Search results for "Chemical engineering"

Effect of pH, cation concentration and sorbent concentration on cadmium and copper removal by a granular activated carbon

1999

The single adsorption of cadmium and copper from aqueous solutions has been investigated on Darco 12-20 mesh granular activated carbon for a wide range of experimental conditions: pH, metal concentration and carbon concentration. The results showed the efficiency of the activated carbon as sorbent for both metals. Metal removal increases on raising pH and carbon concentration, and decreases on raising the initial metal concentration. The adsorption processes have been modelled using the surface complex formation (SCF) Triple Layer Model (TLM) with a single surface bidentate species or with an overall surface species with fractional stoichiometry. Bidentate stoichiometry considering pH, meta…

Molecular Self-Assembly Versus Surface Restructuring During Calcite Dissolution.

2016

Organic additives are known to alter the mineral-water interface in various ways. On the one hand, organic molecules can self assemble into ordered structures wetting the surface. On the other hand, their presence can affect the interfacial morphology, referred to as surface restructuring. Here, we investigate the impact, of a class of calcium-complexing azo dyes on the dissolution of calcite (10.4) using high-resolution atomic force microscopy operated in aqueous solution, with a focus on the two constitutional isomers Eriochrome Black T and Eriochrome Black A. A very pronounced surface restructuring is observed in the presence of the dye solution, irrespective of the specific dye used and…

Molluscan shell proteins

2004

Abstract The shell secreted by molluscs is one of the most remarkable examples of a matrix-mediated mineralisation performed outside living tissues. The calcifying matrix is a mixture of proteins, glycoproteins, and polysaccharides that precisely self-assemble and control the CaCO 3 polymorph (calcite, aragonite), the size, the shapes of the crystallites, and finally, the texture of the shell. In spite of several biochemical studies, the molecular aspects of the shell building are far from being understood. The present article makes an overview of the most recent molecular data on the proteinaceous components of the shell matrix. These data put into question the classical models of mollusca…

Mercaptophenol-Protected Gold Colloids as Nuclei for the Crystallization of Inorganic Minerals:  Templated Crystallization on Curved Surfaces

1999

The self-assembly of monolayers of thiols on gold(111) surfaces yields substrates that are able to template in a controlled manner, the nucleation and growth of crystals of calcium carbonate from solution. In the absence of additives, various factors such as the nature of the thiol, the temperature, and the pH are now established as influencing the nature and relative amounts of the different CaCO3 phases (calcite, vaterite, and aragonite). Recently, we have been able to extend the use of thiol/gold self-assembled monolayers as templates for the growth of inorganic crystals by utilizing protected gold colloids instead of flat gold surfaces. The thiol monolayers that protect the colloids pro…

Structure-Dependent Dissolution and Restructuring of Calcite Surfaces by Organophosphonates

2017

Organophosphonates are well-known to strongly interact with the surfaces of various minerals, such as brucite, gypsum, and barite. In this work, we study the influence of six systematically varied organophosphonate molecules (tetraphosphonates and diphosphonates) on the dissolution process of the (10.4) surface of calcite. In order to pursue a systematic study, we have selected organophosphonates that exhibit similar structural features, but also systematic architectural differences. The effect of this class of additives on the dissolution process of the calcite (10.4) surface is evaluated using in situ dynamic atomic force microscopy. For all of the six organophosphonate derivatives, we ob…

Multianalytical Approach to explain the darkening process of hematite pigment in paintings from ancient Pompeii after accelerated weathering experime…

2013

[EN] In this paper, recently excavated fresco painting fragments from the House of Marcus Lucretius (Pompeii) and not exposed to the atmosphere since the eruption of the Mount Vesuvius were subjected to a controlled SO2 atmosphere and high relative humidity. These experiments were conducted in order to simulate under accelerated conditions the possible deterioration of the hematite pigment and plaster. The mineralogical transformation of the polychromy and plaster was monitored using mainly Raman spectroscopy, a non-destructive technique, but also infrared spectroscopy (FT-IR) and scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS). After different exposure cycles to…

Mechanochemical Access to Defect-Stabilized Amorphous Calcium Carbonate

2018

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. The lifetime of transient ACC in nature is regulated by an organic matrix, to use it as an intermediate storage buffer or as a permanent structural element. The relevance of ACC in material science is related to our understanding of CaCO3 crystallization pathways. ACC can be obtained by liquid–liquid phase separation, and it is typically stabilized with the help of macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. The ball-milled amorphous calcium carbonate (BM-ACC) was stabilized with small amounts of Na2CO3. The addition of foreign ions in form of Na2C…

Pseudomorphic transformation of amorphous calcium carbonate films follows spherulitic growth mechanisms and can give rise to crystal lattice tilting

2015

Amorphous calcium carbonate films synthesized by the polymer-induced liquid-precursor (PILP) process convert into crystallographically complex calcite spherulites. Tuning the experimental parameters allows for the generation of crystal lattice tilting similar to that found in calcareous biominerals. This contribution evidences the role of spherulitic growth mechanisms in pseudomorphic transformations of calcium carbonate.

2018

Synthetic materials based on calcium phosphate (CaP) are frequently used as bone graft substitutes when natural bone grafts are not available or not suitable. Chemical similarity to bone guarantees the biocompatibility of synthetic CaP materials, whereas macroporosity enables their integration into the natural bone tissue. To restore optimum mechanical performance after the grafting procedure, gradual resorption of CaP implants and simultaneous replacement by natural bone is desirable. Mg and Sr ions released from implants support osteointegration by stimulating bone formation. Furthermore, Sr ions counteract osteoporotic bone loss and reduce the probability of related fractures. The presen…

Early homogenous amorphous precursor stages of calcium carbonate and subsequent crystal growth in levitated droplets.

2008

An in situ study of the contact-free crystallization of calcium carbonate in acoustic levitated droplets is reported. The levitated droplet technique allows an in situ monitoring of the crystallization while avoiding any foreign phase boundaries that may influence the precipitation process by heterogeneous nucleation. The diffusion-controlled precipitation of CaCO3 at neutral pH starts in the initial step with the homogeneous formation of a stable, nanosized liquid-like amorphous calcium carbonate phase that undergoes in a subsequent step a solution-assisted transformation to calcite. Cryogenic scanning electron microscopy studies indicate that precipitation is not induced at the solution/a…