to the lake floor correlated with the fluctuations in the allochthonous sediment accumulation rate, indicating that catchment erosion largely controlled lacustrine production during the Holocene history of Lake Lugano. Pollen data show catchment-vegetational transformations at ca. 3500 BC (change from fir to beech forests), at 1400 BC (onset of cereal vegetation) and at ca. A.D. 450 (strong increase in various cultural plants). The first two changes had a relatively large imprint on lacustrine sedimentation. At ca. 3500 BP, erosion increase in the catchment was triggered by vegetation changes in the mountain zone above ca. 1000 m a. s. l., which may have been induced by climatic and human alteration (drop in the treeline altitude). Maximum catchment erosion occurred at ca. 1400 BC which was clearly dominated by human cultivation during the Bronze Age. More oxygenated profundal conditions in the lake after ca. 3000 BC are possibly related to a better mixing of the lake waters during the winter season by increased wind activity.     

Molar refraction of quartz family minerals

Different polymorphs of silica and the opals exhibit significant variations in densities. The paper discusses and correlates the variations in the molar refraction with density. It emerges that in different polymorphs of silica, the increase in density involves a corresponding increase in the characteristic dispersion frequencies leading to a decrease in the molar refraction. The variation of the molar refraction with density can be represented in terms of an elementary formula. The variation of the molar refraction of opals with the water content in them is also accounted for.     

The Source of Silica and Trace Metals in Particulate Matter in Framvaren Fjord

The lake and streams in the Framvarenwatershed were sampled and analyzed in September 1983.The results can be explained by the precipitation ofacid rain containing some sea salts and thedissolution of Farsundite, the dominating rock in thecatchment area. The runoff supplies the surface waterof Framvaren with silica, aluminium, manganese, iron,copper, zinc, cadmium and lead. Reasonablecalculations show that the runoff most likely is themain source for the particulate matter found insediment traps and by filtering off suspendedparticles. Thus the calculations supplement recentstudies of Skei et al. (1996). This revised version was published online in July 2006 with corrections to the Cover Date.     

从太平洋中部热液矿物的表现探讨热液活动的广泛性

位于太平洋中部的克利帕顿和克拉里昂断裂带之间的研究区处于远离大陆的远洋深水环境，区内断裂构造和火山活动非常发育，热水活动强烈，由此形成一系列与热液活动及蚀变沉积有关的矿物，如磷灰石、碳磷灰石、稀土磷灰石、磷铝锶石、重晶石、碳酸钡矿、泡饿矿及非晶质ＳｉＯ２球。这都说明大洋中热水溶液活动并不仅限于洋脊轴部和裂谷地区，只要有较大规模的断裂存在，就会有热水溶液活动或海底热泉出现，随之后形成一系列与热液活动